Re: CNAME Lookup failure for MX microsoft.com

On Tue, Feb 3, 2009 at 11:59 AM, Emilio Perea <[email protected]> wrote:
> On Tue, Feb 03, 2009 at 11:16:52AM +0100, Erwin Hoffmann wrote:
>> I did a little investigation, and it turned out to be the 'oversized' UDP
>> packet problem.
>>
>> Obviously, at least one of the queried NS sends oversized packets.
>>
>> Changing udpbuf to 4096 in dns_transmit.c solves (or at least relaxes) the
>> problem.
>>
>> Comments ?
>
> There was a thread about this last year.  The first proposed solution
> was to change to TCP when this happened:
>
> *----------------------------------------------------------------------*
> --- 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);
> *----------------------------------------------------------------------*
>
> A bit later Someone suggested the same solution you came up with:
>
> *----------------------------------------------------------------------*
> --- dns_transmit.c.orig 2008-10-10 16:59:03.000000000 -0700
> +++ dns_transmit.c      2008-10-10 16:59:11.000000000 -0700
> @@ -240,7 +240,7 @@
>
>  int dns_transmit_get(struct dns_transmit *d,const iopause_fd *x,const
> struct taia *when)
>  {
> -  char udpbuf[513];
> +  char udpbuf[4097];
>   unsigned char ch;
>   int r;
>   int fd;
> *----------------------------------------------------------------------*
>
> That's what I'm using myself, and have not noticed any drawbacks.

Here's the first post in the thread:

http://marc.info/?l=djbdns&m=122329096516302&w=2

From reading the thread, I'd tend to go with the 2nd solution
(enlarging udpbuf).  I'll probably toss that into my next release.