Strange speed problems with ipv6 forwarding

[Frank Steiner]

Hi all,

I've encountered a strange speed problem with ipv6 forwarding. We are using a routing firewall running SLES 11 sp3 at our chair. It has two 10GB network cards with 10G uplinks. We have a subnet behind the firewall and one in front of it and the firewall is configured to forward all traffic between the networks (and has ips from the network on the according devices of course). All hosts/networks have public ipv4 and public ipv6 IPs.

Now I'm at  host A behind the firewall and copy a file from host B outside the firewall. Works with ~ 112MB/s (the hosts have 1 GB uplinks) when I explicitely use the ipv4 address of B in the scp/wget or whatever. When I use the ipv6 address of B (which is the default when I use the host name), the transfer rate drops to ~ 1 MB/s. 

When copying from A to B via ipv6 adresses I get ~ 15 MB/s.

But (let's assume the firewall ist host F) when I copy from A to F, F to A, B to F, F to B, always using ipv6 addresses, I always get the full transfer speed of ~ 112 MB/s.

Thus, both directions from and to the firewall from both subnets are working at full speed when using ipv6 adresses. Only the forwarding through the firewall is slow with ipv6 adresses, while it's fast with ipv4.

I've no idea where to start looking. I flushed all ip6tables rules with no change, /proc/sys/net/ipv6/conf/all/forwarding is "1", default route is set for ipv4 and ipv6.

Any ideas what could be wrong with my setup?  

cu,
Frank

-- 
Dipl.-Inform. Frank Steiner   Web:  http://www.bio.ifi.lmu.de/~steiner/
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