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Email Security
Email Security
Email Security
Email Security
Email Security
Email Security
Email Security
Email Security
Email Security
Email Security
Email Security
Introduction
Email remains the primary attack vector for most organizations. Phishing, business email compromise (BEC), and spam represent significant risks. A robust email security strategy combines authentication protocols, gateway filtering, and user awareness training.
SPF, DKIM, and DMARC
These three DNS-based authentication protocols work together to verify email sender legitimacy and prevent domain spoofing.
SPF (Sender Policy Framework)
SPF specifies which mail servers are authorized to send email for a domain via DNS TXT records.
example.com. TXT "v=spf1 ip4:203.0.113.0/24 include:_spf.google.com ~all"
Mechanisms: ip4, ip6, include, a, mx, exists. Qualifiers: + (pass), - (fail), ~ (softfail), ? (neutral). Use -all for strict enforcement after testing with ~all.
DKIM (DomainKeys Identified Mail)
DKIM adds a digital signature to email headers, allowing receivers to verify the message was not modified in transit.
Generate DKIM key pair with OpenSSL
openssl genrsa -out dkim-private.pem 2048
openssl rsa -in dkim-private.pem -pubout -out dkim-public.pem
Extract public key for DNS record
openssl rsa -pubin -in dkim-public.pem -outform DER | base64
DNS record for DKIM:
default._domainkey.example.com. TXT "v=DKIM1; h=sha256; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA..."
DMARC (Domain-based Message Authentication, Reporting, and Conformance)
DMARC tells receiving mail servers how to handle messages that fail SPF or DKIM checks, and provides reporting on authentication results.
_dmarc.example.com. TXT "v=DMARC1; p=quarantine; rua=mailto:dmarc@example.com; ruf=mailto:forensic@example.com; pct=100; fo=1"
Key tags: p (policy: none/quarantine/reject), rua (aggregate reports), ruf (forensic reports), pct (sampling percentage), sp (subdomain policy), adkim/aspf (strict alignment).
Email Gateway Deployment
Email gateways filter inbound and outbound traffic, applying policy controls, antivirus scanning, URL rewriting, and attachment sandboxing.
Sample gateway filtering policy
inbound_policies:
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