$NetBSD: tlsproxy.8,v 1.5 2023/12/23 20:30:43 christos Exp $

tlsproxy - Postfix TLS proxy

tlsproxy [generic Postfix daemon options]
 DESCRIPTION

The tlsproxy(8) server implements a two-way TLS proxy. It is used by the postscreen(8) server to talk SMTP-over-TLS with remote SMTP clients that are not allowlisted (including clients whose allowlist status has expired), and by the smtp(8) client to support TLS connection reuse, but it should also work for non-SMTP protocols. Although one tlsproxy(8) process can serve multiple sessions at the same time, it is a good idea to allow the number of processes to increase with load, so that the service remains responsive.

The example below concerns postscreen(8). However, the tlsproxy(8) server is agnostic of the application protocol, and the example is easily adapted to other applications. After receiving a valid remote SMTP client STARTTLS command, the postscreen(8) server sends the remote SMTP client endpoint string, the requested role (server), and the requested timeout to tlsproxy(8). postscreen(8) then receives a "TLS available" indication from tlsproxy(8). If the TLS service is available, postscreen(8) sends the remote SMTP client file descriptor to tlsproxy(8), and sends the plaintext 220 greeting to the remote SMTP client. This triggers TLS negotiations between the remote SMTP client and tlsproxy(8). Upon completion of the TLS-level handshake, tlsproxy(8) translates between plaintext from/to postscreen(8) and ciphertext to/from the remote SMTP client.

The tlsproxy(8) server is moderately security-sensitive. It talks to untrusted clients on the network. The process can be run chrooted at fixed low privilege. Problems and transactions are logged to syslogd(8) or postlogd(8).

Changes to main.cf are not picked up automatically, as tlsproxy(8) processes may run for a long time depending on mail server load. Use the command "postfix reload" to speed up a change. The text below provides only a parameter summary. See postconf(5) for more details including examples.

The following settings are global and therefore cannot be overruled by information specified in a tlsproxy(8) client request. Append the system-supplied default Certification Authority certificates to the ones specified with *_tls_CApath or *_tls_CAfile. The number of pseudo-random bytes that an smtp(8) or smtpd(8) process requests from the tlsmgr(8) server in order to seed its internal pseudo random number generator (PRNG). The OpenSSL cipherlist for "high" grade ciphers. The OpenSSL cipherlist for "medium" or higher grade ciphers. The OpenSSL cipherlist for "NULL" grade ciphers that provide authentication without encryption. The elliptic curve used by the Postfix SMTP server for sensibly strong ephemeral ECDH key exchange. The elliptic curve used by the Postfix SMTP server for maximally strong ephemeral ECDH key exchange. List or bit-mask of OpenSSL bug work-arounds to disable. With SSLv3 and later, use the Postfix SMTP server's cipher preference order instead of the remote client's cipher preference order.

Available in Postfix version 2.8..3.7:

The OpenSSL cipherlist for "low" or higher grade ciphers. The OpenSSL cipherlist for "export" or higher grade ciphers.

Available in Postfix version 2.9 and later:

A temporary migration aid for sites that use certificate public-key fingerprints with Postfix 2.9.0..2.9.5, which use an incorrect algorithm.

Available in Postfix version 2.11-3.1:

Configure RFC7671 DANE TLSA digest algorithm agility. Enable support for RFC 6698 (DANE TLSA) DNS records that contain digests of trust-anchors with certificate usage "2".

Available in Postfix version 2.11 and later:

The name of the tlsmgr(8) service entry in master.cf.

Available in Postfix version 3.0 and later:

Algorithm used to encrypt RFC5077 TLS session tickets. The location of the OpenSSL command line program openssl(1).

Available in Postfix version 3.2 and later:

The prioritized list of elliptic curves supported by the Postfix SMTP client and server.

Available in Postfix version 3.4 and later:

Optional lookup tables that map names received from remote SMTP clients via the TLS Server Name Indication (SNI) extension to the appropriate keys and certificate chains.

Available in Postfix 3.5, 3.4.6, 3.3.5, 3.2.10, 3.1.13 and later:

A workaround for implementations that hang Postfix while shutting down a TLS session, until Postfix times out.

Available in Postfix version 3.8 and later:

The prioritized list of finite-field Diffie-Hellman ephemeral (FFDHE) key exchange groups supported by the Postfix SMTP client and server.

Available in Postfix 3.9, 3.8.1, 3.7.6, 3.6.10, 3.5.20 and later:

Optional configuration file with baseline OpenSSL settings. The application name passed by Postfix to OpenSSL library initialization functions.

These settings are clones of Postfix SMTP server settings. They allow tlsproxy(8) to load the same certificate and private key information as the Postfix SMTP server, before dropping privileges, so that the key files can be kept read-only for root. These settings can currently not be overruled by information in a tlsproxy(8) client request, but that limitation may be removed in a future version. A file containing (PEM format) CA certificates of root CAs trusted to sign either remote SMTP client certificates or intermediate CA certificates. A directory containing (PEM format) CA certificates of root CAs trusted to sign either remote SMTP client certificates or intermediate CA certificates. Force the Postfix tlsproxy(8) server to issue a TLS session id, even when TLS session caching is turned off. Ask a remote SMTP client for a client certificate. The verification depth for remote SMTP client certificates. File with the Postfix tlsproxy(8) server RSA certificate in PEM format. The minimum TLS cipher grade that the Postfix tlsproxy(8) server will use with opportunistic TLS encryption. File with the Postfix tlsproxy(8) server DSA certificate in PEM format. File with DH parameters that the Postfix tlsproxy(8) server should use with non-export EDH ciphers. File with DH parameters that the Postfix tlsproxy(8) server should use with export-grade EDH ciphers. File with the Postfix tlsproxy(8) server DSA private key in PEM format. File with the Postfix tlsproxy(8) server ECDSA certificate in PEM format. File with the Postfix tlsproxy(8) server ECDSA private key in PEM format. The Postfix tlsproxy(8) server security grade for ephemeral elliptic-curve Diffie-Hellman (EECDH) key exchange. List of ciphers or cipher types to exclude from the tlsproxy(8) server cipher list at all TLS security levels. The message digest algorithm to construct remote SMTP client-certificate fingerprints. File with the Postfix tlsproxy(8) server RSA private key in PEM format. Enable additional Postfix tlsproxy(8) server logging of TLS activity. The minimum TLS cipher grade that the Postfix tlsproxy(8) server will use with mandatory TLS encryption. Additional list of ciphers or cipher types to exclude from the tlsproxy(8) server cipher list at mandatory TLS security levels. The SSL/TLS protocols accepted by the Postfix tlsproxy(8) server with mandatory TLS encryption. List of TLS protocols that the Postfix tlsproxy(8) server will exclude or include with opportunistic TLS encryption. With mandatory TLS encryption, require a trusted remote SMTP client certificate in order to allow TLS connections to proceed. The SMTP TLS security level for the Postfix tlsproxy(8) server; when a non-empty value is specified, this overrides the obsolete parameters smtpd_use_tls and smtpd_enforce_tls. Files with the Postfix tlsproxy(8) server keys and certificate chains in PEM format.

These settings are clones of Postfix SMTP client settings. They allow tlsproxy(8) to load the same certificate and private key information as the Postfix SMTP client, before dropping privileges, so that the key files can be kept read-only for root. Some settings may be overruled by information in a tlsproxy(8) client request.

Available in Postfix version 3.4 and later:

A file containing CA certificates of root CAs trusted to sign either remote TLS server certificates or intermediate CA certificates. Directory with PEM format Certification Authority certificates that the Postfix tlsproxy(8) client uses to verify a remote TLS server certificate. Files with the Postfix tlsproxy(8) client keys and certificate chains in PEM format. File with the Postfix tlsproxy(8) client RSA certificate in PEM format. File with the Postfix tlsproxy(8) client RSA private key in PEM format. File with the Postfix tlsproxy(8) client DSA certificate in PEM format. File with the Postfix tlsproxy(8) client DSA private key in PEM format. File with the Postfix tlsproxy(8) client ECDSA certificate in PEM format. File with the Postfix tlsproxy(8) client ECDSA private key in PEM format. The message digest algorithm used to construct remote TLS server certificate fingerprints. Enable additional Postfix tlsproxy(8) client logging of TLS activity. The name of the parameter that provides the tlsproxy_client_loglevel value. The verification depth for remote TLS server certificates. Opportunistic mode: use TLS when a remote server announces TLS support. Enforcement mode: require that SMTP servers use TLS encryption. Optional lookup tables with the Postfix tlsproxy(8) client TLS usage policy by next-hop destination and by remote TLS server hostname.

Available in Postfix version 3.4-3.6:

The default TLS security level for the Postfix tlsproxy(8) client. Optional lookup tables with the Postfix tlsproxy(8) client TLS security policy by next-hop destination.

Available in Postfix version 3.7 and later:

The default TLS security level for the Postfix tlsproxy(8) client. Optional lookup tables with the Postfix tlsproxy(8) client TLS security policy by next-hop destination.

These parameters are supported for compatibility with smtpd(8) legacy parameters. Opportunistic TLS: announce STARTTLS support to remote SMTP clients, but do not require that clients use TLS encryption. Mandatory TLS: announce STARTTLS support to remote SMTP clients, and require that clients use TLS encryption. Opportunistic mode: use TLS when a remote server announces TLS support. Enforcement mode: require that SMTP servers use TLS encryption.

How much time a tlsproxy(8) process may take to process local or remote I/O before it is terminated by a built-in watchdog timer.

The default location of the Postfix main.cf and master.cf configuration files. The process ID of a Postfix command or daemon process. The process name of a Postfix command or daemon process. The syslog facility of Postfix logging. A prefix that is prepended to the process name in syslog records, so that, for example, "smtpd" becomes "prefix/smtpd".

Available in Postfix 3.3 and later:

The master.cf service name of a Postfix daemon process.

postscreen(8), Postfix zombie blocker
smtpd(8), Postfix SMTP server
postconf(5), configuration parameters
postlogd(8), Postfix logging
syslogd(8), system logging
 "LICENSE"

The Secure Mailer license must be distributed with this software.
 HISTORY

This service was introduced with Postfix version 2.8.

Wietse Venema
IBM T.J. Watson Research
P.O. Box 704
Yorktown Heights, NY 10598, USA

Wietse Venema
Google, Inc.
111 8th Avenue
New York, NY 10011, USA