The Tcpdump Group git mirrors - tcpdump/blob - README.md

   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
   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.
   8
   9 Anonymous Git is available via
  10
  11         https://github.com/the-tcpdump-group/tcpdump.git
  12
  13 This directory contains source code for tcpdump, a tool for network
  14 monitoring and data acquisition.
  15
  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.
  19
  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.
  25
  26 tcpdump 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/Hurd
  35 * GNU/Linux
  36 * {Mac} OS X / macOS
  37 * NetBSD
  38 * OpenBSD
  39 * Solaris
  40 * Windows (requires WinPcap or Npcap, and Visual Studio with CMake)
  41
  42 In the past tcpdump certainly or likely worked on the following platforms:
  43
  44 * 4.3BSD
  45 * BSD/386, later BSD/OS
  46 * DEC OSF/1, later Digital UNIX, later Tru64 UNIX
  47 * DOS
  48 * IRIX
  49 * SINIX
  50 * SunOS
  51 * Ultrix
  52 * UnixWare
  53
  54 ### Dependency on libpcap
  55 tcpdump uses libpcap, a system-independent interface for user-level
  56 packet capture.  Before building tcpdump, you must first retrieve and
  57 build libpcap.
  58
  59 Once libpcap is built (either install it or make sure it's in
  60 `../libpcap`), you can build tcpdump using the procedure in the
  61 [installation notes](INSTALL.md).
  62
  63 ### Origins of tcpdump
  64 The program is loosely based on SMI's "etherfind" although none of the
  65 etherfind code remains.  It was originally written by Van Jacobson as
  66 part of an ongoing research project to investigate and improve TCP and
  67 Internet gateway performance.  The parts of the program originally
  68 taken from Sun's etherfind were later re-written by Steven McCanne of
  69 LBL.  To insure that there would be no vestige of proprietary code in
  70 tcpdump, Steve wrote these pieces from the specification given by the
  71 manual entry, with no access to the source of tcpdump or etherfind.
  72 ```text
  73 formerly from   Lawrence Berkeley National Laboratory
  74                 Network Research Group <tcpdump@ee.lbl.gov>
  75                 ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z (3.4)
  76 ```
  77
  78 ### See also
  79 Richard Stevens gives an excellent treatment of the Internet protocols
  80 in his book *"TCP/IP Illustrated, Volume 1"*. If you want to learn more
  81 about tcpdump and how to interpret its output, pick up this book.
  82
  83 Another tool that tcpdump users might find useful is
  84 [tcpslice](https://github.com/the-tcpdump-group/tcpslice).
  85 It is a program that can be used to extract portions of tcpdump binary
  86 trace files.
  87
  88 ### The original LBL README by Steve McCanne, Craig Leres and Van Jacobson
  89 ```
  90 This directory also contains some short awk programs intended as
  91 examples of ways to reduce tcpdump data when you're tracking
  92 particular network problems:
  93
  94 send-ack.awk
  95         Simplifies the tcpdump trace for an ftp (or other unidirectional
  96         tcp transfer).  Since we assume that one host only sends and
  97         the other only acks, all address information is left off and
  98         we just note if the packet is a "send" or an "ack".
  99
 100         There is one output line per line of the original trace.
 101         Field 1 is the packet time in decimal seconds, relative
 102         to the start of the conversation.  Field 2 is delta-time
 103         from last packet.  Field 3 is packet type/direction.
 104         "Send" means data going from sender to receiver, "ack"
 105         means an ack going from the receiver to the sender.  A
 106         preceding "*" indicates that the data is a retransmission.
 107         A preceding "-" indicates a hole in the sequence space
 108         (i.e., missing packet(s)), a "#" means an odd-size (not max
 109         seg size) packet.  Field 4 has the packet flags
 110         (same format as raw trace).  Field 5 is the sequence
 111         number (start seq. num for sender, next expected seq number
 112         for acks).  The number in parens following an ack is
 113         the delta-time from the first send of the packet to the
 114         ack.  A number in parens following a send is the
 115         delta-time from the first send of the packet to the
 116         current send (on duplicate packets only).  Duplicate
 117         sends or acks have a number in square brackets showing
 118         the number of duplicates so far.
 119
 120         Here is a short sample from near the start of an ftp:
 121                 3.00    0.20   send . 512
 122                 3.20    0.20    ack . 1024  (0.20)
 123                 3.20    0.00   send P 1024
 124                 3.40    0.20    ack . 1536  (0.20)
 125                 3.80    0.40 * send . 0  (3.80) [2]
 126                 3.82    0.02 *  ack . 1536  (0.62) [2]
 127         Three seconds into the conversation, bytes 512 through 1023
 128         were sent.  200ms later they were acked.  Shortly thereafter
 129         bytes 1024-1535 were sent and again acked after 200ms.
 130         Then, for no apparent reason, 0-511 is retransmitted, 3.8
 131         seconds after its initial send (the round trip time for this
 132         ftp was 1sec, +-500ms).  Since the receiver is expecting
 133         1536, 1536 is re-acked when 0 arrives.
 134
 135 packetdat.awk
 136         Computes chunk summary data for an ftp (or similar
 137         unidirectional tcp transfer). [A "chunk" refers to
 138         a chunk of the sequence space -- essentially the packet
 139         sequence number divided by the max segment size.]
 140
 141         A summary line is printed showing the number of chunks,
 142         the number of packets it took to send that many chunks
 143         (if there are no lost or duplicated packets, the number
 144         of packets should equal the number of chunks) and the
 145         number of acks.
 146
 147         Following the summary line is one line of information
 148         per chunk.  The line contains eight fields:
 149            1 - the chunk number
 150            2 - the start sequence number for this chunk
 151            3 - time of first send
 152            4 - time of last send
 153            5 - time of first ack
 154            6 - time of last ack
 155            7 - number of times chunk was sent
 156            8 - number of times chunk was acked
 157         (all times are in decimal seconds, relative to the start
 158         of the conversation.)
 159
 160         As an example, here is the first part of the output for
 161         an ftp trace:
 162
 163         # 134 chunks.  536 packets sent.  508 acks.
 164         1       1       0.00    5.80    0.20    0.20    4       1
 165         2       513     0.28    6.20    0.40    0.40    4       1
 166         3       1025    1.16    6.32    1.20    1.20    4       1
 167         4       1561    1.86    15.00   2.00    2.00    6       1
 168         5       2049    2.16    15.44   2.20    2.20    5       1
 169         6       2585    2.64    16.44   2.80    2.80    5       1
 170         7       3073    3.00    16.66   3.20    3.20    4       1
 171         8       3609    3.20    17.24   3.40    5.82    4       11
 172         9       4097    6.02    6.58    6.20    6.80    2       5
 173
 174         This says that 134 chunks were transferred (about 70K
 175         since the average packet size was 512 bytes).  It took
 176         536 packets to transfer the data (i.e., on the average
 177         each chunk was transmitted four times).  Looking at,
 178         say, chunk 4, we see it represents the 512 bytes of
 179         sequence space from 1561 to 2048.  It was first sent
 180         1.86 seconds into the conversation.  It was last
 181         sent 15 seconds into the conversation and was sent
 182         a total of 6 times (i.e., it was retransmitted every
 183         2 seconds on the average).  It was acked once, 140ms
 184         after it first arrived.
 185
 186 stime.awk
 187 atime.awk
 188         Output one line per send or ack, respectively, in the form
 189                 <time> <seq. number>
 190         where <time> is the time in seconds since the start of the
 191         transfer and <seq. number> is the sequence number being sent
 192         or acked.  I typically plot this data looking for suspicious
 193         patterns.
 194
 195
 196 The problem I was looking at was the bulk-data-transfer
 197 throughput of medium delay network paths (1-6 sec.  round trip
 198 time) under typical DARPA Internet conditions.  The trace of the
 199 ftp transfer of a large file was used as the raw data source.
 200 The method was:
 201
 202   - On a local host (but not the Sun running tcpdump), connect to
 203     the remote ftp.
 204
 205   - On the monitor Sun, start the trace going.  E.g.,
 206       tcpdump host local-host and remote-host and port ftp-data >tracefile
 207
 208   - On local, do either a get or put of a large file (~500KB),
 209     preferably to the null device (to minimize effects like
 210     closing the receive window while waiting for a disk write).
 211
 212   - When transfer is finished, stop tcpdump.  Use awk to make up
 213     two files of summary data (maxsize is the maximum packet size,
 214     tracedata is the file of tcpdump tracedata):
 215       awk -f send-ack.awk packetsize=avgsize tracedata >sa
 216       awk -f packetdat.awk packetsize=avgsize tracedata >pd
 217
 218   - While the summary data files are printing, take a look at
 219     how the transfer behaved:
 220       awk -f stime.awk tracedata | xgraph
 221     (90% of what you learn seems to happen in this step).
 222
 223   - Do all of the above steps several times, both directions,
 224     at different times of day, with different protocol
 225     implementations on the other end.
 226
 227   - Using one of the Unix data analysis packages (in my case,
 228     S and Gary Perlman's Unix|Stat), spend a few months staring
 229     at the data.
 230
 231   - Change something in the local protocol implementation and
 232     redo the steps above.
 233
 234   - Once a week, tell your funding agent that you're discovering
 235     wonderful things and you'll write up that research report
 236     "real soon now".
 237 ```