dnshack

A hacky solution to nix-community/nix-on-droid#2.

There are two components, the dnshackresolver executable and the dnshackbridge shared library. The dnshackbridge library is compiled against the glibc of nix-on-droid and provides an implementation for the getaddrinfo function. Instead of performing the DNS resolution itself, it serializes the input parameters and passes them to the dnshackresolver program.

dnshackresolver is compiled against Android libc and internally calls the Android libc getaddrinfo function for the DNS resolution. It deserializes the input parameters, calls getaddrinfo, and then serializes the result to return it back via stdout to dnshackbridge.

This method should allow any program compiled against Nix's glibc to use the bionic libc DNS resolution when dnshackbridge is preloaded using LD_PRELOAD. It's also probably horrible for performance and likely not very reliable.

Build it

As dnshackresolver needs to be built with the Android NDK, (which isn't available on aarch64) there is no native package. So just build it on x86 with nix-build ./dnshackresolver. dnshackbridge can be built with nix-build -E 'with import <nixpkgs> {}; callPackage ./dnshackbridge {}'. Set crossSystem if building on x86.

Run it

export DNSHACK_RESOLVER_CMD=/path/to/dnshackresolver
export LD_PRELOAD=/path/to/libdnshackbridge.so
curl hostname_provided_via_android_dns

These exports can also be added to your ~/.profile or turned into a wrapper script.

Binary package

The toplevel default.nix pulls a prebuilt binary for dnshackresolver from releases and creates a ready-to-consume package. So, to use it on nix-on-droid by default (with home-manager), do something like this:

home.file.".bashrc".text =
  let dnshack = pkgs.callPackage (builtins.fetchTarball "https://github.com/ettom/dnshack/tarball/master") { };
  in ''
    export DNSHACK_RESOLVER_CMD="${dnshack}/bin/dnshackresolver"
    export LD_PRELOAD="${dnshack}/lib/libdnshackbridge.so"
  '';