route-update.sh: support hostnames in gateway file via DNS resolution

router/route-update.sh

@@ -68,8 +68,8 @@ MODES OF OPERATION:
   --help, -h      Show this help and exit.
 
 DIRECTORY STRUCTURE:
-  routes.d/GROUP/gateway    Gateway IP(s), one per line. "default" = default gw.
-                            Multiple lines = multipath (per-flow balancing).
+  routes.d/GROUP/gateway    Gateway: IP, hostname, or "default" (one per line).
+                            Hostnames are resolved via dig. Multiple lines = multipath.
   routes.d/GROUP/options    Optional flags, one per line:
                               set-default  Also set system default route via gateway.
   routes.d/GROUP/table      (legacy) Routing table number override.
@@ -137,6 +137,31 @@ resolve_default_gw()
     $1 route show default | awk '/default/ {print $3; exit}'
 }
 
+# Resolve gateway value: "default" → system default, hostname → IP, IP → as-is
+# Usage: resolve_gw VALUE IPCMD
+resolve_gw()
+{
+    local val="$1"
+    local ipcmd="${2:-ip}"
+
+    case "$val" in
+        default)
+            resolve_default_gw "$ipcmd"
+            ;;
+        *[a-zA-Z]*)
+            # Domain name: resolve to IP
+            if [ "$ipcmd" = "ip -6" ] ; then
+                dig +short AAAA "$val" | grep -m1 ':'
+            else
+                dig +short A "$val" | grep -m1 '[0-9]'
+            fi
+            ;;
+        *)
+            echo "$val"
+            ;;
+    esac
+}
+
 # Look up table number: first from rt_tables by name, then from last IP octet
 # If derived from IP, auto-creates entry in rt_tables
 # Usage: lookup_table NAME GATEWAY
@@ -188,9 +213,11 @@ read_group_config()
     gw="" ; table="" ; route_via="" ; opt_set_default=""
 
     if [ -f "$dir/gateway" ] ; then
-        gw=$(grep -v '^#' "$dir/gateway" | grep -m1 .)
-        if [ "$gw" = "default" ] ; then
-            gw=$(resolve_default_gw "$ipcmd")
+        local raw_gw=$(grep -v '^#' "$dir/gateway" | grep -m1 .)
+        gw=$(resolve_gw "$raw_gw" "$ipcmd")
+        if [ -z "$gw" ] ; then
+            echo "[$name] Cannot resolve gateway '$raw_gw', skipping" >&2
+            return 1
         fi
     else
         echo "[$name] No gateway file, skipping" >&2
@@ -220,9 +247,8 @@ read_group_config()
         route_via="table $table"
         local g
         for g in $(grep -v '^#' "$dir/gateway" | grep .) ; do
-            if [ "$g" = "default" ] ; then
-                g=$(resolve_default_gw "$ipcmd")
-            fi
+            g=$(resolve_gw "$g" "$ipcmd")
+            [ -z "$g" ] && continue
             route_via="$route_via nexthop via $g weight 1"
         done
     fi