Resolving dangling CNAME records

This document is for people who are getting a "dangling CNAME" warning, or who wish to have CNAME records that point to records served by other DNS servers.

There are two cases that can cause the "dangling CNAME" message to appear:

When a CNAME points to an external host name.
When a CNAME is misspelled or the host the CNAME points to is misspelled

CNAMEs pointing to external host names

Sometimes, people may wish to have local names point to external entries with CNAME records. For example, it may be desirable to have the shortcut "google" for "www.google.com". One way one may wish to do this is as follows:

	google.example.com. +86400 CNAME www.google.com.

MaraDNS' default configuration will not complete the record. In other words, some people may expect Mara to output the following for "google.example.com":

	google.example.com. +86400 CNAME www.google.com.
	www.google.com. +900 A 66.102.7.104

Instead, Mara simply outputs:

	google.example.com. +86400 CNAME www.google.com.

I call this a "dangling CNAME record", since the CNAME record is not resolvable by some stub resolvers. In order to resolve dangling CNAME records, MaraDNS can be configured thusly:

We run MaraDNS server on one IP and a Deadwood server on another IP.
For the sake of this example, we will suppose that the server people send queries to for resolving hostnames has the IP 192.168.1.1, which will be running Deadwood. We will further suppose that there is a MaraDNS server which has the dangling CNAME issue with the IP 192.168.1.2
Set up 192.168.1.1 to use 192.168.1.2 to resolve all hostnames that end in, say, "example.com.", via the upstream_servers dwood3rc variable.
Set up 192.168.1.2 to have dangling CNAME records in the authoritative half.

This will cause dangling CNAME records to be fully resolved; here is what the two MaraDNS servers do to resolve such a record:

A stub resolver asks 192.168.1.1 the IP address for, say "google.example.com"
192.168.1.1 asks 192.168.1.2 the IP address for "google.example.com" (since the name ends in "example.com")
192.168.1.2 tells 192.168.1.1 "google.example.com is a CNAME for www.google.com, and I don't have an IP for it"
192.168.1.1, seeing that it has a CNAME without an IP, asks nameservers on the internet for the IP for "www.google.com"
When 192.168.1.1 has a complete record, it will send this record to the stub resolver. In other words, 192.168.1.1 will tell the stub resolver that google.example.com is a CNAME for www.google.com, and then give out the IP for www.google.com.

Here is an example dwood3rc file for 192.168.1.1:

ipv4_bind_addresses = "192.168.1.1"
chroot_dir = "/etc/maradns"
recursive_acl = "192.168.1.0/24"
upstream_servers = {}
upstream_servers["example.com."] = "192.168.1.2"

Here is an example mararc file for 192.168.1.2:

ipv4_bind_addresses = "192.168.1.2"
chroot_dir = "/etc/maradns"
csv2 = {}
csv2["example.com."] = "db.example.com"

If dangling CNAMEs are not an issue for a given setup, or if they are resolved by the above setup using both MaraDNS and Deadwood, the warnings about dangling CNAMEs can be turned off by adding this to a mararc file:

no_cname_warnings = 1

A typo can cause a dangling CNAME record

A typo can also cause a dangling CNAME record. E.g:

www.example.com. CNAME hoost1.example.com.
host1.example.com. A 10.1.2.3

Here, we mispelled the CNAME record; we meant to type "host1.example.com" but typed in "hoost1.example.com". We can also get a dangling CNAME thusly:

www.example.com. CNAME host1.example.com.
hoost1.example.com. A 10.1.2.3

Or by a typo in the mararc file:

csv2["exammple.com."] = "db.example.com"

(We typed in exammple.com when we should have typed in "example.com")

Followed by a zone file that looks like this:

www.example.com. CNAME host1.example.com.
host1.% A 10.1.2.3

Since the % will expand to "exammple.com" instead of "example.com".

In all of these cases, the dangling CNAME warning will go away when the typo is found and corrected.