How to disable AAAA lookups.md

How to disable AAAA lookups?

Problem description

The corporate DNS server that is outside of our control doesn't handle AAAA queries properly. When sent a AAAA query, the DNS server doesn't respond. A properly working DNS server returns NOERROR, ANSWER: 0, if there is no AAAA record for a given name. Misconfigured DNS server doesn't send any response.

In an IPv6-enabled environment, the client tries to resolve both A and AAAA addresses. If the DNS server doesn't send any reply, the client repeats the query and eventually times out. Only after the AAAA query times out, the client will use the A address. Waiting for the timeouts renders utilities like curl, kubectl, oc, ... and others unusable.

Identifying the AAAA lookup problem

Check using curl

This command hangs (>10 secs) before it prints out the web page:

$ curl https://google.com

This command fetches the page instantly:

$ curl -4 https://google.com

Check by querying the DNS server

$ NAMESERVER=<your name server here>

This query returns an answers:

$ dig @$NAMESERVER A google.com

This query times out and doesn't return an answer:

dig @$NAMESERVER AAAA google.com

Working around the issue

The idea is to use a custom DNS server which:

1. Forwards the A queries to the broken DNS server
2. Replies to the AAAA queries by itself keeping the clients happy

Dnsmasq can be configured to achieve the above. It can be run on a local machine as a DNS cache server listening on localhost:53.

Configuring Dnsmasq

First, download the list of top-level domains. It will be needed to construct the dnsmasq configuration file:

$ curl -O https://data.iana.org/TLD/tlds-alpha-by-domain.txt

Next, clean up the TLD file:

$ sed -e '/^#/d' -e '/^XN--/d' tlds-alpha-by-domain.txt > tlds-alpha-by-domain.txt.clean

Construct the top part of the dnsmasq configuration file:

$ cat > dnsmasq.conf <<EOF
no-resolv
log-queries
no-hosts
EOF

Generate the rest of the config file:

$ NAMESERVER=<your name server here>
$ while read TLD; do \
    echo address=/$TLD/::; \
    echo server=/$TLD/$NAMESERVER; \
  done < tlds-alpha-by-domain.txt.clean >> dnsmasq.conf

Your dnsmasq.conf file should now look as follows (output was shortened):

$ cat dnsmasq.conf
no-resolv                       
log-queries                                                                                                                                  
no-hosts                   
address=/AAA/::        
server=/AAA/<your name server here>     
address=/AARP/::                                                                                                                             
server=/AARP/<your name server here>
address=/ABARTH/::         
server=/ABARTH/<your name server here>
...

Using the above config file, you can start the DNS cache server:

$ sudo dnsmasq --no-daemon -C dnsmasq.conf

Next, edit the /etc/resolv.conf file to configure your machine to use the DNS cache server instead of the broken DNS server:

$ cat /etc/resolv.conf
...
nameserver 127.0.0.1
...

You are all set. The DNS resolution on your machine should work now!

References

How to disable AAAA lookups?

That's helpful, thanks