Internal dhcp unable to acquire IP from certain routers, breaking network connection for many users

[Mleman <mleman1217 posteo org>]

NetworkManager 1.10 introduced a breaking change by making dhcp=internal 
the default instead of dhclient. Many distros pushed out NM 1.10 
obviously without being aware of that change. Lots of users are 
reporting issues with their wifi connection etc. in support forums and 
lots of guesswork is happening about what is the cause.

For example, Manjaro pushed 1.10 a few weeks ago then recommended 
downgrading to 1.8 after users reporting issues. They pushed a few 
distro upgrades since then without solving the problem for affected 
users. Manjaro by default installs package dhclient and an empty 
Networkmanager.conf so package upgrade of networkmanager changes the 
DHCP client mechanism to internal. I have been affected as well. NM 1.10 
is not able to obtain an IP from my Linksys LRT224 router using 
dhcp=internal, however after a few weeks on a downgraded networkmanager 
I found out that setting dhcp=dhclient fixes the issue.

I am sure there has been put serious consideration of switching default 
to dhcp=internal for NM. However, according to the noise in distro 
support forums there may be quite a number of users affected by this 
issue. Others may notice this only on occasion e.g. when trying to 
connect to a public wifi somewhere. Unfortunately I cannot give a list 
of DHCP server implementations/routers that do not play well with NM 
internal DHCP client, except of my setup with that particular Linksys 
model.

I noticed that the change is not mentioned in 1.10 release notes and 
strongly suggest amending them accordingly. I could only verify the 
change by comparing man pages of 1.8 vs. 1.10.

As for the general issue of not properly working internal DHCP client: 
for me the only notable symptom is simple a timeout after 45sec when 
waiting for an IP. This happens on different machines with different 
network hardware and on wifi as well as on LAN. I have put my logs on 
pastebin here: https://pastebin.com/rQQbUrkv