There was a time, when mail spoofing was an art, was a thing to impress people, was a way to phish attack someone.
With increasing intelligence in spam filters – it became harder, you need good IP reputation to deliver mail to box.
But now it has become almost impossible to spoof address like email@example.com . Why ? Have computer turned intelligent ? No.
The problem of spam protection isn’t new to market. So people came up with DNS based solutions which can allow sender to list IP addresses authorized to send mails.
“Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail” – You can read rfc at https://www.ietf.org/rfc/rfc4408.txt (if you want to dig).
The standard was good, Not good it was best! It block all ways to prank people, but mails were still being delivered, because Network administrator weren’t smart enough to add all server. So as workaround big providers ran algorithms on top to make sure genuine mails which are failing spf are not delivered to spam.
This is all good – but for hardcore phishers it became little hard, people do check mails regularly & getting into network is just distributing malware.
Attacker can perform MITM alter content of mail while its being delivered.
There wasn’t any check.
Solution was DKIM – DomainKeys Identified Mail (DKIM) Signatures , it allows all mail servers to sign messages & certain header fields using defined hashing algorithms & verification using public/private key. Public key is published as DNS record, but private key is kept private.
Acquiring private key is little hard. Its hardest thing. You need to regulate keys to make sure that no one cracks it – if you keep key size 2048 it will make mail delivery slow, if you keep it 512bit with present computing its easy to crack.
DKIM provides way to authorize only certain application to send mail, but there was still no way to get reports on how effective is measure, how many mails are being spoofed & what to do with spoofed mails.
Mails were being delivered even after DKIM failure.
People came with DMARC standard – again it was published using DNS TXT record – it helps in getting reports & also blocking mails. Check the rfc at https://datatracker.ietf.org/doc/rfc7489/
Certainly as every security system comes with an overhead, These standard make mail processing resource intensive. There are many ways to reduce processing cost keeping security upto-date.
There were many spamming attacks originating on behalf of our site, post implementation of DMARC using DMARC Plus, they almost reduced 80% after few months.
One thing to note – if you make a single mistake in any of DNS record you can miss all your mails – So its better to take advice from someone who knows Standard well & can help you in deploying. Make sure you go Slow…