[pmg-devel] [PATCH pmg-api v4] fix-2971: DKIM: Add setting to use From header when signing
Maximiliano Sandoval
m.sandoval at proxmox.com
Tue Jan 16 11:43:01 CET 2024
We add an option to use the address from the `From:` header instead of
the Envelope From address. Following RFC 5322 [2], we use the `Sender:`
header when there are multiple addresses in the `From:` header.
>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 at 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 at bar1 --to EMAIL -s PMG_ADDR:26 --data "Date: %DATE%\nTo: %TO_ADDRESS%\nFrom: foo at bar2\nSubject: test %DATE%\nMessage-Id: <%MESSAGEID%>\nX-Mailer: swaks v%SWAKS_VERSION% jetmore.org/john/code/swaks/\n%NEW_HEADERS%\n%BODY%\n"
[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 at proxmox.com>
---
Do note that for testing one has to restart pmg-smtp-filter after changing the setting value.
Differences from v3:
- Take into account setting
- rename parsing function to parse_headers_for_signing
src/PMG/Config.pm | 7 ++++
src/PMG/DKIMSign.pm | 72 ++++++++++++++++++++++++++++++++++++----
src/PMG/RuleDB/Accept.pm | 3 +-
src/PMG/RuleDB/BCC.pm | 3 +-
src/bin/pmg-smtp-filter | 1 +
5 files changed, 75 insertions(+), 11 deletions(-)
diff --git a/src/PMG/Config.pm b/src/PMG/Config.pm
index 7339e0d..840c0ad 100644
--- a/src/PMG/Config.pm
+++ b/src/PMG/Config.pm
@@ -134,6 +134,11 @@ EODESC
description => "Default DKIM selector",
type => 'string', format => 'dns-name', #see RFC6376 3.1
},
+ dkim_prefer_rfc5322_domain => {
+ description => "Whether to use the address from the From header when DKIM signing outbound mails.",
+ type => 'boolean',
+ default => 0,
+ },
};
}
@@ -152,6 +157,7 @@ sub options {
dkim_sign => { optional => 1 },
dkim_sign_all_mail => { optional => 1 },
dkim_selector => { optional => 1 },
+ dkim_prefer_rfc5322_domain => { optional => 1 },
};
}
@@ -1788,6 +1794,7 @@ my $pmg_service_params = {
dkim_selector => 1,
dkim_sign => 1,
dkim_sign_all_mail => 1,
+ dkim_prefer_rfc5322_domain => 1,
},
};
diff --git a/src/PMG/DKIMSign.pm b/src/PMG/DKIMSign.pm
index 08197f8..2c73101 100644
--- a/src/PMG/DKIMSign.pm
+++ b/src/PMG/DKIMSign.pm
@@ -2,6 +2,7 @@ package PMG::DKIMSign;
use strict;
use warnings;
+use Email::Address::XS;
use Mail::DKIM::Signer;
use Mail::DKIM::TextWrap;
use Crypt::OpenSSL::RSA;
@@ -53,11 +54,16 @@ sub create_signature {
#determines which domain should be used for signing based on the e-mailaddress
sub signing_domain {
- my ($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, $prefer_rfc5322_domain) = @_;
+
+ my $input_domain;
+ if ($prefer_rfc5322_domain) {
+ $input_domain = parse_headers_for_signing($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 $self->{sign_all};
@@ -84,8 +90,60 @@ sub signing_domain {
}
+sub parse_headers_for_signing {
+ # If there is exactly one single address in the 'From' header we use that
+ # address for signing, otherwise we use the 'Sender' header.
+ my ($entity) = @_;
+
+ my @sender_list;
+ 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) {
+ my @send_headers = $entity->head->get('sender');
+ foreach my $send_header (@send_headers) {
+ my @senders = Email::Address::XS->parse($send_header);
+ foreach my $sender (@senders) {
+ push(@sender_list, $sender);
+ }
+ }
+ my $send_count = scalar(@sender_list);
+
+ if ($send_count > 1) {
+ syslog(
+ 'warning',
+ "Email has more than one address in its 'Sender' it won't be signed."
+ ." See RFC 5322 Section 3.6.2"
+ );
+ } elsif ($send_count == 1) {
+ return $sender_list[0]->host();
+ } else {
+ syslog(
+ 'warning',
+ "Email has multiple addresses in its 'From' header and no 'Sender'"
+ ." header, it won't be signed. See RFC 5322 Section 3.6.2"
+ );
+ }
+ } elsif ($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 $prefer_rfc5322_domain = $dkim->{prefer_rfc5322_domain};
+ my $selector = $dkim->{selector};
die "no selector provided\n" if ! $selector;
@@ -110,7 +168,7 @@ sub sign_entity {
$signer->extended_headers($extended_headers);
- if ($signer->signing_domain($sender)) {
+ if ($signer->signing_domain($sender, $entity, $prefer_rfc5322_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..f7502f2 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}->{prefer_rfc5322_domain} = $pmg_cfg->get('admin', 'dkim_prefer_rfc5322_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
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