1. Making non-local sockets work¶

The idea is that you identify packets with destination address matching a localsocket on your box, set the packet mark to a certain value:

# iptables -t mangle -N DIVERT# iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT# iptables -t mangle -A DIVERT -j MARK --set-mark 1# iptables -t mangle -A DIVERT -j ACCEPT

Alternatively you can do this in nft with the following commands:

# nft add table filter# nft add chain filter divert "{ type filter hook prerouting priority -150; }"# nft add rule filter divert meta l4proto tcp socket transparent 1 meta mark set 1 accept

And then match on that value using policy routing to have those packetsdelivered locally:

# ip rule add fwmark 1 lookup 100# ip route add local 0.0.0.0/0 dev lo table 100

Because of certain restrictions in the IPv4 routing output code you’ll have tomodify your application to allow it to send datagrams _from_ non-local IPaddresses. All you have to do is enable the (SOL_IP, IP_TRANSPARENT) socketoption before calling bind:

fd = socket(AF_INET, SOCK_STREAM, 0);/* - 8< -*/int value = 1;setsockopt(fd, SOL_IP, IP_TRANSPARENT, &value, sizeof(value));/* - 8< -*/name.sin_family = AF_INET;name.sin_port = htons(0xCAFE);name.sin_addr.s_addr = htonl(0xDEADBEEF);bind(fd, &name, sizeof(name));

A trivial patch for netcat is available here:http://people.netfilter.org/hidden/tproxy/netcat-ip_transparent-support.patch

2. Redirecting traffic¶

Transparent proxying often involves “intercepting” traffic on a router. This isusually done with the iptables REDIRECT target; however, there are seriouslimitations of that method. One of the major issues is that it actuallymodifies the packets to change the destination address – which might not beacceptable in certain situations. (Think of proxying UDP for example: you won’tbe able to find out the original destination address. Even in case of TCPgetting the original destination address is racy.)

The ‘TPROXY’ target provides similar functionality without relying on NAT. Simplyadd rules like this to the iptables ruleset above:

# iptables -t mangle -A PREROUTING -p tcp --dport 80 -j TPROXY \  --tproxy-mark 0x1/0x1 --on-port 50080

Or the following rule to nft:

# nft add rule filter divert tcp dport 80 tproxy to :50080 meta mark set 1 accept

Note that for this to work you’ll have to modify the proxy to enable (SOL_IP,IP_TRANSPARENT) for the listening socket.

As an example implementation, tcprdr is available here:https://git.breakpoint.cc/cgit/fw/tcprdr.git/This tool is written by Florian Westphal and it was used for testing during thenf_tables implementation.

3. Iptables and nf_tables extensions¶

To use tproxy you’ll need to have the following modules compiled for iptables:

• NETFILTER_XT_MATCH_SOCKET
• NETFILTER_XT_TARGET_TPROXY

Or the floowing modules for nf_tables:

• NFT_SOCKET
• NFT_TPROXY

4. Application support¶