Latest LQ Deal: Complete CCNA, CCNP & Red Hat Certification Training Bundle
Go Back > Forums > Linux Forums > Linux - Networking
User Name
Linux - Networking This forum is for any issue related to networks or networking.
Routing, network cards, OSI, etc. Anything is fair game.


  Search this Thread
Old 07-01-2010, 10:38 AM   #1
LQ Newbie
Registered: Nov 2008
Posts: 6

Rep: Reputation: 0
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
Registered: Apr 2009
Location: Perth, Australia
Distribution: Ubuntu/CentOS
Posts: 208

Rep: Reputation: 51
Can you please post the output of:
ip route
Old 07-02-2010, 10:58 AM   #3
Senior Member
Registered: Jul 2009
Distribution: OpenSuse 11.1, Fedora 14, Ubuntu 12.04/12.10, FreeBSD 9.0
Posts: 1,571

Rep: Reputation: 92
Check, what IP your forwarded port 80 on router to. May be it is not IP of your web server?


firewall, linux, nat, networking

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
NAT port forwarding mrittman Linux - Newbie 3 09-28-2009 06:31 PM
NAT Forwarding not working. MikeyCarter Linux - Networking 2 03-04-2009 09:35 AM
X11 forwarding + NAT technopasta Linux - Networking 9 05-28-2007 06:49 AM
NAT and Port Forwarding aq_mishu Linux - Networking 2 09-16-2005 07:58 AM
NAT forwarding kermitthefrog91 Linux - Networking 4 08-04-2005 03:26 AM > Forums > Linux Forums > Linux - Networking

All times are GMT -5. The time now is 05:50 AM.

Main Menu
Write for LQ is looking for people interested in writing Editorials, Articles, Reviews, and more. If you'd like to contribute content, let us know.
Main Menu
RSS1  Latest Threads
RSS1  LQ News
Twitter: @linuxquestions
Facebook: linuxquestions Google+: linuxquestions
Open Source Consulting | Domain Registration