Comment 29 for bug 1752411

Notes;

when "things are working", host does either:

while on VPN:
{{{
$ LC_ALL=C host -t soa local.
Host local. not found: 3(NXDOMAIN)

$ LC_ALL=C dig -t soa local.

; <<>> DiG 9.11.3-1ubuntu1.1-Ubuntu <<>> -t soa local.
;; global options: +cmd
;; Got answer:
;; WARNING: .local is reserved for Multicast DNS
;; You are currently testing what happens when an mDNS query is leaked to DNS
;; ->>HEADER<<- opcode: QUERY, status: FORMERR, id: 7637
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1
;; WARNING: recursion requested but not available

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
; COOKIE: e1ff5e7222ad62da (echoed)
;; QUESTION SECTION:
;local. IN SOA

;; Query time: 21 msec
;; SERVER: 192.168.194.20#53(192.168.194.20)
;; WHEN: Mon Aug 20 12:01:19 EDT 2018
;; MSG SIZE rcvd: 46
}}}

while off VPN:
{{{
$ LC_ALL=C host -t soa local.
Host local not found: 2(SERVFAIL)

$ LC_ALL=C dig -t soa local.

; <<>> DiG 9.11.3-1ubuntu1.1-Ubuntu <<>> -t soa local.
;; global options: +cmd
;; Got answer:
;; WARNING: .local is reserved for Multicast DNS
;; You are currently testing what happens when an mDNS query is leaked to DNS
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 61619
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;local. IN SOA

;; Query time: 0 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: Mon Aug 20 12:02:24 EDT 2018
;; MSG SIZE rcvd: 34

}}}

=====
while in the broken/hung state:
             ^^^^^^^^^^^
=====

{{{
$ LC_ALL=C host -t soa local.

<HANGS FOREVER>

:(

}}}
 (even hangs w/ "-W 1") ...

dig command augmented returns!:
{{{
$ LC_ALL=C dig -t soa local.

; <<>> DiG 9.11.3-1ubuntu1.1-Ubuntu <<>> -t soa local.
;; global options: +cmd
;; Got answer:
;; WARNING: .local is reserved for Multicast DNS
;; You are currently testing what happens when an mDNS query is leaked to DNS
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 16967
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;local. IN SOA

;; Query time: 0 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: Mon Aug 20 11:56:58 EDT 2018
;; MSG SIZE rcvd: 34
}}}

(I am not familiar enough with SOAL local. lookups though to say if it can replace the host invocation in this method)

/usr/lib/avahi/avahi-daemon-check-dns.sh

dns_has_local() {
  # Some magic to do tests
  if [ -n "${FAKE_HOST_RETURN}" ] ; then
    if [ "${FAKE_HOST_RETURN}" = "true" ]; then
      return 0;
    else
      return 1;
    fi
  fi

  OUT=`LC_ALL=C host -t soa local. 2>&1`
  if [ $? -eq 0 ] ; then
    if echo "$OUT" | egrep -vq 'has no|not found'; then
      return 0
    fi
  else
    # Checking the dns servers failed. Assuming no .local unicast dns, but
    # remove the nameserver cache so we recheck the next time we're triggered
    rm -f ${NS_CACHE}
  fi
  return 1
}