Postfix BDAT (CHUNKING) support


Postfix SMTP server supports RFC 3030 CHUNKING (the BDAT command) without BINARYMIME, in both smtpd(8) and postscreen(8). It is enabled by default.

Topics covered in this document:

Disabling BDAT support

BDAT support is enabled by default. To disable BDAT support globally:

    # The logging alternative:
    smtpd_discard_ehlo_keywords = chunking
    # The non-logging alternative:
    smtpd_discard_ehlo_keywords = chunking, silent_discard

Specify '-o smtpd_discard_ehlo_keywords=' in for the submission and smtps services, if you have clients that benefit from CHUNKING support.

Impact on existing configurations

Example SMTP session

The main differences are that the Postfix SMTP server announces "CHUNKING" support in the EHLO response, and that instead of sending one DATA request, the remote SMTP client may send one or more BDAT requests. In the example below, "S:" indicates server responses, and "C:" indicates client requests (bold font).

    S: 220
    C: EHLO
    S: 250-SIZE 153600000
    S: 250-VRFY
    S: 250-ETRN
    S: 250-STARTTLS
    S: 250-8BITMIME
    S: 250-DSN
    S: 250-SMTPUTF8
    S: 250 CHUNKING
    C: MAIL FROM:<>
    S: 250 2.1.0 Ok
    C: RCPT TO:<>
    S: 250 2.1.5 Ok
    C: BDAT 10000
    C: ..followed by 10000 bytes...
    S: 250 2.0.0 Ok: 10000 bytes
    C: BDAT 123
    C: ..followed by 123 bytes...
    S: 250 2.0.0 Ok: 123 bytes
    C: BDAT 0 LAST
    S: 250 2.0.0 Ok: 10123 bytes queued as 41yYhh41qmznjbD
    C: QUIT
    S: 221 2.0.0 Bye

Internally in Postfix, there is no difference between mail that was received with BDAT or with DATA. Postfix smtpd_mumble_restrictions, policy delegation queries, smtpd_proxy_filter and Milters all behave as if Postfix received (MAIL + RCPT + DATA + end-of-data). However, Postfix will log BDAT-related failures as "xxx after BDAT" to avoid complicating troubleshooting (xxx = 'lost connection' or 'timeout'), and will log a warning when a client sends a malformed BDAT command.

Benefits of CHUNKING (BDAT) support without BINARYMIME

Support for CHUNKING (BDAT) was added to improve interoperability with some clients, a benefit that would reportedly exist even without Postfix support for BINARYMIME. Since June 2018, Wietse's mail server has received BDAT commands from a variety of systems.

Postfix does not support BINARYMIME at this time because:

Downsides of CHUNKING (BDAT) support

The RFC 3030 authors did not specify any limitations on how clients may pipeline commands (i.e. send commands without waiting for a server response). If a server announces PIPELINING support, like Postfix does, then a remote SMTP client can pipeline all commands following EHLO, for example, MAIL/RCPT/BDAT/BDAT/MAIL/RCPT/BDAT, without ever having to wait for a server response. This means that with BDAT, the Postfix SMTP server cannot distinguish between a well-behaved client and a spambot, based on their command pipelining behavior. If you require "reject_unauth_pipelining" to block spambots, then turn off Postfix's CHUNKING announcement as described above.

In RFC 4468, the authors write that a client may pipeline commands, and that after sending BURL LAST or BDAT LAST, a client must wait for the server's response. But as this text does not appear in RFC 3030 which defines BDAT, is it a useless restriction that Postfix will not enforce.