SENDMAIL -- An Internetwork Mail Router
Eric Allman*
University of California, Berkeley
Mammoth Project
Sendmail implements a general internetwork mail routing facility, featuring aliasing and forwarding, 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 different characterstics and managements must communicate. In particular, the syntax and semantics of resource identification change. Certain special cases can be handled trivially by ad hoc techniques, such as providing network names that appear local to hosts on other networks, as with the Ethernet at Xerox PARC. 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 syntax 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 of {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 of the form ``eric@a.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 of each other and the clean, tightly-coupled world of unique network numbers. It can accept old arbitrary address syntaxes, resolving ambiguities using heuristics specified by the system administrator, as well as domain-based addressing. It helps guide the conversion of message formats between disparate networks. In short, sendmail is designed to assist a graceful transition to consistent internetwork addressing schemes.
Section 1 discusses the design goals for sendmail. Section 2 gives an overview of the basic functions of the system. In section 3, details of usage are discussed. Section 4 compares sendmail to other internet mail routers, and an evaluation of sendmail is given in section 5, including future plans.
Design goals for sendmail include:
These goals motivated the architecture illustrated in figure 1.
The user interacts with a mail generating and sending program. When the mail is created, the generator calls sendmail, which routes the message to the correct mailer(s). Since some of the senders may be network servers and some of the mailers may be network clients, sendmail may be used as an internet mail gateway.
Figure 1 -- Sendmail System Structure.
Sendmail neither interfaces with the user nor does actual mail delivery. Rather, it collects a message generated by a user interface program (UIP) such as Berkeley Mail, MS [Crocker77b], or MH [Borden79], edits the message as required by the destination network, and calls appropriate mailers to do mail delivery or queueing for network transmission[1]. This discipline allows the insertion of new mailers at minimum cost. In this sense sendmail resembles the Message Processing Module (MPM) of [Postel79b].
There are three ways sendmail can communicate with the outside world, both in receiving and in sending mail. These are using the conventional UNIX argument vector/return status, speaking SMTP over a pair of UNIX pipes, and speaking SMTP over an interprocess(or) channel.
This technique is the standard UNIX method for communicating with the process. A list of recipients is sent in the argument vector, and the message body is sent on the standard input. Anything that the mailer prints is simply collected and sent back to the sender if there were any problems. The exit status from the mailer is collected after the message is sent, and a diagnostic is printed if appropriate.
The SMTP protocol [Postel82] can be used to run an interactive lock-step interface with the mailer. A subprocess is still created, but no recipient addresses are passed to the mailer via the argument list. Instead, they are passed one at a time in commands sent to the processes standard input. Anything appearing on the standard output must be a reply code in a special format.
This technique is similar to the previous technique, except that it uses a 4.2bsd IPC channel [UNIX83]. This method is exceptionally flexible in that the mailer need not reside on the same machine. It is normally used to connect to a sendmail process on another machine.
When a sender wants to send a message, it issues a request to sendmail using one of the three methods described above. Sendmail operates in two distinct phases. In the first phase, it collects and stores the message. In the second phase, message delivery occurs. If there were errors during processing during the second phase, sendmail creates and returns a new message describing the error and/or returns an status code telling what went wrong.
If sendmail is called using one of the two subprocess techniques, the arguments are first scanned and option specifications are processed. Recipient addresses are then collected, either from the command line or from the SMTP RCPT command, and a list of recipients is created. Aliases are expanded at this step, including mailing lists. As much validation as possible of the addresses is done at this step: syntax is checked, and local addresses are verified, but detailed checking of host names and addresses is deferred until delivery. Forwarding is also performed as the local addresses are verified.
Sendmail appends each address to the recipient list after parsing. When a name is aliased or forwarded, the old name is retained in the list, and a flag is set that tells the delivery phase to ignore this recipient. This list is kept free from duplicates, preventing alias loops and duplicate messages deliverd to the same recipient, as might occur if a person is in two groups.
Sendmail then collects the message. The message should have a header at the beginning. No formatting requirements are imposed on the message except that they must be lines of text (i.e., binary data is not allowed). The header is parsed and stored in memory, and the body of the message is saved in a temporary file.
To simplify the program interface, the message is collected even if no addresses were valid. The message will be returned with an error.
For each unique mailer and host in the recipient list, sendmail calls the appropriate mailer. Each mailer invocation sends to all users receiving the message on one host. Mailers that only accept one recipient at a time are handled properly.
The message is sent to the mailer using one of the same three interfaces used to submit a message to sendmail. Each copy of the message is prepended by a customized header. The mailer status code is caught and checked, and a suitable error message given as appropriate. The exit code must conform to a system standard or a generic message (``Service unavailable'') is given.
If the mailer returned an status that indicated that it might be able to handle the mail later, sendmail will queue the mail and try again later.
If errors occur during processing, sendmail returns the message to the sender for retransmission. The letter can be mailed back or written in the file ``dead.letter'' in the sender's home directory[2].
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 ``From:'' line and a ``Full-name:'' line can be merged under certain circumstances.
Almost all configuration information is read at runtime from an ASCII file, encoding macro definitions (defining the value of macros used internally), header declarations (telling sendmail the format of header lines that it will process specially, i.e., lines that it will add or reformat), mailer definitions (giving information such as the location and characteristics of each mailer), and address rewriting rules (a limited production system to rewrite addresses which is used to parse and rewrite the addresses).
To improve performance when reading the configuration file, a memory image can be provided. This provides a ``compiled'' form of the configuration file.
Arguments may be flags and addresses. Flags set various processing options. Following flag arguments, address arguments may be given, unless we are running in SMTP mode. Addresses follow the syntax in RFC822 [Crocker82] for ARPANET address formats. In brief, the format is:
user name <machine-address>
Parentheses, angle brackets, and double quotes must be properly balanced and nested. The rewriting rules control remaining parsing[3].
Files and programs are legitimate message recipients. Files provide archival storage of messages, useful for project administration and history. Programs are useful as recipients in a variety of situations, for example, to maintain a public repository of systems messages (such as the Berkeley msgs program, or the MARS system [Sattley78]).
Any address passing through the initial parsing algorithm as a local address (i.e, not appearing to be a valid address for another mailer) is scanned for two special cases. If prefixed by a vertical bar (``|'') the rest of the address is processed as a shell command. If the user name begins with a slash mark (``/'') the name is used as a file name, instead of a login name.
Files that have setuid or setgid bits set but no execute bits set have those bits honored if sendmail is running as root.
Sendmail reroutes mail three ways. Aliasing applies system wide. Forwarding allows each user to reroute incoming mail destined for that account. Inclusion directs sendmail to read a file for a list of addresses, and is normally used in conjunction with aliasing.
Aliasing maps names to address lists using a system-wide file. This file is indexed to speed access. Only names that parse as local are allowed as aliases; this guarantees a unique key (since there are no nicknames for the local host).
After aliasing, recipients that are local and valid are checked for the existence of a ``.forward'' file in their home directory. If it exists, the message is not sent to that user, but rather to the list of users in that file. Often this list will contain only one address, and the feature will be used for network mail forwarding.
Forwarding also permits a user to specify a private incoming mailer. For example, forwarding to:
"|/usr/local/newmail myname"
Inclusion is specified in RFC 733 [Crocker77a] syntax:
:Include: pathname
The intent is 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:
project: :include:/usr/project/userlist
It is not necessary to rebuild the index on the alias database when a :include: list is changed.
Once all recipient addresses are parsed and verified, the message is collected. The message comes in two parts: a message header and a message body, separated by a blank line.
The header is formatted as a series of lines of the form
field-name: field-value
The body is a series of text lines. It is completely uninterpreted and untouched, except that lines beginning with a dot have the dot doubled when transmitted over an SMTP channel. This extra dot is stripped by the receiver.
The send queue is ordered by receiving host before transmission to implement message batching. Each address is marked as it is sent so rescanning the list is safe. An argument list is built as the scan proceeds. Mail to files is detected during the scan of the send list. The interface to the mailer is performed using one of the techniques described in section 2.2.
After a connection is established, sendmail makes the per-mailer changes to the header and sends the result to the mailer. If any mail is rejected by the mailer, a flag is set to invoke the return-to-sender function after all delivery completes.
If the mailer returns a ``temporary failure'' exit status, the message is queued. A control file is used to describe the recipients to be sent to and various other parameters. This control file is formatted as a series of lines, each describing a sender, a recipient, the time of submission, or some other salient parameter of the message. The header of the message is stored in the control file, so that the associated data file in the queue is just the temporary file that was originally collected.
Configuration is controlled primarily by a configuration file read at startup. Sendmail should not need to be recomplied except
Adding mailers or changing parsing (i.e., rewriting) or routing information does not require recompilation.
If the mail is being sent by a local user, and the file ``.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 feature is to add header lines.
The configuration file encodes macro definitions, header definitions, mailer definitions, rewriting rules, and options.
Macros can be used in three ways. Certain macros transmit unstructured textual information into the mail system, such as the name sendmail will use to identify itself in error messages. Other macros transmit information from sendmail to the configuration file for use in creating other fields (such as argument vectors to mailers); e.g., 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.
Header declarations inform sendmail of the format of known header lines. Knowledge of a few header lines is built into sendmail, such as the ``From:'' and ``Date:'' lines.
Most configured headers will be automatically inserted in the outgoing message if they don't exist in the incoming message. Certain headers are suppressed by some mailers.
Mailer declarations tell sendmail of the various mailers available to it. The definition specifies 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.
The heart of address parsing in sendmail is a set of rewriting rules. These are an ordered list of pattern-replacement rules, (somewhat like a production system, except that order is critical), which are applied to each address. The address is rewritten textually until it is either rewritten into a special canonical form (i.e., a (mailer, host, user) 3-tuple, such as {arpanet, usc-isif, postel} representing the address ``postel@usc-isif''), or it falls off the end. When a pattern matches, the rule is reapplied until it fails.
The configuration file also supports the editing of addresses into different formats. For example, an address of the form:
ucsfcgl!tef
tef@ucsfcgl.UUCP
There are several options that can be set from the configuration file. These include the pathnames of various support files, timeouts, default modes, etc.
Sendmail is an outgrowth of delivermail. The primary differences are:
MMDF [Crocker79] spans a wider problem set than sendmail. For example, the domain of MMDF includes a ``phone network'' mailer, whereas sendmail calls on preexisting mailers in most cases.
MMDF and sendmail both support aliasing, customized mailers, message batching, automatic forwarding to gateways, queueing, and retransmission. MMDF supports two-stage timeout, which sendmail does not support.
The configuration for MMDF is compiled into the code[4].
Since MMDF does not consider backwards compatibility as a design goal, the address parsing is simpler but much less flexible.
It is somewhat harder to integrate a new channel[5] into MMDF. In particular, MMDF must know the location and format of host tables for all channels, and the channel must speak a special protocol. This allows MMDF to do additional verification (such as verifying host names) at submission time.
MMDF strictly separates the submission and delivery phases. Although sendmail has the concept of each of these stages, they are integrated into one program, whereas in MMDF they are split into two programs.
The Message Processing Module (MPM) discussed by Postel [Postel79b] matches sendmail closely in terms of its basic architecture. However, like MMDF, the MPM includes the network interface software as part of its domain.
MPM also postulates a duplex channel to the receiver, as does MMDF, thus allowing simpler handling of errors by the mailer than is possible in sendmail. When a message queued by 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.
MPM prefers passing the message as a structured object, with type-length-value tuples[6]. Such a convention requires a much higher degree of cooperation between mailers than is required by sendmail. MPM also assumes a universally agreed upon internet name space (with each address in the form of a net-host-user tuple), which sendmail does not.
Sendmail is designed to work in a nonhomogeneous environment. Every attempt is made to avoid imposing unnecessary 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 [Postel79a] and [Postel79b].
A nonuniform address space implies that a path will be specified in all addresses, either explicitly (as part of the address) or implicitly (as with implied forwarding to gateways). This restriction has the unpleasant effect of making replying to messages exceedingly difficult, since there is no one ``address'' for any person, but only a way to get there from wherever you are.
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.
Sendmail has knowledge of a few difficult environments built in. It generates ARPANET FTP/SMTP compatible error messages (prepended with three-digit numbers [Neigus73, Postel74, Postel82]) as necessary, optionally generates UNIX-style ``From'' lines on the front of messages for some mailers, and knows how to parse the same lines on input. Also, error handling has an option customized for BerkNet.
The decision to avoid doing any type of delivery where possible (even, or perhaps especially, local delivery) has turned out to be a good idea. Even with local delivery, there are issues of the location of the mailbox, the format of the mailbox, the locking protocol used, etc., that are best decided by other programs. One surprisingly major annoyance in many internet mailers is that the location and format of local mail is built in. The feeling seems to be that local mail is so common that it should be efficient. This feeling is not born out by our experience; on the contrary, the location and format of mailboxes seems to vary widely from system to system.
The ability to automatically generate a response to incoming mail (by forwarding mail to a program) seems useful (``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. A program could be written to do standard tasks correctly, but this would solve the general case.
It might be desirable to implement some form of load limiting. I am unaware of any mail system that addresses this problem, nor am I aware of any reasonable solution at this time.
The configuration file is currently practically inscrutable; considerable convenience could be realized with a higher-level format.
It seems clear that common protocols will be changing soon to accommodate changing requirements and environments. These changes will include modifications to the message header (e.g., [NBS80]) or to the body of the message itself (such as for multimedia messages [Postel80]). Experience indicates that these changes should be relatively trivial to integrate into the existing system.
In tightly coupled environments, it would be nice to have a name server such as Grapvine [Birrell82] integrated into the mail system. This would allow a site such as ``Berkeley'' to appear as a single host, rather than as a collection of hosts, and would allow people to move transparently among machines without having to change their addresses. Such a facility would require an automatically updated database and some method of resolving conflicts. Ideally this would be effective even without all hosts being under a single management. However, it is not clear whether this feature should be integrated into the aliasing facility or should be considered a ``value added'' feature outside sendmail itself.
As a more interesting case, the CSNET name server [Solomon81] provides an facility that goes beyond a single tightly-coupled environment. Such a facility would normally exist outside of sendmail however.
Thanks are due to Kurt Shoens for his continual cheerful assistance and good advice, Bill Joy for pointing me in the correct direction (over and over), and Mark Horton for more advice, prodding, and many of the good ideas. Kurt and Eric Schmidt are to be credited for using delivermail as a server for their programs (Mail and BerkNet respectively) before any sane person should have, and making the necessary modifications promptly and happily. Eric gave me considerable advice about the perils of network software which saved me an unknown amount of work and grief. Mark did the original implementation of the DBM version of aliasing, installed the VFORK code, wrote the current version of rmail, and was the person who really convinced me to put the work into delivermail to turn it into sendmail. Kurt deserves accolades for using sendmail when I was myself afraid to take the risk; how a person can continue to be so enthusiastic in the face of so much bitter reality is beyond me.
Kurt, Mark, Kirk McKusick, Marvin Solomon, and many others have reviewed this paper, giving considerable useful advice.
Special thanks are reserved for Mike Stonebraker at Berkeley
and Bob Epstein at Britton-Lee,
who both knowingly allowed me to put so much work into this
project
when there were so many other things I really should
have been working on.
REFERENCES