I'm seeing a strange issue with DNAT'ed traffic over a VPN. Incoming packets arrive just fine, but outgoing traffic has trouble for large streams of tcp data. My setup is fairly simple. A group of machines on a private network behind a gateway/firewall (netfilter) connect through an OpenVPN tunnel to a remote group of machines on a different private network. Local subnet: 10.1.2.0/24 Remote Subnet 10.1.1.0/24 Client machines on the local subnet can freely talk to servers on the remote subnet through the vpn with out any problems. Until the vpn tunnel was functional, client machines on the local private network connected to mail.dealnews.com to retrieve and send mail, a public interface of the mail server on the remote private network. Now that the vpn is working, they need to retrieve and send mail using the private address 10.1.1.7. For several reasons, one being laptop administration, I don't want to change all of the mail client's ip addresses to 10.1.1.7. I want to use iptables to DNAT packets headed for the public mail address (mail.dealnews.com) to the private mail address 10.1.1.7 so that packets are routed over the vpn instead of the internet. This is how I attempted to configure iptables: iptables -t nat -A PREROUTING -s 10.1.2.10 -d 129.41.69.137 -p all -j DNAT --to-destination 10.1.1.7 The -s option is there so that I can test the config myself without Borking the rest of the network. This seems to work at first as I can see traffic sent from the client to mail.dealnews.com over the tunnel interface on the remote network. What happens though, is although that I can connect to the remote mail server just fine through IMAP and even send out a very small email message through SMTP, large mail messages just stall and fail. Ftp is the same way. I can transfer files from the remote server, but I can not send any sizeable file to the server. I know for sure that traffic is traveling over the vpn tunnel because I'm dumping the tunnel interface up at the remote network. This sounds like something to do with fragmentation or possibly something along that line of thinking, but I can not for the life of me figure out what this is. I wondered if possibly, I needed another rule to DNAT packets coming from the remote network over the tunnel back to the public mail.dealnews.com ip address: iptables -t nat -A PREROUTING -s 129.41.69.37 -d 10.1.2.10 -p all -j DNAT --to-destination 129.41.69.137 But this didn't seem to help anything. Could anyone help me figure out how I can work around this? Again, incoming traffic through the tunnel seems to work just fine, but outgoing traffic only half seems to work. As strange as that sounds. Thanks, Daniel dealnews.com
