Mark Minasi's Windows Networking Tech Page
Issue #51 October 2005
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- SMTP CD Set At a Great Price
- LAST Active Directory and Security Classes in DC In Two Weeks
- Tech Section
- We Now Have a Newsletter RSS Feed
- ISO Recorder: A Great Free CD/ISO Tool
- Group Policy Management Console Won't Run On Windows x64
- Restart W32Time After Changing the Time Server
- Understanding Sender Policy Framework -- So "AOL Doesn't Reject Your
- Bring a Seminar to
This month brings an announcement -- I finally got around to doing an
RSS feed. You'll also hear about some bad news on x64 and Group
Policy Management Console, some troubles with the Windows Time Service and a lengthy article about anti-spam
techniques followed by a how-to article on what I'm finding is an
interesting contender as an anti-spam, anti-phishing tool, something
called Sender Policy Framework or SPF. Plenty to get to, but first, a word from our sponsor...
Times Running Out! The ONLY Active Directory and Security Classes
Planned So Far Are In DC -- In Just Two Weeks!
I'm trying to stay off the road so I can start working on Server R2
and Vista books, but a fair number of folks have kindly asked for
another seminar... so here goes. The only public seminar I
will do for the rest of the year (and probably a good bit of the next) will be in Washington, DC at the
Marriott near George Washington University this November 14-17.
I'll be running the Active Directory class and the Security class.
You can find the Security class (which is now twice the size and three
times as many PowerPoint slides, all packed into two enlightening days) outline is at
www.minasi.com/secoutln.htm, the Active Directory class outline is
www.minasi.com/2003outln.htm, and you can find the seminar
registration page at
We Now Have a Newsletter RSS Feed
Some of you've asked for it, so now I've set up a simple RSS feed on my Web
site. Just visit the www.minasi.com
home page and you'll see the little XML icon. You'll see updates about
once a month, whenever a new newsletter comes out. For those who've never
played with RSS, it's a simple way to find out when new things appear on the Web
ISO Recorder: A Great Free CD/ISO Tool
I use XP and while it's nice that Microsoft included some support for
CD writing, I find the limited version of Roxio shipped on XP to often be worse
than nothing. Now and then, I either get a CD that I'd like to convert to
a .ISO file for archival and backup, or I may want to take a .ISO file and burn
it to CD. Of course, the built-in CD writing support in XP does none of
That's why I'm so glad that Alex Feinman wrote "ISO Recorder." Visit
http://isorecorder.alexfeinman.com/isorecorder.htm and download his ISO
Recorder v2 and you'll see why. The application, which runs on XP, 2003 or
even the Vista beta, lets you create ISOs from existing CDs, or burn .ISO files
to a CD. He's implemented an x64 version, as well as a Vista version that
uses Vista's DVD writing abilities to let you create ISOs and burn ISOs from/to
DVDs. (Not protected ones, of course.) He's also got a command-line
CD writer utility that I've not played with, but it might just be the answer for
all of those readers of Mastering Windows XP Professional who write to ask me
how to use Windows Backup to back up to a CD. (You can't, not without
Alex has done the community a great service. Give his nifty program a
try and drop him a line to tell him how you like it. (And if you like it a
lot, consider throwing him a few bucks at his "Paypal Donate" link.)
Group Policy Management Console Won't Run On Windows x64
I absolutely love the AMD64 chips. They're reasonably priced and
quite fast -- so fast that I'm less enamored of their 64-bitness than I am of
their speed. I moved from a Mobile Pentium 1.7 GHz-based laptop to a
Turion 2 GHz system recently and am amazed at the difference in speed. The
Turion-based system (Acer's Ferrari 4000 laptop, a very nice portable) has a
clock rate only slightly faster than the Pentium-based system's, but runs
benchmarks 50 to 100 percent faster. (And that not's just a difference
that you only see in benchmarks; it seemed significantly faster from the first
time I set it up.) Add the nice side-effects of 64-bit, like the built-in
anti-worm protection evinced by the "no execute" page flags and, again,
appealing processor prices when compared to Intel's offerings and you've got a
nice combination. So one of my recent hardware purchases -- it was time to
update the network hardware, as for some reason the old 600 MHz DCs won't run
Server 2003 -- was an AMD64-based system.
Now, the AMD64 processors will run 32-bit operating systems without a hitch,
so I could have installed regular old Windows Server 2003 Standard Edition with
SP1, but I figured I'd boldly go where few have gone and all that sort of thing
and install Server 2003 x64. It installed fine and as far as I can see
runs pretty much identically to 32-bit Server, except for a few different
entries in Task Manager, or, better yet, Mark Russinovich's special 64-bit
version of Process Explorer.
So I was stumped by x64's reaction to Group Policy Management Console.
I mean, GPMC is a "given" around my network. Set up a DC, you gotta put
GPMC on it, right? Well, you can surely do that, but it won't work on
Server x64. Right-click on a policy and try to edit it, and GPMC will tell
you that it can't find "gpedit.msc," although it's there. Group Policy
expert Darren Mar-Elia tells me that the problem is that GPMC is built atop the
.NET programming platform and apparently the 64-bit implementation of .NET 1.1
is a bit different (heck, it's probably 32 bits different, y'know?) from
the 32-bit version, sufficiently so to break some .NET apps... and GPMC is
apparently one of them. And, I'm told, there is no fix, nor is one coming
any time soon.
All's not lost, though. You can always run GPMC on a 32-bit workstation
or server and do your work remotely, that seems to work. But it's quite
discouraging to see this sort of thing, particularly as Microsoft's official
stance about 64-bit hardware is that 64-bit is the standard for server hardware,
and that 32-bit server hardware is considered "legacy."
Restart W32Time After Changing the Time Server
Most of the time, Server 2003 behaves as 2000 Server did. But now and
then something small trips me up.
I needed to change the time server for a forest's root domain PDC emulator.
This is usually the first DC that you've ever installed on any domain in the
forest, and it's the only system that doesn't have a system elsewhere in the
network that it uses to automatically synchronize its time. It's the top
of the time synchronization hierarchy in Active Directory, and needs to be told
where to go to synchronize its clock so that all other systems in your AD have
time that is consistent and correct. (In AD, consistent is more important
than correct; every member server or workstation must have a time setting that
is within five minutes of the time setting on their domain controller or
authentications don't work.)
As you may know, you set the time synchronization on the domain controller
holding the PDC emulator FSMO role in your forest's root domain by opening a
command prompt at that system and typing
net time /setsntp:name-or-IP-address-of-external-time-server
Then press Enter. The information goes into the Registry and survives a
reboot, so you needn't type it in every day (a common question). So, for
example, if some kind soul ran a publicly-available time server at
time.bigfirm.com, then I'd tell my system to synchronize to it by typing
net time /setsntp:time.bigfirm.com
None of that's a surprise to anyone who's worked on AD for a while, or to
anyone who's read my Server books. But I found that after typing in the
NET TIME command to the client's DC, the time didn't start synchronizing as I'd
expected it to. After a bit I thought, "well, let's restart the Windows Time
net stop w32time & net start w32time
I then forced the server to try to resynchronize its clock with this command:
And all was well. I'm surprised that I've never stumbled on that
before, but I wanted to pass it along to you. Bottom line: when you do a
NET TIME /SETSNTP: on a 2003 system, stop and start the Windows Time Service.
Understanding Sender Policy Framework -- So "AOL Doesn't Reject Your
Have you heard the latest Internet end-of-the-world rumor? Seems that
if you run your own e-mail server then you absolutely must implement
something called the Sender Policy Framework ("SPF") because if you don't,
then pretty soon (the exact date always varies or is left vague) you won't be
able to send mail to AOL, Yahoo!, Hotmail, and, well, just about anyone.
Panicked yet? Good -- then someone can sell you something, right?
Okay, the truth is that while none of those folks have said that they will
reject all e-mail from people who haven't done SPF, it is true
that SPF is becoming one way for networks to help reduce spam or, more
precisely, to reduce e-mail address forging, wherein someone sends you an e-mail
falsely claiming to be from someone. And it is
true that AOL has been attempting to filter spam by creating a "whitelist" of
domains that it trusts to send it spam-free e-mail, and that to be on that list
AOL wants you to do SPF. You do not, however, have to be on that
whitelist to get your mail delivered to AOL, but it's not a bad idea, as AOL
uses some kind of quirky rules of thumb when deciding whether or not to deliver
a piece of mail to an AOL member, and so right at this very moment AOL's mail
servers are refusing perfectly legitimate e-mails that "smell funny."
Having SPF configured should make your e-mail seem a bit more aromatically
pleasing to AOL's servers.
What's that you say, you don't care about mailing to AOL? I suspect
that whatever AOL ends up doing, other big ISPs will do as well. I don't
think that SPF will ever become an absolute necessity, but I do think
that as time goes by we'll see SPF as part of how spam filters score a
particular message's probability of "spamness." And besides, SPF's really
easy to do once someone explains it, and in this article I hope to make it
extremely easy to understand. But first, a little background. (If
you're already savvy about anti-spam stuff but want to know about SPF, just skip
down to "Sender Policy Framework / Sender ID: Making Forgeries Less Likely.")
The State of Anti-Spam Tools
E-mail's great... except for spam, with the spyware, phishing, viruses and
other garbage that often accompanies it. Everyone hates spam, but apparently
mere hate isn't going to change anything, as it continues to mount in our
mailboxes unabated. Now, part of the reason that spam irritates me stems
from the fact while
once enjoyed what e-mail could do for me, now I spend precious time each day sifting through
hundreds of spam messages a day,
looking for the 50 or so legitimate e-mails. But that source of annoyance is nothing compared to how much spam
makes life tough for people running e-mail servers, or ISPs. Now, most of
the world doesn't have a spam-to-nonspam ratio as bad as mine (a few dozen spam
to each good message), but
imagine the effect of even a 2:1 ratio. That means that the enterprise
setting up this e-mail server must budget hardware and network services three
times the size of their actual needs, must spend three times what they
ought to have to just to accommodate the spam. Ugh. For ISPs it can
be worse, as they've got worry about their own members sending spam in
addition to having to deal with unwanted e-mails to their members' mailboxes.
What'll fix it? Legislation? Probably not, given the
international nature of the Internet. The US's CAN-SPAM act, which took
effect in the beginning of 2004, sure hasn't slowed down the flood of junk
e-mail into my mailbox, and I'll bet it hasn't done much for yours either.
Bad guys just host their stuff on a server in another country, and the threat of
penalties disappears. There are systems with personal whitelists or
"friends and family" lists, but that stuff doesn't scale well and, besides, such
things usually only kick in once the spam-ee's SMTP server has already accepted
the message and wasted hard disk space temporarily storing it before the
whitelist/friends & family list filter gets around to checking it.
Spam filters that learn spam-like behavior, so-called "Bayesian" filters, can
be somewhat effective as an anti-spam tool, but do we really want to address an
Internet-clogging problem like spam by having to buy and run a server whose sole
job is to reject e-mail from people who send us mail that we don't want and
never asked for? I sure don't; I'd rather kill the unwanted mail at the
source, before it gets a chance to chew up bandwidth, CPU cycles, disk space
and, often, my time as I pick through my mailbox looking for the odd legitimate
message. I admit that I may have a greater personal antipathy to the need
for anti-spam softare because I run a 100 percent opt-in newsletter -- the one
that you're reading -- and am terribly frustrated by the fact that probably a
quarter to a half of my subscribers don't get my e-mailed notice that a new
newsletter's available because of those spam filters. (Believe me, that
fact was a contributing factor in deciding to set up RSS.) Don't misunderstand
me, I fully sympathize with the admins who put the spam filters in, it'd just be nice if
they weren't necessary. It'd be nice if it were a bit harder for spammers
to maintain what is essentially a 24/7 denial of service attack on our e-mail
servers and on the Internet itself.
How Spammers Get Away With Spamming
Why can't we easily stop spamming? Mostly because of the trusting
nature of Internet protocols. To understand why, let's look at a
circa-1995 spamming. (We'll get to the present day in no time, I promise
-- unfortunately almost nothing's changed in ten years.)
Suppose I'm a spammer and I intend for your in-box to be my helpless victim.
I'd prefer that you not be able to find out who I am or what SMTP server I'm
using. That's because if you can easily determine who I am or where I'm working from then you
can probably either quickly get me shut down, or, if you were of a more
vigilante bent, you might try to launch some kind of denial of service attack
against me. And heck, if you did that, then how would I perform the
vital service of selling Viagra, offering great mortgage rates, informing people that
they've won a contest that they never entered, or proffer pictures of naked
Suppose my SMTP server is called " "Spammer.bigisp.net" and yours is "Receiver.unwittingvictim.com"
Here's how my server sends yours the junk e-mail.
Spammer to Receiver: "hi there, Receiver. My name is
Hold on -- this machine's name isn't goodserver.bigfirm.com,
it's spammer.bigisp.net." True, but under standard SMTP, Receiver does
not try to verify that. There might even be a machine named
goodserver.bigfirm.com out on the Internet, but it'll never know that
Spammer's claiming to be it. So, returning to our SMTP conversation...
Receiver responds, "hello back; I can
accept the following commands," and lists its capabilities. It
does that because over the years SMTP has changed, and so the receiving SMTP
might not support all of the latest bells and whistles. So when the
sending SMTP server says hello, which is an SMTP statement all in itself --
either HELO for a really old or simple SMTP server, or EHLO ("enhanced HELO")
for a smarter one -- then the receiving SMTP server informs the sending SMTP
server of its strengths and weaknesses when the conversation starts out.
Spammer responds, "I've got some mail here from
Hold on again -- how can we verify this? We can't under standard SMTP.
Back in the early 90's, I used to teach classes about TCP/IP and SMTP and loved
to demonstrate how I could connect to the White House e-mail server and send
myself e-mail from the President. (You can't do that any more, as I'm sure
Anyway, Receiver does nothing to verify this under standard SMTP, and
just says "sounds good, tell me more."
Spammer says, "this mail is destined for firstname.lastname@example.org."
Receiver could in theory say "do we have a user named 'someuser?'"
Or it might even say "do I have any business even accepting e-mail
for the example.com domain? But it doesn't, and says "great, let's see
Spammer sends along the message, and then disconnects. The session's
Notice that Spammer asserted three things:
- Its fully-qualified domain name (FQDN) "goodserver.bigfirm.com."
- The e-mail address of the person that Spammer claims that it is sending
the mail for. This address is called (because of the corresponding
SMTP command) the "MAIL FROM" address, and in this case it's "email@example.com."
- The e-mail address of the desired receiver "firstname.lastname@example.org."
And note once again that none of those are checked by our 1995 SMTP server.
Consider what a fraudulent individual might do:
- Someone could, as I did, get onto an organization's e-mail server and
send bogus e-mail from someone in that organization to someone anywhere on
- Someone could, as many spammers still do, run an SMTP server that
contacts your organization offering mail to you... with a return address
(the MAIL FROM, recall) of... you!
- Someone run an SMTP server that sends out objectionable or criminal
material from a server with some given actual FQDN, while claiming that its
FQDN is anything else, including the FQDN of one of your e-mail servers.
This could potentially affect your organization's reputation, or at least
net you a lot of e-mail and phone calls from some angry people.
So what did we do about it? Not much, unfortunately. But around
the middle of the 90s, we fixed one thing.
1995's Improvement: Closing Relays
In the mid-90s, Internet spam started getting totally out of hand, and that
led many folks to look at that last unverified part -- the receiver address.
After all, if you're the SMTP server for bigfirm.com and I try to send you a
piece of mail for email@example.com, then you ought to be able to say right on
the spot "hey, I'm sorry, but we don't have a Velma here."
That seems reasonable at first glance; why not just build SMTP servers to
reject invalid receiver addresses before the mail's even accepted? It's
perfectly good thinking, except for one thing -- spammers really want to discover
the e-mail addresses on your e-mail servers.
So if I were a spammer and wanted to find out the legitimate e-mail addresses at bigfirm.com, all I'd have to to would be to generate a list of a few thousand
possible user names -- sue, joe, joesmith, janedoe, etc -- and try to send
bigfirm.com a piece of e-mail for each of those names. I'd then note which
ones the bigfirm.com SMTP server accepted, and now I've got a bunch of valid new
e-mail addresses that I can inundate with spam, sell to others, or the like.
In fact, there is a standard SMTP command, VRFY, that lets you ask an SMTP
server to verify whether or not a particular e-mail address is valid, and for a
time spammers used it to test and harvest names, but most e-mail servers at this
point have either redefined the command -- Microsoft's SMTP server will just say
"yup, that's a properly-formatted e-mail address, but I have no idea whether or
not I've got someone here by that name" -- or disabled it altogether. Of
course, that's not stopped the spammers. Now they once again send out
e-mails to random addresses, but they embed in the e-mail body a image link back
to their servers. Click on the e-mail, your e-mail client program
automatically grabs the image -- the request contains your e-mail account's name
in the image request -- and the spammer's server now knows that it's got a "live
one." That's why recent versions of Outlook and Outlook Express do not
show images in e-mails unless you tell them to. This spammer practice of
tricking you into revealing yourself to their servers is called "beaconing."
Validating the recipient's entire e-mail address, then, won't help much. But what about
the domain of the would-be recipient's e-mail address? Ah, that
could be fruitful. The spammer wanted to send e-mail so
firstname.lastname@example.org. But let's suppose that Receiver, our spamming
victim, wasn't the e-mail server for example.com; what happens then?
E-mail Relaying Explained
In the early days of the Internet, where everyone trusted others and spammers
were still making a living trying to sell parcels of swamp land to old folks
and running telemarketing shops, then our Receiver would just say "gosh, I don't
handle e-mail for example.com, but heck, it'll just take me a minute to look up
example.com's MX record in DNS and then I'll just re-send the e-mail to
example.com." In other words, Receiver's just being a nice guy by
assuming some of another SMTP server's burdens. That sounds good, decent, and reasonable, and that's pretty
much how every SMTP server that I know of ran back in 1995. Oh, by the
way, I just said that Receiver "re-sends" e-mail that it got from one SMTP
server to another, but the official e-mail phrase is "relaying." When an
SMTP server "relays," that means that it accepts e-mail for domains that it does
not serve. Receiver does not serve example.com as its SMTP server, so
accepting e-mail for example.com is relaying on Receiver's part.
(More on this in a moment.)
But, of course, the spammers figured out how to ruin that too. Suppose
a spammer wants to send about a million unwanted e-mails. He doesn't,
however, want the address of his computer to be obvious. So instead of
telling his SMTP server to send that million e-mails directly to their proper
destinations -- send spam intended to me directly to my SMTP server, spam intended for
you directly to your SMTP server, and so on -- he chooses some victim running a big, fast
SMTP server on a really fast Internet connection, and sends all million e-mails
to that SMTP server. In effect, the spammer's
SMTP server is dropping all of that mail on the victim's SMTP server, saying
"please look up all of the relevant MX records for all of these recipients'
domains, and then waste a lot of time trying to contact those SMTP servers and
sending this junk e-mail." This, then, was the thanks that an e-mail
administrator got for being helpful and letting his or her SMTP server relay:
wasted CPU time on the server, wasted bandwidth, and let's not forget the angry
people who receive the spam, look at the SMTP headers and see that they got this
spam from the e-mail administrator's SMTP server. As they say, no good
deed goes unpunished.
To combat this, most e-mail administrators adjusted their SMTP server's
behavior vis-a-vis relaying. Now, relaying is a bad word in e-mail
circles, but not all relaying is bad. You see, there is
good relaying and bad relaying. To understand why, remember the
definition of relaying. Most SMTP servers serve a particular domain or set
of domains. For example, my SMTP server receives e-mail for e-mail
addresses in the minasi.com and mmco.com domains; send an e-mail to some address
there and it will, if all goes, well, end up knocking or port 25 of my SMTP
server, and at that point it's reached its final destination. But that
SMTP server has another job, as well: it sends out e-mail from me.
So let's say that I want to send e-mail to email@example.com. Outlook
prepares the message for me and contacts my SMTP server, saying "please send
this to firstname.lastname@example.org." As you'd expect, my e-mail server takes the
mail and sends it along.
But hey, wait -- that's relaying! By accepting a piece of mail
destined for email@example.com, my
SMTP server accepted e-mail for a domain that it doesn't serve. But
there's nothing wrong with that; you'd be pretty annoyed if it worked any other
way. Any time
that you ask your local SMTP server to send e-mail to someone else in your
company, then there's no relaying, because that SMTP server is supposed to serve
your company's e-mail domains. But if you want your e-mail server to send
a message to someone outside of your organization, then you are asking it to
relay. That's good relaying. In other words,
- If you want your SMTP server to accept Internet e-mail for a domain
other than you own, that's good relaying.
- If your SMTP server accepts e-mail from someone outside of your
organization for re-sending to yet another domain outside of your
organization, then spammers can exploit your SMTP server, and that's bad
relaying, and you are said to have an "open relay."
So, as I said before, most e-mail administrators have learned to reconfigure
their servers so that those servers won't relay for the whole world, and instead
will only relay for internal users. But it didn't happen overnight...
Open Relay and Spammer Lists
Anyone who ran an SMTP server program in the early 90's can testify that
closing an SMTP server's open relays wasn't very simple in those days. Worse yet, many e-mail admins just plain didn't understand
what an open relay was. As a result, they didn't understand that they were
dooming their systems and their bandwidth to exploit by spammers, and in the
process making the Internet less usable for the rest of us.
In response, several private groups decided to create a sort of vigilante
effort against people running servers with open relays. They built
publicly-available online databases of IP addresses of
SMTP servers that the database maintainers believe have open relays. The
idea is that you and I can then configure our SMTP servers so that every time
someone tries to send us an e-mail using an SMTP server whose IP address is
listed in those databases, then our SMTP servers reject the message and perhaps
send back an e-mail explaining that the message was rejected and why. Why
do this? Two reasons. First, if an unprotected SMTP server has
become a favorite jumping-off point for spammers, then the chances are good that
the incoming mail is spam, and won't be missed if we reject the communication.
Second, the vigilantes wanted to get the attention of the administrators of
these misconfigured SMTP servers, and one way to do that would be for the users
of that SMTP server to notice that lots of their legitimate e-mail was not being
is supposed to cause users in companies whose mail servers have open relays to
angrily storm the e-mail administrator's office, demanding that he or she fix
This approach has annoyed some people because while getting onto an open
relay database is easy -- you just need someone to rat you out to the open relay
people -- then
getting off can take a while because, as the folks who run the various
open relay databases are prompt to point out, they're running a free service,
and the only reason why they're inconveniencing people is because some e-mail
admins have found their databases useful. In at least one case, ISPs
started blocking those IP addresses on their routers, meaning that no Internet
traffic at all could pass from the organization with the open relaying SMTP
server to that ISP. That was a bit of overkill, as a bit of configuration
ignorance on the e-mail administrator's part meant that that the users of that
ISP could not communicate with the organization at all -- couldn't send them
mail, couldn't visit their Web site, etc. That's bad, but what's worse is
that after the e-mail admins fixed their problem and were no longer running open
relays, the ISPs who blocked them at their routers took months or years to
remove the blocks. As a result, the former open relayers had stepped into
the light, but still couldn't
communicate with clients in the blocking ISP's networks. Some former open
relayers had to leave their ISP -- and their tainted IP addresses -- and go to
another ISP. Furthermore, in my opinion it's all well and good to say
"hey, I'm just doing this for free, nothing is my responsibility," but in fact
one database, Osirusoft, left the field after repeated denial of service attacks
by spammers. That's a shame, but their response in late August 2003 was to
modify their list of spammers to "the whole world" so as to convince everyone
who was using their database to stop using that database, as they took their
football and went home. You kind of have to wonder
how many legitimate e-mails went lost because the Osirusoft database operator
decided to stop the service after spending years convincing the Internet
community to trust it.
Blacklists and Whitelists
The success of open relay databases led to
"blacklisting databases" of known spammers. Places like Spamhaus and
Spamcop run free anti-spam databases used by thousands and thousands of e-mail
While you have to admire the work that these folks do for no pay, I think
they'd be the first people to say that they hope that a day will come when
they're obsolete. So what more can be done to detect and reduce spam?
In addition to blacklists, we've got "whitelists," which essentially say to
the world, "I will reject all e-mail except for this list of people that I
trust." That probably works for some folks, but it surely doesn't work for
me, as I have no idea who's read one of my books, seen a presentation that I've
done, or read something that I've written on the Web. That said, there are
some very clever approaches to whitelist technologies when used in tandem with
Bayesian spam filters. For example, some Bayesian filters watch who's in
your Contacts list and who you send mail to, automatically adding those folks to
your "don't bother checking to see if this is spam" whitelist -- a smart
So far we've got five approaches to reducing spam -- three options we've
discussed, and two for-pay options we haven't but that I'm sure you've already
- Close our open relays and use open relay databases to chivvy others into
closing their open relays
- Use spam blacklists to reject e-mail from systems in the "known
- Use some kind of whitelist either to control entirely what gets in, or
to modify a Bayesian filter's behavior
- Install servers in our network that employ technologies that let them
make decent guesses at what's spam and what's not
- Install servers in our network or rent access to commercially-maintained
databases of spam and spammers
What more can we do? Short of hiring an "e-mail butler" with the
intelligence to be able to filter the wheat from the chaff, there's not a lot
more that will examine the message and make a decision based on that. The
next step seems to be to examine the sender more closely, to ask, "do I
trust this guy?" We're doing some of that now with open relay databases
and spam blacklists, but spammers can move quickly, more quickly than these
databases can respond.
If we could fairly reliably identify the senders, then we could stop the
spammers. But how, given that they can move around so quickly? Well,
they can move around so quickly because it's so simple to fire up a computer on
a random IP address and start spewing spam by lying about who the sender is.
So one answer would be to change that, to say to your SMTP server, "if you
can't positively verify that this sender is who it says that it is, then reject
its mail." How would that work? Here are a few approaches based on
the information that an SMTP server gets when contacted by another SMTP server,
in roughly increasing order of severity in terms of how much the Internet would
have to change for it to happen:
- Look for systems that are not configured according to RFC standards,
such as those lacking reverse-lookup (PTR) records, and reject their
communications. (Discussed below.)
- Verify that the fully qualified domain name specified in the initial
"hello" command matches the IP address of the server. (Discussed
- Go back to the domain that the sending server claims to be sending mail
from, and ask that domain if that machine is authorized to send mail for it.
- Demand and require that a machine sending e-mail from a given domain
present a digital certificate identifying it as an authorized sender of
e-mail from that domain. The technology for this exists, but I
think that digital certs have to become cheaper before this will work.
Sure, I can afford to shell out a few hundred bucks a year for Web SSL certs,
but I make money from my Web site. Saying that the entry fee to
running your own SMTP server is a few annual C-notes to VeriSign, Thawte,
Network Solutions or the like would be, at least in my opinion, a bad thing
-- while I dislike the current anonymity that is part of the warp and woof
of the Internet, I love its egalitarian nature. Saying "you've got to
make VeriSign richer or you can't run a mail server" troubles me a bit.
- Before accepting mail from a given domain, check with a trusted
third-party that the domain has done something to establish a reputation as
a non-spammer. This is an example of what anti-spammers call a
"reputation"-based rule, and it's still very fuzzy how it'd work. The
first firm that I know of that supports this is the "Bonded Sender" program
at www.bondedsender.com. The basic idea seems to be that you want to
send e-mail and be recognized as a good guy, so you post a bond with that
"trusted third-party" as well as furnish some identification information.
If too many people complain that you've spammed them, then there's some
process whereby they figure out whether you truly are a spammer or not.
They then offer a list of trustworthy senders. An interesting idea and
perhaps where we'll go one day, although as far as I can see it'll cost you
at least a grand a year to maintain your "white hat emailer" status, and
that might be a bit rich for many small firms' blood.
I'm not so sure about the last two, although I could imagine things will get
so bad that we'll get there. But let's look at the first one, then take up
the next three in the SPF section.
Requiring Reverse DNS Lookup: Checking PTR records
Many SMTP servers have an option that looks something like "reject incoming
messages with invalid PTR records" or the like. (On MailEnable, which I'm
using now, the option is "Require PTR DNS entry from unauthenticated
connections," but I've seen many variations on mail servers over the years.)
What does this mean? Can it help you repel spam messages?
First, a bit of review and background. According to RFC 1912, every
system visible on the Internet should have an A ("Address") record, so for
example someone who wants to connect to web2.minasi.com can ask DNS, "what's the
IP address corresponding to web2.minasi.com?" so that DNS can respond,
"220.127.116.11." Some DNS zone should also have, according to that same
RFC, a PTR record which would let someone say "hey, what host name is associated
with IP address 18.104.22.168?" and get in return the answer "web2.minasi.com."
Most of us are good about the A records; we're not always so careful about the
PTRs. But it's likely that a spammer who's thrown up an SMTP server for a
few hours of junk mail spewing hasn't had the time or inclination to bother with
PTR records, and we might be able to use that to our advantage.
Again, in the perfect world -- or at least the RFC-compliant world -- then
you should be able to start with a host name like web2.minasi.com, look up its A
record to get an IP address of 22.214.171.124, look up that IP address's PTR
record and get back web2.minasi.com. But accomplishing this A-leads-to-PTR-leads-to-identical-A
equivalence might be tough for a lot of folks, because many of us control our
forward lookup DNS domains, but don't control the reverse lookup domains;
rather, the ISP controls the reverse lookup domains. So it's likely that a
PTR lookup for web2.minasi.com's IP address would yield not "web2.minasi.com"
but instead something more like "wsip-70-168-214-165.hr.hr.cox.net."
If you see oddball PTR results like that, then it's likely that the IP
address in question is controlled by a big ISP that's probably a big DSL and/or
cable provider. In my case, the folks who provide my IP addresses are Cox
Cable. Cox knows that RFC 1912 compliance wants to see both forward and
backward lookups for any given host, but they also know that while their
customers can create their own forward DNS zones, they can't create reverse DNS
zones, as Cox controls them. So Cox worked around it -- as many large
providers do -- by creating an "hr.hr.cox.net" subdomain, and then creating an A
record for each of their IP addresses. They could then create reverse
lookup domains and point the PTR records for each IP address to that address's
corresponding hr.hr.cox.net record.
How does this affect what an SMTP server would do? Over the years,
people have sometimes tried to introduce an element of accountability into
Internet communications by saying to any incoming system, "I see your IP
address; I will then look to see if there is a PTR record that corresponds to
that IP address." And that's all that this setting does. So, for
example, suppose you've got an SMTP server with one of these "I need a valid PTR
or we won't talk" settings, which you've enabled, and I try to send you an
e-mail. The communication could look like this:
- My SMTP server to yours: "Hi, I'm an SMTP server named
web2.minasi.com that you can see has an address of 126.96.36.199. Can
- Your SMTP server: "Hmmm... let's query DNS for a PTR record for
188.8.131.52.in-addr.arpa... ah, I see that it's a system named
"wsip-70-168-214-165.hr.hr.cox.net" ... well, that doesn't match the name
that you sent me, but what the heck, at least you've got some PTR
record, so let's talk.
(In case you're wondering, I don't know of any e-mail servers that have a
rule that says, "I'll take the incoming IP address and look up its PTR record,
then take the FQDN that came from that lookup and compare it to the name that
you claimed when you said hello," although certain SPF settings do that, as
you'll see later.)
Suppose you decided to require that all SMTP servers that tried to talk to
your SMTP server have a PTR record of some kind. What would be the
pros and cons? The benefit would be that you'd stop some spam. In my
experience, 100 percent of the incoming e-mail that I've seen (in the past week,
anyway, when I turned the feature on to test it) is spam -- I know this from
looking at my SMTP logs. The downside? There's a chance that you'll
register a "false positive" -- in other words, you'll reject legitimate e-mail
from people who either don't know how to configure PTR records or who are served
by an ISP who refuses to provide PTR records.
Again, why would this work as an anti-spam tool? Because it assumes
that spammers tend to run poorly-configured servers. Is that a good
assumption? It's not a terrible one, as spamming is becoming more and more
criminalized around the world and so therefore spammers will have to move around
a lot and won't want to invest the time to create servers that follow RFCs.
But it definitely has the side-effect of raising the bar a bit for someone who
wants to set up his or her own SMTP server. For example, people who lack a
static IP address may not be able to set up an SMTP server and have anyone
accept mail from it. That troubles me a bit, but the fact is that inasmuch
as we don't have a "reputation-based" way of finding and filtering spam
producers, we end up using some degree of RFC compliance as a proxy for
reputation. But admittedly, it is just a stopgap; we need some
Well, then, short of creating a certificate- and reputation-based e-mail
infrastructure that significantly raises the cost of entry for people wanting to
run an e-mail server, what can we do? Some people have an answer -- SPF.
Which brings me to the next article.
Sender Policy Framework / Sender ID: Making Forgeries Less Likely
The latest class of proposals to make the Internet a somewhat more
trustworthy place, and to make opening e-mails a bit less scary, is a pair of
ideas called Sender Policy Framework (SPF) and Sender ID. These two
proposals are from different groups and there's a ton of politics between the
two, but I'll skip that for this discussion and focus on SPF because if you're
configured for SPF, then you're also configured for Sender ID.
The goal of SPF is simple: when you get an e-mail from someone at some
domain, then SPF tells you whether or not you can feel fairly certain that this
e-mail did indeed come from that person. Before SPF, you don't know if a
return address for a given piece of mail is trustworthy or forged; SPF tries to
stamp out return address forgery. You've seen forged return addresses if
you've ever seen
- An email addressed to you ... from you
- A fake message from some network support address at Microsoft with an
enclosed "security patch" that is, of course, a virus
- An e-mail from PayPal, your bank, your brokerage firm, or the like with a dire
message exhorting you click somewhere to update your account or it'll be
closed; of course you are
actually directed to some server that will, once you type in your account
name and password, steal your money
- A virus sent from friend A, who never sent the virus because in
actuality friend B opened a virus and the virus found friend A's e-mail
address in friend B's Contacts list and decided to send viruses to everyone
on friend B's Contacts list... but with a forged return address from friend A
This is the kind of thing that SPF and Sender ID want to stop.
So, to exact, SPF isn't an anti-spam tool, it's an anti-return address forging
tool. But a lot of spammer, viruses, and phishing schemes employ forged
return addresses. Yes, even in an all-SPF world, spammers could still spam
you... just so long as they specify their real server and domain names. Or
so long as they forged addresses from a domain that didn't bother to implement
the most basic SPF, as we'll see.
Here's another way to think about SPF. Consider how you'd send me a
piece of e-mail now. You know that my e-mail address is firstname.lastname@example.org.
But what server accepts e-mail for minasi.com? As you probably know, your
SMTP server queries my DNS server for "Mail Exchanger" or MX records. The
minasi.com DNS server responds, "the server that receives e-mail for minasi.com
is web2.minasi.com." Now your SMTP server knows where to send e-mail to me. Notice
that you went to my DNS domain to get that information, so unless someone's monkeyed with DNS then you feel pretty sure that this is indeed the server that
I want you to use to send me e-mail.
SPF does the reverse. Where MX records identify the systems that I've
receive e-mail for minasi.com, SPF adds new DNS records that identify which
servers send e-mail for minasi.com. So now when your SMTP server
receives a piece of e-mail purporting to be from minasi.com, then your SMTP
server can query the minasi.com DNS server and say, "which systems should I
expect to be receiving e-mail from you on?" The DNS server will respond
(in my case) web2.minasi.com, the same system that receives e-mail. If
your SMTP server sees that it's being offered a message from email@example.com
but the server offering it is a system other than web2.minasi.com, then your
SMTP server can safely discard that message. And, once again, notice that
when you receive an e-mail from someone who claims to be from minasi.com and you
want to double-check which SMTP servers to accept mail from, then you come to
the minasi.com DNS zone to get those answers. I control what servers my
domain accepts mail from, no one else. No certificates needed, no seals of
approval from others, no wondering how to get an ISP to add a PTR record.
How SPF Works I: Overview
To see how SPF works, let's review what we know so far about an SMTP message
transfer: what information it offers, and what we can do
with that information to detect deception, which may point to unwanted e-mail.
Let's assume that this a spammer working from a machine named pc42.whocares.com
on IP address 184.108.40.206 wants to send an e-mail to firstname.lastname@example.org. The spammer needs to
use a return address -- not that he's intending on accepting responses! -- so he
picks a random name to forge for as a return address,
email@example.com. Let's even say
that the spammer is smart enough to check around and see that Megacorp, the
place where Jane works, has a mail server called mail.megacorp.com.
- pc42.whocares.com, the spammer's SMTP server, looks up the MX records in
bigfirm.com and finds the name and IP address of Bigfirm's e-mail server, as
would any SMTP server. pc42.whocares.com then says to Bigfirm's SMTP
server, "EHLO mail.megacorp.com." This is, as we've seen, a
correctly-formatted greeting from one SMTP server to another. And yes,
it also contains a big fat lie: the sender's FQDN.
- Bigfirm's SMTP server responds to the spammer's SMTP server and awaits
- Next, the spammer's SMTP server says, "I'd like to send some e-mail from
firstname.lastname@example.org. Another big fat lie: the MAIL FROM address.
At this point, Bigfirm's SMTP server has three pieces of information about
this communication, which might or might not be spam. First, Bigfirm's
SMTP server knows the IP address of the would-be sender, and it's pretty sure
that the address is fairly reliable. Yes, it might be the IP address of a
NAT router or the like, but that's not important; the important thing, you see,
is that the spammer almost certainly can't make Bigfirm's SMTP server think that
the spammer's SMTP server, pc42.whocares.com, has a valid IP address from
Megacorp's network. Bigfirm's SMTP server also knows the FQDN that the
would-be sender's SMTP server claims to have, obtained from the EHLO/HELO
greeting. Finally, Bigfirm's SMTP server also has "MAIL FROM," the e-mail
address that the sender claims is the source of the e-mail.
Next, Bigfirm's SMTP server takes that claimed MAIL FROM address and extracts
that address's domain. As the purported MAIL FROM address claims to be
from megacorp.com, Bigfirm's SMTP server now knows that
- There's data coming from a server with 220.127.116.11
- The system claims to have an FQDN of mail.megacorp.com
- The system claims that it's got an e-mail from megacorp.com
Here's where the SPF stuff comes in. Assuming that Bigfirm's SMTP
server is equipped with SPF-aware code, it contacts the megacorp.com DNS server
and says, "what rules should I use to identify the servers are authorized to
send mail from megacorp.com?" (I'll show you how to do that a bit later.)
Such rules are called "mechanisms" and they're basically just patterns to match.
Here are some examples of what the Bigfirm SMTP server might get back from
megacorp.com's DNS server:
- "Take the IP address of whatever server's contacting you to send mail
for us and do a PTR query on it. If the resulting host name is in the
megacorp.com domain or the mailservers.megacorp.com domain, then the
- "If the IP address of whatever server's contacting you is in XYZ range,
then it's okay."
- "If the IP address of whatever server's contacting you matches the A
record of the megacorp.com domain, then it's okay."
- "Look up the PTR record on whatever server's contacting you. If it
matches the FQDN that it's claiming, then it's okay."
Using this information, the Bigfirm SMTP server passes this information to
its SPF filter code, asking whether the SMTP server from 18.104.22.168 is probably an authorized sender of e-mail for megacorp.com. SPF
answers this question with one of several score values:
- None means that the Bigfirm SMTP server looked for SPF information in
the megacorp.com domain and there wasn't any. This is the most common
answer nowadays, as most people don't do SPF. (But SPF will help you
even if all you want is to block email to you forging your e-mail
address as its return address, as you'll see.)
- Neutral means that megacorp.com included SPF information in their DNS
zone, but it just said "we don't have a comment on this." The effect
is like "none."
- Pass means that the SPF information in the megacorp.com domain allowed
Bigfirm's SMTP server to positively identify the server at 22.214.171.124 as an
authorized sender of e-mail for megacorp.com.
- Fail means that the SPF information in the megacorp.com domain allowed
Bigfirm's SMTP server to conclude that the server at 126.96.36.199 is explicitly
not an authorized sender of e-mail for megacorp.com.
- Soft fail is a state wherein the server can be identified as one that
the domain's owner knows, but that the domain's owner discourages people
receiving SMTP traffic from. You might use it when in transition from a world where
anyone in the organization could run their own SMTP server to one where IT
controlled which servers were allowed to offer SMTP services. In the
transition period your SPF information might tell others "these are our
official e-mail servers -- rate them 'pass' -- but if you see a server from
inside our range of IP addresses that isn't on that list, then don't fail
it, 'soft fail' it." How would that be helpful? We might imagine
that Bigfirm's SMTP server might treat a soft fail as "let the mail pass,
but send a message to the administrator at Megacorp that he's got a rogue
server at XYZ IP address."
Based on the SPF return of "none," "neutral," "pass," "fail," or "soft fail,"
Bigfirm's SMTP server may delete the message, put it in a specific folder, mark
it in some way, or whatever -- that's entirely up to the software implementer.
How SPF Works II: the Two Tests
I gave you a few examples of how SPF would score an e-mail, but let's get
more specific. SPF implementations can score messages based on two things:
the MAIL FROM domain, and the FQDN offered on the EHLO/HELO command. More
specifically, according to a SPF techie on the official SPF forum responding to
a recent question:
- All SPF checkers will check "Mail From" in creating an SPF score
- Some, however, will always check both "Mail From" and the FQDN in
the EHLO/HELO; some will stop at "Mail From"
- Almost all SPF checkers will, if offered a message with a null "Mail
From" value, go check the FQDN in EHLO/HELO
- No SPF checkers check EHLO/HELO, but not "Mail From" values
(Hey, it's an evolving system.)
Sidebar: What You'd Need To Do To Use SPF
That's a quick overview and I'm about to show you more of the nitty-gritty of
SPF, but let's step back for a minute. You're a busy person. How
much work is this going to entail? And what good will it do?
There are two parts to SPF. First, by putting SPF information in your
domain's DNS zone, you advertise to SMTP servers equipped with SPF filters who
can and can't send e-mail from your domain. As I've said, large ISPs are
starting to implement this, so there are a large number of e-mail users who will
never get an e-mail forged in the name of any of your domain accounts.
Since installed SPF, I've stopped getting the near-daily bogus "your Paypal
account will be terminated" e-mails. (Why my dopey bank, BB&T, hasn't
taken five minutes to do this is beyond me.) I
The second part is to install some SPF-aware code for your SMTP server.
There is a free SPF checker that you can put on Exchange's IIS module, and you'd
be surprised how many other systems support SPF. The chances are good that
it won't cost you much more than a download and a few minutes' configuration.
(For a list of SPF support, look at
http://www.spf.pobox.com/downloads.html.) The immediate benefit will
be reduced e-mail with forged return addresses from AOL, Hotmail and a growing
number of other big providers. And, as I've suggested already, if nothing
else you'll never get another e-mail from yourself, at least none that you
didn't actually send!
But even if you only publish your SPF record in your DNS, then you make it a
little bit harder for bad guys. If you're a bank, financial institution of
any kind, or for that matter any firm with a large consumer population who's
tech-aware, then please consider at least publishing SPF records. Won't
cost you a dime.
So let's see how...
Creating SPF Records In DNS
I've said that SPF-aware SMTP servers can "score" an incoming message based
on some rules stated in a DNS record. Those rules are called "mechanisms" by SPFers for some reason.
You state the rules for determining if a given server is authorized using an
"SPF record" that you create and store in a DNS record. SPF records are
just a line of text that looks like (for example)
I'll show you how to construct your own SPF records, but first I want to show
you how to install an SPF record on your DNS domain. (By the way, the SPF
record that I've typed above says "I authorize any system on the entire Internet
to send mail for me," so you won't be using this for long. On the other
hand, though, if you don't have any SPF records currently, then that's about
what you're offering now anyway.)
There isn't any such thing as a SPF type of DNS record, although some people would
like to see such a thing exist. Instead, we'll wrap the SPF records for
our domains inside a DNS record type that you may never have used before -- a
TXT record. Its job is to let you create a record in your DNS zone and put
any kind of text in it that you like.
You can create a DNS TXT record in Windows 2000 or 2003-based DNS servers by
right-clicking your zone, then choosing "Other New Records...," then "Text
(TXT)" and "Create Record..." That offers you a dialog box labeled "New
Resource Record" and two fields, "Record name (uses parent domain if left
blank") and "Text:." The "Record name" field acts as it always does in
DNS, to describe the thing on the left-hand-side of the DNS record. (For
RFC fans, it's called the "owner" in RFC 1034. So, for example, when you
create an A record that looks like "myhost A 10.10.10.10," then "myhost.whateveryourdomainiscalled.com"
is the owner -- remember that any of those things that you put on the
left-hand-side of a DNS record implicitly includes the domain's name.) As
you want to create an SPF record for your entire domain, just leave the "Record
name..." field blank, as, again, blank means "this domain." In the text
And click OK. Look at your DNS zone, and you'll now see something like
(same as parent folder) Text (TXT) v=spf1 +all
Sometimes you may want to create SPF records that apply not to the whole
domain but instead to a particular host. In that case, fill in the host's
name in the "Record name..." field.
Well, congratulations, now you've got an SPF record for your domain.
But it's not a very good one, as I said, so let's see how to make it better.
How SPF Works III: Mechanisms
SPF rules have two parts. The first part says "this is an SPF version
1.0 record." It looks like
It's just "boilerplate," a "must have" set of characters up front.
Following that, you can have a series of pattern-matching rules that SPF uses to
separate the "no good" servers, the "good" servers, and the "I don't care"
servers. Those pattern-matching rules are called "mechanisms."
The basic mechanisms are a, mx, ptr -- which all emply DNS records of
the same name, as you've probably guessed -- ip4, which lets you specify ranges
of IP addresses, and "all" which matches everything. (There are others,
like "ip6," but I'm just trying to get you started here.) Once SPF has
found a matching mechanism, you tell it whether this means "if this matches,
then fail," "if this matches, then pass," "if this passes, then soft fail," or
"if this matches, then consider this neutral" by prefixing it with a -, +, ~, or
?, respectively. If you do not prefix a mechanism with anything, then SPF
assumes that you meant "+," or pass. For example, to use the absolutely
simplest mechanism, "all:"
- "v=spf1 -all" would mean "always match this mechanism" -- remember,
"all" matches everything -- and now that you've matched, return a "fail"
- "v=spf1 +all" would mean "always match" and the "+" would mean "now that
you've matched, return a 'pass' result."
- "v=spf1 all" would have the same effect, as the lack of a
prefix assumes that prefix "+."
Each mechanism uses different algorithms for matching, and I'll explain them
very soon. But here's a point that's not so obvious until we remember that
each mechanism can do two different kinds of matches. Why is this?
Because, recall, SPF filters built into SMTP servers may do two tests -- first, they may try to match the
MAIL FROM domain to something in your domain and, second, they may try to match
the FQDN on the EHLO/HELO command to something in your domain. And recall
that what one piece of SPF-evaluating code doesn't necessarily match what
another vendor's might do;.
So, for example, if our SPF-aware Bigfirm SMTP server were approached by a
system from address 188.8.131.52 offering a server name of "mail.megacorp.com" in its
EHLO/HELO command, then Bigfirm's SMTP server would first query megacorp.com's
DNS zone for a TXT record. It would then examine the TXT record and,
assuming that megacorp.com had a lame SPF record like the one we're using now
("v=spf1 +all") then Bigfirm's SMTP server would ask itself one or two
questions, depending on how its designer built it.
First, it would ask, "does the revealed IP address of this server, 184.108.40.206,
match anything in the SPF record's mechanisms?" Sure it does, because
"all" matches everything. Bingo on the first question. Some SPF
filters would stop here, but some would go further and ask the second question:
"does the FQDN claimed in the EHLO/HELO match anything in the SPF record's
mechanisms?" Again, it would because "all" matches everything.
Bingo on the second, and we've got a definite "pass" on this entry.
Here's a rundown on the other mechanisms. Sometimes they do different
things when applied to the MAIL FROM test than when applied to the EHLO/HELO
test, as you'll see.
The SPF "a" mechanism
Including the "a" mechanism says to SPF,
- Do a DNS query for an A record that matches this.
- If it's there, then you'll get an IP address returned.
- Compare that IP address to the IP address of the would-be SMTP sending
- If they match, great, then this "a" mechanism matches.
But what does it match to? In the case of a MAIL FROM test, it's
looking for an A record for the domain itself. If the minasi.com domain
had an "a" mechanism in its SPF record, that would instruct any SPF code to look
for an A record for "minasi.com," rather than any host inside minasi.com.
The IP address that returns from that query, assuming that there's an A record
for minasi.com, must then match the would-be SMTP server's address.
If testing the EHLO/HELO, the test is a little different. SPF looks for
an A record matching the FQDN claimed in the EHLO/HELO message, and compares the
IP address the A query returns to the actual IP address of the SMTP server
that's trying to send the message.
But what if your SMTP servers live in a different DNS domain, or if you just
want to name a particular mail server? (Most of us don't have a mail
server with an A record for the whole domain.) Then add a colon to the "a,"
followed by a server name. For example, "a:serv1.abc.com" would mean "the
system named 'serv1.abc.com' can send mail for my domain." And you can add
as many of a particular kind of mechanism as you like. For example, this
SPF record for minasi.com:
v=spf1 +a +a:po.otherfirm.com -all
Would say "accept mail claiming to be from my domain from either a system
with an A record of 'minasi.com,' or accept it from a system with an A record of
po.otherfirm.com." SPF scans mechanisms left-to-right and stops as soon as
it gets a "pass." If neither the +a or +a:po.otherfirm.com patterns match,
then the last pattern, "-all," matches -- because "all" always matches -- and
causes SPF to score the message as a "fail." Thus, if the
original "+a" matched, then SPF would stop looking, satisfied, and return a
The SPF "ptr" mechanism
On the MAIL FROM test, a PTR mechanism starts from the IP address of the
sending SMTP server and does a PTR lookup on that address. If the result
(which is a fully qualified domain name like "mail.mychurch.com") has a DNS
domain that matches the domain that the SMTP is trying to send mail from, SPF
scores the message a "pass."
On the EHLO/HELO test, SPF looks up the PTR record for the IP address of the
sending SMTP server. The result must match the FQDN claimed in the EHLO/HELO
For example, if my mail server were called po.minasi.com, it were at IP
address 220.127.116.11, and there was a PTR record allowing you to query 18.104.22.168
and get back "po.minasi.com," then I could use this SPF record
v=spf1 ptr -all
The MAIL FROM test would first try to find a PTR record for 22.214.171.124. If
that returns a FQDN of anythingatall.minasi.com, then there's a match.
The EHLO/HELO test would first do a PTR lookup on 126.96.36.199, and then compare
the FQDN returned to the FQDN claimed in the EHLO/HELO command. They must
be equal for the mechanism to match. And remember, the ptr
mechanism needn't have a + in front of it because if there's nothing at the
front of a mechanism, that means "pass."
The SPF "mx" mechanism
The mx mechanism is a bit more complex. First, it takes the domain of
the purported sender's e-mail address. Then it looks up all of the MX
records in the domain, up to ten MX records. (More could be used by a
spammer to slow down the SPF to a crawl by installing hundreds of bogus MX
records.) Then SPF looks up the A record associated with each of those MX
records, It then examines all of the IP addresses found from all of those
A record queries. If there are any matches to the IP address of the
incoming SMTP server, then SPF scores the message "pass."
It does the same test whether it's a MAIL FROM test, or an EHLO/HELO test.
You can also indicate that e-mail servers from another domain may send e-mail
for your domain by adding a colon to the end of the mx mechanism, and then name
the domain, as in
v=spf1 mx mx:acme.com -all
This would instruct an SPF filter to search for MX records that lead to A
records which produce IP addresses first in your own domain, and then, if that
failed, the acme.com domain.
The SPF "ip4" mechanism
This is the simplest mechanism besides "all" -- it lets you just specify a
range of IP addresses. If the incoming server uses one of those addresses,
the message passes. This test doesn't have to examine the claimed FQDN on the
EHLO/HELO command at all, and it behaves identically whether it's doing a MAIL
FROM or EHLO/HELO test. You specify IP ranges using the CIDR "slash"
notation. For example, to tell SPF to allow any system in the C network
starting at 188.8.131.52, use
Remember you can have as many ip4 mechanisms as you like. Particular IP
addresses do not need a slash. For example, to specify that there the only
two machines, 184.108.40.206 and 220.127.116.11 that can send e-mail from a given domain,
we could create this SPF record:
v=spf1 ip4:18.104.22.168 ip4:22.214.171.124 -all
Testing Your SPF Record
Once you think you've got it set up right, try it out. There's a Web
page with some really useful SPF testing tools at
SPF's not a perfect answer, and it's clearly an interim one. But
despite the common use of the phrase "Internet years," the Internet's become a
place used by so many people that it's becoming pretty difficult to change it in
short time -- so interim solutions aren't a bad idea, in my opinion. But
I'd love to hear your opinion!
Join me at ...
TechTarget's Web Class on SP2 and SP1. As they say it...
"Windows School is in session with Mark Minasi Mark Minasi dissects
Windows XP SP2 and Windows Server 2003 SP1 in five, 15-minute webcast
cram sessions. You can even download a worksheet to follow along with
the lesson you're hearing. Find out the good and bad about XP SP2 and
Windows 2003 SP1 in Mark's inimitable style. Topics cover: Data
execution protection, stack changes, de-anonymizing XP, IE and more."
Windows Connections Fall 2005 (San Diego): I'm not sure how California managed
to snag three of the four best IT shows this year (Spring Connections in San
Francisco, TechMentor in San Jose, Fall Connections in San Diego) but if you're a
fan of the Golden State's weather then 2005's the conference-going year for
you! Information on Connections at
program chair Amy Eisenberg's trying to out-do herself so it'll be a great
show. I'm doing my SMTP talk, the MSDE/SSX talk based on my earlier
newsletter, how to fix broken Active Directories, the Windows Logons
talk, and a keynote on the state of Windows.
Bring Mark to your site to teach
I'm keeping busy doing Active Directory and Security seminars and writing,
but I've still got time to visit your firm. In just two days, I'll make
your current NT techies into 2000, XP, Active Directory and 2003
experts. (And better yet they won't have to sit through any Redmondian propaganda.) To join the large
educational, pharmaceutical, agricultural, aerospace, utility, banking,
government, telecommunication, law enforcement, publishing, transportation,
and other organizations that I've assisted, either take a peek at the course
outlines at www.minasi.com/presentations.htm,
mail our assistant Jean Snead at Assistant@Minasi.com,
or call her at (757) 426-1431 (only between noon-5 Eastern time, weekdays,
Until Next Month...
Have a quiet and safe month.
Please share this newsletter; I'd like very much to expand this periodical
into a useful source of NT/2000/2003/XP information. Please forward it
to any associates who might find it helpful, and accept my thanks. We
are now at over 40,000 subscribers and I hope to use this to get information
to every single Mastering 2003, XP, NT and 2000 Server reader. Thanks for
letting me visit with you, and take care. Many, many thanks to the
readers who have mailed me to offer suggestions, errata, and those kind
reviews. As always, I'm at http://www.minasi.com/gethelp
and please join us at the Forum with technical questions at
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