using a static "fallback /etc/hosts" if all other DNS options failed

Question

If I understand /etc/nsswitch.conf correctly, the configuration

hosts:          files mdns4_minimal [NOTFOUND=return] dns

results in interpreting /etc/hosts, MDNS and finally "regular" DNS servers.

Now I would love to have an option to set some static fallback definitions in case all previous resolving options failed.

Switching DNS and files would probably a bad idea because it would call DNS servers everytime.

So basically I'm looking for a solution that will work in case a DNS server failed (and there was no match in /etc/hosts obviously).

If this solution would result in a file manageable per user it would be even perfect.

Answer 1

You can't, at least with the NSS (Name Service Switch) implemetation of glibc.

You can obviously use an action for a service (like the mdns4_minimal is using [NOTFOUND=return] in your example). It's of the form:

[STATUS=ACTION]

with a preceding ! will negate the status.

The STATUS can be any one of the following four:

• SUCCESS
• NOTFOUND
• UNAVAIL
• TRYAGAIN

and ACTION can be any one of the following two:

• RETURN
• CONTINUE

Here RETURN will return a result without looking any further lookup functions, and CONTINUE will continue to look further. There is no way to mention a file to fallback.

You are basically looking for something like:

hosts:   files mdns4_minimal [NOTFOUND=return] dns [NOTFOUND=/read/for/this/file]

which is not possible.

In a nutshell, you are looking for an non-existent ACTION of nsswitch.