NAME
Mail::SRS - Interface to Sender Rewriting Scheme
SYNOPSIS
use Mail::SRS;
my $srs = new Mail::SRS(
Secret => [ .... ], # scalar or array
MaxAge => 49, # days
HashLength => 4, # base64 characters: 4 x 6bits
HashMin => 4, # base64 characters
);
my $srsaddress = $srs->forward($sender, $alias);
my $sender = $srs->reverse($srsaddress);
DESCRIPTION
The Sender Rewriting Scheme preserves .forward functionality in an
SPF-compliant world.
SPF requires the SMTP client IP to match the envelope sender
(return-path). When a message is forwarded through an intermediate
server, that intermediate server may need to rewrite the return-path to
remain SPF compliant. If the message bounces, that intermediate server
needs to validate the bounce and forward the bounce to the original
sender.
SRS provides a convention for return-path rewriting which allows
multiple forwarding servers to compact the return-path. SRS also
provides an authentication mechanism to ensure that purported bounces
are not arbitrarily forwarded.
SRS is documented at
http://spf.pobox.com/srs.html and many points about
the scheme are discussed at
http://www.anarres.org/projects/srs/
For a better understanding of this code and how it functions, please
read this document and run the interactive walkthrough in eg/simple.pl
in this distribution. To run this from the build directory, type "make
teach".
METHODS
$srs = new Mail::SRS(...)
Construct a new Mail::SRS object and return it. Available parameters
are:
Secret => $string
A key for the cryptographic algorithms. This may be an array or a
single string. A string is promoted into an array of one element.
MaxAge
The maximum number of days for which a timestamp is considered
valid. After this time, the timestamp is invalid.
HashLength => $integer
The number of bytes of base64 encoded data to use for the
cryptographic hash. More is better, but makes for longer addresses
which might exceed the 64 character length suggested by RFC2821.
This defaults to 4, which gives 4 x 6 = 24 bits of cryptographic
information, which means that a spammer will have to make 2^24
attempts to guarantee forging an SRS address.
HashMin => $integer
The shortest hash which we will allow to pass authentication. Since
we allow any valid prefix of the full SHA1 HMAC to pass
authentication, a spammer might just suggest a hash of length 0. We
require at least HashMin characters, which must all be correct.
Naturally, this must be no greater than HashLength and will default
to HashLength unless otherwise specified.
Separator => $character
Specify the initial separator to use immediately after the SRS tag.
SRS uses the = separator throughout EXCEPT for the initial
separator, which may be any of + - or =.
Some MTAs already have a feature by which text after a + or - is
ignored for the purpose of identifying a local recipient. If the
initial separator is set to + or -, then an administrator may
process all SRS mails by creating users SRS0 and SRS1, and using
Mail::SRS in the default delivery rule for these users.
Some notes on the use and preservation of these separators are found
in the perldoc for Mail::SRS::Guarded.
AlwaysRewrite => $boolean
SRS rewriting is not performed by default if the alias host matches
the sender host, since it would be unnecessary to do so, and it
interacts badly with ezmlm if we do. Set this to true if you want
always to rewrite when requested to do so.
IgnoreTimestamp => $boolean
Consider all timestamps to be valid. Defaults to false. It is
STRONGLY recommended that this remain false. This parameter is
provided so that timestamps may be ignored temporarily after a
change in the timestamp format or encoding, until all timestamps in
the old encoding would have become invalid. Note that timestamps
still form a part of the cryptographic data when this is enabled.
AllowUnsafeSrs
This is a backwards compatibility option for an older version of the
protocol where SRS1 was not hash-protected. The 'reverse' method
will detect such addresses, and handle them properly. Deployments
upgrading from version <=0.27 to any version >=0.28 should enable
this for MaxAge+1 days.
When this option is enabled, all new addresses will be generated
with cryptographic protection.
Some subclasses require other parameters. See their documentation for
details.
$srsaddress = $srs->forward($sender, $alias)
Map a sender address into a new sender and a cryptographic cookie.
Returns an SRS address to use as the new sender.
There are alternative subclasses, some of which will return SRS
compliant addresses, some will simply return non-SRS but valid RFC821
addresses. See the interactive walkthrough for more information on this
("make teach").
$sender = $srs->reverse($srsaddress)
Reverse the mapping to get back the original address. Validates all
cryptographic and timestamp information. Returns the original sender
address. This method will die if the address cannot be reversed.
$srs->compile($sendhost, $senduser)
This method, designed to be overridden by subclasses, takes as
parameters the original host and user and must compile a new username
for the SRS transformed address. It is expected that this new username
will be joined on $SRSSEP, and will contain a hash generated from
$self->hash_create(...), and possibly a timestamp generated by
$self->timestamp_create().
$srs->parse($srsuser)
This method, designed to be overridden by subclasses, takes an
SRS-transformed username as an argument, and must reverse the
transformation produced by compile(). It is required to verify any hash
and timestamp in the parsed data, using $self->hash_verify($hash, ...)
and $self->timestamp_check($timestamp).
$srs->timestamp_create([$time])
Return a two character timestamp representing 'today', or $time if
given. $time is a Unix timestamp (seconds since the aeon).
This Perl function has been designed to be agnostic as to base, and in
practice, base32 is used since it can be reversed even if a remote MTA
smashes case (in violation of RFC2821 section 2.4). The agnosticism
means that the Perl uses division instead of rightshift, but in Perl
that doesn't matter. C implementors should implement this operation as a
right shift by 5.
$srs->timestamp_check($timestamp)
Return 1 if a timestamp is valid, undef otherwise. There are 4096
possible timestamps, used in a cycle. At any time, $srs->{MaxAge}
timestamps in this cycle are valid, the last one being today. A
timestamp from the future is not valid, neither is a timestamp from too
far into the past. Of course if you go far enough into the future, the
cycle wraps around, and there are valid timestamps again, but the
likelihood of a random timestamp being valid is 4096/$srs->{MaxAge},
which is usually quite small: 1 in 132 by default.
$srs->time_check($time)
Similar to $srs->timestamp_check($timestamp), but takes a Unix time, and
checks that an alias created at that Unix time is still valid. This is
designed for use by subclasses with storage backends.
$srs->hash_create(@data)
Returns a cryptographic hash of all data in @data. Any piece of data
encoded into an address which must remain inviolate should be hashed, so
that when the address is reversed, we can check that this data has not
been tampered with. You must provide at least one piece of data to this
method (otherwise this system is both cryptographically weak and there
may be collision problems with sender addresses).
$srs->hash_verify($hash, @data)
Verify that @data has not been tampered with, given the cryptographic
hash previously output by $srs->hash_create(); Returns 1 or undef. All
known secrets are tried in order to see if the hash was created with an
old secret.
$srs->set_secret($new, @old)
Add a new secret to the rewriter. When an address is returned, all
secrets are tried to see if the hash can be validated. Don't use "foo",
"secret", "password", "10downing", "god" or "wednesday" as your secret.
$srs->get_secret()
Return the list of secrets. These are secret. Don't publish them.
$srs->separator()
Return the initial separator, which follows the SRS tag. This is only
used as the initial separator, for the convenience of administrators who
wish to make srs0 and srs1 users on their mail servers and require to
use + or - as the user delimiter. All other separators in the SRS
address must be "=".
EXPORTS
Given :all, this module exports the following variables.
$SRSSEP
The SRS separator. The choice of "=" as internal separator was
fairly arbitrary. It cannot be any of the following:
/ + Used in Base64.
- Used in domains.
! % Used in bang paths and source routing.
: Cannot be used in a Windows NT or Apple filename.
; | *
Shell or regular expression metacharacters are probably to be
avoided.
$SRS0TAG
The SRS0 tag.
$SRS1TAG
The SRS1 tag.
$SRSTAG
Deprecated, equal to $SRS0TAG.
$SRSWRAP
Deprecated, equal to $SRS1TAG.
$SRSHASHLENGTH
The default hash length for the SRS HMAC.
$SRSMAXAGE
The default expiry time for timestamps.
NOTES ON SRS
Case Sensitivity
RFC2821 states in section 2.4: "The local-part of a mailbox MUST BE
treated as case sensitive. Therefore, SMTP implementations MUST take
care to preserve the case of mailbox local-parts. [...] In particular,
for some hosts the user "smith" is different from the user "Smith".
However, exploiting the case sensitivity of mailbox local-parts impedes
interoperability and is discouraged."
SRS does not rely on case sensitivity in the local part. It uses base64
for encoding the hash, but allows a case insensitive match, making this
approximately equivalent to base36 at worst. It will issue a warning if
it detects that a remote MTA has smashed case. The timestamp is encoded
in base32.
The 64 Billion Character Question
RFC2821 section 4.5.3.1: Size limits and minimums:
There are several objects that have required minimum/maximum
sizes. Every implementation MUST be able to receive objects
of at least these sizes. Objects larger than these sizes
SHOULD be avoided when possible. However, some Internet
mail constructs such as encoded X.400 addresses [16] will
often require larger objects: clients MAY attempt to transmit
these, but MUST be prepared for a server to reject them if
they cannot be handled by it. To the maximum extent possible,
implementation techniques which impose no limits on the length
of these objects should be used.
local-part
The maximum total length of a user name or other
local-part is 64 characters.
Clearly, by including 2 domain names and a local-part in the rewritten
address, there is no way in which SRS can guarantee to stay under this
limit. However, very few systems are known to actively enforce this
limit, and those which become known to the developers will be listed
here.
Cisco: PIX MailGuard (firewall gimmick)
WebShield [something] (firewall gimmick)
Invalid SRS Addresses
DO NOT MALFORMAT ADDRESSES. This is designed to be an interoperable
format. Certain things are allowed, such as changing the semantics of
the hash or the timestamp. However, both of these fields must be present
and separated by the SRS separator character "=". The purpose of this
section is to illustrate that if a malicious party were to malformat an
address, he would gain nothing by doing so, nor would the network
suffer.
The SRS protocol is predicated on the fact that the first forwarder
provides a cryptographic wrapper on the forward chain for sending mail
to the original sender. So what happens if an SRS address is invalid, or
faked by a spammer?
The minimum parsing of existing SRS addresses is done at each hop. If an
SRS0 address is not valid or badly formatted, it will not affect the
operation of the system: the mail will go out along the forwarder chain,
and return to the invalid or badly formatted address.
If the spammer is not pretending to be the first hop, then he must
somehow construct an SRS0 address to embed within his SRS1 address. The
cryptographic checks on this SRS0 address will fail at the first
forwarder and the mail will be dropped.
If the spammer is pretending to be the first hop, then SPF should
require that any bounces coming back return to his mail server, thus he
wins nothing.
Cryptographic Systems
The hash in the address is designed to prevent the forging of reverse
addresses by a spammer, who might then use the SRS host as a forwarder.
It may only be constructed or validated by a party who knows the secret
key.
The cryptographic system in the default implementation is not mandated.
Since nobody else ever needs to interpret the hash, it is reasonable to
put any binary data into this field (subject to the possible constraint
of case insensitive encoding).
The SRS maintainers have attempted to provide a good system. It
satisfies a simple set of basic requirements: to provide unforgeability
of SRS addresses given that every MTA for a domain shares a secret key.
We prefer SHA1 over MD5 for political, rather than practical reasons.
(Anyone disputing this statement must include an example of a practical
weakness in their mail. We would love to see it.)
If you find a weakness in our system, or you think you know of a better
system, please tell us. If your requirements are different, you may
override hash_create() and hash_verify() to implement a different system
without adversely impacting the network, as long as your addresses still
behave as SRS addresses.
Extending Mail::SRS
Write a subclass. You will probably want to override compile() and
parse(). If you are more familiar with the internals of SRS, you might
want to override hash_create(), hash_verify(), timestamp_create() or
timestamp_check().
CHANGELOG
MINOR CHANGES since v0.29
timestamp_check now explicitly smashes case when verifying. This means
that the base used must be base32, NOT base64.
hash_create and hash_verify now explicitly smash case when creating and
verifying hashes. This does not have a significant cryptographic impact.
MAJOR CHANGES since v0.27
The SRS1 address format has changed to include cryptographic
information. Existing deployments should consider setting AllowUnsafeSrs
for MaxAge+1 days.
MINOR CHANGES since v0.26
parse() and compile() are explicitly specified to die() on error.
MINOR CHANGES since v0.23
Update BASE32 according to RFC3548.
MINOR CHANGES since v0.21
Dates are now encoded in base32.
Case insensitive MAC validation is now allowed, but will issue a
warning.
MINOR CHANGES since v0.18
$SRSTAG and $SRSWRAP are deprecated.
Mail::SRS::Reversable is now Mail::SRS::Reversible
This should not be a problem since people should not be using it!
You must use $SRS0RE and $SRS1RE to detect SRS addresses.
MAJOR CHANGES since v0.15
The separator character is now "=".
The cryptographic scheme is now HMAC with SHA1.
Only a prefix of the MAC is used.
This API is still a release candidate and should remain relatively
stable.
BUGS
Email address parsing for quoted addresses is not yet done properly.
Case insensitive MAC validation should become an option.
TODO
Write a testsuite for testing user-defined SRS implementations.
SEE ALSO
Mail::SRS::Guarded, Mail::SRS::DB, Mail::SRS::Reversable, "make teach",
eg/*,
http://www.anarres.org/projects/srs/
AUTHOR
Shevek
CPAN ID: SHEVEK
[email protected]
http://www.anarres.org/projects/
COPYRIGHT
Copyright (c) 2004 Shevek. All rights reserved.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
