Packets on wrong interface

I had an interesting exercise earlier this week. I'm going to simplify things to make it easier to understand, but the basic idea is that the customer had his ISP's equipment installed some 300 feet from where all the computing equipment was. His Kerio router therefore got its WAN connection at the end of a long wire.

That connection was slow, so he recently had a new, higher speed connection put in. He couldn't just switch over, so temporarily he'd need both. Running a new wire from Kerio Control would have been a major project, but fortunately there was one other unused wire running from where the ISP connections were. Unfortunately that connection did not go near the Kerio but instead reached a LAN switch that serviced exactly one piece of equipment and then joined the rest of the network. Would that work, he asked?

I said it would but it could cause some issues. It wasn't entirely clear to me where this switch was and how everything would flow, but as it turned out most things worked and he was able to test out the new connection using load balancing.

But some computers did not work. The problem was obvious when we looked at the logs.


[13/Nov/2015 06:24:00] from WAN2, proto:TCP, len:52, 192.168.141.103:54315 -> 104.25.139.21:80, flags:[ SYN ], seq:3723491087 ack:0, win:8192, tcplen:0
[13/Nov/2015 06:24:03] from WAN2, proto:TCP, len:52, 192.168.141.103:54314 -> 104.25.138.21:80, flags:[ SYN ], seq:1889551147 ack:0, win:8192, tcplen:0
[13/Nov/2015 06:30:54] from LAN Switch, proto:TCP, len:1489, 192.168.130.101:50185 -> 104.25.139.21:80, flags:[ ACK PSH ], seq:1910086646 ack:73727308, win:4096, tcplen:1437
 

Packets from 192.168.x.x should not be "from WAN2"; they should always be "from LAN Switch". A managed switch could make sure that never happens, but he didn't have that ability. What to do?

I really couldn't think of anything. It wasn't really critical as the new circuit is intended to replace the old in a few weeks, but it woould be easier if he could keep this working until then.

Then it hit me. Why on earth was the Kerio router 300 feet from the two ISP drops? I called the customer back and asked him how difficult it would be to move the router to that space and connect it to the ISP's there? There was a short pause and he answered that yes, of course, that was the way to do it and no it would not be difficult.

Sometimes you have to back up and look at the bigger picture.



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