Postfix Backwards-Compatibility Safety Net

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Purpose of this document

Postfix 3.0 introduces a safety net that runs Postfix programs with backwards-
compatible default settings after an upgrade. The safety net will log a warning
whenever a "new" default setting could have an negative effect on your mail
flow.

This document provides information on the following topics:

  * Detailed descriptions of Postfix backwards-compatibility warnings.

  * What backwards-compatible settings you may have to make permanent in
    main.cf or master.cf.

  * How to turn off Postfix backwards-compatibility warnings.

Overview

With backwards compatibility turned on, Postfix logs a message whenever a
backwards-compatible default setting may be required for continuity of service.
Based on this logging the system administrator can decide if any backwards-
compatible settings need to be made permanent in main.cf or master.cf, before
turning off the backwards-compatibility safety net as described at the end of
this document.

Logged with compatibility_level < 1:

  * Using backwards-compatible default setting append_dot_mydomain=yes

  * Using backwards-compatible default setting chroot=y

  * Using backwards-compatible default setting "smtpd_relay_restrictions =
    (empty)"

  * Using backwards-compatible default setting smtputf8_enable=no

Logged with compatibility_level < 2:

  * Using backwards-compatible default setting mynetworks_style=subnet

  * Using backwards-compatible default setting relay_domains=$mydestination

Logged with compatibility_level < 3.6:

  * Using backwards-compatible default setting smtpd_tls_fingerprint_digest=md5

  * Using backwards-compatible default setting smtp_tls_fingerprint_digest=md5

  * Using backwards-compatible default setting lmtp_tls_fingerprint_digest=md5

  * Using backwards-compatible default setting
    smtpd_relay_before_recipient_restrictions=no

  * Using backwards-compatible default setting respectful_logging=no

Logged with compatibility_level < 3.11:

  * using backwards-compatible default setting
    smtp_tlsrpt_skip_reused_handshakes=yes

  * using backwards-compatible default setting xxx_security_level=(empty)

If such a message is logged in the context of a legitimate request, the system
administrator should make the backwards-compatible setting permanent in main.cf
or master.cf, as detailed in the sections that follow.

When no more backwards-compatible settings need to be made permanent, the
system administrator should turn off the backwards-compatibility safety net as
described at the end of this document.

Using backwards-compatible default setting append_dot_mydomain=yes

The append_dot_mydomain default value has changed from "yes" to "no". This
could result in unexpected non-delivery of email after Postfix is updated from
an older version. The backwards-compatibility safety net is designed to prevent
such surprises.

As long as the append_dot_mydomain parameter is left unspecified at its
implicit default value, and the compatibility_level setting is less than 1,
Postfix may log one of the following messages:

  * Messages about missing "localhost" in mydestination or other address class:

        postfix/trivial-rewrite[14777]: using backwards-compatible
            default setting append_dot_mydomain=yes to rewrite
            "localhost" to "localhost.example.com"; please add
            "localhost" to mydestination or other address class

    If Postfix logs the above message, add "localhost" to mydestination (or
    virtual_alias_domains, virtual_mailbox_domains, or relay_domains) and
    execute the command "postfix reload".

  * Messages about incomplete domains in email addresses:

        postfix/trivial-rewrite[25835]: using backwards-compatible
            default setting append_dot_mydomain=yes to rewrite "foo" to
            "foo.example.com"

    If Postfix logs the above message for domains different from "localhost",
    and the sender cannot be changed to use complete domain names in email
    addresses, then the system administrator should make the backwards-
    compatible setting "append_dot_mydomain = yes" permanent in main.cf:

        # postconf append_dot_mydomain=yes
        # postfix reload

Using backwards-compatible default setting chroot=y

The master.cf chroot default value has changed from "y" (yes) to "n" (no). The
new default avoids the need for copies of system files under the Postfix queue
directory. However, sites with strict security requirements may want to keep
the chroot feature enabled after updating Postfix from an older version. The
backwards-compatibility safety net is designed allow the administrator to
choose if they want to keep the old behavior.

As long as a master.cf chroot field is left unspecified at its implicit default
value, and the compatibility_level setting is less than 1, Postfix may log the
following message while it reads the master.cf file:

    postfix/master[27664]: /etc/postfix/master.cf: line 72: using
        backwards-compatible default setting chroot=y

If this service should remain chrooted, then the system administrator should
make the backwards-compatible setting "chroot = y" permanent in master.cf. For
example, to update the chroot setting for the "smtp inet" service:

    # postconf -F smtp/inet/chroot=y
    # postfix reload

Using backwards-compatible default setting smtpd_relay_restrictions = (empty)

The smtpd_relay_restrictions feature was introduced with Postfix version 2.10,
as a safety mechanism for configuration errors in smtpd_recipient_restrictions
that could make Postfix an open relay.

The smtpd_relay_restrictions implicit default setting forbids mail to remote
destinations from clients that don't match permit_mynetworks or
permit_sasl_authenticated. This could result in unexpected 'Relay access
denied' errors after Postfix is updated from an older Postfix version. The
backwards-compatibility safety net is designed to prevent such surprises.

When the compatibility_level less than 1, and the smtpd_relay_restrictions
parameter is left unspecified at its implicit default setting, Postfix may log
the following message:

    postfix/smtpd[38463]: using backwards-compatible default setting
        "smtpd_relay_restrictions = (empty)" to avoid "Relay access
        denied" error for recipient "user@example.com" from client
        "host.example.net[10.0.0.2]"

If this request should not be blocked, then the system administrator should
make the backwards-compatible setting "smtpd_relay_restrictions=" (i.e. empty)
permanent in main.cf:

    # postconf smtpd_relay_restrictions=
    # postfix reload

Using backwards-compatible default setting smtputf8_enable=no

The smtputf8_enable default value has changed from "no" to "yes". With the new
"yes" setting, the Postfix SMTP server rejects non-ASCII addresses from clients
that don't request SMTPUTF8 support, after Postfix is updated from an older
version. The backwards-compatibility safety net is designed to prevent such
surprises.

As long as the smtputf8_enable parameter is left unspecified at its implicit
default value, and the compatibility_level setting is less than 1, Postfix logs
a warning each time an SMTP command uses a non-ASCII address localpart without
requesting SMTPUTF8 support:

    postfix/smtpd[27560]: using backwards-compatible default setting
        smtputf8_enable=no to accept non-ASCII sender address
        "??@example.org" from localhost[127.0.0.1]

    postfix/smtpd[27560]: using backwards-compatible default setting
        smtputf8_enable=no to accept non-ASCII recipient address
        "??@example.com" from localhost[127.0.0.1]

If the address should not be rejected, and the client cannot be updated to use
SMTPUTF8, then the system administrator should make the backwards-compatible
setting "smtputf8_enable = no" permanent in main.cf:

    # postconf smtputf8_enable=no
    # postfix reload

Using backwards-compatible default setting mynetworks_style=subnet

The mynetworks_style default value has changed from "subnet" to "host". This
parameter is used to implement the "permit_mynetworks" feature. The change
could cause unexpected 'access denied' errors after Postfix is updated from an
older version. The backwards-compatibility safety net is designed to prevent
such surprises.

As long as the mynetworks and mynetworks_style parameters are left unspecified
at their implicit default values, and the compatibility_level setting is less
than 2, the Postfix SMTP server may log one of the following messages:

    postfix/smtpd[17375]: using backwards-compatible default setting
        mynetworks_style=subnet to permit request from client
        "foo.example.com[10.1.1.1]"

    postfix/postscreen[24982]: using backwards-compatible default
        setting mynetworks_style=subnet to permit request from client
        "10.1.1.1"

If the client request should not be rejected, then the system administrator
should make the backwards-compatible setting "mynetworks_style = subnet"
permanent in main.cf:

    # postconf mynetworks_style=subnet
    # postfix reload

Using backwards-compatible default setting relay_domains=$mydestination

The relay_domains default value has changed from "$mydestination" to the empty
value. This could result in unexpected 'Relay access denied' errors or ETRN
errors after Postfix is updated from an older version. The backwards-
compatibility safety net is designed to prevent such surprises.

As long as the relay_domains parameter is left unspecified at its implicit
default value, and the compatibility_level setting is less than 2, Postfix may
log one of the following messages.

  * Messages about accepting mail for a remote domain:

        postfix/smtpd[19052]: using backwards-compatible default setting
            relay_domains=$mydestination to accept mail for domain
            "foo.example.com"

        postfix/smtpd[19052]: using backwards-compatible default setting
            relay_domains=$mydestination to accept mail for address
            "user@foo.example.com"

  * Messages about providing ETRN service for a remote domain:

        postfix/smtpd[19138]: using backwards-compatible default setting
            relay_domains=$mydestination to flush mail for domain
            "bar.example.com"

        postfix/smtp[13945]: using backwards-compatible default setting
            relay_domains=$mydestination to update fast-flush logfile for
            domain "bar.example.com"

If Postfix should continue to accept mail for that domain or continue to
provide ETRN service for that domain, then the system administrator should make
the backwards-compatible setting "relay_domains = $mydestination" permanent in
main.cf:

    # postconf 'relay_domains=$mydestination'
    # postfix reload

Note: quotes are required as indicated above.

Instead of $mydestination, it may be better to specify an explicit list of
domain names.

Using backwards-compatible default setting smtpd_tls_fingerprint_digest=md5

The smtpd_tls_fingerprint_digest default value has changed from "md5" to
"sha256". With the new "sha256" setting, the Postfix SMTP server avoids using
the deprecated "md5" algorithm and computes a more secure digest of the client
certificate.

If you're using the default "md5" setting, or even an explicit "sha1" (also
deprecated) setting, you should consider switching to "sha256". This will
require updating any associated lookup table keys with the "sha256" digests of
the expected client certificate or public key.

As long as the smtpd_tls_fingerprint_digest parameter is left unspecified at
its implicit default value, and the compatibility_level setting is less than
3.6, Postfix logs a warning each time a client certificate or public key
fingerprint is (potentially) used for access control:

    postfix/smtpd[27560]: using backwards-compatible default setting
        smtpd_tls_fingerprint_digest=md5 to compute certificate fingerprints

Since any client certificate fingerprints are passed in policy service lookups,
and Postfix doesn't know whether the fingerprint will be used, the warning may
also be logged when policy lookups are performed for connections that used a
client certificate, even if the policy service does not in fact examine the
client certificate. To reduce the noise somewhat, such warnings are issued at
most once per smtpd(8) process instance.

If you prefer to stick with "md5", you can suppress the warnings by making that
setting explicit. After addressing any other compatibility warnings, you can
update your compatibility level.

    # postconf smtpd_tls_fingerprint_digest=md5
    # postfix reload

Using backwards-compatible default setting smtp_tls_fingerprint_digest=md5

The smtp_tls_fingerprint_digest and lmtp_tls_fingerprint_digest default values
have changed from "md5" to "sha256". With the new "sha256" setting, the Postfix
SMTP and LMTP client avoids using the deprecated "md5" algorithm and computes a
more secure digest of the server certificate.

If you're using the default "md5" setting, or even an explicit "sha1" (also
deprecated) setting, you should consider switching to "sha256". This will
require updating any "fingerprint" security level policies in the TLS policy
table to specify matching "sha256" digests of the expected server certificates
or public keys.

As long as the smtp_tls_fingerprint_digest (or LMTP equivalent) parameter is
left unspecified at its implicit default value, and the compatibility_level
setting is less than 3.6, Postfix logs a warning each time the "fingerprint"
security level is used to specify matching "md5" digests of trusted server
certificates or public keys:

    postfix/smtp[27560]: using backwards-compatible default setting
        smtp_tls_fingerprint_digest=md5 to compute certificate fingerprints

If you prefer to stick with "md5", you can suppress the warnings by making that
setting explicit. After addressing any other compatibility warnings, you can
update your compatibility level.

    # postconf 'smtp_tls_fingerprint_digest = md5' \
        'lmtp_tls_fingerprint_digest = md5'
    # postfix reload

Using backwards-compatible default setting
smtpd_relay_before_recipient_restrictions=no

The smtpd_relay_before_recipient_restrictions feature was introduced in Postfix
version 3.6, to evaluate smtpd_relay_restrictions before
smtpd_recipient_restrictions. Historically, smtpd_relay_restrictions was
evaluated after smtpd_recipient_restrictions, contradicting documented
behavior.

    Background: smtpd_relay_restrictions is primarily designed to enforce a
    mail relaying policy, while smtpd_recipient_restrictions is primarily
    designed to enforce spam blocking policy. Both are evaluated while replying
    to the RCPT TO command, and both support the same features.

To maintain compatibility with earlier versions, Postfix will keep evaluating
smtpd_recipient_restrictions before smtpd_relay_restrictions, as long as the
compatibility_level is less than 3.6, and the
smtpd_relay_before_recipient_restrictions parameter is left unspecified at its
implicit default setting. As a reminder, Postfix may log the following message:

    postfix/smtpd[54696]: using backwards-compatible default setting
        smtpd_relay_before_recipient_restrictions=no to reject recipient
        "user@example.com" from client "host.example.net[10.0.0.2]"

If Postfix should keep evaluating smtpd_recipient_restrictions before
smtpd_relay_restrictions, then the system administrator should make the
backwards-compatible setting "smtpd_relay_before_recipient_restrictions=no"
permanent in main.cf:

    #  postconf smtpd_relay_before_recipient_restrictions=no
    #  postfix reload

Using backwards-compatible default setting respectful_logging=no

Postfix version 3.6 deprecates configuration parameter names and logging that
suggest white is better than black. Instead it prefers 'allowlist, 'denylist',
and variations of those words. While the renamed configuration parameters have
backwards-compatible default values, the changes in logging could affect
logfile analysis tools.

To avoid breaking existing logfile analysis tools, Postfix will keep logging
the deprecated form, as long as the respectful_logging parameter is left
unspecified at its implicit default value, and the compatibility_level setting
is less than 3.6. As a reminder, Postfix may log the following when a remote
SMTP client is allowlisted or denylisted:

    postfix/postscreen[22642]: Using backwards-compatible default setting
        respectful_logging=no for client [address]:port

If Postfix should keep logging the deprecated form, then the system
administrator should make the backwards-compatible setting "respectful_logging
= no" permanent in main.cf.

    # postconf "respectful_logging = no"
    # postfix reload

Using backwards-compatible default setting
smtp_tlsrpt_skip_reused_handshakes=yes

Postfix version 3.11 changes the default value for
smtp_tlsrpt_skip_reused_handshakes from "yes" to "no". The backwards-
compatibility safety net is designed to prevent an unexpected change in
reporting behavior when Postfix is updated from an older version.

As long as the smtp_tlsrpt_skip_reused_handshakes parameter is left unspecified
at its implicit default value, and the compatibility_level setting is less than
3.11, Postfix will log a reminder that it is using the backwards-compatible
default:

    postfix/smtp[388157] using backwards-compatible default setting
        smtp_tlsrpt_skip_reused_handshakes=yes

To keep the old default setting, the system administrator should make the
backwards-compatible setting "smtp_tlsrpt_skip_reused_handshakes = yes"
permanent in main.cf:

    # postconf smtp_tlsrpt_skip_reused_handshakes=yes
    # postfix reload

Using backwards-compatible default setting xxx_security_level=(empty)

When built with TLS support, Postfix version 3.11 changes the default SMTP
client TLS security level from "empty" to "may". The backwards-compatibility
safety net is designed to prevent an unexpected change in mail sending behavior
when Postfix is updated from an older version.

There is no equivalent change for Postfix server TLS security levels, because
changing the level alone is not sufficient. Server-side TLS requires that at
least one private key and one public-key certificate chain are configured.

As long as a TLS security level parameter is left unspecified at its implicit
default value, and the compatibility_level setting is less than 3.11, Postfix
will log one of the following reminders that it is using the backwards-
compatible default:

    postfix/smtp[...] using backwards-compatible default setting
        smtp_tls_security_level=(empty)

    postfix/tlsproxy[...] using backwards-compatible default setting
        tlsproxy_client_security_level=(empty)

To keep the old default setting, the system administrator should make the
backwards-compatible empty setting permanent in main.cf:

    # postconf xxx_security_level=
    # postfix reload

where xxx is taken from the above compatibility message.

Turning off the backwards-compatibility safety net

Backwards compatibility is turned off by updating the compatibility_level
setting in main.cf.

    # postconf compatibility_level=N
    # postfix reload

For N specify the number that is logged in your postfix(1) warning message:

    warning: To disable backwards compatibility use "postconf
    compatibility_level=N" and "postfix reload"

Sites that don't care about backwards compatibility may set
"compatibility_level = 9999" at their own risk.

Starting with Postfix version 3.6, the compatibility level in the above warning
message is the Postfix version that introduced the last incompatible change.
The level is formatted as major.minor.patch, where patch is usually omitted and
defaults to zero. Earlier compatibility levels are 0, 1 and 2.

NOTE: Postfix 3.6 also introduces support for the "<level", "<=level", and
other operators to compare compatibility levels. With the standard operators
"<", "<=", etc., compatibility level "3.10" would be smaller than "3.9" which
is undesirable.

