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openresolv

openresolv is a resolvconf implementation which manages /etc/resolv.conf.

/etc/resolv.conf is a file that holds the configuration for the local resolution of domain names. Normally this file is either static or maintained by a local daemon, normally a DHCP daemon. But what happens if more than one thing wants to control the file? Say you have wired and wireless interfaces to different subnets and run a VPN or two on top of that, how do you say which one controls the file? It's also not as easy as just adding and removing the nameservers each client knows about as different clients could add the same nameservers.

Enter resolvconf, the middleman between the network configuration services and /etc/resolv.conf. resolvconf itself is just a script that stores, removes and lists a full resolv.conf generated for the interface. It then calls all the helper scripts it knows about so it can configure the real /etc/resolv.conf and optionally any local nameservers other than libc.

Reasons for using openresolv

Why openresolv over the Debian implementation? Here's some reasons:

  • Works with POSIX shell and userland
  • Does not need awk, grep or sed which means we can work without /usr mounted
  • Works with other init systems than Debians' out of the box
  • Available as a 2 clause BSD license
  • Prefer configs via IF_METRIC for dynamic ordering
  • Configures zones for local resolvers other than libc

The last point is quite important, especially when running VPN systems. Take the following resolv.conf files which have been generated by a DHCP client and sent to resolvconf:

# resolv.conf from bge0
search foo.com
nameserver 1.2.3.4

# resolv.conf from tap0
domain bar.org
nameserver 5.6.7.8

In this instance, queries for foo.com will go to 1.2.3.4 and queries for bar.org will go to 5.6.7.8. This does require the resolvers to be configured to pickup the resolvconf generated configuration for them though. openresolv ships with helpers for:

See the configuration section for more details.

If openresolv updates /etc/resolv.conf it can notify the following of this: