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// Directory & authentication

Port 636: LDAPS

LDAP over TLS.

636

How LDAPS works

Directory & authentication
userdirectory#%&@!x9OK? userdirectory#%&@!x9OK?

The client proves who it is on port 636 and gets an answer (or a ticket) it can use with other services. Everything is encrypted: someone on the path only sees unreadable bytes.

Transport
TCP · UDP
Exposure risk
5/10 moderate if reachable from the Internet
Encryption
EncryptedTLS or built-in crypto
IANA status
Assigned1 registered service
Range
Well-known (0-1023)
Category
Directory & authentication
01 Security

Is port 636 dangerous?

Common attacks and weaknesses

  • Same directory exposure as LDAP if reachable from untrusted networks.
  • Expired or self-signed certificates disabling validation in clients.

How to secure it

  • Restrict to internal networks.
  • Use certificates from an internal PKI and enforce validation.
02 Registry

IANA registry entries for port 636

ServicePortTransportDescriptionReferenceUpdated
ldaps 636 TCP, UDP ldap protocol over TLS/SSL (was sldap) - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 636, and is it reachable?

Linux

sudo ss -ltnup 'sport = :636'      # process listening on 636
sudo lsof -nP -i :636

macOS

sudo lsof -nP -i :636

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 636 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 636
netstat -ano | findstr :636

From another machine

nmap -p 636 -sV <host>
sudo nmap -sU -p 636 <host>
nc -zv <host> 636

Block it in the firewall

sudo ufw deny 636/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 636" -Direction Inbound -LocalPort 636 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 636 used for?

LDAP over TLS.

Is port 636 TCP or UDP?

Port 636 is registered for TCP and UDP. In practice the service mostly uses TCP.

Is it safe to open port 636?

It is rated 5/10 (moderate) when exposed to the Internet. Same directory exposure as LDAP if reachable from untrusted networks. Restrict to internal networks.

How do I check if port 636 is open?

On Linux run "ss -ltnup 'sport = :636'", on macOS "lsof -nP -i :636", and on Windows "netstat -ano | findstr :636" to see the local process. From another machine, "nmap -p 636 <host>" shows whether it is reachable.

05 Related
:389 LDAP tcp · udp · high risk :88 Kerberos tcp · udp · high risk :3268 Global Catalog (LDAP) tcp · udp · high risk :1812 RADIUS udp · tcp · moderate risk :31 MSG Authentication tcp · udp :49 Login Host Protocol (TACACS) tcp · udp :56 XNS Authentication tcp · udp :113 Authentication Service tcp · udp

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