README
1# tcpdump
2
3[](https://travis-ci.org/the-tcpdump-group/tcpdump)
5
6To report a security issue please send an e-mail to security@tcpdump.org.
7
8To report bugs and other problems, contribute patches, request a
9feature, provide generic feedback etc please see the file
10CONTRIBUTING in the tcpdump source tree root.
11
12TCPDUMP 4.x.y
13Now maintained by "The Tcpdump Group"
14See www.tcpdump.org
15
16Anonymous Git is available via:
17
18 git clone git://bpf.tcpdump.org/tcpdump
19
20formerly from Lawrence Berkeley National Laboratory
21 Network Research Group <tcpdump@ee.lbl.gov>
22 ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z (3.4)
23
24This directory contains source code for tcpdump, a tool for network
25monitoring and data acquisition. This software was originally
26developed by the Network Research Group at the Lawrence Berkeley
27National Laboratory. The original distribution is available via
28anonymous ftp to `ftp.ee.lbl.gov`, in `tcpdump.tar.Z`. More recent
29development is performed at tcpdump.org, http://www.tcpdump.org/
30
31Tcpdump uses libpcap, a system-independent interface for user-level
32packet capture. Before building tcpdump, you must first retrieve and
33build libpcap, also originally from LBL and now being maintained by
34tcpdump.org; see http://www.tcpdump.org/ .
35
36Once libpcap is built (either install it or make sure it's in
37`../libpcap`), you can build tcpdump using the procedure in the `INSTALL.txt`
38file.
39
40The program is loosely based on SMI's "etherfind" although none of the
41etherfind code remains. It was originally written by Van Jacobson as
42part of an ongoing research project to investigate and improve tcp and
43internet gateway performance. The parts of the program originally
44taken from Sun's etherfind were later re-written by Steven McCanne of
45LBL. To insure that there would be no vestige of proprietary code in
46tcpdump, Steve wrote these pieces from the specification given by the
47manual entry, with no access to the source of tcpdump or etherfind.
48
49Over the past few years, tcpdump has been steadily improved by the
50excellent contributions from the Internet community (just browse
51through the `CHANGES` file). We are grateful for all the input.
52
53Richard Stevens gives an excellent treatment of the Internet protocols
54in his book *"TCP/IP Illustrated, Volume 1"*. If you want to learn more
55about tcpdump and how to interpret its output, pick up this book.
56
57Some tools for viewing and analyzing tcpdump trace files are available
58from the Internet Traffic Archive:
59
60* http://www.sigcomm.org/ITA/
61
62Another tool that tcpdump users might find useful is tcpslice:
63
64* https://github.com/the-tcpdump-group/tcpslice
65
66It is a program that can be used to extract portions of tcpdump binary
67trace files. See the above distribution for further details and
68documentation.
69
70Current versions can be found at www.tcpdump.org.
71
72 - The TCPdump team
73
74original text by: Steve McCanne, Craig Leres, Van Jacobson
75
76-------------------------------------
77```
78This directory also contains some short awk programs intended as
79examples of ways to reduce tcpdump data when you're tracking
80particular network problems:
81
82send-ack.awk
83 Simplifies the tcpdump trace for an ftp (or other unidirectional
84 tcp transfer). Since we assume that one host only sends and
85 the other only acks, all address information is left off and
86 we just note if the packet is a "send" or an "ack".
87
88 There is one output line per line of the original trace.
89 Field 1 is the packet time in decimal seconds, relative
90 to the start of the conversation. Field 2 is delta-time
91 from last packet. Field 3 is packet type/direction.
92 "Send" means data going from sender to receiver, "ack"
93 means an ack going from the receiver to the sender. A
94 preceding "*" indicates that the data is a retransmission.
95 A preceding "-" indicates a hole in the sequence space
96 (i.e., missing packet(s)), a "#" means an odd-size (not max
97 seg size) packet. Field 4 has the packet flags
98 (same format as raw trace). Field 5 is the sequence
99 number (start seq. num for sender, next expected seq number
100 for acks). The number in parens following an ack is
101 the delta-time from the first send of the packet to the
102 ack. A number in parens following a send is the
103 delta-time from the first send of the packet to the
104 current send (on duplicate packets only). Duplicate
105 sends or acks have a number in square brackets showing
106 the number of duplicates so far.
107
108 Here is a short sample from near the start of an ftp:
109 3.00 0.20 send . 512
110 3.20 0.20 ack . 1024 (0.20)
111 3.20 0.00 send P 1024
112 3.40 0.20 ack . 1536 (0.20)
113 3.80 0.40 * send . 0 (3.80) [2]
114 3.82 0.02 * ack . 1536 (0.62) [2]
115 Three seconds into the conversation, bytes 512 through 1023
116 were sent. 200ms later they were acked. Shortly thereafter
117 bytes 1024-1535 were sent and again acked after 200ms.
118 Then, for no apparent reason, 0-511 is retransmitted, 3.8
119 seconds after its initial send (the round trip time for this
120 ftp was 1sec, +-500ms). Since the receiver is expecting
121 1536, 1536 is re-acked when 0 arrives.
122
123packetdat.awk
124 Computes chunk summary data for an ftp (or similar
125 unidirectional tcp transfer). [A "chunk" refers to
126 a chunk of the sequence space -- essentially the packet
127 sequence number divided by the max segment size.]
128
129 A summary line is printed showing the number of chunks,
130 the number of packets it took to send that many chunks
131 (if there are no lost or duplicated packets, the number
132 of packets should equal the number of chunks) and the
133 number of acks.
134
135 Following the summary line is one line of information
136 per chunk. The line contains eight fields:
137 1 - the chunk number
138 2 - the start sequence number for this chunk
139 3 - time of first send
140 4 - time of last send
141 5 - time of first ack
142 6 - time of last ack
143 7 - number of times chunk was sent
144 8 - number of times chunk was acked
145 (all times are in decimal seconds, relative to the start
146 of the conversation.)
147
148 As an example, here is the first part of the output for
149 an ftp trace:
150
151 # 134 chunks. 536 packets sent. 508 acks.
152 1 1 0.00 5.80 0.20 0.20 4 1
153 2 513 0.28 6.20 0.40 0.40 4 1
154 3 1025 1.16 6.32 1.20 1.20 4 1
155 4 1561 1.86 15.00 2.00 2.00 6 1
156 5 2049 2.16 15.44 2.20 2.20 5 1
157 6 2585 2.64 16.44 2.80 2.80 5 1
158 7 3073 3.00 16.66 3.20 3.20 4 1
159 8 3609 3.20 17.24 3.40 5.82 4 11
160 9 4097 6.02 6.58 6.20 6.80 2 5
161
162 This says that 134 chunks were transferred (about 70K
163 since the average packet size was 512 bytes). It took
164 536 packets to transfer the data (i.e., on the average
165 each chunk was transmitted four times). Looking at,
166 say, chunk 4, we see it represents the 512 bytes of
167 sequence space from 1561 to 2048. It was first sent
168 1.86 seconds into the conversation. It was last
169 sent 15 seconds into the conversation and was sent
170 a total of 6 times (i.e., it was retransmitted every
171 2 seconds on the average). It was acked once, 140ms
172 after it first arrived.
173
174stime.awk
175atime.awk
176 Output one line per send or ack, respectively, in the form
177 <time> <seq. number>
178 where <time> is the time in seconds since the start of the
179 transfer and <seq. number> is the sequence number being sent
180 or acked. I typically plot this data looking for suspicious
181 patterns.
182
183
184The problem I was looking at was the bulk-data-transfer
185throughput of medium delay network paths (1-6 sec. round trip
186time) under typical DARPA Internet conditions. The trace of the
187ftp transfer of a large file was used as the raw data source.
188The method was:
189
190 - On a local host (but not the Sun running tcpdump), connect to
191 the remote ftp.
192
193 - On the monitor Sun, start the trace going. E.g.,
194 tcpdump host local-host and remote-host and port ftp-data >tracefile
195
196 - On local, do either a get or put of a large file (~500KB),
197 preferably to the null device (to minimize effects like
198 closing the receive window while waiting for a disk write).
199
200 - When transfer is finished, stop tcpdump. Use awk to make up
201 two files of summary data (maxsize is the maximum packet size,
202 tracedata is the file of tcpdump tracedata):
203 awk -f send-ack.awk packetsize=avgsize tracedata >sa
204 awk -f packetdat.awk packetsize=avgsize tracedata >pd
205
206 - While the summary data files are printing, take a look at
207 how the transfer behaved:
208 awk -f stime.awk tracedata | xgraph
209 (90% of what you learn seems to happen in this step).
210
211 - Do all of the above steps several times, both directions,
212 at different times of day, with different protocol
213 implementations on the other end.
214
215 - Using one of the Unix data analysis packages (in my case,
216 S and Gary Perlman's Unix|Stat), spend a few months staring
217 at the data.
218
219 - Change something in the local protocol implementation and
220 redo the steps above.
221
222 - Once a week, tell your funding agent that you're discovering
223 wonderful things and you'll write up that research report
224 "real soon now".
225```
226
README.md
1# tcpdump
2
3[](https://travis-ci.org/the-tcpdump-group/tcpdump)
5
6To report a security issue please send an e-mail to security@tcpdump.org.
7
8To report bugs and other problems, contribute patches, request a
9feature, provide generic feedback etc please see the file
10CONTRIBUTING in the tcpdump source tree root.
11
12TCPDUMP 4.x.y
13Now maintained by "The Tcpdump Group"
14See www.tcpdump.org
15
16Anonymous Git is available via:
17
18 git clone git://bpf.tcpdump.org/tcpdump
19
20formerly from Lawrence Berkeley National Laboratory
21 Network Research Group <tcpdump@ee.lbl.gov>
22 ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z (3.4)
23
24This directory contains source code for tcpdump, a tool for network
25monitoring and data acquisition. This software was originally
26developed by the Network Research Group at the Lawrence Berkeley
27National Laboratory. The original distribution is available via
28anonymous ftp to `ftp.ee.lbl.gov`, in `tcpdump.tar.Z`. More recent
29development is performed at tcpdump.org, http://www.tcpdump.org/
30
31Tcpdump uses libpcap, a system-independent interface for user-level
32packet capture. Before building tcpdump, you must first retrieve and
33build libpcap, also originally from LBL and now being maintained by
34tcpdump.org; see http://www.tcpdump.org/ .
35
36Once libpcap is built (either install it or make sure it's in
37`../libpcap`), you can build tcpdump using the procedure in the `INSTALL.txt`
38file.
39
40The program is loosely based on SMI's "etherfind" although none of the
41etherfind code remains. It was originally written by Van Jacobson as
42part of an ongoing research project to investigate and improve tcp and
43internet gateway performance. The parts of the program originally
44taken from Sun's etherfind were later re-written by Steven McCanne of
45LBL. To insure that there would be no vestige of proprietary code in
46tcpdump, Steve wrote these pieces from the specification given by the
47manual entry, with no access to the source of tcpdump or etherfind.
48
49Over the past few years, tcpdump has been steadily improved by the
50excellent contributions from the Internet community (just browse
51through the `CHANGES` file). We are grateful for all the input.
52
53Richard Stevens gives an excellent treatment of the Internet protocols
54in his book *"TCP/IP Illustrated, Volume 1"*. If you want to learn more
55about tcpdump and how to interpret its output, pick up this book.
56
57Some tools for viewing and analyzing tcpdump trace files are available
58from the Internet Traffic Archive:
59
60* http://www.sigcomm.org/ITA/
61
62Another tool that tcpdump users might find useful is tcpslice:
63
64* https://github.com/the-tcpdump-group/tcpslice
65
66It is a program that can be used to extract portions of tcpdump binary
67trace files. See the above distribution for further details and
68documentation.
69
70Current versions can be found at www.tcpdump.org.
71
72 - The TCPdump team
73
74original text by: Steve McCanne, Craig Leres, Van Jacobson
75
76-------------------------------------
77```
78This directory also contains some short awk programs intended as
79examples of ways to reduce tcpdump data when you're tracking
80particular network problems:
81
82send-ack.awk
83 Simplifies the tcpdump trace for an ftp (or other unidirectional
84 tcp transfer). Since we assume that one host only sends and
85 the other only acks, all address information is left off and
86 we just note if the packet is a "send" or an "ack".
87
88 There is one output line per line of the original trace.
89 Field 1 is the packet time in decimal seconds, relative
90 to the start of the conversation. Field 2 is delta-time
91 from last packet. Field 3 is packet type/direction.
92 "Send" means data going from sender to receiver, "ack"
93 means an ack going from the receiver to the sender. A
94 preceding "*" indicates that the data is a retransmission.
95 A preceding "-" indicates a hole in the sequence space
96 (i.e., missing packet(s)), a "#" means an odd-size (not max
97 seg size) packet. Field 4 has the packet flags
98 (same format as raw trace). Field 5 is the sequence
99 number (start seq. num for sender, next expected seq number
100 for acks). The number in parens following an ack is
101 the delta-time from the first send of the packet to the
102 ack. A number in parens following a send is the
103 delta-time from the first send of the packet to the
104 current send (on duplicate packets only). Duplicate
105 sends or acks have a number in square brackets showing
106 the number of duplicates so far.
107
108 Here is a short sample from near the start of an ftp:
109 3.00 0.20 send . 512
110 3.20 0.20 ack . 1024 (0.20)
111 3.20 0.00 send P 1024
112 3.40 0.20 ack . 1536 (0.20)
113 3.80 0.40 * send . 0 (3.80) [2]
114 3.82 0.02 * ack . 1536 (0.62) [2]
115 Three seconds into the conversation, bytes 512 through 1023
116 were sent. 200ms later they were acked. Shortly thereafter
117 bytes 1024-1535 were sent and again acked after 200ms.
118 Then, for no apparent reason, 0-511 is retransmitted, 3.8
119 seconds after its initial send (the round trip time for this
120 ftp was 1sec, +-500ms). Since the receiver is expecting
121 1536, 1536 is re-acked when 0 arrives.
122
123packetdat.awk
124 Computes chunk summary data for an ftp (or similar
125 unidirectional tcp transfer). [A "chunk" refers to
126 a chunk of the sequence space -- essentially the packet
127 sequence number divided by the max segment size.]
128
129 A summary line is printed showing the number of chunks,
130 the number of packets it took to send that many chunks
131 (if there are no lost or duplicated packets, the number
132 of packets should equal the number of chunks) and the
133 number of acks.
134
135 Following the summary line is one line of information
136 per chunk. The line contains eight fields:
137 1 - the chunk number
138 2 - the start sequence number for this chunk
139 3 - time of first send
140 4 - time of last send
141 5 - time of first ack
142 6 - time of last ack
143 7 - number of times chunk was sent
144 8 - number of times chunk was acked
145 (all times are in decimal seconds, relative to the start
146 of the conversation.)
147
148 As an example, here is the first part of the output for
149 an ftp trace:
150
151 # 134 chunks. 536 packets sent. 508 acks.
152 1 1 0.00 5.80 0.20 0.20 4 1
153 2 513 0.28 6.20 0.40 0.40 4 1
154 3 1025 1.16 6.32 1.20 1.20 4 1
155 4 1561 1.86 15.00 2.00 2.00 6 1
156 5 2049 2.16 15.44 2.20 2.20 5 1
157 6 2585 2.64 16.44 2.80 2.80 5 1
158 7 3073 3.00 16.66 3.20 3.20 4 1
159 8 3609 3.20 17.24 3.40 5.82 4 11
160 9 4097 6.02 6.58 6.20 6.80 2 5
161
162 This says that 134 chunks were transferred (about 70K
163 since the average packet size was 512 bytes). It took
164 536 packets to transfer the data (i.e., on the average
165 each chunk was transmitted four times). Looking at,
166 say, chunk 4, we see it represents the 512 bytes of
167 sequence space from 1561 to 2048. It was first sent
168 1.86 seconds into the conversation. It was last
169 sent 15 seconds into the conversation and was sent
170 a total of 6 times (i.e., it was retransmitted every
171 2 seconds on the average). It was acked once, 140ms
172 after it first arrived.
173
174stime.awk
175atime.awk
176 Output one line per send or ack, respectively, in the form
177 <time> <seq. number>
178 where <time> is the time in seconds since the start of the
179 transfer and <seq. number> is the sequence number being sent
180 or acked. I typically plot this data looking for suspicious
181 patterns.
182
183
184The problem I was looking at was the bulk-data-transfer
185throughput of medium delay network paths (1-6 sec. round trip
186time) under typical DARPA Internet conditions. The trace of the
187ftp transfer of a large file was used as the raw data source.
188The method was:
189
190 - On a local host (but not the Sun running tcpdump), connect to
191 the remote ftp.
192
193 - On the monitor Sun, start the trace going. E.g.,
194 tcpdump host local-host and remote-host and port ftp-data >tracefile
195
196 - On local, do either a get or put of a large file (~500KB),
197 preferably to the null device (to minimize effects like
198 closing the receive window while waiting for a disk write).
199
200 - When transfer is finished, stop tcpdump. Use awk to make up
201 two files of summary data (maxsize is the maximum packet size,
202 tracedata is the file of tcpdump tracedata):
203 awk -f send-ack.awk packetsize=avgsize tracedata >sa
204 awk -f packetdat.awk packetsize=avgsize tracedata >pd
205
206 - While the summary data files are printing, take a look at
207 how the transfer behaved:
208 awk -f stime.awk tracedata | xgraph
209 (90% of what you learn seems to happen in this step).
210
211 - Do all of the above steps several times, both directions,
212 at different times of day, with different protocol
213 implementations on the other end.
214
215 - Using one of the Unix data analysis packages (in my case,
216 S and Gary Perlman's Unix|Stat), spend a few months staring
217 at the data.
218
219 - Change something in the local protocol implementation and
220 redo the steps above.
221
222 - Once a week, tell your funding agent that you're discovering
223 wonderful things and you'll write up that research report
224 "real soon now".
225```
226
