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dkim: improve POD, add dkim_key_gen.sh

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This commit is contained in:
Matt Simerson 2013-04-14 21:42:21 -04:00
parent b8df80d398
commit 2f72f419c3
2 changed files with 108 additions and 23 deletions
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60
config.sample/dkim/dkim_key_gen.sh Executable file
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@ -0,0 +1,60 @@
#!/bin/sh
usage() {
echo " usage: $0 <example.com> [qpsmtpd username]"
echo " "
exit
}
if [ -z $1 ];
then
usage
fi
DOMAIN=$1
SMTPD=$2
if [ -z $SMTPD ];
then
SMTPD="smtpd"
fi
# create a directory for each DKIM signing domain
mkdir -p $DOMAIN
cd $DOMAIN
# create a selector in the format mmmYYYY (apr2013)
date '+%h%Y' | tr "[:upper:]" "[:lower:]" > selector
# generate a private and public keys
openssl genrsa -out private 2048
chmod 400 private
openssl rsa -in private -out public -pubout
# make it really easy to publish the public key in DNS
cat > dns <<EO_DKIM_DNS
`cat selector | tr -d "\n"`._domainkey TXT "v=DKIM1;p=`grep -v -e '^-' public | tr -d "\n"`"
_domainkey TXT "o=-; t=y; r=postmaster@$DOMAIN"
Tell the world that the ONLY mail servers that send mail from this domain bear our MX and A records.
With SPF:
SPF "v=spf1 mx a -all"
TXT "v=spf1 mx a -all"
With DMARC:
_dmarc TXT "v=DMARC1; p=reject; rua=mailto:dmarc-feedback@$DOMAIN; ruf=mailto:dmarc-feedback@'$DOMAIN; adkim=s; aspf=s; pct=100"
For more information about DKIM and SPF policy, the documentation within each plugin contains a longer discussion and links to more detailed information:
perldoc plugins/dkim
perldoc plugins/sender_permitted_from
EO_DKIM_DNS
cd ..
chown -R $SMTPD:$SMTPD $DOMAIN

71
plugins/dkim
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@ -15,8 +15,9 @@ sending policies, and DKIM sign outgoing messages.
dkim reject 0
Reject is a boolean that toggles message rejection on or off, or naughty,
which offloads a deferred rejection to the B<naughty> plugin.
0 - do not reject
1 - reject messages that fail DKIM policy
naughty - defer rejection to the B<naughty> plugin
Default: 1
@ -28,22 +29,23 @@ Default: perm
=head1 HOW TO SIGN
=head2 generate DKIM key(s)
=head2 generate DKIM keys
=head3 the easy way
cd ~smtpd/config/dkim; ./dkim_key_gen.sh example.org
=head3 the manual way
mkdir -p ~smtpd/config/dkim/example.org
cd ~smtpd/config/dkim/example.org
echo 'mar2013' > selector
echo 'may2013' > selector
openssl genrsa -out private 2048
chmod 400 private
openssl rsa -in private -out public -pubout
chown -R smtpd:smtpd ~smtpd/config/dkim/example.org
chown -R smtpd:smtpd ../example.org
After running the commands, you'll have a directory with three files:
example.org
example.org/selector
example.org/private
example.org/public
After generating the keys, there will be three files in the example.org directory: selector, private, and public.
=head3 selector
@ -51,11 +53,19 @@ The selector can be any value that is a valid DNS label.
=head3 key length
The minimum recommended key length for short duration keys (ones that will be replaced within a few months) is 1024. If you are unlikely to rotate your keys frequently, go with 2048, at the expense of more CPU.
The minimum recommended key length for short duration keys (ones that will be replaced within a few months) is 1024. If you are unlikely to rotate your keys frequently, go with 2048, at the expense of a bit more CPU.
=head2 publish public key in DNS
mar2013._domainkey TXT "v=DKIM1;p=[public key stripped of whitespace];"
If the DKIM keys were generated the easy way, there will be a fourth file named I<dns>. The contents contain the DNS formatted record of the public key, as well as suggestions for DKIM, SPF, and DMARC policy records. The records are ready to be copy/pasted into a BIND zone file, or better yet, NicTool, and published to most any DNS server. If you didn't create your keys the easy way, look inside the dkim_key_gen.sh script to see the commands used to format the DKIM public key.
The example DKIM, SPF, and DMARC policy records in the I<dns> file are strict, telling other mail servers that if a sender claims to be from example.org, but the message is not DKIM signed and not SPF aligned, then the message should be rejected. Many email servers, including the largest email providers (Gmail, Yahoo, Outlook/Live/Hotmail) will refuse to accept such messages, greatly reducing the harm caused by miscreants who send out spam with your domain name in the From header.
The DKIM record will look like this:
may2013._domainkey TXT "v=DKIM1;p=[public key stripped of whitespace];"
And the values in the address have the following meaning:
hash: h=[ sha1 | sha256 ]
test; t=[ s | s:y ]
@ -64,19 +74,30 @@ The minimum recommended key length for short duration keys (ones that will be re
services: s=[email]
keytypes: [ rsa ]
Prepare the DNS record with these commands:
cd ~smtpd/config/dkim/example.org
cat selector | tr -d "\n" > dns
echo -n '._domainkey TXT "v=DKIM1;p=' >> dns
grep -v -e '^-' public | tr -d "\n" >> dns
echo '"' >> dns
The contents of I<dns> are ready to be copy/pasted into a BIND zone file, or better yet, NicTool, and published to most any DNS server.
=head2 testing
After confirming that the DKIM public key can be fetched with DNS, send test messages via QP to a Gmail box and check the Authentication-Results header. There are also DKIM relays (check-auth@verifier.port25.com, checkmyauth@auth.returnpath.net) that provide more debugging information in a nice email report.
After confirming that the DKIM public key can be fetched with DNS (dig TXT may2013._domainkey.example.org. @ns1.example.org.), send test messages. There are a number of ways to test your DKIM:
* email to a Gmail address and inspect the Authentication-Results header.
* email to check-auth@verifier.port25.com
* email to checkmyauth@auth.returnpath.net
The two DKIM relays provide a nice email report with additional debugging information.
=head2 publish DKIM policy in DNS
_domainkey TXT "o=~; t=y; r=postmaster@example.org"
o=- - all are signed
o=~ - some are signed
t=y - test mode
r=[email] - responsible email address
n=[notes]
Once DKIM and SPF are tested and working, update the policy, changing o=~ to o=-, so that other mail servers reject unsigned messages claiming to be from your domain.
As of this writing, most mail servers do not reject messages that fail DKIM policy, unless they also fail SPF, and no DMARC policy is published. The same holds true for SPF. There are technical reasons for this. See DMARC for more information, how you can control change that behavior, as well as receiving feedback from remote servers about messages they have accepted and rejected from senders claiming the identity of your domain(s).
=head2 Sign for others
@ -87,6 +108,8 @@ Following the directions above will configure QP to DKIM sign messages from auth
QP will follow the symlink target and sign client.com emails with the example.org DKIM key.
CAUTION: just because you can, doesn't mean you should. Even with a relaxed DKIM policy, if you don't have a suitable DMARC record published for client.com, they may encounter deliverability problems. It is better to have keys generated and published for each domain.
=head1 SEE ALSO
http://www.dkim.org/
@ -111,6 +134,8 @@ http://www.protodave.com/tools/dkim-key-checker/
=head1 AUTHORS
2013 - Matt Simerson - added DKIM signing and key creation script
2012 - Matt Simerson - initial plugin
=head1 ACKNOWLEDGEMENTS