[CentOS] CentOS based router dropping connections

Jesse Cantara jesse_cantara at esupport.com
Fri Jul 20 15:44:08 UTC 2007


I am trying to figure out a problem I'm having using CentOS on a machine 
as a router. The short story is: any traffic routed through the router 
seems to get disconnected at random occasionally.

The hardware setup is:
I have two switches, the router sits between them, the webserver on the 
LAN switch.
The machine I'm using for the router is a Dell 860 1U rackmount with two 
NICs, one NIC on the internet, one NIC on the LAN.

The routing setup is:
I'm using IPTABLES for routing, with the following command:
iptables -t nat -A PREROUTING -p tcp -m tcp -i eth1 --dport 6680 -j DNAT 
Basically, I'm forwarding port 6680 on to the webserver (.10) on the LAN.

What I have tested so far:
If I'm at the router, I can download files from the webserver just fine, 
so the webserver setup and physical connection is OK.
If I'm at the router, I can download files from the internet just fine, 
so the physical connection to the outside is OK as well.
If I'm on the outside of the router (on the internet) I can download 
files directly from the router just fine.

The issue is when I try to download a file from the webserver via the 
router (port 6680). It will work sometimes, but other times it will 
randomly disconnect me, at random points during the download.

Watching the traffic on a packet-sniffer shows that right before the 
download fails, my client computer trying to download the file keeps 
resending "ACK" messages, the router keeps sending the next sequence of 
packets, and eventually the router sends a bunch of "RST" packets.

There aren't any strange messages in /var/log/messages or dmesg in 
either the router or the webserver

I need some help diagnosing this problem. Here's some info about the router:
CentOS 5
latest kernel 2.6.18-8.1.8.el5
iptables v1.3.5

I've tried testing as much as I can before asking for help, but I'm at 
the end of what I know to try. Any leads as to where to look to 
diagnose, or what might cause this would help.

Thanks in advance,

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