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author meillo@marmaro.de
date Thu, 15 Jan 2009 15:47:13 +0100
parents ddfb228a62a4
children 591217f50f69
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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 >>
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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@269 22 \begin{description}
meillo@269 23 \item[\MTA:]
meillo@260 24 \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 25
meillo@269 26 \item[\NAME{MUA}:]
meillo@260 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. 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 28
meillo@269 29 \item[\NAME{MDA}:]
meillo@253 30 \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@269 31 \end{description}
meillo@245 32
meillo@253 33 \begin{figure}
meillo@253 34 \begin{center}
meillo@253 35 \includegraphics[scale=0.75]{img/mail-agents.eps}
meillo@253 36 \end{center}
meillo@253 37 \caption{Mail agents and the way a mail message takes}
meillo@253 38 \label{fig:mail-agents}
meillo@253 39 \end{figure}
meillo@245 40
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meillo@229 45
meillo@245 46 \subsubsection{Mail transfer with SMTP}
meillo@245 47
meillo@245 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}.
meillo@245 49
meillo@245 50 A selection of important concepts of \SMTP\ is explained here.
meillo@245 51
meillo@253 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.
meillo@245 53
meillo@253 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.
meillo@245 55
meillo@253 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.
meillo@245 57
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meillo@245 59
meillo@245 60 \subsubsection{Mail messages}
meillo@245 61
meillo@253 62 Mail messages consist of two parts with defined format. This format is specified in \RFC822, and the successors \RFC2822 and \RFC5322.
meillo@245 63
meillo@253 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.
meillo@245 65
meillo@253 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.
meillo@245 67
meillo@253 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.
meillo@245 69
meillo@269 70 \codeinput{input/sample-email.txt}
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.
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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
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meillo@257 99 \subsection{Target field of \masqmail}
meillo@266 100 \label{sec:masqmail-target-field}
meillo@245 101
meillo@257 102 The intention \person{Kurth} had when creating \masqmail\ is best told in his own words:
meillo@92 103 \begin{quote}
meillo@92 104 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@257 105 \hfill\citeweb{masqmail:homepage2}
meillo@257 106 \end{quote}
meillo@257 107 It is inteded to cover a specific niche: non-permanent internet connection and different \NAME{ISP}s.
meillo@257 108
meillo@257 109 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@257 110 \begin{quote}
meillo@257 111 In these cases, MasqMail is a slim replacement for full-blown MTAs such as sendmail, exim, qmail or postfix.
meillo@257 112 \hfill\citeweb{packages.debian:masqmail}
meillo@257 113 \end{quote}
meillo@257 114 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@257 115
meillo@257 116 \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@257 117
meillo@257 118 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@257 119
meillo@257 120 \masqmail\ is designed to run on workstations and on servers in small networks, like home networks.
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meillo@257 123
meillo@260 124 \subsubsection*{Typical usage scenarios}
meillo@257 125
meillo@269 126 This section describes three common setups that makes senseful use of \masqmail. The first two are shown in figure \ref{fig:masqmail-typical-usage}.
meillo@257 127
meillo@257 128 \begin{figure}
meillo@257 129 \begin{center}
meillo@257 130 \includegraphics[scale=0.75]{img/masqmail-typical-usage.eps}
meillo@257 131 \end{center}
meillo@257 132 \caption{Typical usage scenarios for \masqmail}
meillo@257 133 \label{fig:masqmail-typical-usage}
meillo@257 134 \end{figure}
meillo@257 135
meillo@269 136 Imagine a home network consisting of some workstations which is connected to the Internet.
meillo@260 137
meillo@269 138 \begin{description}
meillo@269 139 \item[Scenario 1:]
meillo@269 140 If no server is present, 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@269 141 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 using the \NAME{POP3} or \NAME{IMAP} protocol.
meillo@269 142
meillo@269 143 \item[Scenario 2:]
meillo@269 144 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 using the \NAME{POP3} or \NAME{IMAP} protocol to fetch the mail in the server's mailbox from the workstations. Mail transfer from the \NAME{ISP} to the local server needs \NAME{POP3} or \NAME{IMAP} as well.
meillo@269 145
meillo@269 146 \item[Scenario 3:]
meillo@269 147 A third scenario is unrelated as it is about notebooks. Notebooks are usually used as mobile workstations. One uses them to work at different locations. With the increasing popularity of wireless networks this gets more and more common. Different networks have different setups: In one network it is best to send mail to an \NAME{ISP} for relay. In another network it might be prefered to use a local mail server. A third network may have no Internet access at all, hence using a local mail server is required. All these different setups can be configured once and then used by simply telling the online state to \masqmail, even automatically within a network setup script.
meillo@269 148 \end{description}
meillo@269 149
meillo@269 150
meillo@269 151 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 trusted network then. For using \masqmail\ on notebooks it is suggested to only accept mail from local users, because notebooks are often in untrusted environments. This limitation leads to the next section.
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meillo@257 156 \subsubsection*{Limitations}
meillo@257 157
meillo@260 158 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@257 159
meillo@260 160 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@257 161
meillo@257 162 \begin{quote}
meillo@257 163 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@257 164 \hfill\citeweb{masqmail:homepage2}
meillo@92 165 \end{quote}
meillo@92 166
meillo@269 167 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. \masqmail\ should not be used for permanent internet connections. Or at least it needs to be secured with care.
meillo@160 168
meillo@269 169 The Internet is the common example for an untrusted network, but this applies to any other untrusted network too.
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meillo@245 182 \subsection{Features}
meillo@238 183
meillo@248 184 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@238 185
meillo@238 186
meillo@238 187 \subsubsection*{The source code}
meillo@238 188
meillo@238 189 \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@238 190
meillo@238 191 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@238 192
meillo@260 193 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@260 194
meillo@260 195 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.
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meillo@238 199 \subsubsection*{Features}
meillo@238 200 \label{sec:masqmail-features}
meillo@238 201
meillo@260 202 \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@260 203
meillo@260 204 \begin{figure}
meillo@260 205 \begin{center}
meillo@260 206 \includegraphics[scale=0.75]{img/masqmail-channels.eps}
meillo@260 207 \end{center}
meillo@260 208 \caption{Incoming and outgoing channels of \masqmail}
meillo@260 209 \label{fig:masqmail-channels}
meillo@260 210 \end{figure}
meillo@238 211
meillo@238 212 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@238 213
meillo@275 214 Mail queuing is essential for \masqmail\ and supported of course, alias expansion is also supported.
meillo@238 215
meillo@260 216 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@238 217 %masqmail: mailq, mailrm, runq, rmail, smtpd/in.smtpd
meillo@238 218 %sendmail: hoststat, mailq, newaliases, purgestat, smtpd
meillo@238 219
meillo@238 220 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@238 221
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meillo@238 223
meillo@245 224 \subsubsection*{Online detection and routes}
meillo@245 225 \label{sec:masqmail-routes}
meillo@238 226
meillo@275 227 \masqmail\ focuses on handling different 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. Mail to destinations outside the local network gets queued until a suitable online connections is available.
meillo@260 228
meillo@275 229 The background of this concept was the send mail to the Internet while using one of a set of dial-up Internet connection from different \NAME{ISP}s. It was quite common that \NAME{ISP}s accepted mail for relay only if it came over a connection they managed. This means, one was not able to relay mail over the mail server of \NAME{ISP}\,1 while being online over the connection of \NAME{ISP}\,2. \masqmail\ is a solution to the wish of switching the relaying mail server easily.
meillo@257 230
meillo@275 231 Related is \masqmail's ability to rewrite the send's email address dependent on which \NAME{ISP} is used. This prevents mail from being likely classified as spam.
meillo@257 232
meillo@275 233 To react on the different situations, \masqmail\ needs to query the current online state. Is an online connection available, and if it is, which one? Three methods are implemented: (1) Reading from a file, (2) reading the output of a command, and (3) by asking an \name{mserver} system. Each method may return a string naming the routes that is online or returning nothing to indicate offline state.
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meillo@275 236 Mail for hosts within the local network or for users on the local machine is not touched by this concept, it is always sent immediately.
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meillo@245 244
meillo@275 245 \section{Why \masqmail\ is worthy}
meillo@245 246
meillo@275 247 First of all, \masqmail\ is better suited for its target field of operation (multiple non-permanent online connections) than every other \MTA. Especially is such usage easy to set up because \masqmail\ was designed for that.
meillo@245 248
meillo@275 249 Additionally does \masqmail\ make it easy to run an \MTA\ on workstations or notebooks. There is no need to do complex configuration or to be a mail server expert. Only a handful of options need to be set; the hostname, the local networks, and one route for relaying are sufficient in most times. %fixme: is that true?
meillo@245 250
meillo@275 251 Not to forget is \masqmail's size. It is much smaller than full-blown \MTA{}s like \sendmail, \postfix, or \exim, and still smaller than \qmail. (See section \ref{sec:mta-comparison} for details.) This makes \masqmail\ a good choice for workstations or even embedded computers.
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meillo@92 253
meillo@275 254 Although development on \masqmail\ stopped in 2003 it still has its users. Having users is alone reason enough for futher development and maintenance. Especially if the software covers a niche, and especially if requirements for such software in general change.
meillo@92 255
meillo@275 256 It is difficult to get numbers about users of Free Software, because no one needs to tell anyone when he uses some software. \debian's \name{popcon} statistics \citeweb{debian:popcon} are a try to provided numbers. The statistics report 61 \masqmail\ installations, in January 2009. If it is assumed that two third of all \debian\ users report their installed software, there would be in total around 90 \masqmail\ installations in \debian. As \masqmail\ is also distributed with \name{Ubuntu} and FIXME %fixme
meillo@275 257 , the number is incremented by the guessed number of further 30 systems with \masqmail\ on it. Including an additional amount of 20 installations on operating systems that do not ship \masqmail\ makes about 140 \masqmail\ installations in total. Of course one person may have \masqmail\ installed on more than one computer, but a total of 100 different users seems to be a good guess.
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meillo@275 259 Software that is used should be developed and maintained.
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meillo@92 261
meillo@275 262 % alternative: http://anfi.homeunix.org/sendmail/dialup10.html
meillo@96 263
meillo@275 264 %<< hikernet >>
meillo@275 265
meillo@275 266 %<< explain why masqmail is old and why it is interesting/important however! >>
meillo@275 267
meillo@275 268 %<< why is it worth to revive masqmail? >>
meillo@175 269
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meillo@92 276 \section{Problems to solve}
meillo@92 277
meillo@275 278 A program, that no one has developed further for nearly six years, that is located in a field of operation that changed during that time, surely needs improvement. Security and spam have now highly increased importance compared to 2003. Dial-up connections became rare---broadband flatrates are common now. Other \MTA{}s evolved in respect to theses changes, \masqmail\ did not.
meillo@96 279
meillo@275 280 The current and trends for a future market situation needs to be identified. Looks at other \MTA{}s need to be taken. And required work on \masqmail\ needs to be defined in combination with the evaluation of strategies to do this work. Finally a plan for further development should be created.
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meillo@245 286 \section{Delimitation}
meillo@96 287
meillo@150 288 << limit against stuff not covered here >>
meillo@96 289
meillo@275 290 No installation guide for \masqmail.
meillo@96 291
meillo@275 292 No description of \masqmail's source code, bit by bit.
meillo@96 293
meillo@275 294 The \NAME{POP3} functionality of \masqmail\ is not regareded.
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