Enable DNS response compression

[aksdb]

Expected Behavior

After enabling AdGuardHome as my main DNS server via DHCP, all devices in my network should still work but receive properly filtered DNS responses.

Actual Behavior

Two devices (a media receiver for IPTV and an electronic door lock) refuse to connect and report network errors. All other devices work fine.

Solution

I played around by putting dnsmasq in front of AdGuardHome and suddenly the two devices in question started working again.
I therefore took Wireguard to analyze what the difference is in the answers I get. And apparently it's only a small thing: with AdGuardHome DNS responses are not compressed, with dnsmasq they are. That means, that the default responses from AdGuardHome are larger and contain the domain name twice (once in the question section and once in the answer section), while with dnsmasq (and apparently most other DNS servers out there) the answer section contains only a pointer into the question section.

So I enabled compression in AdGuardHome:

diff --git a/dnsforward/dnsforward.go b/dnsforward/dnsforward.go
index d2acb04..d9f8b14 100644
--- a/dnsforward/dnsforward.go
+++ b/dnsforward/dnsforward.go
@@ -441,6 +441,8 @@ func (s *Server) handleDNSRequest(p *proxy.Proxy, d *proxy.DNSContext) error {
                }
        }
 
+       d.Res.Compress = true
+
        shouldLog := true
        msg := d.Req

Now all devices in my home are happy.

Additional information

I guess this is somewhere between a bug and a feature, and most likely the actual bug is in one of the client implementations that those two devices use. Since all other DNS servers I tried compress their packets (makes sense I guess, given the max UDP packet size), I presume this small change does not hurt (quite the contrary, probably).

If you prefer a PR instead of this one-line-diff, I can certainly offer that as well. But I guess from a contribution perspective, this ticket with the one-line-diff is easier for you to apply :-)

[ameshkov]

Nice, thank you!