Messages in this thread | | | Date | Sat, 30 Jan 1999 23:57:50 -0800 (PST) | From | David <> | Subject | Re: A router bug? |
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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 -- Look, Windows 98 Buy, lemmings, buy! MCSE, Must Consult Someone Experienced __ (c) 1998 David Ford. Redistribution via the Microsoft Network is prohibited \/ for linux-kernel: please read linux/Documentation/* before posting problems
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