Re: [pmg-devel] [PATCH pmg-api v4] fix-2971: DKIM: Add setting to use From header when signing

[Stoiko Ivanov <s.ivanov@proxmox.com>]

Thanks for the update!

I got around to give it a spin in my testbed - and it looks ok from a
quick spin.

The docs-patch would be great for the next version!

a few comments inline:

On Tue, 16 Jan 2024 11:43:01 +0100
Maximiliano Sandoval <m.sandoval@proxmox.com> wrote:

> 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@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@bar1 --to EMAIL -s PMG_ADDR:26 --data "Date: %DATE%\nTo: %TO_ADDRESS%\nFrom: foo@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@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 => {
I checked what others do for those hard-to-name things - maybe we could
orient ourselves on other mail-filters:
* https://rspamd.com/doc/modules/dkim_signing.html (dkim_use_domain a enum
  with "envelope" and "header") - one upside is that that will remain a
  term even if RFC5322 is obsoleted at some future point...
* opendkim seems to only use header information (but lets you select the
  headers, which are search, which I'd like to avoid for usage-simplicity)
  - http://opendkim.org/opendkim.conf.5.html


> +	    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.
as hinted in my reply to your v1:
https://lists.proxmox.com/pipermail/pmg-devel/2024-January/002625.html
if I read the DMARC-RFC right this fallback is not wanted there
(and DKIM itself does not mention that the dkim-domain (d= tag in the
signature header, would need to be particularly related to either header
or envelope-addresses)


> +    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');
>  	}