Port 5353 (UDP) — mDNS: what it is, how to check

Port 5353 mDNS · UDP

mDNS (multicast DNS): name resolution for .local names and the base of Bonjour/Avahi service discovery (printers, AirPlay, Chromecast, HomeKit, _ssh._tcp announcements).

Port	5353
Transport	UDP
Service	mDNS — Bonjour / Avahi discovery
Category	Infrastructure
Range	registered (1024–49151)
Encrypted	No, plaintext
Exposure	Private network or VPN only

What runs on port 5353

mDNS (multicast DNS): name resolution for .local names and the base of Bonjour/Avahi service discovery (printers, AirPlay, Chromecast, HomeKit, _ssh._tcp announcements). Link-local by design, multicast to 224.0.0.251. It does not cross routers without a reflector, which is why devices on a separate Wi-Fi VLAN cannot find the Chromecast. Exposed on the internet it is another reflection vector.

Software that uses it: Avahi · Bonjour (macOS/iOS/Windows) · systemd-resolved · Chromecast, AirPlay, HomeKit, printers · Kubernetes/Docker mDNS on dev machines

Should port 5353 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.

avahi-browse -a or dns-sd -B _services._dns-sd._udp lists what is being announced.
For cross-VLAN discovery use an mDNS reflector (Avahi reflector, UniFi mDNS, pfSense Avahi package), not routing.

Check port 5353

# on the host: is anything listening, and which process?
ss -lunp 'sport = :5353'
sudo lsof -i :5353

# from another machine: is it reachable?
nc -zuv host 5353
sudo nmap -sU -p 5353 host

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 5353

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 5353 proto udp
sudo ufw deny 5353/udp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 udp dport 5353 accept
nft add rule inet filter input udp dport 5353 drop

# iptables (legacy)
iptables -A INPUT -p udp --dport 5353 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 5353 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=5353 protocol=udp accept'

Replace 10.0.0.0/8 with the networks that need access. Allowing by source address is the rule that matters; "allow 5353" 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 5353 TCP or UDP?

UDP. mDNS is datagram-based; TCP 5353 is not used by it.

Is port 5353 encrypted?

No, plaintext. Anything sent over it can be read by whoever is on the path, so it belongs on a trusted network or inside a VPN or SSH tunnel.

How do I check whether port 5353 is open?

On the server, ss -ltnp (or -lunp for UDP) shows whether something listens on 5353 and which process. From another machine, nc -zv host 5353 -u or nmap -p 5353 -sU host tests reachability. "Open" in a scan means something answered; a firewall usually makes the port filtered (no answer) rather than closed (RST).

Can mDNS 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.