pppoe MTU problems

[Wenzhuo Zhang]

1. If the MTU of eth0 is 1500, network-manager fails to establish a pppoe connection, and the MTU of eth0 (not ppp0) is then automatically lowered to 1492. Now network-manager can successfully establish a pppoe connection. If I manually set the MTU of eth0 back to 1500, network-manager would fail again. See attached log.

$ sed -n '/^Feb 20 08:45/,/^Feb 20 08:51/p' /var/log/syslog | egrep '(pppd|NetworkManager)' | sed 's/.../<username>/g; s/00:11:25:.../00:11:25:aa:bb:cc/g; s/ac:85:3d:.../ac:85:3d:ee:dd:ff/g' > network-manager-pppoe.log

2. The Access Concentrator of my ISP offers MTU values of both 1492 and 1442. pppd-2.4.5 in Ubuntu 14.04 adopts 1492 as the MTU of the ppp device, while pppd-2.4.7 in Xenial adopts 1442. Setting the lower MTU on the WAN link (ppp0) can easily cause PMTU-D problems, especially for remote VPN clients.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: network-manager 1.0.4-0ubuntu9
ProcVersionSignature: Ubuntu 4.4.0-6.21-generic 4.4.1
Uname: Linux 4.4.0-6-generic i686
ApportVersion: 2.20-0ubuntu3
Architecture: i386
CurrentDesktop: LXDE
Date: Sat Feb 20 09:56:32 2016