Alexander Wilms wrote:
> Hi Martin,
> 
> this reminds me of a FTP conntrack problem I had with Xen a while ago.
> It's related to the common Xen checksum offload problem.
> First FTP port packet has incorrect checksum and is dropped. Then the resent 
> packet is ignored by the ftp_conntrack module and doesn't get masq'ed. E 
> voila, you have your internal address in the port command.
> 
> Diagnosis: run tcpdump -vv (or even better wireshark) on the involved 
> interfaces and you'll see a lot of invalid checksums
> 
> Solution: disable tx-checksumming on ALL interfaces (ethtool -K <device> tx 
> off).

Hi Alexander,

Voila! That did it. Now it works. I ran
# ethtool -K <device> tx off
... on the firewalls both network interfaces as well as on the FTP server 
network interface.

Big thanks to you, Andrew and Tom for your time and input.

Tom, I think this is something for the documentation.

/Martin Leben


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