RE: Cursed Checksums

[Albert Cahalan]

On Sun, 2004-08-22 at 18:38, Josan Kadett wrote:
> In normal conditions, when NAT translates an IP header, it should do correct
> checksumming. However; when the source IP address of a packet is
> manipulated, Iptables still seems to use the "original" IP address and
> calculate the checksum accordingly. This way even if I use 3 three boxes;
> 
> Original Packet --> NAT 1 --> NAT 2 --> NAT3 --> Still incorrect checksum

Sure. That's why I think this is a job for ebtables.
Note: EBtables, not IPtables. This is lower level,
dealing with raw Ethernet packets. It should be able
to mangle anything in the packet.

Just as iptables lets you change any 32-bit value in
an IP packet, ebtables should let you change any 32-bit
value in an Ethernet packet. Unlike iptables, it could
operate prior to the IP checksum verification.

Sadly, it looks like u32 mangle rules are missing.


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