Get original destination IP and port with Linux 2.4 iptables redirect?

[Lincoln Yeoh]

Hi,

Say I use iptables to redirect tcp connections to my perl proxy
servers. How then do I get the original destination IP address and tcp
port?

On FreeBSD I just use ipfw and fwd and then following works:
$daddr=$client->sockhost;
$dport=$client->sockport;

And then my various proxies work transparently.

But on Linux I'm supposed to use some FD options:
e.g.
getsockopt(fd, SOL_IP, SO_ORIGINAL_DST, &dst_addr, &slen)

What's a good way to do this with perl? Working examples would be very
helpful.

I've tried perl's getsockopt but replacing OPTNAME with
SO_ORIGINAL_DST doesn't work - it's not defined.

perl -f getsockopt
getsockopt SOCKET,LEVEL,OPTNAME

I've tried specifying a numerical 80 for OPTNAME but not sure how to
get the address etc.

Thanks,
Link.