Ubuntu
ppp package

pppd sends packets exceeding MTU

Bug #317567 reported by Philipp Kohlbecher
This bug report is a duplicate of:  Bug #258743: NM 0.7 Fails To Set Custom MTU. Edit Remove
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ppp (Ubuntu)
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Bug Description

Binary package hint: ppp

When I log on to a certain VPN of type "Microsoft VPN (PPTP)", the connection is always dropped after some time. It turns out that this happens when my computer sends a GRE-encapsulated PPP packet with a payload length of 1417. The MTU of the network interface ppp0 is 1412. I believe the VPN gateway accepts packets with a payload length up to 1416. (How can I verify that?)

I am not sure whether pptp or pppd is the culprit here, but I suspect pppd. This looks like an off-by-one error.

Does anyone know why this might happen? Any pointers where I might look? Is there any information I can provide you with?

I am using Ubuntu 8.04 "hardy heron" with ppp 2.4.4rel-9ubuntu2.

Revision history for this message
Philipp Kohlbecher (xt28) wrote :

After manually setting the interface MTU to 1411, I found out that the gateway does not accept packets with a payload length of 1416 either.

Also, when pppd receives an ICMP message with message code 4 ("Fragmentation needed"), why doesn't it react by fragmenting and re-sending the offending packet?

Revision history for this message
Philipp Kohlbecher (xt28) wrote :

Trial and error: The magic number for the payload length is 1400. Anything exceeding that terminates the connection. (Thus, the interface MTU needs to be set to 1395.)

One more question: Why does pppd set the interface MTU to 1412 in the first place, when it is called with "mtu 1200 mru 1500" in its command line?

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