From: Johannes Thyssen Tishman <jtt@openbsd.org>
Subject: Re: smtpd: fix using modifiers with partial expansion in format specifiers
To: Omar Polo <op@omarpolo.com>
Cc: tech@openbsd.org
Date: Fri, 3 Apr 2026 10:21:37 +0000

2026-04-03T11:32:22+0200 Omar Polo <op@omarpolo.com>:
> Hello,
> 
> Johannes Thyssen Tishman <jtt@openbsd.org> wrote:
> > As described in smtpd.conf(5), format specifiers, e.g. '%{sender.user}',
> > support partial expansion using bracket notation (%{sender.user[n:m]}),
> > as well as modifiers (%{sender.user:strip}). However using both at the
> > same time (%{sender.user[n:m]:strip}) gives the following error:
> > 
> >   "smtpd: mda command line could not be expanded"
> > 
> > This occurs because the leading colon from the modifier is not stripped,
> > resulting in a "modifier not found" error. The diff below fixes this. I
> > also added a comment to smtpd.conf(5) to clarify that modifiers and
> > partial expansion *can* be used together and that, when used together,
> > modifiers are applied before partial expansion.
> > 
> > For those interested, a good example of how this can be used is sorting
> > emails from OpenBSD mailing lists into their own maildirs, e.g.:
> > 
> >   action "obsd_sort" maildir "~/mail/.OpenBSD.%{sender.user[6:]:strip}"
> >   match from mail-from regex "^owner-[^@]*@example\.org$" \
> >           for local action "obsd_sort"
> 
> nice =)
> 
> I have sieve filters for this, but it's nice that it can be done also
> with just the modifiers!
> 
> diffs make sense to me, nits on the manpage:
> 
> > [...]
> > --- usr.sbin/smtpd/smtpd.conf.5
> > +++ usr.sbin/smtpd/smtpd.conf.5
> > @@ -1144,6 +1144,13 @@ For example, with recipient
> >  .It %{rcpt:lowercase|strip} Ta expands to Dq user@example.org
> >  .El
> >  .Pp
> > +Modifiers and partial expansion can be used together. In such cases, the
> 
> new sentence, new line
> 
> > +modifiers are applied before the partial expansion. For example, with sender
> 
> ditto
> 
> > +.Dq owner-tech+M123456@openbsd.org :
> > +.Bl -column %{sender.user[6:]:strip} -offset indent
> > +.It %{sender.user[6:]:strip} Ta expands to Dq tech
> > +.El
> > +.Pp
> >  For security concerns, expanded values are sanitized and potentially
> >  dangerous characters are replaced with
> >  .Sq \&: .
> 
> please run this through `man -Tlint -l smtpd.conf.5' to verify :P
> 
> with this fixed, the diff is okay op@, thanks!

Thanks op@, committed!