11.2 dhclient MTU behavior is broken

[Jim Pingle]

The DHCP client, dhclient, in 11.2 was changed to support server-side 
MTU ( https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=206721 ), but 
this MTU feature being enabled by default causes an unexpected change in 
behavior on upgrade, which to me is a POLA violation.

My ISP sends a bogus MTU of 576, and I've seen reports of others that do 
the same. Previously this was ignored since the client didn't support 
processing the MTU, but now it's respected and breaks connectivity since 
the MTU should really be 1500.

There doesn't appear to be any way to override the client behavior 
either. Using a request line that doesn't include interface-mtu doesn't 
help since if the server always sends it, it is still read and 
respected. Any supersede value in dhclient.conf appears to be ignored in 
favor of the server-supplied value.

The link bounces when the MTU is set as well, at least on e1000. I see 
dhclient was changed to help cope with that but it also affects other 
things that key off link up/down events and can lead to a loop depending 
on what those scripts do.

Can this feature be changed so that it's optional and disabled by 
default? Either a command-line option or a dhclient.conf directive would 
work. As-is, it's making the stock dhclient unusable here without 
patching out that feature. I can see people wanting to use that, but it 
shouldn't be on for everyone out of the box.

Jim P.