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+# 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 security@tcpdump.org, 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](https://github.com/the-tcpdump-group/tcpdump/issues)
+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](https://www.tcpdump.org/#mailing-lists)
+and ask!
+
+
+## How to add new code and to update existing code
+
+0) Check that there isn't a pull request already opened for the changes you
+ intend to make.
+
+1) [Fork](https://help.github.com/articles/fork-a-repo/) the Tcpdump
+ [repository](https://github.com/the-tcpdump-group/tcpdump).
+
+2) 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).
+
+3) Setup your git working copy
+ ```
+ git clone https://github.com/<username>/tcpdump.git
+ cd tcpdump
+ git remote add upstream https://github.com/the-tcpdump-group/tcpdump
+ git fetch upstream
+ ```
+
+4) 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.
+
+5) Configure and build
+ ```
+ ./configure && make -s && make check
+ ```
+
+6) 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 `./update-test.sh test-name`
+
+ It is often useful to have test outputs with different verbosity levels
+ (none, `-v`, `-vv`, `-vvv`, etc.) depending on the code.
+
+7) Test using `make check` (current build options) and `./build_matrix.sh`
+ (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.
+
+8) 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.)
+
+9) Once 100% happy, put your work into your forked repository using `git push`.
+
+10) [Initiate and send](https://help.github.com/articles/using-pull-requests/)
+ 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_U_1(p)
+ GET_S_1(p)
+ 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_SIZE(p)
+ 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