1# TCPDUMP 4.x.y by [The Tcpdump Group](https://www.tcpdump.org/) 2 3**To report a security issue please send an e-mail to security@tcpdump.org.** 4 5To report bugs and other problems, contribute patches, request a 6feature, provide generic feedback etc please see the 7[guidelines for contributing](CONTRIBUTING.md) in the tcpdump source tree root. 8 9Anonymous Git is available via 10 11 https://github.com/the-tcpdump-group/tcpdump.git 12 13This directory contains source code for tcpdump, a tool for network 14monitoring and data acquisition. 15 16Over the past few years, tcpdump has been steadily improved by the 17excellent contributions from the Internet community (just browse 18through the [change log](CHANGES)). We are grateful for all the input. 19 20### Supported platforms 21In many operating systems tcpdump is available as a native package or port, 22which simplifies installation of updates and long-term maintenance. However, 23the native packages are sometimes a few versions behind and to try a more 24recent snapshot it will take to compile tcpdump from the source code. 25 26tcpdump compiles and works on at least the following platforms: 27 28* AIX 29* DragonFly BSD 30* FreeBSD 31* Haiku 32* HP-UX 11i 33* illumos (OmniOS, OpenIndiana) 34* GNU/Linux 35* {Mac} OS X / macOS 36* NetBSD 37* OpenBSD 38* OpenWrt 39* Solaris 40* Windows (requires WinPcap or Npcap, and Visual Studio with CMake) 41 42### Dependency on libpcap 43Tcpdump uses libpcap, a system-independent interface for user-level 44packet capture. Before building tcpdump, you must first retrieve and 45build libpcap. 46 47Once libpcap is built (either install it or make sure it's in 48`../libpcap`), you can build tcpdump using the procedure in the 49[installation notes](INSTALL.md). 50 51### Origins of tcpdump 52The program is loosely based on SMI's "etherfind" although none of the 53etherfind code remains. It was originally written by Van Jacobson as 54part of an ongoing research project to investigate and improve TCP and 55Internet gateway performance. The parts of the program originally 56taken from Sun's etherfind were later re-written by Steven McCanne of 57LBL. To insure that there would be no vestige of proprietary code in 58tcpdump, Steve wrote these pieces from the specification given by the 59manual entry, with no access to the source of tcpdump or etherfind. 60```text 61formerly from Lawrence Berkeley National Laboratory 62 Network Research Group <tcpdump@ee.lbl.gov> 63 ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z (3.4) 64``` 65 66### See also 67Richard Stevens gives an excellent treatment of the Internet protocols 68in his book *"TCP/IP Illustrated, Volume 1"*. If you want to learn more 69about tcpdump and how to interpret its output, pick up this book. 70 71Another tool that tcpdump users might find useful is 72[tcpslice](https://github.com/the-tcpdump-group/tcpslice). 73It is a program that can be used to extract portions of tcpdump binary 74trace files. 75 76### The original LBL README by Steve McCanne, Craig Leres and Van Jacobson 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