Using TLS on port 636 for LDAP, often referred to as LDAP over SSL (LDAPS), versus using StartTLS over the standard LDAP port 389, reflects different approaches to securing LDAP communications. Both methods aim to encrypt data in transit to prevent interception or eavesdropping. Note that LDAPS is not specified as part of the protocol, which is (I guess) why some will claim it’s “deprecated”, but that’s irrelevant, since all LDAP servers and clients support it. Here’s why using TLS on port 636 can be considered better than StartTLS on port 389 in some scenarios:

1. Dedicated Secure Channel from the Start

  • Immediate Security Context: With LDAPS (port 636), the security context is established immediately as the connection is initiated. This means that the communication is encrypted from the very beginning. It provides no opportunity for unencrypted data to be transmitted. (This is the main benefit.)
  • Clear Operational Intent: Using a dedicated port for encrypted communications (port 636) makes it clear that the service expects a secure connection from the outset. This can simplify firewall and network policy configurations because the port explicitly denotes secure traffic.

2. Compatibility and Simplicity

  • Simplified Client Configuration: Clients connecting to port 636 know in advance that the connection will be secure, simplifying client configuration since they don’t need to negotiate the upgrade of the connection to a secure one as with StartTLS.
  • Broader Compatibility: Some older LDAP clients or systems may handle LDAPS (port 636) better because they are designed to initiate connections with encryption from the start, rather than upgrading an existing connection as required by StartTLS.

3. Avoids StartTLS Pitfalls

  • Mitigation against Downgrade Attacks: While StartTLS is designed to upgrade a plain connection to a secure one, it could be susceptible to downgrade attacks if improperly configured or if a client does not properly validate the security upgrade process. An attacker could potentially interfere with the StartTLS command to prevent encryption, though this requires specific vulnerabilities or misconfigurations to be exploitable.
  • Consistent Security Posture: By using LDAPS, organizations ensure that all data is encrypted by default without relying on the client to initiate a StartTLS command. This reduces the risk of accidental unencrypted communications due to client misconfigurations or software limitations.


Choosing between LDAPS (port 636) and StartTLS (port 389) depends on specific organizational needs, infrastructure, and compatibility requirements. LDAPS offers a straightforward, secure-by-default approach that may be preferable in environments where simplicity and guaranteed encryption from the start of the connection are priorities. However, it’s important to note that modern security practices recommend using StartTLS where possible due to its flexibility and because it aligns with the modern approach of upgrading connections to secure channels. Ultimately, ensuring that the LDAP service and clients are correctly configured and up-to-date is key to securing LDAP communications, regardless of the method used.