Port 53 DNS · TCP and UDP

What runs on port 53

DNS, the name system every connection starts with. Queries normally use UDP; TCP is used when an answer does not fit (large DNSSEC responses), for zone transfers (AXFR) and increasingly by resolvers that prefer it. An authoritative server must answer the world on both; a recursive resolver must not, because open resolvers are abused for amplification attacks. Encrypted variants are DNS over TLS on 853 and DNS over HTTPS on 443.

Software that uses it: BIND · Unbound · Knot · PowerDNS · CoreDNS · dnsmasq · systemd-resolved

Should port 53 be open?

Designed to face the internet. This port exists to be reachable from anywhere, so the usual question is not whether to open it but how the service behind it is configured and patched.

• Allow TCP 53 as well as UDP; blocking it breaks DNSSEC and large answers.
• Recursive resolvers: listen on internal interfaces only or use an allow-list.
• dig +tcp and dig +short are the quickest checks.

Check port 53

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :53'
ss -lunp 'sport = :53'
sudo lsof -i :53

# from another machine: is it reachable?
nc -zv host 53
nmap -p 53 host
nc -zuv host 53
sudo nmap -sU -p 53 host
curl -v telnet://host:53      # 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 53

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 53 proto tcp
sudo ufw allow from 10.0.0.0/8 to any port 53 proto udp
sudo ufw deny 53/tcp
sudo ufw deny 53/udp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 meta l4proto { tcp, udp } th dport 53 accept
nft add rule inet filter input meta l4proto { tcp, udp } th dport 53 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 53 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 53 -j DROP
iptables -A INPUT -p udp --dport 53 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 53 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=53 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 53" with no source is just "open".

Related ports

• 853 DNS over TLS
• 443 HTTPS — Web over TLS; UDP for HTTP/3 (QUIC)
• 5353 mDNS — Bonjour / Avahi discovery

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