I have moved houses, and thereby internet providers. Choosing a 4G connection rather than a landline, I bought a Huawei B310 for my home network. My LAN is unusual in that I run my own DHCP and DNS in order to run all my traffic over a VPN. In my previous home, this all worked perfectly on Virgin Media.
After ensuring everything is on the correct subnet (192.168.1.0/24), I plugged the B310 into my unmanaged switch and very quickly lost access to the web UI. After tracing to my server, and then to the specific service that kicked the router offline, I was very surprised to see it is
isc-dhcp-server (4.3.1-6+deb8u2, running on Debian v8 Jessie). Reproducibly, starting up the daemon causes the router to cease responding to ping after 2-3 seconds.
I have switched off DHCP on the router and this does not cause any problems, but as soon as I switch on my own, the router stops working - it seems to drop all traffic. I have run
nmap -Pn 192.168.1.1 and this returns the expected ports, showing them as 'filtered', but the router does not respond to anything. Shutting off DHCP, unplugging ethernet and plugging it back in brings it back.
My question to you network people is - is this actually possible? If so, how? I don't understand how or why the router would care about DHCP working externally. If it's a setting I've gotten wrong, I could accept that, but my understanding of DHCP is that the server simply waits for a request, and at this point everything connected to the switch had an IP assigned. I'm very confused.