From: "Omar Polo" <op@omarpolo.com>
Subject: Re: smtpd: fix using modifiers with partial expansion in format specifiers
To: Johannes Thyssen Tishman <jtt@openbsd.org>
Cc: tech@openbsd.org
Date: Fri, 03 Apr 2026 11:32:22 +0200

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!