Port 8883 (TCP) — MQTT over TLS: what it is, how to check

Port 8883 MQTT over TLS · TCP

MQTT over TLS: the port to use for any MQTT traffic that leaves the LAN.

Port	8883
Transport	TCP
Service	MQTT over TLS
Category	IoT and home automation
Range	registered (1024–49151)
Encrypted	Yes, always (TLS or equivalent)
Exposure	Internet-facing only with authentication and care

What runs on port 8883

MQTT over TLS: the port to use for any MQTT traffic that leaves the LAN. Cloud brokers (AWS IoT, Azure IoT Hub, HiveMQ Cloud) accept only this (or WebSocket on 443), usually with client certificates or per-device tokens.

Software that uses it: Mosquitto · EMQX · HiveMQ · AWS IoT Core, Azure IoT Hub · Tasmota/ESPHome (TLS builds)

Should port 8883 be open?

Internet-facing only with authentication and care. It can be exposed when the service behind it is authenticated, rate-limited and kept up to date; most deployments are better off behind a VPN, an allow-list or a reverse proxy.

mosquitto_pub -h broker -p 8883 --cafile ca.crt -u user -P pass -t test -m hi is the test; certificate name mismatch is the usual failure.

Check port 8883

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :8883'
sudo lsof -i :8883

# from another machine: is it reachable?
nc -zv host 8883
nmap -p 8883 host
curl -v telnet://host:8883      # 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 8883

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 8883 proto tcp
sudo ufw deny 8883/tcp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 8883 accept
nft add rule inet filter input tcp dport 8883 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 8883 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 8883 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=8883 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 8883" with no source is just "open".

Related ports

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

Is port 8883 TCP or UDP?

TCP. MQTT over TLS is a connection-oriented protocol; there is no UDP service on 8883 by default (a scanner reporting UDP 8883 open is usually seeing a filtered port).

Is port 8883 encrypted?

Yes, always (TLS or equivalent). The port number does not make it safe by itself: certificate validation, authentication and patching still decide.

How do I check whether port 8883 is open?

On the server, ss -ltnp (or -lunp for UDP) shows whether something listens on 8883 and which process. From another machine, nc -zv host 8883 or nmap -p 8883 host tests reachability. "Open" in a scan means something answered; a firewall usually makes the port filtered (no answer) rather than closed (RST).

Can MQTT over TLS run on a different port?

Yes; the number is a convention, not a requirement. Changing it reduces log noise from scanners but does not add security, and every client, firewall rule and monitoring check then needs the new number. Keep the default unless you have a concrete reason.