Question

FreeBSD gateway/routed connections

Asked by: oobayly

I've a FreeBSD machine to be set up as a firewall/router box for our office. It's using 7.2 with a generic kernel (for the moment). It routes all traffic to the internet via an ADSL modem in bridging mode (as all outbound connections are NATed to our fixed IP).

Everything seems to be working fine apart from an issue with TCP connections remaining ESTABLISHED on client machines.

For example, we have a .Net app that connects to a MySQL database. Very occasionally, if there's a hiccup on the line, the app will freeze. Using Netstat, Process Explorer or TCP View from Systinternals, I can see that the connection to the MySQL server is ESTABLISHED.
The same also happens on a Linux machine in our network that hosts another MySQL database which replicates the remote DB.
Note, this is rare behaviour as normally what happens is that the connection is closed due a TCP ACK not being received after the given number of retries.
I'm assuming the client freezes as it's waiting for data to be sent, as when I use tcpdump (or Wireshark on a Windows machine), I see that no traffic is being sent.

However, when I log onto the remote MySQL server, I can see that that ip/port is no longer connected. Netstat on the server also shows that the connection is no longer there.

Can anyone suggest why the client's TCP connection is still established, while the connection at the server end appears to have been reset.

I also can't for the life of me find out how to list all the connections that are using the FreeBSD machine as a gateway to the internet. netstat -naf inet only shows connections where the FreeBSD machine is one of the endpoints.

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Asked On
2009-06-26 at 07:52:28ID24525328
Tags

FreeBSD TCP ipnat routing gateway

Topics

FreeBSD

,

DSL Lines / Cable Internet

,

TCP/IP

Participating Experts
1
Points
500
Comments
8

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Answers

 

by: gheistPosted on 2009-06-27 at 05:32:01ID: 24727445

> connection is closed due a TCP ACK not being received after the given number of retries
No - it is closed only after 6 hours in this case.

Router operates at lower level and unless you employ state tracking like ipfw or ipf or pf you really only see IP addresses in routing tables.

 

by: oobaylyPosted on 2009-07-12 at 04:56:37ID: 24834092

Sorry for taking so long to get back to this. Just back from a holiday (and have spent the 1st week back telling BT that there's an issue in their exchange).

As far as I was aware, if a TCP packet is sent, and no ACK is received, the packet is retransmitted at increasing intervals and after a given system specified time, the connection is closed. I tried this with Telnet while watching the traffic using Wireshark. I connected and then diconnected the phone jack from the ADSL router to simulate a network outage. I then watched the TCP retransmissions which were at the following intervals before Telnet dropped me back at the prompt
3s, 9s, 21s, 45s, 93

Anyhow, I see what you're saying. I'm guessing that the server attempts to send a packet, and closes the connection

 

by: oobaylyPosted on 2009-07-12 at 05:20:11ID: 24834153

Sorry, I didn't finish that post, pressed enter to attach the file, and it posted the response, grrr.

Anyhow, what I was going to say was this:
While the server has closed the connection as it never received an ACK for a PSH packet that it sent, while the client will sit there for (6 hours?) waiting for the packet to arrive. Am I correct?

Can you see any way of getting around the problems I'm having. The root of these problems is the fact that our ADSL connection has been immensely flakey (which has been isolated to our local Exchange, which I'm hoping will eventually be resolved).

 

by: gheistPosted on 2009-07-12 at 14:13:53ID: 24835859

Implement some sort of keepalive like select count(now()); once in a while on clientside.
more definite solution is removing crap firewall in the middle.

 

by: oobaylyPosted on 2009-07-13 at 02:03:21ID: 24838023

"crap firewall in the middle". Ouch.
I'll accept your opinion due to your rank regarding FreeBSD, but could you expand on that?

 

by: gheistPosted on 2009-07-13 at 03:42:32ID: 24838512

Dump traffic at both ends until problem pops up.
Then compare input and output and go after one who ate packets...

 

by: oobaylyPosted on 2009-07-13 at 04:04:24ID: 24838600

Yeah, I'd been doing that on the firewall's WAN & LAN interfaces, but not on the remote server. BT now seems to have sorted the issue out (touch wood), so it could take some time to get a result.

However, what I had meant was can you expand on the "crap firewall" comment?

 

by: gheistPosted on 2009-07-13 at 05:13:33ID: 24838924

That it drops states and messes DB connection up. It is then .net programming to establish keep-alive (not my call)

20120131-EE-VQP-002

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