SPF Syntax
From: http://www.open-spf.org/ 





SPF Record Syntax
(Sender Policy Framework)

Note:
This page serves as an introduction and quick overview of SPF mechanism
syntax. For the complete and definitive picture, please see the
specification.

Domains define zero or more mechanisms. Mechanisms can be used to describe
the set of hosts which are designated outbound mailers for the domain.
	



all | 
ip4 | 
ip6 | 
a | 
mx | 
ptr | 
exists | 
include| 



Domains may also define modifiers. Each modifier can appear only once.
	



redirect |  
exp | 






Mechanisms

Mechanisms can be prefixed with one of four qualifiers:
	

If a mechanism results in a hit, its qualifier value is used. The default
qualifier is "+", i.e. "Pass". For example:
	

Mechanisms are evaluated in order. If no mechanism or modifier matches, the
default result is "Neutral".

If a domain has no SPF record at all, the result is "None". If a domain has a
temporary error during DNS processing, you get the result "TempError" (called "error" in
earlier drafts). If some kind of syntax or evaluation error occurs (eg. the
domain specifies an unrecognized mechanism) the result is "PermError" (formerly "unknown").

Evaluation of an SPF record can return any of these results:



Result
Explanation
Intended action

PassThe SPF record designates the host to be allowed to sendaccept

FailThe SPF record has designated the host as NOT being allowed to
sendreject

SoftFailThe SPF record has designated the host as NOT being allowed to
send but is in transitionaccept but mark

NeutralThe SPF record specifies explicitly that nothing can be said about
validityaccept

NoneThe domain does not have an SPF record or the SPF record does not
evaluate to a resultaccept

PermErrorA permanent error has occured (eg. badly formatted SPF
record)unspecified

TempErrorA transient error has occuredaccept or reject






The "all" mechanism (edit)
	all

This mechanism always matches. It usually goes at the end of the SPF
record.


Examples:
	
"v=spf1 mx -all"
Allow domain's MXes to send mail for the domain, prohibit all others.

"v=spf1 -all"
The domain sends no mail at all.

"v=spf1 +all"
The domain owner thinks that SPF is useless and/or doesn't care.



The "ip4" mechanism (edit)
	ip4:<ip4-address>
ip4:<ip4-network>/<prefix-length>

The argument to the "ip4:" mechanism is an IPv4 network range. If no prefix-length
is given, /32 is assumed (singling out an individual host address).




Examples:
	"v=spf1 ip4:192.168.0.1/16 -all"
Allow any IP address between 192.168.0.1 and 192.168.255.255.





The "ip6" mechanism (edit)
	ip6:<ip6-address>
ip6:<ip6-network>/<prefix-length>

The argument to the "ip6:" mechanism is an IPv6 network range. If no prefix-length
is given, /128 is assumed (singling out an individual host address).




Examples:
	"v=spf1 ip6:1080::8:800:200C:417A/96 -all"
	Allow any IPv6 address between 1080::8:800:0000:0000 and
1080::8:800:FFFF:FFFF.

"v=spf1 ip6:1080::8:800:68.0.3.1/96 -all"
	Allow any IPv6 address between 1080::8:800:0000:0000 and
1080::8:800:FFFF:FFFF.




The "a" mechanism (edit)
	a
a/<prefix-length>
a:<domain>
a:<domain>prefix-length>

All the A records for domain are tested. If the client IP is found among
them, this mechanism matches. If the connection is made over IPv6, then an
AAAA lookup is performed instead.

If domain is not specified, the current-domain is used.

The A records have to match the client IP exactly, unless a prefix-length is
provided, in which case each IP address returned by the A lookup will be
expanded to its corresponding CIDR prefix, and the client IP will be sought
within that subnet.
	"v=spf1 a -all"
The current-domain is used.

"v=spf1 a:example.com -all"
Equivalent if the current-domain is example.com.

"v=spf1 a:mailers.example.com -all"
Perhaps example.com has chosen to explicitly list all the outbound mailers
in a special A record under mailers.example.com.

"v=spf1 a/24 a:offsite.example.com/24 -all"
If example.com resolves to 192.0.2.1, the entire class C of 192.0.2.0/24
would be searched for the client IP. Similarly for offsite.example.com. If
more than one A record were returned, each one would be expanded to a CIDR
subnet.




The "mx" mechanism (edit)
	mx
mx/<prefix-length>
mx:<domain>
mx:<domain>/<prefix-length>

All the A records for all the MX records for domain are tested in order of
MX priority. If the client IP is found among them, this mechanism matches.

If domain is not specified, the current-domain is used.

The A records have to match the client IP exactly, unless a prefix-length is
provided, in which case each IP address returned by the A lookup will be
expanded to its corresponding CIDR prefix, and the client IP will be sought
within that subnet.




Examples:
	"v=spf1 mx mx:deferrals.domain.com -all"
Perhaps a domain sends mail through its MX servers plus another set of
servers whose job is to retry mail for deferring domains.

"v=spf1 mx/24 mx:offsite.domain.com/24 -all"
Perhaps a domain's MX servers receive mail on one IP address, but send mail
on a different but nearby IP address.




The "ptr" mechanism (edit)
	ptr
ptr:<domain>

The hostname or hostnames for the client IP are looked up using PTR queries.
The hostnames are then validated: at least one of the A records for a PTR
hostname must match the original client IP. Invalid hostnames are discarded.
If a valid hostname ends in domain, this mechanism matches.

If domain is not specified, the current-domain is used.

If at all possible, you should avoid using this mechanism in your SPF
record, because it will result in a larger number of expensive DNS lookups.




Examples:
	"v=spf1 ptr -all"
A domain which directly controls all its machines (unlike a dialup or
broadband ISP) allows all its servers to send mail. For example, hotmail.com
or paypal.com might do this.

"v=spf1 ptr:otherdomain.com -all"
Any server whose hostname ends in otherdomain.com is designated.




The "exists" mechanism (edit)
	exists:<domain>

Perform an A query on the provided domain. If a result is found, this
constitutes a match. It doesn't matter what the lookup result is – it
could be 127.0.0.2.

When you use macros with this mechanism, you can perform RBL-style reversed
-IP lookups, or set up per-user exceptions.




Examples:

In the following example, the client IP is 1.2.3.4 and the current-domain is
example.com.
	"v=spf1 exists:example.com -all"
If example.com does not resolve, the result is fail. If it does resolve,
this mechanism results in a match.




The "include" mechanism (edit)
	include:<domain>

The specified domain is searched for a match. If the lookup does not return
a match or an error, processing proceeds to the next directive. Warning: If
the domain does not have a valid SPF record, the result is a permanent
error. Some mail receivers will reject based on a PermError.




Examples:
	In the following example, the client IP is 1.2.3.4 and the current-domain
is example.com.

"v=spf1 include:example.com -all"
If example.com has no SPF record, the result is PermError.
Suppose example.com's SPF record were "v=spf1 a -all".
Look up the A record for example.com. If it matches 1.2.3.4, return Pass.
If there is no match, other than the included domain's "-all", the include as a
whole fails to match; the eventual result is still Fail from the outer
directive set in this example.

Trust relationships — The "include:" mechanism is meant to cross administrative
boundaries. Great care is needed to ensure that "include:" mechanisms do not place
domains at risk for giving SPF Pass results to messages that result from
cross user forgery. Unless technical mechanisms are in place at the
specified otherdomain to prevent cross user forgery, "include:" mechanisms should give
a Neutral rather than Pass result. This is done by adding "?" in front of "include:". The
example above would be:
	"v=spf1 ?include:example.com -all"

In hindsight, the name "include" was poorly chosen. Only the evaluated result of the
referenced SPF record is used, rather than acting as if the referenced SPF
record was literally included in the first. For example, evaluating a "-all"
directive in the referenced record does not terminate the overall processing
and does not necessarily result in an overall Fail. (Better names for this
mechanism would have been "if-pass", "on-pass", etc.)




Modifiers
Modifiers are optional. A modifier may appear only once per record. Unknown
modifiers are ignored.




The "redirect" modifier (edit)
	redirect=<domain>

The SPF record for domain replace the current record. The macro-expanded
domain is also substituted for the current-domain in those look-ups.




Examples:
	In the following example, the client IP is 1.2.3.4 and the current-domain
is example.com.

"v=spf1 redirect=example.com"
If example.com has no SPF record, that is an error; the result is unknown.
Suppose example.com's SPF record was "v=spf1 a -all".
Look up the A record for example.com. If it matches 1.2.3.4, return Pass.
If there is no match, the exec fails to match, and the -all value is used.




The "exp" modifier (edit)
	exp=<domain>

If an SMTP receiver rejects a message, it can include an explanation. An SPF
publisher can specify the explanation string that senders see. This way, an
ISP can direct nonconforming users to a web page that provides further
instructions about how to configure SASL.

The domain is expanded; a TXT lookup is performed. The result of the TXT
query is then macro-expanded and shown to the sender. Other macros can be
used to provide an customized explanation.