Port 53: DNS

Domain name lookups; UDP for most queries, TCP for large answers and zone transfers. An open resolver answering anyone on the internet gets used for DNS amplification floods. Authoritative servers must be public; resolvers should only answer your own network.

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Port Lookup

Port 53 at a glance

Port53
ProtocolTCP/UDP
ServiceDNS
Open to the Internet?Only to addresses that need it
See What Is Listeningss -lntup | grep ':53 '
Test From Outsidenc -zv your-server 53
Block with UFWufw deny 53/tcp; ufw deny 53/udp
Block with nftablesnft add rule inet filter input meta l4proto { tcp, udp } th dport 53 drop

DNS uses TCP/UDP port 53. Domain name lookups; UDP for most queries, TCP for large answers and zone transfers.

An open resolver answering anyone on the internet gets used for DNS amplification floods. Authoritative servers must be public; resolvers should only answer your own network.

Allow port 53 only from the addresses that need it: your office, a VPN or a bastion host. The firewall rule generator builds those rules for nftables, iptables and UFW.

Other Ports

20 FTP Data21 FTP22 SSH23 Telnet25 SMTP53 DNS67 DHCP Server80 HTTP110 POP3123 NTP143 IMAP161 SNMP389 LDAP443 HTTPS445 SMB465 SMTPS514 Syslog587 Mail Submission853 DNS over TLS993 IMAPS995 POP3S1194 OpenVPN1433 Microsoft SQL Server1900 SSDP / UPnP2049 NFS2083 cPanel (SSL)2087 WHM (SSL)3306 MySQL / MariaDB3389 RDP3478 STUN / TURN5060 SIP5432 PostgreSQL5900 VNC6379 Redis8006 Proxmox VE8080 HTTP Alternate8443 HTTPS Alternate11211 Memcached25565 Minecraft27017 MongoDB51820 WireGuard

Questions

What is port 53 used for?
DNS. Domain name lookups; UDP for most queries, TCP for large answers and zone transfers.
Is it safe to open port 53?
Only to addresses that need it. An open resolver answering anyone on the internet gets used for DNS amplification floods. Authoritative servers must be public; resolvers should only answer your own network.
How do I check if port 53 is open?
On the server, ss -lntup | grep ':53 ' shows what is listening. From another machine, nc -zv your-server 53 shows whether the firewall lets it through.
How do I block port 53?
With UFW: ufw deny 53/tcp and ufw deny 53/udp. With nftables, add a drop rule for dport 53 in your input chain.

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