Port 636 LDAPS · TCP

What runs on port 636

LDAPS: LDAP over implicit TLS. Required by many applications for binding with credentials, and by Windows for password changes via LDAP. Certificate validation is the usual stumbling block: the server certificate must match the hostname the client uses, and the client must trust the CA.

Software that uses it: Active Directory · OpenLDAP · FreeIPA · Samba AD

Should port 636 be open?

Private network or VPN only. Nothing good comes from this port being reachable from the internet. Keep it on an internal network, reach it over a VPN or an SSH tunnel, and firewall it from everything that does not need it.

• openssl s_client -connect dc:636 shows the certificate the clients will see.
• ldapsearch -H ldaps://dc:636 with LDAPTLS_CACERT set to the CA file is the standard test.

Check port 636

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :636'
sudo lsof -i :636

# from another machine: is it reachable?
nc -zv host 636
nmap -p 636 host
curl -v telnet://host:636      # 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 636

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 636 proto tcp
sudo ufw deny 636/tcp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 636 accept
nft add rule inet filter input tcp dport 636 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 636 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 636 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=636 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 636" with no source is just "open".

Related ports

• 389 LDAP — Directory services; plaintext/STARTTLS
• 88 Kerberos — Authentication (Active Directory)

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