c-ares/test/README.md

c-ares Unit Test Suite

This directory holds unit tests for the c-ares library. To build the tests:

• Build the main c-ares library first, in the directory above this. To enable tests of internal functions, configure the library build to expose hidden symbols with ./configure --disable-symbol-hiding.
• Generate a configure file by running autoreconf -iv (which requires a local installation of autotools).
• ./configure
• make
• Run the tests with ./arestest, or ./arestest -v for extra debug info.

Points to note:

• The tests are written in C++11, and so need a C++ compiler that supports this. To avoid adding this as a requirement for the library, the configuration and build of the tests is independent from the library.
• The tests include some live queries, which will fail when run on a machine without internet connectivity. To skip live tests, run with ./arestest --gtest_filter=-*.Live*.
• The tests include queries of a mock DNS server. This server listens on port 5300 by default, but the port can be changed with the -p 5300 option to arestest.

Test Types

The test suite includes various different types of test.

• There are live tests (ares-test-live.cc), which assume that the current machine has a valid DNS setup and connection to the internet; these tests issue queries for real domains but don't particularly check what gets returned. The tests will fail on an offline machine.
• There are some mock tests (ares-test-mock.cc) that set up a fake DNS server and inject its port into the c-ares library configuration. These tests allow specific response messages to be crafted and injected, and so are likely to be used for many more tests in future.
  • To make this generation/injection easier, the dns-proto.h file includes C++ helper classes for building DNS packets.
• Other library entrypoints that don't require network activity (e.g. ares_parse_*_reply) are tested directly.
• A couple of the tests use a helper method of the test fixture to inject memory allocation failures, using a recent change to the c-ares library that allows override of malloc/free.
• There are some tests of the internal entrypoints of the library (ares-test-internal.c), but these are only enabled if the library was configured with --disable-symbol-hiding and/or --enable-expose-statics.
• There is also an entrypoint to allow Clang's libfuzzer to drive the packet parsing code in ares_parse_*_reply, together with a standalone wrapper for it (./aresfuzz) to allow use of command line fuzzers (such as afl-fuzz) for further fuzz testing.

Code Coverage Information

To generate code coverage information:

• Configure both the library and the tests with ./configure --enable-code-coverage before building. This requires the relevant code coverage tools (gcov, lcov) to be installed locally.
• Run the tests with test/arestest.
• Generate code coverage output with make code-coverage-capture in the library directory (i.e. not in test/).

Fuzzing

libFuzzer

To fuzz the packet parsing code with libFuzzer, follow the main libFuzzer build instructions:

• Configure the c-ares library and test suite with a recent Clang and a sanitizer, for example:

  % export CFLAGS="-fsanitize=address -fsanitize-coverage=edge"
  % export CC=clang
  % ./configure --disable-shared && make
  

• Download and build the libFuzzer code:

  % cd test
  % svn co http://llvm.org/svn/llvm-project/llvm/trunk/lib/Fuzzer
  % clang++ -c -g -O2 -std=c++11 Fuzzer/*.cpp -IFuzzer
  % ar ruv libFuzzer.a Fuzzer*.o
  

• Link the fuzzer entrypoint in with ares-fuzz.cc:

  % $CC $CFLAGS -I.. -c ares-test-fuzz.c
  % clang++ $CFLAGS ares-test-fuzz.o ../.libs/libcares.a libFuzzer.a -o ares-libfuzzer
  

• Run the fuzzer using the starting corpus with:

  % ./ares-libfuzzer fuzzinput/
  

AFL

To fuzz using AFL, follow the AFL quick start guide:

• Download and build AFL.

• Configure the c-ares library and test tool to use AFL's compiler wrappers:

  % export CC=$AFLDIR/afl-gcc
  % ./configure --disable-shared && make
  % cd test && ./configure && make aresfuzz
  

• Run the AFL fuzzer against the starting corpus:

  % mkdir fuzzoutput
  % $AFLDIR/afl-fuzz -i fuzzinput -o fuzzoutput -- ./aresfuzz
  

AFL Persistent Mode

If a recent version of Clang is available, AFL can use its built-in compiler instrumentation; this configuration also allows the use of a (much) faster persistent mode, where multiple fuzz inputs are run for each process invocation.

• Download and build a recent AFL, and run make in the llvm_mode subdirectory to ensure that afl-clang-fast gets built.

• Configure the c-ares library and test tool to use AFL's clang wrappers that use compiler instrumentation:

  % export CC=$AFLDIR/afl-clang-fast
  % ./configure --disable-shared && make
  % cd test && ./configure && make aresfuzz
  

• Run the AFL fuzzer (in persistent mode) against the starting corpus:

  % mkdir fuzzoutput
  % $AFLDIR/afl-fuzz -i fuzzinput -o fuzzoutput -- ./aresfuzz