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早期freebsd实现

construct has been adopted from IDA to defer macro evaluation.
Normally, macros in rulesets are bound when the rule is first parsed
during startup.
Some macros change during processing and are uninteresting during startup.
However, that macro can be referenced using
.q $&x
to defer the evaulation of
$x
until the rule is processed.
.pp
The tokens
.b $(
and
.b $)
have been added to allow specification of map rewriting.
.pp
Version 8 allows
.b $@
on the Left Hand Side of an `R' line to match
zero tokens.
This is intended to be used to match the null input.
.sh 2 "Bigger Defaults
.pp
Version 8 allows up to 100 rulesets instead of 30.  It is recommended
that rulesets 0\-9 be reserved for sendmail's dedicated use in future
releases.
.pp
The total number of MX records that can be used has been raised to
20.
.pp
The number of queued messages that can be handled at one time has
been raised from 600 to 1000.
.sh 2 "Different Default Tuning Parameters
.pp
Version 8 has changed the default parameters for tuning queue costs
to make the number of recipients more important than the size of
the message (for small messages).  This is reasonable if you are
connected with reasonably fast links.
.sh 2 "Auto-Quoting in Addresses
.pp
Previously, the ``Full Name <email address>'' syntax would generate
incorrect protocol output if ``Full Name'' had special characters
such as dot.  This version puts quotes around such names.
.sh 2 "Symbolic Names On Error Mailer
.pp
Several names have been built in to the $@ portion of the $#error
mailer.  For example:
.(b
$#error $@NOHOST $: Host unknown
.)b
Prints the indicated message
and sets the exit status of
.i sendmail
to
.sm EX_NOHOST .
.sh 2 "New Built-In Mailers"
.pp
Two new mailers, *file* and *include*, are included to define options
when mailing to a file or a :include: file respectively.  Previously
these were overloaded on the local mailer.
.sh 2 "SMTP VRFY Doesn't Expand
.pp
Previous versions of sendmail treated VRFY and EXPN the same.  In
this version, VRFY doesn't expand aliases or follow .forward files.
.pp
As an optimization, if you run with your default delivery mode
being queue-only, the RCPT command will also not chase aliases and
\&.forward files.
It will chase them when it processes the queue.
This speeds up RCPT processing.
.sh 2 "[IPC] Mailers Allow Multiple Hosts
.pp
When an address resolves to a mailer that has ``[IPC]'' as its
``Path'', the $@ part (host name) can be a colon-separated list of
hosts instead of a single hostname.  This asks sendmail to search
the list for the first entry that is available exactly as though
it were an MX record.  The intent is to route internal traffic
through internal networks without publishing an MX record to the
net.  MX expansion is still done on the individual items.
.sh 2 "Aliases Extended"
.pp
The implementation has been merged with maps.  Among other things,
this supports multiple alias files and NIS-based aliases.  For
example:
.(b
OA/etc/aliases,nis:mail.aliases
.)b
will search first the local database
.q /etc/aliases
followed by the NIS map

.sh 2 "Portability and Security Enhancements
.pp
A number of internal changes have been made to enhance portability.
.pp
Several fixes have been made to increase the paranoia factor.
.pp
In particular, the permissions required for .forward and :include:
files have been tightened up considerably.  V5 would pretty much
read any file it could get to as root, which exposed some security
holes.  V8 insists that all directories leading up to the .forward
or :include: file be searchable ("x" permission) by the controlling
user" (defined below), that the file itself be readable by the
controlling user, and that .forward files be owned by the user
who is being forwarded to or root.
.pp
The "controlling user" is the user on whose behalf the mail is
being delivered.  For example, if you mail to "user1" then the
controlling user for ~user1/.forward and any mailers invoked
by that .forward file, including :include: files.
.pp
Previously, anyone who had a home directory could create a .forward
could forward to a program.  Now, sendmail checks to make sure
that they have an "approved shell", that is, a shell listed in
the /etc/shells file.
.sh 2 "Miscellaneous Fixes and Enhancements"
.pp
A number of small bugs having to do with things like backslash-escaped
quotes inside of comments have been fixed.
.pp
The fixed size limit on header lines
(such as
.q To:
and
.q Cc: )
has been eliminated;
those buffers are dynamically allocated now.
.pp
Sendmail writes a /etc/sendmail.pid file with the current process id
and the current invocation flags.
.pp
Two people using the same program (e.g., submit) are considered
"different" so that duplicate elimination doesn't delete one of
them.  For example, two people forwarding their email to
|submit will be treated as two recipients.
.pp
The mailstats program prints mailer names and gets the location of
the sendmail.st file from /etc/sendmail.cf.
.pp
Many minor bugs have been fixed, such as handling of backslashes
inside of quotes.
.pp
A hook has been added to allow rewriting of local addresses after
aliasing.
.sh 1 "FUTURE WORK"
.pp
The previous section describes
.i sendmail
as of version 8.6.6.
There is still much to be done.
Some high points are described below.
This list is by no means exhaustive.
.sh 2 "Full MIME Support"
.pp
Currently
.i sendmail
only supports seven bit MIME messages.
Although it can pass eight bit MIME messages,
it cannot advertise that fact because the standards say
that the mail agent must be able to do 8- to 7-bit conversion
to have full 8-bit support.
This requires far more extensive modification of the message body
than is currently supported.
.pp
The best way to do this would be to support the general concept
of an external
``message filter''
that could do arbitrary modifications of the message.
This would allow MIME conversion as well as such things as
automatic encryption of messages sent over external links.
This is probably an extremely non-trivial change.
.sh 2 "Service Switch Abstraction"
.pp
Most modern systems include some concept of a
.q "service switch"
\*- for example, to look up host names you can try
DNS, NIS, NIS+, text tables, NetInfo,
or other services in some arbitrary order.
This is currently very clumsy in
.i sendmail ,
with only limited control of the services provided.
.sh 2 "More Control of Local Addresses"
.pp
Currently some addresses are declared as
.q local
and are handled specially \*-
for example, they may have .forward files,
may be translated into program calls or file deliveries,
and so forth.
These should be broken out into separate flags
to allow the local system administrator
to have more fine-grained control over operations.
.sh 2 "More Run-Time Configuration Options"
.pp
There are many options that are configured at compile time,
such as the method of file locking
and the use of the IDENT protocol
[RFC1413].
These should be transfered to run time
by adding new options.
.pp
Similarly, some options are currently overloaded,
that is, a single option controls more than one thing.
These should probably be broken out into separate options.
.pp
This implies that options will change from single characters
to words.
.sh 2 "More Configuration Control Over Errors"
.pp
Currently,
the configuration file can generate an error message during parsing.
However,
it cannot tweak other operations,
such as issuing a warning message to the system postmaster.
Similarly,
some errors should not be triggered if they are in aliases
during an alias file rebuild,
but should be triggered if that alias is actually used.
.sh 2 "Long Term Host State"
.pp
Currently,
.i sendmail
only remembers host status during a single queue run.
This should be converted to long term status
stored on disk
so it can be shared between instantiations of
.i sendmail .
Entries will have to be timestamped
so they can time out.
This will allow
.i sendmail
to implement exponential backoff on queue runs
on a per-host basis.
.sh 2 "Connection Control"
.pp
Modern networks have different types of connectivity
than the past.
In particular, the rising prominence of dialup IP
has created certain challenges for automated servers.
It is not uncommon to try to make a connection to a host
and have it fail, even though if you tried again it would succeed.
The connection management could be a bit cleverer
to try to adapt to such situations.
.sh 2 "Other Caching"
.pp
When you do an MX record lookup,
the name server automatically returns the IP addresses
of the associated MX servers.
This information is currently ignored,
and another query is done to get this information.
It should be cached to avoid excess name server traffic.
.sh 1 "REFERENCES"
.ip [Allman83a]
.q "Sendmail \*- An Internetwork Mail Router."
E. Allman.
In
.ul
Unix Programmers's Manual,
4.2 Berkeley Software Distribution,
volume 2C.
August 1983.
.ip [Allman83b]
.q "Mail Systems and Addressing in 4.2BSD."
E. Allman
In
.ul
UNICOM Conference Proceedings.
San Diego, California.
January 1983.
.ip [Allman&Amos85]
``Sendmail Revisited.''
E. Allman and M. Amos.
In
.ul
Usenix Summer 1985 Conference Proceedings.
Portland, Oregon.
June 1985.
.ip [IDA87]
.ul 3
Electronic Mail Addressing in Theory and Practice
with the IDA Sendmail Enhancement Kit
(or The Postmaster's Last Will and Testament).
Lennart Lo\*:vstrand.
Department of Computer and Information Science,
University of Linko\*:ping,
Sweden,
Report no. LiTH-IDA-Ex-8715.
May 1987.
.ip [RFC821]
.ul
Simple Mail Transport Protocol.
J. Postel.
August 1982.
.ip [RFC1123]
.ul
Requirements for Internet Hosts \*- Application and Support.
Internet Engineering Task Force,
R. Braden, Editor.
October 1989.
.ip [RFC1344]
.ul
Implications of MIME for Internet Mail Gateways.
N. Borenstein.
June 1992.
.ip [RFC1413]
.ul
Identification Protocol.
M. St. Johns.
February 1993.
.ip [RFC1425]
.ul
SMTP Service Extensions.
J. Klensin, N. Freed, M. Rose, E. Stefferud, and D. Crocker.
February 1993.
.ip [RFC1426]
.ul
SMTP Service Extension for 8bit-MIMEtransport.
J. Klensin, N. Freed, M. Rose, E. Stefferud, and D. Crocker.
February 1993.
.ip [RFC1427]
.ul
SMTP Service Extension for Message Size Declaration.
J. Klensin, N. Freed, and K. Moore.
February 1993.
.ip [RFC1521]
.ul 3
MIME (Multipurpose Internet Mail Extensions) Part One:
Mechanisms for Specifying and Describing
the Format of Internet Message Bodies.
N. Borenstein and N. Freed.
September 1993.