Impact of Adding a Second SPF Record on DNS: Risks and Best Practices


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SPF (Sender Policy Framework) records are TXT records in DNS that specify which mail servers are authorized to send emails for a domain. A common question among sysadmins is whether adding a second SPF record could cause DNS issues, similar to how multiple nameservers provide redundancy.

Unlike nameservers where multiple entries are beneficial, having multiple SPF records for a single domain violates RFC 7208. Email receivers will typically only check the first SPF record they encounter, ignoring subsequent ones. This can lead to email delivery problems if your legitimate sending sources are listed in the ignored records.

Here's what happens in DNS when you have duplicate SPF records:

example.com. IN TXT "v=spf1 include:_spf.google.com ~all"
example.com. IN TXT "v=spf1 ip4:192.0.2.0/24 ~all"

Instead of creating multiple records, you should merge all authorized senders into a single SPF record. The correct approach would be:

example.com. IN TXT "v=spf1 include:_spf.google.com ip4:192.0.2.0/24 ~all"

Adding a second SPF record won't technically "break" your DNS in the way that incorrect NS or MX records would. However, it will:

  • Increase DNS response size unnecessarily
  • Potentially cause email delivery failures
  • Violate SPF specifications

Always verify your SPF records using tools like:

dig TXT example.com +short
nslookup -type=TXT example.com

Or online SPF validators that will warn you about multiple records.

If your combined SPF record exceeds the DNS lookup limit (10 mechanisms/32 includes), consider:

; Main record
example.com. IN TXT "v=spf1 include:spf.example.net ~all"

; Additional includes in separate record
spf.example.net. IN TXT "v=spf1 ip4:192.0.2.0/24 ip4:198.51.100.0/24 ..."

Unlike DNS nameservers where multiple entries provide redundancy, SPF records have strict technical requirements. According to RFC 7208, a domain must not have more than one SPF record. If multiple TXT records containing SPF syntax exist, receivers will typically evaluate only the first record they encounter.

When you mistakenly create multiple SPF records:

example.com. IN TXT "v=spf1 include:_spf.google.com ~all"
example.com. IN TXT "v=spf1 ip4:192.0.2.0/24 -all"

Email receivers may:

  • Use only the first record
  • Combine both records (rare implementation)
  • Treat it as a permanent error (hard fail)

Instead of adding multiple records, combine mechanisms in a single record:

example.com. IN TXT "v=spf1 include:_spf.google.com ip4:192.0.2.0/24 -all"

When modifying SPF records:

; Before update (dig output)
example.com. 300 IN TXT "v=spf1 include:spf.protection.outlook.com -all"

; Update command (BIND syntax)
$TTL 300
@ IN SOA ns1.example.com. admin.example.com. (
  2023081501 ; serial
  3600       ; refresh
  900        ; retry
  604800     ; expire
  300 )      ; minimum

@ IN TXT "v=spf1 include:spf.protection.outlook.com include:_spf.salesforce.com -all"

Always verify your SPF configuration with:

nslookup -type=TXT example.com
dig TXT example.com +short
Online tools like MXToolbox or SPF Survey

For complex cases requiring multiple includes:

; Correct approach
"v=spf1 include:_spf.google.com include:mailgun.org include:sendgrid.net -all"

; Problematic approach
"v=spf1 include:_spf.google.com -all"
"v=spf1 include:mailgun.org -all"