$ tar xf $(nix-build '<nixpkgs>' -A iproute.src)
$ cd iproute2-4.15.0/ip/
$ gdb ip
(gdb) b main
Breakpoint 1 at 0x408d60: file ip.c, line 172.
(gdb) r route
Starting program: /nix/store/x7gjcsxd48fp1wnpwjidqpn7nar4483y-iproute2-4.15.0/bin/ip route

Breakpoint 1, main (argc=2, argv=0x7fffffffc6c8) at ip.c:172
warning: Source file is more recent than executable.
172	{