[pmg-devel] applied: [PATCH pmg-api v8] fix #2971: DKIM: add setting to use From header when signing

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

applied with a few follow-ups on top, which hopefully make things clearer
(and shorter). Maybe you want to check them out.

tested this in my setup and managed to get a mail signed with this code
delivered to my private gmail inbox (through a few intermediate relays)

Thanks for seeing this through and the continued improvements!


On Mon, 26 Feb 2024 15:03:02 +0100
Maximiliano Sandoval <m.sandoval@proxmox.com> wrote:

> 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>
> ---
> Differences from v7:
>  - Simplify code counting the number of senders
>  - `die` when the sender count is not 1
> 
> Differences from v6:
>  - Do not use uppercase in commit title
>  - Rename dkim_use_domain to dkim-use-domain
> 
>  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 71d4c0b..0c5419e 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 },
>      };
>  }
>  
> @@ -1808,6 +1815,7 @@ my $pmg_service_params = {
>  	dkim_selector => 1,
>  	dkim_sign => 1,
>  	dkim_sign_all_mail => 1,
> +	'dkim-use-domain' => 1,
>      },
>  };
>  
> diff --git a/src/PMG/DKIMSign.pm b/src/PMG/DKIMSign.pm
> index 08197f8..093ff34 100644
> --- a/src/PMG/DKIMSign.pm
> +++ b/src/PMG/DKIMSign.pm
> @@ -2,6 +2,8 @@ package PMG::DKIMSign;
>  
>  use strict;
>  use warnings;
> +use Email::Address::XS qw(parse_email_addresses);
> +use Email::Address::XS;
>  use Mail::DKIM::Signer;
>  use Mail::DKIM::TextWrap;
>  use Crypt::OpenSSL::RSA;
> @@ -53,11 +55,16 @@ sub create_signature {
>  
>  #determines which domain should be used for signing based on the e-mailaddress
>  sub signing_domain {
> -    my ($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 $self->{sign_all};
> @@ -84,8 +91,35 @@ sub signing_domain {
>  }
>  
>  
> +sub parse_headers_for_signing {
> +    # Following RFC 7489 [1], we only sign emails with exactly one sender in the
> +    # From header.
> +    #
> +    # [1] https://datatracker.ietf.org/doc/html/rfc7489#section-6.6.1
> +    my ($entity) = @_;
> +
> +    my $from_count = 0;
> +    my $domain;
> +
> +    my @from_headers = $entity->head->get('from');
> +    foreach my $from_header (@from_headers) {
> +	my @addresses = parse_email_addresses($from_header);
> +	$from_count += scalar(@addresses);
> +	$domain = $addresses[0]->host() if scalar(@addresses) > 0;
> +    }
> +
> +    die "there is more than one sender in the header" if $from_count > 1;
> +    die "there is no sender in the header" if $from_count == 0;
> +    return $domain;
> +}
> +
> +
>  sub sign_entity {
> -    my ($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 dc7128c..6061459 100755
> --- a/src/bin/pmg-smtp-filter
> +++ b/src/bin/pmg-smtp-filter
> @@ -636,6 +636,7 @@ 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');
>  	}