How to use e-mail security features in EXIM SMTP with BIND DNS.

Sender policy framework uses a DNS text entry within a domain detailing the authorised IP addresses and hosts that can send email for that domain.

Using the “a” directive, a receiving server will look up the IP4 or IPV6 address of the hostname and match it to the origin for validation.

Zone

1H IN TXT "v=spf1 a:hostname -all"

Test

dig in txt DOMAIN

If more than one source needs to be authorised the “include” directive can be used (refer to RFC 7208).

DomainKeys Identified Mail is signing process used when sending email to determine authenticity and detect tampering on the receiving server.

Openssl can be used to generate the private signing key and public key published in DNS.

#! /bin/bash

rm -f private.pem public.pem public.der

# PEM printable encoding, RFC 7468
openssl genrsa -out private.pem 2048 1>/dev/null 2>&1
openssl rsa -in private.pem -pubout -out public.pem 1>/dev/null 2>&1

# Distinguished Encoding Rules, OPENSSL-FORMAT-OPTIONS(1SSL)
openssl rsa -in private.pem -pubout -outform der -out public.der 1>/dev/null 2>&1

# DKIM
base64 -w 0 public.der | awk '
{
        print "dkim._domainkey IN TXT (\"v=DKIM1; k=rsa; p=\""
        do {
                printf "\t\"" substr($0, 1, 64) "\""
                $0 = substr($0, 65)
                if (length)
                        printf "\n"
                else
                        print ")"
        }
        while (length)
}'

The sub-domain “_domainkey” is used to provide a “selector” to use with DKIM. In this example the selector is simply “dkim”.

Zone

dkim._domainkey IN TXT ("v=DKIM1; k=rsa; p="
        "use output from above")

The “p” directive contains a base64 encoded public key which can be created by openssl in the previous section.

Test

dig in txt dkim._domainkey.DOMAIN

The text record format for BIND is explained in RFC 6376.

Exim can provide transports which support DKIM selectively. This example will use DKIM signing for a specific domain.

routers

dnslookup_dkim_DOMAIN:
 driver          = dnslookup
 domains         = !+local_domains
 condition       = ${if eq{$sender_address_domain}{DOMAIN}}
 transport       = remote_smtp_dkim_DOMAIN
 ignore_target_hosts = 0.0.0.0:127.0.0.0/8
 no_more

transports

This transport signs the message using the private key and indicates which selector should be examined on delivery in DNS.

remote_smtp_dkim_DOMAIN:
 driver           = smtp
 dkim_domain      = DOMAIN
 dkim_selector    = dkim
 dkim_private_key = /etc/exim4/private.pem

The DKIM domain needn't be the same as the sender domain.

DMARC defines a policy used for e-mails that fail SPF and DKIM tests.

Providers such as google send report of emails that arrive at their user mailboxes detailing those that pass and fail. In google's case, the reports originate from noreply-dmarc-support@google.com.

The simplest policy is to do nothing with failed e-mails, this can be used to determine if the feature is working before applying stricter rules which can tell a receiver to reject or quarantine.

Zone

_dmarc IN TXT "v=DMARC1; p=none; rua=mailto:postmaster@DOMAIN"

Test

dig in txt _dmarc.DOMAIN

See RFC 7489 for more information.

DKIM validator