link-local IPv6 addresses need a '%interface' specification to be useful

setup/functions.sh

@@ -85,6 +85,9 @@ function get_default_privateip {
 	# Return the IP address of the network interface connected
 	# to the Internet.
 	#
+	# Pass '4' or '6' as an argument to this function to specify
+	# what type of address to get (IPv4, IPv6).
+	#
 	# We used to use `hostname -I` and then filter for either
 	# IPv4 or IPv6 addresses. However if there are multiple
 	# network interfaces on the machine, not all may be for
@@ -99,11 +102,16 @@ function get_default_privateip {
 	# assigned to an interface. `ip route get` reports the
 	# preferred. That's good enough for us. See issue #121.
 	#
+	# With IPv6, the best route may be via an interface that
+	# only has a link-local address (fe80::*). These addresses
+	# are only unique to an interface and so need an explicit
+	# interface specification in order to use them with bind().
+	# In these cases, we append "%interface" to the address.
+	# See the Notes section in the man page for getaddrinfo and
+	# https://discourse.mailinabox.email/t/update-broke-mailinabox/34/9.
+	#
 	# Also see ae67409603c49b7fa73c227449264ddd10aae6a9 and
 	# issue #3 for why/how we originally added IPv6.
-	#
-	# Pass '4' or '6' as an argument to this function to specify
-	# what type of address to get (IPv4, IPv6).
 
 	target=8.8.8.8
 
@@ -112,9 +120,21 @@ function get_default_privateip {
 	# as it's an address on the public Internet.
 	if [ "$1" == "6" ]; then target=2001:4860:4860::8888; fi
 
-	ip -$1 -o route get $target \
-		| grep -v unreachable \
-		| sed "s/.* src \([^ ]*\).*/\1/"
+	# Get the route information.
+	route=$(ip -$1 -o route get $target | grep -v unreachable)
+
+	# Parse the address out of the route information.
+	address=$(echo $route | sed "s/.* src \([^ ]*\).*/\1/")
+
+	if [[ "$1" == "6" && $address == fe80:* ]]; then
+		# For IPv6 link-local addresses, parse the interface out
+		# of the route information and append it with a '%'.
+		interface=$(echo $route | sed "s/.* dev \([^ ]*\).*/\1/")
+		address=$address%$interface
+	fi
+
+	echo $address
+		
 }
 
 function ufw_allow {