Re: A router bug?

David (david@killerlabs.com)
Sat, 30 Jan 1999 23:57:50 -0800 (PST)

On 31 Jan 1999, Andi Kleen wrote:
> This explains it. Your other box is connected to an ethernet with MTU of
> 1500. Thus it sends a MSS option of 1500-X into the initial SYN of the
> TCP connection. Now the other server correctly sets its MSS to 1500-X
> and sends packets with that size with the Dont-Fragment bit set for PMTU
> discovery. Once they reach the other end of your slip link they get dropped
> and the router sends back a ICMP frag-needed to tell the other box to lower
> its pmtu - but because of the ICMP blocking firewalls set up by people that
> were so nicely described by Alan it never sees them. It does not happen
> from your router box, because there the first interface has a 576 MTU which
> means that TCP only puts a small MSS option in the first SYN, and all packets
> that are exchanged are small and no pmtu discovery is needed.

This describes the problem more accurately. However, I didn't notice any
outbound frag-needed icmps. That was several kernels ago and we had since
simply put up with it.

> One workaround: set the mtu on the gateway route you use on the other box
> (route add default gw ROUTER mss 576), then the MSS will contain small values
> from the beginning. It would be better to complain to the sites that don't
> work, because it is clearly a misconfiguration on their part. Another workaround
> is to increase the sl0 MTU to 1500bytes.

Doing the above works. We are ecstatic.

Being naive that I am and not getting feedback *hint* *hint :), why did
setting -all- interfaces on the net devs to an MTU of 576 still fail but
setting the mss on the route work?

Now if only dhcp supported that in the options :S

-d

-- 
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__ (c) 1998 David Ford.  Redistribution via the Microsoft Network is prohibited
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