Status changed to 'Confirmed' because the bug affects multiple users.
** Changed in: network-manager (Ubuntu)
Status: New => Confirmed
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https://bugs.launchpad.net/bugs/1110787
Title:
VPN tunnel connections are not properly setting MTU values
Status in “network-manager” package in Ubuntu:
Confirmed
Bug description:
I regularly connect to a cisco anyconnect vpn using vpnc through the
network manager. I am often at various different location using wifi,
including connecting via wifi to a cellular hotspot. Usually, when
connecting to a WIFI connection the MTU value (on the wifi connection)
is 1500. When connecting to the vpn, the MTU value of the tun0
connection is set automatically and it usually sets itself to
something like 1412. This works with no problem.
When the wifi connection has a lower MTU value (and the cellular
hotspot usually has an MTU of 1430, as do connections in various
coffee shops), the tun0 connection still sets itself to 1412. For
read-intensive operations (e.g. accessing websites and downloading
pages, pulling from a subversion repository) there is no apparent
problem. But when attempting to post to a web page (for instance,
writing a bug report in bugzilla) or commit to a repository over the
VPN, the connection will fail. The MTU value for a VPN connections
seems to want to be 78 less than the MTU value of the connection that
it is working over, and having MTU greater than this causes the upload
failures.
I use the VPN via the network manager (using network-manager-vpnc and
network-manager-vpnc-gnome). I am able to work around the issue by
setting the MTU value of the VPN connection manually from the command
line after connecting with the following command:
sudo ifconfig tun0 mtu 1330
It is tedious to do this each time, but it does allow me to
successfully upload or commit when connected. (I also find that RDP
connections fail much less often when the MTU is set correctly.) I
should note that I pick an MTU value that is 100 less than the MTU
value of the WIFI connection--it should be 78 but I opt for the easier
math and don't really notice the difference.
And for those that don't know, you can find the current MTU value of
your connections by issuing the "ifconfig" command.
If you do not have access to a cellular hotspot or other wifi
connection that has a low MTU value, it is likely you can replicate
this issue by manually setting the primary network connection's MTU
value to 1430 (using the above command) though I have not tried to
replicate it under these conditions.
Ideally, the VPN connection would properly calculate an appropriate
MTU value to use with the network adapter it will be communicating
over. Perhaps it is trying that and simply cannot identify the proper
connection--while writing this it occurs to me that the eth0
connection has an MTU value of 1500 even though I don't have a network
cable plugged in, but lowering the eth0 MTU value had no effect on the
tun0 MTU value chosen when I connected to VPN over the WIFI
connection.
To replicate the error:
* connect to a wifi connection that gives you an MTU value of 1430
* connect to VPN using vpnc via network manager
* verify that the MTU value of your tun0 connection was set to 1412
* Attempt a large upload (a large file, a large bugzilla comment or other
html form submission, or a sizeable svn commit) and it should fail
Important version info requested:
Ubuntu 12.10
vpnc is 0.5.3r512-2ubuntu1
network-manager, network-manager-vpnc, and network-manager-vpnc-gnome are all
0.9.6.0-ubuntu1
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