This section defines HOWTO configure a Sender Policy Framework (SPF) record for a domain and its mail servers.
<grovelling apology> The macro feature of SPF was incorrectly documented using parenthesis, these should have been braces ({}).</grovelling apology>
SPF was initiated by Meng Weng Wong of pobox.com and is now an IETF standard (RFC 4408) to enable validation of legitimate sources of email for a domain.
Briefly the design intent of the SPF record is to allow a receiving MTA (Message Transfer Agent) to interrogate the Name Server of the domain which appears in the email (the sender) and determine if the originating IP of the mail (the source) is authorized to send mail for the sender's domain.
The SPF information SHOULD be defined in a standard TXT RR and MAY now be defined in an SPF RR type (BIND releases from 9.4.0 support the SPF RR type - see also RFC 4408).
If a SPF (TXT) RR exists and authorizes the source IP address the mail can be accepted by the MTA. If the SPF (TXT) RR does not authorize the IP address the mail can be bounced - it did not originate from an authorized source for the sender's domain. If the domain does not have an SPF RR the situation is no worse than before.
Many Open Source MTAs have already been modified to use the SPF record and there is no down-side and plenty of potential up-side to implement the proposed record format now.
We use the following terminology to try and simplify the descriptions below:
- sender - the full email address of the originator of the mail item (typically uses return-path in the actual SPF checks)
- source-ip - the IP address of the SMTP server trying to send this message
- sender-domain the domain name part of the sender's email address e.g. assume the sender is info@example.com the sender-domain is example.com.
The SPF record defines one or more tests to carry out to verify the sender. Each test returns a condition code (pre below). The first test to pass will terminate SPF processing.
TXT RR Format
The standard TXT and SPF record format is defined as:
name ttl class TXT text
name ttl class SPF text
The SPF RR is functionally identical to a TXT record with SPF data. BIND 9.4+ supports the SPF RR type, however previous versions, and most other DNS software (as of July 2007), do not yet support the SPF RR type. Thus the RFC's recommendation is to always provide a TXT based SPF RR and, if your DNS software supports the SPF RR type, duplicate the information from the TXT version of the SPF RR in a native SPF RR. The reason for this procedure is simply because while the master/slave may support the SPF RR, querying name servers - such as name servers used by receiving MTAs - may not. Some, but not all examples, below have been updated to reflect the use of both record types to illustrate usage. In all cases the TXT and SPF RRs are shown with a comment line between containing the word AND as a reminder of the currently policy recommendation. It is safe to assume for the foreseeable future that only using a TXT version of the SPF will always work.
The SPF data is entirely contained in the text field (a quoted string). SPF defines the contents of the quoted string as follows:
v=spf1 [[pre] type ] ... [mod]
Where:
v=spf1 | ||||||||||||||
| Mandatory. Defines the version being used. Currently the only version supported is spf1. | ||||||||||||||
pre | ||||||||||||||
Optional (defaults to +). pre defines the code to return when a match occurs. If a test is conclusive either add + or omit (defaults to +). If a test might not be conclusive use "?" or "~" (tilde). "-"(minus) is typically only used with -all to indicate that if we have had no previous matches - fail.
| ||||||||||||||
type | ||||||||||||||
Defines the mechanism type to use for verification of the sender. May take one of the following values: Basic MechanismsThese types do NOT define a verification mechanism but affect the verification sequence.
Sender MechanismsThese types define a verification mechanism.
| ||||||||||||||
mod | ||||||||||||||
Two optional record modifiers are defined. If present they should follow the last type directive i.e. after the all. The current values defined are as follows:
|
Macro-Expansion
SPF defines a number of macro-expansion features as defined below:
Note: all macro-expansion delimiters use braces {}.
| Modifier | Description |
| %{c} | Only allowed in TXT records referenced by the exp field. The IP of the receiving MTA. |
| %{d} | The current domain, normally the sender-domain %{o} but replaced by the value of any domain argument in the type above. |
| %{h} | The domain name supplied on HELO or EHLO, normally the hostname of the sending SMTP server. |
| %{i} | sender-ip The IP of SMTP server sending mail for user info@example.com. |
| %{l} | replace with local part of sender e.g. if sender is info@example.com, the local part is info. |
| %{o} | The sender-domain e.g. if email address is info@example.com the sender-domain is example.com. |
| %{p} | The validated domain name. The name obtained using the PTR RR of the sender-ip. Use of this macro will require an additional query unless a ptr type is used. |
| %{r} | Only allowed in TXT records referenced by the exp field. The name of the host performing the SPF check. Normally the same as the receiving MTA. |
| %{t} | Only allowed in TXT records referenced by the exp field. Current timestamp. |
| %{s} | Replace with sender email address e.g. info@example.com |
| %{v} | Replaced with "in-addr" if sender-ip is an IPv4 address and "ipv6" if an IPv6 address. Used to construct reverse map strings. |
The above macros may take one or more additional arguments as follows:
r - Indicates reverse the order of the field, for instance, %{or} would display example.com as com.example and %{ir} would display 192.168.0.2 as 2.0.168.192. The normal split uses "." (dot) as the separator but any other character may be used to define the split but a "." (dot) is always used when rejoining so, for instance, %{sr@} would display info@example.com as example.com.info.
digit - the presence of a digit (range 1 to 128) limits the number of right most elements displayed, for instance, %{d1} displays only com only from example.com but %{d5} would display five right hand elements up to the maximum available, in this case it will display example.com since that is all that is available.
Examples
Example 1
Example 1: Assumes a single mail server which both sends and receives mail for the domain.
; zone file fragment for example.com
$ORIGIN example.com.
IN MX 10 mail.example.com.
....
mail IN A 192.168.0.4
; SPF stuff
; domain SPF
example.com. IN TXT "v=spf1 mx -all"
; AND
example.com. IN SPF "v=spf1 mx -all"
; mail host SPF
mail IN TXT "v=spf1 a -all"
; AND
mail IN SPF "v=spf1 a -all"
Notes:
- the domain SPF is returned from a sender-domain query using the sender's email address e.g. sender = info@example.com sender-domain = example.com. The SPF record only allows the MX host to send for the domain.
- the mail host SPF is present in case the receiving MTA uses a reverse query to obtain the source-ip host name and then does a query for the SPF record of that host. The SPF record states that the A record of mail.example.com alone is permitted to send mail for the domain.
If the domain contains multiple MX servers the domain SPF would stay the same but each mail host should have a SPF record.
Example 2
Example 2: Assumes the domain will send mail through an offsite mail server e.g. an ISP:
; zone file fragment for example.com
$ORIGIN example.com.
IN MX 10 mail.offsite.com.
....
; SPF stuff
; domain SPF
example.com. IN TXT "v=spf1 include:offsite.com -all"
; AND
example.com. IN SPF "v=spf1 include:offsite.com -all"
; WARNING: offsite.com MUST have a valid SPF definition
Notes:
- This format should be used IF AND ONLY IF you know that offsite.com has a valid SPF configuration.
- include recurses (restarts) verification using the SPF records for offsite.com. Mail configuration changes are localised at offsite.com which may simplify administration.
- include could have been replaced with redirect.
Example 3
Example 3: Assumes we are the host for a number of virtual mail domains and that we can send mail from any host in our subnet.
Zone file fragment for one of the virtual mail domains:
; zone file fragment for vhost1.com
$ORIGIN example.com.
IN MX 10 mail.example.com.
....
; SPF stuff
; domain SPF
vhost1.com. IN TXT "v=spf1 include:example.com -all"
; AND
vhost1.com. IN SPF "v=spf1 include:example.com -all"
Notes:
- the domain SPF is returned from a sender-domain query using the sender's email e.g. sender = info@vhost1.com, sender-domain = vhost1.com. The SPF record recurses to the DOMAIN example.com for verification.
Zone file for example.com
; zone file fragment for example.com
IN MX 10 mail.example.com.
....
; SPF stuff
; domain SPF - any host from
; 192.168.0.1 to 192.168.0.30 (32 - bcast and mcast = 30)
; can send mail
example.com. IN TXT "v=spf1 ip4:192.168.0.3/27 -all"
; AND
example.com. IN SPF "v=spf1 ip4:192.168.0.3/27 -all"
; mail SPF
mail IN TXT "v=spf1 ip4:192.168.0.3/27 -all"
; AND
mail IN SPF "v=spf1 ip4:192.168.0.3/27 -all"
Notes:
- the domain SPF is returned from a sender-domain query using the sender's email e.g. sender = info@example.com sender-domain = example.com. The SPF record allows any host in the 32 address subnet which contains 192.168.0.3 to send mail for this and any host virtual domain e.g virtual1.com in the above example. NOTE: while /27 allows 32 IP addresses subnet rules remove 192.168.0.0 and 192.168.0.31 as the multicast and broadcast addresses respectively. [read more about IPv4 Classes]
- In the above scenario we could have used a slightly shorter version such as:
example.com. IN TXT "v=spf1 mx/27 -all"
This record has the same effect as a:192.168.0.3/27 above but will cost a further DNS look up operation whereas the IP is already available.
; AND
example.com. IN SPF "v=spf1 mx/27 -all" - The above scenario relies on the fact that customers will only send mail via the domain example.com i.e. they will NOT send via another ISP at home or when travelling. If you are not sure if this is the case you can terminate the sequence with ?all which says kinda pass (soft fail) and let the mail go through - perhaps logging the incident to capture statistics.
If the domain contains multiple MX servers the domain SPF would stay the same but each mail host should have a SPF record.
Example 4
Example 4: Assumes that the domain never sends mail from ANY location - ever. Typically you would do this to prevent bogus mail for everyone else - it is a supreme act of self-sacrifice!
; zone file fragment for example.com
; zone does NOT contain MX record(s)
...
; SPF stuff
; domain SPF
example.com. IN TXT "v=spf1 -all"
; AND
example.com. IN SPF "v=spf1 -all"
Notes:
- This SPF test will always fail since the only condition it tests is the all which results in a fail.
Example 5
Example 5: Illustrates various macro expansion features:
; zone file fragment for example.com
$ORIGIN example.org.
IN MX 10 mail.example.com.
....
; SPF records
; domain SPF
@ IN TXT "v=spf1 exists:%{ir}.%{v}.arpa -all exp=badguy.example.com"
; AND
@ IN SPF "v=spf1 exists:%{ir}.%{v}.arpa -all exp=badguy.example.com"
badguy IN TXT "The email from %{s} using SMTP server at %{i}
was rejected by %{c} (%{r}) at %{t} because it failed
the SPF records check for the domain %{p}.
Please visit http://abuse.example.com/badguys.html
for more information"
Notes:
- The badguy TXT above is split across multiple lines for presentation reasons only and should appear on a single line in the zone file.
- The exists:%{ir}.%{v}.arpa tests is a great example BUT IT WILL NOT WORK because the exists type uses an A RR. But it's a great example to show the power of macro-expansion and we can't think of a better one. Sorry about that.
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