ppp_mppe osize issue

(I sent this earlier this week under the subject "osize too small" and now realize that most spam filters probably kicked it out. Resending...

I'm using ppp with pptp to connect to a MS server at work. Kernel is 2.6.3 AMD64 on Mandrake 10.0, ppp 2.4.2. I built this kernel myself to
fix the 64-bit issues with ppp (fix previously reported.) When I have
VPN up and run a browser, it will retrieve a particular page fine the first time. However, if I click the submit button on that page again (which transmits a bunch of data - I authored this page so know all about it) the browser just goes into continuous wait and never actually transmits anything. I see the following in dmesg:

PPP generic driver version 2.4.2
ppp_mppe: module license 'BSD without advertisement clause' taints kernel.
PPP MPPE Compression module registered
Module ppp_mppe cannot be unloaded due to unsafe usage in include/linux/module.h:503
mppe_compress[0]: osize too small! (have: 1400 need: 1404)
ppp: compressor dropped pkt
mppe_compress[0]: osize too small! (have: 1400 need: 1404)
ppp: compressor dropped pkt

What is the appropriate fix for this? ifconfig shows an MTU of 1396 for ppp0. I tried specifying mtu 1500 when starting pppd, but it still ends up 1396. I suppose the server end is constraining it. Appreciate all suggestions - I can rebuild the kernel if necessary.

Guy Rouillier

PPPD reduces the MTU of the interface, because MPPE encryption needs additional 4 bytes. Windows clients do not see if that way, however. This leads to problems, because the Windows client sends a TCP MSS of 4 bytes too large for PPPD.
What you can do is change the PPPD, so that it does not reduce the MTU anymore. See the patch below.

Alternatively, set the MTU of the PPP interface in the ip-up script.

Stephan

diff -u -r1.1.1.1 -r1.1.1.1.4.1
@@ -1191,7 +1191,12 @@
  		     */
  		    mtu = netif_get_mtu(f->unit);
  		    if (mtu)
-			netif_set_mtu(f->unit, mtu - MPPE_PAD);
+		    {
+			// Removed the lowering of the MTU, because it causes
+			// problems with routing (TCP/MSS: Windows clients
+			// do not care about this reduced packet size)
+			//netif_set_mtu(f->unit, mtu - MPPE_PAD);
+		    }
  		    else
  			newret = CONFREJ;
  		}

Stephan Scholz <[email protected]> | Development
Astaro AG | www.astaro.com | Phone +49-721-490069-0 | Fax -55

Documentation: http://docs.astaro.org
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