filter-masquerade

OpenSMTPD filter to rewrite sender headers for subdomain masquerading.

Description

Rewrites From:, Sender:, and Reply-To: headers to use a parent domain when the original address is from a subdomain. This is useful for organizations that want outgoing mail to appear from the main domain regardless of which internal host sent it.

Example: With target domain @example.com:

• user@mail.example.com → user@example.com (subdomain - rewritten)
• user@example.com → user@example.com (exact match - unchanged)
• user@other.org → user@other.org (different domain - unchanged)

Only message headers are rewritten. For SMTP envelope (MAIL FROM) rewriting, use OpenSMTPD's native sender action.

Requirements

• OpenSMTPD 6.6 or later
• POSIX-compatible awk

Installation

make install

Default installation path: /usr/local/libexec/smtpd/filter-masquerade

To install to a different location:

make PREFIX=/usr install

Configuration

Add to /etc/mail/smtpd.conf:

filter "masquerade" proc-exec "/usr/local/libexec/smtpd/filter-masquerade @example.com"

listen on all filter "masquerade"

Replace @example.com with your target domain (must include the @).

Combined with envelope rewriting

To also rewrite the SMTP envelope sender:

action "relay" relay host smtp://relay.example.com \
    mail-from "@example.com"

filter "masquerade" proc-exec "/usr/local/libexec/smtpd/filter-masquerade @example.com"

match from local action "relay" filter "masquerade"

Testing

make test

License

ISC License. See source file for details.