1 # TCPDUMP 4.x.y by [The Tcpdump Group](https://www.tcpdump.org/)
3 **To report a security issue please send an e-mail to security@tcpdump.org.**
5 To report bugs and other problems, contribute patches, request a
6 feature, provide generic feedback etc please see the
7 [guidelines for contributing](CONTRIBUTING.md) in the tcpdump source tree root.
9 Anonymous Git is available via
11 https://github.com/the-tcpdump-group/tcpdump.git
13 This directory contains source code for tcpdump, a tool for network
14 monitoring and data acquisition.
16 Over the past few years, tcpdump has been steadily improved by the
17 excellent contributions from the Internet community (just browse
18 through the [change log](CHANGES)). We are grateful for all the input.
20 ### Supported platforms
21 In many operating systems tcpdump is available as a native package or port,
22 which simplifies installation of updates and long-term maintenance. However,
23 the native packages are sometimes a few versions behind and to try a more
24 recent snapshot it will take to compile tcpdump from the source code.
26 tcpdump compiles and works on at least the following platforms:
33 * illumos (OmniOS, OpenIndiana)
39 * Windows (requires WinPcap or Npcap, and Visual Studio with CMake)
41 In the past tcpdump certainly or likely worked on the following platforms:
44 * BSD/386, later BSD/OS
45 * DEC OSF/1, later Digital UNIX, later Tru64 UNIX
53 ### Dependency on libpcap
54 Tcpdump uses libpcap, a system-independent interface for user-level
55 packet capture. Before building tcpdump, you must first retrieve and
58 Once libpcap is built (either install it or make sure it's in
59 `../libpcap`), you can build tcpdump using the procedure in the
60 [installation notes](INSTALL.md).
62 ### Origins of tcpdump
63 The program is loosely based on SMI's "etherfind" although none of the
64 etherfind code remains. It was originally written by Van Jacobson as
65 part of an ongoing research project to investigate and improve TCP and
66 Internet gateway performance. The parts of the program originally
67 taken from Sun's etherfind were later re-written by Steven McCanne of
68 LBL. To insure that there would be no vestige of proprietary code in
69 tcpdump, Steve wrote these pieces from the specification given by the
70 manual entry, with no access to the source of tcpdump or etherfind.
72 formerly from Lawrence Berkeley National Laboratory
73 Network Research Group <tcpdump@ee.lbl.gov>
74 ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z (3.4)
78 Richard Stevens gives an excellent treatment of the Internet protocols
79 in his book *"TCP/IP Illustrated, Volume 1"*. If you want to learn more
80 about tcpdump and how to interpret its output, pick up this book.
82 Another tool that tcpdump users might find useful is
83 [tcpslice](https://github.com/the-tcpdump-group/tcpslice).
84 It is a program that can be used to extract portions of tcpdump binary
87 ### The original LBL README by Steve McCanne, Craig Leres and Van Jacobson
89 This directory also contains some short awk programs intended as
90 examples of ways to reduce tcpdump data when you're tracking
91 particular network problems:
94 Simplifies the tcpdump trace for an ftp (or other unidirectional
95 tcp transfer). Since we assume that one host only sends and
96 the other only acks, all address information is left off and
97 we just note if the packet is a "send" or an "ack".
99 There is one output line per line of the original trace.
100 Field 1 is the packet time in decimal seconds, relative
101 to the start of the conversation. Field 2 is delta-time
102 from last packet. Field 3 is packet type/direction.
103 "Send" means data going from sender to receiver, "ack"
104 means an ack going from the receiver to the sender. A
105 preceding "*" indicates that the data is a retransmission.
106 A preceding "-" indicates a hole in the sequence space
107 (i.e., missing packet(s)), a "#" means an odd-size (not max
108 seg size) packet. Field 4 has the packet flags
109 (same format as raw trace). Field 5 is the sequence
110 number (start seq. num for sender, next expected seq number
111 for acks). The number in parens following an ack is
112 the delta-time from the first send of the packet to the
113 ack. A number in parens following a send is the
114 delta-time from the first send of the packet to the
115 current send (on duplicate packets only). Duplicate
116 sends or acks have a number in square brackets showing
117 the number of duplicates so far.
119 Here is a short sample from near the start of an ftp:
121 3.20 0.20 ack . 1024 (0.20)
122 3.20 0.00 send P 1024
123 3.40 0.20 ack . 1536 (0.20)
124 3.80 0.40 * send . 0 (3.80) [2]
125 3.82 0.02 * ack . 1536 (0.62) [2]
126 Three seconds into the conversation, bytes 512 through 1023
127 were sent. 200ms later they were acked. Shortly thereafter
128 bytes 1024-1535 were sent and again acked after 200ms.
129 Then, for no apparent reason, 0-511 is retransmitted, 3.8
130 seconds after its initial send (the round trip time for this
131 ftp was 1sec, +-500ms). Since the receiver is expecting
132 1536, 1536 is re-acked when 0 arrives.
135 Computes chunk summary data for an ftp (or similar
136 unidirectional tcp transfer). [A "chunk" refers to
137 a chunk of the sequence space -- essentially the packet
138 sequence number divided by the max segment size.]
140 A summary line is printed showing the number of chunks,
141 the number of packets it took to send that many chunks
142 (if there are no lost or duplicated packets, the number
143 of packets should equal the number of chunks) and the
146 Following the summary line is one line of information
147 per chunk. The line contains eight fields:
149 2 - the start sequence number for this chunk
150 3 - time of first send
151 4 - time of last send
152 5 - time of first ack
154 7 - number of times chunk was sent
155 8 - number of times chunk was acked
156 (all times are in decimal seconds, relative to the start
157 of the conversation.)
159 As an example, here is the first part of the output for
162 # 134 chunks. 536 packets sent. 508 acks.
163 1 1 0.00 5.80 0.20 0.20 4 1
164 2 513 0.28 6.20 0.40 0.40 4 1
165 3 1025 1.16 6.32 1.20 1.20 4 1
166 4 1561 1.86 15.00 2.00 2.00 6 1
167 5 2049 2.16 15.44 2.20 2.20 5 1
168 6 2585 2.64 16.44 2.80 2.80 5 1
169 7 3073 3.00 16.66 3.20 3.20 4 1
170 8 3609 3.20 17.24 3.40 5.82 4 11
171 9 4097 6.02 6.58 6.20 6.80 2 5
173 This says that 134 chunks were transferred (about 70K
174 since the average packet size was 512 bytes). It took
175 536 packets to transfer the data (i.e., on the average
176 each chunk was transmitted four times). Looking at,
177 say, chunk 4, we see it represents the 512 bytes of
178 sequence space from 1561 to 2048. It was first sent
179 1.86 seconds into the conversation. It was last
180 sent 15 seconds into the conversation and was sent
181 a total of 6 times (i.e., it was retransmitted every
182 2 seconds on the average). It was acked once, 140ms
183 after it first arrived.
187 Output one line per send or ack, respectively, in the form
189 where <time> is the time in seconds since the start of the
190 transfer and <seq. number> is the sequence number being sent
191 or acked. I typically plot this data looking for suspicious
195 The problem I was looking at was the bulk-data-transfer
196 throughput of medium delay network paths (1-6 sec. round trip
197 time) under typical DARPA Internet conditions. The trace of the
198 ftp transfer of a large file was used as the raw data source.
201 - On a local host (but not the Sun running tcpdump), connect to
204 - On the monitor Sun, start the trace going. E.g.,
205 tcpdump host local-host and remote-host and port ftp-data >tracefile
207 - On local, do either a get or put of a large file (~500KB),
208 preferably to the null device (to minimize effects like
209 closing the receive window while waiting for a disk write).
211 - When transfer is finished, stop tcpdump. Use awk to make up
212 two files of summary data (maxsize is the maximum packet size,
213 tracedata is the file of tcpdump tracedata):
214 awk -f send-ack.awk packetsize=avgsize tracedata >sa
215 awk -f packetdat.awk packetsize=avgsize tracedata >pd
217 - While the summary data files are printing, take a look at
218 how the transfer behaved:
219 awk -f stime.awk tracedata | xgraph
220 (90% of what you learn seems to happen in this step).
222 - Do all of the above steps several times, both directions,
223 at different times of day, with different protocol
224 implementations on the other end.
226 - Using one of the Unix data analysis packages (in my case,
227 S and Gary Perlman's Unix|Stat), spend a few months staring
230 - Change something in the local protocol implementation and
231 redo the steps above.
233 - Once a week, tell your funding agent that you're discovering
234 wonderful things and you'll write up that research report
projects / tcpdump / blob