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Sendmail Tutorial

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From: Eliot Lear <[email protected]>

The following was written by Dr. Charles Hedrick of Rutgers University
sometime in 1985.  Please read it with the understanding that rule 
numbers are nothing more than function names.  For further reference,
I suggest the Sun Tutorial on Sendmail in their manuals.

Command: followup
Newsgroups: net.unix-wizards,net.mail
To: [email protected]
Subject: a brief tutorial on sendmail rules
References: <[email protected]> <[email protected]>

A previous message suggested using "sendmail -bt" to see how sendmail
is going to process an address.  This is indeed a handy command for
testing how an address will be processed.  However the instructions
given were not quite right.  To see how sendmail is going to deliver
mail to a given address, a reasonable thing to type is
	sendmail -bt
	0,4 address
Even this isn't quite right, but with "normal" rule sets it should work.

Because there is so much confusion about sendmail rules, the rest of
this message contains a brief tutorial.  My own opinion of sendmail is
that it is quite a good piece of work.  Many people have complained
about the difficulty of understanding sendmail rule sets.  However I
have also worked with mailers that code address processing directly
into the program.  I much prefer sendmail.  The real problem is not
with sendmail, but with the rules.  The rules normally shipped from
Berkeley have lots of code that does strange Berkeley-specific things,
and they are not commented.  Also, typical complex rule sets are
trying to handle lots of things, forwarding mail among several
different mail systems with incompatible addressing conventions.  A
rule set to handle just old-style (non-domain) UUCP mail would be very
simple and easy to understand.  But real rule sets are not doing
simple things, so they are not simple.

For those not familiar with sendmail, -bt invokes the rule tester.  It
lets you type a set of rule numbers and an address, and then shows you
what the rules will do to that address.  In addition, rule test mode
automatically applies rule 3 before whatever rule you ask it to apply.
As we will see shortly, this is a reasonable thing to do.

Before describing the rule sets, let me define two terms: "header" and
"envelope".  Header refers to the lines at the beginning of the
message, starting with "from:", "to:", "subject:", etc.  Sendmail does
process these lines.  E.g. with uucp mail it will add its own host
name at the beginning of the from line, so that the final recipient
stands some change of replying to the message.  However sendmail
normally does not depend upon the from and to lines to perform its
actual delivery.  It has more direct knowledge, passed on to it from
the program that generated the mail, or if it came from another site,
the mailer at that site.  This information is referred to as the
"envelope", since it is like the addresses on the outside of an
envelope.  For Arpanet mail, the envelope is passed to the next site
by the MAIL FROM: and RCPT TO: commands.  For UUCP mail, it is passed
on as arguments to the remote rmail command.  To see why there have to
be separate addresses "on the envelope", consider what happens when
you send mail to "[email protected], [email protected]".  Two copies of the message will
be dispatched, one to vax and the other to sun.  The "to: " line in
the headers will show both addresses.  However the envelope will show
only the right address that we want this copy to go to.  The copy sent
to vax will show "[email protected]" and the copy sent to sun will show
"[email protected]".  If sendmail had to look at the "to: " line, it would
never know which of the addresses shown there it was responsible for

Anyway, here is what the rules do:

3: always done first.  This turns addresses from their normal textual
form into a form that the rest of the rules understand.  In most
cases, all it does it put < > around the name of the host that is next
in line.  Thus [email protected] turns into foo<@bar>.  However it also does a
few transformations.  E.g. it turns foo!bar!user into
bar!user<@foo.UUCP>.  Since sendmail accepts either ! syntax or
@....UUCP syntax, rule 3 standardizes on @ syntax.  It also does a few
other minor things.  But you won't be far off if you just think of it
as adding < > around the host name.

4: always done last.  This turns addresses from internal form  back
into external form.  It removes the < > around the host name, and
turns [email protected] back into bar!foo.  Again, there are one or two
other minor things, but you won't be too far off if you think of 4 as
just removing the < > around the host name.

0: This is the rule that handles the destination address on the
envelope.  It is in some sense the primary rule.  It returns a triple:
protocol, host, user.  The protocol is usually one of local, TCP, or
UUCP.  At the moment, it figures this out syntactically.  In our rule
set, hosts ending in .UUCP are handled by UUCP, the current host is
local, and everything else is TCP.  As domains are integrated into
UUCP, obviously this rule is going to change.  This rule does very
little other than simply look at the format of the host name, though
as usual a few other details are involved (e.g. it removes the local
host.  So myhost!foo!bar will be sent directly to foo).

1 and 2 are protocol-independent transformations used for sender and
recipient lines in the header (i.e. from: and to: lines).  In our
rule sets, they don't do anything.

Each protocol has its own rules to use for sender and recipient lines
in the header.  E.g. UUCP rules might add the local host name to the
beginning of the from line and remove it from the to line.  In our
rule set, the complexities in these rules are primarily caused by
forwarding between UUCP and TCP.  The line that defines the mailer for
a protocol lists the rule to use for source and recipient, in the S=
and R=.

Finally, here is the exact sequence in which these rules are used.
For example, the first line means that the destination specified in
the envelope is processed first by rule 3, then rule 0, then rule 4.

envelope recipient:    3,0,4  [actually rule 4 is applied only to the
				user name portion of what rule 0 returns]
envelope sender:       3,1,4
header recipient:      3,2,xx,4  [xx is the rule number specified in R=]
header sender:         3,1,xx,4  [xx is the rule number specified in S=]

I have the impression that the sender from the envelope (the
return-path) may actually get processed twice, once by 3,1,4 and the
second time by 3,1,xx,4.  However I'm not sure about that.

Now for the format of the rules themselves.  I'm just going to show
some examples, since sendmail comes with a reference manual, which you
can refer to.  However these examples are probably enough to let you
understand any set of rules that makes sense in the first place (which
the normal rules do not).  This example is from our UUCP definition.
It a simplified version of the set of rules used to process the sender
specification.  As such, the major thing it has to do is to add our
host name to the beginning, so that the guy at the end will know that
the mail went through us.

R$+<@$-.UUCP>		$2!$1				[email protected] => host!u
R$=U!$+			$2				strip local name
R$+			$:$U!$1				stick on our host name

Briefly, the first rule turns the address from the form foo<@bar.UUCP>
back into bar!foo.  The second rule removes our local host name, if
it happens to be there already, so we don't get it twice.  The third
rule adds our host name to the beginning.

S13 says that this is the beginning of a new rule set, number 13.

R$+<@$-.UUCP>		$2!$1				[email protected] => host!u

R says that this is a rule.  The thing immediately after it,
$+<@$-.UUCP> is a pattern.  If this pattern matches the address, then
the rule "triggers".  If the rule triggers, the address is replaced
with the "right hand side", i.e. what is after the tab(s).  In this
rule, the right hand sie is $2!$1.  The thing after the next tab(s) is
a comment.  This rule is used in processing UUCP addresses.  As noted
above, by the time we get to it, rule 3 has already been applied.  So
if we had a UUCP address of the form host1!host2!user, it would now be
in the form host2!user<@host1.UUCP>.  This does match the pattern:

	$+	  <@$-   .UUCP>   

$+ and $- are "wildcards" that match anything.  $- will match exactly
one word, while $+ will match any number.  (By the way, with the
increasing use of domains, this production should probably use
$+.UUCP, not $-.UUCP.)  Since the pattern matches, we replace this
with the "right hand side" of the rule, $2!$1.  $ followed by a digit
means the Nth thing matched by a wildcard.  In this case there were
two wildcards, so
   $1 = host2!user
   $2 = host1
The final result is 
As you can see, we have simply turned UUCP addresses from the format
produced by rule 3 back into normal ! format.

The second rule is

R$=U!$+			$2				strip local name

This is needed because there are situations in which our host name
ends up on the beginning of the recipient address.  Since we are
about to add our host name, we don't want it to be there twice.
So if it was there before, we remove it.  $= is used to see if
something is a member of a specified "class".  U happens to be a list
of our UUCP host name and any nicknames.  So $=U!$+ matches
any address that begins with our host name or nickname, then !, then
anything else.  Suppose we had  topaz!host1!host2!user.  The
match would be

     $=U  !$+

The result of the match is that

     $1 = topaz
     $2 = host1!host2!user

Since the right hand side of this rule is simply "$2", the result is


I.e. we have removed the topaz from the beginning.  By the way, the
class U used by the rule would have been defined earlier in the file
by the statement

CUtopaz ru-topaz

C defines a class.  U is the name of the class.  The rest of the
line is the list of things that will be in the class.

Finally we have the rule

R$+			$:$U!$1				stick on our host name

The $+ matches anything.  In this case the name is host1!host2!user, so the
result of the match is

	$1 = host1!host2!user

The result looks slightly obscure.  $: is a tag that says to do this
only once.  The problem is that this rule always applies, since the
pattern matches anything.  Normally, rules are applied over and
over, as long as they apply.  In this case, the result would be
an infinite loop.  Putting $: at the beginning says to do it only
once.  $U says to use the value of the macro U.  Earlier in the
file we defined U as our UUCP host name, with a definition


Note that there can be a class and a macro with the same name.
$=U tests whether something is in the class U.  $U is replaced
by the value of the macro U.

So the final value of this rule, $:$U!$1, is


So this rule has managed to add our host name to the beginning, as it
was supposed to.  Since there are no further rules in the set (the
next line is the end of file or the beginning of a new rule set),
this value is returned.

There are several more magic things that can appear in a pattern.
The most important are:

$* - this is another wild card.  It is similar to $+, but $+ matches
anything, whereas $* matches both anything and nothing.  I.e. $+
matches 1 or more tokens and $* matches 0 or more tokens.  So here
is a list of the wildcards I have mentioned:

   $*   0 or more
   $+   1 or more
   $-   exactly 1
   $=x  any member of class x

A typical example of $* is a production where we aren't sure whether
the user name is before or after the host name:

R$*<@$+.UUCP>$*		[email protected]$1<@$2.UUCP>$3

This production would test for the host name ending in .UUCP, and
return immediately.  [email protected] is a flag you haven't seen yet.  It is simply
a return statement.  It causes the right hand side of this rule to be
returned as the final value of this rule set.

The other magic thing I will mention is $>.  This is a subroutine
call.  Here is an example taken from rule set 24, which is used to
process recipients in TCP mail.  Its purpose is to handle the
situation where we might have an address like [email protected]  (Our
host name is topaz.  Red is a local host that we talk to via TCP.)
I.e. someone is asking us to relay mail to red.  Rule 3 will have
turned this into [email protected]<@topaz.UUCP>.  What we want to do is
get rid of the topaz.UUCP and treat red as the host.  (Rule set 0
would do this for the recipient on the envelope.  This rule is
used for the to: field in the header.)  Here is the rule.

R$+<@$=U.UUCP>		[email protected]$>9$1				in case [email protected]

The pattern matches our example, as follows:

   $+      <@$=U  .UUCP>
   [email protected]<@topaz.UUCP>

Recall that $+ matches anything and $=U tests whether something is our
UUCP host name or one of our nicknames.  The result of the match is

   $1 = [email protected]
   $2 = topaz

The right hand side is [email protected]$>9$1.  The [email protected] is the tag saying to stop the
rule set here and return this value.  $>9 is a subroutine call.  It
says to take the right hand side, pass it to rule set 9, and then
use the value of rule set 9.  The actual right hand side is simply
$1, which in this case is [email protected]  Here is rule set 9:

R$*<$*>$*		$1$2$3				defocus
R$+			$:$>3$1				make canonical
R$+			[email protected]$>24$1			and do 24 again

The first rule simply removes < >.  It is sort of a quick and dirty
version of rule 4.  In fact we have no < > left, since we have removed
the <@topaz.UUCP>.  So this rule does not trigger.  (Now that I think
about it, I suspect it is probably never going to trigger, and so is
not needed.)  

The next rule is a simple subroutine call.  It matches anything ($+
matches any 1 or more token).  The right hand side is $:$>3$1 The $:
says to do it only once.  Since the rule matches anything, you need
this, or you will have an infinite loop.  The $>3 says to call rule 3
as a subroutine.  The $1 is the actual right hand side.  Since the
left hand side matched the whole address, what this rule does is
simply call rule set 3 on the whole address.  Recall that rule set 3
basically locates the host name and puts < > around it.  So in this
case the result is user<@red>.  As you can see, it was not enough to
remove <@topaz.UUCP>.  That leaves us with no  host name.  We have to
call rule 3 to find the current host name and put < > around it.

The last rule is really just a goto statement.  The pattern is $+,
which matches anything, so it always triggers.  The right hand side is
[email protected]$>24$1.  The [email protected] is the return tag.  It says to stop this rule set
and return that value.  $>24 says to call rule set 24.  The actual
right hand side is $1, so we call rule set 24 with the whole address.
If you recall, this ruleset (9) was called from the  middle of 24 when
we found [email protected]<@topaz.UUCP>.  So what we have done is to change
this into user<@red> and say to start rule set 24 over again.

I hope you have found this exposition useful.  As a final convenience,
here is a "reference card" for reading rule sets.  Note that this
contains only operators used by the rules.  There are plenty of
other facilities used in the configuration section which I am
not documenting here.  (I'd love to see someone produce a complete
reference card.)

   $*   0 or more tokens
   $+   1 or more tokens
   $-   exactly one token
   $=x  member of class x (x must be a letter, lower/upper case distinct)
   $~x  not a member of class x

macro values (usable in pattern or on right hand side)
   $x	value of macro x (x must be a letter, lower/upper case distinct)
	At least on the Pyramid, $x is replaced by the macro's value
	when the file is being read in.

on the right hand side:
   $n	string matched by the Nth wildcard
   $>n	call rule set N as a subroutine
   [email protected]   return
   $:   only do this rule once

in rule 0, defining the return value
   $#   protocol
   [email protected]   host
   $:   user

Rutgers extensions, usable only on right hand side
   $%n  take the string matched by the Nth wildcard, look it up in
	/etc/hosts, and if found use the primary host name
   $&x	use the current value of macro x.  x must be a letter.
	upper and lower case are treated as distinct.