TCPDUMP 4.x.y byThe Tcpdump Group

https://github.com/the-tcpdump-group/tcpdump.git

Supported platforms

Dependency on libpcap

Origins of tcpdump

formerly fromLawrence Berkeley National LaboratoryNetwork Research Group <tcpdump@ee.lbl.gov>ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z (3.4)

See also

The original LBL README by Steve McCanne, Craig Leres and Van Jacobson

This directory also contains some short awk programs intended asexamples of ways to reduce tcpdump data when you're trackingparticular network problems:send-ack.awkSimplifies the tcpdump trace for an ftp (or other unidirectionaltcp transfer).  Since we assume that one host only sends andthe other only acks, all address information is left off andwe just note if the packet is a "send" or an "ack".There is one output line per line of the original trace.Field 1 is the packet time in decimal seconds, relativeto the start of the conversation.  Field 2 is delta-timefrom last packet.  Field 3 is packet type/direction."Send" means data going from sender to receiver, "ack"means an ack going from the receiver to the sender.  Apreceding "*" indicates that the data is a retransmission.A preceding "-" indicates a hole in the sequence space(i.e., missing packet(s)), a "#" means an odd-size (not maxseg size) packet.  Field 4 has the packet flags(same format as raw trace).  Field 5 is the sequencenumber (start seq. num for sender, next expected seq numberfor acks).  The number in parens following an ack isthe delta-time from the first send of the packet to theack.  A number in parens following a send is thedelta-time from the first send of the packet to thecurrent send (on duplicate packets only).  Duplicatesends or acks have a number in square brackets showingthe number of duplicates so far.Here is a short sample from near the start of an ftp:3.00    0.20   send . 5123.20    0.20    ack . 1024  (0.20)3.20    0.00   send P 10243.40    0.20    ack . 1536  (0.20)3.80    0.40 * send . 0  (3.80) [2]3.82    0.02 *  ack . 1536  (0.62) [2]Three seconds into the conversation, bytes 512 through 1023were sent.  200ms later they were acked.  Shortly thereafterbytes 1024-1535 were sent and again acked after 200ms.Then, for no apparent reason, 0-511 is retransmitted, 3.8seconds after its initial send (the round trip time for thisftp was 1sec, +-500ms).  Since the receiver is expecting1536, 1536 is re-acked when 0 arrives.packetdat.awkComputes chunk summary data for an ftp (or similarunidirectional tcp transfer). [A "chunk" refers toa chunk of the sequence space -- essentially the packetsequence number divided by the max segment size.]A summary line is printed showing the number of chunks,the number of packets it took to send that many chunks(if there are no lost or duplicated packets, the numberof packets should equal the number of chunks) and thenumber of acks.Following the summary line is one line of informationper chunk.  The line contains eight fields:   1 - the chunk number   2 - the start sequence number for this chunk   3 - time of first send   4 - time of last send   5 - time of first ack   6 - time of last ack   7 - number of times chunk was sent   8 - number of times chunk was acked(all times are in decimal seconds, relative to the startof the conversation.)As an example, here is the first part of the output foran ftp trace:# 134 chunks.  536 packets sent.  508 acks.1       1       0.00    5.80    0.20    0.20    4       12       513     0.28    6.20    0.40    0.40    4       13       1025    1.16    6.32    1.20    1.20    4       14       1561    1.86    15.00   2.00    2.00    6       15       2049    2.16    15.44   2.20    2.20    5       16       2585    2.64    16.44   2.80    2.80    5       17       3073    3.00    16.66   3.20    3.20    4       18       3609    3.20    17.24   3.40    5.82    4       119       4097    6.02    6.58    6.20    6.80    2       5This says that 134 chunks were transferred (about 70Ksince the average packet size was 512 bytes).  It took536 packets to transfer the data (i.e., on the averageeach chunk was transmitted four times).  Looking at,say, chunk 4, we see it represents the 512 bytes ofsequence space from 1561 to 2048.  It was first sent1.86 seconds into the conversation.  It was lastsent 15 seconds into the conversation and was senta total of 6 times (i.e., it was retransmitted every2 seconds on the average).  It was acked once, 140msafter it first arrived.stime.awkatime.awkOutput one line per send or ack, respectively, in the form<time> <seq. number>where <time> is the time in seconds since the start of thetransfer and <seq. number> is the sequence number being sentor acked.  I typically plot this data looking for suspiciouspatterns.The problem I was looking at was the bulk-data-transferthroughput of medium delay network paths (1-6 sec.  round triptime) under typical DARPA Internet conditions.  The trace of theftp transfer of a large file was used as the raw data source.The method was:  - On a local host (but not the Sun running tcpdump), connect to    the remote ftp.  - On the monitor Sun, start the trace going.  E.g.,      tcpdump host local-host and remote-host and port ftp-data >tracefile  - On local, do either a get or put of a large file (~500KB),    preferably to the null device (to minimize effects like    closing the receive window while waiting for a disk write).  - When transfer is finished, stop tcpdump.  Use awk to make up    two files of summary data (maxsize is the maximum packet size,    tracedata is the file of tcpdump tracedata):      awk -f send-ack.awk packetsize=avgsize tracedata >sa      awk -f packetdat.awk packetsize=avgsize tracedata >pd  - While the summary data files are printing, take a look at    how the transfer behaved:      awk -f stime.awk tracedata | xgraph    (90% of what you learn seems to happen in this step).  - Do all of the above steps several times, both directions,    at different times of day, with different protocol    implementations on the other end.  - Using one of the Unix data analysis packages (in my case,    S and Gary Perlman's Unix|Stat), spend a few months staring    at the data.  - Change something in the local protocol implementation and    redo the steps above.  - Once a week, tell your funding agent that you're discovering    wonderful things and you'll write up that research report    "real soon now".