DKIM - domainkeys identified mail

Looks like I never wrote a blog posting on setting up DKIM. I just realised one of my servers wasn't set up after a re-install, so I'm having to remember how to do it again.

I'm using sendmail. (yes, shut up) and DKIM hooks in using its milter (mail filter) mechanism.

# apt-get install dkim-filter

Now wire it into sendmail.mc:
INPUT_MAIL_FILTER(`dkim', `S=/var/run/dkim-filter/dkim-filter.sock')

Now when mail comes in, you should see it gets headers like this added by your mail server (dildano.hawaga.org.uk in this case) when DKIM verification happens (eg in mail from gmail).:

Authentication-Results: dildano.hawaga.org.uk; dkim=pass (1024-bit key)
 header.i=@hawaga.org.uk; dkim-adsp=none

The other half of the equation is DKIM signing my outbound mail, so that other people who do checks like this can verify/not-verify my email.

DKIM needs a public/private keypaid

# dkim-genkey -b 1024 -d hawaga.org.uk -s hampshire

-s specifies a selector name. This is a fairly arbitrary identifier used to identify this keypair, because a domain can have multiple keypairs (for example, one per mail server). In the hawaga.org.uk domain, I seem to use names of English counties.

# ls
hampshire.private  hampshire.txt
# cat hampshire.txt
hampshire._domainkey IN TXT "v=DKIM1; g=*; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDUP+5f0nEWyYICxr8rLN8xannlteBg4WF2Fat/MS8CiAa1lE2wgvhKYJJD/ydJ//5B9fBZAwSXTAq2ZCQYIfRf985Yip0BK80ECTlOunaSnMY/4/RzmkXGpndJaHIFqmSWDhML1yBP6W6owJDXIPDCAbV80kd5Z5aAkv8518lk+wIDAQAB" ; ----- DKIM hampshire for hawaga.org.uk

That .txt file is a DNS record to install under hawaga.org.uk. When you've installed it, you can check with:
dig -t txt hampshire._domainkey.hawaga.org.uk @localhost

That's the public key installed. Now the private key.

In /etc/dkim-filter.conf:

Domain hawaga.org.uk
KeyFile /etc/mail/hampshire.private
Selector hampshire

# /etc/init.d/dkim-filter restart

Now send out some mail to some other account. It should have a DKIM signature header added like this:

DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=hawaga.org.uk;
 s=hampshire; t=1328349841;
 bh=hGo8Oadbgx3cVNwLr3hGDRfMX5LwWwXuz2PzqEowx0I=;
 h=Date:From:To:Subject:Message-ID:MIME-Version:Content-Type;
 b=oBeSDSzxz7/awSnxuos6jyJuBoYH2MbiB3HDpbZfLQnTTdEJdx2WD0ubSVAaKAJmV
  ma5xuSaNGeS7X3Xg49obL6nWA89tiOeVAq9FO+7NP+v2DmUPFxEYkLeQJUANYKzAw/
  r8ag9XnbRkxvY+J/rrmeaAjJdnfgUQlKSHlV5CWE=

... and if that other account happens to do DKIM verification, you should see its version of:

Authentication-Results: paella.hawaga.org.uk; dkim=pass (1024-bit key) header.i=@hawaga.org.uk