OpenConnect ignores the gateway's proposed MTU

Michael Helmling helmling at
Fri Feb 15 09:21:45 EST 2013


when connecting to a VPN server with OpenConnect, the MTU on the created 
tun interface is too high (1418) and TCP connections are broken as soon 
as large packages are transmitted. The gateway operator told me that the 
gateway sends a MTU of 1330 which is also what the Cisco AnyConnect 
client sets on the cscotun0 interface. Manually setting the MTU to 1330 
on OpenConnect's interface fixes all problems.

I have seen that in the changelog of v4.03 it says "Fix handling of 
requested vs. received MTU settings.", however I do still have the 
problem with openconnect v4.07 (on 64-bit Arch Linux).

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