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Getting some really weird behavior from our Ubuntu server... it's behind a Verizon router firewall with port forwarding (port 8080 to port 80 on the server), and we've been having issues accessing it via this external IP. From within the network, it appears to respond normally (I can access it via web browser and SSH), but refuses connections through port forwarding (using our static external IP). The strangest thing is that it actually responds to external port-forwarded connections right after being restarted, but quickly lapses back into this pattern of refusing external connections.

I'm a bit of a server newbie (I'm actually a programmer in a small startup that just lost their server ops guy, urgh) so this is all trial by fire for me. Does anyone have any advice on what could be going wrong here? Any help would be appreciated, thanks.

EDIT:

We have another server being forwarded on port 80, and it hasn't had any accessibility problems. So now I'm beginning to think it's a problem with the server. But what would prevent a server from accepting port-forwarded connections?

EDIT 2:

Okay, now this is really bizarre... it seems that the :8080 external address works in my browser immediately after I SSH into the server via the local IP in the terminal... and then it stops working shortly thereafter. Any idea what could be behind this?

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  • I'm having the same issue. With one of my Ubuntu servers. No clue yet why. Commented Mar 28, 2012 at 7:53
  • What does a tcpdump -n 'port 8080' on the server say when you try and open the external address in a web browser? You should at least see incoming packets, if the port forwarding works. Commented Jun 14, 2020 at 5:33

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Does restarting the ubuntu server or the verizon firewall make it start working again temporarily?

To check the status of your firewall rules, use

sudo /sbin/iptables -nvL |less

what do you see there? Save that output right after restart and then compare to when it stops working - is some other process mucking with your firewall rules (via crontab maybe)? Seems odd, but that could explain the behavior you are seeing.

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  • iptables look empty... there don't seem to be any restrictive rules in place. Also, I should've been more clear in my original question: the problem is momentarily relieved by restarting the SERVER, not the Verizon router. However, I'm starting to wonder if the problem lies with the router, since this started happening shortly after our internet connection mysteriously dropped out (though it has since come back and the problem persists). Commented Jan 17, 2011 at 2:00
  • Hm it's entirely possible the verizon router is doing something evil to block your setup. Not sure how to check that, other than setting up another server behind it and seeing it it has the same sort of failure. Running nmap on the router from outside and inside your network might also be informative. Commented Jan 17, 2011 at 2:24
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You might also want to check your server's routing configuration to make sure there are no custom made routing tables being used:

Any new routing tables should be defined in /etc/iproute2/rt_tables

rules defining what tables are used when :

"ip rule show"

if u find any custom tables u can see whats in there using :

"ip route show table tablename"

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