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Old 07-01-2010, 10:38 AM   #1
mboudro
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Registered: Nov 2008
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NAT Forwarding Issue (SYN_SENT)


My setup is as follows:

I have an outside WAN connection on the /29 subnet on eth1 of my Endian Linux router (for the most part, an IPTables based router) and an internal connection on the /24 subnet on eth0 of the router. I set up an SNAT masquerade and I can successfully access the internet on computers on my internal /24 network.

I am relatively well versed in firewalls and NAT so I proceeded to set up a DNAT in order to forward packets from the external interface to my internal web server on port 80. I also set up a firewall rule to allow the movement of TCP packets on port 80 into the internal network.

Unfortunately, I cannot seem to connect to the web server through the external IP. I monitored the TCP packets through the Endian interface and also lsof and found that the TCP packets were being destroyed in the SYN_SENT state, whereas they never received a TCP "handshake" (SYN/ACK) from the internal web server in order to begin TCP transmission. For the life of me, I cannot figure out why this is happening.

Some additional notes, the router feeds into the /24 network through a Cisco Catalyst switch. I currently have a Draytek "dumb" router fully functioning including NAT, firewall and IPSec VPN. This router has no trouble forwarding packets to the web server and is currently in production. Also, I tested this same appliance with a Vyatta linux firewall and received the same SYN_SENT packet problem so I don't believe it is a firewall configuration problem.

Feel free to ask if you need any additional information.

Thanks in advance for your help.
 
Old 07-02-2010, 10:36 AM   #2
SuperJediWombat!
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Registered: Apr 2009
Location: Perth, Australia
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Can you please post the output of:
Code:
ifconfig
ip route
iptables-save
 
Old 07-02-2010, 10:58 AM   #3
nimnull22
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Registered: Jul 2009
Distribution: OpenSuse 11.1, Fedora 14, Ubuntu 12.04/12.10, FreeBSD 9.0
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Check, what IP your forwarded port 80 on router to. May be it is not IP of your web server?
 
  


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