Port 4200 Angular dev · TCP

What runs on port 4200

The Angular CLI development server (ng serve) by default. Localhost only unless started with --host 0.0.0.0, which exposes a server with hot reload and source maps to the network. Also used by some other tools (Pentaho, Jupyter Enterprise Gateway).

Software that uses it: Angular CLI

Should port 4200 be open?

Loopback or the host's own network only. The service should listen on 127.0.0.1 (or a Unix socket) and be reached by a reverse proxy or by the application on the same host. If it answers on a public address, that is a misconfiguration.

• Use --host only on a trusted LAN, and never in production: ng serve is not a production server.

Check port 4200

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :4200'
sudo lsof -i :4200

# from another machine: is it reachable?
nc -zv host 4200
nmap -p 4200 host
curl -v telnet://host:4200      # raw TCP connect test, Ctrl-C to leave

A port that answers from the internet when it should not is the finding; nmap from a machine outside your network (or a VPS) is the honest test, because scans from inside the LAN pass the firewall you are trying to verify.

Allow or block port 4200

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 4200 proto tcp
sudo ufw deny 4200/tcp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 4200 accept
nft add rule inet filter input tcp dport 4200 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 4200 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 4200 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=4200 protocol=tcp accept'

Replace 10.0.0.0/8 with the networks that need access. Allowing by source address is the rule that matters; "allow 4200" with no source is just "open".

Putting it behind nginx

server {
    listen 443 ssl;
    server_name app.example.com;
    location / {
        proxy_pass http://127.0.0.1:4200;
        proxy_set_header Host $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto https;
    }
}

Then make the application listen on 127.0.0.1:4200 only. Test which location block a request hits with the nginx location tester.

Related ports

• 5173 Vite dev
• 3000 dev servers — Grafana, Rails, Node apps
• 8080 HTTP alt — Tomcat, proxies, dev servers

Part of the port number reference. Ports are 16-bit numbers; see also the subnet calculator and the packet decoder, which shows ports inside TCP and UDP headers.

Questions