[GTALUG] Rogers dhcp mystery

[D. Hugh Redelmeier]

My internet connection stopped working.  After some hunting, and some 
pecking, it works again.  But I'm not sure why.

Setup:

- Rogers Cable

- modem is set in bridge mode (i.e. it is not acting as a router and does 
  not do Network Address Translation).

- my router is a little PC with two ethernet ports, running the latest 
  CentOS 7.

- I'm using Network Manager.

What went wrong?

My router got an RFC 1918 IP address, 192.168.100.100.  So it looked
as if the modem forgot it was in bridge mode and started acting like a
router.  I don't know if it was doing NATing, but it certainly acted
as a DHCP server, something it should not have done.  I have no idea
what precipitated this.

I logged into the modem and it said that it was in bridge mode.  (I had 
rebooted it a couple of times but with no obvious benefit).

Connecting to the modem's web page was easy since the PC had been
given an IP address on the same LAN.  Normally I'd have to add the
subnet onto the interface (something that I've not done done in a
long time).

I don't exactly know why my router couldn't work behind NAT.  Double
NAT is dumb but I think that it should work.  Perhaps NATing was not
being done.  If not, packets from 192.168.100.100 were going nowhere.

I rebooted my PC, hoping to lose the 192.168.100.100 IP address, but
it stuck.  This was one of many false starts -- I don't even remember
all the details.

I tried this command and my PC got the proper address:

	sudo nmcli c up WAN

- c is short for "connection"

- "up" means "Activate a device with a connection".  I don't know what
  that means.  Maybe it released the lease and acquired a new one.
  Maybe not.

- WAN is the name of my Rogers-facing interface

- I previously found the name of my connections with this command:

	sudo nmcli c show

I don't actually know for sure that this command fixed by problem or
something else did.

I wanted to tell dhclient to release its leases and acquire a new one.
The -r flag should do that.  But dhclient is being run by
NetworkManager and I didn't know how to properly do a -r.

Observation: Network Manager mostly works but it makes it hard for me
to apply my bottom-up knowledge of networking.