Port / TCP
6667IRC (common)
The most widely used IRC port in practice, despite 194 being the registered assignment.
If it is internet-facing: Port 6667 is legitimate in the right place, but exposed carelessly it widens the attack surface noticeably.
Exposure
Behind a gateway
Transport
Cleartext
Risk if public
Elevated
Exposure verdict
Port 6667 should sit behind a reverse proxy, VPN, or source allow-list rather than being forwarded at the perimeter. Publishing it directly puts an administrative or internal interface on the open internet.
Traffic crosses the network readable and modifiable. Anything sensitive on this port needs a tunnel, a VPN, or a different protocol.
Registered with IANA on request, but bindable by any unprivileged user. Registration records intent — it does not reserve the port, so collisions between products are common.
Grouped with the other messaging & media ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
1 note- Long associated with botnet command-and-control, so outbound 6667 from a server is worth explaining.
Recommended firewall rule
Allow only from 10.0.0.0/8, drop the rest
Swap in the CIDR your administrators or application tier actually come from — the point is that the default answer is no.
ufw
sudo ufw allow from 10.0.0.0/8 to any port 6667 proto tcpiptables
sudo iptables -A INPUT -p tcp -s 10.0.0.0/8 --dport 6667 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 6667 -j DROPIf you do need it open to everyone
sudo ufw allow 6667/tcpWrite down why. An unexplained allow rule outlives the person who added it, and the next reviewer has no way to tell a deliberate exception from an accident.
Check what is really there
Listening locally
sudo ss -tlpn 'sport = :6667'The bind address is the answer that matters: 127.0.0.1 is local-only, 0.0.0.0 and :: mean every interface.
Listening (macOS)
sudo lsof -nP -iTCP:6667 -sTCP:LISTENmacOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sV -Pn -p 6667 <host>A version probe confirms whether IRC (common) is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vz <host> 6667The quick yes/no when you only need to know whether a path exists through the firewall.
The safer path
port 6697Move to port 6697 — IRC over TLS
IRC over TLS does the same job with the session protected from the first byte, so there is no plaintext phase for an on-path attacker to read or strip. Migrating is usually a client configuration change plus a firewall swap.