Re: dnscache and oversized answers

>On Mon, Oct 06, 2008 at 10:55:12 -0000, John Levine wrote:
>> One of my users noticed that MX lookups for xo.net were timing out.
>> The problem turns out to be that XO's name servers are putting in too
>> many additional section records and are returning oversized answers of
>> about 540 bytes, which dnscache then throws away.
>
>Try this.
>
>--- dns_transmit.c~	2008-01-09 00:50:09.559806518 +0200
>+++ dns_transmit.c	2008-10-06 14:16:28.539802762 +0300
>@@ -352,7 +352,7 @@ have sent query to curserver on UDP sock
>       if (errno == error_connrefused) if (d->udploop == 2) return 0;
>       return nextudp(d);
>     }
>-    if (r + 1 > sizeof udpbuf) return 0;
>+    if (r + 1 > sizeof udpbuf) return firsttcp(d);
> 
>     if (irrelevant(d,udpbuf,r)) return 0;
>     if (serverwantstcp(udpbuf,r)) return firsttcp(d);

Thanks, that works great.

From a security point of view, it'd probably be better to check the
irrelevant bit before falling back to TCP, but that seems like a minor
bug since at worst it'd provoke redundant lookups to the real server.

R's,
John