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<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix
Backwards-Compatibility Safety Net</h1>

<hr>

<h2>Purpose of this document </h2>

<p> 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. </p>

<p>This document provides information on the following topics: </p>

<ul>

<li> <p> <a href="#overview">Detailed descriptions</a> of Postfix
backwards-compatibility warnings.

<li> <p> What backwards-compatible settings you may have to make
permanent in main.cf or master.cf.  </p>

<li> <p> <a href="#turnoff">How to turn off</a> Postfix
backwards-compatibility warnings. </p>

</ul>

<h2> <a name="overview"> Overview </a> </h2>

<p>  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 <a href="#turnoff">turning
off the backwards-compatibility safety net</a> as described at the
end of this document. </p>

<p> Logged with compatibility_level &lt; 1: </p>

<ul>

<li> <p> <a href="#append_dot_mydomain"> Using backwards-compatible
default setting append_dot_mydomain=yes </a> </p>

<li> <p> <a href="#chroot"> Using backwards-compatible default setting
chroot=y</a> </p>

</ul>

<p> Logged with compatibility_level &lt; 2: </p>

<ul>

<li><p> <a href="#relay_restrictions"> Using backwards-compatible
default setting "smtpd_relay_restrictions = (empty)"</a> </p>

<li> <p> <a href="#mynetworks_style"> Using backwards-compatible
default setting mynetworks_style=subnet </a> </p>

<li> <p> <a href="#relay_domains"> Using backwards-compatible default
setting relay_domains=$mydestination </a> </p>

<li> <p> <a href="#smtputf8_enable"> Using backwards-compatible
default setting smtputf8_enable=no</a> </p>

</ul>

<p> Logged with compatibility_level &lt; 3.6: </p>

<ul>

<li> <p> <a href="#smtpd_digest"> Using backwards-compatible
default setting smtpd_tls_fingerprint_digest=md5</a> </p>

<li> <p> <a href="#smtp_digest"> Using backwards-compatible
default setting smtp_tls_fingerprint_digest=md5</a> </p>

<li> <p> <a href="#smtp_digest"> Using backwards-compatible
default setting lmtp_tls_fingerprint_digest=md5</a> </p>

<li> <p> <a href="#relay_before_rcpt"> Using backwards-compatible
default setting smtpd_relay_before_recipient_restrictions=no</a> </p>

<li> <p> <a href="#respectful_logging"> Using backwards-compatible
default setting respectful_logging=no</a> </p>

</ul>

<p> 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. </p>

<p> When no more backwards-compatible settings need to be made
permanent, the system administrator should <a href="#turnoff">turn
off the backwards-compatibility safety net</a> as described at the
end of this document. </p>

<h2> <a name="append_dot_mydomain"> Using backwards-compatible default
setting append_dot_mydomain=yes</a> </h2>

<p> 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. </p>

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

<ul>

<li> <p> Messages about missing "localhost" in mydestination or
other address class: </p>

<blockquote>
<pre>
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
</pre>
</blockquote>

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

<li> <p> Messages about incomplete domains in email addresses: </p>

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

<p> 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: </p>

<blockquote>
<pre>
# <b>postconf append_dot_mydomain=yes</b>
# <b>postfix reload</b>
</pre>
</blockquote>

</ul>

<h2> <a name="chroot"> Using backwards-compatible default
setting chroot=y</a> </h2>

<p> 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. </p>

<p> As long as a master.cf chroot field is left 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: </p>

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

<p> 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: </p>

<blockquote>
<pre>
# <b>postconf -F smtp/inet/chroot=y</b>
# <b>postfix reload</b>
</pre>
</blockquote>

<h2> <a name="relay_restrictions"> Using backwards-compatible default
setting smtpd_relay_restrictions = (empty)</a> </h2>

<p> 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.
</p>

<p> 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. </p>

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

<blockquote>
<pre>
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]"
</pre>
</blockquote>

<p> 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:

<blockquote>
<pre>
# <b>postconf smtpd_relay_restrictions=</b>
# <b>postfix reload</b>
</pre>
</blockquote>

<h2> <a name="mynetworks_style"> Using backwards-compatible default
setting mynetworks_style=subnet</a> </h2>

<p> 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. </p>

<p> As long as the mynetworks and mynetworks_style parameters are
left 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: </p>

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

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

<p> 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: </p>

<blockquote>
<pre>
# <b>postconf mynetworks_style=subnet</b>
# <b>postfix reload</b>
</pre>
</blockquote>

<h2><a name="relay_domains"> Using backwards-compatible default
setting relay_domains=$mydestination </a> </h2>

<p> 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. </p>

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

<ul>

<li> <p> Messages about accepting mail for a remote domain:</p>

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

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

<li> <p> Messages about providing ETRN service for a remote domain:</p>

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

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

</ul>

<p> 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: </p>

<blockquote>
<pre>
# <b>postconf 'relay_domains=$mydestination'</b>
# <b>postfix reload</b>
</pre>
</blockquote>

<p> Note: quotes are required as indicated above. </p>

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

<h2> <a name="smtputf8_enable"> Using backwards-compatible default
setting smtputf8_enable=no</a> </h2>

<p> 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. </p>

<p> As long as the smtputf8_enable parameter is left 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: </p>

<blockquote>
<pre>
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]
</pre>
</blockquote>

<blockquote>
<pre>
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]
</pre>
</blockquote>

<p> 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:

<blockquote>
<pre>
# <b>postconf smtputf8_enable=no</b>
# <b>postfix reload</b>
</pre>
</blockquote>

<h2> <a name="smtpd_digest"> Using backwards-compatible
default setting smtpd_tls_fingerprint_digest=md5</a> </h2>

<p> 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.  </p>

<p> 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.  </p>

<p> As long as the smtpd_tls_fingerprint_digest parameter is left 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: </p>

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

<p> 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.  </p>

<p> 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 <a href="#turnoff">update</a> your compatibility level.
</p>

<blockquote>
<pre>
# <b>postconf smtpd_tls_fingerprint_digest=md5</b>
# <b>postfix reload</b>
</pre>
</blockquote>

<h2> <a name="smtp_digest"> Using backwards-compatible
default setting smtp_tls_fingerprint_digest=md5</a> </h2>

<p> 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.  </p>

<p> 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.  </p>

<p> As long as the smtp_tls_fingerprint_digest (or LMTP equivalent)
parameter is left 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: </p>

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

<p> 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 <a href="#turnoff">update</a> your compatibility level.
</p>

<blockquote>
<pre>
# <b>postconf 'smtp_tls_fingerprint_digest = md5' \
    'lmtp_tls_fingerprint_digest = md5' </b>
# <b>postfix reload</b>
</pre>
</blockquote>

<h2> <a name="relay_before_rcpt"> Using backwards-compatible
default setting smtpd_relay_before_recipient_restrictions=no</a> </h2>

<p> 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. </p>

<blockquote> <p> 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. </p> </blockquote>

<p> 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 at its implicit default setting. As a reminder,
Postfix may log the following message: </p>

<blockquote>
<pre>
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]"
</pre>
</blockquote>

<p> 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: </p>

<blockquote>
<pre>
# <b> postconf smtpd_relay_before_recipient_restrictions=no </b>
# <b> postfix reload </b>
</pre>
</blockquote>

<h2> <a name="respectful_logging"> Using backwards-compatible
default setting respectful_logging=no</a> </h2>

<p> 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. </p>

<p> To avoid breaking existing logfile analysis tools, Postfix will keep
logging the deprecated form, as long as the respectful_logging parameter
is left 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: </p>

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

<p> 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.

<blockquote>
<pre>
# <b>postconf "respectful_logging = no"</b>
# <b>postfix reload</b>
</pre>
</blockquote>

<h2> <a name="turnoff">Turning off the backwards-compatibility safety net</a> </h2>

<p> Backwards compatibility is turned off by updating the
compatibility_level setting in main.cf. </p>

<blockquote>
<pre>
# <b>postconf compatibility_level=<i>N</i></b>
# <b>postfix reload</b>
</pre>
</blockquote>

<p> For <i>N</i> specify the number that is logged in your postfix(1)
warning message: </p>

<blockquote>
<pre>
warning: To disable backwards compatibility use "postconf compatibility_level=<i>N</i>" and "postfix reload"
</pre>
</blockquote>

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

<p> 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
<i>major.minor.patch</i>, where <i>patch</i> is usually omitted and
defaults to zero. Earlier compatibility levels are 0, 1 and 2. </p>

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

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