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) 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 * GNU/Linux
  34 * {Mac} OS X / macOS
  35 * NetBSD
  36 * OpenBSD
  37 * OpenIndiana
  38 * OpenWrt
  39 * Solaris
  40 * Windows (requires WinPcap or Npcap, and Visual Studio with CMake)
  41
  42 ### Dependency on libpcap
  43 Tcpdump uses libpcap, a system-independent interface for user-level
  44 packet capture.  Before building tcpdump, you must first retrieve and
  45 build libpcap.
  46
  47 Once 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
  52 The program is loosely based on SMI's "etherfind" although none of the
  53 etherfind code remains.  It was originally written by Van Jacobson as
  54 part of an ongoing research project to investigate and improve TCP and
  55 Internet gateway performance.  The parts of the program originally
  56 taken from Sun's etherfind were later re-written by Steven McCanne of
  57 LBL.  To insure that there would be no vestige of proprietary code in
  58 tcpdump, Steve wrote these pieces from the specification given by the
  59 manual entry, with no access to the source of tcpdump or etherfind.
  60 ```text
  61 formerly 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
  67 Richard Stevens gives an excellent treatment of the Internet protocols
  68 in his book *"TCP/IP Illustrated, Volume 1"*. If you want to learn more
  69 about tcpdump and how to interpret its output, pick up this book.
  70
  71 Another tool that tcpdump users might find useful is
  72 [tcpslice](https://github.com/the-tcpdump-group/tcpslice).
  73 It is a program that can be used to extract portions of tcpdump binary
  74 trace files.
  75
  76 ### The original LBL README by Steve McCanne, Craig Leres and Van Jacobson
  77 ```
  78 This directory also contains some short awk programs intended as
  79 examples of ways to reduce tcpdump data when you're tracking
  80 particular network problems:
  81
  82 send-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
 123 packetdat.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
 174 stime.awk
 175 atime.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
 184 The problem I was looking at was the bulk-data-transfer
 185 throughput of medium delay network paths (1-6 sec.  round trip
 186 time) under typical DARPA Internet conditions.  The trace of the
 187 ftp transfer of a large file was used as the raw data source.
 188 The 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 ```