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System Administration Guide: Network Services
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Changes in Version 8.13 of sendmail

Starting in the Solaris 10 release, version 8.13 is the default. Although this new version of sendmail provides many new features, the FallBackSmartHost option is the most significant addition. Because of this option you no longer need to use main.cf and subsidiary.cf. The main.cf file was used in environments that supported MX records. The subsidiary.cf file was used in environments without a fully operative DNS. In such environments a smart host was used instead of MX records. The FallBackSmartHost option provides unified configuration. It operates like an MX record of last possible preference for all environments. To ensure that mail gets delivered to clients, this option, if enabled, provides a well-connected (or smart) host that serves as a backup (or failover) for MX records that fail.

For more information about version 8.13, see the following sections:

Additionally, starting in the Solaris 10 1/06 release, SMTP can run with Transport Layer Security (TLS). See the following description.

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:

  • Private, authenticated communications over the Internet

  • Protection from eavesdroppers and attackers


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:

  • 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.

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:

  • O OptionName=argument # for the configuration file

  • -O OptionName=argument # for the command line

  • define(`m4Name',argument) # for m4 configuration

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:

  • CACertPath

  • CACertFile

  • ServerCertFile

  • ClientKeyFile

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.

  • NONESTARTTLS has not been performed.

  • TEMP – Temporary error occurred.

  • PROTOCOL – SMTP error occurred.

  • SOFTWARESTARTTLS 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 the following:

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.

Additional Command-Line Options in Version 8.13 of sendmail

The following table describes additional command-line options that are available in version 8.13 of sendmail. Other command-line options are described in the sendmail(1M) man page.

Table 14-16 Command-Line Options Available in Version 8.13 of sendmail

Option

Description

-D logfile

Sends debugging output to the indicated logfile, instead of including this information with the standard output.

-q[!]Qsubstr

Specifies the processing of quarantined jobs that have this substr, which is a substring of the quarantine reason. See the description of the -Qreason option. If ! is added, this option processes quarantined jobs that do not have this substr.

-Qreason

Quarantines a normal queue item with this reason. If no reason is given, the quarantined queue item is unquarantined. This option works with the -q[!]Qsubstr option. The substr is a portion (or substring) of the reason.

Additional and Revised Configuration File Options in Version 8.13 of sendmail

The following table describes the added and revised configuration file options. 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
Table 14-17 Configuration File Options Available in Version 8.13 of sendmail

Option

Description

ConnectionRateWindowSize

m4 name: confCONNECTION_RATE_WINDOW_SIZE

Argument: number

Default value: 60

Sets the number of seconds for incoming connections to be maintained.

FallBackSmartHost

m4 name: confFALLBACK_SMARTHOST

Argument: hostname

To ensure that mail gets delivered to the clients, this option provides a well-connected host that serves as a backup (or failover) for MX records that fail.

InputMailFilters

m4 name: confINPUT_MAIL_FILTERS

Argument: filename

Lists the input mail filters for the sendmail daemon.

PidFile

m4 name: confPID_FILE

Argument: filename

Default value: /var/run/sendmail.pid

As in previous releases, the file name is macro-expanded before it is opened. Additionally, in version 8.13, the file is unlinked when sendmail exits.

QueueSortOrder

m4 name: confQUEUE_SORT_ORDER

Added argument: none

In version 8.13 none is used to specify no sorting order.

RejectLogInterval

m4 name: confREJECT_LOG_INTERVAL

Argument: period-of-time

Default value: 3h, which represents three hours.

When a daemon connection is refused for the period-of-time specified, the information is logged.

SuperSafe

m4 name: confSAFE_QUEUE

Short name: s

Added argument: postmilter

Default value: true

If postmilter is set, sendmail defers synchronizing the queue file until all milters have signaled acceptance of the message. For this argument to be useful, sendmail must be running as an SMTP server. Otherwise, postmilter operates as if you are using the true argument.

Additional and Revised FEATURE() Declarations in Version 8.13 of sendmail

The following table describes the added and revised FEATURE() declarations. This m4 macro uses the following syntax.

FEATURE(`name', `argument')
Table 14-18 FEATURE() Declarations Available in Version 8.13 of sendmail

Name of FEATURE()

Description

conncontrol

Works with the access_db rule set to check the number of incoming SMTP connections. For details, see /etc/mail/cf/README.

greet_pause

Adds the greet_pause rule set, which enables open proxy and SMTP slamming protection. For details, see /etc/mail/cf/README.

local_lmtp

The default argument continues to be mail.local, which is the LMTP-capable mailer in this Solaris release. However, in version 8.13, if a different LMTP-capable mailer is used, its path name can be specified as a second parameter and the arguments that are passed to the second parameter can be specified in the third parameter. For example:

FEATURE(`local_lmtp', `/usr/local/bin/lmtp', `lmtp')

mtamark

Provides experimental support for “Marking Mail Transfer Agents in Reverse DNS with TXT RRs” (MTAMark). For details, see /etc/mail/cf/README.

ratecontrol

Works with the access_db rule set to control connection rates for hosts. For details, see /etc/mail/cf/README.

use_client_ptr

If this FEATURE() is enabled, the rule set check_relay overrides its first argument with this argument, $&{client_ptr}.

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