diff --git a/setup/server.py b/setup/server.py
index 020da4a3..e983c60d 100644
--- a/setup/server.py
+++ b/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"])
diff --git a/setup/templates/base.html b/setup/templates/base.html
index 5576421a..b4b4ad3e 100644
--- a/setup/templates/base.html
+++ b/setup/templates/base.html
@@ -1,5 +1,6 @@
 {% extends "bootstrap/base.html" %}
 {% import "macros.html" as macros %}
+{% from 'bootstrap/utils.html' import flashed_messages %}
 
 {% block title %}Mailu setup{% endblock %}
 
@@ -15,6 +16,7 @@
 
 <div id="container" class="container">
   <h1>Mailu configuration</h1>
+  {{ flashed_messages() }}
   <p>
     Version
     <select id=version_select onchange="window.location.href=this.value;" class="btn btn-primary dropdown-toggle">
diff --git a/setup/templates/steps/compose/03_expose.html b/setup/templates/steps/compose/03_expose.html
index 77b43640..b00ce7d7 100644
--- a/setup/templates/steps/compose/03_expose.html
+++ b/setup/templates/steps/compose/03_expose.html
@@ -16,12 +16,10 @@ avoid generic all-interfaces addresses like <code>0.0.0.0</code> or <code>::</co
 <div class="form-group">
   <label>IPv4 listen address</label>
 <!--   Validates IPv4 address -->
-  <input class="form-control" type="text" name="bind4" value="127.0.0.1"
-  		pattern="^((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$">
+  <input class="form-control" type="text" name="bind4" value="127.0.0.1" >
   <label>Subnet of the docker network. This should not conflict with any networks to which your system is connected. (Internal and external!)
     Fourth number is always 0. Normally the format is '*.*.*.0/24'</label>
-  <input class="form-control" type="text" name="subnet" required pattern="^([0-9]{1,3}\.){3}[0](\/([0-9]|[1-2][0-9]|3[0-2]))$"
-    value="192.168.203.0/24">
+  <input class="form-control" type="text" name="subnet" value="192.168.203.0/24">
 </div>
 
 <div class="form-check form-check-inline">