[PVE-User] Solved proxmox KVM VNC problem / issue

Jesper Dangaard Brouer jdb at comx.dk
Tue Feb 8 12:24:32 CET 2011

Hi Proxmox users

I have (hopefully) identified the problem, with proxmox, where the "Open
VNC console" does NOT work for KVM virtual machines, but it works for
OpenVZ containers.

The problem is related to IPv6 socket bindings, which has been up on the
mailing list before see [1] and [2], and via a blog [3].

The real cause of the problem is that eth0 gets assigned an IPv6 address
automatically (related to its MAC).  The interface eth0 is part of a
bridge, and thus must not be assigned any IPs (due to, how the kernel
bridge code works, one must use the bridge interface vmbr0).  Somehow
the netcat/nc choose to bind to this address and screws up.

My solution was to disable IPv6 on eth0
via /proc/sys/net/ipv6/conf/eth0/disable_ipv6.  This can be done via the

 echo "net.ipv6.conf.eth0.disable_ipv6 = 1" > /etc/sysctl.conf
 sysctl -p

One of the reasons eth0 gets assigned an IPv6, is most likely because,
the default install is missing a line in /etc/network/interfaces.  The
file should contain:

 iface eth0 inet manual

This line will propably also solve the issue.

 Problem found on a freshly installed Proxmox version 1.7

 [1] http://pve.proxmox.com/pipermail/pve-user/2010-February/001236.html
 [2] http://pve.proxmox.com/pipermail/pve-user/2010-February/001237.html
 [3] http://blog.stalkr.net/2010/02/proxmox-kvm-reach-vnc-in-ipv6.html

Best regards
  Jesper Brouer
  ComX Networks A/S
  Linux Network Kernel Developer
  Cand. Scient Datalog / MSc.CS
  Author of http://adsl-optimizer.dk
  LinkedIn: http://www.linkedin.com/in/brouer

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