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System Administration Guide: Network Services

Support for Running SMTP With TLS in Version 8.13 of sendmail

Communications between SMTP servers and clients are not usually controlled or trusted on either end. This lack of security might allow a third party to monitor and even alter a communication between a server and a client. Starting in the Solaris 10 1/06 release, SMTP can use Transport Layer Security (TLS) in version 8.13 of sendmail to resolve this problem. This extended service to SMTP servers and clients provides the following:

Note –

The implementation of TLS is based on the Secure Sockets Layer (SSL) protocol.

STARTTLS is the SMTP keyword that initiates a secure SMTP connection by using TLS. This secure connection might be between two servers or between a server and a client. A secure connection is defined as follows:

When the client issues the STARTTLS command, the server responds with one of the following:

The 220 response requires the client to start the TLS negotiation. The 501 response notes that the client incorrectly issued the STARTTLS command. STARTTLS is issued with no parameters. The 454 response necessitates that the client apply rule set values to determine whether to accept or maintain the connection.

Note that to maintain the Internet's SMTP infrastructure, publicly used servers must not require a TLS negotiation. However, a server that is used privately might require the client to perform a TLS negotiation. In such instances, the server returns this response:


530 Must issue a STARTTLS command first

The 530 response instructs the client to issue the STARTTLS command to establish a connection.

The server or client can refuse a connection if the level of authentication and privacy is not satisfactory. Alternately, because most SMTP connections are not secure, the server and client might maintain an unsecure connection. Whether to maintain or refuse a connection is determined by the configuration of the server and the client.

Support for running SMTP with TLS is not enabled by default. TLS is enabled when the SMTP client issues the STARTTLS command. Before the SMTP client can issue this command, you must set up the certificates that enable sendmail to use TLS. See How to Set SMTP to Use TLS. Note that this procedure includes defining new configuration file options and rebuilding your sendmail.cf file.

Configuration File Options for Running SMTP With TLS

The following table describes the configuration file options that are used to run SMTP with TLS. If you declare any of these options, use one of the following syntaxes:

Table 14–13 Configuration File Options for Running SMTP With TLS

Option 

Description 

CACertFile

m4 name: confCACERT

Argument: filename

Default value: undefined 

Identifies the file that contains one CA certificate. 

CACertPath

m4 name: confCACERT_PATH

Argument: path

Default value: undefined 

Identifies the path to the directory that contains certificates of CAs. 

ClientCertFile

m4 name: confCLIENT_CERT

Argument: filename

Default value: undefined 

Identifies the file that contains the certificate of the client. Note that this certificate is used when sendmail acts as a client.

ClientKeyFile

m4 name: confCLIENT_KEY

Argument: filename

Default value: undefined 

Identifies the file that contains the private key that belongs to the client certificate. 

CRLFile

m4 name: confCRL

Argument: filename

Default value: undefined 

Identifies the file that contains the certificate revocation status, which is used for X.509v3 authentication. 

DHParameters

m4 name: confDH_PARAMETERS

Argument: filename

Default value: undefined 

Identifies the file that contains the Diffie-Hellman (DH) parameters. 

RandFile

m4 name: confRAND_FILE

Argument: file:filename or egd:UNIX socket

Default value: undefined 

Uses the file: prefix to identify the file that contains random data or uses the egd: prefix to identify the UNIX socket. Note that because the Solaris OS supports the random number generator device, this option does not need to be specified. See the random(7D) man page.

ServerCertFile

m4 name: confSERVER_CERT

Argument: filename

Default value: undefined 

Identifies the file that contains the server's certificate. This certificate is used when sendmail acts as a server.

Timeout.starttls

m4 name: confTO_STARTTLS

Argument: amount of time

Default value: 1h

Sets the amount of time the SMTP client waits for a response to the STARTTLS command.

TLSSrvOptions

m4 name: confTLS_SRV_OPTIONS

Argument: V

Default value: undefined 

Determines whether the server asks for a certificate from the client. If this option is set to V, no client verification is performed.

For sendmail to support SMTP's use of TLS, the following options must be defined:

Other options are not required.

Macros for Running SMTP With TLS

The following table describes the macros that are used by the STARTTLS command.

Table 14–14 Macros for Running SMTP With TLS

Macro 

Description 

${cert_issuer}

Holds the distinguished name (DN) of the certification authority (CA), which is the certificate issuer. 

${cert_subject}

Holds the DN of the certificate that is called the cert subject.

${cn_issuer}

Holds the common name (CN) of the CA, which is the cert issuer.

${cn_subject}

Holds the CN of the certificate that is called the cert subject.

${tls_version}

Holds the version of TLS that is used for the connection. 

${cipher}

Holds a set of cryptographic algorithms (known as a cipher suite) that is used for the connection.

${cipher_bits}

Holds in bits the key length of the symmetric encryption algorithm that is used for the connection. 

${verify}

Holds the result of the verification of the certificate that was presented. Possible values are as follows: 

  • OK – The verification succeeded.

  • NO – No certificate was presented.

  • NOT – No certificate was requested.

  • FAIL – The certificate that was presented could not be verified.

  • NONE – STARTTLS has not been performed.

  • TEMP – Temporary error occurred.

  • PROTOCOL – SMTP error occurred.

  • SOFTWARE – STARTTLS handshake failed.

${server_name}

Holds the name of the server with the current outgoing SMTP connection. 

${server_addr}

Holds the address of the server with the current outgoing SMTP connection. 

Rule Sets for Running SMTP With TLS

The following table describes rule sets that determine whether an SMTP connection that uses TLS should be accepted, continued, or refused.

Table 14–15 Rule Sets for Running SMTP With TLS

Rule Set 

Description 

tls_server

Acting as a client, sendmail uses this rule set to determine whether the server is currently supported by TLS.

tls_client

Acting as a server, sendmail uses this rule set to determine whether the client is currently supported by TLS.

tls_rcpt

This rule set requires verification of the recipient's MTA. This recipient restriction makes attacks such as DNS spoofing impossible. 

TLS_connection

This rule set checks the requirement that is specified by the RHS of the access map against the actual parameters of the current TLS connection. 

try_tls

sendmail uses this rule set to determine the feasibility of using STARTTLS when connecting to another MTA. If the MTA cannot properly implement STARTTLS, then STARTTLS is not used.

For more information, see http://www.sendmail.org/m4/starttls.html.

Security Considerations Related to Running SMTP With TLS

As a standard mail protocol that defines mailers that run over the Internet, SMTP is not an end-to-end mechanism. Because of this protocol limitation, TLS security through SMTP does not include mail user agents. Mail user agents act as an interface between users and a mail transfer agent such as sendmail.

Also, mail might be routed through multiple servers. For complete SMTP security the entire chain of SMTP connections must have TLS support.

Finally, the level of negotiated authentication and privacy between each pair of servers or a client and server pair must be considered. For more information, see Authentication Services in System Administration Guide: Security Services.