IP Sysctl¶

/proc/sys/net/ipv4/* Variables¶

ip_forward - BOOLEAN

Forward Packets between interfaces.

This variable is special, its change resets all configurationparameters to their default state (RFC1122 for hosts, RFC1812for routers)

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

ip_default_ttl - INTEGER

Default value of TTL field (Time To Live) for outgoing (but notforwarded) IP packets. Should be between 1 and 255 inclusive.Default: 64 (as recommended by RFC1700)

ip_no_pmtu_disc - INTEGER

Disable Path MTU Discovery. If enabled in mode 1 and afragmentation-required ICMP is received, the PMTU to thisdestination will be set to the smallest of the old MTU tothis destination and min_pmtu (see below). You will needto raise min_pmtu to the smallest interface MTU on your systemmanually if you want to avoid locally generated fragments.

In mode 2 incoming Path MTU Discovery messages will bediscarded. Outgoing frames are handled the same as in mode 1,implicitly setting IP_PMTUDISC_DONT on every created socket.

Mode 3 is a hardened pmtu discover mode. The kernel will onlyaccept fragmentation-needed errors if the underlying protocolcan verify them besides a plain socket lookup. Currentprotocols for which pmtu events will be honored are TCP andSCTP as they verify e.g. the sequence number or theassociation. This mode should not be enabled globally but isonly intended to secure e.g. name servers in namespaces whereTCP path mtu must still work but path MTU information of otherprotocols should be discarded. If enabled globally this modecould break other protocols.

Possible values: 0-3

Default: FALSE

min_pmtu - INTEGER

default 552 - minimum Path MTU. Unless this is changed manually,each cached pmtu will never be lower than this setting.

ip_forward_use_pmtu - BOOLEAN

By default we don’t trust protocol path MTUs while forwardingbecause they could be easily forged and can lead to unwantedfragmentation by the router.You only need to enable this if you have user-space softwarewhich tries to discover path mtus by itself and depends on thekernel honoring this information. This is normally not thecase.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

fwmark_reflect - BOOLEAN

Controls the fwmark of kernel-generated IPv4 reply packets that are notassociated with a socket for example, TCP RSTs or ICMP echo replies).If disabled, these packets have a fwmark of zero. If enabled, they have thefwmark of the packet they are replying to.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

fib_multipath_use_neigh - BOOLEAN

Use status of existing neighbor entry when determining nexthop formultipath routes. If disabled, neighbor information is not used andpackets could be directed to a failed nexthop. Only valid for kernelsbuilt with CONFIG_IP_ROUTE_MULTIPATH enabled.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

fib_multipath_hash_policy - INTEGER

Controls which hash policy to use for multipath routes. Only validfor kernels built with CONFIG_IP_ROUTE_MULTIPATH enabled.

Default: 0 (Layer 3)

Possible values:

0 - Layer 3
1 - Layer 4
2 - Layer 3 or inner Layer 3 if present
3 - Custom multipath hash. Fields used for multipath hash calculationare determined by fib_multipath_hash_fields sysctl

fib_multipath_hash_fields - UNSIGNED INTEGER

When fib_multipath_hash_policy is set to 3 (custom multipath hash), thefields used for multipath hash calculation are determined by thissysctl.

This value is a bitmask which enables various fields for multipath hashcalculation.

Possible fields are:

0x0001	Source IP address
0x0002	Destination IP address
0x0004	IP protocol
0x0008	Unused (Flow Label)
0x0010	Source port
0x0020	Destination port
0x0040	Inner source IP address
0x0080	Inner destination IP address
0x0100	Inner IP protocol
0x0200	Inner Flow Label
0x0400	Inner source port
0x0800	Inner destination port

Default: 0x0007 (source IP, destination IP and IP protocol)

fib_multipath_hash_seed - UNSIGNED INTEGER

The seed value used when calculating hash for multipath routes. Appliesto both IPv4 and IPv6 datapath. Only present for kernels built withCONFIG_IP_ROUTE_MULTIPATH enabled.

When set to 0, the seed value used for multipath routing defaults to aninternal random-generated one.

The actual hashing algorithm is not specified -- there is no guaranteethat a next hop distribution effected by a given seed will keep stableacross kernel versions.

Default: 0 (random)

fib_sync_mem - UNSIGNED INTEGER

Amount of dirty memory from fib entries that can be backlogged beforesynchronize_rcu is forced.

Default: 512kB Minimum: 64kB Maximum: 64MB

ip_forward_update_priority - INTEGER

Whether to update SKB priority from “TOS” field in IPv4 header after itis forwarded. The new SKB priority is mapped from TOS field valueaccording to an rt_tos2priority table (see e.g. man tc-prio).

Default: 1 (Update priority.)

Possible values:

0 - Do not update priority.
1 - Update priority.

route/max_size - INTEGER

Maximum number of routes allowed in the kernel. Increasethis when using large numbers of interfaces and/or routes.

From linux kernel 3.6 onwards, this is deprecated for ipv4as route cache is no longer used.

From linux kernel 6.3 onwards, this is deprecated for ipv6as garbage collection manages cached route entries.

neigh/default/gc_thresh1 - INTEGER

Minimum number of entries to keep. Garbage collector will notpurge entries if there are fewer than this number.

Default: 128

neigh/default/gc_thresh2 - INTEGER

Threshold when garbage collector becomes more aggressive aboutpurging entries. Entries older than 5 seconds will be clearedwhen over this number.

Default: 512

neigh/default/gc_thresh3 - INTEGER

Maximum number of non-PERMANENT neighbor entries allowed. Increasethis when using large numbers of interfaces and when communicatingwith large numbers of directly-connected peers.

Default: 1024

neigh/default/unres_qlen_bytes - INTEGER

The maximum number of bytes which may be used by packetsqueued for each unresolved address by other network layers.(added in linux 3.3)

Setting negative value is meaningless and will return error.

Default: SK_WMEM_DEFAULT, (same as net.core.wmem_default).

Exact value depends on architecture and kernel options,but should be enough to allow queuing 256 packetsof medium size.

neigh/default/unres_qlen - INTEGER

The maximum number of packets which may be queued for eachunresolved address by other network layers.

(deprecated in linux 3.3) : use unres_qlen_bytes instead.

Prior to linux 3.3, the default value is 3 which may causeunexpected packet loss. The current default value is calculatedaccording to default value of unres_qlen_bytes and true size ofpacket.

Default: 101

neigh/default/interval_probe_time_ms - INTEGER

The probe interval for neighbor entries with NTF_MANAGED flag,the min value is 1.

Default: 5000

mtu_expires - INTEGER

Time, in seconds, that cached PMTU information is kept.

min_adv_mss - INTEGER

The advertised MSS depends on the first hop route MTU, but willnever be lower than this setting.

fib_notify_on_flag_change - INTEGER

Whether to emit RTM_NEWROUTE notifications whenever RTM_F_OFFLOAD/RTM_F_TRAP/RTM_F_OFFLOAD_FAILED flags are changed.

After installing a route to the kernel, user space receives anacknowledgment, which means the route was installed in the kernel,but not necessarily in hardware.It is also possible for a route already installed in hardware to changeits action and therefore its flags. For example, a host route that istrapping packets can be “promoted” to perform decapsulation followingthe installation of an IPinIP/VXLAN tunnel.The notifications will indicate to user-space the state of the route.

Default: 0 (Do not emit notifications.)

Possible values:

0 - Do not emit notifications.
1 - Emit notifications.
2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change.

IP Fragmentation:

ipfrag_high_thresh - LONG INTEGER

Maximum memory used to reassemble IP fragments.

ipfrag_low_thresh - LONG INTEGER

(Obsolete since linux-4.17)Maximum memory used to reassemble IP fragments before the kernelbegins to remove incomplete fragment queues to free up resources.The kernel still accepts new fragments for defragmentation.

ipfrag_time - INTEGER

Time in seconds to keep an IP fragment in memory.

ipfrag_max_dist - INTEGER

ipfrag_max_dist is a non-negative integer value which defines themaximum “disorder” which is allowed among fragments which share acommon IP source address. Note that reordering of packets isnot unusual, but if a large number of fragments arrive from a sourceIP address while a particular fragment queue remains incomplete, itprobably indicates that one or more fragments belonging to that queuehave been lost. When ipfrag_max_dist is positive, an additional checkis done on fragments before they are added to a reassembly queue - ifipfrag_max_dist (or more) fragments have arrived from a particular IPaddress between additions to any IP fragment queue using that sourceaddress, it’s presumed that one or more fragments in the queue arelost. The existing fragment queue will be dropped, and a new onestarted. An ipfrag_max_dist value of zero disables this check.

Using a very small value, e.g. 1 or 2, for ipfrag_max_dist canresult in unnecessarily dropping fragment queues when normalreordering of packets occurs, which could lead to poor applicationperformance. Using a very large value, e.g. 50000, increases thelikelihood of incorrectly reassembling IP fragments that originatefrom different IP datagrams, which could result in data corruption.Default: 64

bc_forwarding - INTEGER

bc_forwarding enables the feature described in rfc1812#section-5.3.5.2and rfc2644. It allows the router to forward directed broadcast.To enable this feature, the ‘all’ entry and the input interface entryshould be set to 1.Default: 0

INET peer storage¶

inet_peer_threshold - INTEGER: The approximate size of the storage. Starting from this thresholdentries will be thrown aggressively. This threshold also determinesentries’ time-to-live and time intervals between garbage collectionpasses. More entries, less time-to-live, less GC interval.
inet_peer_minttl - INTEGER: Minimum time-to-live of entries. Should be enough to cover fragmenttime-to-live on the reassembling side. This minimum time-to-live isguaranteed if the pool size is less than inet_peer_threshold.Measured in seconds.
inet_peer_maxttl - INTEGER: Maximum time-to-live of entries. Unused entries will expire afterthis period of time if there is no memory pressure on the pool (i.e.when the number of entries in the pool is very small).Measured in seconds.

TCP variables¶

somaxconn - INTEGER

Limit of socketlisten() backlog, known in userspace as SOMAXCONN.Defaults to 4096. (Was 128 before linux-5.4)See also tcp_max_syn_backlog for additional tuning for TCP sockets.

tcp_abort_on_overflow - BOOLEAN

If listening service is too slow to accept new connections,reset them. Default state is FALSE. It means that if overflowoccurred due to a burst, connection will recover. Enable thisoption _only_ if you are really sure that listening daemoncannot be tuned to accept connections faster. Enabling thisoption can harm clients of your server.

tcp_adv_win_scale - INTEGER

Obsolete since linux-6.6Count buffering overhead as bytes/2^tcp_adv_win_scale(if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),if it is <= 0.

Possible values are [-31, 31], inclusive.

Default: 1

tcp_allowed_congestion_control - STRING

Show/set the congestion control choices available to non-privilegedprocesses. The list is a subset of those listed intcp_available_congestion_control.

Default is “reno” and the default setting (tcp_congestion_control).

tcp_app_win - INTEGER

Reserve max(window/2^tcp_app_win, mss) of window for applicationbuffer. Value 0 is special, it means that nothing is reserved.

Possible values are [0, 31], inclusive.

Default: 31

tcp_autocorking - BOOLEAN

Enable TCP auto corking :When applications do consecutive small write()/sendmsg() system calls,we try to coalesce these small writes as much as possible, to lowertotal amount of sent packets. This is done if at least one priorpacket for the flow is waiting in Qdisc queues or device transmitqueue. Applications can still use TCP_CORK for optimal behaviorwhen they know how/when to uncork their sockets.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

tcp_available_congestion_control - STRING

Shows the available congestion control choices that are registered.More congestion control algorithms may be available as modules,but not loaded.

tcp_base_mss - INTEGER

The initial value of search_low to be used by the packetization layerPath MTU discovery (MTU probing). If MTU probing is enabled,this is the initial MSS used by the connection.

tcp_mtu_probe_floor - INTEGER

If MTU probing is enabled this caps the minimum MSS used for search_lowfor the connection.

Default : 48

tcp_min_snd_mss - INTEGER

TCP SYN and SYNACK messages usually advertise an ADVMSS option,as described in RFC 1122 and RFC 6691.

If this ADVMSS option is smaller than tcp_min_snd_mss,it is silently capped to tcp_min_snd_mss.

Default : 48 (at least 8 bytes of payload per segment)

tcp_congestion_control - STRING

Set the congestion control algorithm to be used for newconnections. The algorithm “reno” is always available, butadditional choices may be available based on kernel configuration.Default is set as part of kernel configuration.For passive connections, the listener congestion control choiceis inherited.

[see setsockopt(listenfd, SOL_TCP, TCP_CONGESTION, “name” ...) ]

tcp_dsack - BOOLEAN

Allows TCP to send “duplicate” SACKs.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

tcp_early_retrans - INTEGER

Tail loss probe (TLP) converts RTOs occurring due to taillosses into fast recovery (RFC8985). Note thatTLP requires RACK to function properly (see tcp_recovery below)

Possible values:

0 disables TLP
3 or 4 enables TLP

Default: 3

tcp_ecn - INTEGER

Control use of Explicit Congestion Notification (ECN) by TCP.ECN is used only when both ends of the TCP connection indicate supportfor it. This feature is useful in avoiding losses due to congestion byallowing supporting routers to signal congestion before having to droppackets. A host that supports ECN both sends ECN at the IP layer andfeeds back ECN at the TCP layer. The highest variant of ECN feedbackthat both peers support is chosen by the ECN negotiation (Accurate ECN,ECN, or no ECN).

The highest negotiated variant for incoming connection requestsand the highest variant requested by outgoing connectionattempts:

Value	Incoming connections	Outgoing connections
0	No ECN	No ECN
1	ECN	ECN
2	ECN	No ECN
3	AccECN	AccECN
4	AccECN	ECN
5	AccECN	No ECN

Default: 2

tcp_ecn_option - INTEGER

Control Accurate ECN (AccECN) option sending when AccECN has beensuccessfully negotiated during handshake. Send logic inhibitssending AccECN options regarless of this setting when no AccECNoption has been seen for the reverse direction.

Possible values are:

0	Never send AccECN option. This also disables sending AccECNoption in SYN/ACK during handshake.
1	Send AccECN option sparingly according to the minimum optionrules outlined in draft-ietf-tcpm-accurate-ecn.
2	Send AccECN option on every packet whenever it fits into TCPoption space.

Default: 2

tcp_ecn_option_beacon - INTEGER

Control Accurate ECN (AccECN) option sending frequency per RTT and ittakes effect only when tcp_ecn_option is set to 2.

Default: 3 (AccECN will be send at least 3 times per RTT)

tcp_ecn_fallback - BOOLEAN

If the kernel detects that ECN connection misbehaves, enable fallback to non-ECN. Currently, this knob implements the fallbackfrom RFC3168, section 6.1.1.1., but we reserve that in future,additional detection mechanisms could be implemented under thisknob. The value is not used, if tcp_ecn or per route (or congestioncontrol) ECN settings are disabled.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

tcp_fack - BOOLEAN

This is a legacy option, it has no effect anymore.

tcp_fin_timeout - INTEGER

The length of time an orphaned (no longer referenced by anyapplication) connection will remain in the FIN_WAIT_2 statebefore it is aborted at the local end. While a perfectlyvalid “receive only” state for an un-orphaned connection, anorphaned connection in FIN_WAIT_2 state could otherwise waitforever for the remote to close its end of the connection.

Cf. tcp_max_orphans

Default: 60 seconds

tcp_frto - INTEGER

Enables Forward RTO-Recovery (F-RTO) defined in RFC5682.F-RTO is an enhanced recovery algorithm for TCP retransmissiontimeouts. It is particularly beneficial in networks where theRTT fluctuates (e.g., wireless). F-RTO is sender-side onlymodification. It does not require any support from the peer.

By default it’s enabled with a non-zero value. 0 disables F-RTO.

tcp_fwmark_accept - BOOLEAN

If enabled, incoming connections to listening sockets that do not have asocket mark will set the mark of the accepting socket to the fwmark ofthe incoming SYN packet. This will cause all packets on that connection(starting from the first SYNACK) to be sent with that fwmark. Thelistening socket’s mark is unchanged. Listening sockets that alreadyhave a fwmark set via setsockopt(SOL_SOCKET, SO_MARK, ...) areunaffected.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

tcp_invalid_ratelimit - INTEGER

Limit the maximal rate for sending duplicate acknowledgmentsin response to incoming TCP packets that are for an existingconnection but that are invalid due to any of these reasons:

out-of-window sequence number,
out-of-window acknowledgment number, or
PAWS (Protection Against Wrapped Sequence numbers) check failure

This can help mitigate simple “ack loop” DoS attacks, whereina buggy or malicious middlebox or man-in-the-middle canrewrite TCP header fields in manner that causes each endpointto think that the other is sending invalid TCP segments, thuscausing each side to send an unterminating stream of duplicateacknowledgments for invalid segments.

Using 0 disables rate-limiting of dupacks in response toinvalid segments; otherwise this value specifies the minimalspace between sending such dupacks, in milliseconds.

Default: 500 (milliseconds).

tcp_keepalive_time - INTEGER

How often TCP sends out keepalive messages when keepalive is enabled.Default: 2hours.

tcp_keepalive_probes - INTEGER

How many keepalive probes TCP sends out, until it decides that theconnection is broken. Default value: 9.

tcp_keepalive_intvl - INTEGER

How frequently the probes are send out. Multiplied bytcp_keepalive_probes it is time to kill not responding connection,after probes started. Default value: 75sec i.e. connectionwill be aborted after ~11 minutes of retries.

tcp_l3mdev_accept - BOOLEAN

Enables child sockets to inherit the L3 master device index.Enabling this option allows a “global” listen socket to workacross L3 master domains (e.g., VRFs) with connected socketsderived from the listen socket to be bound to the L3 domain inwhich the packets originated. Only valid when the kernel wascompiled with CONFIG_NET_L3_MASTER_DEV.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

tcp_low_latency - BOOLEAN

This is a legacy option, it has no effect anymore.

tcp_max_orphans - INTEGER

Maximal number of TCP sockets not attached to any user file handle,held by system. If this number is exceeded orphaned connections arereset immediately and warning is printed. This limit existsonly to prevent simple DoS attacks, you _must_ not rely on thisor lower the limit artificially, but rather increase it(probably, after increasing installed memory),if network conditions require more than default value,and tune network services to linger and kill such statesmore aggressively. Let me to remind again: each orphan eatsup to ~64K of unswappable memory.

tcp_max_syn_backlog - INTEGER

Maximal number of remembered connection requests (SYN_RECV),which have not received an acknowledgment from connecting client.

This is a per-listener limit.

The minimal value is 128 for low memory machines, and it willincrease in proportion to the memory of machine.

If server suffers from overload, try increasing this number.

Remember to also check /proc/sys/net/core/somaxconnA SYN_RECV request socket consumes about 304 bytes of memory.

tcp_max_tw_buckets - INTEGER

Maximal number of timewait sockets held by system simultaneously.If this number is exceeded time-wait socket is immediately destroyedand warning is printed. This limit exists only to preventsimple DoS attacks, you _must_ not lower the limit artificially,but rather increase it (probably, after increasing installed memory),if network conditions require more than default value.

tcp_mem - vector of 3 INTEGERs: min, pressure, max

min: below this number of pages TCP is not bothered about itsmemory appetite.

pressure: when amount of memory allocated by TCP exceeds this numberof pages, TCP moderates its memory consumption and enters memorypressure mode, which is exited when memory consumption fallsunder “min”.

max: number of pages allowed for queueing by all TCP sockets.

Defaults are calculated at boot time from amount of availablememory.

tcp_min_rtt_wlen - INTEGER

The window length of the windowed min filter to track the minimum RTT.A shorter window lets a flow more quickly pick up new (higher)minimum RTT when it is moved to a longer path (e.g., due to trafficengineering). A longer window makes the filter more resistant to RTTinflations such as transient congestion. The unit is seconds.

Possible values: 0 - 86400 (1 day)

Default: 300

tcp_moderate_rcvbuf - BOOLEAN

If enabled, TCP performs receive buffer auto-tuning, attempting toautomatically size the buffer (no greater than tcp_rmem[2]) tomatch the size required by the path for full throughput.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

tcp_rcvbuf_low_rtt - INTEGER

rcvbuf autotuning can over estimate final socket rcvbuf, whichcan lead to cache trashing for high throughput flows.

For small RTT flows (below tcp_rcvbuf_low_rtt usecs), we can relaxrcvbuf growth: Few additional ms to reach the final (and smaller)rcvbuf is a good tradeoff.

Default : 1000 (1 ms)

tcp_mtu_probing - INTEGER

Controls TCP Packetization-Layer Path MTU Discovery. Takes threevalues:

0 - Disabled
1 - Disabled by default, enabled when an ICMP black hole detected
2 - Always enabled, use initial MSS of tcp_base_mss.

tcp_probe_interval - UNSIGNED INTEGER

Controls how often to start TCP Packetization-Layer Path MTUDiscovery reprobe. The default is reprobing every 10 minutes asper RFC4821.

tcp_probe_threshold - INTEGER

Controls when TCP Packetization-Layer Path MTU Discovery probingwill stop in respect to the width of search range in bytes. Defaultis 8 bytes.

tcp_no_metrics_save - BOOLEAN

By default, TCP saves various connection metrics in the route cachewhen the connection closes, so that connections established in thenear future can use these to set initial conditions. Usually, thisincreases overall performance, but may sometimes cause performancedegradation. If enabled, TCP will not cache metrics on closingconnections.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

tcp_no_ssthresh_metrics_save - BOOLEAN

Controls whether TCP saves ssthresh metrics in the route cache.If enabled, ssthresh metrics are disabled.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

tcp_orphan_retries - INTEGER

This value influences the timeout of a locally closed TCP connection,when RTO retransmissions remain unacknowledged.See tcp_retries2 for more details.

The default value is 8.

If your machine is a loaded WEB server,you should think about lowering this value, such socketsmay consume significant resources. Cf. tcp_max_orphans.

tcp_recovery - INTEGER

This value is a bitmap to enable various experimental loss recoveryfeatures.

RACK: 0x1	enables RACK loss detection, for fast detection of lostretransmissions and tail drops, and resilience toreordering. currently, setting this bit to 0 has noeffect, since RACK is the only supported loss detectionalgorithm.
RACK: 0x2	makes RACK’s reordering window static (min_rtt/4).
RACK: 0x4	disables RACK’s DUPACK threshold heuristic

Default: 0x1

tcp_reflect_tos - BOOLEAN

For listening sockets, reuse the DSCP value of the initial SYN messagefor outgoing packets. This allows to have both directions of a TCPstream to use the same DSCP value, assuming DSCP remains unchanged forthe lifetime of the connection.

This options affects both IPv4 and IPv6.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

tcp_reordering - INTEGER

Initial reordering level of packets in a TCP stream.TCP stack can then dynamically adjust flow reordering levelbetween this initial value and tcp_max_reordering

Default: 3

tcp_max_reordering - INTEGER

Maximal reordering level of packets in a TCP stream.300 is a fairly conservative value, but you might increase itif paths are using per packet load balancing (like bonding rr mode)

Default: 300

tcp_retrans_collapse - BOOLEAN

Bug-to-bug compatibility with some broken printers.On retransmit try to send bigger packets to work around bugs incertain TCP stacks.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

tcp_retries1 - INTEGER

This value influences the time, after which TCP decides, thatsomething is wrong due to unacknowledged RTO retransmissions,and reports this suspicion to the network layer.See tcp_retries2 for more details.

RFC 1122 recommends at least 3 retransmissions, which is thedefault.

tcp_retries2 - INTEGER

This value influences the timeout of an alive TCP connection,when RTO retransmissions remain unacknowledged.Given a value of N, a hypothetical TCP connection followingexponential backoff with an initial RTO of TCP_RTO_MIN wouldretransmit N times before killing the connection at the (N+1)th RTO.

The default value of 15 yields a hypothetical timeout of 924.6seconds and is a lower bound for the effective timeout.TCP will effectively time out at the first RTO which exceeds thehypothetical timeout.If tcp_rto_max_ms is decreased, it is recommended to alsochange tcp_retries2.

RFC 1122 recommends at least 100 seconds for the timeout,which corresponds to a value of at least 8.

tcp_rfc1337 - BOOLEAN

If enabled, the TCP stack behaves conforming to RFC1337. If unset,we are not conforming to RFC, but prevent TCP TIME_WAITassassination.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

tcp_rmem - vector of 3 INTEGERs: min, default, max

min: Minimal size of receive buffer used by TCP sockets.It is guaranteed to each TCP socket, even under moderate memorypressure.

Default: 4K

default: initial size of receive buffer used by TCP sockets.This value overrides net.core.rmem_default used by other protocols.Default: 131072 bytes.This value results in initial window of 65535.

max: maximal size of receive buffer allowed for automaticallyselected receiver buffers for TCP socket.Callingsetsockopt() with SO_RCVBUF disablesautomatic tuning of that socket’s receive buffer size, in whichcase this value is ignored.Default: between 131072 and 32MB, depending on RAM size.

tcp_sack - BOOLEAN

Enable select acknowledgments (SACKS).

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

tcp_comp_sack_rtt_percent - INTEGER

Percentage of SRTT used for the compressed SACK feature.See tcp_comp_sack_nr, tcp_comp_sack_delay_ns, tcp_comp_sack_slack_ns.

Possible values : 1 - 1000

Default : 33 %

tcp_comp_sack_delay_ns - LONG INTEGER

TCP tries to reduce number of SACK sent, using a timer basedon tcp_comp_sack_rtt_percent of SRTT, capped by this sysctlin nano seconds.The default is 1ms, based on TSO autosizing period.

Default : 1,000,000 ns (1 ms)

tcp_comp_sack_slack_ns - LONG INTEGER

This sysctl control the slack used when arming thetimer used by SACK compression. This gives extra timefor small RTT flows, and reduces system overhead by allowingopportunistic reduction of timer interrupts.Too big values might reduce goodput.

Default : 10,000 ns (10 us)

tcp_comp_sack_nr - INTEGER

Max number of SACK that can be compressed.Using 0 disables SACK compression.

Default : 44

tcp_backlog_ack_defer - BOOLEAN

If enabled, user thread processing socket backlog tries sendingone ACK for the whole queue. This helps to avoid potentiallong latencies at end of a TCP socket syscall.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

tcp_slow_start_after_idle - BOOLEAN

If enabled, provide RFC2861 behavior and time out the congestionwindow after an idle period. An idle period is defined atthe current RTO. If unset, the congestion window will notbe timed out after an idle period.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

tcp_stdurg - BOOLEAN

Use the Host requirements interpretation of the TCP urgent pointer field.Most hosts use the older BSD interpretation, so if enabled,Linux might not communicate correctly with them.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

tcp_synack_retries - INTEGER

Number of times SYNACKs for a passive TCP connection attempt willbe retransmitted. Should not be higher than 255. Default valueis 5, which corresponds to 31seconds till the last retransmissionwith the current initial RTO of 1second. With this the final timeoutfor a passive TCP connection will happen after 63seconds.

tcp_syncookies - INTEGER

Only valid when the kernel was compiled with CONFIG_SYN_COOKIESSend out syncookies when the syn backlog queue of a socketoverflows. This is to prevent against the common ‘SYN flood attack’Default: 1

Note, that syncookies is fallback facility.It MUST NOT be used to help highly loaded servers to standagainst legal connection rate. If you see SYN flood warningsin your logs, but investigation shows that they occurbecause of overload with legal connections, you should tuneanother parameters until this warning disappear.See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.

syncookies seriously violate TCP protocol, do not allowto use TCP extensions, can result in serious degradationof some services (f.e. SMTP relaying), visible not by you,but your clients and relays, contacting you. While you seeSYN flood warnings in logs not being really flooded, your serveris seriously misconfigured.

If you want to test which effects syncookies have to yournetwork connections you can set this knob to 2 to enableunconditionally generation of syncookies.

tcp_migrate_req - BOOLEAN

The incoming connection is tied to a specific listening socket whenthe initial SYN packet is received during the three-way handshake.When a listener is closed, in-flight request sockets during thehandshake and established sockets in the accept queue are aborted.

If the listener has SO_REUSEPORT enabled, other listeners on thesame port should have been able to accept such connections. Thisoption makes it possible to migrate such child sockets to anotherlistener after close() orshutdown().

The BPF_SK_REUSEPORT_SELECT_OR_MIGRATE type of eBPF program shouldusually be used to define the policy to pick an alive listener.Otherwise, the kernel will randomly pick an alive listener only ifthis option is enabled.

Note that migration between listeners with different settings maycrash applications. Let’s say migration happens from listener A toB, and only B has TCP_SAVE_SYN enabled. B cannot read SYN data fromthe requests migrated from A. To avoid such a situation, cancelmigration by returning SK_DROP in the type of eBPF program, ordisable this option.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

tcp_fastopen - INTEGER

Enable TCP Fast Open (RFC7413) to send and accept data in the openingSYN packet.

The client support is enabled by flag 0x1 (on by default). The clientthen must usesendmsg() orsendto() with the MSG_FASTOPEN flag,rather thanconnect() to send data in SYN.

The server support is enabled by flag 0x2 (off by default). Theneither enable for all listeners with another flag (0x400) orenable individual listeners via TCP_FASTOPEN socket option withthe option value being the length of the syn-data backlog.

The values (bitmap) are

0x1	(client)	enables sending data in the opening SYN on the client.
0x2	(server)	enables the server support, i.e., allowing data ina SYN packet to be accepted and passed to theapplication before 3-way handshake finishes.
0x4	(client)	send data in the opening SYN regardless of cookieavailability and without a cookie option.
0x200	(server)	accept data-in-SYN w/o any cookie option present.
0x400	(server)	enable all listeners to support Fast Open bydefault without explicit TCP_FASTOPEN socket option.

Default: 0x1

Note that additional client or server features are onlyeffective if the basic support (0x1 and 0x2) are enabled respectively.

tcp_fastopen_blackhole_timeout_sec - INTEGER

Initial time period in second to disable Fastopen on active TCP socketswhen a TFO firewall blackhole issue happens.This time period will grow exponentially when more blackhole issuesget detected right after Fastopen is re-enabled and will reset toinitial value when the blackhole issue goes away.0 to disable the blackhole detection.

By default, it is set to 0 (feature is disabled).

tcp_fastopen_key - list of comma separated 32-digit hexadecimal INTEGERs

The list consists of a primary key and an optional backup key. Theprimary key is used for both creating and validating cookies, while theoptional backup key is only used for validating cookies. The purpose ofthe backup key is to maximize TFO validation when keys are rotated.

A randomly chosen primary key may be configured by the kernel ifthe tcp_fastopen sysctl is set to 0x400 (see above), or if theTCP_FASTOPENsetsockopt() optname is set and a key has not beenpreviously configured via sysctl. If keys are configured viasetsockopt() by using the TCP_FASTOPEN_KEY optname, then thoseper-socket keys will be used instead of any keys that are specified viasysctl.

A key is specified as 4 8-digit hexadecimal integers which are separatedby a ‘-’ as: xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx. Leading zeros may beomitted. A primary and a backup key may be specified by separating themby a comma. If only one key is specified, it becomes the primary key andany previously configured backup keys are removed.

tcp_syn_retries - INTEGER

Number of times initial SYNs for an active TCP connection attemptwill be retransmitted. Should not be higher than 127. Default valueis 6, which corresponds to 67seconds (with tcp_syn_linear_timeouts = 4)till the last retransmission with the current initial RTO of 1second.With this the final timeout for an active TCP connection attemptwill happen after 131seconds.

tcp_timestamps - INTEGER

Enable timestamps as defined in RFC1323.

0: Disabled.
1: Enable timestamps as defined in RFC1323 and use random offset foreach connection rather than only using the current time.
2: Like 1, but without random offsets.

Default: 1

tcp_min_tso_segs - INTEGER

Minimal number of segments per TSO frame.

Since linux-3.12, TCP does an automatic sizing of TSO frames,depending on flow rate, instead of filling 64Kbytes packets.For specific usages, it’s possible to force TCP to build bigTSO frames. Note that TCP stack might split too big TSO packetsif available window is too small.

Default: 2

tcp_tso_rtt_log - INTEGER

Adjustment of TSO packet sizes based on min_rtt

Starting from linux-5.18, TCP autosizing can be tweakedfor flows having small RTT.

Old autosizing was splitting the pacing budget to send 1024 TSOper second.

tso_packet_size = sk->sk_pacing_rate / 1024;

With the new mechanism, we increase this TSO sizing using:

distance = min_rtt_usec / (2^tcp_tso_rtt_log)tso_packet_size += gso_max_size >> distance;

This means that flows between very close hosts can use biggerTSO packets, reducing their cpu costs.

If you want to use the old autosizing, set this sysctl to 0.

Default: 9 (2^9 = 512 usec)

tcp_pacing_ss_ratio - INTEGER

sk->sk_pacing_rate is set by TCP stack using a ratio appliedto current rate. (current_rate = cwnd * mss / srtt)If TCP is in slow start, tcp_pacing_ss_ratio is appliedto let TCP probe for bigger speeds, assuming cwnd can bedoubled every other RTT.

Default: 200

tcp_pacing_ca_ratio - INTEGER

sk->sk_pacing_rate is set by TCP stack using a ratio appliedto current rate. (current_rate = cwnd * mss / srtt)If TCP is in congestion avoidance phase, tcp_pacing_ca_ratiois applied to conservatively probe for bigger throughput.

Default: 120

tcp_syn_linear_timeouts - INTEGER

The number of times for an active TCP connection to retransmit SYNs witha linear backoff timeout before defaulting to an exponential backofftimeout. This has no effect on SYNACK at the passive TCP side.

With an initial RTO of 1 and tcp_syn_linear_timeouts = 4 we wouldexpect SYN RTOs to be: 1, 1, 1, 1, 1, 2, 4, ... (4 linear timeouts,and the first exponential backoff using 2^0 * initial_RTO).Default: 4

tcp_tso_win_divisor - INTEGER

This allows control over what percentage of the congestion windowcan be consumed by a single TSO frame.The setting of this parameter is a choice between burstiness andbuilding larger TSO frames.

Default: 3

tcp_tw_reuse - INTEGER

Enable reuse of TIME-WAIT sockets for new connections when it issafe from protocol viewpoint.

0 - disable
1 - global enable
2 - enable for loopback traffic only

It should not be changed without advice/request of technicalexperts.

Default: 2

tcp_tw_reuse_delay - UNSIGNED INTEGER

The delay in milliseconds before a TIME-WAIT socket can be reused by anew connection, if TIME-WAIT socket reuse is enabled. The actual reusethreshold is within [N, N+1] range, where N is the requested delay inmilliseconds, to ensure the delay interval is never shorter than theconfigured value.

This setting contains an assumption about the other TCP timestamp clocktick interval. It should not be set to a value lower than the peer’sclock tick for PAWS (Protection Against Wrapped Sequence numbers)mechanism work correctly for the reused connection.

Default: 1000 (milliseconds)

tcp_window_scaling - BOOLEAN

Enable window scaling as defined in RFC1323.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

tcp_shrink_window - BOOLEAN

This changes how the TCP receive window is calculated.

RFC 7323, section 2.4, says there are instances when a retractedwindow can be offered, and that TCP implementations MUST ensurethat they handle a shrinking window, as specified in RFC 1122.

Possible values:

0 (disabled) - The window is never shrunk.
1 (enabled) - The window is shrunk when necessary to remain withinthe memory limit set by autotuning (sk_rcvbuf).This only occurs if a non-zero receive windowscaling factor is also in effect.

Default: 0 (disabled)

tcp_wmem - vector of 3 INTEGERs: min, default, max

min: Amount of memory reserved for send buffers for TCP sockets.Each TCP socket has rights to use it due to fact of its birth.

Default: 4K

default: initial size of send buffer used by TCP sockets. Thisvalue overrides net.core.wmem_default used by other protocols.

It is usually lower than net.core.wmem_default.

Default: 16K

max: Maximal amount of memory allowed for automatically tunedsend buffers for TCP sockets. This value does not overridenet.core.wmem_max. Callingsetsockopt() with SO_SNDBUF disablesautomatic tuning of that socket’s send buffer size, in which casethis value is ignored.

Default: between 64K and 4MB, depending on RAM size.

tcp_notsent_lowat - UNSIGNED INTEGER

A TCP socket can control the amount of unsent bytes in its write queue,thanks to TCP_NOTSENT_LOWAT socket option. poll()/select()/epoll()reports POLLOUT events if the amount of unsent bytes is below a persocket value, and if the write queue is not full.sendmsg() willalso not add new buffers if the limit is hit.

This global variable controls the amount of unsent data forsockets not using TCP_NOTSENT_LOWAT. For these sockets, a changeto the global variable has immediate effect.

Default: UINT_MAX (0xFFFFFFFF)

tcp_workaround_signed_windows - BOOLEAN

If enabled, assume no receipt of a window scaling option means theremote TCP is broken and treats the window as a signed quantity.If disabled, assume the remote TCP is not broken even if we donot receive a window scaling option from them.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

tcp_thin_linear_timeouts - BOOLEAN

Enable dynamic triggering of linear timeouts for thin streams.If enabled, a check is performed upon retransmission by timeout todetermine if the stream is thin (less than 4 packets in flight).As long as the stream is found to be thin, up to 6 lineartimeouts may be performed before exponential backoff mode isinitiated. This improves retransmission latency fornon-aggressive thin streams, often found to be time-dependent.For more information on thin streams, seeThin-streams and TCP

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

tcp_limit_output_bytes - INTEGER

Controls TCP Small Queue limit per tcp socket.TCP bulk sender tends to increase packets in flight until itgets losses notifications. With SNDBUF autotuning, this canresult in a large amount of packets queued on the local machine(e.g.: qdiscs, CPU backlog, or device) hurting latency of otherflows, for typical pfifo_fast qdiscs. tcp_limit_output_byteslimits the number of bytes on qdisc or device to reduce artificialRTT/cwnd and reduce bufferbloat.

Default: 4194304 (4 MB)

tcp_challenge_ack_limit - INTEGER

Limits number of Challenge ACK sent per second, as recommendedin RFC 5961 (Improving TCP’s Robustness to Blind In-Window Attacks)Note that this per netns rate limit can allow some side channelattacks and probably should not be enabled.TCP stack implements per TCP socket limits anyway.Default: INT_MAX (unlimited)

tcp_ehash_entries - INTEGER

Show the number of hash buckets for TCP sockets in the currentnetworking namespace.

A negative value means the networking namespace does not own itshash buckets and shares the initial networking namespace’s one.

tcp_child_ehash_entries - INTEGER

Control the number of hash buckets for TCP sockets in the childnetworking namespace, which must be set before clone() orunshare().

If the value is not 0, the kernel uses a value rounded up to 2^nas the actual hash bucket size. 0 is a special value, meaningthe child networking namespace will share the initial networkingnamespace’s hash buckets.

Note that the child will use the global one in case the kernelfails to allocate enough memory. In addition, the global hashbuckets are spread over available NUMA nodes, but the allocationof the child hash table depends on the current process’s NUMApolicy, which could result in performance differences.

Note also that the default value of tcp_max_tw_buckets andtcp_max_syn_backlog depend on the hash bucket size.

Possible values: 0, 2^n (n: 0 - 24 (16Mi))

Default: 0

tcp_plb_enabled - BOOLEAN

If enabled and the underlying congestion control (e.g. DCTCP) supportsand enables PLB feature, TCP PLB (Protective Load Balancing) isenabled. PLB is described in the following paper:https://doi.org/10.1145/3544216.3544226. Based on PLB parameters,upon sensing sustained congestion, TCP triggers a change inflow label field for outgoing IPv6 packets. A change in flow labelfield potentially changes the path of outgoing packets for switchesthat use ECMP/WCMP for routing.

PLB changes socket txhash which results in a change in IPv6 Flow Labelfield, and currently no-op for IPv4 headers. It is possibleto apply PLB for IPv4 with other network header fields (e.g. TCPor IPv4 options) or using encapsulation where outer header is usedby switches to determine next hop. In either case, further hostand switch side changes will be needed.

If enabled, PLB assumes that congestion signal (e.g. ECN) is madeavailable and used by congestion control module to estimate acongestion measure (e.g. ce_ratio). PLB needs a congestion measure tomake repathing decisions.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

tcp_plb_idle_rehash_rounds - INTEGER

Number of consecutive congested rounds (RTT) seen after whicha rehash can be performed, given there are no packets in flight.This is referred to as M in PLB paper:https://doi.org/10.1145/3544216.3544226.

Possible Values: 0 - 31

Default: 3

tcp_plb_rehash_rounds - INTEGER

Number of consecutive congested rounds (RTT) seen after whicha forced rehash can be performed. Be careful when setting thisparameter, as a small value increases the risk of retransmissions.This is referred to as N in PLB paper:https://doi.org/10.1145/3544216.3544226.

Possible Values: 0 - 31

Default: 12

tcp_plb_suspend_rto_sec - INTEGER

Time, in seconds, to suspend PLB in event of an RTO. In order to avoidhaving PLB repath onto a connectivity “black hole”, after an RTO a TCPconnection suspends PLB repathing for a random duration between 1x and2x of this parameter. Randomness is added to avoid concurrent rehashingof multiple TCP connections. This should be set corresponding to theamount of time it takes to repair a failed link.

Possible Values: 0 - 255

Default: 60

tcp_plb_cong_thresh - INTEGER

Fraction of packets marked with congestion over a round (RTT) totag that round as congested. This is referred to as K in the PLB paper:https://doi.org/10.1145/3544216.3544226.

The 0-1 fraction range is mapped to 0-256 range to avoid floatingpoint operations. For example, 128 means that if at least 50% ofthe packets in a round were marked as congested then the roundwill be tagged as congested.

Setting threshold to 0 means that PLB repaths every RTT regardlessof congestion. This is not intended behavior for PLB and should beused only for experimentation purpose.

Possible Values: 0 - 256

Default: 128

tcp_pingpong_thresh - INTEGER

The number of estimated data replies sent for estimated incoming datarequests that must happen before TCP considers that a connection is a“ping-pong” (request-response) connection for which delayedacknowledgments can provide benefits.

This threshold is 1 by default, but some applications may need a higherthreshold for optimal performance.

Possible Values: 1 - 255

Default: 1

tcp_rto_min_us - INTEGER

Minimal TCP retransmission timeout (in microseconds). Note that therto_min route option has the highest precedence for configuring thissetting, followed by the TCP_BPF_RTO_MIN and TCP_RTO_MIN_US socketoptions, followed by this tcp_rto_min_us sysctl.

The recommended practice is to use a value less or equal to 200000microseconds.

Possible Values: 1 - INT_MAX

Default: 200000

tcp_rto_max_ms - INTEGER

Maximal TCP retransmission timeout (in ms).Note that TCP_RTO_MAX_MS socket option has higher precedence.

When changing tcp_rto_max_ms, it is important to understandthat tcp_retries2 might need a change.

Possible Values: 1000 - 120,000

Default: 120,000

UDP variables¶

udp_l3mdev_accept - BOOLEAN

Enabling this option allows a “global” bound socket to workacross L3 master domains (e.g., VRFs) with packets capable ofbeing received regardless of the L3 domain in which theyoriginated. Only valid when the kernel was compiled withCONFIG_NET_L3_MASTER_DEV.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

udp_mem - vector of 3 INTEGERs: min, pressure, max

Number of pages allowed for queueing by all UDP sockets.

min: Number of pages allowed for queueing by all UDP sockets.

pressure: This value was introduced to follow format of tcp_mem.

max: This value was introduced to follow format of tcp_mem.

Default is calculated at boot time from amount of available memory.

udp_rmem_min - INTEGER

Minimal size of receive buffer used by UDP sockets in moderation.Each UDP socket is able to use the size for receiving data, even iftotal pages of UDP sockets exceed udp_mem pressure. The unit is byte.

Default: 4K

udp_wmem_min - INTEGER

UDP does not have tx memory accounting and this tunable has no effect.

udp_hash_entries - INTEGER

Show the number of hash buckets for UDP sockets in the currentnetworking namespace.

A negative value means the networking namespace does not own itshash buckets and shares the initial networking namespace’s one.

udp_child_hash_entries - INTEGER

Control the number of hash buckets for UDP sockets in the childnetworking namespace, which must be set before clone() orunshare().

Possible values: 0, 2^n (n: 7 (128) - 16 (64K))

Default: 0

RAW variables¶

raw_l3mdev_accept - BOOLEAN

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

CIPSOv4 Variables¶

cipso_cache_enable - BOOLEAN

If enabled, enable additions to and lookups from the CIPSO label mappingcache. If disabled, additions are ignored and lookups always result in amiss. However, regardless of the setting the cache is stillinvalidated when required when means you can safely toggle this on andoff and the cache will always be “safe”.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

cipso_cache_bucket_size - INTEGER

The CIPSO label cache consists of a fixed size hash table with eachhash bucket containing a number of cache entries. This variable limitsthe number of entries in each hash bucket; the larger the value is, themore CIPSO label mappings that can be cached. When the number ofentries in a given hash bucket reaches this limit adding new entriescauses the oldest entry in the bucket to be removed to make room.

Default: 10

cipso_rbm_optfmt - BOOLEAN

Enable the “Optimized Tag 1 Format” as defined in section 3.4.2.6 ofthe CIPSO draft specification (see Documentation/netlabel for details).This means that when set the CIPSO tag will be padded with emptycategories in order to make the packet data 32-bit aligned.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

cipso_rbm_strictvalid - BOOLEAN

If enabled, do a very strict check of the CIPSO option whenip_options_compile() is called. If disabled, relax the checks done duringip_options_compile(). Either way is “safe” as errors are caught elsewhere in the CIPSO processing code but setting this to 0 (False) shouldresult in less work (i.e. it should be faster) but could cause problemswith other implementations that require strict checking.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

IP Variables¶

ip_local_port_range - 2 INTEGERS

Defines the local port range that is used by TCP and UDP tochoose the local port. The first number is the first, thesecond the last local port number.If possible, it is better these numbers have different parity(one even and one odd value).Must be greater than or equal to ip_unprivileged_port_start.The default values are 32768 and 60999 respectively.

ip_local_reserved_ports - list of comma separated ranges

Specify the ports which are reserved for known third-partyapplications. These ports will not be used by automatic portassignments (e.g. when callingconnect() orbind() with portnumber 0). Explicit port allocation behavior is unchanged.

The format used for both input and output is a comma separatedlist of ranges (e.g. “1,2-4,10-10” for ports 1, 2, 3, 4 and10). Writing to the file will clear all previously reservedports and update the current list with the one given in theinput.

Note that ip_local_port_range and ip_local_reserved_portssettings are independent and both are considered by the kernelwhen determining which ports are available for automatic portassignments.

You can reserve ports which are not in the currentip_local_port_range, e.g.:

$ cat /proc/sys/net/ipv4/ip_local_port_range32000       60999$ cat /proc/sys/net/ipv4/ip_local_reserved_ports8080,9148

although this is redundant. However such a setting is usefulif later the port range is changed to a value that willinclude the reserved ports. Also keep in mind, that overlappingof these ranges may affect probability of selecting ephemeralports which are right after block of reserved ports.

Default: Empty

ip_unprivileged_port_start - INTEGER

This is a per-namespace sysctl. It defines the firstunprivileged port in the network namespace. Privileged portsrequire root or CAP_NET_BIND_SERVICE in order to bind to them.To disable all privileged ports, set this to 0. They must notoverlap with the ip_local_port_range.

Default: 1024

ip_nonlocal_bind - BOOLEAN

If enabled, allows processes tobind() to non-local IP addresses,which can be quite useful - but may break some applications.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

ip_autobind_reuse - BOOLEAN

By default,bind() does not select the ports automatically even ifthe new socket and all sockets bound to the port have SO_REUSEADDR.ip_autobind_reuse allowsbind() to reuse the port and this is usefulwhen you usebind()+connect(), but may break some applications.The preferred solution is to use IP_BIND_ADDRESS_NO_PORT and thisoption should only be set by experts.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

ip_dynaddr - INTEGER

If set non-zero, enables support for dynamic addresses.If set to a non-zero value larger than 1, a kernel logmessage will be printed when dynamic address rewritingoccurs.

Default: 0

ip_early_demux - BOOLEAN

Optimize input packet processing down to one demux forcertain kinds of local sockets. Currently we only do thisfor established TCP and connected UDP sockets.

It may add an additional cost for pure routing workloads thatreduces overall throughput, in such case you should disable it.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

ping_group_range - 2 INTEGERS

Restrict ICMP_PROTO datagram sockets to users in the group range.The default is “1 0”, meaning, that nobody (not even root) maycreate ping sockets. Setting it to “100 100” would grant permissionsto the single group. “0 4294967294” would enable it for the world, “1004294967294” would enable it for the users, but not daemons.

tcp_early_demux - BOOLEAN

Enable early demux for established TCP sockets.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

udp_early_demux - BOOLEAN

Enable early demux for connected UDP sockets. Disable this ifyour system could experience more unconnected load.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

icmp_echo_ignore_all - BOOLEAN

If enabled, then the kernel will ignore all ICMP ECHOrequests sent to it.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

icmp_echo_enable_probe - BOOLEAN

If enabled, then the kernel will respond to RFC 8335 PROBErequests sent to it.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

icmp_echo_ignore_broadcasts - BOOLEAN

If enabled, then the kernel will ignore all ICMP ECHO andTIMESTAMP requests sent to it via broadcast/multicast.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

icmp_ratelimit - INTEGER

Limit the maximal rates for sending ICMP packets whose type matchesicmp_ratemask (see below) to specific targets.0 to disable any limiting,otherwise the minimal space between responses in milliseconds.Note that another sysctl, icmp_msgs_per_sec limits the numberof ICMP packets sent on all targets.

Default: 1000

icmp_msgs_per_sec - INTEGER

Limit maximal number of ICMP packets sent per second from this host.Only messages whose type matches icmp_ratemask (see below) arecontrolled by this limit. For security reasons, the precise countof messages per second is randomized.

Default: 1000

icmp_msgs_burst - INTEGER

icmp_msgs_per_sec controls number of ICMP packets sent per second,while icmp_msgs_burst controls the burst size of these packets.For security reasons, the precise burst size is randomized.

Default: 50

icmp_ratemask - INTEGER

Mask made of ICMP types for which rates are being limited.

Significant bits: IHGFEDCBA9876543210

Default mask: 0000001100000011000 (6168)

Bit definitions (see include/linux/icmp.h):

0
Echo Reply
3
Destination Unreachable[1]
4
Source Quench[1]
5
Redirect
8
Echo Request
B
Time Exceeded[1]
C
Parameter Problem[1]
D
Timestamp Request
E
Timestamp Reply
F
Info Request
G
Info Reply
H
Address Mask Request
I
Address Mask Reply

[1](1,2,3,4)

These are rate limited by default (see default mask above)

icmp_ignore_bogus_error_responses - BOOLEAN

Some routers violate RFC1122 by sending bogus responses to broadcastframes. Such violations are normally logged via a kernel warning.If enabled, the kernel will not give such warnings, whichwill avoid log file clutter.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

icmp_errors_use_inbound_ifaddr - BOOLEAN

If disabled, icmp error messages are sent with the primary address ofthe exiting interface.
If enabled, the message will be sent with the primary address ofthe interface that received the packet that caused the icmp error.This is the behaviour many network administrators will expect froma router. And it can make debugging complicated network layoutsmuch easier.
Note that if no primary address exists for the interface selected,then the primary address of the first non-loopback interface thathas one will be used regardless of this setting.
Possible values:
0 (disabled)
1 (enabled)
Default: 0 (disabled)

icmp_errors_extension_mask - UNSIGNED INTEGER

Bitmask of ICMP extensions to append to ICMPv4 error messages(“Destination Unreachable”, “Time Exceeded” and “Parameter Problem”).The original datagram is trimmed / padded to 128 bytes in order to becompatible with applications that do not comply with RFC 4884.

Possible extensions are:

0x01	Incoming IP interface information according to RFC 5837.Extension will include the index, IPv4 address (if present),name and MTU of the IP interface that received the datagramwhich elicited the ICMP error.

Default: 0x00 (no extensions)

igmp_max_memberships - INTEGER

Change the maximum number of multicast groups we can subscribe to.Default: 20

Theoretical maximum value is bounded by having to send a membershipreport in a single datagram (i.e. the report can’t span multipledatagrams, or risk confusing the switch and leaving groups you don’tintend to).

The number of supported groups ‘M’ is bounded by the number of groupreport entries you can fit into a single datagram of 65535 bytes.

M = 65536-sizeof (ip header)/(sizeof(Group record))

Group records are variable length, with a minimum of 12 bytes.So net.ipv4.igmp_max_memberships should not be set higher than:

(65536-24) / 12 = 5459

The value 5459 assumes no IP header options, so in practicethis number may be lower.

igmp_max_msf - INTEGER

Maximum number of addresses allowed in the source filter list for amulticast group.

Default: 10

igmp_qrv - INTEGER

Controls the IGMP query robustness variable (see RFC2236 8.1).

Default: 2 (as specified by RFC2236 8.1)

Minimum: 1 (as specified by RFC6636 4.5)

force_igmp_version - INTEGER

0 - (default) No enforcement of a IGMP version, IGMPv1/v2 fallbackallowed. Will back to IGMPv3 mode again if all IGMPv1/v2 QuerierPresent timer expires.
1 - Enforce to use IGMP version 1. Will also reply IGMPv1 report ifreceive IGMPv2/v3 query.
2 - Enforce to use IGMP version 2. Will fallback to IGMPv1 if receiveIGMPv1 query message. Will reply report if receive IGMPv3 query.
3 - Enforce to use IGMP version 3. The same react with default 0.

Note

this is not the same with force_mld_version because IGMPv3 RFC3376Security Considerations does not have clear description that we couldignore other version messages completely as MLDv2 RFC3810. So makethis value as default 0 is recommended.

conf/interface/*

changes special settings per interface (whereinterface” is the name of your network interface)

conf/all/*

is special, changes the settings for all interfaces

log_martians - BOOLEAN

Log packets with impossible addresses to kernel log.log_martians for the interface will be enabled if at least one ofconf/{all,interface}/log_martians is set to TRUE,it will be disabled otherwise

accept_redirects - BOOLEAN

Accept ICMP redirect messages.accept_redirects for the interface will be enabled if:

both conf/{all,interface}/accept_redirects are TRUE in the caseforwarding for the interface is enabled

at least one of conf/{all,interface}/accept_redirects is TRUE in thecase forwarding for the interface is disabled

accept_redirects for the interface will be disabled otherwise

default:

TRUE (host)
FALSE (router)

forwarding - BOOLEAN

Enable IP forwarding on this interface. This controls whether packetsreceived _on_ this interface can be forwarded.

mc_forwarding - BOOLEAN

Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTEand a multicast routing daemon is required.conf/all/mc_forwarding must also be set to TRUE to enable multicastrouting for the interface

medium_id - INTEGER

Integer value used to differentiate the devices by the medium theyare attached to. Two devices can have different id values whenthe broadcast packets are received only on one of them.The default value 0 means that the device is the only interfaceto its medium, value of -1 means that medium is not known.

Currently, it is used to change the proxy_arp behavior:the proxy_arp feature is enabled for packets forwarded betweentwo devices attached to different media.

proxy_arp - BOOLEAN

Do proxy arp.

proxy_arp for the interface will be enabled if at least one ofconf/{all,interface}/proxy_arp is set to TRUE,it will be disabled otherwise

proxy_arp_pvlan - BOOLEAN

Private VLAN proxy arp.

Basically allow proxy arp replies back to the same interface(from which the ARP request/solicitation was received).

This is done to support (ethernet) switch features, like RFC3069, where the individual ports are NOT allowed tocommunicate with each other, but they are allowed to talk tothe upstream router. As described in RFC 3069, it is possibleto allow these hosts to communicate through the upstreamrouter by proxy_arp’ing. Don’t need to be used together withproxy_arp.

This technology is known by different names:

In RFC 3069 it is called VLAN Aggregation.
Cisco and Allied Telesyn call it Private VLAN.
Hewlett-Packard call it Source-Port filtering or port-isolation.
Ericsson call it MAC-Forced Forwarding (RFC Draft).

proxy_delay - INTEGER

Delay proxy response.

Delay response to a neighbor solicitation when proxy_arpor proxy_ndp is enabled. A random value between [0, proxy_delay)will be chosen, setting to zero means reply with no delay.Value in jiffies. Defaults to 80.

shared_media - BOOLEAN

Send(router) or accept(host) RFC1620 shared media redirects.Overrides secure_redirects.

shared_media for the interface will be enabled if at least one ofconf/{all,interface}/shared_media is set to TRUE,it will be disabled otherwise

default TRUE

secure_redirects - BOOLEAN

Accept ICMP redirect messages only to gateways listed in theinterface’s current gateway list. Even if disabled, RFC1122 redirectrules still apply.

Overridden by shared_media.

secure_redirects for the interface will be enabled if at least one ofconf/{all,interface}/secure_redirects is set to TRUE,it will be disabled otherwise

default TRUE

send_redirects - BOOLEAN

Send redirects, if router.

send_redirects for the interface will be enabled if at least one ofconf/{all,interface}/send_redirects is set to TRUE,it will be disabled otherwise

Default: TRUE

bootp_relay - BOOLEAN

Accept packets with source address 0.b.c.d destinednot to this host as local ones. It is supposed, thatBOOTP relay daemon will catch and forward such packets.conf/all/bootp_relay must also be set to TRUE to enable BOOTP relayfor the interface

default FALSE

Not Implemented Yet.

accept_source_route - BOOLEAN

Accept packets with SRR option.conf/all/accept_source_route must also be set to TRUE to accept packetswith SRR option on the interface

default

TRUE (router)
FALSE (host)

accept_local - BOOLEAN

Accept packets with local source addresses. In combination withsuitable routing, this can be used to direct packets between twolocal interfaces over the wire and have them accepted properly.default FALSE

route_localnet - BOOLEAN

Do not consider loopback addresses as martian source or destinationwhile routing. This enables the use of 127/8 for local routing purposes.

default FALSE

rp_filter - INTEGER

0 - No source validation.
1 - Strict mode as defined in RFC3704 Strict Reverse PathEach incoming packet is tested against the FIB and if the interfaceis not the best reverse path the packet check will fail.By default failed packets are discarded.
2 - Loose mode as defined in RFC3704 Loose Reverse PathEach incoming packet’s source address is also tested against the FIBand if the source address is not reachable via any interfacethe packet check will fail.

Current recommended practice in RFC3704 is to enable strict modeto prevent IP spoofing from DDos attacks. If using asymmetric routingor other complicated routing, then loose mode is recommended.

The max value from conf/{all,interface}/rp_filter is usedwhen doing source validation on the {interface}.

Default value is 0. Note that some distributions enable itin startup scripts.

src_valid_mark - BOOLEAN

0 - The fwmark of the packet is not included in reverse pathroute lookup. This allows for asymmetric routing configurationsutilizing the fwmark in only one direction, e.g., transparentproxying.
1 - The fwmark of the packet is included in reverse path routelookup. This permits rp_filter to function when the fwmark isused for routing traffic in both directions.

This setting also affects the utilization of fmwark whenperforming source address selection for ICMP replies, ordetermining addresses stored for the IPOPT_TS_TSANDADDR andIPOPT_RR IP options.

The max value from conf/{all,interface}/src_valid_mark is used.

Default value is 0.

arp_filter - BOOLEAN

1 - Allows you to have multiple network interfaces on the samesubnet, and have the ARPs for each interface be answeredbased on whether or not the kernel would route a packet fromthe ARP’d IP out that interface (therefore you must use sourcebased routing for this to work). In other words it allows controlof which cards (usually 1) will respond to an arp request.
0 - (default) The kernel can respond to arp requests with addressesfrom other interfaces. This may seem wrong but it usually makessense, because it increases the chance of successful communication.IP addresses are owned by the complete host on Linux, not byparticular interfaces. Only for more complex setups like load-balancing, does this behaviour cause problems.

arp_filter for the interface will be enabled if at least one ofconf/{all,interface}/arp_filter is set to TRUE,it will be disabled otherwise

arp_announce - INTEGER

Define different restriction levels for announcing the localsource IP address from IP packets in ARP requests sent oninterface:

0 - (default) Use any local address, configured on any interface
1 - Try to avoid local addresses that are not in the target’ssubnet for this interface. This mode is useful when targethosts reachable via this interface require the source IPaddress in ARP requests to be part of their logical networkconfigured on the receiving interface. When we generate therequest we will check all our subnets that include thetarget IP and will preserve the source address if it is fromsuch subnet. If there is no such subnet we select sourceaddress according to the rules for level 2.
2 - Always use the best local address for this target.In this mode we ignore the source address in the IP packetand try to select local address that we prefer for talks withthe target host. Such local address is selected by lookingfor primary IP addresses on all our subnets on the outgoinginterface that include the target IP address. If no suitablelocal address is found we select the first local addresswe have on the outgoing interface or on all other interfaces,with the hope we will receive reply for our request andeven sometimes no matter the source IP address we announce.

The max value from conf/{all,interface}/arp_announce is used.

Increasing the restriction level gives more chance forreceiving answer from the resolved target while decreasingthe level announces more valid sender’s information.

arp_ignore - INTEGER

Define different modes for sending replies in response toreceived ARP requests that resolve local target IP addresses:

0 - (default): reply for any local target IP address, configuredon any interface
1 - reply only if the target IP address is local addressconfigured on the incoming interface
2 - reply only if the target IP address is local addressconfigured on the incoming interface and both with thesender’s IP address are part from same subnet on this interface
3 - do not reply for local addresses configured with scope host,only resolutions for global and link addresses are replied
4-7 - reserved
8 - do not reply for all local addresses

The max value from conf/{all,interface}/arp_ignore is usedwhen ARP request is received on the {interface}

arp_notify - BOOLEAN

Define mode for notification of address and device changes.

0
(default): do nothing
1
Generate gratuitous arp requests when device is brought upor hardware address changes.

arp_accept - INTEGER

Define behavior for accepting gratuitous ARP (garp) frames from devicesthat are not already present in the ARP table:

0 - don’t create new entries in the ARP table
1 - create new entries in the ARP table
2 - create new entries only if the source IP address is in the samesubnet as an address configured on the interface that received thegarp message.

Both replies and requests type gratuitous arp will trigger theARP table to be updated, if this setting is on.

If the ARP table already contains the IP address of thegratuitous arp frame, the arp table will be updated regardlessif this setting is on or off.

arp_evict_nocarrier - BOOLEAN

Clears the ARP cache on NOCARRIER events. This option is important forwireless devices where the ARP cache should not be cleared when roamingbetween access points on the same network. In most cases this shouldremain as the default (1).

Possible values:

0 (disabled) - Do not clear ARP cache on NOCARRIER events
1 (enabled) - Clear the ARP cache on NOCARRIER events

Default: 1 (enabled)

mcast_solicit - INTEGER

The maximum number of multicast probes in INCOMPLETE state,when the associated hardware address is unknown. Defaultsto 3.

ucast_solicit - INTEGER

The maximum number of unicast probes in PROBE state, whenthe hardware address is being reconfirmed. Defaults to 3.

app_solicit - INTEGER

The maximum number of probes to send to the user space ARP daemonvia netlink before dropping back to multicast probes (seemcast_resolicit). Defaults to 0.

mcast_resolicit - INTEGER

The maximum number of multicast probes after unicast andapp probes in PROBE state. Defaults to 0.

disable_policy - BOOLEAN

Disable IPSEC policy (SPD) for this interface

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

disable_xfrm - BOOLEAN

Disable IPSEC encryption on this interface, whatever the policy

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

igmpv2_unsolicited_report_interval - INTEGER

The interval in milliseconds in which the next unsolicitedIGMPv1 or IGMPv2 report retransmit will take place.

Default: 10000 (10 seconds)

igmpv3_unsolicited_report_interval - INTEGER

The interval in milliseconds in which the next unsolicitedIGMPv3 report retransmit will take place.

Default: 1000 (1 seconds)

ignore_routes_with_linkdown - BOOLEAN

Ignore routes whose link is down when performing a FIB lookup.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

promote_secondaries - BOOLEAN

When a primary IP address is removed from this interfacepromote a corresponding secondary IP address instead ofremoving all the corresponding secondary IP addresses.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

drop_unicast_in_l2_multicast - BOOLEAN

Drop any unicast IP packets that are received in link-layermulticast (or broadcast) frames.

This behavior (for multicast) is actually a SHOULD in RFC1122, but is disabled by default for compatibility reasons.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

drop_gratuitous_arp - BOOLEAN

Drop all gratuitous ARP frames, for example if there’s a knowngood ARP proxy on the network and such frames need not be used(or in the case of 802.11, must not be used to prevent attacks.)

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

tag - INTEGER

Allows you to write a number, which can be used as required.

Default value is 0.

xfrm4_gc_thresh - INTEGER

(Obsolete since linux-4.14)The threshold at which we will start garbage collecting for IPv4destination cache entries. At twice this value the system willrefuse new allocations.

igmp_link_local_mcast_reports - BOOLEAN

Enable IGMP reports for link local multicast groups in the224.0.0.X range.

Default TRUE

Alexey Kuznetsov.kuznet@ms2.inr.ac.ru

Updated by:

Andi Kleenak@muc.de
Nicolas Delondelon.nicolas@wanadoo.fr

/proc/sys/net/ipv6/* Variables¶

IPv6 has no global variables such as tcp_*. tcp_* settings under ipv4/ alsoapply to IPv6 [XXX?].

bindv6only - BOOLEAN

Default value for IPV6_V6ONLY socket option,which restricts use of the IPv6 socket to IPv6 communicationonly.

Possible values:

0 (disabled) - enable IPv4-mapped address feature
1 (enabled) - disable IPv4-mapped address feature

Default: 0 (disabled)

flowlabel_consistency - BOOLEAN

Protect the consistency (and unicity) of flow label.You have to disable it to use IPV6_FL_F_REFLECT flag on theflow label manager.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

auto_flowlabels - INTEGER

Automatically generate flow labels based on a flow hash of thepacket. This allows intermediate devices, such as routers, toidentify packet flows for mechanisms like Equal Cost MultipathRouting (see RFC 6438).

0	automatic flow labels are completely disabled
1	automatic flow labels are enabled by default, they can bedisabled on a per socket basis using the IPV6_AUTOFLOWLABELsocket option
2	automatic flow labels are allowed, they may be enabled on aper socket basis using the IPV6_AUTOFLOWLABEL socket option
3	automatic flow labels are enabled and enforced, they cannotbe disabled by the socket option

Default: 1

flowlabel_state_ranges - BOOLEAN

Split the flow label number space into two ranges. 0-0x7FFFF isreserved for the IPv6 flow manager facility, 0x80000-0xFFFFFis reserved for stateless flow labels as described in RFC6437.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

flowlabel_reflect - INTEGER

Control flow label reflection. Needed for Path MTUDiscovery to work with Equal Cost Multipath Routing in anycastenvironments. See RFC 7690 and:https://tools.ietf.org/html/draft-wang-6man-flow-label-reflection-01

This is a bitmask.

1: enabled for established flows
Note that this prevents automatic flowlabel changes, as donein “tcp: change IPv6 flow-label upon receiving spurious retransmission”and “tcp: Change txhash on every SYN and RTO retransmit”
2: enabled for TCP RESET packets (no active listener)If set, a RST packet sent in response to a SYN packet on a closedport will reflect the incoming flow label.
4: enabled for ICMPv6 echo reply messages.

Default: 0

fib_multipath_hash_policy - INTEGER

Controls which hash policy to use for multipath routes.

Default: 0 (Layer 3)

Possible values:

0 - Layer 3 (source and destination addresses plus flow label)
1 - Layer 4 (standard 5-tuple)
2 - Layer 3 or inner Layer 3 if present
3 - Custom multipath hash. Fields used for multipath hash calculationare determined by fib_multipath_hash_fields sysctl

fib_multipath_hash_fields - UNSIGNED INTEGER

When fib_multipath_hash_policy is set to 3 (custom multipath hash), thefields used for multipath hash calculation are determined by thissysctl.

This value is a bitmask which enables various fields for multipath hashcalculation.

Possible fields are:

0x0001	Source IP address
0x0002	Destination IP address
0x0004	IP protocol
0x0008	Flow Label
0x0010	Source port
0x0020	Destination port
0x0040	Inner source IP address
0x0080	Inner destination IP address
0x0100	Inner IP protocol
0x0200	Inner Flow Label
0x0400	Inner source port
0x0800	Inner destination port

Default: 0x0007 (source IP, destination IP and IP protocol)

anycast_src_echo_reply - BOOLEAN

Controls the use of anycast addresses as source addresses for ICMPv6echo reply

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

idgen_delay - INTEGER

Controls the delay in seconds after which time to retryprivacy stable address generation if a DAD conflict isdetected.

Default: 1 (as specified in RFC7217)

idgen_retries - INTEGER

Controls the number of retries to generate a stable privacyaddress if a DAD conflict is detected.

Default: 3 (as specified in RFC7217)

mld_qrv - INTEGER

Controls the MLD query robustness variable (see RFC3810 9.1).

Default: 2 (as specified by RFC3810 9.1)

Minimum: 1 (as specified by RFC6636 4.5)

max_dst_opts_number - INTEGER

Maximum number of non-padding TLVs allowed in a Destinationoptions extension header. If this value is less than zerothen unknown options are disallowed and the number of knownTLVs allowed is the absolute value of this number.

Default: 8

max_hbh_opts_number - INTEGER

Maximum number of non-padding TLVs allowed in a Hop-by-Hopoptions extension header. If this value is less than zerothen unknown options are disallowed and the number of knownTLVs allowed is the absolute value of this number.

Default: 8

max_dst_opts_length - INTEGER

Maximum length allowed for a Destination options extensionheader.

Default: INT_MAX (unlimited)

max_hbh_length - INTEGER

Maximum length allowed for a Hop-by-Hop options extensionheader.

Default: INT_MAX (unlimited)

skip_notify_on_dev_down - BOOLEAN

Controls whether an RTM_DELROUTE message is generated for routesremoved when a device is taken down or deleted. IPv4 does notgenerate this message; IPv6 does by default. Setting this sysctlto true skips the message, making IPv4 and IPv6 on par in relyingon userspace caches to track link events and evict routes.

Possible values:

0 (disabled) - generate the message
1 (enabled) - skip generating the message

Default: 0 (disabled)

nexthop_compat_mode - BOOLEAN

New nexthop API provides a means for managing nexthops independent ofprefixes. Backwards compatibility with old route format is enabled bydefault which means route dumps and notifications contain the newnexthop attribute but also the full, expanded nexthop definition.Further, updates or deletes of a nexthop configuration generate routenotifications for each fib entry using the nexthop. Once a systemunderstands the new API, this sysctl can be disabled to achieve fullperformance benefits of the new API by disabling the nexthop expansionand extraneous notifications.

Note that as a backward-compatible mode, dumping of modern featuresmight be incomplete or wrong. For example, resilient groups will not beshown as such, but rather as just a list of next hops. Also weights thatdo not fit into 8 bits will show incorrectly.

Default: true (backward compat mode)

fib_notify_on_flag_change - INTEGER

Whether to emit RTM_NEWROUTE notifications whenever RTM_F_OFFLOAD/RTM_F_TRAP/RTM_F_OFFLOAD_FAILED flags are changed.

Default: 0 (Do not emit notifications.)

Possible values:

0 - Do not emit notifications.
1 - Emit notifications.
2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change.

ioam6_id - INTEGER

Define the IOAM id of this node. Uses only 24 bits out of 32 in total.

Possible value range:

Min: 0
Max: 0xFFFFFF

Default: 0xFFFFFF

ioam6_id_wide - LONG INTEGER

Define the wide IOAM id of this node. Uses only 56 bits out of 64 intotal. Can be different from ioam6_id.

Possible value range:

Min: 0
Max: 0xFFFFFFFFFFFFFF

Default: 0xFFFFFFFFFFFFFF

IPv6 Fragmentation:

ip6frag_high_thresh - INTEGER

Maximum memory used to reassemble IPv6 fragments. Whenip6frag_high_thresh bytes of memory is allocated for this purpose,the fragment handler will toss packets until ip6frag_low_threshis reached.

ip6frag_low_thresh - INTEGER

See ip6frag_high_thresh

ip6frag_time - INTEGER

Time in seconds to keep an IPv6 fragment in memory.

conf/default/*:

Change the interface-specific default settings.

These settings would be used during creating new interfaces.

conf/all/*:

Change all the interface-specific settings.

[XXX: Other special features than forwarding?]

conf/all/disable_ipv6 - BOOLEAN

Changing this value is same as changingconf/default/disable_ipv6setting and also all per-interfacedisable_ipv6 settings to the samevalue.

Reading this value does not have any particular meaning. It does not saywhether IPv6 support is enabled or disabled. Returned value can be 1also in the case when some interface hasdisable_ipv6 set to 0 andhas configured IPv6 addresses.

conf/all/forwarding - BOOLEAN

Enable global IPv6 forwarding between all interfaces.

IPv4 and IPv6 work differently here; theforce_forwarding flag mustbe used to control which interfaces may forward packets.

This also sets all interfaces’ Host/Router setting‘forwarding’ to the specified value. See below for details.

This referred to as global forwarding.

proxy_ndp - BOOLEAN

Do proxy ndp.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

force_forwarding - BOOLEAN

Enable forwarding on this interface only -- regardless of the setting onconf/all/forwarding. When settingconf.all.forwarding to 0,theforce_forwarding flag will be reset on all interfaces.

fwmark_reflect - BOOLEAN

Controls the fwmark of kernel-generated IPv6 reply packets that are notassociated with a socket for example, TCP RSTs or ICMPv6 echo replies).If disabled, these packets have a fwmark of zero. If enabled, they have thefwmark of the packet they are replying to.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

conf/interface/*:

Change special settings per interface.

The functional behaviour for certain settings is differentdepending on whether local forwarding is enabled or not.

accept_ra - INTEGER

Accept Router Advertisements; autoconfigure using them.

It also determines whether or not to transmit RouterSolicitations. If and only if the functional setting is toaccept Router Advertisements, Router Solicitations will betransmitted.

Possible values are:

0
Do not accept Router Advertisements.
1
Accept Router Advertisements if forwarding is disabled.
2
Overrule forwarding behaviour. Accept Router Advertisementseven if forwarding is enabled.

Functional default:

enabled if local forwarding is disabled.
disabled if local forwarding is enabled.

accept_ra_defrtr - BOOLEAN

Learn default router in Router Advertisement.

Functional default:

enabled if accept_ra is enabled.
disabled if accept_ra is disabled.

ra_defrtr_metric - UNSIGNED INTEGER

Route metric for default route learned in Router Advertisement. This valuewill be assigned as metric for the default route learned via IPv6 RouterAdvertisement. Takes affect only if accept_ra_defrtr is enabled.

Possible values:

1 to 0xFFFFFFFF

Default: IP6_RT_PRIO_USER i.e. 1024.

accept_ra_from_local - BOOLEAN

Accept RA with source-address that is found on local machineif the RA is otherwise proper and able to be accepted.

Default is to NOT accept these as it may be an un-intendednetwork loop.

Functional default:

enabled if accept_ra_from_local is enabledon a specific interface.
disabled if accept_ra_from_local is disabledon a specific interface.

accept_ra_min_hop_limit - INTEGER

Minimum hop limit Information in Router Advertisement.

Hop limit Information in Router Advertisement less than thisvariable shall be ignored.

Default: 1

accept_ra_min_lft - INTEGER

Minimum acceptable lifetime value in Router Advertisement.

RA sections with a lifetime less than this value shall beignored. Zero lifetimes stay unaffected.

Default: 0

accept_ra_pinfo - BOOLEAN

Learn Prefix Information in Router Advertisement.

Functional default:

enabled if accept_ra is enabled.
disabled if accept_ra is disabled.

ra_honor_pio_life - BOOLEAN

Whether to use RFC4862 Section 5.5.3e to determine the validlifetime of an address matching a prefix sent in a RouterAdvertisement Prefix Information Option.

Possible values:

0 (disabled) - RFC4862 section 5.5.3e is used to determinethe valid lifetime of the address.
1 (enabled) - the PIO valid lifetime will always be honored.

Default: 0 (disabled)

ra_honor_pio_pflag - BOOLEAN

The Prefix Information Option P-flag indicates the network canallocate a unique IPv6 prefix per client using DHCPv6-PD.This sysctl can be enabled when a userspace DHCPv6-PD clientis running to cause the P-flag to take effect: i.e. theP-flag suppresses any effects of the A-flag within the samePIO. For a given PIO, P=1 and A=1 is treated as A=0.

Possible values:

0 (disabled) - the P-flag is ignored.
1 (enabled) - the P-flag will disable SLAAC autoconfigurationfor the given Prefix Information Option.

Default: 0 (disabled)

accept_ra_rt_info_min_plen - INTEGER

Minimum prefix length of Route Information in RA.

Route Information w/ prefix smaller than this variable shallbe ignored.

Functional default:

0 if accept_ra_rtr_pref is enabled.
-1 if accept_ra_rtr_pref is disabled.

accept_ra_rt_info_max_plen - INTEGER

Maximum prefix length of Route Information in RA.

Route Information w/ prefix larger than this variable shallbe ignored.

Functional default:

0 if accept_ra_rtr_pref is enabled.
-1 if accept_ra_rtr_pref is disabled.

accept_ra_rtr_pref - BOOLEAN

Accept Router Preference in RA.

Functional default:

enabled if accept_ra is enabled.
disabled if accept_ra is disabled.

accept_ra_mtu - BOOLEAN

Apply the MTU value specified in RA option 5 (RFC4861). Ifdisabled, the MTU specified in the RA will be ignored.

Functional default:

enabled if accept_ra is enabled.
disabled if accept_ra is disabled.

accept_redirects - BOOLEAN

Accept Redirects.

Functional default:

enabled if local forwarding is disabled.
disabled if local forwarding is enabled.

accept_source_route - INTEGER

Accept source routing (routing extension header).

>= 0: Accept only routing header type 2.
< 0: Do not accept routing header.

Default: 0

autoconf - BOOLEAN

Autoconfigure addresses using Prefix Information in RouterAdvertisements.

Functional default:

enabled if accept_ra_pinfo is enabled.
disabled if accept_ra_pinfo is disabled.

dad_transmits - INTEGER

The amount of Duplicate Address Detection probes to send.

Default: 1

forwarding - INTEGER

Configure interface-specific Host/Router behaviour.

Note

It is recommended to have the same setting on allinterfaces; mixed router/host scenarios are rather uncommon.

Possible values are:

0 Forwarding disabled
1 Forwarding enabled

FALSE (0):

By default, Host behaviour is assumed. This means:

IsRouter flag is not set in Neighbour Advertisements.
If accept_ra is TRUE (default), transmit RouterSolicitations.
If accept_ra is TRUE (default), accept RouterAdvertisements (and do autoconfiguration).
If accept_redirects is TRUE (default), accept Redirects.

TRUE (1):

If local forwarding is enabled, Router behaviour is assumed.This means exactly the reverse from the above:

IsRouter flag is set in Neighbour Advertisements.
Router Solicitations are not sent unless accept_ra is 2.
Router Advertisements are ignored unless accept_ra is 2.
Redirects are ignored.

Default: 0 (disabled) if global forwarding is disabled (default),otherwise 1 (enabled).

hop_limit - INTEGER

Default Hop Limit to set.

Default: 64

mtu - INTEGER

Default Maximum Transfer Unit

Default: 1280 (IPv6 required minimum)

ip_nonlocal_bind - BOOLEAN

If enabled, allows processes tobind() to non-local IPv6 addresses,which can be quite useful - but may break some applications.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

router_probe_interval - INTEGER

Minimum interval (in seconds) between Router Probing describedin RFC4191.

Default: 60

router_solicitation_delay - INTEGER

Number of seconds to wait after interface is brought upbefore sending Router Solicitations.

Default: 1

router_solicitation_interval - INTEGER

Number of seconds to wait between Router Solicitations.

Default: 4

router_solicitations - INTEGER

Number of Router Solicitations to send until assuming norouters are present.

Default: 3

use_oif_addrs_only - BOOLEAN

When enabled, the candidate source addresses for destinationsrouted via this interface are restricted to the set of addressesconfigured on this interface (vis. RFC 6724, section 4).

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

use_tempaddr - INTEGER

Preference for Privacy Extensions (RFC3041).

<= 0 : disable Privacy Extensions
== 1 : enable Privacy Extensions, but prefer publicaddresses over temporary addresses.
> 1 : enable Privacy Extensions and prefer temporaryaddresses over public addresses.

Default:

0 (for most devices)
-1 (for point-to-point devices and loopback devices)

temp_valid_lft - INTEGER

valid lifetime (in seconds) for temporary addresses. If less than theminimum required lifetime (typically 5-7 seconds), temporary addresseswill not be created.

Default: 172800 (2 days)

temp_prefered_lft - INTEGER

Preferred lifetime (in seconds) for temporary addresses. Iftemp_prefered_lft is less than the minimum required lifetime (typically5-7 seconds), the preferred lifetime is the minimum required. Iftemp_prefered_lft is greater than temp_valid_lft, the preferred lifetimeis temp_valid_lft.

Default: 86400 (1 day)

keep_addr_on_down - INTEGER

Keep all IPv6 addresses on an interface down event. If set staticglobal addresses with no expiration time are not flushed.

>0 : enabled
0 : system default
<0 : disabled

Default: 0 (addresses are removed)

max_desync_factor - INTEGER

Maximum value for DESYNC_FACTOR, which is a random valuethat ensures that clients don’t synchronize with eachother and generate new addresses at exactly the same time.value is in seconds.

Default: 600

regen_min_advance - INTEGER

How far in advance (in seconds), at minimum, to create a new temporaryaddress before the current one is deprecated. This value is added tothe amount of time that may be required for duplicate address detectionto determine when to create a new address. Linux permits setting thisvalue to less than the default of 2 seconds, but a value less than 2does not conform to RFC 8981.

Default: 2

regen_max_retry - INTEGER

Number of attempts before give up attempting to generatevalid temporary addresses.

Default: 5

max_addresses - INTEGER

Maximum number of autoconfigured addresses per interface. Settingto zero disables the limitation. It is not recommended to set thisvalue too large (or to zero) because it would be an easy way tocrash the kernel by allowing too many addresses to be created.

Default: 16

disable_ipv6 - BOOLEAN

Disable IPv6 operation. If accept_dad is set to 2, this valuewill be dynamically set to TRUE if DAD fails for the link-localaddress.

Default: FALSE (enable IPv6 operation)

When this value is changed from 1 to 0 (IPv6 is being enabled),it will dynamically create a link-local address on the giveninterface and start Duplicate Address Detection, if necessary.

When this value is changed from 0 to 1 (IPv6 is being disabled),it will dynamically delete all addresses and routes on the giveninterface. From now on it will not possible to add addresses/routesto the selected interface.

accept_dad - INTEGER

Whether to accept DAD (Duplicate Address Detection).

0
Disable DAD
1
Enable DAD (default)
2
Enable DAD, and disable IPv6 operation if MAC-based duplicatelink-local address has been found.

DAD operation and mode on a given interface will be selected accordingto the maximum value of conf/{all,interface}/accept_dad.

force_tllao - BOOLEAN

Enable sending the target link-layer address option even whenresponding to a unicast neighbor solicitation.

Default: FALSE

Quoting from RFC 2461, section 4.4, Target link-layer address:

“The option MUST be included for multicast solicitations in order toavoid infinite Neighbor Solicitation “recursion” when the peer nodedoes not have a cache entry to return a Neighbor Advertisementsmessage. When responding to unicast solicitations, the option can beomitted since the sender of the solicitation has the correct link-layer address; otherwise it would not have be able to send the unicastsolicitation in the first place. However, including the link-layeraddress in this case adds little overhead and eliminates a potentialrace condition where the sender deletes the cached link-layer addressprior to receiving a response to a previous solicitation.”

ndisc_notify - BOOLEAN

Define mode for notification of address and device changes.

Possible values:

0 (disabled) - do nothing
1 (enabled) - Generate unsolicited neighbour advertisements when device is broughtup or hardware address changes.

Default: 0 (disabled)

ndisc_tclass - INTEGER

The IPv6 Traffic Class to use by default when sending IPv6 NeighborDiscovery (Router Solicitation, Router Advertisement, NeighborSolicitation, Neighbor Advertisement, Redirect) messages.These 8 bits can be interpreted as 6 high order bits holding the DSCPvalue and 2 low order bits representing ECN (which you probably wantto leave cleared).

0 - (default)

ndisc_evict_nocarrier - BOOLEAN

Clears the neighbor discovery table on NOCARRIER events. This option isimportant for wireless devices where the neighbor discovery cache shouldnot be cleared when roaming between access points on the same network.In most cases this should remain as the default (1).

Possible values:

0 (disabled) - Do not clear neighbor discovery cache on NOCARRIER events.
1 (enabled) - Clear neighbor discover cache on NOCARRIER events.

Default: 1 (enabled)

mldv1_unsolicited_report_interval - INTEGER

The interval in milliseconds in which the next unsolicitedMLDv1 report retransmit will take place.

Default: 10000 (10 seconds)

mldv2_unsolicited_report_interval - INTEGER

The interval in milliseconds in which the next unsolicitedMLDv2 report retransmit will take place.

Default: 1000 (1 second)

force_mld_version - INTEGER

0 - (default) No enforcement of a MLD version, MLDv1 fallback allowed
1 - Enforce to use MLD version 1
2 - Enforce to use MLD version 2

suppress_frag_ndisc - INTEGER

Control RFC 6980 (Security Implications of IPv6 Fragmentationwith IPv6 Neighbor Discovery) behavior:

1 - (default) discard fragmented neighbor discovery packets
0 - allow fragmented neighbor discovery packets

optimistic_dad - BOOLEAN

Whether to perform Optimistic Duplicate Address Detection (RFC 4429).

Optimistic Duplicate Address Detection for the interface will be enabledif at least one of conf/{all,interface}/optimistic_dad is set to 1,it will be disabled otherwise.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

use_optimistic - BOOLEAN

If enabled, do not classify optimistic addresses as deprecated duringsource address selection. Preferred addresses will still be chosenbefore optimistic addresses, subject to other ranking in the sourceaddress selection algorithm.

This will be enabled if at least one ofconf/{all,interface}/use_optimistic is set to 1, disabled otherwise.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

stable_secret - IPv6 address

This IPv6 address will be used as a secret to generate IPv6addresses for link-local addresses and autoconfiguredones. All addresses generated after setting this secret willbe stable privacy ones by default. This can be changed via theaddrgenmode ip-link. conf/default/stable_secret is used as thesecret for the namespace, the interface specific ones canoverwrite that. Writes to conf/all/stable_secret are refused.

It is recommended to generate this secret during installationof a system and keep it stable after that.

By default the stable secret is unset.

addr_gen_mode - INTEGER

Defines how link-local and autoconf addresses are generated.

0	generate address based on EUI64 (default)
1	do no generate a link-local address, use EUI64 for addressesgenerated from autoconf
2	generate stable privacy addresses, using the secret fromstable_secret (RFC7217)
3	generate stable privacy addresses, using a random secret if unset

drop_unicast_in_l2_multicast - BOOLEAN

Drop any unicast IPv6 packets that are received in link-layermulticast (or broadcast) frames.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

drop_unsolicited_na - BOOLEAN

Drop all unsolicited neighbor advertisements, for example if there’sa known good NA proxy on the network and such frames need not be used(or in the case of 802.11, must not be used to prevent attacks.)

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled).

accept_untracked_na - INTEGER

Define behavior for accepting neighbor advertisements from devices thatare absent in the neighbor cache:

0 - (default) Do not accept unsolicited and untracked neighboradvertisements.
1 - Add a new neighbor cache entry in STALE state for routers onreceiving a neighbor advertisement (either solicited or unsolicited)with target link-layer address option specified if no neighbor entryis already present for the advertised IPv6 address. Without this knob,NAs received for untracked addresses (absent in neighbor cache) aresilently ignored.
This is as per router-side behavior documented in RFC9131.
This has lower precedence than drop_unsolicited_na.
This will optimize the return path for the initial off-linkcommunication that is initiated by a directly connected host, byensuring that the first-hop router which turns on this setting doesn’thave to buffer the initial return packets to do neighbor-solicitation.The prerequisite is that the host is configured to send unsolicitedneighbor advertisements on interface bringup. This setting should beused in conjunction with the ndisc_notify setting on the host tosatisfy this prerequisite.
2 - Extend option (1) to add a new neighbor cache entry only if thesource IP address is in the same subnet as an address configured onthe interface that received the neighbor advertisement.

enhanced_dad - BOOLEAN

Include a nonce option in the IPv6 neighbor solicitation messages used forduplicate address detection per RFC7527. A received DAD NS will only signala duplicate address if the nonce is different. This avoids any falsedetection of duplicates due to loopback of the NS messages that we send.The nonce option will be sent on an interface unless both ofconf/{all,interface}/enhanced_dad are set to FALSE.

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

`icmp/*`:¶

ratelimit - INTEGER

Limit the maximal rates for sending ICMPv6 messages.

0 to disable any limiting,otherwise the minimal space between responses in milliseconds.

Default: 1000

ratemask - list of comma separated ranges

For ICMPv6 message types matching the ranges in the ratemask, limitthe sending of the message according to ratelimit parameter.

The format used for both input and output is a comma separatedlist of ranges (e.g. “0-127,129” for ICMPv6 message type 0 to 127 and129). Writing to the file will clear all previous ranges of ICMPv6message types and update the current list with the input.

Refer to:https://www.iana.org/assignments/icmpv6-parameters/icmpv6-parameters.xhtmlfor numerical values of ICMPv6 message types, e.g. echo request is 128and echo reply is 129.

Default: 0-1,3-127 (rate limit ICMPv6 errors except Packet Too Big)

echo_ignore_all - BOOLEAN

If enabled, then the kernel will ignore all ICMP ECHOrequests sent to it over the IPv6 protocol.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

echo_ignore_multicast - BOOLEAN

If enabled, then the kernel will ignore all ICMP ECHOrequests sent to it over the IPv6 protocol via multicast.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

echo_ignore_anycast - BOOLEAN

If enabled, then the kernel will ignore all ICMP ECHOrequests sent to it over the IPv6 protocol destined to anycast address.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

error_anycast_as_unicast - BOOLEAN

If enabled, then the kernel will respond with ICMP Errorsresulting from requests sent to it over the IPv6 protocol destinedto anycast address essentially treating anycast as unicast.

Possible values:

0 (disabled)
1 (enabled)

Default: 0 (disabled)

errors_extension_mask - UNSIGNED INTEGER

Bitmask of ICMP extensions to append to ICMPv6 error messages(“Destination Unreachable” and “Time Exceeded”). The original datagramis trimmed / padded to 128 bytes in order to be compatible withapplications that do not comply with RFC 4884.

Possible extensions are:

0x01	Incoming IP interface information according to RFC 5837.Extension will include the index, IPv6 address (if present),name and MTU of the IP interface that received the datagramwhich elicited the ICMP error.

Default: 0x00 (no extensions)

xfrm6_gc_thresh - INTEGER

(Obsolete since linux-4.14)The threshold at which we will start garbage collecting for IPv6destination cache entries. At twice this value the system willrefuse new allocations.

IPv6 Update by:Pekka Savola <pekkas@netcore.fi>YOSHIFUJI Hideaki / USAGI Project <yoshfuji@linux-ipv6.org>

/proc/sys/net/bridge/* Variables:¶

bridge-nf-call-arptables - BOOLEAN

Possible values:
0 (disabled) - disable this.
1 (enabled) - pass bridged ARP traffic to arptables’ FORWARD chain.
Default: 1 (enabled)

bridge-nf-call-iptables - BOOLEAN

Possible values:
0 (disabled) - disable this.
1 (enabled) - pass bridged IPv4 traffic to iptables’ chains.
Default: 1 (enabled)

bridge-nf-call-ip6tables - BOOLEAN

Possible values:
0 (disabled) - disable this.
1 (enabled) - pass bridged IPv6 traffic to ip6tables’ chains.
Default: 1 (enabled)

bridge-nf-filter-vlan-tagged - BOOLEAN

Possible values:
0 (disabled) - disable this.
1 (enabled) - pass bridged vlan-tagged ARP/IP/IPv6 traffic to {arp,ip,ip6}tables
Default: 0 (disabled)

bridge-nf-filter-pppoe-tagged - BOOLEAN

Possible values:
0 (disabled) - disable this.
1 (enabled) - pass bridged pppoe-tagged IP/IPv6 traffic to {ip,ip6}tables.
Default: 0 (disabled)

bridge-nf-pass-vlan-input-dev - BOOLEAN

1: if bridge-nf-filter-vlan-tagged is enabled, try to find a vlaninterface on the bridge and set the netfilter input device to thevlan. This allows use of e.g. “iptables -i br0.1” and makes theREDIRECT target work with vlan-on-top-of-bridge interfaces. When nomatching vlan interface is found, or this switch is off, the inputdevice is set to the bridge interface.
0: disable bridge netfilter vlan interface lookup.

Default: 0

`proc/sys/net/sctp/*` Variables:¶

addip_enable - BOOLEAN

Enable or disable extension of Dynamic Address Reconfiguration(ADD-IP) functionality specified in RFC5061. This extension providesthe ability to dynamically add and remove new addresses for the SCTPassociations.

Possible values:

0 (disabled) - disable extension.
1 (enabled) - enable extension

Default: 0 (disabled)

pf_enable - INTEGER

Enable or disable pf (pf is short for potentially failed) state. A valueof pf_retrans > path_max_retrans also disables pf state. That is, one ofboth pf_enable and pf_retrans > path_max_retrans can disable pf state.Since pf_retrans and path_max_retrans can be changed by userspaceapplication, sometimes user expects to disable pf state by the value ofpf_retrans > path_max_retrans, but occasionally the value of pf_retransor path_max_retrans is changed by the user application, this pf state isenabled. As such, it is necessary to add this to dynamically enableand disable pf state. See:https://datatracker.ietf.org/doc/draft-ietf-tsvwg-sctp-failover fordetails.

Possible values:

1: Enable pf.
0: Disable pf.

Default: 1

pf_expose - INTEGER

Unset or enable/disable pf (pf is short for potentially failed) stateexposure. Applications can control the exposure of the PF path statein the SCTP_PEER_ADDR_CHANGE event and access of SCTP_PF-statetransport info via SCTP_GET_PEER_ADDR_INFO sockopt.

Possible values:

0: Unset pf state exposure (compatible with old applications). Noevent will be sent but the transport info can be queried.
1: Disable pf state exposure. No event will be sent and trying toobtain transport info will return -EACCESS.
2: Enable pf state exposure. The event will be sent for a transportbecoming SCTP_PF state and transport info can be obtained.

Default: 0

addip_noauth_enable - BOOLEAN

Dynamic Address Reconfiguration (ADD-IP) requires the use ofauthentication to protect the operations of adding or removing newaddresses. This requirement is mandated so that unauthorized hostswould not be able to hijack associations. However, olderimplementations may not have implemented this requirement whileallowing the ADD-IP extension. For reasons of interoperability,we provide this variable to control the enforcement of theauthentication requirement.

1	Allow ADD-IP extension to be used without authentication. Thisshould only be set in a closed environment for interoperabilitywith older implementations.
0	Enforce the authentication requirement

Default: 0

auth_enable - BOOLEAN

Enable or disable Authenticated Chunks extension. This extensionprovides the ability to send and receive authenticated chunks and isrequired for secure operation of Dynamic Address Reconfiguration(ADD-IP) extension.

Possible values:

0 (disabled) - disable extension.
1 (enabled) - enable extension

Default: 0 (disabled)

prsctp_enable - BOOLEAN

Enable or disable the Partial Reliability extension (RFC3758) whichis used to notify peers that a given DATA should no longer be expected.

Possible values:

0 (disabled) - disable extension.
1 (enabled) - enable extension

Default: 1 (enabled)

max_burst - INTEGER

The limit of the number of new packets that can be initially sent. Itcontrols how bursty the generated traffic can be.

Default: 4

association_max_retrans - INTEGER

Set the maximum number for retransmissions that an association canattempt deciding that the remote end is unreachable. If this valueis exceeded, the association is terminated.

Default: 10

max_init_retransmits - INTEGER

The maximum number of retransmissions of INIT and COOKIE-ECHO chunksthat an association will attempt before declaring the destinationunreachable and terminating.

Default: 8

path_max_retrans - INTEGER

The maximum number of retransmissions that will be attempted on a givenpath. Once this threshold is exceeded, the path is consideredunreachable, and new traffic will use a different path when theassociation is multihomed.

Default: 5

pf_retrans - INTEGER

The number of retransmissions that will be attempted on a given pathbefore traffic is redirected to an alternate transport (should oneexist). Note this is distinct from path_max_retrans, as a path thatpasses the pf_retrans threshold can still be used. Its onlydeprioritized when a transmission path is selected by the stack. Thissetting is primarily used to enable fast failover mechanisms withouthaving to reduce path_max_retrans to a very low value. See:http://www.ietf.org/id/draft-nishida-tsvwg-sctp-failover-05.txtfor details. Note also that a value of pf_retrans > path_max_retransdisables this feature. Since both pf_retrans and path_max_retrans canbe changed by userspace application, a variable pf_enable is used todisable pf state.

Default: 0

ps_retrans - INTEGER

Primary.Switchover.Max.Retrans (PSMR), it’s a tunable parameter comingfrom section-5 “Primary Path Switchover” in rfc7829. The primary pathwill be changed to another active path when the path error counter onthe old primary path exceeds PSMR, so that “the SCTP sender is allowedto continue data transmission on a new working path even when the oldprimary destination address becomes active again”. Note this featureis disabled by initializing ‘ps_retrans’ per netns as 0xffff by default,and its value can’t be less than ‘pf_retrans’ when changing by sysctl.

Default: 0xffff

rto_initial - INTEGER

The initial round trip timeout value in milliseconds that will be usedin calculating round trip times. This is the initial time intervalfor retransmissions.

Default: 3000

rto_max - INTEGER

The maximum value (in milliseconds) of the round trip timeout. Thisis the largest time interval that can elapse between retransmissions.

Default: 60000

rto_min - INTEGER

The minimum value (in milliseconds) of the round trip timeout. Thisis the smallest time interval the can elapse between retransmissions.

Default: 1000

hb_interval - INTEGER

The interval (in milliseconds) between HEARTBEAT chunks. These chunksare sent at the specified interval on idle paths to probe the state ofa given path between 2 associations.

Default: 30000

sack_timeout - INTEGER

The amount of time (in milliseconds) that the implementation will waitto send a SACK.

Default: 200

valid_cookie_life - INTEGER

The default lifetime of the SCTP cookie (in milliseconds). The cookieis used during association establishment.

Default: 60000

cookie_preserve_enable - BOOLEAN

Enable or disable the ability to extend the lifetime of the SCTP cookiethat is used during the establishment phase of SCTP association

Possible values:

0 (disabled) - disable.
1 (enabled) - enable cookie lifetime extension.

Default: 1 (enabled)

cookie_hmac_alg - STRING

Select the hmac algorithm used when generating the cookie value sent bya listening sctp socket to a connecting client in the INIT-ACK chunk.Valid values are:

sha256
none

Default: sha256

rcvbuf_policy - INTEGER

Determines if the receive buffer is attributed to the socket or toassociation. SCTP supports the capability to create multipleassociations on a single socket. When using this capability, it ispossible that a single stalled association that’s buffering a lotof data may block other associations from delivering their data byconsuming all of the receive buffer space. To work around this,the rcvbuf_policy could be set to attribute the receiver buffer spaceto each association instead of the socket. This prevents the describedblocking.

1: rcvbuf space is per association
0: rcvbuf space is per socket

Default: 0

sndbuf_policy - INTEGER

Similar to rcvbuf_policy above, this applies to send buffer space.

1: Send buffer is tracked per association
0: Send buffer is tracked per socket.

Default: 0

sctp_mem - vector of 3 INTEGERs: min, pressure, max

Number of pages allowed for queueing by all SCTP sockets.

min: Below this number of pages SCTP is not bothered about itsmemory usage. When amount of memory allocated by SCTP exceedsthis number, SCTP starts to moderate memory usage.
pressure: This value was introduced to follow format of tcp_mem.
max: Maximum number of allowed pages.

Default is calculated at boot time from amount of available memory.

sctp_rmem - vector of 3 INTEGERs: min, default, max

Only the first value (“min”) is used, “default” and “max” areignored.

min: Minimal size of receive buffer used by SCTP socket.It is guaranteed to each SCTP socket (but not association) evenunder moderate memory pressure.

Default: 4K

sctp_wmem - vector of 3 INTEGERs: min, default, max

Only the first value (“min”) is used, “default” and “max” areignored.

min: Minimum size of send buffer that can be used by SCTP sockets.It is guaranteed to each SCTP socket (but not association) evenunder moderate memory pressure.

Default: 4K

addr_scope_policy - INTEGER

Control IPv4 address scoping (seehttps://datatracker.ietf.org/doc/draft-stewart-tsvwg-sctp-ipv4/00/for details).

0 - Disable IPv4 address scoping
1 - Enable IPv4 address scoping
2 - Follow draft but allow IPv4 private addresses
3 - Follow draft but allow IPv4 link local addresses

Default: 1

udp_port - INTEGER

The listening port for the local UDP tunneling sock. Normally it’susing the IANA-assigned UDP port number 9899 (sctp-tunneling).

This UDP sock is used for processing the incoming UDP-encapsulatedSCTP packets (from RFC6951), and shared by all applications in thesame net namespace. This UDP sock will be closed when the value isset to 0.

The value will also be used to set the src port of the UDP headerfor the outgoing UDP-encapsulated SCTP packets. For the dest port,please refer to ‘encap_port’ below.

Default: 0

encap_port - INTEGER

The default remote UDP encapsulation port.

This value is used to set the dest port of the UDP header for theoutgoing UDP-encapsulated SCTP packets by default. Users can alsochange the value for each sock/asoc/transport by using setsockopt.For further information, please refer to RFC6951.

Note that when connecting to a remote server, the client should setthis to the port that the UDP tunneling sock on the peer server islistening to and the local UDP tunneling sock on the client alsomust be started. On the server, it would get the encap_port fromthe incoming packet’s source port.

Default: 0

plpmtud_probe_interval - INTEGER

The time interval (in milliseconds) for the PLPMTUD probe timer,which is configured to expire after this period to receive anacknowledgment to a probe packet. This is also the time intervalbetween the probes for the current pmtu when the probe searchis done.

PLPMTUD will be disabled when 0 is set, and other values for itmust be >= 5000.

Default: 0

reconf_enable - BOOLEAN

Enable or disable extension of Stream Reconfiguration functionalityspecified in RFC6525. This extension provides the ability to “reset”a stream, and it includes the Parameters of “Outgoing/Incoming SSNReset”, “SSN/TSN Reset” and “Add Outgoing/Incoming Streams”.

Possible values:

0 (disabled) - Disable extension.
1 (enabled) - Enable extension.

Default: 0 (disabled)

intl_enable - BOOLEAN

Enable or disable extension of User Message Interleaving functionalityspecified in RFC8260. This extension allows the interleaving of usermessages sent on different streams. With this feature enabled, I-DATAchunk will replace DATA chunk to carry user messages if also supportedby the peer. Note that to use this feature, one needs to set this optionto 1 and also needs to set socket options SCTP_FRAGMENT_INTERLEAVE to 2and SCTP_INTERLEAVING_SUPPORTED to 1.

Possible values:

0 (disabled) - Disable extension.
1 (enabled) - Enable extension.

Default: 0 (disabled)

ecn_enable - BOOLEAN

Control use of Explicit Congestion Notification (ECN) by SCTP.Like in TCP, ECN is used only when both ends of the SCTP connectionindicate support for it. This feature is useful in avoiding lossesdue to congestion by allowing supporting routers to signal congestionbefore having to drop packets.

Possible values:

0 (disabled) - Disable ecn.
1 (enabled) - Enable ecn.

Default: 1 (enabled)

l3mdev_accept - BOOLEAN

Possible values:

0 (disabled)
1 (enabled)

Default: 1 (enabled)

`/proc/sys/net/core/*`¶

Please see:Documentation for /proc/sys/net/ for descriptions of these entries.

`/proc/sys/net/unix/*`¶

max_dgram_qlen - INTEGER

The maximum length of dgram socket receive queue

Default: 10

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IP Sysctl¶

/proc/sys/net/ipv4/* Variables¶

INET peer storage¶

TCP variables¶

UDP variables¶

RAW variables¶

CIPSOv4 Variables¶

IP Variables¶

/proc/sys/net/ipv6/* Variables¶

`icmp/*`:¶

/proc/sys/net/bridge/* Variables:¶

`proc/sys/net/sctp/*` Variables:¶

`/proc/sys/net/core/*`¶

`/proc/sys/net/unix/*`¶

0	Echo Reply
3	Destination Unreachable[1]
4	Source Quench[1]
5	Redirect
8	Echo Request
B	Time Exceeded[1]
C	Parameter Problem[1]
D	Timestamp Request
E	Timestamp Reply
F	Info Request
G	Info Reply
H	Address Mask Request
I	Address Mask Reply

0	(default): do nothing
1	Generate gratuitous arp requests when device is brought upor hardware address changes.

0	Do not accept Router Advertisements.
1	Accept Router Advertisements if forwarding is disabled.
2	Overrule forwarding behaviour. Accept Router Advertisementseven if forwarding is enabled.

0	Disable DAD
1	Enable DAD (default)
2	Enable DAD, and disable IPv6 operation if MAC-based duplicatelink-local address has been found.

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IP Sysctl¶

/proc/sys/net/ipv4/* Variables¶

INET peer storage¶

TCP variables¶

UDP variables¶

RAW variables¶

CIPSOv4 Variables¶

IP Variables¶

/proc/sys/net/ipv6/* Variables¶

icmp/*:¶

/proc/sys/net/bridge/* Variables:¶

proc/sys/net/sctp/* Variables:¶

/proc/sys/net/core/*¶

/proc/sys/net/unix/*¶

`icmp/*`:¶

`proc/sys/net/sctp/*` Variables:¶

`/proc/sys/net/core/*`¶

`/proc/sys/net/unix/*`¶