.nr DR 1
.he 'SENDMAIL''%'
.if \n(DR .fo 'For Your Eyes Only'\*-DRAFT\*-'\*(td'
.ls 2
.+c
.(l C
.sz 12
SENDMAIL \*- An Internet Mail Router
.sz
.sp
Eric Allman
.i
Project INGRES
Electronics Research Lab
University of California
Berkeley, California  94720
.)l
.(f
This is
.if \n(DR draft
version 3.1,
last modified on 08/24/81.
.if \n(DR Please do not distribute this version without permission
.if \n(DR of the author.
.)f
.sp 2
.pp
.i Sendmail
implements a general internetwork mail routing facility.
Features include aliasing and forwarding,
automatic routing to network gateways,
and flexible configuration.
.pp
Section 1 discusses the design goals for
.i sendmail .
Section 2 gives an overview of the basic functions of the system.
In section 3,
details of usage are discussed.
A detailed description of the configuration file
is given in section 4,
including a walkthrough of a specific configuration file.
Section 5 compares
.i sendmail
to other internet mail routers,
and an evaluation of
.i sendmail
is given in section 6,
including future plans.
.sh 1 "DESIGN GOALS"
.pp
.i Sendmail
was an outgrowth of
.i delivermail,
a previous incarnation of a UNIX internetwork mail router.
.i Delivermail
was written relatively quickly.
The first version only knew about taking apart addresses
for explicit forwarding
and limited aliasing;
automatic forwarding and other features
came later.
.pp
Design goals for
.i delivermail
included:
.np
Compatibility with the existing mail system,
including Bell version 6 mail,
Bell version 7 mail,
Berkeley mail,
BerkNet mail,
and hopefully UUCP mail.
ARPANET mail was also required,
and the difference in format
drove the decision to put all such formatting
into the low-level mailer.
.np
Because of time constraints,
utilize as much existing software as possible.
The changes to the existing software were minimal:
Berkeley mail,
BerkNet,
UUCP, and the ARPANET FTP server
had to be modified to call
.i delivermail
as their server instead of
/bin/mail
(or in the case of the ARPANET,
writing the mail into a file
which has no meaning to standard UNIX mailers).
The only major modifications
were to /bin/mail,
which was maintained both as a user interface sender
and as a
.i delivermail
mailer.
As a sender,
it calls
.i delivermail .
.i Delivermail
calls it in turn to do local delivery,
so a
.b \-d
flag was added to avoid loops.
.np
Reliability was considered essential
because of the emphasis on mail in our environment.
This turned out to be one of the hardest goals to satisfy,
especially in the face of the many anomalous message formats
produced by various ARPANET sites.
For example,
MIT and CMU allow mail from people who are not logged in
(and which have meaningless from addresses),
which caused error message loops.
WHARTON changes our host name from
.q Berkeley
to
.q Berkel-
which creates interesting problems under certain circumstances \*-
not to mention rendering any
.q reply
feature unworkable.
CMU puts blanks in names,
which created amazing problems,
since many UNIX mail programs assume that an address
is one word.
And at least one person lists his address as
.q "From: the TTY of ..." ,
giving a
.q Sender:
field with his real address.
In summary,
the obscure aspects of the ARPANET mail protocol
really are used,
are difficult to support,
but must be supported.
.pp
There were certain non-goals in
.i delivermail
also.
Many of these resulted from the expectation that
it would only be used at Berkeley,
and probably only at a few sites at Berkeley.
.np
It was fair game to compile configuration information
into the code,
even to assume that they were running BerkNet.
.np
The problem of multiple connections to a single network
was not foreseen.
For example,
on a host with no UUCP connection,
all UUCP mail was sent to a single host.
In fact,
Berkeley is running UUCP on at least three hosts.
.np
No attempt was made to reduce the volume of mail across a network link.
Besides the difficulty of doing this,
we failed to appreciate how much volume there would be.
.pp
.i Sendmail
maintained the goals of
.i delivermail.
Time was less of a constraint,
but not reimplementing the wheel
(or other mailers)
had proven to be a wise move in many ways.
For example,
many internet mailers deliver local mail directly.
This is more efficient,
but builds in the design decisions
of the local mailer,
and makes it difficult to concentrate
on the
.q "real problems"
(such as locking).
Other design goals were:
.np
.i Sendmail
should operate in more complex environments,
including multiple
(but not equivalent)
connections to a single network.
This required that the contents of a host field
be considered,
as well as just the syntax of an address.
This results both from a desire to simplify use by other sites,
and to anticipate the environment Berkeley is moving towards.
.np
Configuration should not be compiled into the code.
A single binary should be able to run as is at any site
(modulo such basic changes as the CPU type or the operating system).
We have found this apparently unimportant goal
to be critical in real life.
.np
.i Delivermail
only knows about one alias file.
Berkeley is a sufficiently open environment
that this can be writable by everyone,
but other environments are not so lax.
Thus,
.i sendmail
must be able to let various groups maintain their own mailing lists,
and individuals their own forwarding,
without writing the system alias file.
.np
Customized incoming mailers should be supported.
.np
Network traffic should be minimized
by batching addresses to a single host where possible,
without assistance by the user.
.sh 1 "OVERVIEW"
.sh 2 "System Organization"
.pp
.i Sendmail
neither interfaces with the user
nor does actual mail delivery.
Rather,
it collects a message
generated by a user interface program (UIP),
does editing as required by the internet environment,
and calls appropriate mailers
to do mail delivery or queueing for network transmission
(the exception is when mailing to a file).
This discipline allows the insertion of new mailers
at minimum cost.
In this sense it is like the Message Processing Module (MPM)
of [1].
.(d
[1] Postel -- internet message structure
.)d
See Figure 1.
.(z
.(c
	       +-------+
	       | user  |
	       +-------+
		   |
	        +-----+
	        | UIP |
	        +-----+
		   |
             +-----------+
             | sendmail  |
             +-----------+
		|  |  |
     +----------+  +  +----------+
     |  	   |             |
+---------+   +---------+   +---------+
| mailer  |   | mailer  |   | mailer  |
+---------+   +---------+   +---------+
.)c

.ce
Figure 1 \*- System Structure.
.)z
.sh 2 "Operational Description"
.sh 3 "Argument processing and address parsing"
.pp
The arguments are first scanned,
and flag arguments are processed.
The remaining arguments are addresses.
They are parsed in turn,
and a list of recipients is created.
Aliasing is done at this step.
As much validity checking of the addresses as possible
is done at this step.
Syntax is checked, and local addresses can be verified,
but detailed checking of host names and addresses
is not checked until later.
Forwarding is also done
as the local addresses are verified.
.pp
As each address is parsed,
it is appended to the recipient list.
When a name is aliased or forwarded,
the old name is not removed from the list,
but a flag is set in the address header
that tells the delivery phase
to not actually deliver the message to this recipient.
This list is kept without duplicates,
preventing alias loops
and eliminating people receiving two copies of a message,
as might happen if a person were in two groups.
.pp
The recipient list is kept partitioned by mailer;
this simplifies the task of sending one copy of a message
across network links.
.sh 3 "Message collection"
.pp
The message is then collected from the standard input.
Parsing of the message header is done at this point.
The header is stored in memory,
and the body of the message is saved
in a temp file.
.pp
Collection occurs even if no addresses were valid
to simplify program interface.
The message will be returned with an error.
.sh 3 "Message delivery"
.pp
For each mailer known to the system,
the part of the recipient list for this mailer is scanned.
For each unique host,
a call is made to the mailer.
Each call contains the users on that host.
Mailers that only accept one recipient at a time
are handled properly.
.pp
.i Sendmail
then forks a process for each mailer
with a non-empty send list.
The message is then sent to the mailer
(which must read its standard input)
prepended by a customized header.
The exit code is caught and checked,
and a suitable error message given as appropriate.
The exit code must conform to a system standard
or a meaningless message
(\c
.q "Service unavailable" )
is given.
.pp
Delivery to files is handled directly.
.sh 3 "Return to sender"
.pp
If errors occurred during processing,
the message is returned to the sender for retransmission.
The letter can be mailed back
or written in the file
.q dead.letter
in the sender's home directory.
.sh 2 "Configuration File"
.pp
Almost all configuration information is read at runtime
from an ASCII file.
Information encoded in this file includes
macro definitions,
header declarations,
mailer definitions,
and address rewriting rules.
.sh 3 Macros
.pp
Macros can be used in three ways.
Certain macros transmit
unstructured textual information
into the mail system,
such as the name
.i sendmail
will use to identify itself in error messages.
Other macros transmit information from
.i sendmail
to the configuration file
for use in creating other fields
(such as argument vectors to mailers);
examples of these are the name of the sender and the host and user
of the recipient.
Other macros are unused internally,
and can be used as shorthand in the configuration file.
.sh 3 "Header declarations"
.pp
Header declarations declare to
.i sendmail
the set of known header lines.
Knowledge of a few header lines
is built into
.i sendmail ,
such as the
.q From:
and
.q Date:
lines.
.pp
Most headers declared in the configuration file
will be automatically inserted
in the outgoing message
if they don't exist in the incoming message.
Certain headers are suppressed by some mailers.
.sh 3 "Mailer declarations"
.pp
Mailer declarations tell
.i sendmail
of the various mailers available to it.
The definition includes the internal name of the mailer,
the pathname of the program to call,
some flags associated with the mailer,
and an argument vector to be used on the call;
this vector is macro expanded before use.
.sh 3 "Address rewriting rules"
.pp
The heart of address parsing in
.i sendmail
is the rewriting rules.
These are an ordered list of pattern-replacement rules.
Each address is applied successively to these rules
until it resolves into a canonical address
(i.e.,
a [mailer, host, user]
3-tuple),
or it falls off the end.
When a pattern matches,
the rule is reapplied until it fails.
.sh 2 "Message Header Editing"
.pp
Certain editing of the message header
occurs automatically.
Header lines can be inserted
under control of the configuration file.
Some lines can be merged;
for example,
a
.q From:
line and a
.q Full-name:
line can be merged under certain circumstances.
.sh 1 USAGE
.sh 2 "Arguments"
.pp
Arguments must be presented with flags before addresses.
The flags are:
.nr ii 1i
.ip "\-f addr"
The mail is from
.i addr .
This flag is ignored unless the real user
is root,
network,
or uucp,
or if
.i addr
contains an exclamation point
(because of certain restrictions in UUCP).
.ip "\-r addr"
An obsolete form of
.b \-f .
.ip "\-h cnt"
Sets the
.q "hop count"
to
.i cnt .
This represents the number of times this message has been processed
by
.i sendmail
(to the extent that it is supported by the underlying networks).
.i Cnt
is incremented during processing,
and if it reaches
MAXHOP
(currently 30)
.i sendmail
throws away the message with an error.
.ip "\-F\&name"
Sets the full name of this user to
.i name .
.ip \-e\&p
Print error messages (default).
.ip \-e\&q
Throw away error messages.
The only response is the exit status.
.ip \-e\&m
Mail back errors.
.ip \-e\&w
.q Write
back errors \*- or mail them if the user is not logged in.
.ip \-e\&e
Do special error processing for BerkNet.
This involves mailing back the errors
but always returning a zero exit status.
.ip \-n
Don't do aliasing or forwarding.
.ip \-m
Include me in alias expansions.
Normally
.i sendmail
suppresses the sender
if in a group being sent to.
.ip \-i
Don't take a dot to end a message.
.ip \-t
Read the header for
.q To: ,
.q Cc: ,
and
.q Bcc:
lines, and send to everyone listed in those lists.
The
.q Bcc:
line will be deleted before sending.
.ip \-a\&m
Do special processing for the
ARPANET.
This includes taking the
.q "From:"
person from the header,
printing
ARPANET
style messages
(preceded by three digits),
and ending lines with <CRLF>.
.ip \-a\&f
Same as
.b \-a\&m ,
except print out message numbers appropriate for the MLFL command.
.ip \-s
Save UNIX-style
.q From
lines at the beginning of headers.
Normally they are assumed redundant
and discarded.
.ip \-v
Give a blow-by-blow description of function.
This gives information of interest to the user
rather than for the
.i sendmail
maintainer;
for example,
aliases are printed as expanded
and mailer functions are printed as they run.
.ip \-C\&file
Use a different configuration file.
.ip \-A\&file
Use a different alias file.
.ip \-I
Initialize the DBM version
of the alias file.
If
.b \-I
is given,
no delivery is attempted.
.ip \-V
Verify the addresses only.
Only partial verification is done:
syntax is checked, and local names are verified,
but no checking normally done by the mailer is attempted.
.ip \-d\&level
Set debugging level.
.ip \-D\&x\&val
Define macro
.i x
to have value
.i val .
.nr ii 5n
.pp
Following flag arguments,
address arguments may be given.
These follow the syntax in RFC733
[7]
.(d
[7] RFC733
.)d
for ARPANET
address formats.
In brief, the format is:
.np
Anything in parentheses is thrown away
(as a comment).
.np
Anything in angle brackets (\ <>\ ) is preferred
over anything else.
.np
Double quotes
(\ "\ )
quote phrases;
backslashes quote characters.
Backslashes are more powerful \*- for example,
.i user
and
.i
"user"
.r
are equivalent,
but
.i \euser
is different from either of them.
.np
The word
.q at
is converted to
.q @ .
.pp
All other processing is controlled by the rewriting rules
(disclaimer: some special processing is done
after rewriting local names).
Parentheses, angle brackets, and double quotes
must be properly balanced and nested.
.sh 2 "Aliasing, Forwarding, Inclusion"
.pp
.i Sendmail
supports three methods for implicitly rerouting mail.
Aliasing applies system wide.
Forwarding allows each user to reroute incoming mail
destined for that account.
Inclusion directs
.i sendmail
to read a file for a list of addresses,
and would normal be used
in conjunction with aliasing.
.sh 3 "Aliasing"
.pp
Aliasing uses a system-wide file
mapping names to address lists.
This file is inverted to speed access.
Only names that appear to be local
are allowed as aliases;
this guarantees a unique key.
.pp
The inverted form of the file
must be recreated when the text copy is changed.
The
.b \-I
option to
.i sendmail
rebuilds the database.
.sh 3 "Forwarding"
.pp
After aliasing,
users that are found to be local and valid
are checked for the existence of a
.q .forward
file in their home directory.
If it exists,
the message is
.i not
sent to that user,
but rather to the list of users in that file.
The expectation is that this will normally
be one user only,
and the use will be for network mail forwarding.
.pp
Forwarding also permits a user to specify a private incoming mailer.
For example,
forwarding to:
.(b
"\^|\|/usr/local/newmail myname"
.)b
will use a different incoming mailer.
.sh 3 "Inclusion"
.pp
Inclusion is specified in ARPANET syntax:
.(b
:Include: pathname
.)b
An address of this form reads the file specified by
.i pathname
and sends to all users listed in that file.
.pp
The intent is
.i not
to support direct use of this feature,
but rather to use this as a subset of aliasing.
For example,
an alias of the form:
.(b
project: :include:/usr/project/userlist
.)b
is a method of letting a project maintain a mailing list
without interaction with the system administration,
even if the alias file is protected.
.pp
It is not necessary to rebuild the alias database
when a :include: list is changed.
.sh 2 "Exit Status"
.pp
An exit status is returned that corresponds to the system standard
used by the other mailers.
.sh 1 CONFIGURATION
.pp
Configuration is controlled primarily by the file
/usr/lib/sendmail.cf.
.i Sendmail
should not need to be recomplied except
.np
To change operating systems
(V6, V7/32V, 4BSD).
.np
To remove or insert the DBM library.
.np
To change ARPANET reply codes.
.np
To add headers requiring special processing.
.lp
Adding mailers or changing parsing or routing information
does not require recompilation.
.pp
If the mail is being sent by a local user,
and the file
.q .mailcf
exists in the sender's home directory,
that file is read as a configuration file
after the system configuration file.
The primary use of this is to add header lines.
.sh 2 "Configuration File Description"
.pp
The configuration file is formatted
as a series of text lines,
each beginning with a character describing its semantics.
Blank lines and lines beginning with a sharp sign
(#)
are ignored.
.pp
See figure 2 for an example configuration file.
.(z
##########################################
# sendmail configuration file
#	@(#)sendmail.cf	3.6	8/17/81
##########################################

### local hosts on various nets
DABerkeley
DBIngVAX
DUucbvax

### special macros
# my name
D\&n\&MAILER-DAEMON
# UNIX header format
D\&l\&From $g  $d
# delimiter (operator) characters
D\&o\&.:%@!^

### format of headers:
H\&Date: $a
H\&From: $g$?x ($x)$.
H\&Full-Name: $x
H\&Message-Id: <$t.$p.$B@$A>
H\&Posted-Date: $a

### name classifications
# arpanet hostnames
C\&A\&ucb berkeley
# list of local host names
C\&B\&j IngVax
# berknet hosts on the arpanet
C\&C\&i ingres ing70
# uucp hostnames
C\&U\&ucbvax ernie

###  mailers
# local mail -- must be zero
M\&local	/bin/mail		rlsAmn	$f	...local\&mail -d $u
# program mail -- must be one
M\&prog	/bin/csh		lA	$f	...prog\&mail -fc $u
# berkeley net mail
M\&berk	/usr/net/bin/sendberkmail fxs	$B:$f	...berk\&mail -m $h -h $c -t $u
# arpanet mail
M\&arpa	/usr/lib/mailers/arpa	sAu	$f@$A	...arpa\&mail $f $h $u
# uucp mail
M\&uucp	/usr/bin/uux		rsDxm	$U!$f	...uucp\&mail - $h!rmail ($u)

### rewriting rules
R\&$-h.$+u	$+h:$+u			change "." to ":"
R\&$=C:$+u@$-h	$+u@$+h			delete ing70: on arpanet addresses
R\&$+u@$=A	ing70:$+u		delete local arpa hosts
R\&$+u@$-h	$#berk$@ing70$:$+u@$+h	send arpa mail to ing70
R\&$+h^$+u	$+h!$+u			change "^" to "!"
R\&$-x!$=U!$+y	csvax:$+y		delete uucp loops through csvax
R\&$-h!$+u	csvax:$+h!$+u		send uucp mail to csvax
R\&$-x:$-h:$+u	$+h:$+u			delete multiple berk hosts
R\&$=B:$+u	$+u			delete local berk hosts
R\&$-h:$+u	$#berk$@$+h$:$+u	resolve berk mail
R\&$+u		$#local$:$+u		resolve local mail

### rewriting rules for from host
S\&1
R\&ing70:$+u@$-h	$+u@$+h			arpanet mail is automatic
R\&CSVAX:$-h!$+u	$+h!$+u			uucp mail is automatic


.ce
Figure 2.  Sample configuration file.
.)z
.sh 3 "D \*- define macro"
.pp
This line defines a macro.
Macros have single character names.
They can be interpolated using the escape
.b $\c
.i x ,
where
.i x
is the macro name.
By convention,
all upper-case letters are unused by
.i sendmail
and may be used freely by the user;
all other names are reserved for use by sendmail.
Certain macros
.i must
be defined,
and are used internally.
These are:
.(b
$l	UNIX-style \*(lqFrom\*(rq line.
$n	My address in error messages.
$o	\*(lqOperators\*(rq in addresses.
.)b
The
.b $l
macro is expanded when
.i sendmail
wants to insert a UNIX-style
.q From
line on messages.
This typically expands to something like:
.(b
From joe  Wed Aug 12 09:15:13 1981
.)b
The
.b $n
macro is used as the name of this process
when error messages are being mailed back.
Typically,
it is wise to include an alias
so that mail to this address will be sent to root.
The
.b $o
macro defines the characters
that will separate words when addresses are being broken up.
Each of these becomes a word by itself when scanned.
Blanks and tabs are built-in separators
but are ignored,
i.e., are not turned into words.
For example, the input:
.(b
Ing70:ZRM @ MIT-MC SRI-KL
.)b
Is broken up into the six words:
.(b
Ing70, :, ZRM, @, MIT-MC, SRI-KL
.)b
assuming that colon and at-sign are operators
(but hyphen is not).
.pp
A number of macros are defined by
.i sendmail
for use as primitives.
These are:
.(l
$a	The date in ARPANET format.
$c	The hop count.
$d	The date in UNIX (ctime) format.
$f	The sender's (from) address.
$g	The sender's address translated by the mailer.
$h	The host of the recipient.
$p	The process id of sendmail in decimal.
$t	The time in seconds in decimal.
$u	The user part of the recipient.
$v	The version number of sendmail.
$x	The full name of the sender.
$y	The id of the sender's terminal.
$z	The home directory of the recipient.
.)l
.pp
The
.b $p
and
.b $t
macros are used to create unique strings.
The
.b $f
macro is the id of the sender
as originally determined;
when mailing to a specific person,
the
.b $g
macro is the address of the sender
with respect to the receiver.
For example,
if I send to
.q csvax:samwise
the
.b $f
and
.b $g
macros are:
.(b
$f	eric
$g	IngVAX:eric
.)b
This only applies to the first step in the link.
For example,
sending to Ing70:drb@bbn-unix,
we have
.b $f
and
.b $g
as above for the transfer to Ing70, but:
.(b
$f	IngVAX:eric
$g	IngVAX:eric@Berkeley
.)b
For transfer to the ARPANET.
When sending, the
.b $u ,
.b $h ,
and
.b $z
macros get set to the user, host, and home directory
(respectively)
of the receiver.
The host is only set if the user is not local,
and the home directory is only set if the user is local.
.pp
A primitive conditional is available during macro expansion.
The construct:
.(b
$?x text1 $: text2 $.
.)b
tests if macro
.b $\c
.i x
is defined.
If it is,
text1 is interpolated;
otherwise,
text2 is interpolated.
.sh 3 "H \*- define header"
.pp
The remainder of the
.b H
line looks like a regular header line,
except that the field value is macro expanded
before use.
All headers mentioned in this way
are automatically inserted
into every message
except for headers mentioned in the compile-time
configuration file
.i conf.c .
These headers are
Date,
From,
Full-Name,
Message-Id,
and
Received-Date.
To get these fields the appropriate flag
must be specified
for the receiving mailer.
.pp
Since the file
.q ".mailcf"
in the sender's home directory is read and processed,
it is possible to add customized header lines.
For example,
the .mailcf consisting of:
.(b
H\&Phone: (415) 642-7520
.)b
will add that line to every outgoing message.
.sh 3 "M \*- define mailer"
.pp
This line is structured into fields
separated by white space (spaces or tabs).
The fields are:
.np
The internal name of the mailer,
referred to in the rewriting rules.
.np
The pathname of the program to execute for this mailer.
.np
The flags for this mailer,
described below.
.np
The macro string to become the
.b $g
macro (translated sender)
for this mailer.
.np
The argument vector passed to the mailer
(macro expanded).
.pp
The flags are a series of characters:
.ls 1
.ip f
The mailer wants a
.b \-f
.i from
flag,
but only if this is a network forward operation
(i.e.,
the mailer will give an error
if the executing user does not have special permissions).
.ip r
Same as
.b f ,
but sends a
.b \-r
flag.
.ip q
Don't print errors \*- the mailer will do it for us.
.ip S
Don't reset your userid before calling the mailer.
This would be used in a secure environment where
.i sendmail
ran as a special user.
This could be used to prevent
(or at least complicate)
forged addresses.
.ip n
This mailer does not want a UNIX-style
.q From
line on the message.
.ip l
This mailer is local,
so no host will be specified.
.ip s
Strip quote characters off of addresses
before calling the mailer.
.ip m
This mailer can send to multiple users
(on the same host)
in one call.
.ip F
This mailer wants a
.q From:
header line.
.ip D
This mailer wants a
.q Date:
header line.
.ip M
This mailer wants a
.q Message-Id:
header line.
.ip x
This mailer wants a
.q Full-Name:
header line.
.ip u
Upper case should be preserved in user names.
.ip h
Upper case should be preserved in host names.
.ip A
This mailer wants an ARPANET standard header
(equivalent to the
.b F
and
.b D
flags).
.ls
.sh 3 "S \*- use rewriting set"
.pp
There are two sets of rewriting rules.
Set zero is used to rewrite recipient addresses.
Set one is used to rewrite sender addresses.
Set one can be used to eliminate implicit links.
For example,
if there exists a site on on the BerkNet called
.q Ing70
which is an ARPANET gateway,
and we are on a site called
.q IngVAX ,
ARPANET mail coming into
.q Ing70
for someone on
.q IngVAX
will read:
.(b
From: Ing70:auser@ahost
.)b
Rewriting set one can rewrite this as:
.(b
From: auser@ahost
.)b
since
.q Ing70
will be implied.
.pp
When you change to a new set,
the previous content of that set is cleared.
.sh 3 "R \*- rewriting rule"
.pp
The heart of parsing is the rewriting rules.
The process is essentially textual.
First,
the address to be rewritten is broken up into words.
Words are defined as strings of non-special characters
separated by white space or single special characters
as defined by the
.b $o
macro.
Then,
the words are rewritten using simple pattern matching.
Words in the pattern match themselves
unless they begin with dollar sign.
The dollar escapes have the following meanings\**:
.(f
\**These dollar escapes have nothing to do with macro expansion.
.)f
.(b
$-x	Match a single word (and call it x).
$+x	Match one or more words (and call them x).
$=c	Match any word in class c (see below).
.)b
The case of letters is ignored in pattern matching
(including class comparisons).
.pp
When a pattern (also called a left hand side or LHS)
matches,
the input is rewritten as defined by the right hand side (RHS).
Acceptable escapes in the RHS are:
.(b
$+x	Replace from corresponding match in LHS.
$#word	Canonical mailer name.
$@word	Canonical host name.
$:word	Canonical user name.
.)b
Patterns are reexecuted until it either resolves to
a canonical name
(i.e.,
.q "$#mailer$@host$:user" )
or fails.
As soon as the input resolves to a canonical name,
matching ends;
otherwise,
the next pattern is tried.
The
.q "$@host"
part is not needed
if the mailer does not require a host.
The special mailer
.q error
causes the user part to be printed as an error.
.sh 3 "C \*- define word class"
.pp
There are twenty six word classes,
represented as
.q A
through
.q Z .
For example:
.(b
CVcsvax ingvax esvax
.)b
defines the words
.q csvax ,
.q ingvax ,
and
.q esvax
to all be in class
.q V ,
so that
.q $=V
on the LHS of a rewriting rule
will match any of these words.
.sh 2 "A Detailed Example"
.pp
We will now follow the configuration file
in figure 2
through in detail.
.sh 3 "Macro definitions"
.(b
DABerkeley
DBIngVAX
DUucbvax
DnMAILER-DAEMON
DlFrom $g  $d
Do.:%@!^
.)b
The first three macros are for convenience only,
and are used to define the local host names
on the ARPANET, BerkNet, and the UUCP net
respectively.
.pp
Macro
.b n
defines the name of this entity
when error messages are sent.
Macro
.b l
defines what the first line
of a message in UNIX format looks like,
in this case the version 7 standard of:
.(b
From sender-name  time-of-submission
.)b
The
.b o
macro
tells what characters will be distinct from names
when scanning addresses.
In this case,
dot and colon will be used
to distinguish BerkNet addresses,
at sign for ARPANET addresses,
and exclamation point and caret for UUCP addresses.
.sh 3 "Header definitions"
.(b
H\&Date: $a
H\&From: $g$?x ($x)$.
H\&Full-Name: $x
H\&Message-Id: <$t.$p.$B@$A>
H\&Posted-Date: $a
.)b
These define the headers
that may be added to a message.
The
.q Date:
is just the ARPANET idea of the date.
The
.q From:
line is the translated version of the sender,
followed by the sender's full name if known.
The
.q Message-Id:
field has the time and process id's concatenated
with the BerkNet and ARPANET addresses
to make a unique string.
Finally, the
.q Posted-Date:
is the date in ARPANET format;
it differs from
.q Date:
in that it is always output as soon as the message is submitted,
and hence indicates the time that the message first enters
the mail delivery system [4].
.(d
[4] NBS standard
.)d
.sh 3 "Name classifications"
.(b
C\&A\&ucb berkeley
C\&B\&j IngVax
C\&C\&i ingres ing70
C\&U\&ucbvax ernie
.)b
These commands put the words
.q ucb
and
.q berkeley
into class
.q A ,
the valid names of this site on the ARPANET.
Words
.q j
and
.q ingvax
are in class
.q B ,
the local names on BerkNet.
Class
.q C ,
the names of the site which has the ARPANET link,
has the words
.q i ,
.q ingres ,
and
.q ing70 .
Finally,
.q ucbvax
and
.q ernie
are the UUCP names of our UUCP gateway,
and are in class
.q U .
.pp
The classes will be used in the patterns of the rewriting rules
as described below.
.sh 3 "Mailer definitions"
.(b
M\&local	/bin/mail		rlsAmn	$f	...localmail -d $u
M\&prog	/bin/csh		lA	$f	...progmail -fc $u
M\&berk	/usr/net/bin/sendberkmail fxs	$B:$f	...berkmail -m $h -h $c -t $u
M\&arpa	/usr/lib/mailers/arpa	sAu	$f@$A	...arpamail $f $h $u
M\&uucp	/usr/bin/uux		rsDxm	$U!$f	...uucpmail - $h!rmail ($u)
.)b
Five mailers are known in the configuration file.
The first two
.i must
be declared as
.b local
and
.b prog
and must come as the first and second mailers respectively.
.pp
Local mail is sent using
/bin/mail.
It takes a
.b \-r
flag,
is local,
quote characters are stripped before sending,
takes ARPANET standard headers,
can deliver to multiple recipients at once,
and does not want a UNIX-style
.q From
line since it will add one itself.
The translated from address is the same as the raw from address,
since no network hops are made.
The argument vector has a program name,
a
.b \-d
flag (\c
.q "really deliver" ,
which must be added to /bin/mail),
and the list of recipients \*- one recipient per argument.
.pp
Mail piped through programs
is interpreted by /bin/csh.
It does not take a
.b \-r
flag,
quotes should be left,
it can only deal with one user,
and it does want a UNIX-style
.q From
line,
but is still local and still wants an ARPANET style header.
.pp
BerkNet mail is processed by
/usr/net/bin/sendberkmail.
It takes a
.b \-f
flag,
wants a
.q Full-Name:
header line,
and wants quotes stripped.
The
.q Full-Name:
is used here because if it were given as a comment
in a
.q From:
line it might be discarded by later instantiations of
.i sendmail .
The from address as seen by the receiver is
.q IngVAX:sender ,
and it takes a flag-oriented
rather than a positional
command list.
.pp
The ARPANET wants quotes stripped,
ARPANET standard headers,
and wants the user name left with case intact.
It takes a positional command list.
.pp
UUCP mail calls
.i uux
with a
.b \-r
flag,
quotes stripped,
a
.q Date:
line,
a
.q Full-Name:
line,
and with multiple users listed.
.sh 3 "Rewriting rules for recipient addresses"
.(b
R\&$-h.$+u	$+h:$+u			change "." to ":"
R\&$=C:$+u@$-h	$+u@$+h			delete ing70: on arpanet addresses
R\&$+u@$=A	ing70:$+u		delete local arpa hosts
R\&$+u@$-h	$#berk$@ing70$:$+u@$+h	send arpa mail to ing70
R\&$+h^$+u	$+h!$+u			change "^" to "!"
R\&$-x!$=U!$+y	csvax:$+y		delete uucp loops through csvax
R\&$-h!$+u	csvax:$+h!$+u		send uucp mail to csvax
R\&$-x:$-h:$+u	$+h:$+u			delete multiple berk hosts
R\&$=B:$+u	$+u			delete local berk hosts
R\&$-h:$+u	$#berk$@$+h$:$+u	resolve berk mail
R\&$+u		$#local$:$+u		resolve local mail
.)b
Dots in addresses are translated to colons
in the first rule.
Redundant explicit routing to the ARPANET is deleted
in the second rule.
Hops out over the ARPANET
back to us are deleted in the third rule \*-
note that the host that we would have come in on
is inserted.
Real ARPANET mail is resolved immediately with no further ado \*-
it is sent out over the BerkNet to the ing70,
and further rewriting stops immediately.
.pp
Carets are changed to exclamation points
for UUCP addresses in the fifth rule.
The sixth rule deletes loops out into UUCP land
and back to us \*- noting that we will be left on CSVAX.
Multiple BerkNet hosts are deleted in rule seven \*-
this can occur internally quite easily
as a side effect of a rewriting rule.
Rule eight deletes local BerkNet hosts.
The last two rules resolve BerkNet and local mail.
.pp
Consider the following examples:
.(b
	esvax.asa
[1]	esvax:asa
[10]	$#berk$@esvax$:asa
.)b
.(b
	research^vax135^dmr
[5]	research!vax135^dmr
[5]	research!vax135!dmr
[7]	$#berk$@csvax$:research!vax135!dmr
.)b
.(b
	research!ucbvax!j:eric
[6]	csvax:j:eric
[8]	j:eric
[9]	eric
[11]	$#local$:eric
.)b
.(b
	ing70:wnj@Berkeley
[2]	wnj@Berkeley
[3]	ing70:wnj
[10]	$#berk$@ing70$:wnj
.)b
.sh 3 "Rewriting rules for sender addresses"
.(b
S\&1
R\&ing70:$+u@$-h	$+u@$+h			arpanet mail is automatic
R\&CSVAX:$-h!$+u	$+h!$+u			uucp mail is automatic
.)b
The
.b S
line starts putting the rules into set one.
These rules strip off the
.q ing70:
from incoming ARPANET mail
and the
.q CSVAX:
off of incoming UUCP mail.
.sh 1 "COMPARISON WITH OTHER MAILERS"
.sh 2 "Delivermail"
.pp
.i Sendmail
is an outgrowth of
.i delivermail .
The primary differences are:
.np
Configuration information is not compiled in.
This simplifies many of the problems
of moving to other machines.
It also allows easy debugging of new mailers.
.np
Address parsing is more flexible.
For example,
.i delivermail
only supported one gateway to any network,
whereas
.i sendmail
can be sensitive to host names
and reroute to different gateways.
.np
Forwarding and
:include:
support eliminate the requirement that the system alias file
be writable by any user
(or that an update program be written,
or that the system administration make all changes).
.np
.i Sendmail
supports message batching across networks
when a message is being sent to multiple recipients.
.sh 2 "MMDF"
.pp
MMDF [ref]
spans a much wider problem set than
.i sendmail .
For example,
MMDF includes a
.q "phone network"
mailer, whereas
.i sendmail
calls on preexisting mailers in most cases.
.i Sendmail
is approximately equivalent to the
SUBMIT and DELIVER phases of MMDF.
Because of this difference in design goals,
some of the important features of MMDF
(queueing, retransmission, and two-phase timeout)
are unimplemented by
.i sendmail .
.pp
MMDF and
.i sendmail
both support aliasing,
customized mailers,
message batching,
and automatic forwarding to gateways.
.sh 2 "Message Processing Module"
.pp
The Message Processing Module (MPM)
discussed by Postel [ref]
matches
.i sendmail
closely in terms of its basic architecture.
However,
like MMDF,
the MPM includes the network interface software
as part of its domain.
.pp
MPM also postulates a duplex channel to the receiver,
as does MMDF.
This allows simpler handling of errors
by the mailer
than possible in
.i sendmail ;
when a message queued by
.i sendmail
is sent,
any errors must be returned to the sender
by the mailer itself.
Both MPM and MMDF mailers
can return an immediate error response,
and a single error processor can create an appropriate response.
.pp
MPM prefers passing the message as a structured message,
with type-length-value tuples.
This implies a much higher degree of cooperation
between mailers than required by
.i sendmail .
MPM also assumes a universally agreed upon internet name space
(with each address a net-host-user tuple),
which
.i sendmail
does not.
.sh 1 "EVALUATIONS AND FUTURE PLANS"
.pp
.i Sendmail
is designed to work in a nonhomogeneous environment.
Every attempt is made to avoid imposing any constraints
on the underlying mailers.
This goal has driven much of the design.
One of the major problems
has been the lack of a uniform address space,
as postulated in [IP]
.(d
[IP] -- internet protocol
.)d
and [PostelIMS].
.(d
[PostelIMS] -- Internet Message Structure
.)d
.pp
A nonuniform address space implies that path will be specified
in all addresses,
either explicitly (as part of the address)
or implicitly
(as with implied forwarding to gateways).
This has the unpleasant effect of making replying to messages
exceedingly difficult,
since there is no one
.q address
for any person,
but only a way to get there from wherever you are.
.pp
Interfacing to mail programs
that were not initially intended to be applied
in an internet environment
has been amazingly successful,
and has reduced the job to a manageable task.
.pp
However,
many of these mailers implement their own queueing and retransmission.
In networks that support store-and-forward file transfer,
such as BerkNet and UUCP,
this feature must be supplied already.
However,
networks that transfer in real time,
such as the ARPANET or an Ether-based network
[ref],
generally do not provide these features.
Also,
networks which provide these generally do not understand
timeouts or returning the text of the message on error,
both highly desirable features\**.
.(f
\**We have implemented an ARPANET mailer which
returns the message on error and does one-stage timeout
(returning the message after three days).
.)f
Such queueing, retransmission, and two-phase timeout
may be integrated into
.i sendmail
if it seems desirable.
.pp
.i Sendmail
has knowledge of a few difficult environments
built in.
It generates ARPANET FTP compatible error messages
(prepended with three-digit numbers
[FTP1, FTP2])
.(d
[FTP1] -- FTP description
.)d
.(d
[FTP2] -- revised FTP codes
.)d
as necessary,
optionally generates UNIX-style
.q From
lines on the front of messages for some mailers,
and knows how to parse the same lines on input.
Although it still adds and understands ARPANET-style
.q From:
lines,
this can be inconvenient to sites which have abandoned UNIX mail.
Also,
error handling has an option customized for BerkNet.
.pp
One surprisingly major annoyance in most internet mailers
(such as MMDF)
is that the location and format of local mail is built in\**.
.(f
\**For example,
MMDF puts local mail in the file
.q .mail
\*- useful if you are running version 6.
.)f
.i Sendmail
eliminates all knowledge of location
and can function successfully with different formats.
.pp
The ability to automatically generate a response to incoming mail
(by forwarding mail to a program)
seems useful
(\c
.q "I am on vacation until late August...." )
but can create problems
such as forwarding loops
(two people on vacation whose programs send notes back and forth,
for instance)
if these programs are not well written.
It might be desirable to implement some form of load limiting.
I am unaware of any mail system that addresses this problem.
.pp
.i Sendmail
should be modified to run as a daemon,
reading an MPX file
(or other IPC scheme)
to receive mail and process it.
This would reduce the cost of sending mail to writing the message
into a known file.
.i Sendmail
would be modified to have a very different argument structure.
It already has an option to read the recipients
from the message header.
A more palatable technique for giving error messages
would also have to be devised.
.pp
The configuration file is currently practically inscrutable;
considerable convenience could be realized
with a higher-level format.
For example, a description might read:
.(b
(MACRO name value)
(HEADER name value
	(OPTION option) ...
	(NEEDS option) ... )
(MAILER name path xlatstring
	(OPTION option) ...
	(ARGV arg ... ))
(CLASS name word ...)
(REWRITE setname
	(RULE lhs rhs) ... )
.)b
.pp
Many other nice features could be implemented.
For example,
if we were sure that the alias file were writable by the effective user
(i.e., if
.i sendmail
were to run setuid)
then the inverted form could be rebuilt automatically when the
text copy was changed.
However, this appears to be little more than frosting.
.sp 2i
.pd