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Old 10-25-2005, 06:17 AM   #1
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Registered: Oct 2005
Posts: 3

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Problem with Port Forwarding

I am using Slackware 10.2 with a 2.4 kernel for NAT and a WIN XP SP2 workstation which runs a DC client.
My iptables configuration looks something like this:

#Enable Forward
iptables -A FORWARD -i $INTIF -o $EXTIF -j ACCEPT

iptables -t nat -A POSTROUTING-o $EXTIF -J SNAT --to

#Port Forwarding
iptables -t nat -A PREROUTING -d -p tcp --dport 6783 -j DNAT --to-destination
iptables -t nat -A PREROUTING -d -p udp --dport 5758 -j DNAT --to-destination

The DC client is configured as follows:
Firewall with manual port forwarding
TCP port: 6783
UDP port: 5758
External/WAN IP:

The NAT works without any problems, meaning that I can write this text from the Win dows workstation.
Unfortunatey, the port forwarding part is not working meaning that even if I set the above ports (6783 - TCP, 5758 - UDP), the DC client doesn't have any traffic on these ports. It works well when it's active with a real IP or when passive through the NAT server.
As far as I know, this whole configuration should work. What am I doing wrong?
Old 10-25-2005, 07:18 AM   #2
Registered: Jan 2005
Location: Tasmania
Distribution: Xen Debian Lenny/Sid
Posts: 578

Rep: Reputation: 30
Have you checked
# cat /proc/sys/net/ipv4/ip_forward

it needs to be set to 1 for forwarding:
# echo 1 > /proc/sys/net/ipv4/ip_forward


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