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| author | meillo@marmaro.de |
|---|---|
| date | Wed, 14 Jan 2009 21:39:50 +0100 |
| parents | b390fb627f10 |
| children | ddfb228a62a4 |
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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 that are the nodes of the email infrastructure. Here they are explained with references to the snail mail system which is known from everyday's life. Figure \ref{fig:mail-agents} shows the relation between those three mail agents and the way an email message takes trough the system.
22 \paragraph{MTA}
23 \name{Mail Tranfer Agents} are the post offices for electronic mail. The basic job of an \MTA\ is to transport mail from senders to recipients, or more pedantic: from \MTA\ to \MTA. \sendmail, \exim, \qmail, \postfix, and of course \masqmail\ are \MTA{}s. \MTA{}s are explained in more detail in chapter \ref{chap:mail-transfer-agents}.
25 \paragraph{MUA}
26 \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. Also 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}.
28 \paragraph{MDA}
29 \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.
31 \begin{figure}
32 \begin{center}
33 \includegraphics[scale=0.75]{img/mail-agents.eps}
34 \end{center}
35 \caption{Mail agents and the way a mail message takes}
36 \label{fig:mail-agents}
37 \end{figure}
44 \subsubsection{Mail transfer with SMTP}
46 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}.
48 A selection of important concepts of \SMTP\ is explained here.
50 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.
52 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.
54 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.
58 \subsubsection{Mail messages}
60 Mail messages consist of two parts with defined format. This format is specified in \RFC822, and the successors \RFC2822 and \RFC5322.
62 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.
64 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.
66 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.
68 \begin{quote}
69 \footnotesize
70 \input{input/sample-email.txt}
71 \end{quote}
73 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 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.
75 Each \MTA\ on the way reads envelopes it receives and generates new ones. If a message has recipients on different hosts, then the message gets copied and sent within multiple envelopes, one for each host.
77 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.
84 \section{The \masqmail\ project}
85 \label{sec:masqmail}
87 The \masqmail\ project was by \person{Oliver Kurth} in 1999. His aim was to create a small \MTA\ that is especially focused on computers with dial-up Internet connections. Throughout the next four years, he worked steadily on it, releasing new versions every few weeks. In total it were 53 releases, which is in average a new version every 20 days.
89 This thesis bases on the latest release of \masqmail---version 0.2.21 from November 2005. It was released after a 28 month gap. The source code of 0.2.21 is the same as of 0.2.20, only build documents were modified. The release tarball can be retrieved from the \debian\ package pool\footnote{The \NAME{URL} is: \url{http://ftp.de.debian.org/debian/pool/main/m/masqmail/masqmail\_0.2.21.orig.tar.gz}\,.} \citeweb{debian:packages}. Probably was only put into public in the \debian\ pool because \masqmail's homepage \citeweb{masqmail:homepage2} does not include it.
91 \masqmail\ is covered by the \name{General Public License} (short: \GPL), which qualifies it as \freesw.
93 \person{Kurth} abandonned \masqmail\ after 2005, and no one addopted the project since then. Thus, the author of this thesis decided to take responsibility for \masqmail\ now. He received \person{Kurth}'s permission to do so.
95 The program's new homepage \citeweb{masqmail:homepage} is a collection of available information about this \MTA.
100 \subsection{Target field of \masqmail}
101 \label{sec:masqmail-target-field}
103 The intention \person{Kurth} had when creating \masqmail\ is best told in his own words:
104 \begin{quote}
105 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.
106 \hfill\citeweb{masqmail:homepage2}
107 \end{quote}
108 It is inteded to cover a specific niche: non-permanent internet connection and different \NAME{ISP}s.
110 Although it can basically replace other \MTA{}s, it is not \emph{generally} aimed to do so. The package description of \debian\ states this more clearly by changing the last sentence to:
111 \begin{quote}
112 In these cases, MasqMail is a slim replacement for full-blown MTAs such as sendmail, exim, qmail or postfix.
113 \hfill\citeweb{packages.debian:masqmail}
114 \end{quote}
115 The program is a good replacement ``in these cases'', but not generally, since is lacks essential features for running on mail servers. It is primarily not secure enough for being accessable from untrusted locations.
117 \masqmail\ is best used in home networks, which are non-permanently connected to the Internet. It is easy configurable for situations which are rarely solveable with the common \MTA{}s. Such include different handling of mail to local or remote destination and respecting different routes of being online connection. These features are explained in more detail in the following \name{Features} section on page \ref{sec:masqmail-features}. %fixme: is it still called ``features''?
119 While many other \MTA{}s are general purpose \MTA{}s, \masqmail\ aims on special situations. Nevertheless, it can be used as general purpose \MTA, too. Especially this was a design goal of \masqmail: To be a replacement for \sendmail, or similar well known \MTA{}s.
121 \masqmail\ is designed to run on workstations and on servers in small networks, like home networks.
125 \subsubsection*{Typical usage scenarios}
127 This section describes two common setups that makes senseful use of \masqmail. They are shown in figure \ref{fig:masqmail-typical-usage}.
129 \paragraph{Scenario 1}
130 Imagine a home network consisting of some workstations without a server. The network is connected to the Internet.
132 Every workstation would be equiped with \masqmail. Mail transfer within the same machine or within the local net works straight forward using direkt transfer. Outgoing mail to the internet is sent, to an \name{Internet Service Provider} (short: \NAME{ISP}) for relaying whenever the router goes online. The configuration of \masqmail\ would be the same on every computer, except different hostnames.
133 To receive mail from the Internet requires a mailbox on the \NAME{ISP}'s mail server. Mail needs to be fetched from the \NAME{ISP}'s server onto the workstation. % pop3/imap
135 \paragraph{Scenario 2}
136 In the same network but with a server, one could have \masqmail\ running on the server and using simple forwarders (see \ref{subsec:relay-only}) on the workstations to tranfer mail to the server. The server would then, dependent on the desination of the message, deliver locally or relay to an \NAME{ISP}'s server for further relay. This setup does only support mail transfer to the server, but not back to a workstation. However, it can be solved by mounting the users mailbox from the server to the workstation, or by making the workstations fetch mail from the server. % pop3/imap
138 \begin{figure}
139 \begin{center}
140 \includegraphics[scale=0.75]{img/masqmail-typical-usage.eps}
141 \end{center}
142 \caption{Typical usage scenarios for \masqmail}
143 \label{fig:masqmail-typical-usage}
144 \end{figure}
146 %fixme: what about notebooks?
147 << notebooks >>
149 In general, all kinds of usage scenarios within a trusted network are possible. Important to notice is that mail can not be send from outside into the local network. This limitation leads to the next section.
154 \subsubsection*{Limitations}
156 Although \masqmail\ is seen as a replacement for other general purpose \MTA{}s, it should not be used on large mail servers. The reasons are that it implements only a basic subset of features, and that its performance and security are not as needed for such usage.
158 The author, \person{Kurth}, warns on the old project's website about using \masqmail\ to accept connections from the Internet, because of the risk of being an open relay:
160 \begin{quote}
161 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.
162 \hfill\citeweb{masqmail:homepage2}
163 \end{quote}
165 The actual problem is not the permanent Internet connection, but listening for incomming mail on it. If a firewall is closed for incoming mail, then the permanent Internet connection is no problem.
167 Hence, \masqmail\ should not be used for permanent internet connections. Or at least it needs to be secured with care.
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 Some functionality of \masqmail\ can be included or excluded at compile time by defining symbols. To enable maildir support for example, one has to add \verb_--enable-maildir_ to the configure call. Otherwise the concerning code gets removed during preprocessing.
193 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 parts of \masqmail's source code; they only add a file with a \verb+main()+ function each.
197 \subsubsection*{Features}
198 \label{sec:masqmail-features}
200 \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. Figure \ref{fig:masqmail-channels} shows this as a picture. (The ``online state'' input is explained a bit later.)
202 \begin{figure}
203 \begin{center}
204 \includegraphics[scale=0.75]{img/masqmail-channels.eps}
205 \end{center}
206 \caption{Incoming and outgoing channels of \masqmail}
207 \label{fig:masqmail-channels}
208 \end{figure}
210 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}.
212 Mail queuing and alias expansion is both supported.
214 The \masqmail\ executable can be called under various names for sendmail-compatibility reasons (see section \ref{sec:sendmail-compat}). 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.
215 %masqmail: mailq, mailrm, runq, rmail, smtpd/in.smtpd
216 %sendmail: hoststat, mailq, newaliases, purgestat, smtpd
218 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.
222 \subsubsection*{Online detection and routes}
223 \label{sec:masqmail-routes}
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 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.
227 \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.
229 \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.
231 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.
232 ---
234 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.
236 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.
238 ---
240 \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.
251 \section{Why \masqmail?}
253 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.
255 Workstations use %FIXME
257 \textbf{Alternatives?}
258 http://anfi.homeunix.org/sendmail/dialup10.html
261 << explain why masqmail is old and why it is interesting/important however! >>
263 << why is it worth to revive masqmail? >>
271 \section{Problems to solve}
273 << what problems has masqmail? >>
275 << what's the intention of this document? >>
277 << why is it worth the effort? >>
283 \section{Delimitation}
285 << limit against stuff not covered here >>
287 pop3 stuff of masqmail is not regarded.