filter-rewrite-from

#!/usr/bin/awk -f
# vim: set ft=awk: ts=4:
#
# USAGE:
#   filter <filter-name> proc-exec "/path/to/filter-rewrite-from <mailaddr>"
#
# OpenSMTPD filter that rewrites email address in both MAIL FROM command and
# From header with the specified email address.
#
# ARGUMENTS:
#   <mailaddr>  Email address to use in MAIL FROM and From header. It must be
#               just an email address without a display name or angle brackets.
#
# Please report bugs at <https://github.com/jirutka/opensmtpd-rewrite-from/issues>.


function die(msg) {
	printf("mail-from: %s\n", msg) > "/dev/stderr"
	exit 1
}

# If the *address* contains a display name without "@" and "<", and an email
# address in angle brackets, replaces the email address with *mailaddr* and
# preserves the display name. Otherwise it's defensive and returns
# just *mailaddr*.
function replace_mailaddr(address, mailaddr) {
	if (address ~ /^[^@<]+<[^>]+> *$/) {
		res = address ""
		sub(/<[^>]+>/, "<" mailaddr ">", res)
		return res
	}
	return mailaddr
}

BEGIN {
	ARGC = 0  # don't treat additional CLI args as input files
	FS = "|"
	OFS = FS
	_ = FS
	MAILADDR = ARGV[1]
	in_body[""] = 0

	if (!MAILADDR) {
		die("invalid usage: mail address must be provided!")
	}
}

"config|ready" == $0 {
	print("register|filter|smtp-in|mail-from")
	print("register|filter|smtp-in|data-line")
	print("register|ready")
	fflush()
	next
}
"config" == $1 {
	next
}
"filter" == $1 {
	if (NF < 7) {
		die("invalid filter command: expected >6 fields!")
	}
	sid = $6
	token = $7
	line = substr($0, length($1$2$3$4$5$6$7) + 8)

	# continue with next rule...
}
"filter|smtp-in|mail-from" == $1_$4_$5 {
	print("filter-result", sid, token, "rewrite", "<" MAILADDR ">")
	fflush()
	next
}
"filter|smtp-in|data-line" == $1_$4_$5 {
	if (line == "") {  # end of headers
		in_body[sid] = 1
	}
	if (!in_body[sid] && match(toupper(line), /^FROM:[\t ]*/)) {
		line = "From: " replace_mailaddr(substr(line, RLENGTH + 1), MAILADDR)
	}
	if (line == ".") {  # end of data
		delete in_body[sid]
	}
	print("filter-dataline", sid, token, line)
	fflush()
	next
}