rc: Teach netwait to wait for DAD

In some configurations, especially in jails, it is possible for the
system to boot so fast that we end up launching daemons while duplicate
address detection is still ongoing.  If that happens, said daemons may
fail to bind to IPv6 addresses, as they are still tentative.  Teach the
netwait service to wait (up to 10 seconds, by default) for the tentative
flag to vanish.

MFC after:	1 week
Reviewed by:	olce
Differential Revision:	https://reviews.freebsd.org/D51889

libexec/rc/rc.conf

@@ -500,6 +500,8 @@ netwait_enable="NO"		# Enable rc.d/netwait (or NO)
 netwait_timeout="60"		# Total number of seconds to perform pings.
 #netwait_if=""			# Wait for active link on each intf in this list.
 netwait_if_timeout="30"		# Total number of seconds to monitor link state.
+netwait_dad="NO"		# Wait for DAD to complete
+netwait_dad_timeout="10"	# Total number of seconds to wait for DAD.
 
 ### Miscellaneous network options: ###
 icmp_bmcastecho="NO"	# respond to broadcast ping packets

libexec/rc/rc.d/netwait

@@ -2,12 +2,14 @@
 #
 # PROVIDE: netwait
 # REQUIRE: devd ipfw pf routing
-# KEYWORD: nojail
 #
-# The netwait script helps handle two situations:
+# The netwait script helps handle three situations:
 #  - Systems with USB or other late-attaching network hardware which
 #    is initialized by devd events.  The script waits for all the
 #    interfaces named in the netwait_if list to appear.
+#  - Systems with IPv6 addresses, especially jails, where we need to
+#    wait for DAD to complete before starting daemons, as they will
+#    otherwise fail to bind to IN6ADDR_ANY.
 #  - Systems with statically-configured IP addresses in rc.conf(5).
 #    The IP addresses in the netwait_ip list are pinged.  The script
 #    waits for any single IP in the list to respond to the ping.  If your
@@ -29,28 +31,36 @@ netwait_start()
 {
 	local ip rc count output link wait_if got_if any_error
 
-	if [ -z "${netwait_if}" ] && [ -z "${netwait_ip}" ]; then
-		err 1 "No interface or IP addresses listed, nothing to wait for"
+	if [ -z "${netwait_if}" ] && [ -z "${netwait_ip}" ] &&
+	   ! checkyesno netwait_dad ; then
+		err 1 "Nothing to wait for"
 	fi
 
-	if [ ${netwait_timeout} -lt 1 ]; then
+	if ! [ "${netwait_if_timeout}" -ge 1 ]; then
+		err 1 "netwait_if_timeout must be >= 1"
+	fi
+	if ! [ "${netwait_dad_timeout}" -ge 1 ]; then
+		err 1 "netwait_dad_timeout must be >= 1"
+	fi
+	if ! [ "${netwait_timeout}" -ge 1 ]; then
 		err 1 "netwait_timeout must be >= 1"
 	fi
 
+	any_error=false
+
 	if [ -n "${netwait_if}" ]; then
-		any_error=0
 		for wait_if in ${netwait_if}; do
 			echo -n "Waiting for ${wait_if}"
 			link=""
-			got_if=0
+			got_if=false
 			count=1
-			# Handle SIGINT (Ctrl-C); force abort of while() loop
+			# Handle SIGINT (Ctrl-C); force abort of while loop
 			trap break SIGINT
 			while [ ${count} -le ${netwait_if_timeout} ]; do
 				if output=`/sbin/ifconfig ${wait_if} 2>/dev/null`; then
-					if [ ${got_if} -eq 0 ]; then
+					if ! ${got_if}; then
 						echo -n ", interface present"
-						got_if=1
+						got_if=true
 					fi
 					link=`expr "${output}" : '.*[[:blank:]]status: \(no carrier\)'`
 					if [ -z "${link}" ]; then
@@ -63,22 +73,45 @@ netwait_start()
 			done
 			# Restore default SIGINT handler
 			trap - SIGINT
-			if [ ${got_if} -eq 0 ]; then
+			if ! ${got_if}; then
 				echo ", wait failed: interface never appeared."
-				any_error=1
+				any_error=true
 			elif [ -n "${link}" ]; then
 				echo ", wait failed: interface still has no link."
-				any_error=1
+				any_error=true
 			fi
 		done
-		if [ ${any_error} -eq 1 ]; then
-		    warn "Continuing with startup, but be aware you may not have "
-		    warn "a fully functional networking layer at this point."
-		fi
 	fi
 	
+	if checkyesno netwait_dad; then
+		got_dad=false
+		# Handle SIGINT (Ctrl-C); force abort of while loop
+		trap break SIGINT
+
+		echo -n "Waiting for DAD to complete"
+		count=1
+		while [ ${count} -le ${netwait_dad_timeout} ]; do
+			if ! ifconfig | grep -q 'inet6.*tentative'; then
+				echo ', done.'
+				got_dad=true
+				break
+			fi
+			sleep 1
+			count=$((count+1))
+		done
+
+		# Restore default SIGINT handler
+		trap - SIGINT
+
+		if ! ${got_dad}; then
+			echo ', timed out.'
+			any_error=true
+		fi
+	fi
+
 	if [ -n "${netwait_ip}" ]; then
-		# Handle SIGINT (Ctrl-C); force abort of for() loop
+		got_ip=false
+		# Handle SIGINT (Ctrl-C); force abort of for loop
 		trap break SIGINT
 
 		for ip in ${netwait_ip}; do
@@ -90,11 +123,9 @@ netwait_start()
 				rc=$?
 
 				if [ $rc -eq 0 ]; then
-					# Restore default SIGINT handler
-					trap - SIGINT
-
 					echo ', got response.'
-					return
+					got_ip=false
+					break 2
 				fi
 				count=$((count+1))
 			done
@@ -104,10 +135,15 @@ netwait_start()
 		# Restore default SIGINT handler
 		trap - SIGINT
 
-		warn "Exhausted IP list.  Continuing with startup, but be aware you may"
-		warn "not have a fully functional networking layer at this point."
+		if ! ${got_ip}; then
+			any_error=true
+		fi
 	fi
 
+	if ${any_error}; then
+		warn "Continuing with startup, but be aware you may not have "
+		warn "a fully functional networking layer at this point."
+	fi
 }
 
 load_rc_config $name

share/man/man5/rc.conf.5

@@ -22,7 +22,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd September 25, 2025
+.Dd October 2, 2025
 .Dt RC.CONF 5
 .Os
 .Sh NAME
@@ -4563,20 +4563,16 @@ If set to
 .Dq Li YES ,
 delays the start of network-reliant services until
 .Va netwait_if
-is up and ICMP packets to a destination defined in
+is up, duplicate address discovery (DAD) has completed, and ICMP
+packets to a destination defined in
 .Va netwait_ip
 are flowing.
-Link state is examined first, followed by
+Link state is examined first, followed by DAD, then
 .Dq Li pinging
 an IP address to verify network usability.
 If no destination can be reached or timeouts are exceeded,
 network services are started anyway with no guarantee that
 the network is usable.
-Use of this variable requires both
-.Va netwait_ip
-and
-.Va netwait_if
-to be set.
 .It Va netwait_ip
 .Pq Vt str
 Empty by default.
@@ -4612,6 +4608,18 @@ interface if desired.
 Defines the total number of seconds to wait for link to become usable,
 polled at a 1-second interval.
 The default is 30.
+.It Va netwait_dad
+.Pq Vt str
+Set to
+.Dq Li NO
+by default.
+Set to
+.Dq Li YES
+to enable waiting for DAD to complete.
+.It Va netwait_dad_timeout
+.Pq Vt int
+Indicates the total number of seconds to wait for DAD to complete.
+The default is 10.
 .It Va rctl_enable
 .Pq Vt bool
 If set to