Filter out AAAA DNS responses

November 7, 2020

Tl;DR: with RPZ we can hide AAAA records while preserving other records

For those who look up the way to filter out AAAA records in their local caching DNS server here is my unbound snippet for a single ipv6 address:

$ cat /etc/unbound/rpz.home
$ORIGIN rpz.home.
;; fails to resolve: https://bugs.gentoo.org/742326
;; block 2607:fcc0:4:ffff::4/128 -> NODATA (CNAME *.)
128.4.0.0.0.ffff.4.fcc0.2607.rpz-ip.rpz.home. CNAME *.

$ cat /etc/unbound/unbound.conf
server:
    ... usual configuration goes here
    module-config: "respip validator iterator"

rpz:
    name: rpz.home.
    zonefile: /etc/unbound/rpz.home

Resolver can’t see the ipv6 address, only ipv4:

$ dig bugs.gentoo.org AAAA @8.8.8.8
gannet.gentoo.org.      21599   IN      AAAA    2607:fcc0:4:ffff::4
$ dig bugs.gentoo.org AAAA | fgrep AAAA
# nothing

$ dig bugs.gentoo.org A @8.8.8.8
gannet.gentoo.org.      21599   IN      A       204.187.15.4
$ dig bugs.gentoo.org A
gannet.gentoo.org.      20091   IN      A       204.187.15.4

You can do full sub-networks as well.

Why would I need it?

The other day I tried to reach bugs.gentoo.org programmatically over HTTP API and observed 100% traffic loss. wget was also showing hangups:

$ wget bugs.gentoo.org
--2020-09-19 09:24:22--  http://bugs.gentoo.org/
Resolving bugs.gentoo.org... 2607:fcc0:4:ffff::4, 204.187.15.4
Connecting to bugs.gentoo.org|2607:fcc0:4:ffff::4|:80...
# hung up
^C

$ wget -4 bugs.gentoo.org
--2020-09-19 09:25:02--  http://bugs.gentoo.org/
Resolving bugs.gentoo.org... 204.187.15.4
Connecting to bugs.gentoo.org|204.187.15.4|:80... connected.
HTTP request sent, awaiting response... 301 Moved Permanently
...
2020-09-19 09:25:03 (1.08 MB/s) - 'index.html' saved [13934]

As a workaround I added AAAA response filtering locally as specified in TL;DR to unblock the scripts.