brandt / awsresolver

DNS server that decodes AWS EC2 internal hostnames on-the-fly

awsresolver

Very basic DNS server that resolves AWS's EC2 internal FQDNs (ex: ip-192-0-2-1.us-west-2.compute.internal) by extracting the IP out of the hostname.

Includes an optional subcommand that sets up macOS to automatically direct all "*.internal" queries to this daemon.

Requirements

• Go 1.13 or later to build.

NOTE: Only macOS is currently supported.

Installation

1. Run: brew install brandt/personal/awsresolver
2. Run: sudo awsresolver setup (installs /etc/resolver/internal)
3. Run: brew services start awsresolver

Confirm it's working

This hooks into macOS's resolver. That means things like ping and ssh will do what you expect, but dig will not.

To confirm it's working, run: ping ip-192-0-2-1.us-west-2.compute.internal

If awsresolver is correctly setup, you will see ping attempt to reach 192.0.2.1:

# SUCCESS                                     vvvvvvvvv
PING ip-192-0-2-1.us-west-2.compute.internal (192.0.2.1): 56 data bytes
Request timeout for icmp_seq 0
Request timeout for icmp_seq 1

How it works

This tool listens for A record requests ending in .internal, extracts the IP from the requested name, and returns it as a response. It binds to UDP and TCP 127.0.0.1:1053.

Mac OS X has a cool feature that allows you to configure different resolvers by domain. (See: man 5 resolver) When you run sudo awsresolver setup, it writes a config file to /etc/resolver/internal that steers *.internal requests to 127.0.0.1:1053.

Note that the resolver(5) config only applies to DNS resolution performed through the built-in OS facilities. So ping, ssh, and Chrome will be routed to this resolver, but by default dig will not.

To query with dig, point it directly at the resolver like so: dig @127.0.0.1 -p 1053 ip-192-0-2-1.us-west-2.compute.internal

Building

To build from source, simply run these commands from inside this repo:

go mod vendor # optional
make

The compiled binary is here: bin/awsresolver

Uninstalling

To uninstall:

1. Run: sudo rm -f /etc/resolver/internal
2. Run: brew uninstall awsresolver

Author

• J. Brandt Buckley