ACCESS(5)

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version 3.11.7 · 5 versions
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🌈 ACCESS(5) — Postfix SMTP server access table 💅

okay bestie, gather round, because this is the access(5) page from the File Formats Manual and it is lowkey iconic. We're talking access control tables for the Postfix SMTP server. Let's get into it ✨

🎀 SYNOPSIS 🎀

postmap /etc/postfix/access
postmap -q "string" /etc/postfix/access
postmap -q - /etc/postfix/access <inputfile

💖 DESCRIPTION 💖

So here's the vibe: this document describes access control based on remote SMTP client info — we're talking host names, network addresses, and envelope sender or recipient addresses. It's implemented by the Postfix SMTP server, no cap. 🫶

And if you were wondering about access control on the content of email messages? That's a different girl entirely — go check out header_checks(5) or body_checks(5). 📬

Normally, the access(5) table is specified as a text file that serves as input to the postmap(1) command, which creates an indexed file so lookups go zoom. ⚡

Run the command "postmap /etc/postfix/access" to rebuild a default-type indexed file after you change the text file, or run "postmap type:/etc/postfix/access" to specify an explicit type. Easy peasy.

The default indexed file type is configured with the default_database_type parameter. Depending on the platform, this may be one of lmdb:, cdb:, hash:, or dbm: — and note, those are listed without the trailing ':'. 💅

When the table is provided via other means such as NIS, LDAP or SQL, the same lookups are done as for ordinary indexed files. But managing those databases? That's outside the scope of Postfix, bestie. Not our problem. Managing such databases is outside the scope of Postfix. 💀

Alternatively, the table can be provided as a regular-expression map where patterns are given as regular expressions, or lookups can be directed to a TCP-based server. In those cases, the lookups are done in a slightly different way, as described below under "REGULAR EXPRESSION TABLES" or "TCP-BASED TABLES". 👀

🌸 CASE FOLDING 🌸

The search string is folded to lowercase before database lookup. 📉

But — as of Postfix 2.3, the search string is not case folded with database types such as regexp: or pcre:, because those lookup fields can match both upper and lower case. So there. 💅

✨ TABLE FORMAT ✨

The input format for the postmap(1) command is as follows, bestie:

  • pattern action — when pattern matches a mail address, domain or host address, perform the corresponding action. Periodt.
  • blank lines and comments — empty lines and whitespace-only lines are ignored, and so are lines whose first non-whitespace character is a '#'. 🙈
  • multi-line text — a logical line starts with non-whitespace text. A line that starts with whitespace continues a logical line. (yes, it's giving continuation energy 🦋)

👑 EMAIL ADDRESS PATTERNS IN INDEXED TABLES 👑

With lookups from indexed files such as DB or DBM, or from networked tables such as NIS, LDAP or SQL, patterns are tried in the order as listed below:

  • user@domain — matches the specified mail address. 🎯
  • domain.tld — matches domain.tld as the domain part of an email address.

The pattern domain.tld also matches subdomains, but only when the string smtpd_access_maps is listed in the Postfix parent_domain_matches_subdomains configuration setting. 👑

  • .domain.tld — matches subdomains of domain.tld, but only when the string smtpd_access_maps is not listed in the Postfix parent_domain_matches_subdomains configuration setting. 🦋
  • user@ — matches all mail addresses with the specified user part.
Note: lookup of the null sender address is not possible with some types of lookup table. By default, Postfix uses <> as the lookup key for such addresses. That value is specified with the smtpd_null_access_lookup_key parameter in the Postfix main.cf file. 🫶

🌈 EMAIL ADDRESS EXTENSION 🌈

When a mail address localpart contains the optional recipient delimiter (e.g., user+foo@domain), the lookup order becomes: user+foo@domain, user@domain, domain, user+foo@, and user@. That's the whole lineup, in that exact order. ⭐

💜 HOST NAME/ADDRESS PATTERNS IN INDEXED TABLES 💜

With lookups from indexed files such as DB or DBM, or from networked tables such as NIS, LDAP or SQL, the following lookup patterns are examined in the order as listed:

  • domain.tld — matches domain.tld.

The pattern domain.tld also matches subdomains, but only when the string smtpd_access_maps is listed in the Postfix parent_domain_matches_subdomains configuration setting. 👑

  • .domain.tld — matches subdomains of domain.tld, but only when the string smtpd_access_maps is not listed in the Postfix parent_domain_matches_subdomains configuration setting.
  • net.work.addr.ess, net.work.addr, net.work, net — matches a remote IPv4 host address or network address range. Specify one to four decimal octets separated by ".". Do not specify "[]", "/", leading zeros, or hexadecimal forms. 🚫

Network ranges are matched by repeatedly truncating the last ".octet" from a remote IPv4 host address string, until a match is found in the access table, or until further truncation is not possible. 🔍

NOTE: use the cidr lookup table type to specify network/netmask patterns. See cidr_table(5) for details.
  • net:work:addr:ess, net:work:addr, net:work, net — matches a remote IPv6 host address or network address range. Specify three to eight hexadecimal octet pairs separated by ":", using the compressed form "::" for a sequence of zero-valued octet pairs. Do not specify "[]", "/", leading zeros, or non-compressed forms. 💀

A network range is matched by repeatedly truncating the last ":octetpair" from the compressed-form remote IPv6 host address string, until a match is found in the access table, or until further truncation is not possible. 🔍

NOTE: use the cidr lookup table type to specify network/netmask patterns. See cidr_table(5) for details.

IPv6 support is available in Postfix 2.2 and later. ⭐

🫶 ACCEPT ACTIONS 🫶

  • OK — accept the address etc. that matches the pattern. Ate. 💅
  • all-numerical — an all-numerical result is treated as OK. This format is generated by address-based relay authorization schemes such as pop-before-smtp. 🔥

For other accept actions, see "OTHER ACTIONS" below. 👇

😭 REJECT ACTIONS 😭

<b>Postfix version 2.3 and later</b> support enhanced status codes as defined in RFC 3463. When no code is specified at the beginning of the text below, Postfix inserts a default enhanced status code of "5.7.1" in the case of reject actions, and "4.7.1" in the case of defer actions. See "ENHANCED STATUS CODES" below. 🎀

  • 4NN text, 5NN text — reject the address etc. that matches the pattern, and respond with the numerical three-digit code and text. 4NN means "try again later", while 5NN means "do not try again". 😤

The following responses have special meaning for the Postfix SMTP server:

  • 421 text (Postfix 2.3 and later)
  • 521 text (Postfix 2.6 and later) — after responding with the numerical three-digit code and text, disconnect immediately from the SMTP client. This frees up SMTP server resources so that they can be made available to another SMTP client. ✂️
Note: The "521" response should be used only with botnets and other malware where interoperability is of no concern. The "send 521 and disconnect" behavior is NOT defined in the SMTP standard. 💀
  • REJECT optional text... — reject the address etc. that matches the pattern. Reply with "$access_map_reject_code optional text..." when the optional text is specified, otherwise reply with a generic error response message. 🚫
  • DEFER optional text... — reject the address etc. that matches the pattern. Reply with "$access_map_defer_code optional text..." when the optional text is specified, otherwise reply with a generic error response message. ⏳

This feature is available in Postfix 2.6 and later.

  • DEFER_IF_REJECT optional text... — defer the request if some later restriction would result in a REJECT action. Reply with "$access_map_defer_code 4.7.1 optional text..." when the optional text is specified, otherwise reply with a generic error response message. 🙈

Prior to Postfix 2.6, the SMTP reply code is 450.

This feature is available in Postfix 2.1 and later.

  • DEFER_IF_PERMIT optional text... — defer the request if some later restriction would result in an explicit or implicit PERMIT action. Reply with "$access_map_defer_code 4.7.1 optional text..." when the optional text is specified, otherwise reply with a generic error response message. 🦋

Prior to Postfix 2.6, the SMTP reply code is 450.

This feature is available in Postfix 2.1 and later.

For other reject actions, see "OTHER ACTIONS" below. 👇

🫶 OTHER ACTIONS 🫶

Ok so these are the other things a table can make Postfix do, besides accept and reject. Buckle up bestie, there's like ten of them 💅

  • restriction... — Apply the named UCE restriction(s) (permit, reject, reject_unauth_destination, and so on).
  • BCC user@domain — Send one copy of the message to the specified recipient.

If multiple BCC actions are specified within the same SMTP MAIL transaction, with Postfix 3.0 only the last action will be used. 😭

This feature is available in Postfix 3.0 and later.

  • DISCARD optional text... — Claim successful delivery and silently discard the message. Log the optional text if specified, otherwise log a generic message. Ghosting the whole entire message, periodt 👻
Note: this action currently affects all recipients of the message. To discard only one recipient without discarding the entire message, use the transport(5) table to direct mail to the discard(8) service.

This feature is available in Postfix 2.0 and later.

  • DUNNO — Pretend that the lookup key was not found. This prevents Postfix from trying substrings of the lookup key (such as a subdomain name, or a network address subnetwork). It's giving "no thoughts, head empty, idk her" 🙈

This feature is available in Postfix 2.0 and later.

  • FILTER transport:destination — After the message is queued, send the entire message through the specified external content filter. The transport name specifies the first field of a mail delivery agent definition in master.cf; the syntax of the next-hop destination is described in the manual page of the corresponding delivery agent. More information about external content filters is in the Postfix FILTER_README file.
Note 1: do not use $number regular expression substitutions for transport or destination unless you know that the information has a trusted origin. (⚠️ untrusted interpolation is how you get got, bestie)
Note 2: this action overrides the main.cf content_filter setting, and affects all recipients of the message. In the case that multiple FILTER actions fire, only the last one is executed.
Note 3: the purpose of the FILTER command is to override message routing. To override the recipient's transport but not the next-hop destination, specify an empty filter destination (Postfix 2.7 and later), or specify a transport:destination that delivers through a different Postfix instance (Postfix 2.6 and earlier). Other options are using the recipient-dependent transport_maps or the sender-dependent sender_dependent_default_transport_maps features.

This feature is available in Postfix 2.0 and later.

  • HOLD optional text... — Place the message on the hold queue, where it will sit until someone either deletes it or releases it for delivery. Log the optional text if specified, otherwise log a generic message. It's just sitting there. Vibing. Waiting. ✨

Mail that is placed on hold can be examined with the postcat(1) command, and can be destroyed or released with the postsuper(1) command.

Note: use postsuper -r to release mail that was kept on hold for a significant fraction of $maximal_queue_lifetime or $bounce_queue_lifetime, or longer. Use postsuper -H only for mail that will not expire within a few delivery attempts.
Note: this action currently affects all recipients of the message.

This feature is available in Postfix 2.0 and later.

  • PREPEND headername: headervalue — Prepend the specified message header to the message. When more than one PREPEND action executes, the first prepended header appears before the second etc. prepended header. So the first one to show up is first in line, no cuts 💅
Note: this action must execute before the message content is received; it cannot execute in the context of smtpd_end_of_data_restrictions.

This feature is available in Postfix 2.1 and later.

  • REDIRECT user@domain — After the message is queued, send the message to the specified address instead of the intended recipient(s). When multiple REDIRECT actions fire, only the last one takes effect.
Note 1: this action overrides the FILTER action, and currently overrides all recipients of the message.
Note 2: a REDIRECT address is subject to canonicalization (add missing domain) but NOT subject to canonical, masquerade, bcc, or virtual alias mapping.

This feature is available in Postfix 2.1 and later.

  • INFO optional text... — Log an informational record with the optional text, together with client information and if available, with helo, sender, recipient and protocol information. Just a lil note for the group chat 📝

This feature is available in Postfix 3.0 and later.

  • WARN optional text... — Log a warning with the optional text, together with client information and if available, with helo, sender, recipient and protocol information.

This feature is available in Postfix 2.1 and later.

💜 ENHANCED STATUS CODES 💜

Postfix version 2.3 and later support enhanced status codes as defined in RFC 3463. When an enhanced status code is specified in an access table, it is subject to modification. The following transformations are needed when the same access table is used for client, helo, sender, or recipient access restrictions; they happen regardless of whether Postfix replies to a MAIL FROM, RCPT TO or other SMTP command. (Postfix said "I'm gonna fix it for you" and honestly? slay 🎀)

  • When a sender address matches a REJECT action, the Postfix SMTP server will transform a recipient DSN status (e.g., 4.1.1-4.1.6) into the corresponding sender DSN status, and vice versa.
  • When non-address information matches a REJECT action (such as the HELO command argument or the client hostname/address), the Postfix SMTP server will transform a sender or recipient DSN status into a generic non-address DSN status (e.g., 4.0.0).

🌈 REGULAR EXPRESSION TABLES 🌈

This section describes how the table lookups change when the table is given in the form of regular expressions. For a description of regular expression lookup table syntax, see regexp_table(5) or pcre_table(5).

Each pattern is a regular expression that is applied to the entire string being looked up. Depending on the application, that string is an entire client hostname, an entire client IP address, or an entire mail address. Thus, no parent domain or parent network search is done, user@domain mail addresses are not broken up into their user@ and domain constituent parts, nor is user+foo broken up into user and foo. No splitting, no substrings, we're doing the whole thing in one bite fr fr 🍓

Patterns are applied in the order as specified in the table, until a pattern is found that matches the search string.

Actions are the same as with indexed file lookups, with the additional feature that parenthesized substrings from the pattern can be interpolated as $1, $2 and so on. That's the special sauce 👑

🦋 TCP-BASED TABLES 🦋

This section describes how the table lookups change when lookups are directed to a TCP-based server. For a description of the TCP client/server lookup protocol, see tcp_table(5). This feature is not available up to and including Postfix version 2.4. (rip to the old heads 💀)

Each lookup operation uses the entire query string once. Depending on the application, that string is an entire client hostname, an entire client IP address, or an entire mail address. Thus, no parent domain or parent network search is done, user@domain mail addresses are not broken up into their user@ and domain constituent parts, nor is user+foo broken up into user and foo.

Actions are the same as with indexed file lookups.

✨ EXAMPLE ✨

The following example uses an indexed file, so that the order of table entries does not matter. The example permits access by the client at address 1.2.3.4 but rejects all other clients in 1.2.3.0/24. Instead of hash lookup tables, some systems use dbm. Use the command postconf -m to find out what lookup tables Postfix supports on your system.

/etc/postfix/main.cf:

smtpd_client_restrictions =
    check_client_access hash:/etc/postfix/access

/etc/postfix/access:

1.2.3   REJECT
1.2.3.4 OK

Execute the command postmap /etc/postfix/access after editing the file. Don't skip that step bestie, I'm literally begging 🙏

💀 BUGS 💀

The table format does not understand quoting conventions. That's it. That's the bug. No cap.

👑 SEE ALSO 👑

🌸 README FILES 🌸

Use postconf readme_directory or postconf html_directory to locate this information.

  • SMTPD_ACCESS_README, built-in SMTP server access control
  • DATABASE_README, Postfix lookup table overview

🎀 LICENSE 🎀

The Secure Mailer license must be distributed with this software.

💖 AUTHOR(S) 💖

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

And that's a wrap on ACCESS(5) in all its glory, the Postfix access table main character moment it deserves. Go forth and table responsibly 🫶✨💅

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