Port 43 (TCP) — WHOIS: what it is, how to check

Port 43 WHOIS · TCP

WHOIS: a plaintext query/response protocol for domain, IP block and AS registration data.

Port	43
Transport	TCP
Service	WHOIS
Category	Infrastructure
Range	well-known (0–1023, needs root to bind on Unix)
Encrypted	No, plaintext
Exposure	Designed to face the internet

What runs on port 43

WHOIS: a plaintext query/response protocol for domain, IP block and AS registration data. The whois command connects to the registry's server on port 43, which often refers you to the registrar's server. RDAP over HTTPS is the JSON-based replacement and is now mandatory for gTLDs.

Software that uses it: whois (client) · registry/registrar WHOIS servers · RIR servers (whois.ripe.net, whois.arin.net)

Should port 43 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.

Rate limits are strict; bulk lookups get you blocked.
For IP ownership, query the RIR directly (whois -h whois.ripe.net 1.2.3.4).

Check port 43

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :43'
sudo lsof -i :43

# from another machine: is it reachable?
nc -zv host 43
nmap -p 43 host
curl -v telnet://host:43      # 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 43

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 43 proto tcp
sudo ufw deny 43/tcp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 43 accept
nft add rule inet filter input tcp dport 43 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 43 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 43 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=43 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 43" 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 43 TCP or UDP?

TCP. WHOIS is a connection-oriented protocol; there is no UDP service on 43 by default (a scanner reporting UDP 43 open is usually seeing a filtered port).

Is port 43 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 43 is open?

On the server, ss -ltnp (or -lunp for UDP) shows whether something listens on 43 and which process. From another machine, nc -zv host 43 or nmap -p 43 host tests reachability. "Open" in a scan means something answered; a firewall usually makes the port filtered (no answer) rather than closed (RST).

Can WHOIS 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.