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Fuzzing the PowerDNS products

This repository contains several fuzzing targets that can be used with generic fuzzing engines like AFL and libFuzzer.

These targets are built by passing the --enable-fuzz-targets option to the configure of the authoritative server and dnsdist, then building them as usual. You can also build only these targets manually by going into the pdns/ directory and issuing a make fuzz_targets command for the authoritative server, or going into the pdns/dnsdistdist and issuing a make fuzz_targets command for dnsdist.

The current targets cover:

  • the auth and rec packet cache (fuzz_target_packetcache) ;
  • MOADNSParser (fuzz_target_moadnsparser) ;
  • the Proxy Protocol parser (fuzz_target_proxyprotocol) ;
  • the HTTP parser we use (YaHTTP, fuzz_target_yahttp) ;
  • ZoneParserTNG (fuzz_target_zoneparsertng).
  • Parts of the ragel-generated parser (parseRFC1035CharString in fuzz_target_dnslabeltext) ;
  • the dnsdist packet cache (fuzz_target_dnsdistcache).

By default the targets are linked against a standalone target, standalone_fuzz_target_runner.cc, which does no fuzzing but makes it easy to check a given test file, or just that the fuzzing targets can be built properly.

This behaviour can be changed via the LIB_FUZZING_ENGINE variable, for example by setting it to -lFuzzer, building with clang by setting CC=clang CXX=clang++ before running the configure and adding -fsanitize=fuzzer-no-link to CFLAGS and CXXFLAGS. Doing so instructs the compiler to instrument the code for efficient fuzzing but not to link directly with -lFuzzer, which would make the compilation tests done during the configure phase fail.

Sanitizers

In order to catch the maximum of issues during fuzzing, it makes sense to enable the ASAN and UBSAN sanitizers via --enable-asan and --enable-ubsan options to the configure, or to set the appropriate flags directly.

Corpus

This directory contains a few files used for continuous fuzzing of the PowerDNS products.

The corpus directory contains three sub-directories:

  • http-raw-payloads/ contains HTTP payloads of queries, used by fuzz_target_yahttp ;
  • proxy-protocol-raw-packets/ contains DNS queries prefixed with a Proxy Protocol v2 header, used by fuzz_target_proxyprotocol ;
  • raw-dns-packets/ contains DNS queries and responses as captured on the wire. These are used by the fuzz_target_dnsdistcache, fuzz_target_moadnsparser and fuzz_target_packetcache targets ;
  • zones/ contains DNS zones, used by the fuzz_target_zoneparsertng target.

When run in the OSS-Fuzz environment, the zone files from the regression-tests/zones/ directory are added to the ones present in the fuzzing/corpus/zones/ directory.

Quickly getting started (using clang 11)

First, configure the authoritative server:

LIB_FUZZING_ENGINE="/usr/lib/clang/11.0.1/lib/linux/libclang_rt.fuzzer-x86_64.a" \
  CC=clang \
  CXX=clang++ \
  CFLAGS='-fsanitize=fuzzer-no-link' \
  CXXFLAGS='-fsanitize=fuzzer-no-link' \
  ./configure --without-dynmodules --with-modules= --disable-lua-records --disable-ixfrdist --enable-fuzz-targets --disable-dependency-tracking --disable-silent-rules --enable-asan --enable-ubsan

If you build the fuzzing targets only, you will need to issue the following commands first:

make -j2 -C ext/arc4random/
make -j2 -C ext/yahttp/

Then build:

LIB_FUZZING_ENGINE="/usr/lib/clang/11.0.1/lib/linux/libclang_rt.fuzzer-x86_64.a" \
  make -C pdns -j2 fuzz_targets

Now you're ready to run one of the fuzzing targets. First, copy the starting corpus:

mkdir new-corpus
./pdns/fuzz_target_XXXXXXX -merge=1 new-corpus fuzzing/corpus/YYYYY

Then run the thing:

./pdns_fuzz_target_XXXXXXX new-corpus

The LLVM docs have more info.