The topic of this post is much more narrow than any I’ve done before, but I think it’s a good idea to publish this collection of keywords and pointers regarding a tricky configuration problem I encountered.
Background: Launching a Resolver
As most of my readers would know, the Domain Name System (DNS) is the distributed system that converts Internet hostnames such as http://www.google.com into IP addresses such as 188.8.131.52, so that it’s possible to route traffic from your host to the desired destination (e.g., a search engine site).
A resolver is the software component that submits queries to DNS on behalf of a user’s app or other programs. I am working with an excellent open source resolver called unbound.
The problem I ran into was that unbound would shutdown immediately after starting. A look at the log messages showed the following:
May 9 17:54:11 foo.bar.org unbound: [3482:0] error: failed to read /root.key May 9 17:54:11 foo.bar.org unbound: [3482:0] error: error reading auto-trust-anchor-file: /var/unbound/root.key May 9 17:54:11 foo.bar.org unbound: [3482:0] error: validator: error in trustanchors config May 9 17:54:11 foo.bar.org unbound: [3482:0] error: validator: could not apply configuration settings. May 9 17:54:11 foo.bar.org unbound: [3482:0] error: module init for module validator failed
Root.key? Trust anchor??
Diagnosis and Treatment
Hitting Google shows that a corrupted root key is a common affliction for unbound, see this forum article. Unsurprisingly, the cure is to replace the trashed root.key file with a good one, as suggested here. But how?
Answer: the tool, unbound-anchor! Run
…and a legit root.key will be created, allowing unbound to start.
No rest for the weary
This solved my problem, which was getting unbound to start on a test system. But the purpose of the root.key is to support the optional DNSSEC feature of unbound — the ability for DNS message content to be selectively “signed” by cryptographic hashes. If you are deploying unbound on a production system, you have the obligation to verify that you are using an appropriate trust anchor.