Re: CNAME Lookup failure for MX microsoft.com

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.