Bug 241792

Summary: dhclient/dhclient-script: dhclient default route not working when given a /32 netmask
Product: Base System Reporter: sigsys
Component: binAssignee: freebsd-bugs mailing list <bugs>
Status: Open ---    
Severity: Affects Many People CC: bz, dch, emaste
Priority: --- Keywords: patch
Version: 12.1-RELEASE   
Hardware: Any   
OS: Any   

Description sigsys 2019-11-07 21:44:40 UTC
There's a certain VM where autoconfiguration via DHCP works with Linux but not FreeBSD.  I have no information on how this VM is setup apart that it's hosted with KVM/QEMU.

The DHCP server sends a lease with a /32 netmask.  This makes adding the default route fail since it is not reachable via any interface.  Linux's dhclient-script seem to usually have a special case for that and explicitly adds an interface route to the router's address.

FreeBSD's dhclient-script has a special case for when the router address is the same as the leased address, but not when it's a different address that doesn't fall in the interface's subnet.

With this change, DHCP just works on this VM:

Index: sbin/dhclient/dhclient-script
===================================================================
--- sbin/dhclient/dhclient-script	(revision 354408)
+++ sbin/dhclient/dhclient-script	(working copy)
@@ -173,6 +173,9 @@
			if [ "$new_ip_address" = "$router" ]; then
				route add default -iface $router >/dev/null 2>&1
			else
+				if [ "$new_subnet_mask" = "255.255.255.255" ]; then
+					route add "$router" -iface "$interface" >/dev/null 2>&1
+				fi
				route add default $router >/dev/null 2>&1
			fi
		fi
Comment 1 Dave Cottlehuber freebsd_committer 2019-11-28 23:45:20 UTC
I have seen this in Google Cloud, Packet.net and other infrastructures. In the first case Google patched their DHCP setup to accommodate this. In the past
I have manually overridden this using settings in dhclient.conf.

Patch seems reasonable to me.
Comment 2 Bjoern A. Zeeb freebsd_committer 2019-11-29 00:47:35 UTC
I thought Linux used to simply arp for an off-link gateway and then use it.

I've seen this elsewhere as well with recovery images and documentation, e.g., https://wiki.hetzner.de/index.php/Cloud_IP_static#FreeBSD for static configuration.

While I don't do much IPv4 anymore, it seems a proper workaround.  +1 on the idea of the change (not reviewed).