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view thesis/tex/1-Introduction.tex @ 256:68ef2040912a
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| author | meillo@marmaro.de |
|---|---|
| date | Tue, 13 Jan 2009 17:18:25 +0100 |
| parents | 724cc6057105 |
| children | f4966e84815d |
line source
1 \chapter{Introduction}
2 \label{chap:introduction}
4 << say what you want to say >>
6 << the overall goal of the document >>
13 \section{Email prerequisites}
15 email and everything is defined in RFCs
18 \subsubsection{Mail agents}
20 This thesis will frequently use the three terms: \MTA, \NAME{MUA}, and \NAME{MDA}. The name the three different kinds of software on which the email system depends. Here they are explained with references to the snail mail system one knows from everyday's life. Figure \ref{fig:mail-agents} shows the relation between them and the way an email message takes trough the system.
22 \paragraph{MTA}
23 \name{Mail Tranfer Agents} are for electronic mail what post offices are for snail mail. The basic job of an \MTA\ is to transport mail from senders to recipients, or more pedantic: from \MTA\ to \MTA. This is the definition of such kind of software, and this is how \MTA{}s are generally seen \cite[page 19]{dent04} \cite[pages 3-5]{hafiz05}. \MTA{}s are explained in more detail in chapter \ref{chap:mail-transfer-agents}.
26 \paragraph{MUA}
27 \name{Mail User Agents} are the software the user deals with. He writes and reads email with it. The \NAME{MUA} passes outgoing mail to the nearest \MTA, and the \NAME{MUA} displays the contents of the user's mailbox. Well known \NAME{MUA}s are \name{Mozilla Thunderbird} and \name{mutt} on \unix\ systems, and \name{Microsoft Outlook} on \name{Windows}.
30 \paragraph{MDA}
31 \name{Mail Delivery Agents} correspond to postmen in the real world. They receive mail, destinated to recipients they are responsible for, from an \MTA, and deliver it to the mailboxes of those recipients. Many \MTA{}s include an own \NAME{MDA}, but specialized ones exist: \name{procmail} and \name{maildrop} are examples.
33 \begin{figure}
34 \begin{center}
35 \includegraphics[scale=0.75]{img/mail-agents.eps}
36 \end{center}
37 \caption{Mail agents and the way a mail message takes}
38 \label{fig:mail-agents}
39 \end{figure}
46 \subsubsection{Mail transfer with SMTP}
48 Today most of the email is transfered using the \name{Simple Mail Transfer Protocol} (short: \SMTP), which is defined in \RFC821 and the successors \RFC2821 and \RFC5321. A good entry point for further information is \citeweb{wikipedia:smtp}.
50 A selection of important concepts of \SMTP\ is explained here.
52 First the \name{store and forward} transfer concept. This means mail messages are sent from \MTA\ to \MTA, until the final \MTA\ (the one which is responsible for the recipient) is reached. The message is gets stored for some time on each \MTA, until it is forwarded to the next \MTA.
54 This leads to the concept of \name{responsibility}. A mail message is always in the responsibility of one system. First it is the \NAME{MUA}. After it was transfered to the first \MTA, it takes the responsibility for the message over. The \NAME{MUA} can then delete its copy of the message. This is the same for each transfer, from \MTA\ to \MTA\ and finally from \MTA\ to the \NAME{MDA}, the message gets transfered and if the transfer was successful, the responsibility for the message is transfered as well. The responsibility chain ends at a user's mailbox, where he himself has control on the message.
56 A third concept is about failure handling. At any step on the way, an \MTA\ may receive a message it is unable to handle. In such a case, this receiving \MTA\ will \name{reject} the message before it takes responsibility for it. The sending \MTA\ still has responsibility for the message and may try other ways for sending the message. If none succeeds, the \MTA\ will send a \name{bounce message} back to the original sender with information on the type of failure. Bounces are only sent if the failure is expected to be permanent, or if the transfer still was unsuccessful after many tries.
60 \subsubsection{Mail messages}
62 Mail messages consist of two parts with defined format. This format is specified in \RFC822, and the successors \RFC2822 and \RFC5322.
64 The two parts of a message are the \name{header} and the \name{body}. The header of an email message is similar to the header of a (formal) letter. It spans the first lines of the message up to the first empty line. The header consists of several lines, called \name{header lines} or simply \name{headers}. They specify the sender, the address(es) of the recipient(s), the date, and possibly further information. Their order is irrelevant. Headers are named after the colon separated start of those lines, for example the ``\texttt{Date:}'' header. A user may write the header himself, but normally the \NAME{MUA} does this job.
66 The body is the payload of the message. It is under full control of the user. From the view point of the \SMTP\ protocol, it must consist of only 7-bit \NAME{ASCII} text. But arbitrary content can be included by encoding it to 7-bit \NAME{ASCII}. \NAME{MIME} is the common \SMTP\ extension to handle such convertion automatically in \NAME{MUA}s.
68 Following is a sample mail message with four header lines (\texttt{From:}, \texttt{To:}, \texttt{Date:}, and \texttt{Subject:}) and three lines of message body.
70 \input{input/sample-email.txt}
72 Email messages are put into envelopes for transfer. This concept is derived from the real world, so it is easy to understand. The envelope is what is used to route the message from sender to recipient. It contains the sender's address and addresses of one or more recipients. Envelopes are generated by \MTA{}s, usually by using mail header data. The user has not to deal with them.
74 The sample message would would lead to two envelopes, one from \name{markus@host01} to \name{alice@host02}, the other from \name{markus@host01} to \name{bob@host03}. Both envelopes would contain the same message. There is no difference to how it would be done for snail mail.
81 \section{The \masqmail\ project}
82 \label{sec:masqmail}
84 << about masqmail (some history) >>
86 (include history of email, definition of MTA and sendmail-compatibility in text)
88 The \masqmail\ program was written by \person{Oliver Kurth}, starting in 1999. His aim was to create a small \mta\ which is especially focused on computers with dial-up connections to the internet. \masqmail\ is easy configurable for situations which are rarely solveable with the common \MTA{}s.
90 \masqmail\ queues mail for destinations outside the local network if no connection to the internet is online. If the machine goes online, this mail is sent. Mail to local machines is sent immediately.
92 While the other \MTA{}s are more general purpose \MTA{}s, \masqmail\ aims on special situations only. Nevertheless can it handle ordinary mail transfers too.
94 \masqmail\ is released under the \GPL, which makes it \freesw. The latest stable version is 0.2.21 from November 2005.
96 The program's new homepage \citeweb{masqmail:homepage} provides further information about this \MTA.
98 << specify the really important external documents here >> %FIXME
102 \subsection{Target field / When to use \masqmail}
104 Its original author, \person{Oliver Kurth}, sees \masqmail\ so:
105 \begin{quote}
106 MasqMail is a mail server designed for hosts that do not have a permanent internet connection eg. a home network or a single host at home. It has special support for connections to different ISPs. It replaces sendmail or other MTAs such as qmail or exim.
107 \end{quote}
109 \masqmail\ is inteded to cover a specific niche: non-permanent internet connection and different \NAME{ISP}s.
111 Although it can basically replace other \MTA{}s, it is not generally aimed to do so. The package description of \debian\citeweb{packages.debian:masqmail} states this more clearly by changing the last sentence to:
112 \begin{quote}
113 In these cases, MasqMail is a slim replacement for full-blown MTAs such as sendmail, exim, qmail or postfix.
114 \end{quote}
115 \masqmail\ is a good replacement ``in these cases'', but not generally, since is lacks features essential for running on mail servers. It is primarily not secure enough for being accessable from untrusted locations.
117 The program is best used in home networks, which are non-permanently connected to the internet. \masqmail\ sends mail to local destinations, like users on the same machine and on other machines in the local net, immediately. Email to recipients outside the local net are queued when offline and sent when a online connection gets established.
119 Further more does \masqmail\ respect online connections through different \NAME{ISP}s; a common thing for dial-up connections. In particular can different sender addresses be set, dependent on the \NAME{ISP} that is used. This prevents mail to be likely classified as spam.
124 \subsubsection*{\masqmail's main goal}
126 \masqmail\ does have similar requirements, by being a \sendmail\ replacement, which is a basic goal of the project. The main difference is that \masqmail\ is intended to be used on workstations and in small networks, but \sendmail, \qmail, and \postfix\ are designed to run on large mail servers to handle masses of email. The author of \masqmail, \person{Kurth}, in contrast, warns on the old project's website \citeweb{masqmail:homepage2} about using it to accept connections from the Internet, because of the risk to be an open relay:
127 \begin{quote}
128 MasqMail is not designed to run on a host with a permanent internet connection. It does not have the ability to check for spam mail and it will relay everything from everywhere to everywhere. Use another mail server such as exim for permanent connections.
129 \end{quote}
130 Even if some relay control will be added, ``is not designed to'' is a clear indicator for being careful. Issues like high memory consumption, low performance, and denial-of-service attacks---things not regarded by design---may cause serious problems.
132 Here shows a misfit off: On the one hand does \masqmail\ want to be a \sendmail\ replacement. But on the other hand, is it not designed to be used like \sendmail. If \masqmail\ is inteded to replace other \MTA{}s, then one may replace another one with it. Hence it must be secure enough. It either needs the security features or must drop the unsecure funtionality. The second option, however, leads to being \emph{no} replacement for other \MTA{}s. It is a valid decision to not be a replacement for \sendmail\ or thelike, but this is a design decision---the change of a primary goal.
134 If \masqmail\ should be an \MTA\ to replace others, a switch to a better suited architecture that provides good security and extendability by design, seems required. But if \masqmail\ is wanted to cover some special jobs, not to replace common \MTA{}s, then its architecture depends on the special requirements of the specific job; \MTA\ architectures, like discussed by \person{Hafiz}, may be inadequate.
137 \subsubsection*{Full featured or stripped down}
139 What future is to choose for \masqmail---one to be a full featured \MTA, or one to be a stipped down \MTA\ for special jobs?
141 The critical point to discuss upon is surely the listening on a port to accepte messages from outside via \NAME{SMTP} (herafter also refered to as the \NAME{SMTP}-in channel). This feature is required for an \MTA\ to be a \name{smart host}, to relay mail. But running as deamon and listening on a port requires much more security effort, because the program is put in direct contact with attackers and other bad guys.
143 \MTA{}s without \SMTP-in channels can not receive mail from arbitrary outside hosts. They are only invoked by local users. This lowers the security need a lot---however, security is a general goal and still required, but on a lower level. Unfortunately, as they do not receive mail anymore (except by local submission), they are just better \name{forwarders} that are able to send mail directly to the destination.
145 This is not what \masqmail\ was intended to be. Programs that cover this purpose are available; one is \name{msmtp}.
147 \masqmail\ shall be a complete \mta. It shall be able to replace ones like \sendmail.
151 \subsubsection*{Typical usage}
152 This section describes situations that make senseful use of \masqmail.
154 A home network consisting of some workstations without a server. The network is connected to the internet by dial-up or broadband. Going online is initiated by computers inside the local net. \NAME{IP} addresses change at least once every day.
156 Every workstation would be equiped with \masqmail. Mail transfer within the same machine or within the local net works straight forward. Outgoing mail to the internet is sent, to the concerning \NAME{ISP} for relaying, whenever the router goes online. Receiving of mail from outside needs to be done by a mail fetch program, like the \masqmail\ internal \NAME{POP3} client or \name{fetchmail} for example. The configuration for \masqmail\ would be the same on every computer, except the hostname.
158 For the same network but having a server, one could have \masqmail\ running on the server and using simple forwarders (see \ref{subsec:relay-only}) to the server on the workstations. This setup does only support mail transfer to the server, but not back to a workstation; also sending mail to another user on the same workstation is not possible.
160 A better setup is to run \masqmail\ on every machine %FIXME
167 \subsection{When not to use \masqmail}
169 ...
180 \subsection{Features}
182 Here regarded is version 0.2.21 of \masqmail. This is the last version released by \person{Oliver Kurth}, and the basis for my thesis.
185 \subsubsection*{The source code}
187 \masqmail\ is written in the C programming language. The program, as of version 0.2.21, consists of 34 source code and eight header files, containing about 9,000 lines of code\footnote{Measured with \name{sloccount} by David A.\ Wheeler.}. Additionally, it includes a \name{base64} implementation (about 300 lines) and \name{md5} code (about 150 lines). For systems that do not provide \name{libident}, this library is distributed as well (circa 600 lines); an available shared library has higher precedence in linking, though.
189 The only mandatory dependency is \name{glib}---a cross-platform software utility library, originated in the \NAME{GTK+} project. It provides safe replacements for many standard library functions, especially for the string functions. It also offers handy data containers, easy-to-use implementations of data structures, and much more.
191 With \masqmail\ comes the small tool \path{mservdetect}; it helps setting up a configuration that uses the \name{mserver} system to detect the online state. Two other binaries get compiled for testing purposes: \path{readtest} and \path{smtpsend}. All three programs use \masqmail\ source code; they only add a file with a \verb+main()+ function each.
195 \subsubsection*{Features}
196 \label{sec:masqmail-features}
198 \masqmail\ supports two channels for incoming mail: (1) Standard input, used when \path{masqmail} is executed on the command line and (2) a \NAME{TCP} socket, used by local or remote clients that talk \SMTP. The outgoing channels for mail are: (1) direct delivery to local mailboxes (in \name{mbox} or \name{maildir} format), (2) local pipes to pass mail to a program (e.g.\ gateways to \NAME{UUCP}, gateways to fax, or \NAME{MDA}s), and (3) \NAME{TCP} sockets to transfer mail to other \MTA{}s using the \SMTP\ protocol.
200 Outgoing \SMTP\ connections feature \SMTP-\NAME{AUTH} and \SMTP-after-\NAME{POP} authentication, but incoming connections do not. Using wrappers for outgoing connections is supported. This allows encrypted communication through a gateway application like \name{openssl}.
202 Mail queuing and alias expansion is both supported.
204 \masqmail\ focuses on non-permanent online connections, thus a concept of online routes is used. One may configure any number of routes to send mail. Each route can have criteria to determine if some message is allowed to be sent over it. This concept is explained in section \ref{sec:masqmail-routes} in detail. Mail to destinations outside the local network gets queued until an online connections is available.
206 The \masqmail\ executable can be called under various names for sendmail-compatibility reasons. This is organized by symbolic links with different names pointing to the \masqmail\ executable. The \sendmail\ names are \path{/usr/lib/sendmail} and \path{/usr/sbin/sendmail} because many programs expect the \mta\ to be located there. Further more \sendmail\ supports calling it with a different name instead of supplying command line arguments. The best known of this shortcuts is \path{mailq}, which is equivalent to calling it with the argument \verb+-bq+. \masqmail\ recognizes the shortcuts \path{mailq}, \path{smtpd}, \path{mailrm}, \path{runq}, \path{rmail}, and \path{in.smtpd}. The first two are inspired by \sendmail. Not implemented is the shortcut \path{newaliases} because \masqmail\ does not generate binary representations of the alias file.\footnote{A shell script named \path{newaliases}, that invokes \texttt{masqmail -bi}, can provide the command to satisfy other software needing it.} \path{hoststat} and \path{purgestat} are missing for complete sendmail-compatibility.
207 %masqmail: mailq, mailrm, runq, rmail, smtpd/in.smtpd
208 %sendmail: hoststat, mailq, newaliases, purgestat, smtpd
210 Additional to the \mta\ job, \masqmail\ also offers mail retrieval services by being a \NAME{POP3} client. It can fetch mail from different remote locations, dependent on the active online connection.
214 \subsubsection*{Online detection and routes}
215 \label{sec:masqmail-routes}
217 ---
219 As \masqmail\ is focused on non-permanent Internet connections, online state can be queried by three methods: reading from a file, reading the output of a command, or by asking an \name{mserver}. Each method may return a string indicating one of the available routes being online, or returning nothing to indicate offline state.
221 Delivery to recipients on the local host or in local nets is done at once; delivery to recipients on the Internet is only done when being online, and queued otherwise. Each online route may have a different mail server to which mail is relayed. Return address headers are modified appropriate if wished.
223 ---
225 \masqmail\ focuses on non-permanent online connections, thus a concept of online routes is used. One may configure any number of routes to send mail. Each route can have criteria, like matching \texttt{From:} or \texttt{To:} headers, to determine if some message is allowed to be sent over it. Mail to destinations outside the local network gets queued until an online connections is available.
236 \section{Why \masqmail?}
238 As main advantage, \masqmail\ makes it easy to set up an \MTA\ on workstations or notebooks without the need to do complex configuration or to be an mail server expert.
240 Workstations use %FIXME
242 \textbf{Alternatives?}
243 http://anfi.homeunix.org/sendmail/dialup10.html
246 << explain why masqmail is old and why it is interesting/important however! >>
248 << why is it worth to revive masqmail? >>
256 \section{Problems to solve}
258 << what problems has masqmail? >>
260 << what's the intention of this document? >>
262 << why is it worth the effort? >>
268 \section{Delimitation}
270 << limit against stuff not covered here >>