[geeks] Further info on Sol8 networking problem

[Joshua D Boyd]

On Fri, Mar 22, 2002 at 12:55:42AM +0100, Martijn Pronk wrote:

> 
> If I read this correctly, it's trying to send a tcp packet with 1460 bytes
> and the Don't Fragment bit on. This looks like it is thying to do Path 
> MTU discovery. (Send a big TCP packet with DF set down the line and if 
> there is a link between the two hosts with a smaller MTU the router will 
> send a ICMP message Fragmentation Needed. The OS will try to send a 
> smaller packet and the proces is repeated until the tcp connection is 
> complete and the smaller MTU size is used until the tcp connection is 
> dropped.)
> If you're somehow not able to recieve these ICMP messages, your OS is 
> going to be confused...
> 
> Since ping and DNS packets are generally small these packets are not 
> affected by this.
> 
> How to test for this? Try pinging the host with a bigger than normal 
> Ping packet. (Your ping program will be able to create bigger ping packets)
> So you could try (Solaris 8 Ping syntax)
> ping -s obelix.sillywalks.org   (normal size, should work if not blocked 
> by network)
> ping -s obelix.sillywalks.org 1472 (Should work if the mtu is at least 
> 1500 bytes)
> ping -s obelix.sillywalks.org 500 (Should work...)
> 
> Try to find the biggest packet that comes through and you have found the 
> mtu of the path between you and the other host.

Bingo.  That nailed it.  That's the problem.  The standard (64bytes) and 500
bytes work, but 1472 bytes doesn't.  Just have to figure out how to set the 
Solaris MTU.

-- 
Joshua D. Boyd