Don't warn when the "hostname" rc variable is unset, but the hostname

is already non-empty (common in jails).

etc/rc.d/hostname

@@ -60,9 +60,11 @@ hostname_start()
 	# Have we got a hostname yet?
 	#
 	if [ -z "${hostname}" ]; then
-		# Null hostname is probably OK if DHCP is in use.
+		# Null hostname is probably OK if DHCP is in use,
+		# or when hostname is already set (common for jails).
 		#
-		if [ -z "`list_net_interfaces dhcp`" ]; then
+		if [ -z "`list_net_interfaces dhcp`" -a \
+		     -z "`/bin/hostname`" ]; then
 			warn "\$hostname is not set -- see rc.conf(5)."
 		fi
 		return

share/man/man5/rc.conf.5

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd February 15, 2018
+.Dd March 10, 2018
 .Dt RC.CONF 5
 .Os
 .Sh NAME
@@ -421,6 +421,9 @@ If
 .Xr dhclient 8
 is used to set the hostname via DHCP,
 this variable should be set to an empty string.
+Within a
+.Xr jail 8
+the hostname is generally already set and this variable may absent.
 If this value remains unset when the system is done booting
 your console login will display the default hostname of
 .Dq Amnesiac .