Some Information for Contributors

Thank you for considering to make a contribution to tcpdump! Please use the guidelines below to achieve the best results and experience for everyone.

How to report bugs and other problems

To report a security issue (segfault, buffer overflow, infinite loop, arbitrary code execution etc) please send an e-mail to, do not use the bug tracker!

To report a non-security problem (failure to compile, incorrect output in the protocol printout, missing support for a particular protocol etc) please check first that it reproduces with the latest stable release of tcpdump and the latest stable release of libpcap. If it does, please check that the problem reproduces with the current git master branch of tcpdump and the current git master branch of libpcap. If it does (and it is not a security-related problem, otherwise see above), please navigate to the bug tracker and check if the problem has already been reported. If it has not, please open a new issue and provide the following details:

  • tcpdump and libpcap version (tcpdump --version)
  • operating system name and version and any other details that may be relevant (uname -a, compiler name and version, CPU type etc.)
  • custom configure/cmake flags, if any
  • statement of the problem
  • steps to reproduce

Please note that if you know exactly how to solve the problem and the solution would not be too intrusive, it would be best to contribute some development time and to open a pull request instead as discussed below.

Still not sure how to do? Feel free to subscribe to the mailing list and ask!

How to add new code and to update existing code

  1. Check that there isn't a pull request already opened for the changes you intend to make.

  2. Fork the Tcpdump repository.

  3. The easiest way to test your changes on multiple operating systems and architectures is to let the upstream CI test your pull request (more on this below).

  4. Setup your git working copy

    git clone<username>/tcpdump.git
    cd tcpdump
    git remote add upstream
    git fetch upstream
  5. Do a touch .devel in your working directory. Currently, the effect is

    • add (via configure, in Makefile) some warnings options (-Wall, -Wmissing-prototypes, -Wstrict-prototypes, ...) to the compiler if it supports these options,
    • have the Makefile support make depend and the configure script run it.
  6. Configure and build

    ./configure && make -s && make check
  7. Add/update tests The tests directory contains regression tests of the dissection of captured packets. Those captured packets were saved running tcpdump with option -w sample.pcap. Additional options, such as -n, are used to create relevant and reproducible output; -# is used to indicate which particular packets have output that differs. The tests are run with the TZ environment variable set to GMT0, so that UTC, rather than the local time where the tests are being run, is used when “local time” values are printed. The actual test compares the current text output with the expected result (sample.out) saved from a previous version.

    Any new/updated fields in a dissector must be present in a sample.pcap file and the corresponding output file.

    Configuration is set in tests/TESTLIST. Each line in this file has the following format:

    test-name   sample.pcap   sample.out   tcpdump-options

    The sample.out file can be produced as follows:

    (cd tests && TZ=GMT0 ../tcpdump -# -n -r sample.pcap tcpdump-options > sample.out)

    Or, for convenience, use ./ test-name

    It is often useful to have test outputs with different verbosity levels (none, -v, -vv, -vvv, etc.) depending on the code.

  8. Test using make check (current build options) and ./ (a multitude of build options, build systems and compilers). If you can, test on more than one operating system. Don't send a pull request until all tests pass.

  9. Try to rebase your commits to keep the history simple.

    git fetch upstream
    git rebase upstream/master

    (If the rebase fails and you cannot resolve, issue git rebase --abort and ask for help in the pull request comment.)

  10. Once 100% happy, put your work into your forked repository using git push.

  11. Initiate and send a pull request. This will trigger the upstream repository CI tests.

Code style and generic remarks

  • A thorough reading of some other printers code is useful.

  • Put the normative reference if any as comments (RFC, etc.).

  • Put the format of packets/headers/options as comments if there is no published normative reference.

  • The printer may receive incomplete packet in the buffer, truncated at any random position, for example by capturing with -s size option. If your code reads and decodes every byte of the protocol packet, then to ensure proper and complete bounds checks it would be sufficient to read all packet data using the GET_*() macros, typically:

    GET_BE_U_n(p), n in { 2, 3, 4, 5, 6, 7, 8 }
    GET_BE_S_n(p), n in { 2, 3, 4, 5, 6, 7, 8 }

    If your code uses the macros above only on some packet data, then the gaps would have to be bounds-checked using the ND_TCHECK_*() macros:

    ND_TCHECK_n(p), n in { 1, 2, 3, 4, 5, 6, 7, 8, 16 }
    ND_TCHECK_LEN(p, l)

    For the ND_TCHECK_* macros (if not already done):

    • Assign: ndo->ndo_protocol = "protocol";
    • Define: ND_LONGJMP_FROM_TCHECK before including netdissect.h
    • Make sure that the intersection of GET_*() and ND_TCHECK_*() is minimal, but at the same time their union covers all packet data in all cases.

    You can test the code via:

    sudo ./tcpdump -s snaplen [-v][v][...] -i lo # in a terminal
    sudo tcpreplay -i lo sample.pcap             # in another terminal

    You should try several values for snaplen to do various truncation.

  • Do invalid packet checks in code: Think that your code can receive in input not only a valid packet but any arbitrary random sequence of octets (packet

    • built malformed originally by the sender or by a fuzz tester,
    • became corrupted in transit or for some other reason).

    Print with: nd_print_invalid(ndo); /* to print " (invalid)" */

  • Use struct tok for indexed strings and print them with tok2str() or bittok2str() (for flags).

  • Avoid empty lines in output of printers.

  • A commit message must have:

    First line: Capitalized short summary in the imperative (50 chars or less)
    If the commit concerns a protocol, the summary line must start with
    "protocol: ".
    Body: Detailed explanatory text, if necessary. Fold it to approximately
    72 characters. There must be an empty line separating the summary from
    the body.
  • Avoid non-ASCII characters in code and commit messages.

  • Use the style of the modified sources.

  • Don't mix declarations and code.

  • Don't use // for comments. Not all C compilers accept C++/C99 comments by default.

  • Avoid trailing tabs/spaces