board/common: adjust metric on IPv4LL and DHCP routes to match Frr

Apply a distance adjustment to routes retrieved from IPv4LL/ZeroConf and
DHCP to ensure that Frr (Zebra) can compare other protocol routes, e.g.,
static.  Without this Frr will consider all routes read from the kernel
to have a protocol distance of 0, meaning better than static routes that
have a distance of 1.

Initial proposal to fix issue #640

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

board/common/rootfs/etc/avahi/avahi-autoipd.action

@@ -7,6 +7,10 @@
 #          STOP:     The daemon is terminating
 #   $2 interface name
 #   $3 IP adddress
+#
+# For details on the distance adjustment below, please see the following
+# section in the Frr documentation on distance + prefence => to metric.
+# https://docs.frrouting.org/en/stable-8.0/zebra.html#administrative-distance
 
 PATH="$PATH:/usr/bin:/usr/sbin:/bin:/sbin"
 
@@ -15,9 +19,14 @@ log()
     logger -I $$ -t zeroconf -p user.notice "$*"
 }
 
-# Use a different metric for each interface, so that we can set
-# identical routes to multiple interfaces.
-METRIC=$((1000 + `cat "/sys/class/net/$2/ifindex" 2>/dev/null || echo 0`))
+# Use a different metric for each interface as tie breaker, to ensure we
+# can set identical routes to multiple interfaces.
+IFINDEX=$(cat "/sys/class/net/$2/ifindex" 2>/dev/null || echo 0)
+
+# Distance adjustment to ensure that static routes set by Zebra and any
+# routes learned from DHCP take precedence over these interface routes.
+# A distance of 255 ensures these routes are never redistributed.
+METRIC=$(((255 << 24) + 1000 + IFINDEX))
 
 case "$1" in
     BIND)

board/common/rootfs/usr/share/udhcpc/default.script

@@ -1,5 +1,9 @@
 #!/bin/sh
 # This script expect a system with resolvconf (openresolv) and iproute2
+#
+# For details on the distance adjustment below, please see the following
+# section in the Frr documentation on distance + prefence => to metric.
+# https://docs.frrouting.org/en/stable-8.0/zebra.html#administrative-distance
 
 [ -z "$1" ] && echo "Error: should be called from udhcpc" && exit 1
 
@@ -11,6 +15,10 @@ NTPFILE="/run/chrony/dhcp-sources.d/${interface}.sources"
 [ -n "$subnet" ] || subnet=32
 [ -n "$metric" ] || metric=100
 
+# Distance adjustment to ensure that static routes set by Zebra
+# can take precedence over routes learned from DHCP.
+metric=$(((5 << 24) + metric))
+
 # Handle stateful DHCPv6 like DHCPv4
 [ -n "$ipv6" ] && ip="$ipv6/128"