An open mail relay is an SMTP server configured in such a way that it allows
anyone on the Internet to send e-mail through it, not just mail destined to or
originating from known users. This used to be the default configuration in many
mail servers; indeed, it was the way the Internet was initially set up, but open
mail relays have become unpopular because of their exploitation by
spammers and worms. Many relays were closed, or were placed on blacklists by
other servers. History and technology
Until the 1990s, mail servers were commonly intentionally configured as
open relays; this configuration was also frequently the installation default
setting on UNIX systems. The traditional store-and-forward method of relaying
e-mail to its destination required that it was passed from computer to computer
via modems on telephone lines. For many early networks, such as UUCPNET, FidoNet
and BITNET, lists of machines that were open relays were a core part of those
networks. Filtering and speed of e-mail delivery were not priorities at that
time and in any case the government and educational servers that started the
Internet were covered by a federal edict forbidding the transfer of commercial
messages. Abuse by spammers
In the mid-1990s, with the rise of spamming, spammers resorted to
re-routing their e-mail through third party e-mail servers to avoid detection
and to exploit the additional resources of these open relay servers. Spammers
would send one e-mail to the open relay and include a large blind carbon copy
list, then the open relay would relay that spam to the entire list. While this
greatly reduced the bandwidth requirements for spammers at a time when
Internet connections were limited, it forced each spam to be an exact copy and
thus easier to detect. After abuse by spammers became widespread, operating an
open relay came to be frowned upon among the majority of Internet server
administrators and other prominent users. Open relays are recommended
against in RFC 2505 and RFC 5321. The exact copy nature of spam using open
relays made it easy to create bulk e-mail detection systems such as Vipul's
Razor and the Distributed Checksum Clearinghouse. To counter this, spammers
were forced to switch to using hash busters to make them less effective and
the advantage of using open relays was removed since every copy of spam was
"unique" and had to be sent individually.
Since open mail relays make no effort to authenticate the sender of an e-mail,
open mail relays are vulnerable to address spoofing.
Anti-spam efforts Many Internet service providers use
DNSBLs to disallow mail from open relays. Once a mail server is detected
or reported that allows third parties to send mail through them, they will be
added to one or more such lists, and other e-mail servers using those lists
will reject any mail coming from those sites. The relay need not actually be
used for sending spam to be blacklisted: instead, it may be blacklisted after
simple test that just confirms open access.
This trend reduced the percentage of mail senders that were open relays from
over 90% down to well under 1% over several years. This led to spammers
adopting other techniques, such as the use of botnets of zombie computers to
send spam. One consequence of the new
unacceptability of open relays was an inconvenience for some end users and
certain internet service providers. To allow customers to use their e-mail
addresses at Internet locations other than the company's systems, many mail
sites explicitly allowed open relaying so that customers could send e-mail via
the ISP from any location. Once open relay became unacceptable because of
abuse, ISPs and other sites had to adopt new protocols to allow remote users to
send mail. These include smart hosts, SMTP-AUTH, POP before SMTP, and the use
of virtual private networks. The IETF has written a best current practices
covering Email Submission Operations in RFC 5068.
Note that the above only becomes an issue if the user wishes to continue to
send e-mail remotely, using the same SMTP server which they were previously
accessing locally. If they have valid access to some other SMTP server from
their new, remote location, then they will typically be able to use that new
server to send e-mails as if from their old address, even when this server is
properly secured. The Can Spam Act of 2003 makes it
illegal to send spam through an open relay in the United States, but makes no
provision regarding sending personal e-mail through them or regarding their
operation, although the effectiveness of the act has been questioned.
Modern-day proponents The most famous open mail relay
operating today is probably that of John Gilmore, who argues that running an open
relay is a free speech issue. His server is included on many open relay
blacklists test e-mail to other servers to see if they will be relayed). These
measures cause much of his outgoing e-mail to be blocked. Along with his
further deliberate configuration of the server, his open relay enables people to
send e-mail without their IP address being directly visible to the recipient
and thereby send e-mail anonymously. In 2002, his open relay, along with 24
others, was used by a computer worm to propagate.
John Gilmore and other open relay proponents declare that they do not
support spam and spamming, but see bigger threat in attempts to limit web
capabilities that may block evolution of the new, next generation technologies.
They compare the network communication restrictions with restrictions that some
phone companies tried to place on their lines in the past, preventing
transferring of computer data rather than speech.
Closing relays In order not to be considered "open", an
e-mail relay should be secure and configured to accept and forward only
the following messages: Messages from local IP addresses to
local mailboxes Messages from local IP addresses to
non-local mailboxes Messages from non-local IP addresses to
local mailboxes Messages from clients that are
authenticated and authorized In particular, a properly secured SMTP
mail relay should not accept and forward arbitrary e-mails from non-local IP
addresses to non-local mailboxes by an unauthenticated or unauthorized user.
In general, any other rules which an administrator chooses to enforce must be
in addition to, rather than instead of, the above. If not, the relay is still
effectively open: it is easy to forge e-mail header and envelope information,
it is considerably harder to successfully forge an IP address in a
TCP/IP transaction because of the three-way handshake that occurs as a
connection is started. Open relays have also resulted from
security flaws in software, rather than misconfiguration by system
administrators. In these cases, security patches need to be applied to close the
relay. Internet initiatives to close open
relays have ultimately missed their intended purpose because spammers have
created distributed botnets of zombie computers that contain malware with mail
relaying capability. The number of clients under spammers' control is now
so great that previous anti-spam countermeasures that focused on closing
open relays are no longer effective. References