[pmg-devel] [PATCH pmg-api v5] fix-2971: DKIM: Add setting to use From header when signing

[Maximiliano Sandoval <m.sandoval@proxmox.com>]

Following RFC 5322 [2], we add an option to use the address from the
`From:` header instead of the Envelope From address.

From RFC 7489 [1]:

   To illustrate, in relaxed mode, if a validated DKIM signature
   successfully verifies with a "d=" domain of "example.com", and the
   RFC5322.From address is "alerts@news.example.com", the DKIM "d="
   domain and the RFC5322.From domain are considered to be "in
   alignment".  In strict mode, this test would fail, since the "d="
   domain does not exactly match the FQDN of the address.

Tested with the following command:

    swaks --from foo@envelope.domain --to EMAIL -s PMG_ADDR:26 --data "Date: %DATE%\nTo: %TO_ADDRESS%\nFrom: bar@header.domain\nSubject: test %DATE%\nMessage-Id: <%MESSAGEID%>\nX-Mailer: swaks v%SWAKS_VERSION% jetmore.org/john/code/swaks/\n%NEW_HEADERS%\n%BODY%\n"

Where EMAIL and PMG_ADDR are given adequate values, and envelope.domain
and header.domain are in the 'Sign Domains' list.

[1] https://datatracker.ietf.org/doc/html/rfc7489#section-3.1.1
[2] https://datatracker.ietf.org/doc/html/rfc5322#section-3.6.2

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
---
Do note that for testing one has to restart pmg-smtp-filter after changing the setting value.

Differences from v4:
 - Rename setting to dkim_use_domain and make it an enum
 - Improved commit message
 - Do not use 'Sender' field as a fallback

Differences from v3:
 - Take into account setting
 - rename parsing function to parse_headers_for_signing

 src/PMG/Config.pm        |  8 +++++++
 src/PMG/DKIMSign.pm      | 48 ++++++++++++++++++++++++++++++++++------
 src/PMG/RuleDB/Accept.pm |  3 +--
 src/PMG/RuleDB/BCC.pm    |  3 +--
 src/bin/pmg-smtp-filter  |  1 +
 5 files changed, 52 insertions(+), 11 deletions(-)

diff --git a/src/PMG/Config.pm b/src/PMG/Config.pm
index 7339e0d..e9e740d 100644
--- a/src/PMG/Config.pm
+++ b/src/PMG/Config.pm
@@ -134,6 +134,12 @@ EODESC
 	    description => "Default DKIM selector",
 	    type => 'string', format => 'dns-name', #see RFC6376 3.1
 	},
+	dkim_use_domain => {
+	    description => "Whether to sign using the address from the header or the envelope.",
+	    type => 'string',
+	    enum => [qw(header envelope)],
+	    default => 'envelope',
+	},
     };
 }
 
@@ -152,6 +158,7 @@ sub options {
 	dkim_sign => { optional => 1 },
 	dkim_sign_all_mail => { optional => 1 },
 	dkim_selector => { optional => 1 },
+	dkim_use_domain => { optional => 1 },
     };
 }
 
@@ -1788,6 +1795,7 @@ my $pmg_service_params = {
 	dkim_selector => 1,
 	dkim_sign => 1,
 	dkim_sign_all_mail => 1,
+	dkim_use_domain => 1,
     },
 };
 
diff --git a/src/PMG/DKIMSign.pm b/src/PMG/DKIMSign.pm
index 08197f8..5df9aa8 100644
--- a/src/PMG/DKIMSign.pm
+++ b/src/PMG/DKIMSign.pm
@@ -2,6 +2,7 @@ package PMG::DKIMSign;
 
 use strict;
 use warnings;
+use Email::Address::XS;
 use Mail::DKIM::Signer;
 use Mail::DKIM::TextWrap;
 use Crypt::OpenSSL::RSA;
@@ -53,11 +54,16 @@ sub create_signature {
 
 #determines which domain should be used for signing based on the e-mailaddress
 sub signing_domain {
-    my ($self, $sender_email) = @_;
-
-    my @parts = split('@', $sender_email);
-    die "no domain in sender e-mail\n" if scalar(@parts) < 2;
-    my $input_domain = $parts[-1];
+    my ($self, $sender_email, $entity, $use_domain) = @_;
+
+    my $input_domain;
+    if ($use_domain eq 'header') {
+	$input_domain = parse_headers_for_signing($entity);
+    } else {
+	my @parts = split('@', $sender_email);
+	die "no domain in sender e-mail\n" if scalar(@parts) < 2;
+	$input_domain = $parts[-1];
+    }
 
     if ($self->{sign_all}) {
 	    $self->domain($input_domain) if $self->{sign_all};
@@ -84,8 +90,36 @@ sub signing_domain {
 }
 
 
+sub parse_headers_for_signing {
+    # Following RFC 7489 [1], we only sign emails with exactly one sender in the
+    # From header.
+    #
+    # [1] https://datatracker.ietf.org/doc/html/rfc7489#section-6.6.1
+    my ($entity) = @_;
+
+    my @from_list;
+
+    my @from_headers = $entity->head->get('from');
+    foreach my $from_header (@from_headers) {
+	my @senders = Email::Address::XS->parse($from_header);
+	foreach my $sender (@senders) {
+	    push(@from_list, @senders);
+	}
+    }
+    my $from_count = scalar(@from_list);
+
+    if ($from_count == 1) {
+	return $from_list[0]->host();
+    }
+}
+
+
 sub sign_entity {
-    my ($entity, $selector, $sender, $sign_all) = @_;
+    my ($entity, $dkim, $sender) = @_;
+
+    my $sign_all = $dkim->{sign_all};
+    my $use_domain = $dkim->{use_domain};
+    my $selector = $dkim->{selector};
 
     die "no selector provided\n" if ! $selector;
 
@@ -110,7 +144,7 @@ sub sign_entity {
 
     $signer->extended_headers($extended_headers);
 
-    if ($signer->signing_domain($sender)) {
+    if ($signer->signing_domain($sender, $entity, $use_domain)) {
 	$entity->print($signer);
 	my $signature = $signer->create_signature();
 	$entity->head->add('DKIM-Signature', $signature, 0);
diff --git a/src/PMG/RuleDB/Accept.pm b/src/PMG/RuleDB/Accept.pm
index 4ebd6da..d14c2fb 100644
--- a/src/PMG/RuleDB/Accept.pm
+++ b/src/PMG/RuleDB/Accept.pm
@@ -102,8 +102,7 @@ sub execute {
 
 	if ($dkim->{sign}) {
 	    eval {
-		$entity = PMG::DKIMSign::sign_entity($entity,
-		    $dkim->{selector}, $msginfo->{sender}, $dkim->{sign_all});
+		$entity = PMG::DKIMSign::sign_entity($entity, $dkim, $msginfo->{sender});
 	    };
 	    syslog('warning',
 		"Could not create DKIM-Signature - disabling Signing: $@") if $@;
diff --git a/src/PMG/RuleDB/BCC.pm b/src/PMG/RuleDB/BCC.pm
index 0f016f8..65b6fb5 100644
--- a/src/PMG/RuleDB/BCC.pm
+++ b/src/PMG/RuleDB/BCC.pm
@@ -142,8 +142,7 @@ sub execute {
 	my $dkim = $msginfo->{dkim} // {};
 	if ($dkim->{sign}) {
 	    eval {
-		$entity = PMG::DKIMSign::sign_entity($entity,
-		    $dkim->{selector}, $msginfo->{sender}, $dkim->{sign_all});
+		$entity = PMG::DKIMSign::sign_entity($entity, $dkim, $msginfo->{sender});
 	    };
 	    syslog('warning',
 		"Could not create DKIM-Signature - disabling Signing: $@") if $@;
diff --git a/src/bin/pmg-smtp-filter b/src/bin/pmg-smtp-filter
index 7da3de8..aedae0c 100755
--- a/src/bin/pmg-smtp-filter
+++ b/src/bin/pmg-smtp-filter
@@ -627,6 +627,7 @@ sub handle_smtp {
 	my $dkim_sign = $msginfo->{trusted} && $pmg_cfg->get('admin', 'dkim_sign');
 	if ($dkim_sign) {
 	    $msginfo->{dkim}->{sign} = $dkim_sign;
+	    $msginfo->{dkim}->{use_domain} = $pmg_cfg->get('admin', 'dkim_use_domain');
 	    $msginfo->{dkim}->{sign_all} = $pmg_cfg->get('admin', 'dkim_sign_all_mail');
 	    $msginfo->{dkim}->{selector} = $pmg_cfg->get('admin', 'dkim_selector');
 	}
-- 
2.39.2