DNS Override for a Single Process

The Problem

I needed to run a mobile emulator on my laptop, in order to test some DNS server changes before releasing them.
However, since the emulator had no option to override its DNS settings it used my laptop’s DNS settings, so to make it query a different server I had to modify my laptop’s settings.
This meant that I broke a lot of other programs running on my laptop, so for example I couldn’t edit the DNS server and test the emulator at the same time. Very frustrating.

The Solution

I made a script that’s using LD_PRELOAD to monkey-patch fopen and return a custom version of /etc/resolv.conf.
It basically works like this:

1. Create a new resolv.conf file and store it in an environment variable
2. Append my shared library to LD_PRELOAD to make my fopen hide the standard one
3. exec to the binary you want to run
4. When the binary calls fopen, our function is called. If the requested file is not /etc/resolv.conf, we pass everything to the original function. If it is, we open a pointer to the modified file instead

The code is available under dns-override

Usage

Before:

nitz@mars:~$ nslookup google.com
Server:   127.0.1.1
Address:  127.0.1.1#53

Non-authoritative answer:
Name: google.com
Address: 172.217.22.78

nitz@mars:~$

After:

nitz@mars:~$ ~/projects/dns-override/dns-override.sh -s 8.8.8.8 nslookup google.com
Server:		8.8.8.8
Address:	8.8.8.8#53

Non-authoritative answer:
Name:	google.com
Address: 216.58.212.174

nitz@mars:~$ 

Interesting points:

1. Took me a while to figure out there are fopen and fopen64
2. I’m opening read streams for /etc/resolv.conf, even when write streams are required. Could produce some weird behaviour
3. My original goal was passing something like DNS_SERVERS=1.2.3.4 to the library, but it was much easier generating the entire resolv.conf file in a shell script.
4. I’m only targeting glibc’s resolv. However, that should cover all of my use cases so I’m happy.