Add ability for user to select root servers for iterative mode

[phillip-stephens]

Description

A user may want to select a set of root name servers other than the full 13 . roots, either to only use one root or to test against a different server. This PR adds that.

Testing

When selecting a root server, ZDNS uses it

$ echo "cloudflare.com" | ./zdns A --iterative --name-servers="198.41.0.4" --verbosity=5

INFO[0000] using local addresses: [192.168.1.243]       
INFO[0000] for non-iterative lookups, using external nameservers: 198.41.0.4:53 
INFO[0000] for iterative lookups, using nameservers: 198.41.0.4:53 
DEBU[0000] DEPTH 00:[MIEKG-IN: starting a C/DNAME following lookup for  cloudflare.com  ( 1 )] 
DEBU[0000] DEPTH 01:    [MIEKG-IN: following iterative lookup for  cloudflare.com  ( 1 )] 
DEBU[0000] DEPTH 01:    [iterative lookup for  cloudflare.com  ( 1 ) against  198.41.0.4:53  layer  .] 
DEBU[0000] DEPTH 02:        [Cached retrying lookup. Name:  {1 1 cloudflare.com} , Layer:  . , Nameserver:  198.41.0.4:53] 
DEBU[0000] DEPTH 03:            [Cache request for:  cloudflare.com  ( 1 )] 

Notice how we start the lookup against the 1 root provided.

Normal usage isn't broken

$ echo "cloudflare.com" | ./zdns A --iterative --verbosity=5

INFO[0000] using local addresses: [192.168.1.243]       
INFO[0000] for non-iterative lookups, using external nameservers: 192.168.1.1:53 
INFO[0000] for iterative lookups, using nameservers: 198.41.0.4:53, 170.247.170.2:53, 192.33.4.12:53, 199.7.91.13:53, 192.203.230.10:53, 192.5.5.241:53, 192.112.36.4:53, 198.97.190.53:53, 192.36.148.17:53, 192.58.128.30:53, 193.0.14.129:53, 199.7.83.42:53, 202.12.27.33:53 
DEBU[0000] DEPTH 00:[MIEKG-IN: starting a C/DNAME following lookup for  cloudflare.com  ( 1 )] 
DEBU[0000] DEPTH 01:    [MIEKG-IN: following iterative lookup for  cloudflare.com  ( 1 )] 
DEBU[0000] DEPTH 01:    [iterative lookup for  cloudflare.com  ( 1 ) against  202.12.27.33:53  layer  .] 
DEBU[0000] DEPTH 02:        [Cached retrying lookup. Name:  {1 1 cloudflare.com} , Layer:  . , Nameserver:  202.12.27.33:53] 

[zakird]

Does this need resolution?

[phillip-stephens]

Thanks for the catch, looked back over this and yeah this needed addressing.

In the case that there was a loopback NS in /etc/resolv.conf, without the check added in a2ada2c, ZDNS would over-write the nameservers.

Added a guard and tested by setting my local /etc/resolv.conf to loopback. The old code overwrites the nameservers, with the guard added we don't and it performs as expected.