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This directory contains source code for tcpdump, a tool for network monitoring and data acquisition.
Over the past few years, tcpdump has been steadily improved by the excellent contributions from the Internet community (just browse through the change log). We are grateful for all the input.
In many operating systems tcpdump is available as a native package or port, which simplifies installation of updates and long-term maintenance. However, the native packages are sometimes a few versions behind and to try a more recent snapshot it will take to compile tcpdump from the source code.
tcpdump compiles and works on at least the following platforms:
Tcpdump uses libpcap, a system-independent interface for user-level packet capture. Before building tcpdump, you must first retrieve and build libpcap.
Once libpcap is built (either install it or make sure it's in
../libpcap), you can build tcpdump using the procedure in the installation guide.
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.
formerly from Lawrence Berkeley National Laboratory Network Research Group <firstname.lastname@example.org> ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z (3.4)
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.
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.
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)  3.82 0.02 * ack . 1536 (0.62)  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 <time> <seq. number> where <time> 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: - 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".