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tcpdump

Build Status

TCPDUMP 4.x.y
Now maintained by “The Tcpdump Group”
See www.tcpdump.org

Please send inquiries/comments/reports to:

Anonymous Git is available via:

git clone git://bpf.tcpdump.org/tcpdump

Please submit patches by forking the branch on GitHub at:

and issuing a pull request.

formerly from Lawrence Berkeley National Laboratory
Network Research Group tcpdump@ee.lbl.gov
ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z (3.4)

This directory contains source code for tcpdump, a tool for network monitoring and data acquisition. This software was originally developed by the Network Research Group at the Lawrence Berkeley National Laboratory. The original distribution is available via anonymous ftp to ftp.ee.lbl.gov, in tcpdump.tar.Z. More recent development is performed at tcpdump.org, http://www.tcpdump.org/

Tcpdump uses libpcap, a system-independent interface for user-level packet capture. Before building tcpdump, you must first retrieve and build libpcap, also originally from LBL and now being maintained by tcpdump.org; see http://www.tcpdump.org/ .

Once libpcap is built (either install it or make sure it’s in ../libpcap), you can build tcpdump using the procedure in the INSTALL.txt file.

The program is loosely based on SMI’s “etherfind” although none of the etherfind code remains. It was originally written by Van Jacobson as part of an ongoing research project to investigate and improve tcp and internet gateway performance. The parts of the program originally taken from Sun’s etherfind were later re-written by Steven McCanne of LBL. To insure that there would be no vestige of proprietary code in tcpdump, Steve wrote these pieces from the specification given by the manual entry, with no access to the source of tcpdump or etherfind.

Over the past few years, tcpdump has been steadily improved by the excellent contributions from the Internet community (just browse through the CHANGES file). We are grateful for all the input.

Richard Stevens gives an excellent treatment of the Internet protocols in his book “TCP/IP Illustrated, Volume 1”. If you want to learn more about tcpdump and how to interpret its output, pick up this book.

Some tools for viewing and analyzing tcpdump trace files are available from the Internet Traffic Archive:

Another tool that tcpdump users might find useful is tcpslice:

It is a program that can be used to extract portions of tcpdump binary trace files. See the above distribution for further details and documentation.

Problems, bugs, questions, desirable enhancements, etc. should be sent to the address "tcpdump-workers@lists.tcpdump.org". Bugs, support requests, and feature requests may also be submitted on the GitHub issue tracker for tcpdump at:

Source code contributions, etc. should be sent to the email address above or submitted by forking the branch on GitHub at:

and issuing a pull request.

Current versions can be found at www.tcpdump.org.

original text by: Steve McCanne, Craig Leres, Van Jacobson

————————————- ``` This directory also contains some short awk programs intended as examples of ways to reduce tcpdump data when you’re tracking particular network problems:

send-ack.awk Simplifies the tcpdump trace for an ftp (or other unidirectional tcp transfer). Since we assume that one host only sends and the other only acks, all address information is left off and we 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, relative
to the start of the conversation.  Field 2 is delta-time
from 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.  A
preceding "*" 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 max
seg size) packet.  Field 4 has the packet flags
(same format as raw trace).  Field 5 is the sequence
number (start seq. num for sender, next expected seq number
for acks).  The number in parens following an ack is
the delta-time from the first send of the packet to the
ack.  A number in parens following a send is the
delta-time from the first send of the packet to the
current send (on duplicate packets only).  Duplicate
sends or acks have a number in square brackets showing
the number of duplicates so far.

Here is a short sample from near the start of an ftp:
    3.00    0.20   send . 512
    3.20    0.20    ack . 1024  (0.20)
    3.20    0.00   send P 1024
    3.40    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 1023
were sent.  200ms later they were acked.  Shortly thereafter
bytes 1024-1535 were sent and again acked after 200ms.
Then, for no apparent reason, 0-511 is retransmitted, 3.8
seconds after its initial send (the round trip time for this
ftp was 1sec, +-500ms).  Since the receiver is expecting
1536, 1536 is re-acked when 0 arrives.

packetdat.awk Computes chunk summary data for an ftp (or similar unidirectional tcp transfer). [A “chunk” refers to a chunk of the sequence space – essentially the packet sequence 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 number
of packets should equal the number of chunks) and the
number of acks.

Following the summary line is one line of information
per 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 start
of the conversation.)

As an example, here is the first part of the output for
an ftp trace:

# 134 chunks.  536 packets sent.  508 acks.
1       1       0.00    5.80    0.20    0.20    4       1
2       513     0.28    6.20    0.40    0.40    4       1
3       1025    1.16    6.32    1.20    1.20    4       1
4       1561    1.86    15.00   2.00    2.00    6       1
5       2049    2.16    15.44   2.20    2.20    5       1
6       2585    2.64    16.44   2.80    2.80    5       1
7       3073    3.00    16.66   3.20    3.20    4       1
8       3609    3.20    17.24   3.40    5.82    4       11
9       4097    6.02    6.58    6.20    6.80    2       5

This says that 134 chunks were transferred (about 70K
since the average packet size was 512 bytes).  It took
536 packets to transfer the data (i.e., on the average
each chunk was transmitted four times).  Looking at,
say, chunk 4, we see it represents the 512 bytes of
sequence space from 1561 to 2048.  It was first sent
1.86 seconds into the conversation.  It was last
sent 15 seconds into the conversation and was sent
a total of 6 times (i.e., it was retransmitted every
2 seconds on the average).  It was acked once, 140ms
after it first arrived.

stime.awk atime.awk Output one line per send or ack, respectively, in the form <seq. number> where is the time in seconds since the start of the transfer and <seq. number> is the sequence number being sent or acked. I typically plot this data looking for suspicious patterns.

The problem I was looking at was the bulk-data-transfer throughput of medium delay network paths (1-6 sec. round trip time) under typical DARPA Internet conditions. The trace of the ftp transfer of a large file was used as the raw data source. The method was:

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