forwarding traffic from one port to another on the same box

[Andrew <andrew@arda.homeunix.net>]

I would like to forward all tcp traffic arriving on a particular port to 
another port on the same machine. This has worked for me in the past but 
I can't get it working on my current machine.

Here are the two commands I'm using to try to create the forward.

iptables -I FORWARD -p tcp -d 192.168.10.34 --dport 26 -j ACCEPT

iptables -t nat -A PREROUTING -p tcp -i eth0 -s 0/0 -d 192.168.10.34 
--dport 26 -j DNAT --to 192.168.10.34:25

The first command is accepted but the second command results in an 
'Invalid argument' error.

The computer has only one interface, eth0. Here are its particulars:
Mandrake Linux 9.2
Iptables 1.2.8
kernel 2.4.24 patched with super-freeswan 1.99.8

The value of /proc/sys/net/ipv4/conf/eth0/forwarding is 0. Changing it 
to 1 has no impact.
The value of /proc/sys/net/ipv4/conf/eth0/rp_filter is 0.

I hope someone out there has some ideas about what's going on because 
I'm all out.

Andrew