[krbdev.mit.edu #6566] UDP datagrams > 4K do not work.

[elric@mournblade.imrryr.org via RT]

On 1253075737 seconds since the Beginning of the UNIX epoch
"Ken Raeburn via RT" wrote:
>
>On Sep 15, 2009, at 22:02, elric at mournblade.imrryr.org via RT wrote:
>> Unfortunately, if you receive a datagram of over sizeof(pktbuf)
>> you will succeed with cc == sizeof(pktbuf) not detecting the fact
>> that there was additional data.  This results in an ASN.1 parse
>> error.  What should happen is that the KDC should return an
>> appropriate error to the client indicating that TCP should be used.
>
>Regardless of other options, it sounds like cc==sizeof(pktbuf) should  
>trigger the use-TCP error, since we can't distinguish between a packet  
>equal in size to the buffer and a packet that was larger but got  
>truncated.  Either that, or we could peek at the size of the next  
>datagram and grow the buffer as needed, but I'm not sure that peeking  
>can be done portably.

Yes, this sounds like exactly the approach I would think about
implementing.

>> I noticed this while debugging a JGSS problem.  Apparently, the
>> Java Kerberos libraries do not fail over from UDP to TCP unless
>> the KDC specifically tells them to.  And they have no default
>> setting for udp_preference_limit.  And so, if you are constructing
>> tickets of over 4K because, let's say, a user is in a lot of groups
>> in Windows, JGSS will just fail against an MIT KDC.
>
> From what I've read, the common wisdom still seems to be that some  
>gateways/routers/NAT boxes/firewalls/whatever will not properly  
>process UDP fragments, so UDP traffic over ~1500 bytes (or less) may  
>never get to the KDC.  So this sounds like a bug in the Java Kerberos  
>libraries.

It's most certainly a bug in the Java Kerberos libraries.  I've also
run into them breaking when frags are dropped, etc.

Thanks,

--
    Roland Dowdeswell                      http://Imrryr.ORG/~elric/