info > CANONICAL

📛 NAME

canonical – Postfix canonical table format

🚀 Quick Reference

Use Case	Command	Description
Replace login names with Firstname.Lastname	postmap /etc/postfix/canonical	Rebuild indexed map after editing the text file
Query a mapping	postmap -q "string" /etc/postfix/canonical	Look up a specific address
Query from stdin	postmap -q - /etc/postfix/canonical <inputfile	Supply query strings via file
Clean up legacy addresses	user@domain address in table	Replace user@domain with canonical form
Wild‑card domain rewrite	@domain address in table	Rewrite all addresses in a domain (⚠️ backscatter risk)

📋 SYNOPSIS

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

📝 DESCRIPTION

The optional canonical(5) table specifies an address mapping for local and non‑local addresses. The mapping is used by the cleanup(8) daemon, before mail is stored into the queue. The address mapping is recursive.

Normally, the canonical(5) table is specified as a text file that serves as input to the postmap(1) command. The result, an indexed file in dbm or db format, is used for fast searching by the mail system. Execute the command postmap /etc/postfix/canonical to rebuild an indexed file after changing the corresponding text file.

When the table is provided via other means such as NIS, LDAP or SQL, the same lookups are done as for ordinary indexed files.

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.

By default the canonical(5) mapping affects both message header addresses (i.e. addresses that appear inside messages) and message envelope addresses (for example, the addresses that are used in SMTP protocol commands). This is controlled with the canonical_classes parameter.

⚠️ NOTE: Postfix versions 2.2 and later rewrite message headers from remote SMTP clients only if the client matches the local_header_rewrite_clients parameter, or if the remote_header_rewrite_domain configuration parameter specifies a non‑empty value. To get the behavior before Postfix 2.2, specify local_header_rewrite_clients = static:all.

Typically, one would use the canonical(5) table to replace login names by Firstname.Lastname, or to clean up addresses produced by legacy mail systems.

⚠️ The canonical(5) mapping is not to be confused with virtual alias support or with local aliasing. To change the destination but not the headers, use the virtual(5) or aliases(5) map instead.

🔠 Case Folding

The search string is folded to lowercase before database lookup. As of Postfix 2.3, the search string is not case folded with database types such as regexp: or pcre: whose lookup fields can match both upper and lower case.

📄 Table Format

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

• pattern address — When pattern matches a mail address, replace it by the corresponding address.
• blank lines and comments — Empty lines and whitespace‑only lines are ignored, as 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.

🔍 Table Search Order

With lookups from indexed files such as DB or DBM, or from networked tables such as NIS, LDAP or SQL, each user@domain query produces a sequence of query patterns as described below.

Each query pattern is sent to each specified lookup table before trying the next query pattern, until a match is found.

• user@domain address — Replace user@domain by address. This form has the highest precedence. Useful to clean up addresses produced by legacy mail systems. It can also be used to produce Firstname.Lastname style addresses, but see below for a simpler solution.
• user address — Replace user@site by address when site is equal to $myorigin, when site is listed in $mydestination, or when it is listed in $inet_interfaces or $proxy_interfaces. This form is useful for replacing login names by Firstname.Lastname.
• @domain address — Replace other addresses in domain by address. This form has the lowest precedence. ⚠️ Note: @domain is a wild‑card. When this form is applied to recipient addresses, the Postfix SMTP server accepts mail for any recipient in domain, regardless of whether that recipient exists. This may turn your mail system into a backscatter source: Postfix first accepts mail for non‑existent recipients and then tries to return that mail as “undeliverable” to the often forged sender address.

To avoid backscatter with mail for a wild‑card domain, replace the wild‑card mapping with explicit 1:1 mappings, or add a reject_unverified_recipient restriction for that domain:

smtpd_recipient_restrictions =
    ...
    reject_unauth_destination
    check_recipient_access
        inline:{example.com=reject_unverified_recipient}
unverified_recipient_reject_code = 550

In the above example, Postfix may contact a remote server if the recipient is rewritten to a remote address.

✏️ Result Address Rewriting

The lookup result is subject to address rewriting:

• 🔹 When the result has the form @otherdomain, the result becomes the same user in otherdomain.
• 🔹 When append_at_myorigin=yes, append @$myorigin to addresses without @domain.
• 🔹 When append_dot_mydomain=yes, append .$mydomain to addresses without .domain.

🔗 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, user+foo, user, and @domain.

The propagate_unmatched_extensions parameter controls whether an unmatched address extension (+foo) is propagated to the result of table lookup.

📐 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 address being looked up. Thus, 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.

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

Results 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.

🌐 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.

Each lookup operation uses the entire address once. Thus, 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.

Results are the same as with indexed file lookups.

🐛 Bugs

The table format does not understand quoting conventions.

⚙️ Configuration Parameters

The following main.cf parameters are especially relevant. The text below provides only a parameter summary. See postconf(5) for more details including examples.

• canonical_classes (envelope_sender, envelope_recipient, header_sender, header_recipient) — What addresses are subject to canonical_maps address mapping.
• canonical_maps (empty) — Optional address mapping lookup tables for message headers and envelopes.
• recipient_canonical_maps (empty) — Optional address mapping lookup tables for envelope and header recipient addresses.
• sender_canonical_maps (empty) — Optional address mapping lookup tables for envelope and header sender addresses.
• propagate_unmatched_extensions (canonical, virtual) — What address lookup tables copy an address extension from the lookup key to the lookup result.

Other parameters of interest:

• inet_interfaces (all) — The network interface addresses that this mail system receives mail on.
• local_header_rewrite_clients (permit_inet_interfaces) — Rewrite message header addresses in mail from these clients and update incomplete addresses with the domain name in $myorigin or $mydomain; either don't rewrite message headers from other clients at all, or rewrite message headers and update incomplete addresses with the domain specified in the remote_header_rewrite_domain parameter.
• proxy_interfaces (empty) — The network interface addresses that this mail system receives mail on by way of a proxy or network address translation unit.
• masquerade_classes (envelope_sender, header_sender, header_recipient) — What addresses are subject to address masquerading.
• masquerade_domains (empty) — Optional list of domains whose subdomain structure will be stripped off in email addresses.
• masquerade_exceptions (empty) — Optional list of user names that are not subjected to address masquerading, even when their address matches $masquerade_domains.
• mydestination ($myhostname, localhost.$mydomain, localhost) — The list of domains that are delivered via the $local_transport mail delivery transport.
• myorigin ($myhostname) — The domain name that locally‑posted mail appears to come from, and that locally posted mail is delivered to.
• owner_request_special (yes) — Enable special treatment for owner‑listname entries in the aliases(5) file, and don't split owner‑listname and listname‑request address localparts when the recipient_delimiter is set to “-”.
• remote_header_rewrite_domain (empty) — Don't rewrite message headers from remote clients at all when this parameter is empty; otherwise, rewrite message headers and append the specified domain name to incomplete addresses.

📚 See Also

• cleanup(8) — canonicalize and enqueue mail
• postmap(1) — Postfix lookup table manager
• postconf(5) — configuration parameters
• virtual(5) — virtual aliasing

📖 Readme Files

Use postconf readme_directory or postconf html_directory to locate this information.

• DATABASE_README — Postfix lookup table overview
• ADDRESS_REWRITING_README — address rewriting guide

📄 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