Comment 13 for bug 1787739

I'll see if I can get a chance to do some packet captures later this week, however, I did notice something that may be relevant.

The domains that seem to fail do not have IPV6 look addresses. The domains that succeed have IPV6 address returns:

vagrant@ubuntu-bionic:~$ nslookup google.com
Server: 127.0.0.53
Address: 127.0.0.53#53

Non-authoritative answer:
Name: google.com
Address: 172.217.12.14
Name: google.com
Address: 2607:f8b0:400f:805::200e

vagrant@ubuntu-bionic:~$ nslookup yahoo.com
Server: 127.0.0.53
Address: 127.0.0.53#53

Non-authoritative answer:
Name: yahoo.com
Address: 98.138.219.231
Name: yahoo.com
Address: 98.137.246.7
Name: yahoo.com
Address: 98.137.246.8
Name: yahoo.com
Address: 72.30.35.9
Name: yahoo.com
Address: 72.30.35.10
Name: yahoo.com
Address: 98.138.219.232
Name: yahoo.com
Address: 2001:4998:44:41d::4
Name: yahoo.com
Address: 2001:4998:58:1836::11
Name: yahoo.com
Address: 2001:4998:c:1023::4
Name: yahoo.com
Address: 2001:4998:c:1023::5
Name: yahoo.com
Address: 2001:4998:44:41d::3
Name: yahoo.com
Address: 2001:4998:58:1836::10

However, the servers that fail are not returning IPV6 information:
vagrant@ubuntu-bionic:~$ nslookup ubuntu.com
Server: 127.0.0.53
Address: 127.0.0.53#53

Non-authoritative answer:
Name: ubuntu.com
Address: 91.189.94.40
** server can't find ubuntu.com: SERVFAIL

vagrant@ubuntu-bionic:~$ nslookup oracle.com
Server: 127.0.0.53
Address: 127.0.0.53#53

Non-authoritative answer:
Name: oracle.com
Address: 137.254.120.50
** server can't find oracle.com: SERVFAIL

vagrant@ubuntu-bionic:~$ nslookup amazon.com
Server: 127.0.0.53
Address: 127.0.0.53#53

Non-authoritative answer:
Name: amazon.com
Address: 176.32.103.205
Name: amazon.com
Address: 176.32.98.166
Name: amazon.com
Address: 205.251.242.103
** server can't find amazon.com: SERVFAIL

There's also a pause after the last "Address" output line and the "** server" line, where the IPV6 address would be.