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...or, Making it harder to forge email
I am no longer maintaining this page. You should refer to more recent work on RMX or SPF for further information.
Initial publication: 2003.May.01
Page last updated: 2003.June.02
When I first wrote this document in Feb. 2003, I intended to submit an
Internet Draft to the IETF, formalizing what is discussed here. However, I
soon discovered that a draft
suggesting practically the same thing had been proposed just two months
earlier! Hadmut Danisch had, apparently, beaten me to the punch. Well,
RMX records are a good idea, so I'm posting this
document to increase awareness.
Since posting this article, I have learned about two other proposed
anti-forgery mechanisms, those of Fecyk and Vixie, which are listed in the
References section. These are functionally equivalent to
but are implemented in subtly different ways; either of these solutions
would also be satisfactory to me. Update: 2003.06.02: It seems there are
even earlier RMX-like proposals! Here's one
from David Green; he wanted to call the records
MT. And Paul
that Jim Miller suggested the concept as early as 1998, although I
cannot find a written proposal.
identall over again!
This document supports and encourages the adoption of Hadmut Danisch's
RMX resource records for DNS.
RMX records are
intended to make email forgery difficult by providing a mechanism for a
domain owner to list all mail servers authorized to send email on behalf of
his or her domain name. They eliminate the most serious drawback of
breaking any core internet protocols.
A few short months ago, I had the unpleasant experience of getting email-jacked. Apparently, someone sent thousands of unsolicited advertisements, all claiming to have come from webmaster (at) www.mikerubel.org. The messages in question had never passed through my mail server (which does not relay), and they were certainly not sent by me. They were forged. It was as if the spammer had done a mass-mailing and had printed my home address on the envelopes to lend them credibility.
The only reason I know about this episode is that over the course of a day, I received a number of bounces from recipients' vacation programs and spam filters. I traced the headers on one of the bounce messages to a machine in South Korea, which either belonged to the original spammer or, more likely, had also been hijacked by him. There was nothing more I could do at that point, so I let the matter slide.
Unfortunately, the episode was more than just annoying. Supporting the
reasonable inclination to refuse additional mail from the sender of an
unwanted message, some email programs provide a "refuse sender" feature.
But since the
From: field of spam is almost always forged,
recipients end up blocking the wrong person. (For the spammers, this was
the idea all along, of course). How many computers will no longer accept my
email, I wonder?
The big sites get this all the time. I'm sure that a large fraction of your
spam appears to come from
hotmail.com. You might be tempted to conclude that such sites
are major spam havens. They aren't (see here). If you check the full headers on the
messages you've received, you'll see that
hotmail did not send them. But the misconception persists--I
know people who block all email claiming to originate from those sites!
While limiting email forgery would not, by itself, put an end to spam, it
would remove one of the spammers' more potent weapons: the ability to
masquerade as others. The constant target
for example, would finally get her name back. Robust forgery detection
might also help improve the accuracy of such powerful filters as spamassassin by providing them with
more information on which to base their decisions.
Email forgery is a serious problem. Larry Seltzer has recently gone so far as to propose replacing the whole Internet because, in his words:
"...the biggest failure in this regard is SMTP, the dominant mail protocol of the net [...] We know how to fix these problems; the problem is that doing so would break existing applications, which means email in general."
And Seltzer has plenty of company. I disagree, though; I believe that it is indeed possible to fix the email forgery problem without breaking, or even substantially changing, the Internet, and without having to change users' behavior.
Throughout this document, I will refer to the
From: address on
an email as if there were only one such address. Of course, this is a
simplification; there may be an envelope
MAIL FROM: address, a
From: address, and even other, more obscure sender
addresses. It is the envelope address we're interested in. These and other
details are discussed at length in the notes section.
We begin by learning how forgery works.
Every self-respecting computer geek has, at some point in the course of his
or her education, sent an email with a forged
The prank generally goes something like this:
$ telnet myfriendsmachine.college.edu 25 Trying 192.168.1.2... Connected to myfriendsmachine.college.edu. Escape character is '^]' 220 myfriendsmachine.college.edu ESMTP Sendmail 8.12.3/8.12.3; Thu, 27 Feb 2003 23:05:02 -0800 (PST) HELO mymachine.college.edu 250 myfriendsmachine.college.edu Hello mymachine.college.edu [192.168.1.1], pleased to meet you MAIL FROM: email@example.com 250 2.1.0 firstname.lastname@example.org... Sender ok RCPT TO: email@example.com 250 2.1.5 firstname.lastname@example.org... Recipient ok DATA 354 Enter mail, end with "." on a line by itself From: "Mr. Deity" <email@example.com> To: "Poor Sod" <firstname.lastname@example.org> Date: Thu, 27 Feb 2003 23:05:03 -0800 (PST) Subject: some friendly advice Hello my child, I know about that stack of magazines under your bed. For shame! Repent now and free yourself from sin. Send the magazines away with your roommate. He'll know what to do with them. Yours in virtue, God ps> stop borrowing his toothbrush. that's disgusting. . 250 2.0.0 h1S7238N127934 Message accepted for delivery QUIT 221 2.0.0 myfriendsmachine.college.edu closing connection Connection closed by foreign host. $
The reason this works is historical. Email is old; SMTP is one of the
oldest protocols on the net. It was born at a time when senders could
generally be trusted to identify themselves and spam was only a serious
issue to vegetarians. At
that time, routing (now a completely automatic process) needed
application-layer help in the form of bangpaths, and the mail relay which
delivered your message was not necessarily associated with the original
source. Only the weakest of background checks--such as verifying that the
From: domain actually existed--were put in place to
authenticate the sender.
Now, of course, the Internet is a shadier and more autonomous place. The need for public mail relays evaporated just as their services began to suffer at the hands of dishonest marketers. Black-hole lists have emerged to shun sloppy sysadmins and their service providers. Stronger security measures, such as digital signatures, now provide both a means to authenticate the sender of a message and a way to prevent adulteration enroute. Despite their potential, however, digital signatures are seldom seen in email nowadays, owing mainly to the level of understanding they require of end-users.
Our apathy in this area has given rise to an unfortunate new type of
problem. Where email header forgery was once mostly the domain of bored
college students, it has now become the modus operandi of spammers,
virus-writers, and con artists. End users are understandably confused: why
From: address on an email header be less trustworthy
http:// URL in their web browser? Users tend to trust
an email if it claims to come from a friend, a co-worker, or a well-known
company. And their trust is easily exploited. Witness chain letters
claiming to originate from microsoft.com,
false security announcements claiming to originate from organizations
like Symantec or CERT, and viruses or worms such as Nimda which use
forged email headers to trick users into opening them. And witness, of
course, the torrent of forged spam currently flooding your inbox.
One begins by scanning the full message headers for
lines, each of which looks something like this:
Received: from foo.com (aardvark.foo.com [192.168.1.222]) by bar.com (Postfix) with ESMTP id 27Y1C46CB for <email@example.com> Sat, 1 Mar 2003 17:25:48 -0800 (PST)
Received: entry is added to the top of the email by every
relay the message visits on its trip across the Internet, giving the
recipient a (reversed) postal history. For example, In the situation
illustrated below, four headers would be added: one by the sender's outgoing
server (#1), two by the public mail relays (#2 and #3), and one by the
recipient's incoming mail server (#4).
Received: header will be the recipient's mail server,
which is the (real) machine he or she contacted to read the message.
Proceeding down through earlier
Received: lines, the recipient
should be able to trace back to the original sender's mail server. If the
message is forged, this process breaks down at some point; one of the real,
trustworthy mail relays (probably #4, the one belonging to the recipient's
company or ISP) will have accepted the message from a machine through which
it had no reason to pass. To cover their tracks, spammers sometimes add a
Received: lines below that as well.
Detecting a forgery begins by realizing that email systems never
legitimately use third-party relays anymore (certain exceptions to this
rule, discussed below, are the reason forgery detection is currently an
inexact science). Now that the Internet has standardized on TCP/IP, routing
is automatic (blue dots in the picture below), and so open relays are no
longer needed. If a message is being delivered from someone at
sender.org to someone at
recipient.com, then at
some point one of the
sender.org mail servers will have passed
the message directly to one of the
recipient.com mail servers.
Routers, not mail relays, will be the intermediaries. This situation is
Thus, detecting a forgery comes down to the following rule:
To Identify Forged Email: Look back through the
Received: lines to the last trusted relay--i.e., the last relay
belonging to your company or ISP (#2 in the above picture). Check the IP
address--not the reported name--from which that relay received the message.
In the example above, this would be the IP address of machine #1. If that
IP address does not belong to the sender's netblock, then the message is
The reason we check the IP address instead of the name is that under TCP (a
basic protocol on which higher-level protocols like SMTP are built), the
source IP address is difficult to forge. You may use the
utility to determine whether the IP address of the sending relay belongs to
the netblock of the sender. Details on the use of
be outside the scope of this article, but please see the References section below.
The problem with the forgery check just described is that while most sites obey this convention, there is no shortage of exceptions and marginal cases. One issue is that small sites sometimes use their hosting company's mail server, yet the business relationship might not be obvious from whois records. Also, some organizations do not (yet) provide remote authenticated SMTP services for traveling members, relying instead on laptops, which send their messages directly.
However, there is a way to make the forgery check robust, which we shall now demonstrate by way of example.
The management of
foo.com is growing tired of spammers using
its good name to promote such unsavory products as earlobe enlargement gel,
herbal spleen supplements, and sugar-free pizza. To make matters worse, a
loosely-knit group of twelve-year-old internet vigilantes has begun
foo.com's customers with false safety recall notices
for its "productz". Irate customers and primary school spelling teachers
have been ringing all day, and the staff is exasperated.
A company-wide meeting is called to address the problem. Early suggestions, such as hiring one secretary's large, muscle-bound nephew to track down the perpetrators, receive widespread support but are ultimately rejected for assorted legal reasons.
Finally, a network administrator pipes up. "What if we post a page on our web site that lists the IP addresses of all of our outbound mail servers?
"Spammers can still forge our company name, but ultimately they're sending the spam from somewhere else. They can't make it appear to come directly from our servers' IP addresses unless they break into our network, which would be a lot harder. So customers will be able to check the source from which their ISP received the message (in the email header), and verify that it's one of the IP addresses on our list."
A dramatic pause ensues, during which the the entire staff contemplates the network administrator's suggestion. One-by-one, the participants seem to light up with understanding. There is a great deal of appreciative nodding and chin-scratching. Finally, the company founder stands up and clears his throat. "What the heck is an IP address?"
The core idea of the network administrator's plan--publishing a list of the company's real outbound mail servers--is sound. But for a number of reasons, the web is probably not the place to do it. Web sites usually exist to communicate information to humans, and if this strategy is going to work, it needs to happen automatically. Also, it might not be obvious where to look; not all mail systems have associated web sites, and those that do might adopt different conventions.
Fortunately, there exists a system which is better suited to this task:
DNS. The Domain Name System is the "phone directory" of the Internet,
responsible for translating names (such as
mikerubel.org) to IP
addresses (such as
220.127.116.11). DNS actually stores all
kinds of information about a name: the IP address just mentioned is called
A record. Nameservers (
NS records) provide
directory information for machines in the domain. Mail servers
MX records) are configured to recieve its email. Each piece
of information associated with a name is called a resource record,
RR, and is identified by a short abbreviation. To learn
more about the different kinds of resource records available, check out the
official specification, RFC 1035, which is linked-to in the references section below.
One of the first actions a system takes when receiving an email is to query
DNS for the name given in the
From: address, to verify that it
actually exists. If we add one new type of resource record, tentatively
RMX (or "reverse-
MX"), for the IP addresses
of servers authorized to send mail on behalf of that name, then we're
set--the recipient's email server will verify that the sending IP address
matches one of the
RMX records of the name given in the
This process is illustrated below.
Had the actual source not been among the
RMX records for
foo.com, the message would have been registered as a forgery.
Net-minded readers may recognize the reason this works: it's a type of three-way handshake, similar in principle to the one which makes it fundamentally difficult for the initiator of a TCP transaction to conceal his or her identity.
The most significant property of
RMX records is that they
do not break the Internet! The standard email protocol,
SMTP, does not change. DNS simply adds a new type of resource
record--something it is designed to do both easily and backwards-compatibly
(clarification). The protocol remains the
same. If the sender has not configured
RMX records, or the
recipient's mail server does not bother to check them, then nothing changes.
RMX records have been configured, and the recipient
does check them, then forgeries will be detected. What the recipient
chooses to do with that information--throwing the message out immediately,
passing it on as a special header for use by (for example) spamassassin or procmail, or ignoring it--is up to the
recipient. Without breaking the Internet,
RMX records make
email forgery much more difficult.
RMX records are also attractive because they don't have to be
implemented everywhere to be useful. There is a small, incremental
advantage to upgrading to
RMX-aware software, both for senders
and for recipients. Senders get some protection against the illegitimate
use of their names, and recipients improve their ability to filter spam and
avoid being deceived. This property makes the eventual widespread use of
RMX records a realistic goal.
The efficacy of this scheme rests with the security of DNS. That's an
important observation--not necessarily a drawback, because the era of secure
DNS (DNSSEC) is slowly coming--but it is only fair to note that if the DNS
system can be compromised, then
RMX records break down. The
recipient's mail server must handle inability to access the sender's name
server (caused, for example, by a DDOS attack) gracefully, perhaps by
placing the message in temporary quarantine until the name server is
RMX is not a strong authentication mechanism, but it should be
sufficient to stop most forgery. It is not intended to replace stronger
mechanisms such as digital signatures. With
From: address on an email will become exactly as trustworthy as
the URL in a web browser, which meshes with user expectation. Things
that ought to work, such as "refuse sender," will work.
One particularly nice feature of
RMX records is that they add
to the technical infrastructure governments will have at their disposal to
apply legal and economic pressure to would-be email forgers. Domain names
could be forfeited (possibly along with a deposit) if they are used to send
misrepresented email. Or perhaps a credit-like system could be established
to prevent frequent abusers from registering new domain names.
Fundamental difference: with
ident, you call back the forger's
RMX, you call back the forgee's machine. The
former approach assumes that the forger is one user on a server administered
by someone trustworthy--which, on the Internet, is often not the case.
RMX requires no such assumption.
There seems to be a very common gut reaction to this proposal: that
RMX would be vulnerable to trivial attacks like this
one. It isn't, and I wonder if the confusion doesn't stem from a
lingering bad taste over
Scott Nelson was kind enough to inform me that some older versions of BIND
do not properly handle unknown
RR's, crashing or otherwise
failing when they receive one. While I assume that very few nameservers on
the Internet are so easy to break from remotely, it would be more accurate
to say, "RMX records are not as disruptive to the Internet as competing
approaches might be."
Before an organization posts
RMX records, it must ensure that
all outbound mail passes through one of the designated outgoing mail
servers. Right now, certain situations, such as travel, often lead to a
break in this convention. Before an organization can implement
RMX, it must shore up these leaks. There are many technologies
now available (VPN, SMTP-AUTH, SASL, SSL/TLS, and even webmail come to mind)
which can be used to satisfy this requirement. By posting
records, a domain declares that it has adopted the necessary measures.
No, but it will help. The stated goal is to give system admins a way to
prevent malicious misuse of their names by spammers, and it does accomplish
that. However, by giving intelligent tools like spamassassin more
information to work with, it will help to reduce spam, particularly as the
RMX-protected sites grows. Because
is such a simple concept (particularly for the recipient), it stands a much
better chance of being adopted than some of the more complicated proposals.
RMX helps against spam by making compromised,
poorly-secured machines--most connected home desktop machines fall into this
category--less useful as spam tools. Spammers often trick these machines
into sending spam for them.
RMX does not stop this practice,
but it makes spammers unable to claim that the messages originated from any
RMX-protected domain, since otherwise the messages would be
treated as forgeries by their recipients. This property is more useful than
it would first seem, as we shall now show.
Suppose that you are a responsible domain administrator, and further suppose the owner of a compromised home desktop computer uses your email service. In this situation, many anti-forgery proposals break; the spammer could steal the desktop owner's credentials and begin spamming as him or her immediately. Since your domain name is attached to his or her email address, this abuse could affect you; in some cases it will lead to your domain being blacklisted.
RMX, however, the spammer can still steal credentials,
but to make the messages legit he must send them through your mail servers.
Being a responsible domain administrator, you have automated tools that
detect and flag attempts by a single user to send a lot of messages at once,
and cut that user off (with a message like this)
until the situation is resolved. Thus, the spammer cannot get away with
sending a large number of spams appearing to come from your domain;
RMX has given you both the ability and the responsibility to
secure your domain.
Suppose Bob, upon graduating from
foo.edu, opens an account
emailprovider.com. It is common practice for Bob to put a
.forward file in his old
foo.edu account, which
tells it to forward mail to the new account.
emailprovider.com wants to start checking
RMX records. What happens? If
foo.edu has an
old mail server which doesn't do sender rewriting, then
emailprovider.com will see messages claiming to be from
original senders but forwarded from
foo.edu, and will conclude
that the messages are forgeries.
How should this situation be handled? Well, ideally,
should update its mail server, but we can't count on that happening. In the
meantime, the best course of action for
emailprovider.com is to
not reject apparently-forged email, but to pass it to the
client with an
X-header noting that the
check failed. This gives Bob the option of accepting mail that arrives from
foo.edu mail servers addressed to his old address even if
it does not pass
It is for this reason, and for certain situations involving mailing lists
and anonymous senders, that I believe bouncing messages immediately--or even
during the SMTP session, as has been proposed--would be a bad idea. Better
to pass the information in an
X-header, and let the
client/spamfilter sort it out, at least during the transition period. I
acknowledge that this position puts me at odds with RFC-2505.
How much of spam is really forged? Here's one clue. David Walker sent the following breakdown of spam he received to IETF's anti-spam research group:
With regards to spoofing being a minor problem. Out of 3130 denied messages (to accounts I had to stop because they were receiving 100% spam) @juno.com | 36 @netscape.com | 38 @email.com | 40 @excite.com | 50 @lycos.com | 50 @earthlink.net | 71 @msn.com | 72 @yemenmail.com | 93 @hotmail.com | 241 @aol.com | 298 @yahoo.com | 311 Total | 1300 1300 out of 3130 = 41% of all my denies are very high likelyhood spoofs from the popular domains 1050 out of 3130 = 34% are guaranteed spoofs (The helo name is not remotely associated with the spoofed domain) from the popular domains. (These numbers do not represent all spoofing I receive but rather just the spoofing to popular domains)
The FTC also reports in their2003 study [pdf] on misleading information in spam that "one-third of the spam messages contained false text in the From line." Thanks to Phill Hallam-Baker for referring me to this article.
Daniel Erat raises a very reasonable and interesting point. We have shown
that by using
RMX, a domain may prevent its name from being
forged on the email envelope (the
MAIL FROM address of
RFC-821). However, a lot of mail reading programs do not show the
envelope header; they show only the
From: address given inside
the message (RFC-822). So we've succeeded in making the envelope return
address trustworthy, but in many cases the butler is throwing out the
envelope before passing the message to the recipient!
This fact does remove one incentive for senders to implement
RMX records--right now, recipients might not notice. On the
other hand, forgeries will still be obvious to automated filters, vacation
programs, and such. Once
RMX is implemented, the true meaning
of the envelope
MAIL FROM will be "he who forwarded this
message to you" and the message
From: will be "where the
message claims to have originated." The two may differ; for example, in the
case of a mailing list, the envelope return address is that of the list but
From: is that of the original sender. As
RMX records become widespread, hopefully mail reader software
will evolve to show recipients both pieces of information in a way that
makes their meanings clear.
Roger Standridge (homepage) and your author had a long discussion about this issue. In its current form, DNS is known to be vulnerable to certain types of spoofing and cache poisoning attacks. Joe Stewart has a very informative article on this topic, which you can find at Security Focus.
RMX is particularly sensitive to this problem. Because an
attacker controls the timing of
RMX requests--by choosing when
to send forged email--he or she knows when to send spoofed DNS replies.
Turns out this has already been
mentioned before. However, in most cases the attacker already has this
advantage for other reasons. Recursive queries, reverse-lookups on incoming
mail, and honeypot-based techniques could all be used to give the attacker
much the same advantage. In the end,
RMX gives the
URL in a web
browser, which is to say, not much right now.
When upgrading to an
RMX-aware mail server or posting
RMX records, administrators must also verify that their name
servers are up to date and properly configured, or the
check will be essentially meaningless. Once
widespread, this problem will be less of an issue.
All email leaving an
RMX-protected domain must pass through one
of its designated mail servers to be certified as a non-forgery. This
property was a design goal. One oft-raised objection, though, is that it
gives a domain owner the ability to filter outgoing mail, should he or she
While I agree that this technically true, I do not agree that it is a problem. Consider the following:
MX) mail servers. Why should outgoing mail be different?
Note that cryptography-based alternatives have been proposed which claim to afford more privacy at the cost of somewhat more disruptive changes.
The act of destroying a person's online reputation by sending spam claiming to come from him or her is called a Joe Job. Joe Jobbers are con artists--they aren't trying to sell something, they're trying to smear someone. It is important to recognize the difference; next time you become furious with someone who apparently sent you an offensive spam, realize that he or she may be the innocent victim of a Joe Job. For an interesting discussion, see this article. Brian T Glenn was kind enough to send me information about the origin of the term: here is a page from the person who coined it, and a a newsgroup discussion on the topic.
In response to the IPv4 address shortage, the fact that home users do not run servers, and a general desire to keep things plug-n-play, most home and small-business ISP's offer only temporary, DHCP-assigned IP addresses to their clients. This is a problem for the small body of homes and businesses that do wish to run their own servers on such systems, since they need a permanent way to refer to their machines from outside. Under dyndns, or Dynamic DNS, the home or small business registers a permanent domain name, and obtains name service from a special DNS provider that responds very quickly (and automatically) to changes in the host's IP address, and keeps the cache time very small.
If a dyndns user wishes to implement
RMX, he or she may need to
update the appropriate
RMX record dynamically along with the
A record. Note that it is not currently necessary to
MX records under dyndns, because they point to
A records indirectly, through
RMX, several field types are possible (see Hadmut's draft for
the proposed types). Some would need to be dynamically updated, and others
(which provide a level of indirection though a permanent name) would not.
Thanks to Tim Long for pointing this issue
out to me.
hotwaydbecause I don't like to use the web interface directly; will
RMXbreak my system?
Possibly; this is a complicated issue.
If your email provider allows non-web-based access and decides to post
RMX records, then it must provide a way for you to send email
(SMTP) as well as receive it (POP3 or IMAP), or recipients will refuse to
accept your mail. There are a few providers out there--Hotmail in
particular--which do not explicitly prohibit non-web-based access, but at
the same time provide only a nonstandard way to access it. And there are a
number of workarounds for this problem, such as the
hotwayd project. If
Hotmail decides to post
RMX records, then it will either
provide remote access through (e.g.) secure
remote access through some kind of nonstandard protocol which can be worked
around (such as a web form), or change the terms of service to prohibit
If your webmail provider refuses to permit non-web-based access, then you
will have to use their web interface or switch to a different provider. If
it opts to permit non-web-based access but provides only a nonstandard way
to send mail, then projects such as
hotwayd will need to
develop a workaround, and users of such projects will need to use the web
interface to send mail until the needed tools are ready. Such users also
have the option of sending from a different (but also legitimate) address
and including an appropriate
If your email provider allows secure
SMTP-AUTH access, then
you're set, unless your ISP refuses to allow outbound TCP port 465
connections. In this last case, you'll need to convince your ISP to unblock
port 465 connections to your email provider or find a less brain-dead ISP.
A somewhat more common problem would be ISP's blocking outbound TCP port 25
connections; webmail providers should keep this issue in mind when
configuring their remote access systems. It would be best to explicitly
SMTPS access, which has vast security advantages over
Please understand that sending
MAIL FROM a machine which is not
authorized to send from that domain is a benevolent form of forgery--one
which is indistinguishable from its less innocuous cousins. If we want to
end forgery, then benevolent forgery must also go. It is becoming
increasingly common for recipients to block email from popular domains when
the emails do not arrive directly from the domains' mail servers; see, for
example, the system described here.
This is a manual form of
RMX, applied to only a few popular and
frequently-forged domains. It can be a problem, though, because it blocks
some legitimate uses, such as the
hotwayd workaround described
RMX suffers no such problem; by posting
RMX records, a domain declares that it has already provided all
of the remote access facilities that it intends to provide, and that
therefore any other email claiming to come from it should be treated as a
By the way, if you work for Hotmail, Yahoo mail, AOL, or one of the other
major providers, I'd be very interested to hear whether your company
intends to implement
RMX, and if so, how it intends to handle
Thanks to RenE J.V. Bertin for bringing up this issue.
When I originally wrote this document in late February 2003, I intended to
call the new records
MO, for "mail origin." I began to write
an Internet Draft to the IETF to formally propose the new method at the same
time I wrote this page to bring it to the general public. In the process of
finding references, however, I stumbled upon Hadmut Danisch's draft,
which proposed essentially the same thing, but called the new records
RMX instead of
MO. I'd been scooped! I contacted
Hadmut to explain the situation, and suggested that I post this page (since
it was already mostly written!) in support of
instead. Hadmut was kind enough to reply, and he agreed with the
RMX records are a simple, practical, and elegant solution that
should have been implemented a long time ago. To make them happen, the
following changes need to take place:
RMXresource record must be added to
DNSsoftware such as bind.
RMXchecks must be added to mail server software such as sendmail and postfix.
Update (2003.06.01): Hadmut has released first bind9 and sendmail implementations. See his site to download the patches.
With this document, I hope to accelerate the process a bit, by making people
aware of the existence and potential benefits of
I hope you'll agree that now is the time to make this happen--but if you
disagree, I'd like to hear that, too. Please feel free to write to me at the
address below (webmaster) with comments.
From:nameserver with the IP address from which it received an email, rather than requesting the list and making the decision itself. This is slightly more complicated and hits DNS harder, but has advantages too. An organization would quickly detect the source of attempts to forge its name, and there would be no need to add an additional
RR(just a new convention). I would be equally satisfied with this solution to the forgery problem. Thanks to Scott Nelson for pointing this out to me.
MXrecords instead. I like this approach as well; it has the same net effect as
RMXwithout introducing the additional resource record. Notice, however, that it might hit DNS harder.
RMX. His page also has a good discussion and comparisons of the two proposals, worth reading.
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