Check IPv4 and subnet server side, flash message if these are invalid.

setup/server.py

@@ -10,11 +10,13 @@ import random
 import ipaddress
 import hashlib
 import time
+import secrets
 from flask_bootstrap import StaticCDN
 
 version = os.getenv("this_version", "master")
 static_url_path = "/" + version + "/static"
 app = flask.Flask(__name__, static_url_path=static_url_path)
+app.secret_key = secrets.token_hex(16)
 flask_bootstrap.Bootstrap(app)
 # Load our jQuery. Do not use jQuery 1.
 app.extensions['bootstrap']['cdns']['jquery'] = StaticCDN()
@@ -92,12 +94,33 @@ def build_app(path):
     def submit():
         data = flask.request.form.copy()
         data['uid'] = str(uuid.uuid4())
+        valid = True
+        try:
+            ipaddress.ip_address(data['bind4'])
+        except:
+            flask.flash('Configured IPv4 address is invalid', 'error')
+            valid = False
+        try:
+            ipaddress.ip_network(data['subnet'])
+        except:
+            flask.flash('Configured subnet is invalid.', 'error')
+            valid = False
         try:
             data['dns'] = str(ipaddress.IPv4Network(data['subnet'], strict=False)[-2])
         except ValueError as err:
-            return "Error while generating files: " + str(err)
-        db.set(data['uid'], json.dumps(data))
-        return flask.redirect(flask.url_for('.setup', uid=data['uid']))
+            flask.flash('Invalid configuration: ' + str(err))
+            valid = False
+        if valid:
+            db.set(data['uid'], json.dumps(data))
+            return flask.redirect(flask.url_for('.setup', uid=data['uid']))
+        else:
+            return flask.render_template(
+                'wizard.html',
+                flavor="compose",
+                steps=sorted(os.listdir(os.path.join(path, "templates", "steps", "compose"))),
+                subnet6=random_ipv6_subnet()
+            )
+
 
     @prefix_bp.route("/setup/<uid>", methods=["GET"])
     @root_bp.route("/setup/<uid>", methods=["GET"])