docs/diploma: 18b7b517e2dd thesis/tex/4-MasqmailsFuture.tex

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wrote about discussion on architecture

author	meillo@marmaro.de
date	Wed, 17 Dec 2008 18:48:17 +0100
parents	0b17f6e5edae
children	5681a18270b5

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1 \chapter{\masqmail's present and future}

3 \section{Existing code base}

4 Here regarded is version 0.2.21 of \masqmail. This is the last version released by Oliver \person{Kurth}, and the basis for my thesis.

7 \subsubsection*{Features}

9 \masqmail\ accepts mail on the command line and via \SMTP. Mail queueing and alias expansion is supported. \masqmail\ is able to deliver mail to local mailboxes (in \name{mbox} or \name{maildir} format) or pass it to a \name{mail delivery agent} (like \name{procmail}). Mail destinated to remote locations is sent using \SMTP\ or can be piped to commands, being gatesways to \NAME{UUCP} or \NAME{FAX} for example.

11 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 offers a two way communication through a wrapper application like \name{openssl}.

12 %todo: what about SSL/TLS encryption?

14 \masqmail\ focuses on non-permanent online connections, thus a concept of online routes is used. One may configure any amount of routes to send mail. Each route can have criterias, like matching \texttt{From:} or \texttt{To:} headers, to determine if mail is allowed to be sent using it. Mail to destinations outside the local net gets queued until \masqmail\ is informed about the existance of a online connection.

16 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 equivilent to calling it with the argument \verb+-bq+. \masqmail\ recognizes the names \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 name \path{newaliases} because \masqmail\ does not generate binary representations of the alias file.\footnote{A shell script located 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 sendmail-compatibility.

17 %masqmail: mailq, mailrm, runq, rmail, smtpd/in.smtpd

18 %sendmail: hoststat, mailq, newaliases, purgestat, smtpd

20 Additional to the \mta\ job, \masqmail\ also offers mail retrieval services with being a \NAME{POP3} client. It can fetch mail from different remote locations, dependent on the active online route.

24 \subsubsection*{The code}

26 \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 however has higher precedence in linking.

28 The only mandatory dependency is \name{glib}---a cross-platform software utility library, originated in the \NAME{GTK+} project. It provides safer replacements for many standard library functions. It also offers handy data containers, easy-to-use implementations of data structures, and much more.

31 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 programms use \masqmail\ source code; they only add a file with a \verb+main()+ function each.

34 \masqmail\ does not provide an interface to plug in modules with additional functionality. There exists no add-on or module system. The code is only separated by function to the various source files. Some functional parts can be included or excluded by defining symbols. Adding maildir support at compile time, means giving the option \verb+--enable-maildir+ to the \path{configure} call. This preserves the concerning code to get removed by the preprocessor. Unfortunately the \verb+#ifdef+s are scattered through all the source, leading to a FIXME(holperig) code base.

41 \section{Requirements}

43 Following is a list of current and future requirements to make \masqmail\ ready for the future.

46 \subsubsection*{Large message handling}

47 Trends in the market for electronic communication go towards consolidated communication, hence email will be used more to transfer voice and video messages. This leads to larger messages. The store-and-forward transport of email is not good suited for large data. Thus new protocols, like \NAME{QMTP} (described in section %\ref{FIXME}

48 ), may become popular.

51 \subsubsection*{Ressource friendly software}

52 The merge of communication hardware and the move of email services from providers to homes, demands smaller and more resource-friendly software. The amount of mail will be lower, even if much more mail will be sent. More important will be the energy consumption and heat emission. These topics increased in relevance during the past years and they are expected to become more central. \masqmail\ is not a program to be used on large servers, but to be used on small devices. Thus focusing on energy and heat, not on performance, is the direction to go.

55 \subsubsection*{New mail transfer protocols}

56 Large messages demand more efficient transport through the net. As well is a final solution needed to defeat the spam problem. New mail transport protocols may be the only good solutions for both problems. They also can improve reliability, authentication, and verification issues. \masqmail\ should be able to support new protocols as they appear and are used.

59 \subsubsection*{Spam handling}

60 Spam is a major threat. According to the \NAME{SWOT} analysis, the goal is to reduce it to a bearable level. Spam fighting is a war are where the good guys tend to lose. Putting too much effort there will result in few gain. Real success will only be possible with new---better---protocols and abandonning the weak legacy technologies. Hence \masqmail\ should be able to provide state-of-the-art spam protection, but not more.

63 \subsubsection*{Security}

64 \MTA{}s are critical points for computer security, as they are accessable from external networks. They must be secured with high effort. Properties like high priviledge level, work load influenced from extern, work on unsafe data, and demand for reliability, increase the security needed. Unsecure and unreliable \mta{}s are of no value. \masqmail\ needs to b e secure enough for its target field of operation.

67 \subsubsection*{Easy configuration}

68 Having \mta{}s on many home servers and clients, requires easy and standardized configuration. The common setups should be configurable with single actions by the user. Complex configuration should be possible, but focused must be the most common form of configuration: choosing one of several standard setups.

75 \section{Discussion on architecture}

77 A program's architecture is maybe the most influencing design decision, and has the greatest impact on the program's future capabilities. %fixme: search quote ... check if good

79 \masqmail's current artitecture is monolitic like \sendmail's and \exim's. But more than the other two, is it one block of interweaved code. \sendmail\ provides now, with its \name{milter} interface, standardized connection channels to external modules. \exim\ has a highly structured code with many internal interfaces, like the one for supported authentication ``modules''. \masqmail\ has none of them; it is what \sendmail\ was in the beginning: a single large block.

81 Figure \ref{fig:masqmail-arch} is an attempt to depict \masqmail's internal structure.

83 \begin{figure}

84 \begin{center}

85 \input{input/masqmail-arch.tex}

86 \end{center}

87 \caption{Internal architecture of \masqmail}

88 \label{fig:masqmail-arch}

89 \end{figure}

91 \sendmail\ improved its old architecture, for example by adding the milter interface. \exim\ was designed and is carefully maintained with a modular-like code structure in mind. \qmail\ started from scratch with a security-first approach, \postfix\ improved on it, and \name{sendmail X}/\name{MeTA1} tries to adopt the best of \qmail\ and \postfix, to completely replace the old \sendmail\ architecture. \person{Hafiz} \cite{hafiz05}. describes this evolution of \mta\ architecture very well.

93 Every one of the popular \MTA{}s is more modular, or became more modular, than \masqmail. The logical step is to rewrite \masqmail\ using a modern, modular architecture to get a modern \MTA\ satisfying nowadays needs. But how is the effort of this complete rewrite compared to what is gained afterwards?

98 A secure architecture is of need.

100

101

102

103

104

105

106 (ssl)

107 -> msg-in (local or remote protocol handlers)

108 -> spam-filter (and more)

109 -> queue

110 -> msg-out (local-delivery by MDA, or remote-protocol-handlers)

111 (ssl)

112

113 A design from scratch?

114

115 << what would be needed (effort) >>

116

117 << would one create it at all? >>

118

119 << should it be done? >>

120

121

122 http://fanf.livejournal.com/50917.html %how not to design an mta - the sendmail command

123 http://fanf.livejournal.com/51349.html %how not to design an mta - partitioning for security

124 http://fanf.livejournal.com/61132.html %how not to design an mta - local delivery

125 http://fanf.livejournal.com/64941.html %how not to design an mta - spool file format

126 http://fanf.livejournal.com/65203.html %how not to design an mta - spool file logistics

127 http://fanf.livejournal.com/65911.html %how not to design an mta - more about log-structured MTA queues

128 http://fanf.livejournal.com/67297.html %how not to design an mta - more log-structured MTA queues

129 http://fanf.livejournal.com/70432.html %how not to design an mta - address verification

130 http://fanf.livejournal.com/72258.html %how not to design an mta - content scanning

131

132

133 \subsubsection*{local mail delivery}

134 But for example delivery of mail to local users is \emph{not} what \mta{}s should care about, although most \MTA\ are able to deliver mail, and many do. (\name{mail delivery agents}, like \name{procmail} and \name{maildrop}, are the right programs for this job.)

135

136

137

138

139

140 \subsection{Access and Auth}

141

142 easiest: restricting by static IP addresses (Access control via hosts.allow/hosts.deny)

143 if dynamic remote hosts need access: some auth is needed

144 - SASL

145 - POP/IMAP: pop-before-smtp, DRAC, WHOSON

146 - TLS (certificates)

147

148 ``None of these add-ons is an ideal solution. They require additional code compiled into your existing daemons that may then require special write accesss to system files. They also require additional work for busy system administrators. If you cannot use any of the nonauthenticating alternatives mentioned earlier, or your business requirements demand that all of thyour users' mail pass through your system no matter where they are on the Internet, SASL is probably the solution that offers the most reliable and scalable method to authenticate users.'' (Dent: Postfix, page 44, ch04)

149

150

151

152 postfix: after-queue-content-filter (smtp communication)

153 exim: content-scan-feature

154 sendmail: milter (tcp or unix sockets)

155

156 checks while smtp dialog (pre-queue): in MTA implemented (need to be fast)

157 checks when mail is accepted and queued: external (amavis, spamassassin)

158

159 anti-virus: clamav

160

161 AMaViS (amavisd-new): email filter framework to integrate spam and virus scanner

162 internet -->25 MTA -->10024 amavis -->10025 MTA --> reciptient

163 | |

164 +----------------------------+

165 mail scanner:

166 incoming queue --> mail scanner --> outgoing queue

167

168 mimedefang: uses milter interface with sendmail

169

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174

175

176 \subsection{spam and malicious content}

177

178 The same for malicious content (\name{malware}) like viruses, worms, trojan horses. They are related to spam, but affect the \MTA less, as they are in the mail body.

179

180 message body <-> envelope, header

181

182 where to filter what

183

184

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188

189

190 \section{Directions to go}