From: Johannes Thyssen Tishman <jtt@openbsd.org>
Subject: Re: smtpd: fix using modifiers with partial expansion in format specifiers
To: tech@openbsd.org
Date: Fri, 3 Apr 2026 10:15:26 +0000

2026-04-02T16:24:45+0000 Johannes Thyssen Tishman <jtt@openbsd.org>:
> 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"

Just noticed that the above example should use openbsd.org instead of
example.org if it is to do what I suggest. I forgot to update it after
testing. Updated for posterity below.

  action "obsd_sort" maildir "~/mail/.OpenBSD.%{sender.user[6:]:strip}"
  match from mail-from regex "^owner-[^@]*@openbsd\.org$" \
          for local action "obsd_sort"

> 
> diff /usr/src
> path + /usr/src
> commit - b47f74b4221666678dbec685b662de7d14a8b253
> blob - 0542f95de3337ec47fd0b7573178885eed868bec
> file + usr.sbin/smtpd/mda_variables.c
> --- usr.sbin/smtpd/mda_variables.c
> +++ usr.sbin/smtpd/mda_variables.c
> @@ -94,7 +94,8 @@ mda_expand_token(char *dest, size_t len, const char *t
>  			return -1;
>  
>  		/* token:mod_1,mod_2,mod_n -> extract modifiers */
> -		mods = strchr(rbracket + 1, ':');
> +		if ((mods = strchr(rbracket + 1, ':')) != NULL)
> +			*mods++ = '\0';
>  	} else {
>  		if ((mods = strchr(rtoken, ':')) != NULL)
>  			*mods++ = '\0';
> commit - b47f74b4221666678dbec685b662de7d14a8b253
> blob - 1cdaf067c5585ca69bb19baf72014d29adfe6747
> file + usr.sbin/smtpd/smtpd.conf.5
> --- 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
> +modifiers are applied before the partial expansion. For example, with sender
> +.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 \&: .
>