Port / TCP
636LDAPS
LDAP wrapped in TLS from connect time, used for directory binds and queries that must not cross the network in the clear.
If it is internet-facing: Port 636 is routine where it is expected. Its risk lives in the service configuration behind it, not the port itself.
Exposure
Trusted network only
Transport
Encrypted by default
Risk if public
Low
Exposure verdict
Port 636 belongs on a trusted internal network. If a perimeter scan finds it answering, treat that as a firewall defect rather than a configuration preference.
The session is protected from the first byte, so there is no plaintext phase for an on-path attacker to strip or read.
Assigned by IANA through a review process. On Unix systems a process needs root or the CAP_NET_BIND_SERVICE capability to bind here, which is why containers so often remap these to a higher port.
Grouped with the other directory & auth ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
1 note- Encrypted transport does not make the directory public data — keep it on internal networks.
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 636 proto tcpiptables
sudo iptables -A INPUT -p tcp -s 10.0.0.0/8 --dport 636 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 636 -j DROPIf you do need it open to everyone
sudo ufw allow 636/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 = :636'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:636 -sTCP:LISTENmacOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sV -Pn -p 636 <host>A version probe confirms whether LDAPS is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vz <host> 636The quick yes/no when you only need to know whether a path exists through the firewall.