OpenConnect client for Cisco's AnyConnect SSL VPN

If we need to connect Cisco's AnyConnect SSL VPN from an AWS CentOS instance without using Cisco AnyConnect client, there's the OpenConnect as an alternative.

On CentOS 5.4, it's easy to install OpenConnect from enabled Red Hat Enterprise Linux / CentOS Linux Enable EPEL (Extra Packages for Enterprise Linux) repository.
If it is not, then install it:

rpm -ivh http://dl.fedoraproject.org/pub/epel/5/i386/epel-release-5-4.noarch.rpm

And then install openconnect: yum install openconnect

Then use it to connect VPN: openconnect -u auser cisco-vpn-server
But after enter the password, it throws an error: Failed to open tun device: No such device

Let's check the tun module:

[root@myhost ~]# modprobe tun
FATAL: Module tun not found.

[root@myhost ~]# cat /dev/net/tun
cat: /dev/net/tun: No such device

The module tun is /lib/modules/2.6.16-xenU/kernel/drivers/net/tun.ko. For some reason, if it's missed, so get it from http://s3.amazonaws.com/ec2-downloads/modules-2.6.16-ec2.tgz and unpack/copy entire folder lib/modules/2.6.16-xenU/kernel to /lib/modules/2.6.16-xenU

Then we need to reload modules:
[root@myhost ~]# depmod -ae 2.6.16-xenU

[root@myhost ~]# modprobe tun

Then connect VPN again, and it works: Connected tun0 as a.b.c.d, using SSL

Then just open another terminal and ssh to a server in the network.