Port 1883 MQTT · TCP

What runs on port 1883

MQTT: the publish/subscribe protocol of IoT and home automation. Devices publish to topics on a broker; subscribers receive. Port 1883 is plaintext; 8883 is MQTT over TLS, and WebSocket variants usually sit on 9001 or 8083. A broker with "allow_anonymous true" exposed to the internet lets anyone read every sensor and control every device, which Shodan shows thousands of.

Software that uses it: Mosquitto · EMQX · HiveMQ · VerneMQ · Home Assistant · Tasmota/ESPHome devices · AWS IoT Core

Should port 1883 be open?

Private network or VPN only. Nothing good comes from this port being reachable from the internet. Keep it on an internal network, reach it over a VPN or an SSH tunnel, and firewall it from everything that does not need it.

• mosquitto_sub -h broker -t "#" -v shows everything on the broker; if that works without a password, fix it.
• Use 8883 with TLS and per-device credentials for anything leaving the LAN.

Check port 1883

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :1883'
sudo lsof -i :1883

# from another machine: is it reachable?
nc -zv host 1883
nmap -p 1883 host
curl -v telnet://host:1883      # 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 1883

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 1883 proto tcp
sudo ufw deny 1883/tcp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 1883 accept
nft add rule inet filter input tcp dport 1883 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 1883 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 1883 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=1883 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 1883" with no source is just "open".

Related ports

• 8883 MQTT over TLS
• 9001 Tor ORPort / Supervisor

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