l2tp vpm

You should describe your setup in more details. The config of iptables you are troubleshooting -- is it on the vpn client host, or an intermediate router?

we have one server for windows 2003 act as a vpn server
we have one server for redhat act as a firewall
we are unable to connect vpn using l2tp, but pptp vpn works fine

this is a config

-A PREROUTING -d 12.12.12.2 -i eth0 -p udp -m udp --dport 1701 -j DNAT --to-destination 192.168.200.1

-A FORWARD -p esp -j ACCEPT
-A FORWARD -p ah -j ACCEPT
-A FORWARD -p udp -m udp --dport 1701 -j ACCEPT
-A FORWARD -p udp -m udp --dport 500 -j ACCEPT
-A FORWARD -p udp -m udp --dport 4500 -j ACCEPT

I would suggest you to sniff the traffic on both interfaces of the firewall and see what packets get blocked when you try to establish the VPN. Check if IKE (UDP 500 or 4500) and IPSEC (ESP).

By the way, UDP 1701 packets travel encapsulated in IPSEC and are invisible to IPTABLES, the corresponding statements in your policy have no effect on L2TP. If you need DNAT, can try to apply it to IKE/ESP traffic.

we are unable to connect till now,pls tell me the above config correct or not.If not pls give me a correct config .

And then we have checked the traffic the show ony the isakmp and UDP

Please provide more details about your setup.

What OS do you use as a client? Does the L2TP client software support NAT-T (nat traversal)?
What is IP address of your client?
What IP address your client tries to connect to?

What is the IP address of the interface of your router facing L2TP client?
What is the purpose of DNAT statement, what address translation you are trying to achieve?
What is the IP address of the interface of your router facing L2TP server?

What is the IP address of the L2TP server?

ok i give

1.windows vista
2.public ip (internet work no issues)
3.public ip 12.12.12.2 (example)
4.12.12.12.2
5.
6.12.12.12.2
7.192.168.200.1 (windows 2003 server)

we understand the IPSEC servers behind NAT.thanks..

thanks..