meillo@26: \chapter{Introduction}
meillo@42: \label{chap:introduction}
meillo@26: 
meillo@26: \section{History of electronic mail}
meillo@30: %TODO: have a quote from Bell Labs about email here
meillo@27: 
meillo@71: Electronic mail\index{electronic mail} (short: \name{email})\citeweb{wikipedia:email} is a basic concept in \unix.\citeweb{unix-mail-intro} On \unix\ machines, a lot of information is distributed by \name{system mail}, which is email sent by the operating system. Beside that, email is the common communication system between humans working on computers.
meillo@27: 
meillo@71: The \unix\ operating system supports email through the \name{mail user agent} (short: \NAME{MUA}) \name{/bin/mail}.
meillo@27: 
meillo@71: Development of \unix\ was not only made in the \name{Bell Labratories} of \NAME{AT\&T}. The \name{Univerity of California at Berkeley} worked on their version of a \unix\ operating system, too. It is refered to as \NAME{UCB} \unix, or \name{Berkeley} \unix\index{Berkeley Unix}.
meillo@27: 
meillo@74: The few features of \name{/bin/mail} lead to a second \NAME{MUA} from Berkeley: \name{Mail} (with a capital `M'). Later, the superior functionality of \name{Mail} went back to \name{Bell Labs} and into the program \name{mailx}, the successor of \name{/bin/mail}.
meillo@27: 
meillo@55: Nowadays, \name{mailx} and \name{Mail} are quite equivalent and \name{/bin/mail} is linked to either of them---whichever is installed.
meillo@55: 
meillo@71: At that time, computers were connected by various kinds of networks. \name{Bell Labs} had invented the \NAME{UUCP} program and protocol suite (for ``\unix\ to \unix\ copy'')\citeweb{wikipedia:uucp}. Berkeley however had an own creation called \name{Berknet} in use. And the \name{United States Department of Defence Advanded Research Projects Agency}'s (\NAME{ARPA}) effort on designing a new wide area network, led to the \NAME{ARPANET}\citeweb{wikipedia:arpanet}, based on the \name{transmission control protocol} (\NAME{TCP}). There were also other, minor, kinds of networks in use.
meillo@30: 
meillo@71: Email was transfered between different machines within the same networks. The file transfer itself was made uniformly using \NAME{FTP}, but the higher layered logic of the transfer was different. For example was addressing done different: \NAME{UUCP} used a flat-style schema, while \NAME{ARPANET}'s was hierachical.
meillo@30: 
meillo@71: Mail transport from one machine connected to one kind of network to a second machine connected to another was a problem. This showed up at Berkeley where some departments of the university had switched to \NAME{ARPANET}, and some to \NAME{UUCP}, while the rest used \name{Berknet}.
meillo@30: 
meillo@71: It was around 1982, when Eric Allman, then a student at Berkeley, wrote \name{delivermail}. Its purpose was to transform email from one network to another. \name{delivermail}, like its successor---the more flexible \sendmail---intermediated between the different networks. They were able to transform email messages from any network to any other.
meillo@30: 
meillo@71: Todays email structure is basicly the same as then. The major difference is the uniformity of the underlying network, which is nearly always the \NAME{ARPANET}-based \name{Internet}. Hence lowering the importance of the transformation capabilities of \MTA{}s, that was essential to \sendmail's success---yet being the primary motivation for the program.
meillo@30: 
meillo@71: More information about the history of electronic mail can be found at: \citeweb{email:griffiths}, \citeweb{email:crocker}, \citeweb{email:vleck}, \citeweb{email:akkad}, \citeweb{email:murakami}, and \citeweb{email:tomlinson}. A good starting point for general information on internet history is \citeweb{wikipedia:historyoftheinternet}.
meillo@71: %TODO: check the websites which ones are the important ones; remove unnessesary ones
meillo@30: 
meillo@27: 
meillo@26: 
meillo@26: \section{Transporting mail}
meillo@31: %TODO: when was the term ``mail transfer agent'' established?
meillo@27: 
meillo@71: This thesis is about a \name{mail transfer agent} (or \index{mail transport agent|see{mail transfer agent}}\name{mail transport agent}, short \NAME{MTA}): \masqmail. \sendmail\ is one too---the most important one.
meillo@71: 
meillo@71: The basic job of a \mta\ is to transfer/transport electronic mail from one host to another.
meillo@71: 
meillo@71: Here are definitions from others:
meillo@27: 
meillo@78: \begin{quote}
meillo@71: A mail transfer agent (MTA) is a highly specialized program that delivers mail and transports it between machines, like the post office.
meillo@71: \cite{costales97}
meillo@78: \end{quote}
meillo@55: 
meillo@78: \begin{quote}
meillo@55: A mail transfer agent (MTA) (also called a mail transport agent, message transfer agent, or smtpd (short for SMTP daemon)), is a computer program or software agent that transfers electronic mail messages from one computer to another.
meillo@55: \citeweb{wikipedia:mta}
meillo@78: \end{quote}
meillo@55: 
meillo@78: \begin{quote}
meillo@55: mail server (also known as a mail transfer agent or MTA, a mail transport agent, a mail router or an Internet mailer) is an application that receives incoming e-mail from local users (people within the same domain) and remote senders and forwards outgoing e-mail for delivery.
meillo@55: \citeweb{website:techtarget}
meillo@78: \end{quote}
meillo@55: 
meillo@78: \begin{quote}
meillo@71: Message Transfer Agent - (MTA, Mail Transfer Agent): Any program responsible for delivering e-mail messages. Upon receiving a message from a Mail User Agent or another MTA, [...] it [...] delivers it to any local addressees and/or forwards it to other remote MTAs (routing) for delivery to remote recipients.
meillo@71: %Any program responsible for delivering e-mail messages. Upon receiving a message from a Mail User Agent or another MTA, often by SMTP over the Internet, it stores it temporarily locally and analyses the recipients and delivers it to any local addressees and/or forwards it to other remote MTAs (routing) for delivery to remote recipients. In either case it may edit and/or add to the message headers.
meillo@71: %
meillo@71: %The most widely used MTA for Unix is sendmail, which communicates using SMTP.
meillo@71: %
meillo@71: %RFC 2821 (SMTP) expands MTA as ``Mail Transfer Agent'' though this is less common. Alternatives with ``Transport'' are also seen but less correct.
meillo@55: \citeweb{website:thefreedictionary}
meillo@78: \end{quote}
meillo@55: 
meillo@71: Common is the transfer of mail to other machines; this is the actual job. \MTA{}s work with mail, received from local users and/or remote machines. Mail delivery however is \emph{not} what \mta{}s are for, although probably every \MTA\ is able to deliver mail, and many do. \name{mail delivery agents} (short: \NAME{MDA}) are the programs for this job. Two of the best known \NAME{MDA}s are \name{procmail} and \name{maildrop}.
meillo@55: 
meillo@26: 
meillo@30: 
meillo@34: \section{\sendmail}
meillo@42: \label{sec:sendmail}
meillo@27: 
meillo@71: Allman wrote it to transfer emails between different networks, thus giving \sendmail\ mighty address rewriting abilities. In contrast to its predecessor \name{delivermail}, was \sendmail\ designed to offer greatest flexiblity in configuration; this enabled it to deal with any type of network.
meillo@27: 
meillo@71: \sendmail\ was, and still is, very successful. So successful that it stands, like no other, for the whole group of \MTA{}s: \name{sendmail} actually is the \emph{de facto standard} for \mta{}s.
meillo@27: 
meillo@71: Its author, Allman, sees three reasons for the huge success: the ``sloopy'' approach (accepting badly formed messages); its focus on the routing function; and the flexible configuration (this was important in \sendmail's early days).
meillo@71: \cite[page xviii]{costales97}
meillo@27: 
meillo@71: Others see \sendmail's success more critical. One of them is quoted in the \name{MMDF} FAQs \citeweb{faqs:mmdf}:
meillo@78: \begin{quote}
meillo@71: Sendmail was once compared by one old Internet hand to ``those killer bees that escaped from the laboratory---and now they're everywhere and you can't get rid of 'em''.
meillo@78: \end{quote}
meillo@71: He definately hints here at \sendmail's many security vulnerabilities that came to light and on its complexity, in particular its obscure configuration file \path{sendmail.cf}.
meillo@71: 
meillo@71: No matter how \sendmail\ is seen, one must admit its influence on \unix\ emailing programs. Most existing substitutes mimic \sendmail's interface and behavior. Most notable, they create a symbolic link named ``sendmail'' pointing to their own executable. The reason herefor are the many programs assuming an executable called ``sendmail'' on every computer system existing.
meillo@71: 
meillo@71: \sendmail\ is not only ported to many platforms, even including \name{Microsoft Windows}, but also it is still the prefered \MTA\ on many systems.
meillo@71: 
meillo@71: For deeper knowledge on \sendmail's history, see \cite{costales97} and \cite{vixie01}.