docs/diploma

annotate thesis/tex/1-Introduction.tex @ 260:4931d56b61ea

rework in ch01
author meillo@marmaro.de
date Wed, 14 Jan 2009 12:31:52 +0100
parents f4966e84815d
children b390fb627f10
rev   line source
meillo@26 1 \chapter{Introduction}
meillo@42 2 \label{chap:introduction}
meillo@26 3
meillo@96 4 << say what you want to say >>
meillo@92 5
meillo@102 6 << the overall goal of the document >>
meillo@92 7
meillo@92 8
meillo@92 9
meillo@229 10
meillo@229 11
meillo@245 12
meillo@245 13 \section{Email prerequisites}
meillo@245 14
meillo@245 15 email and everything is defined in RFCs
meillo@245 16
meillo@245 17
meillo@245 18 \subsubsection{Mail agents}
meillo@245 19
meillo@260 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.
meillo@253 21
meillo@245 22 \paragraph{MTA}
meillo@260 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}.
meillo@245 24
meillo@245 25 \paragraph{MUA}
meillo@260 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}.
meillo@245 27
meillo@245 28 \paragraph{MDA}
meillo@253 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.
meillo@245 30
meillo@253 31 \begin{figure}
meillo@253 32 \begin{center}
meillo@253 33 \includegraphics[scale=0.75]{img/mail-agents.eps}
meillo@253 34 \end{center}
meillo@253 35 \caption{Mail agents and the way a mail message takes}
meillo@253 36 \label{fig:mail-agents}
meillo@253 37 \end{figure}
meillo@245 38
meillo@229 39
meillo@253 40
meillo@229 41
meillo@229 42
meillo@229 43
meillo@245 44 \subsubsection{Mail transfer with SMTP}
meillo@245 45
meillo@245 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}.
meillo@245 47
meillo@245 48 A selection of important concepts of \SMTP\ is explained here.
meillo@245 49
meillo@253 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.
meillo@245 51
meillo@253 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.
meillo@245 53
meillo@253 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.
meillo@245 55
meillo@245 56
meillo@245 57
meillo@245 58 \subsubsection{Mail messages}
meillo@245 59
meillo@253 60 Mail messages consist of two parts with defined format. This format is specified in \RFC822, and the successors \RFC2822 and \RFC5322.
meillo@245 61
meillo@253 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.
meillo@245 63
meillo@253 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.
meillo@245 65
meillo@253 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.
meillo@245 67
meillo@260 68 \begin{quote}
meillo@245 69 \input{input/sample-email.txt}
meillo@260 70 \end{quote}
meillo@245 71
meillo@260 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 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.
meillo@253 73
meillo@260 74 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.
meillo@260 75
meillo@260 76 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.
meillo@245 77
meillo@245 78
meillo@245 79
meillo@245 80
meillo@229 81
meillo@229 82
meillo@92 83 \section{The \masqmail\ project}
meillo@102 84 \label{sec:masqmail}
meillo@96 85
meillo@260 86 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.
meillo@96 87
meillo@260 88 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.
meillo@96 89
meillo@257 90 \masqmail\ is covered by the \name{General Public License} (short: \GPL), which qualifies it as \freesw.
meillo@102 91
meillo@257 92 \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.
meillo@102 93
meillo@260 94 The program's new homepage \citeweb{masqmail:homepage} is a collection of available information about this \MTA.
meillo@102 95
meillo@102 96
meillo@96 97
meillo@92 98
meillo@257 99 \subsection{Target field of \masqmail}
meillo@245 100
meillo@257 101 The intention \person{Kurth} had when creating \masqmail\ is best told in his own words:
meillo@92 102 \begin{quote}
meillo@92 103 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.
meillo@257 104 \hfill\citeweb{masqmail:homepage2}
meillo@257 105 \end{quote}
meillo@257 106 It is inteded to cover a specific niche: non-permanent internet connection and different \NAME{ISP}s.
meillo@257 107
meillo@257 108 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:
meillo@257 109 \begin{quote}
meillo@257 110 In these cases, MasqMail is a slim replacement for full-blown MTAs such as sendmail, exim, qmail or postfix.
meillo@257 111 \hfill\citeweb{packages.debian:masqmail}
meillo@257 112 \end{quote}
meillo@257 113 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.
meillo@257 114
meillo@257 115 \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''?
meillo@257 116
meillo@257 117 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.
meillo@257 118
meillo@257 119 \masqmail\ is designed to run on workstations and on servers in small networks, like home networks.
meillo@257 120
meillo@257 121
meillo@257 122
meillo@260 123 \subsubsection*{Typical usage scenarios}
meillo@257 124
meillo@260 125 This section describes two common setups that makes senseful use of \masqmail. They are shown in figure \ref{fig:masqmail-typical-usage}.
meillo@257 126
meillo@260 127 \paragraph{Scenario 1}
meillo@260 128 Imagine a home network consisting of some workstations without a server. The network is connected to the Internet.
meillo@257 129
meillo@260 130 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.
meillo@260 131 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
meillo@257 132
meillo@260 133 \paragraph{Scenario 2}
meillo@260 134 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
meillo@257 135
meillo@257 136 \begin{figure}
meillo@257 137 \begin{center}
meillo@257 138 \includegraphics[scale=0.75]{img/masqmail-typical-usage.eps}
meillo@257 139 \end{center}
meillo@257 140 \caption{Typical usage scenarios for \masqmail}
meillo@257 141 \label{fig:masqmail-typical-usage}
meillo@257 142 \end{figure}
meillo@257 143
meillo@260 144 %fixme: what about notebooks?
meillo@260 145 << notebooks >>
meillo@260 146
meillo@260 147 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.
meillo@257 148
meillo@257 149
meillo@257 150
meillo@257 151
meillo@257 152 \subsubsection*{Limitations}
meillo@257 153
meillo@260 154 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.
meillo@257 155
meillo@260 156 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:
meillo@257 157
meillo@257 158 \begin{quote}
meillo@257 159 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.
meillo@257 160 \hfill\citeweb{masqmail:homepage2}
meillo@92 161 \end{quote}
meillo@92 162
meillo@260 163 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.
meillo@160 164
meillo@257 165 Hence, \masqmail\ should not be used for permanent internet connections. Or at least it needs to be secured with care.
meillo@160 166
meillo@160 167
meillo@160 168
meillo@245 169
meillo@245 170
meillo@245 171
meillo@245 172
meillo@245 173
meillo@245 174
meillo@245 175
meillo@245 176
meillo@245 177
meillo@245 178 \subsection{Features}
meillo@238 179
meillo@248 180 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.
meillo@238 181
meillo@238 182
meillo@238 183 \subsubsection*{The source code}
meillo@238 184
meillo@238 185 \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.
meillo@238 186
meillo@238 187 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.
meillo@238 188
meillo@260 189 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.
meillo@260 190
meillo@260 191 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 parts of \masqmail's source code; they only add a file with a \verb+main()+ function each.
meillo@238 192
meillo@238 193
meillo@238 194
meillo@238 195 \subsubsection*{Features}
meillo@238 196 \label{sec:masqmail-features}
meillo@238 197
meillo@260 198 \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. Figure \ref{fig:masqmail-channels} shows this as a picture. (The ``online state'' input is explained a bit later.)
meillo@260 199
meillo@260 200 \begin{figure}
meillo@260 201 \begin{center}
meillo@260 202 \includegraphics[scale=0.75]{img/masqmail-channels.eps}
meillo@260 203 \end{center}
meillo@260 204 \caption{Incoming and outgoing channels of \masqmail}
meillo@260 205 \label{fig:masqmail-channels}
meillo@260 206 \end{figure}
meillo@238 207
meillo@238 208 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}.
meillo@238 209
meillo@238 210 Mail queuing and alias expansion is both supported.
meillo@238 211
meillo@260 212 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.
meillo@238 213 %masqmail: mailq, mailrm, runq, rmail, smtpd/in.smtpd
meillo@238 214 %sendmail: hoststat, mailq, newaliases, purgestat, smtpd
meillo@238 215
meillo@238 216 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.
meillo@238 217
meillo@238 218
meillo@238 219
meillo@245 220 \subsubsection*{Online detection and routes}
meillo@245 221 \label{sec:masqmail-routes}
meillo@238 222
meillo@260 223 \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.
meillo@260 224
meillo@257 225 \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.
meillo@257 226
meillo@257 227 \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.
meillo@257 228
meillo@257 229 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.
meillo@245 230 ---
meillo@238 231
meillo@245 232 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.
meillo@238 233
meillo@245 234 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.
meillo@238 235
meillo@245 236 ---
meillo@238 237
meillo@245 238 \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.
meillo@238 239
meillo@238 240
meillo@245 241
meillo@245 242
meillo@245 243
meillo@245 244
meillo@245 245
meillo@245 246
meillo@245 247
meillo@245 248
meillo@245 249 \section{Why \masqmail?}
meillo@92 250
meillo@92 251 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.
meillo@92 252
meillo@92 253 Workstations use %FIXME
meillo@92 254
meillo@96 255 \textbf{Alternatives?}
meillo@245 256 http://anfi.homeunix.org/sendmail/dialup10.html
meillo@92 257
meillo@92 258
meillo@245 259 << explain why masqmail is old and why it is interesting/important however! >>
meillo@96 260
meillo@175 261 << why is it worth to revive masqmail? >>
meillo@175 262
meillo@96 263
meillo@96 264
meillo@245 265
meillo@245 266
meillo@245 267
meillo@245 268
meillo@92 269 \section{Problems to solve}
meillo@92 270
meillo@245 271 << what problems has masqmail? >>
meillo@96 272
meillo@245 273 << what's the intention of this document? >>
meillo@96 274
meillo@245 275 << why is it worth the effort? >>
meillo@96 276
meillo@96 277
meillo@245 278
meillo@245 279
meillo@245 280
meillo@245 281 \section{Delimitation}
meillo@96 282
meillo@150 283 << limit against stuff not covered here >>
meillo@96 284
meillo@260 285 pop3 stuff of masqmail is not regarded.
meillo@96 286
meillo@96 287
meillo@150 288