Packet loss when connecting via IPv6

Johannes Brechtmann brechtmann at
Thu Jan 28 12:01:17 PST 2016

On Thu, 28 Jan 2016 01:04:24 +0000
David Woodhouse <dwmw2 at> wrote:

> On Thu, 2016-01-28 at 02:02 +0100, Johannes Brechtmann wrote:
> > When connecting to a vpn server via IPv6, I get only small packets
> > (I guess), so ping works fine, http redirects work but http 200
> > responses get lost. With ssh I can log into a server and use basic
> > commands like uptime but e.g. top doesn't work.
> > 
> > If I connect to the server via IPv4 everything works fine.
> > If I connect to the same server with the Cisco AnyConnect client (no
> > matter if IPv6 or IPv4), everything works fine, too. 
> > 
> > I'm using openconnect v7.06 on Debian testing and openconnect for
> > android v1.11.
> > 
> > I'm wondering if this is a problem of openconnect or the server I
> > try to connect to.
> > 
> > Can anyone confirm or deny this behaviour with any vpn server?  
> What MTU does the AnyConnect client set for the IPv6 link. And what
> MTU does OpenConnect set?

Sorry, got something wrong: AnyConnect doesn't connect to the server
via IPv6 at all.
Openconnect sets the MTU to 1350, setting it manually after connextion
is established to 1178 fixed it for me.
Thanks for the hint to look at the MTU.

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