Bug 1674067

I'm filing this bug based on some testing with dnsmasq 2.80.

It is my belief that dnsmasq is incorrectly converting NXDOMAIN responses from authoritative dns servers into NODATA.

This can result in bad resolution for ipv4-only or ipv6-only hostnames when searching through the search path (a correct dns client library aborts the search at NODATA but continues with the next search path element at NXDOMAIN - any other behaviour results in bugs [flakiness] in the case of server timeouts and other errors).

tea6.foo. and tea7.foo. don't exist.

athina:~$ for i in srv txt aaaa a aaaa a txt srv; do host -t $i tea6.foo. 127.0.0.1 | tail -n 1; done
Host tea6.foo. not found: 3(NXDOMAIN)
Host tea6.foo. not found: 3(NXDOMAIN)
Host tea6.foo. not found: 3(NXDOMAIN)
tea6.foo has no A record
Host tea6.foo. not found: 3(NXDOMAIN)
tea6.foo has no A record
tea6.foo has no TXT record
tea6.foo has no SRV record

athina:~$ for i in srv txt a aaaa a aaaa txt srv; do host -t $i tea7.foo. 127.0.0.1 | tail -n 1; done
Host tea7.foo. not found: 3(NXDOMAIN)
Host tea7.foo. not found: 3(NXDOMAIN)
Host tea7.foo. not found: 3(NXDOMAIN)
tea7.foo has no AAAA record
Host tea7.foo. not found: 3(NXDOMAIN)
tea7.foo has no AAAA record
tea7.foo has no TXT record
tea7.foo has no SRV record

yeah somehow A/AAAA are special (127.0.0.1 is dnsmasq 2.80)

Here's some more detail:

https://umbrella.cisco.com/blog/2014/06/23/nxdomain-nodata-debugging-dns-dual-stacked-hosts/

I'm guessing this bug is introduced by (but unverified):

commit b6f926fbefcd2471699599e44f32b8d25b87b471
Author: Simon Kelley <simon.uk>
Date:   Tue Aug 21 17:46:52 2018 +0100

    Don't return NXDOMAIN to empty non-terminals.
    
    When a record is defined locally, eg an A record for one.two.example then
    we already know that if we forward, eg an AAAA query for one.two.example,
    and get back NXDOMAIN, then we need to alter that to NODATA. This is handled
    by  check_for_local_domain(). But, if we forward two.example, because
    one.two.example exists, then the answer to two.example should also be
    a NODATA.
    
    For most local records this is easy, just to substring matching.
    for A, AAAA and CNAME records that are in the cache, it's more difficult.
    The cache has no efficient way to find such records. The fix is to
    insert empty (none of F_IPV4, F_IPV6 F_CNAME set) records for each
    non-terminal.