Error if a loopback name server is used with a non-loopback local address, or vice versa

[phillip-stephens]

Closes #309

Changes

• use built-in loopback detection
• error if name server is loopback and local address. is not, or vice versa.

Description

In #389, we added this sort of "can't use a nameserver/local address that aren't both loopback or both not" validation to the config. But it is also possible to specify a name server in an individual input line: echo "cloudflare.com,127.0.0.53" | ./zdns A. In this case, our config validation will not error if the local address mismatches, since we're bypassing the config/CLI inputs to ZDNS.

Solution is to validate in the resolver.Lookup itself. I think this should be done in addition to the config validation rather than replacing it, since catching the config error earlier leads to better UX.

Even prior to this change, it was not possible to specify both a loopback and non-loopback name server in the same scan. The mismatched query would timeout. This change just adds a descriptive error for the user to remediate the issue.

Example post-fix:

~/zdns on  phillip/309-local-address-selection! ⌚ 22:43:25
$ echo "cloudflare.com,127.0.0.53\nyahoo.com,1.1.1.1" | ./zdns A --verbosity 5
INFO[0000] No name servers specified. will use: 1.1.1.1:53
INFO[0000] using local address(es): [171.67.71.209]
{"data":null,"error":"nameserver 127.0.0.53 must be reachable from the local address 171.67.71.209, ie. both must be loopback or not loopback","name":"cloudflare.com","status":"ILLEGAL_INPUT","timestamp":"2024-07-10T22:45:00Z"}