SENDMAIL -- An Internetwork Mail Router Eric Allman* _U_n_i_v_e_r_s_i_t_y _o_f _C_a_l_i_f_o_r_n_i_a_, _B_e_r_k_e_l_e_y _M_a_m_m_o_t_h _P_r_o_j_e_c_t ABSTRACT Routing mail through a heterogenous internet pre- sents many new problems. Among the worst of these is that of address mapping. Historically, this has been handled on an _a_d _h_o_c basis. However, this ap- proach has become unmanageable as internets grow. Sendmail acts a unified "post office" to which all mail can be submitted. Address interpretation is controlled by a production system, which can parse both domain-based addressing and old-style _a_d _h_o_c addresses. The production system is powerful enough to rewrite addresses in the message header to con- form to the standards of a number of common target networks, including old (NCP/RFC733) Arpanet, new (TCP/RFC822) Arpanet, UUCP, and Phonenet. Sendmail also implements an SMTP server, message queueing, and aliasing. _S_e_n_d_m_a_i_l implements a general internetwork mail routing facility, featuring aliasing and forwarding, automatic rout- ing 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 ____________________ *A considerable part of this work was done while under the employ of the INGRES Project at the University of Cali- fornia at Berkeley and at Britton Lee. SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSMMMM::99--11 SSMMMM::99--22 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr particular, the mail system can refer to users using a host- username pair. Host names and numbers have to be adminis- tered by a central authority, but usernames can be assigned locally to each host. In an internet, multiple networks with different char- acterstics and managements must communicate. In particular, the syntax and semantics of resource identification change. Certain special cases can be handled trivially by _a_d _h_o_c 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 com- plex. 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 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSMMMM::99--33 logical addressing. For example, an address of the form "eric@a.cc.berkeley.arpa" describes only the logical organi- zation of the address space. _S_e_n_d_m_a_i_l is intended to help bridge the gap between the totally _a_d _h_o_c 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 sys- tem administrator, as well as domain-based addressing. It helps guide the conversion of message formats between dis- parate networks. In short, _s_e_n_d_m_a_i_l is designed to assist a graceful transition to consistent internetwork addressing schemes. Section 1 discusses the design goals for _s_e_n_d_m_a_i_l. Section 2 gives an overview of the basic functions of the system. In section 3, details of usage are discussed. Sec- tion 4 compares _s_e_n_d_m_a_i_l to other internet mail routers, and an evaluation of _s_e_n_d_m_a_i_l is given in section 5, including future plans. 11.. DDEESSIIGGNN GGOOAALLSS Design goals for _s_e_n_d_m_a_i_l include: (1) Compatibility with the existing mail programs, including Bell version 6 mail, Bell version 7 mail [UNIX83], Berkeley _M_a_i_l [Shoens79], BerkNet mail SSMMMM::99--44 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr [Schmidt79], and hopefully UUCP mail [Nowitz78a, Nowitz78b]. ARPANET mail [Crocker77a, Postel77] was also required. (2) Reliability, in the sense of guaranteeing that every message is correctly delivered or at least brought to the attention of a human for correct disposal; no message should ever be completely lost. This goal was considered essential because of the emphasis on mail in our environment. It has turned out to be one of the hardest goals to satisfy, especially in the face of the many anoma- lous message formats produced by various ARPANET sites. For example, certain sites generate improperly formated addresses, occasionally caus- ing error-message loops. Some hosts use blanks in names, causing problems with UNIX mail programs that assume that an address is one word. The semantics of some fields are interpreted slightly differently by different sites. In summary, the obscure features of the ARPANET mail protocol really _a_r_e used and are difficult to support, but must be supported. (3) Existing software to do actual delivery should be used whenever possible. This goal derives as much from political and practical considerations as technical. SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSMMMM::99--55 (4) Easy expansion to fairly complex environments, including multiple connections to a single network type (such as with multiple UUCP or Ether nets [Metcalfe76]). This goal requires consideration of the contents of an address as well as its syn- tax in order to determine which gateway to use. For example, the ARPANET is bringing up the TCP protocol to replace the old NCP protocol. No host at Berkeley runs both TCP and NCP, so it is neces- sary to look at the ARPANET host name to determine whether to route mail to an NCP gateway or a TCP gateway. (5) Configuration should not be compiled into the code. A single compiled program should be able to run as is at any site (barring such basic changes as the CPU type or the operating system). We have found this seemingly unimportant goal to be criti- cal in real life. Besides the simple problems that occur when any program gets recompiled in a different environment, many sites like to "fiddle" with anything that they will be recompiling any- way. (6) _S_e_n_d_m_a_i_l must be able to let various groups main- tain their own mailing lists, and let individuals specify their own forwarding, without modifying the system alias file. SSMMMM::99--66 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr (7) Each user should be able to specify which mailer to execute to process mail being delivered for him. This feature allows users who are using spe- cialized mailers that use a different format to build their environment without changing the sys- tem, and facilitates specialized functions (such as returning an "I am on vacation" message). (8) Network traffic should be minimized by batching addresses to a single host where possible, without assistance from the user. These goals motivated the architecture illustrated in figure 1. The user interacts with a mail generating ____________________________________________________________ +---------+ +---------+ +---------+ | sender1 | | sender2 | | sender3 | +---------+ +---------+ +---------+ | | | +----------+ + +----------+ | | | v v v +-------------+ | sendmail | +-------------+ | | | +----------+ + +----------+ | | | v v v +---------+ +---------+ +---------+ | mailer1 | | mailer2 | | mailer3 | +---------+ +---------+ +---------+ Figure 1 -- Sendmail System Structure. ____________________________________________________________ SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSMMMM::99--77 and sending program. When the mail is created, the gen- erator calls _s_e_n_d_m_a_i_l, which routes the message to the correct mailer(s). Since some of the senders may be net- work servers and some of the mailers may be network clients, _s_e_n_d_m_a_i_l may be used as an internet mail gate- way. 22.. OOVVEERRVVIIEEWW 22..11.. SSyysstteemm OOrrggaanniizzaattiioonn _S_e_n_d_m_a_i_l neither interfaces with the user nor does actual mail delivery. Rather, it collects a mes- sage generated by a user interface program (UIP) such as Berkeley _M_a_i_l, MS [Crocker77b], or MH [Borden79], edits the message as required by the destination net- work, and calls appropriate mailers to do mail deliv- ery or queueing for network transmission1. This dis- cipline allows the insertion of new mailers at minimum cost. In this sense _s_e_n_d_m_a_i_l resembles the Message Processing Module (MPM) of [Postel79b]. 22..22.. IInntteerrffaacceess ttoo tthhee OOuuttssiiddee WWoorrlldd There are three ways _s_e_n_d_m_a_i_l can communicate with the outside world, both in receiving and in send- ing mail. These are using the conventional UNIX ____________________ 1except when mailing to a file, when _s_e_n_d_m_a_i_l does the delivery directly. SSMMMM::99--88 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr argument vector/return status, speaking SMTP over a pair of UNIX pipes, and speaking SMTP over an inter- process(or) channel. 22..22..11.. AArrgguummeenntt vveeccttoorr//eexxiitt ssttaattuuss This technique is the standard UNIX method for communicating with the process. A list of recipi- ents is sent in the argument vector, and the mes- sage 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. 22..22..22.. SSMMTTPP oovveerr ppiippeess 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. SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSMMMM::99--99 22..22..33.. SSMMTTPP oovveerr aann IIPPCC ccoonnnneeccttiioonn 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 send- mail process on another machine. 22..33.. OOppeerraattiioonnaall DDeessccrriippttiioonn When a sender wants to send a message, it issues a request to _s_e_n_d_m_a_i_l using one of the three methods described above. _S_e_n_d_m_a_i_l 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, _s_e_n_d_m_a_i_l creates and returns a new message describing the error and/or returns an status code telling what went wrong. 22..33..11.. AArrgguummeenntt pprroocceessssiinngg aanndd aaddddrreessss ppaarrssiinngg If _s_e_n_d_m_a_i_l 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 SSMMMM::99--1100 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr 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 deliv- ery. Forwarding is also performed as the local addresses are verified. _S_e_n_d_m_a_i_l appends each address to the recipient list after parsing. When a name is aliased or for- warded, 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 dupli- cates, preventing alias loops and duplicate mes- sages deliverd to the same recipient, as might occur if a person is in two groups. 22..33..22.. MMeessssaaggee ccoolllleeccttiioonn _S_e_n_d_m_a_i_l then collects the message. The mes- sage 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 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSMMMM::99--1111 message will be returned with an error. 22..33..33.. MMeessssaaggee ddeelliivveerryy For each unique mailer and host in the recipi- ent list, _s_e_n_d_m_a_i_l 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. 22..33..44.. QQuueeuueeiinngg ffoorr rreettrraannssmmiissssiioonn If the mailer returned an status that indi- cated that it might be able to handle the mail later, _s_e_n_d_m_a_i_l will queue the mail and try again later. 22..33..55.. RReettuurrnn ttoo sseennddeerr If errors occur during processing, _s_e_n_d_m_a_i_l returns the message to the sender for SSMMMM::99--1122 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr retransmission. The letter can be mailed back or written in the file "dead.letter" in the sender's home directory2. 22..44.. MMeessssaaggee HHeeaaddeerr EEddiittiinngg 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. 22..55.. CCoonnffiigguurraattiioonn FFiillee 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 lim- ited production system to rewrite addresses which is used to parse and rewrite the addresses). ____________________ 2Obviously, if the site giving the error is not the orig- inating site, the only reasonable option is to mail back to the sender. Also, there are many more error disposition op- tions, but they only effect the error message -- the "return to sender" function is always handled in one of these two ways. SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSMMMM::99--1133 To improve performance when reading the configu- ration file, a memory image can be provided. This provides a "compiled" form of the configuration file. 33.. UUSSAAGGEE AANNDD IIMMPPLLEEMMEENNTTAATTIIOONN 33..11.. AArrgguummeennttss 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: (1) Anything in parentheses is thrown away (as a comment). (2) Anything in angle brackets ("<>") is preferred over anything else. This rule implements the ARPANET standard that addresses of the form user name will send to the electronic "machine-address" rather than the human "user name." (3) Double quotes ( " ) quote phrases; backslashes quote characters. Backslashes are more power- ful in that they will cause otherwise equiva- lent phrases to compare differently -- for SSMMMM::99--1144 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr example, _u_s_e_r and _"_u_s_e_r_" are equivalent, but _\_u_s_e_r is different from either of them. Parentheses, angle brackets, and double quotes must be properly balanced and nested. The rewriting rules control remaining parsing3. 33..22.. MMaaiill ttoo FFiilleess aanndd PPrrooggrraammss Files and programs are legitimate message recipi- ents. Files provide archival storage of messages, useful for project administration and history. Pro- grams are useful as recipients in a variety of situa- tions, for example, to maintain a public repository of systems messages (such as the Berkeley _m_s_g_s 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 com- mand. If the user name begins with a slash mark ("/") the name is used as a file name, instead of a login name. ____________________ 3Disclaimer: Some special processing is done after rewriting local names; see below. SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSMMMM::99--1155 Files that have setuid or setgid bits set but no execute bits set have those bits honored if _s_e_n_d_m_a_i_l is running as root. 33..33.. AAlliiaassiinngg,, FFoorrwwaarrddiinngg,, IInncclluussiioonn _S_e_n_d_m_a_i_l reroutes mail three ways. Aliasing applies system wide. Forwarding allows each user to reroute incoming mail destined for that account. Inclusion directs _s_e_n_d_m_a_i_l to read a file for a list of addresses, and is normally used in conjunction with aliasing. 33..33..11.. AAlliiaassiinngg 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). 33..33..22.. FFoorrwwaarrddiinngg 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 _n_o_t 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. SSMMMM::99--1166 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr Forwarding also permits a user to specify a private incoming mailer. For example, forwarding to: "|/usr/local/newmail myname" will use a different incoming mailer. 33..33..33.. IInncclluussiioonn Inclusion is specified in RFC 733 [Crocker77a] syntax: :Include: pathname An address of this form reads the file specified by _p_a_t_h_n_a_m_e and sends to all users listed in that file. The intent is _n_o_t 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 is a method of letting a project maintain a mailing list without interaction with the system adminis- tration, even if the alias file is protected. It is not necessary to rebuild the index on the alias database when a :include: list is changed. SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSMMMM::99--1177 33..44.. MMeessssaaggee CCoolllleeccttiioonn Once all recipient addresses are parsed and veri- fied, the message is collected. The message comes in two parts: a message header and a message body, sepa- rated by a blank line. The header is formatted as a series of lines of the form field-name: field-value Field-value can be split across lines by starting the following lines with a space or a tab. Some header fields have special internal meaning, and have appro- priate special processing. Other headers are simply passed through. Some header fields may be added auto- matically, such as time stamps. The body is a series of text lines. It is com- pletely uninterpreted and untouched, except that lines beginning with a dot have the dot doubled when trans- mitted over an SMTP channel. This extra dot is stripped by the receiver. 33..55.. MMeessssaaggee DDeelliivveerryy 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 SSMMMM::99--1188 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr 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 per- formed using one of the techniques described in sec- tion 2.2. After a connection is established, _s_e_n_d_m_a_i_l 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. 33..66.. QQuueeuueedd MMeessssaaggeess 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 recipi- ent, 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. 33..77.. CCoonnffiigguurraattiioonn Configuration is controlled primarily by a con- figuration file read at startup. _S_e_n_d_m_a_i_l should not SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSMMMM::99--1199 need to be recomplied except (1) To change operating systems (V6, V7/32V, 4BSD). (2) To remove or insert the DBM (UNIX database) library. (3) To change ARPANET reply codes. (4) To add headers fields requiring special pro- cessing. 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 direc- tory, 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. 33..77..11.. MMaaccrrooss Macros can be used in three ways. Certain macros transmit unstructured textual information into the mail system, such as the name _s_e_n_d_m_a_i_l will use to identify itself in error messages. SSMMMM::99--2200 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr Other macros transmit information from _s_e_n_d_m_a_i_l 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. 33..77..22.. HHeeaaddeerr ddeeccllaarraattiioonnss Header declarations inform _s_e_n_d_m_a_i_l of the format of known header lines. Knowledge of a few header lines is built into _s_e_n_d_m_a_i_l, 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. 33..77..33.. MMaaiilleerr ddeeccllaarraattiioonnss Mailer declarations tell _s_e_n_d_m_a_i_l of the vari- ous mailers available to it. The definition speci- fies 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. SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSMMMM::99--2211 33..77..44.. AAddddrreessss rreewwrriittiinngg rruulleess The heart of address parsing in _s_e_n_d_m_a_i_l is a set of rewriting rules. These are an ordered list of pattern-replacement rules, (somewhat like a pro- duction 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 edit- ing of addresses into different formats. For exam- ple, an address of the form: ucsfcgl!tef might be mapped into: tef@ucsfcgl.UUCP to conform to the domain syntax. Translations can also be done in the other direction. 33..77..55.. OOppttiioonn sseettttiinngg There are several options that can be set from the configuration file. These include the SSMMMM::99--2222 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr pathnames of various support files, timeouts, default modes, etc. 44.. CCOOMMPPAARRIISSOONN WWIITTHH OOTTHHEERR MMAAIILLEERRSS 44..11.. DDeelliivveerrmmaaiill _S_e_n_d_m_a_i_l is an outgrowth of _d_e_l_i_v_e_r_m_a_i_l. The primary differences are: (1) Configuration information is not compiled in. This change simplifies many of the problems of moving to other machines. It also allows easy debugging of new mailers. (2) Address parsing is more flexible. For example, _d_e_l_i_v_e_r_m_a_i_l only supported one gateway to any network, whereas _s_e_n_d_m_a_i_l can be sensitive to host names and reroute to different gateways. (3) Forwarding and :include: features 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). (4) _S_e_n_d_m_a_i_l supports message batching across net- works when a message is being sent to multiple recipients. SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSMMMM::99--2233 (5) A mail queue is provided in _s_e_n_d_m_a_i_l_. Mail that cannot be delivered immediately but can potentially be delivered later is stored in this queue for a later retry. The queue also provides a buffer against system crashes; after the message has been collected it may be reli- ably redelivered even if the system crashes during the initial delivery. (6) _S_e_n_d_m_a_i_l uses the networking support provided by 4.2BSD to provide a direct interface net- works such as the ARPANET and/or Ethernet using SMTP (the Simple Mail Transfer Protocol) over a TCP/IP connection. 44..22.. MMMMDDFF MMDF [Crocker79] spans a wider problem set than _s_e_n_d_m_a_i_l. For example, the domain of MMDF includes a "phone network" mailer, whereas _s_e_n_d_m_a_i_l calls on pre- existing mailers in most cases. MMDF and _s_e_n_d_m_a_i_l both support aliasing, cus- tomized mailers, message batching, automatic forward- ing to gateways, queueing, and retransmission. MMDF supports two-stage timeout, which _s_e_n_d_m_a_i_l does not support. SSMMMM::99--2244 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr The configuration for MMDF is compiled into the code4. Since MMDF does not consider backwards compati- bility as a design goal, the address parsing is sim- pler but much less flexible. It is somewhat harder to integrate a new channel5 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 deliv- ery phases. Although _s_e_n_d_m_a_i_l has the concept of each of these stages, they are integrated into one program, whereas in MMDF they are split into two programs. 44..33.. MMeessssaaggee PPrroocceessssiinngg MMoodduullee The Message Processing Module (MPM) discussed by Postel [Postel79b] matches _s_e_n_d_m_a_i_l closely in terms of its basic architecture. However, like MMDF, the MPM includes the network interface software as part of its domain. ____________________ 4Dynamic configuration tables are currently being consid- ered for MMDF; allowing the installer to select either com- piled or dynamic tables. 5The MMDF equivalent of a _s_e_n_d_m_a_i_l "mailer." SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSMMMM::99--2255 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 _s_e_n_d_m_a_i_l. When a message queued by _s_e_n_d_m_a_i_l 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 cre- ate an appropriate response. MPM prefers passing the message as a structured object, with type-length-value tuples6. Such a con- vention requires a much higher degree of cooperation between mailers than is required by _s_e_n_d_m_a_i_l. MPM also assumes a universally agreed upon internet name space (with each address in the form of a net-host- user tuple), which _s_e_n_d_m_a_i_l does not. 55.. EEVVAALLUUAATTIIOONNSS AANNDD FFUUTTUURREE PPLLAANNSS _S_e_n_d_m_a_i_l 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]. ____________________ 6This is similar to the NBS standard. SSMMMM::99--2266 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr 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. _S_e_n_d_m_a_i_l has knowledge of a few difficult environ- ments built in. It generates ARPANET FTP/SMTP compatible error messages (prepended with three-digit numbers [Nei- gus73, Postel74, Postel82]) as necessary, optionally gen- erates 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 deliv- ery) has turned out to be a good idea. Even with local delivery, there are issues of the location of the mail- box, the format of the mailbox, the locking protocol used, etc., that are best decided by other programs. One SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSMMMM::99--2277 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 pro- gram 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 chang- ing soon to accommodate changing requirements and envi- ronments. These changes will include modifications to the message header (e.g., [NBS80]) or to the body of the SSMMMM::99--2288 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr message itself (such as for multimedia messages [Pos- tel80]). Experience indicates that these changes should be relatively trivial to integrate into the existing sys- tem. In tightly coupled environments, it would be nice to have a name server such as Grapvine [Birrell82] inte- grated 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 resolv- ing conflicts. Ideally this would be effective even without all hosts being under a single management. How- ever, it is not clear whether this feature should be integrated into the aliasing facility or should be con- sidered a "value added" feature outside _s_e_n_d_m_a_i_l itself. As a more interesting case, the CSNET name server [Solomon81] provides an facility that goes beyond a sin- gle tightly-coupled environment. Such a facility would normally exist outside of _s_e_n_d_m_a_i_l however. AACCKKNNOOWWLLEEDDGGEEMMEENNTTSS Thanks are due to Kurt Shoens for his continual cheer- ful assistance and good advice, Bill Joy for pointing me in the correct direction (over and over), and Mark Horton for SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSMMMM::99--2299 more advice, prodding, and many of the good ideas. Kurt and Eric Schmidt are to be credited for using _d_e_l_i_v_e_r_m_a_i_l as a server for their programs (_M_a_i_l and BerkNet respectively) before any sane person should have, and making the necessary modifications promptly and happily. Eric gave me consider- able advice about the perils of network software which saved me an unknown amount of work and grief. Mark did the origi- nal implementation of the DBM version of aliasing, installed the VFORK code, wrote the current version of _r_m_a_i_l, and was the person who really convinced me to put the work into _d_e_l_i_v_e_r_m_a_i_l to turn it into _s_e_n_d_m_a_i_l. Kurt deserves acco- lades for using _s_e_n_d_m_a_i_l 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 [Birrell82] Birrell, A. D., Levin, R., Needham, R. M., and Schroeder, M. D., "Grapevine: An Exercise in Distributed Computing." In _C_o_m_m_. _A_._C_._M_. _2_5_, 4, April 82. [Borden79] Borden, S., Gaines, R. S., and Shapiro, N. Z., _T_h_e _M_H _M_e_s_s_a_g_e _H_a_n_d_l_i_n_g _S_y_s_t_e_m_: _U_s_e_r_s_' _M_a_n_u_a_l_. R-2367-PAF. Rand Corporation. October 1979. [Crocker77a] Crocker, D. H., Vittal, J. J., Pogran, K. T., and Henderson, D. A. Jr., _S_t_a_n_d_a_r_d _f_o_r _t_h_e _F_o_r_m_a_t _o_f _A_R_P_A _N_e_t_w_o_r_k _T_e_x_t _M_e_s_s_a_g_e_s_. RFC 733, NIC 41952. In [Feinler78]. November 1977. [Crocker77b] Crocker, D. H., _F_r_a_m_e_w_o_r_k _a_n_d _F_u_n_c_t_i_o_n_s _o_f _t_h_e _M_S _P_e_r_s_o_n_a_l _M_e_s_s_a_g_e _S_y_s_t_e_m_. R-2134-ARPA, Rand Corporation, Santa Monica, California. 1977. [Crocker79] Crocker, D. H., Szurkowski, E. S., and Far- ber, D. J., _A_n _I_n_t_e_r_n_e_t_w_o_r_k _M_e_m_o _D_i_s_t_r_i_b_u_t_i_o_n _F_a_c_i_l_i_t_y _-_- _M_M_D_F_. 6th Data Communication Symposium, Asilomar. November 1979. SSMMMM::99--3300 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSMMMM::99--3311 [Crocker82] Crocker, D. H., _S_t_a_n_d_a_r_d _f_o_r _t_h_e _F_o_r_m_a_t _o_f _A_r_p_a _I_n_t_e_r_n_e_t _T_e_x_t _M_e_s_s_a_g_e_s_. RFC 822. Net- work Information Center, SRI International, Menlo Park, California. August 1982. [Metcalfe76] Metcalfe, R., and Boggs, D., "Ethernet: Dis- tributed Packet Switching for Local Computer Networks", _C_o_m_m_u_n_i_c_a_t_i_o_n_s _o_f _t_h_e _A_C_M _1_9_, 7. July 1976. [Feinler78] Feinler, E., and Postel, J. (eds.), _A_R_P_A_N_E_T _P_r_o_t_o_c_o_l _H_a_n_d_b_o_o_k_. NIC 7104, Network Infor- mation Center, SRI International, Menlo Park, California. 1978. [NBS80] National Bureau of Standards, _S_p_e_c_i_f_i_c_a_t_i_o_n _o_f _a _D_r_a_f_t _M_e_s_s_a_g_e _F_o_r_m_a_t _S_t_a_n_d_a_r_d_. Report No. ICST/CBOS 80-2. October 1980. [Neigus73] Neigus, N., _F_i_l_e _T_r_a_n_s_f_e_r _P_r_o_t_o_c_o_l _f_o_r _t_h_e _A_R_P_A _N_e_t_w_o_r_k_. RFC 542, NIC 17759. In [Fein- ler78]. August, 1973. [Nowitz78a] Nowitz, D. A., and Lesk, M. E., _A _D_i_a_l_-_U_p _N_e_t_w_o_r_k _o_f _U_N_I_X _S_y_s_t_e_m_s_. Bell Laboratories. In UNIX Programmer's Manual, Seventh Edition, Volume 2. August, 1978. [Nowitz78b] Nowitz, D. A., _U_u_c_p _I_m_p_l_e_m_e_n_t_a_t_i_o_n _D_e_s_c_r_i_p_- _t_i_o_n_. Bell Laboratories. In UNIX SSMMMM::99--3322 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr Programmer's Manual, Seventh Edition, Volume 2. October, 1978. [Postel74] Postel, J., and Neigus, N., Revised FTP Reply Codes. RFC 640, NIC 30843. In [Feinler78]. June, 1974. [Postel77] Postel, J., _M_a_i_l _P_r_o_t_o_c_o_l_. NIC 29588. In [Feinler78]. November 1977. [Postel79a] Postel, J., _I_n_t_e_r_n_e_t _M_e_s_s_a_g_e _P_r_o_t_o_c_o_l_. RFC 753, IEN 85. Network Information Center, SRI International, Menlo Park, California. March 1979. [Postel79b] Postel, J. B., _A_n _I_n_t_e_r_n_e_t_w_o_r_k _M_e_s_s_a_g_e _S_t_r_u_c_- _t_u_r_e_. In _P_r_o_c_e_e_d_i_n_g_s _o_f _t_h_e _S_i_x_t_h _D_a_t_a _C_o_m_- _m_u_n_i_c_a_t_i_o_n_s _S_y_m_p_o_s_i_u_m_, IEEE. New York. November 1979. [Postel80] Postel, J. B., _A _S_t_r_u_c_t_u_r_e_d _F_o_r_m_a_t _f_o_r _T_r_a_n_s_- _m_i_s_s_i_o_n _o_f _M_u_l_t_i_-_M_e_d_i_a _D_o_c_u_m_e_n_t_s_. RFC 767. Network Information Center, SRI Interna- tional, Menlo Park, California. August 1980. [Postel82] Postel, J. B., _S_i_m_p_l_e _M_a_i_l _T_r_a_n_s_f_e_r _P_r_o_t_o_c_o_l_. RFC821 (obsoleting RFC788). Network Informa- tion Center, SRI International, Menlo Park, California. August 1982. SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr SSMMMM::99--3333 [Schmidt79] Schmidt, E., _A_n _I_n_t_r_o_d_u_c_t_i_o_n _t_o _t_h_e _B_e_r_k_e_l_e_y _N_e_t_w_o_r_k_. University of California, Berkeley California. 1979. [Shoens79] Shoens, K., _M_a_i_l _R_e_f_e_r_e_n_c_e _M_a_n_u_a_l_. Univer- sity of California, Berkeley. In UNIX Pro- grammer's Manual, Seventh Edition, Volume 2C. December 1979. [Sluizer81] Sluizer, S., and Postel, J. B., _M_a_i_l _T_r_a_n_s_f_e_r _P_r_o_t_o_c_o_l_. RFC 780. Network Information Cen- ter, SRI International, Menlo Park, Califor- nia. May 1981. [Solomon81] Solomon, M., Landweber, L., and Neuhengen, D., "The Design of the CSNET Name Server." CS-DN-2, University of Wisconsin, Madison. November 1981. [Su82] Su, Zaw-Sing, and Postel, Jon, _T_h_e _D_o_m_a_i_n _N_a_m_i_n_g _C_o_n_v_e_n_t_i_o_n _f_o_r _I_n_t_e_r_n_e_t _U_s_e_r _A_p_p_l_i_c_a_- _t_i_o_n_s_. RFC819. Network Information Center, SRI International, Menlo Park, California. August 1982. [UNIX83] _T_h_e _U_N_I_X _P_r_o_g_r_a_m_m_e_r_'_s _M_a_n_u_a_l_, _S_e_v_e_n_t_h _E_d_i_- _t_i_o_n_, Virtual VAX-11 Version, Volume 1. Bell Laboratories, modified by the University of California, Berkeley, California. March, SSMMMM::99--3344 SSEENNDDMMAAIILL ---- AAnn IInntteerrnneettwwoorrkk MMaaiill RRoouutteerr 1983.