Fixing My Network after expanding the LAN

Authored by Edwin Kofler

Published on 2020-2-4

Somewhat recently, I finalized the purchase of some computers and computer parts on Craigslist. I was able to bargain for some cheap discounts - and I wanted to plug these computers into my network. Little did I know that I would be in for a wild ride…

At the time, my LAN did not consist of much: just an old (but still great!) MacBook Pro 2012, a Raspberry Pi 3B+ and a Surface Pro 5, which is my daily driver. The Pi was the only computer on the network with a statically set IP (10.0.0.2) because it was running my DHCP, DDNS, and DNS servers.

When it came to hooking up the new computers to my switch and router, I had no choice to unplug and rearrange all my computers (space is pretty tight in a college dorm room). I had to get creative and hang my router from some hangers…

Photograph of computer setup

So I plugged in everything, and turned all the machines on all at once. Of course, several problems popped up. At the time, I only saw one problem, which eventually caused me to misdiagnose some problems down the line.

Immediately, I ran into problems. I could not ICMP ping my router from my Surface Pro, even though I was able to do so from my Raspberry Pi. Both were connected directly to the router (not the switch). I checked the ARP table, and the mapping (for my router, 10.0.0.1) was in a FAILED state on my Surface, but REACHABLE from my Pi. After testing the CAT 6 ethernet cable (it was still functional), I wasn’t sure what else to do - so i just reset the router, which seemed to work.

But then there was another problem. I was not able to ping my Raspberry Pi from my Surface. I think I was able to ping other hosts from my Raspberry Pi though, but that doesn’t make too much sense now that I think about it. It’s because the Raspberry Pi’s static IP address conflicted with another computer. I had planned to move the DHCP, DDNS, and DNS servers from the Pi to another computer (my trusty Macintosh G5 Tower), and I forgot the operating system I installed onto it had the same IP address. Changing the Pi’s IP to prevent collisions fixed this issue.

But, you guessed it! - there were more problems. These were related to the services I was running on the Pi. The Kea DHCP4 server would no longer start. However, the issue was a simple one to fix. The configuration specified the wrong IP to listen to

{
	"interfaces-config": {
		"interfaces": ["eth0/10.0.0.2"],
		"dhcp-socket-type": "raw"
	}
}

It was no longer 10.0.0.2, but a separate one that did not collide with my Mac G5 Tower. Because specifying the IP address was optional in the first place, I just fixed it to "eth0".

Frustratingly, BIND9 DNS also stopped working, for a similar reason.

include "/usr/local/etc/bind/named.conf.log";

options {
  directory "/usr/local/var/cache/bind";

  listen-on port 53 { 10.0.0.2; };
  listen-on-v6 port 53 { ::1; };
}
...

At first, I changed the IP address to the loopback interface 127.0.0.1. That didn’t work because I forgot the loopback interface is not associated with an actual hardware component that can be connected to from the reset of the network. I changed it to the actual IP address of the Pi (10.0.0.10).

And lastly, there is one more issue that I almost forgot to mention. Inside of some option-data in my Kea DHCP server, it was configured to continue broadcasting the old Domain Name Server IP.

{
	"name": "domain-name-servers",
	"code": 6,
	"space": "dhcp4",
	"csv-format": true,
	"data": "10.0.0.2"
}

Of course, I did not want to give the clients to my DHCP server the wrong IP address, so I had to change that as well.

So in conclusion, after changing my network topology, I faced several issues:

Could not ping my router (router bugged?)
Two devices on the network had the same IP address
DHCP servers were listening on the correct interface, but at the wrong (old) IP address
DNS servers were listening on the wrong (old) IP address
DHCP servers were broadcasting the old IP of the DNS servers

The one key takeaway I learned is that it’s a terrible idea to update / change operating systems (especially multiple at once!) while simultaneously changing the network topology.