feat(netconf): add input validation, 'b' to go back, 'a' to abort

- All prompts accept 'a' = abort, 'b' = back to previous step
- Interface input: validate numeric range and name existence, re-prompt on bad input
- IP address: regex check x.x.x.x/prefix format
- Gateway: regex check x.x.x.x format
- Main loop: 'b' at mode selection goes back to interface list

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/overlay/usr/local/bin/netconf

@@ -1,10 +1,50 @@
 #!/bin/sh
 # Quick network configurator for the local console.
+# Type 'a' at any prompt to abort, 'b' to go back.
 set -e
 
-# List interfaces (exclude lo)
+abort() { echo "Aborted."; exit 0; }
-IFACES=$(ip -o link show | awk -F': ' '$2 != "lo" {print $2}' | cut -d@ -f1)
 
+ask() {
+    # ask VARNAME "prompt" [default]
+    # Sets VARNAME. Returns 1 on 'b' (back), calls abort on 'a'.
+    _var="$1"; _prompt="$2"; _default="$3"
+    while true; do
+        if [ -n "$_default" ]; then
+            printf "%s [%s] (b=back a=abort): " "$_prompt" "$_default"
+        else
+            printf "%s (b=back a=abort): " "$_prompt"
+        fi
+        read _input
+        case "$_input" in
+            a|A) abort ;;
+            b|B) return 1 ;;
+            "")
+                if [ -n "$_default" ]; then
+                    eval "$_var=\"\$_default\""
+                    return 0
+                else
+                    echo "  Required — please enter a value."
+                fi
+                ;;
+            *)
+                eval "$_var=\"\$_input\""
+                return 0
+                ;;
+        esac
+    done
+}
+
+# ── Step 1: choose interface ───────────────────────────────────────────────────
+
+choose_iface() {
+    IFACES=$(ip -o link show | awk -F': ' '$2 != "lo" {print $2}' | cut -d@ -f1)
+    if [ -z "$IFACES" ]; then
+        echo "No network interfaces found."
+        exit 1
+    fi
+
+    echo ""
     echo "Interfaces:"
     i=1
     for iface in $IFACES; do
@@ -13,49 +53,111 @@ for iface in $IFACES; do
         i=$((i+1))
     done
     echo ""
-printf "Interface number or name [Enter = first]: "
-read INPUT
 
-if [ -z "$INPUT" ]; then
+    FIRST=$(echo "$IFACES" | head -1)
-    IFACE=$(echo "$IFACES" | head -1)
+    while true; do
-elif echo "$INPUT" | grep -qE '^[0-9]+$'; then
+        printf "Interface number or name [%s] (a=abort): " "$FIRST"
-    # Numeric input — resolve to name
+        read INPUT
+        case "$INPUT" in
+            a|A) abort ;;
+            "")
+                IFACE="$FIRST"
+                break
+                ;;
+            *)
+                if echo "$INPUT" | grep -qE '^[0-9]+$'; then
                     IFACE=$(echo "$IFACES" | awk "NR==$INPUT")
                     if [ -z "$IFACE" ]; then
-        echo "Error: no interface with number $INPUT"
+                        echo "  No interface #$INPUT — try again."
-        exit 1
+                        continue
                     fi
                 else
+                    # Validate name exists
+                    if ! echo "$IFACES" | grep -qx "$INPUT"; then
+                        echo "  Unknown interface '$INPUT' — try again."
+                        continue
+                    fi
                     IFACE="$INPUT"
                 fi
-
+                break
+                ;;
+        esac
+    done
     echo "Selected: $IFACE"
+}
+
+# ── Step 2: choose mode ────────────────────────────────────────────────────────
+
+choose_mode() {
     echo ""
     echo "  1) DHCP"
-echo "  2) Static"
+    echo "  2) Static IP"
-printf "Mode [1]: "
+    echo ""
-read MODE
+    while true; do
-MODE=${MODE:-1}
+        printf "Mode [1] (b=back a=abort): "
+        read INPUT
+        case "$INPUT" in
+            a|A) abort ;;
+            b|B) return 1 ;;
+            ""|1) MODE=dhcp; break ;;
+            2)    MODE=static; break ;;
+            *) echo "  Enter 1 or 2." ;;
+        esac
+    done
+}
 
-if [ "$MODE" = "1" ]; then
+# ── Step 3a: DHCP ─────────────────────────────────────────────────────────────
+
+run_dhcp() {
     echo "Running DHCP on $IFACE..."
     dhclient -v "$IFACE"
-else
+}
-    printf "IP address (e.g. 192.168.1.100/24): "
+
-    read ADDR
+# ── Step 3b: Static ───────────────────────────────────────────────────────────
-    printf "Gateway (e.g. 192.168.1.1): "
+
-    read GW
+run_static() {
-    printf "DNS [8.8.8.8]: "
+    while true; do
-    read DNS
+        ask ADDR "IP address (e.g. 192.168.1.100/24)" || return 1
-    DNS=${DNS:-8.8.8.8}
+        # Basic format check: must contain a dot and a /
+        if ! echo "$ADDR" | grep -qE '^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+/[0-9]+$'; then
+            echo "  Invalid format — use x.x.x.x/prefix (e.g. 192.168.1.10/24)."
+            continue
+        fi
+        break
+    done
+
+    while true; do
+        ask GW "Gateway (e.g. 192.168.1.1)" || return 1
+        if ! echo "$GW" | grep -qE '^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$'; then
+            echo "  Invalid IP address."
+            continue
+        fi
+        break
+    done
+
+    ask DNS "DNS server" "8.8.8.8" || return 1
 
     ip addr flush dev "$IFACE"
     ip addr add "$ADDR" dev "$IFACE"
     ip link set "$IFACE" up
-    ip route add default via "$GW"
+    ip route add default via "$GW" 2>/dev/null || true
     echo "nameserver $DNS" > /etc/resolv.conf
     echo "Done."
+}
+
+# ── Main loop ─────────────────────────────────────────────────────────────────
+
+choose_iface
+
+while true; do
+    choose_mode || { choose_iface; continue; }
+
+    if [ "$MODE" = "dhcp" ]; then
+        run_dhcp && break
+    else
+        run_static && break || continue
     fi
+done
 
 echo ""
 ip -4 addr show "$IFACE"