Sun Microsystems Sun Workstation 100U System Manager's Manual page 316

SENDMAIL - An Internetwork Mail Router
Eric Allmant
Britton-Lee, Inc.
1919 Addi,on Street, Suite 105.
Berkele,l, California 9-170-1.
ABSTRACT
Routing mail through a heterogenous internet presents many new problems. Among the
worst of these is that of address mapping. Historically, this has been handled on an ad
hoc basis. However, this approach has become unmanageable as internets grow.
Sendmail acts a unified "post office" to which all mail can be submitted. Address in-
terpretation is controlled by a production system, which can parse both domain-based
addressing and old-style ad hoc addresses. The production system is powerful enough to
rewrite addresses in the message header to conform to the standards of a number of com-
mon target networks, including old (NCP /RFC733) Arpanet, new (TCP /RFC822) Ar-
panet, UUCP, and Phonenet. Send mail also implements an SMTP server, message
queueing, and aliasing.
Sendmail implements a general internetwork mail routing facility, featuring aliasing and for-
warding, automatic routing to network gateways, and flexible configuration.
In a simple network, each node has an address, and resources can be identified with a host-
resource pair; in particular, the mail system can refer to users using a host-username pair. Host
names and numbers have to be administered by a central authority, but usernames can be
assigned locally to each host.
In an internet, multiple networks with difl'erent characterstics and managements must com-
municate. In particular, the syntax and semantics of resource identification change. Certain spe-
cial cases can be handled trivially by ad hoc techniques, such
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providing network names that
appear local to hosts on other networks,
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with the Ethernet at Xerox P ARC. However, the
general case is extremely complex. For example, some networks require point-to-point routing,
which simplifies the database update problem since only adjacent hosts must be entered into the
system tables, while others use end-to-end addressing. Some networks use a left-associative syn-
tax and others use a right-associative syntax, causing ambiguity in mixed addresses.
Internet standards seek to eliminate these problems. Initially, these proposed expanding the
address pairs to address triples, consisting 01 {network, host, resource} triples. Network numbers
must be universally agreed upon, and hosts can be assigned locally on each network. The user-
level presentation was quickly expanded to address domains, comprised of a local resource
identification and a hierarchical domain specification with a common static root. The domain
technique separates the issue of physical versus logical addressing. For example, an address 01 the
form "ericOa.cc.berkeley.arpa" describes only the logical organization of the address space.
Sendmail is intended to help bridge the gap between the totally ad hoc world of networks
that know nothing 01 each other and the clean, tightly-coupled world 01 unique network numbers.
It can accept old arbitrary address syntaxes, resolving ambiguities using heuristics specified by the
tA considerable pari of this work was done while under the employ or the INGRES Projed
a~
the Univenity or
California a~ Berkeley.
SENDMAIL 1