tech stuff.

I’ve been struggling with the following warning in postfix for a while now:

postfix/trivial-rewrite[xxxx]: warning: do not list domain subdomain.blarg.org in BOTH virtual_alias_domains and relay_domains

The problem is, I’m not listing it in both.  I swear.

% postconf relay_domains
relay_domains = $mydestination
% postconf mydestination
mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain
% postconf virtual_alias_domains
virtual_alias_domains = subdomain.blarg.org

I finally tracked it down today to a relatively obscure (to me, anyway) feature in postfix that causes subdomains to be implicitly matched for certain coniguration parameters. From postconf(5):

parent_domain_matches_subdomains (default: see postconf -d output)
What Postfix features match subdomains of “domain.tld” automatically, instead of requiring an explicit “.domain.tld” pattern. This is planned backwards compatibility: eventually, all Postfix features are expected to require explicit “.domain.tld” style patterns when you really want to match subdomains.

This was exactly what was happening in my case.  I was being explicit in my relay_domains and mydestinations, but postfix was matching subdomains for relay_domains:

% postconf parent_domain_matches_subdomains
parent_domain_matches_subdomains = debug_peer_list,fast_flush_domains,mynetworks,permit_mx_backup_networks,qmqpd_authorized_clients,relay_domains,smtpd_access_maps

Since this is a feature with planned obsolescence that I’m not really using anyway (afaik), I just completely disabled it:

# Fixes the "do not list domain in BOTH" nags, also the future default behavior
# To enable subdomain matching, use .domain.com
parent_domain_matches_subdomains =

Apparently it’s still possible to explicitly use subdomain matching by specifying the domain as .blarg.org, which makes things a lot more self-evident, anyway.