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:
Private, authenticated communications over the Internet
Protection from eavesdroppers and attackers
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:
The source email address and the destination address are encrypted.
The content of the email message is encrypted.
When the client issues the STARTTLS command, the server responds with one of the following:
220 Ready to start TLS
501 Syntax error (no parameters allowed)
454 TLS not available due to temporary reason
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.
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:
O OptionName=argument # for the configuration file
-O OptionName=argument # for the command line
define(`m4Name',argument) # for m4 configuration
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:
CACertPath
CACertFile
ServerCertFile
ClientKeyFile
Other options are not required.
The following table describes the macros that are used by the STARTTLS command.
Table 14–14 Macros 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.
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.