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Selection of Outbound Anonymous TLS Certificates

 

Applies to: Exchange Server 2007 SP3, Exchange Server 2007 SP2, Exchange Server 2007 SP1, Exchange Server 2007

Topic Last Modified: 2007-04-25

The selection of an outbound anonymous Transport Layer Security (TLS) certificate occurs in the following scenarios:

  • Simple Mail Transfer Protocol (SMTP) sessions between Edge Transport servers and Hub Transport servers for authentication
  • SMTP sessions between Hub Transport servers for encryption only by using public keys

For communication between Hub Transport servers, anonymous TLS and the public keys from certificates are used to encrypt the session. But the next authentication is Kerberos authentication. When an SMTP session is established, the receiving server initiates a certificate selection process to determine which certificate to use in the TLS negotiation. The receiving server also performs a certificate selection process. For more information about that process, see Selection of Inbound Anonymous TLS Certificates.

This topic describes the selection process for outbound anonymous TLS certificates. All the steps are performed on the sending server. The following figure shows the steps of this process.

noteNote:
During the initial loading of the certificate, the outbound certificate selection process is different for the Edge Transport server role and the Hub Transport server role. The following figure shows the starting point for each server role.
Selection of an outbound anonymous TLS certificate

  1. When the SMTP session is established from a Hub Transport server or Edge Transport server, Microsoft Exchange calls a process to load the certificates.
  2. The certificate loading process depends on whether the SMTP session is initiated from a Hub Transport server or an Edge Transport server.
    On a Hub Transport server     The following checks are made:
    1. The Send connector to which the session is connected is checked to see whether the SmartHostAuthMechanism property is configured for ExchangeServer. You can set the SmartHostAuthMechanism property on the Send connector by using the Set-SendConnector cmdlet. You can also set the SmartHostAuthMechanism property to ExchangeServer by selecting Exchange Server Authentication on the Configure Smart Host Authentication Settings page of a given Send connector. To open the Configure Smart Host Authentication Settings page, click Change on the Network tab of the Send connector properties page.
    2. The DeliveryType property of the message is checked to determine whether it is set to a value of SmtpRelayWithinAdSitetoEdge. You can view the DeliveryType property by running the Get-Queue cmdlet with the format list argument ( | FL).
      Both of these conditions must be met. If ExchangeServer is not enabled as an authentication mechanism, or if the DeliveryType property is not set to SmtpRelayWithinAdSitetoEdge, the sending Hub Transport server does not use anonymous TLS and no certificate is loaded. If both conditions are met, the certificate selection process continues to step 3.
    On an Edge Transport server     The following checks are made:
    1. The Send connector to which the session is connected is checked to see whether the SmartHostAuthMechanism property is configured for ExchangeServer. As noted earlier in this topic, you can set the SmartHostAuthMechanism property on the Send connector by using the Set-SendConnector cmdlet. You can also set the SmartHostAuthMechanism property to ExchangeServer by selecting Exchange Server Authentication on the Configure Smart Host Authentication Settings page of a given Send connector. To open the Configure Smart Host Authentication Settings page, click Change on the Network tab of the Send connector properties page.
    2. The Send connector to which the session is connected is checked to determine whether the SmartHost address space property contains “- -“.
      Both of these conditions must be met. If ExchangeServer is not enabled as an authentication mechanism and the address space does not contain “- -“, the sending Edge Transport server does not use anonymous TLS and no certificate is loaded. If both conditions are met, the certificate selection process continues to step 3.
  3. Microsoft Exchange queries the Active Directory directory service to retrieve the thumbprint of the certificate on the server. The msExchServerInternalTLSCert attribute on the server object stores the certificate thumbprint.
    If the msExchServerInternalTLSCert attribute cannot be read or if the value is null, Microsoft Exchange does not advertise X-ANONYMOUSTLS in the SMTP session and no certificate is loaded.
    noteNote:
    If the msExchServerInternalTLSCert attribute cannot be read or if the value is null during startup of the Microsoft Exchange Transport service, instead of during the SMTP session, Event ID 12012 is logged in the Application log.
  4. If a thumbprint is found, the certificate selection process searches the local computer certificate store for a certificate that matches the thumbprint. If the certificate is not found, the server does not advertise X-ANONYMOUSTLS, no certificate is loaded, and Event ID 12013 is logged in the Application log.
  5. After a certificate is loaded from the certificate store, it is checked to see whether it has expired. The Valid to field on the certificate is compared to the current date and time. If the certificate has expired, Event ID 12015 is logged in the Application log. But the certificate selection process does not fail and continues with the remaining checks.
  6. The certificate is checked to see whether it is the latest in the local computer’s certificate store. As part of this check, a domain list is built for potential certificate domains. The domain list is based on the following computer configuration:
    • Fully qualified domain name (FQDN), such as mail.contoso.com
    • Host name, such as EdgeServer01
    • Physical FQDN, such as EdgeServer01.contoso.com
    • Physical host name, such as EdgeServer01
    noteNote:
    If the server is configured as a cluster or for a computer that is running Microsoft Windows Load Balancing, the following registry key is checked instead of the DnsFullyQualifiedDomainName setting: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\WLBS\Parameters\Interface\{GUID}\ClusterName
  7. After the domain list is built, the certificate selection process performs a search to find all certificates in the certificate store that have a matching FQDN. From this list, the certificate selection process identifies a list of eligible certificates. Eligible certificates must meet the following criteria:
    • The certificate is an X.509 version 3 or a later version certificate.
    • The certificate has an associated private key.
    • The Subject or Subject Alternate Name fields contain the FQDN that was retrieved in step 6.
    • The certificate is enabled for Secure Sockets Layer (SSL)/TLS use. Specifically, the SMTP service has been enabled for this certificate by using the Enable-ExchangeCertificate cmdlet.
  8. From the eligible certificates, the best certificate is selected based on the following sequence:
    • Sort eligible certificates by most recent Valid from date. Valid from is a Version 1 field on the certificate.
    • The first valid public key infrastructure (PKI) certificate that is found in this list is used.
    • If no valid PKI certificates are found, the first self-signed certificate is used.
  9. After the best certificate has been determined, another check is made to determine whether its thumbprint matches the certificate that is stored in msExchServerInternalTLSCert attribute. If the certificate matches, the certificate is used for X-AnonymousTLS. If it does not match, Event ID 1037 is logged in the Application log. However, this does not cause X-AnonymousTLS to fail.

For more information about how certificates are selected for other TLS scenarios, see the following topics:

To ensure that you are reading the most up-to-date information and to find additional Exchange Server 2007 documentation, visit the Exchange Server TechCenter.
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