docs/diploma

annotate thesis/tex/4-MasqmailsFuture.tex @ 211:f2b8481789f6

better diagrams and other small changes
author meillo@marmaro.de
date Sun, 04 Jan 2009 10:28:40 +0100
parents 2e006e8f5250
children 711f0d3f5dfd
rev   line source
meillo@109 1 \chapter{\masqmail's present and future}
meillo@93 2
meillo@185 3 This chapter \dots %fixme write text here
meillo@185 4
meillo@185 5
meillo@185 6
meillo@185 7
meillo@137 8 \section{Existing code base}
meillo@185 9
meillo@142 10 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.
meillo@142 11
meillo@93 12
meillo@185 13 \subsubsection*{The source code}
meillo@185 14
meillo@185 15 \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.
meillo@185 16
meillo@185 17 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.
meillo@185 18
meillo@185 19 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.
meillo@185 20
meillo@185 21 \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 source code that is hard to read.
meillo@185 22 %fixme: refer to ifdef-considered-harmful ?
meillo@185 23
meillo@185 24
meillo@185 25
meillo@137 26 \subsubsection*{Features}
meillo@93 27
meillo@142 28 \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.
meillo@93 29
meillo@142 30 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}.
meillo@137 31 %todo: what about SSL/TLS encryption?
meillo@93 32
meillo@142 33 \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.
meillo@142 34
meillo@137 35 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.
meillo@109 36 %masqmail: mailq, mailrm, runq, rmail, smtpd/in.smtpd
meillo@109 37 %sendmail: hoststat, mailq, newaliases, purgestat, smtpd
meillo@109 38
meillo@137 39 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.
meillo@109 40
meillo@137 41
meillo@132 42
meillo@132 43
meillo@132 44
meillo@132 45
meillo@177 46 \section{Requirements}
meillo@146 47
meillo@185 48 This section identifies the requirements for a modern \masqmail. Most of them will apply to modern \MTA{}s in general.
meillo@185 49
meillo@185 50
meillo@177 51
meillo@177 52 \subsection{General requirements}
meillo@146 53
meillo@189 54 Here follows a list of quality requirements for \masqmail, or other kinds of programs in similar environment and with similar jobs. These requirements specify the non-functional properties of the software, thus they are also called \name{non-functional requirements}. The list is based on \person{Hafiz} \cite[page~2]{hafiz05}, with insperation from \person{Spinellis} \cite[page~6]{spinellis06}.
meillo@185 55 %fixme: refer to ch01 and ch02
meillo@146 56
meillo@146 57
meillo@177 58 \subsubsection*{Security}
meillo@196 59 \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 need for security. \masqmail\ needs to be secure enough for its target field of operation.
meillo@177 60
meillo@177 61
meillo@177 62 \subsubsection*{Reliability}
meillo@196 63 Reliability is the second essential quality property for an \MTA. Mail for which the \MTA\ took responsibility must never get lost. The \MTA\ must not be \emph{the cause} of any mail loss, no matter what happens. Unreliable \mta{}s are of no value.
meillo@177 64
meillo@189 65
meillo@189 66 \subsubsection*{Robustness}
meillo@196 67 Being robust means handling errors properly. Small errors may get tolerated, large errors may kill a process. Killed processes should be automatically restarted and lead to a clean state again. Log messages should be written in every case. Robust software does not need a special environment, it creates the right environment itself. \person{Raymond}'s \name{Rule of Robustness} and his \name{Rule of Repair} are good descriptions.\cite[pages~18--21]{raymond03}
meillo@177 68
meillo@177 69
meillo@177 70 \subsubsection*{Extendability}
meillo@196 71 Modern needs like large messages demand for more efficient mail transport through the net. Aswell is a final solution needed to defeat the spam problem. New mail transport protocols seem to be the only good solutions for both problems. They also can improve reliability, authentication, and verification issues. \masqmail's architecture needs to be extendable, to allow new features to be added afterwards. For example new mail transfer protocols as they appear and are used.
meillo@196 72
meillo@146 73
meillo@189 74 \subsubsection*{Maintainability}
meillo@196 75 Maintaining software takes much time and effort. \person{Spinellis} guesses ``40\,\% to 70\,\% of the effort that goes into a software system is expended after the system is written first time.'' \cite[page~1]{spinellis03}. This work is called \emph{maintaining}. Hence making software good to maintain is effort that will easy 40 to 70\,\% of the work afterwards.
meillo@146 76
meillo@189 77
meillo@189 78 \subsubsection*{Testability}
meillo@196 79 Good testability make maintainance easier too, because functionality is directly verifiable when changes are done, thus removing the uncertainty. Modularized software makes testing easier, because parts can be testet without external influences. \person{Spinellis} sees testability as a subquality of maintainability.
meillo@189 80
meillo@189 81
meillo@189 82 \subsubsection*{Performance}
meillo@196 83 Also called ``efficiency''. Efficient software requires few time and few resources. The merge of communication hardware and its move from service providers to homes and to mobile devices, demand 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.
meillo@146 84
meillo@146 85
meillo@189 86 \subsubsection*{Availability}
meillo@196 87 Availability is important for server programs. They must stay operational, even during \name{denial of service} attacks.
meillo@146 88
meillo@146 89
meillo@189 90 \subsubsection*{Portability}
meillo@196 91 Source code that compiles and runs on various operationg systems is called portable. Portability can be achieved by using standard features of the programming language and common libraries. Basic rules for portable code are defined by \person{Kerighan} and \person{Pike} \cite{kernighan99}. Portable code lets software spread faster.
meillo@189 92
meillo@189 93
meillo@189 94 \subsubsection*{Usability}
meillo@196 95 Usability, not mentioned by \person{Hafiz} (he focuses on architecture) but by \person{Spinellis} and \person{Kan}, is a property very important from the user's point of view. Software with bad usability is rarely used, no matter how good it is. If roughly equivilent substitutes with better usability exist, the user will switch to one of them. Usability here means easy to set up and configure, too; users are also administrators here. 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.
meillo@146 96
meillo@146 97
meillo@146 98
meillo@146 99
meillo@185 100
meillo@185 101 \subsection{Functional requirements}
meillo@185 102
meillo@185 103 This section identifies the needed functionality for a modern \MTA. The basic job of a \mta\ is to tranport mail from a sender to a recipient. 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}.
meillo@185 104
meillo@185 105 An \MTA\ therefore needs at least a mail receiving facility and a mail sending facility.
meillo@185 106
meillo@185 107
meillo@185 108
meillo@185 109 \subsubsection*{Incoming channels}
meillo@185 110
meillo@185 111 \sendmail-compatible \mta{}s must support at least two incoming channels: mail submitted using the \sendmail\ command, and mail received via the \SMTP\ daemon. Thus it is common to split the incoming channel into local and remote. This is done by \qmail\ and \postfix. The same way is \person{Hafiz}'s view. \SMTP\ is the primary mail transport protocol today, but with the increasing need for new protocols\ref{FIXME} in mind, support for more than just \SMTP\ is good to have. This leads to more than one remote channel.
meillo@185 112
meillo@185 113
meillo@185 114 \subsubsection*{Outgoing channels}
meillo@185 115
meillo@185 116 Outgoing mail is commonly either sent using \SMTP, piped into local commands (for example \texttt{uucp}), or delivered locally by appending to a mailbox.
meillo@185 117
meillo@185 118 Outgoing channels are similar for \qmail, \postfix, and \name{sendmail X}: All of them have a module to send mail using \SMTP, and one for writing into a local mailbox. Local mail delivery is a job that requires root priveledge to be able to switch to any user in order to write to his mailbox.
meillo@185 119
meillo@185 120 As mail delivery to local users, is \emph{not} included in the basic job of an \MTA{}, why should it care about it? In order to keep the system simple and to have programs that do one job well, the local delivery job should be handed over to a specialist: the \name{mail delivery agent}. \NAME{MDA}s know about the various mailbox formats and are aware of the problems of concurrent write access and thelike. Hence handling the message and the responsiblity over to a \NAME{MDA}, like \name{procmail} or \name{maildrop}, seems to be the right way to go.
meillo@185 121
meillo@185 122 This means an outgoing connection that pipes mail into local commands is required. Other outgoing channels, one for each supportet protocol, may be designed like it was done in other \MTA{}s.
meillo@185 123
meillo@185 124
meillo@185 125
meillo@185 126 \subsubsection*{Mail queue}
meillo@185 127
meillo@185 128 Additionally to the mail receiving and sending facilities, mail queues are a basic feature. A mail queue removes the need to deliver intantly as a message is received. They provide fail-safe storage of mails until they are delivered. Mail queues are probably used in all \mta{}s, excluding the simple forwarders. A mail queue is a essential requirement for \masqmail, as it is to be used for non-permanent online connections. This means, mail must be queued until a online connection is available to send the message.
meillo@185 129
meillo@185 130 The mail queue and the module to manage it are the central part of the whole system. This demands especially for robustness and reliability, as a failure here can lead to loosing mail. An \MTA\ takes over responsibility for mail in accepting it, hence loosing mail messages is absolutely to avoid. This covers any kind of crash situation too. The worst thing acceptable to happen is a mail to be sent twice.
meillo@185 131
meillo@185 132 \sendmail, \exim, \qmail, \name{sendmail X}, and \masqmail\ feature one single mail queue. \postfix\ has three of them: \name{incoming}, \name{active}, and \name{deferred}. (The \name{maildrop} queue is excluded, as it is only used for the \texttt{sendmail} command.)
meillo@185 133
meillo@185 134 \MTA\ setups that do external content scanning tend to require two separate queues. To use \sendmail\ in such setups requires two independent instances, with two separate queues, running. \exim\ can handle it with special \name{router} and \name{transport} rules, but the data flow gets complicated. Having two independent queues seems to be preferable.
meillo@185 135
meillo@185 136
meillo@185 137
meillo@185 138
meillo@185 139 \subsubsection*{Header sanitizing}
meillo@185 140
meillo@185 141 Mail coming into the system often lacks important header lines. At least the required ones must be added from the \MTA. One example is the \texttt{Date:} header, another is the, not required but recommended \texttt{Message-ID:} header. Apart from adding missing headers, rewriting headers is important too, to change the locally known domain part of email addresses to globally known ones for example. \masqmail\ needs also the ability to rewrite the domain part dependent on the route used to send the message.
meillo@185 142
meillo@185 143 Generating the envelope is a related job. The envelope specifies the actual recipient of the mail, no matter what the \texttt{To:}, \texttt{Cc:}, and \texttt{Bcc:} headers tell. Multiple reciptients lead to multiple different envelopes, containing all the same mail message.
meillo@185 144
meillo@185 145
meillo@185 146
meillo@185 147
meillo@185 148 \subsubsection*{Aliasing}
meillo@185 149
meillo@185 150 Email addresses can have aliases and need to be expanded. Aliases can be of different kind: different local user, remote user, list of local and/or remote users, or a command. Most important are the aliases in the \path{aliases} file, usually located at \path{/etc/aliases}. Addresses expanding to lists of users lead to more envelopes. Aliases changing the reciptients domain part may require a different route to use.
meillo@185 151
meillo@185 152
meillo@185 153
meillo@185 154
meillo@185 155 \subsubsection*{Choose route to use}
meillo@185 156
meillo@185 157 One key feature of \masqmail\ is its ability to send mail out in different ways. The decision is based on the current online state and whether a route may be used for a message or not. The online state can be retrieved in tree ways, explained in \ref{sec:fixme}. A route to send is found by checking every available route for being able to transfer the current message, until one matches.
meillo@185 158
meillo@185 159
meillo@185 160
meillo@185 161
meillo@185 162 \subsubsection*{Authentication}
meillo@185 163
meillo@185 164 One thing to avoid is being an \name{open relay}. Open relays allow to relay mail from everywhere to everywhere. This is a major source of spam. The solution is restricting relay\footnote{Relaying is passing mail, that is not from and not for the own system, through it.} access.
meillo@185 165
meillo@185 166 Several ways to restrict access are available. The most simple one is restrictiction relaying by the \NAME{IP} address. No extra complexity is added this way, but static \NAME{IP} addresses are needed.
meillo@185 167
meillo@185 168 If static access restriction is not possible, for example mail from locations with changing \NAME{IP} addresses, some kind of authentication mechanism is required. Three common kinds exist:
meillo@185 169 \begin{itemize}
meillo@185 170 \item \SMTP-after-\NAME{POP}: Uses authenication on the \NAME{POP} protocol to permit incoming \SMTP\ connections for a limited time afterwards.
meillo@185 171 \item \SMTP authentication: An extension to \SMTP. Authentication can be requested before mail is accepted.
meillo@185 172 \item Certificates: They confirm the identity of someone.
meillo@185 173 \end{itemize}
meillo@185 174
meillo@185 175
meillo@185 176
meillo@185 177 \subsubsection*{Encryption}
meillo@185 178
meillo@185 179 Electronic mail is very weak to sniffing attacks, because all data transfer is unencrypted. This concerns the message's content, as well as the email addresses in header and envelope, but also authentication dialogs that may transfer plain text passwords (\NAME{PLAIN} and \NAME{LOGIN} are examples). Thus encryption is wanted.
meillo@185 180
meillo@185 181 The common way to encrypt \SMTP\ dialogs is using \name{Transport Layer Security} (short: \TLS, successor of \NAME{SSL}). \TLS\ encrypts the datagrams of the \name{transport layer}. This means it works below the application protocols and can be used by any of them\citeweb{wikipedia:tls}.
meillo@185 182
meillo@185 183 Outgoing \SMTP\ connections can get encrypted using a secure tunnel, created by an external application. Incoming connections, can not use this technique because the remote \NAME{IP} address is hidden then; \NAME{STARTTLS}---defined in \RFC2487---is what \mta{}s implement.
meillo@185 184
meillo@185 185
meillo@185 186
meillo@185 187 \subsubsection*{Spam prevention}
meillo@185 188
meillo@185 189 Spam is a major threat nowadays, but it is a war in which the good guys tend to lose. Putting much effort in fighting spam results in few gain. Real success will only be possible with new---better---protocols and abandonning the weak legacy technologies. The goal is to provide state-of-the-art spam protection, but not more (see section \ref{sec:swot-analysis}). As spam is not just a nuisance for end users, but also for the infrastructure---the \mta{}s---by increasing the amount of mail messages, \MTA{}s need to protect themself.
meillo@185 190
meillo@185 191 Filtering spam can be done in two ways: Refusing spam during the \SMTP\ dialog or checking for spam after the mail was accepted and queued. Both have advantages and disadvantages, so modern \MTA{}s use them in combination. Spam is identified by the results of a set of checks. Static rules, querying databases (\NAME{DNS} blacklists), requesting special client behavior (\name{greylisting}, \name{hashcash}), or statistical analysis (\name{bogofilter}) are checks that may be used. Running more checks leads to better results, but takes more system resources and time.
meillo@185 192
meillo@185 193 Doing some basic checks during the \SMTP\ dialog seems to be a must.
meillo@185 194
meillo@185 195
meillo@185 196
meillo@185 197 \subsubsection*{Virus checking}
meillo@185 198
meillo@185 199 Related to spam is malicous content (short: \name{malware}) like viruses, worms, trojan horses. They, in contrast to spam, do not affect the \MTA\ itself, as they are in the mail body. \MTA{}s searching for malware is equal to real world's post offices opening letters to check if they contain something that could harm the recipient. This is not a mail transport job. But the last \MTA---the one responsible for the recipient---seems to be at a good position to do this work.
meillo@185 200
meillo@185 201 In any way should malware checking be done by external programs that may be invoked by the \mta. But using mail deliver and processing agents, like \name{procmail}, a better suited locations to invoke content scanners.
meillo@185 202
meillo@185 203 A popular email filter framework is \name{amavis} which integrates various spam and virus scanners. The common setup includes a receiving \MTA\ which sends it to \name{amavis} using \SMTP, \name{amavis} processes the mail and sends it then to a second \MTA\ that does the outgoing transfer. Having interfaces to such scanners is, for sure, good to have.
meillo@185 204
meillo@185 205
meillo@185 206
meillo@185 207 \subsubsection*{Archiving}
meillo@185 208
meillo@185 209 Mail archiving and auditability become more important as electronic mail becomes more important. The ability to archive verbatim copies of every mail coming into and every mail going out of the system, with relation between them, appears to be a goal to achieve.
meillo@185 210
meillo@185 211 \postfix\ for example has a \texttt{always\_bcc} feature, to send a copy of every mail to a definable reciptient. At least this funtionality should be given, although a more complete approach is preferable.
meillo@185 212
meillo@185 213
meillo@185 214
meillo@185 215
meillo@185 216
meillo@185 217
meillo@185 218
meillo@185 219
meillo@185 220 \section{Work to do}
meillo@185 221
meillo@187 222 After \masqmail's features were presented in section \ref{sec:fixme} and the requirements for modern \mta{}s were identified in section \ref{sec:fixme}, here the differences between them are shown.
meillo@185 223
meillo@185 224
meillo@187 225
meillo@187 226 \subsubsection*{Fulfilled requirements}
meillo@187 227
meillo@200 228 \masqmail's incoming and outgoing channels are the ones common to most \MTA{}s: the \texttt{sendmail} command and \SMTP\ for incoming mail; local delivery, piping to commands, and \SMTP\ for outgoing mail. Support for other protocols is not available. To add it, modifications at many places in the source are needed.
meillo@187 229
meillo@187 230 One single mail queue is used in \masqmail. The envelope and mail headers are generated when the mail is put into the queue. Aliasing is done on delivery, after the route to be used was determined. Headers may be rewritten then. These parts do all provide the functionality required.
meillo@187 231
meillo@187 232 Static authentication, based on \NAME{IP} addresses, can be set up using the \path{hosts.allow} and \path{hosts.deny} files. Dynamic authentication is supported in form of \NAME{SMTP-AUTH} and \SMTP-after-\NAME{POP}, but only for outgoing connections. The same for encryption which is also only available for outgoing \SMTP\ connections; here a wrapper application like \name{openssl} needs to be used. Support for authentication and encryption of incoming connections is completely missing, but a basic requirement for all secure emailing.
meillo@187 233
meillo@187 234 \masqmail\ does not provide special support for spam filter or content checking. But it is possible to invoke external filter applications by running two independent instances of \masqmail, connected by the filter application. The receiving \MTA\ instance accepts mail and pushes it into the filter. The filter application receives mail, processes it, possible modifies it, and pushes it over to a second \MTA\ instance. The second \MTA\ is responsible for further delivery of the mail. Appendix \ref{app:FIXME} shows configuration files to create such a setup. This is a concept that works in general. However, real spam \emph{prevention}---to not accept spam mail at all---or good filter interfaces are not available, but are nessesary for using \masqmail\ in an unsafe environment.
meillo@187 235
meillo@187 236 There is currently no way of archiving every message going through \masqmail.
meillo@187 237
meillo@187 238
meillo@196 239 Non-functional requirements are not so easy to be marked as fulfilled or not. Instead they are discussed here.
meillo@196 240
meillo@196 241 \masqmail\ needs to be ``secure enough'', but what is ``secure enough''? This depends on its target field. Currently \masqmail\ is targeted to workstations and private networks, with explicit warning for use on permanent online hosts \citeweb{masqmail:homepage2}. \masqmail's current security is bad. (For instance does a long time known attack against \sendmail, described by \person{Sill} \cite[page~4]{sill02}, still outwit \masqmail). The security, however, seems acceptable for use on workstations and private networks, if the environment is trusted. In environments where untrusted components or persons have access to \masqmail, its security is too low.
meillo@196 242
meillo@196 243 Similar for its reliability. It has been reported that \masqmail\ has not sent mail under some circumstances \citeweb{FIXME}. %fixme
meillo@196 244 The author also noticed problems with where only one part of the queued message was removed, the other part remained as garbage. Fortunately, reports abou lost mail are not known. Current reliability is not good enough.
meillo@196 245
meillo@196 246 The logging behavior of \masqmail\ is good, but does not cover everything. Also it trusts its environment. For example, if the queue directory is world writeable by accident (or as action of an intruder), any user can remove messages from the queue or replace them with own ones. \masqmail\ does not even write a debug message in this case.
meillo@196 247
meillo@196 248 \masqmail's extendability is very poor. This is a general problem of monolithic software, but can thus be provided with high effort. \exim\ is an example for good extendability for a monolithic program.
meillo@196 249
meillo@196 250 The maintainability of \masqmail\ appears to be equivilent to other software of similar kind. Missing modularity and therefore more complexity makes the maintainer's work harder. In summary is \masqmail's maintainability bearable, and in view of its missing modularity good. The testability suffers from the same. Testing program parts is hard, but done by compiling source parts to special test programs.
meillo@196 251
meillo@196 252 The performance---effenciency---of \masqmail\ is good enough for its target field of operation, where this is a minor goal. As well is its availability. Hence no further work needs to be done her.
meillo@196 253
meillo@196 254 The code's protability is good with view on \unix-like operation systems. At least \name{Debian}, \name{Redhat}, \NAME{SUSE}, \name{slackware}, \name{Free}\NAME{BSD}, \name{Open}\NAME{BSD}, and \name{Net}\NAME{BSD} are reported to be able to compile and run \masqmail\ \citeweb{masqmail:homepage2}. About special requirements for the underlying file system is not heard of. The portability is already good.
meillo@196 255
meillo@196 256 The usability, from the administator's point of view, is very good. \masqmail\ was developed to suite a specific, limited job, its configuration does perfect match. The user's view does not reach to the \MTA, as it is hidden behind the \name{mail user agent}.
meillo@196 257
meillo@187 258
meillo@187 259
meillo@187 260 \subsubsection*{Missing parts}
meillo@187 261
meillo@187 262 Support for other protocols than \SMTP\ seems not to be nessesary at the moment. Adding such support will need lots of work in all parts of \masqmail, hence delaying it until the support is needed appears to be the best solution.
meillo@187 263
meillo@187 264 Authentication of incoming \SMTP\ connections is definately needed and should be added soon. The same applies to encryption of incomming connections. These two features are essential for restricting relaying and providing privacy.
meillo@187 265
meillo@187 266 As authentication can be a guard against spam, filter facilities have lower priority. But basic spam filtering and interfaces for external tools should be implemented in future. Content checking, if really nessesary, should be left over to the \NAME{MDA}, to deal with it in local delivery.
meillo@187 267
meillo@187 268 Archiving again is prefered to be implemented soon. It does not require much work, but enables all kinds of statistical analysis. Also it is a requirement for companies to archive their mail communication.
meillo@187 269
meillo@200 270 Non-functional requirements need improvement too.
meillo@185 271
meillo@196 272 \masqmail's security is bad and limits it thus to a field of operation that shrinks. Security becomes more important and networking and interaction increases. Save and trusted environment become rare. Improving security is an important thing to do in future.
meillo@185 273
meillo@196 274 Reliability is also to improve. It is a key quality property for an \MTA, and not good enough in \masqmail. Also is the program lacking robustness. Checking the environment and reporting bad characteristics is wanted. Especially improving robustness inrelation to the queue is favorable; applying ideas of \name{crash-only software}\cite{candea03} will be a good step.
meillo@185 275
meillo@196 276 Extendability, maintainability, and testability do all lack from the monolithic architecture and are nearly impossible to improve without changing the programs structure. These properties can hardly be retrofitted into software. But extendability might become important in the future, and the other two make all further work on the software easier.
meillo@185 277
meillo@196 278 Performance is a nice-to-have property, but as performance improvements are in contrast to many other quality poperties (reliability, maintainability, usability, capability \cite[page~5]{kan03}), jeopardizing these to gain some more performance should not be done. \person{Kernighan} and \person{Pike} state clear: ``[T]he first principle of optimization is \emph{don't}.''\cite[page~165]{kernighan99}. \masqmail\ is not a program to be used on large servers, but on small devices. Thus important for \masqmail\ is focusing on energy and heat, maybe also system resources, not on performance.
meillo@196 279
meillo@196 280 Focusing on being portable amoung the various flavors of \unix\ systems seems to be okay, because these systems are the ones \MTA{}s run on usually. Problems with non-\unix platforms are primary expected to come from file systems lacking required features. But no special care should be taken here.
meillo@196 281
meillo@196 282 Configuration could be eased more, by providing configuration generators to be able to run \masqmail\ right ``out of the box'' with only running one of several configuration scripts for common setups. This would make \masqmail\ better usable by people not technical educated.
meillo@196 283
meillo@196 284
meillo@196 285
meillo@200 286 \subsubsection*{The need for structural changes}
meillo@196 287
meillo@200 288 Adding authentication and encryption support, for example, is limited to a narrow region in the code. Such features are addable to the current code base without much problem. In contrast does adding support for new protocols or mail processing interfaces to external programs require a lot of effort. Changes in many parts of the source code are required. It is a bad idea to implement large retro-fitted features into software that is critical in security and reliability, like \masqmail. Worse if these features need changes in the program's structure, like adding mail scanning interfaces would do.
meillo@196 289
meillo@200 290 If such features are needed, it is best do redesign the program's structure and rebuild it. A program's structure is primary its architecture. Which is probably the most influencing design decision, and has the greatest impact on the program's future capabilities. The architecture defines what the program can do, and how it can be used. If the architecture does not fit the requirements, develpement reached a dead end \dots\ further work on this basis will make everything worse. The only good solution is to change the architecture, which, sadley but most likely, means a redesign from scratch.
meillo@196 291
meillo@200 292 Quality properties, like security and reliability, as well as extendability and maintainablity, can hardly be added afterwards---if at all. Only structural changes will improve them. Hence, if security, reliability, extendability (to add support for future mail transfer protocols), or maintainability shall be improved, a redesign of \masqmail\ is the only sane way to go.
meillo@146 293
meillo@200 294 %\person{Hafiz} adds: ``The major idea is that security cannot be retrofitted into an architecture.''\cite[page 64]{hafiz05}
meillo@187 295
meillo@200 296
meillo@200 297
meillo@200 298
meillo@200 299 \section{Discussion on MTA architecture}
meillo@187 300
meillo@188 301 \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, a good example is the one for authentication ``modules''. %fixme: add ref
meillo@188 302 \masqmail\ has none of them; it is what \sendmail\ was in the beginning: a single large block.
meillo@161 303
meillo@200 304 Figure \ref{fig:masqmail-arch} is a call graph generated from \masqmail's source code. It gives a impression of how interweaved the internal structure is.
meillo@161 305
meillo@161 306 \begin{figure}
meillo@161 307 \begin{center}
meillo@211 308 %\input{input/masqmail-arch.tex}
meillo@211 309 \includegraphics[scale=0.75]{img/callgraph.eps}
meillo@161 310 \end{center}
meillo@200 311 \caption{Call graph of \masqmail\ to show its internal structure}
meillo@161 312 \label{fig:masqmail-arch}
meillo@161 313 \end{figure}
meillo@161 314
meillo@200 315 \sendmail\ improved its old architecture, for example by adding the milter interface, to add further functionality by invocing external programs. \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.
meillo@161 316
meillo@188 317 Every one of the popular \MTA{}s is more modular, or became more modular over time, than \masqmail\ is. Modern requirements like spam protection and future requirements like---probably---the use of new mail transport protocols demand modular designs for keeping the software simple. Simplicity is a key property for security. ``[T]he essence of security engenieering is to build systems that are as simple as possible.''\cite[page 45]{graff03}
meillo@161 318
meillo@188 319 \person{Hafiz} agrees: ``The goal of making software secure can be better achieved by making the design simple and easier to understand and verify.''\cite[page 64]{hafiz05} He identifies the security of \qmail\ to come from it's \name{compartmentalization}, which goes hand in hand with modularity:
meillo@163 320 \begin{quote}
meillo@188 321 A perfect example is the contrast between the feature envy early \sendmail\ architecture implemented as one process and the simple, modular architecture of \qmail. The security of \qmail\ comes from its compartmentalized simple processes that perform one task only and are therefore testable for security. \cite[page 64]{hafiz05}
meillo@163 322 \end{quote}
meillo@200 323 As well does \person{Dent} for \postfix: ``The modular architecture of Postfix forms the basis for much of its security.'' \cite[page 7]{dent04}
meillo@200 324
meillo@200 325 Modularity is also needed to satisfy modern \MTA\ requirements, in providing a clear interface to add functionality without increasing the overall complexity much.
meillo@161 326
meillo@188 327 Security comes from good design, as \person{Graff} and \person{van Wyk} explain:
meillo@163 328 \begin{quote}
meillo@163 329 Good design is the sword and shield of the security-conscious developer. Sound design defends your application from subversion or misuse, protecting your network and the information on it from internal and external attacks alike. It also provides a safe foundation for future extensions and maintainance of the software.
meillo@163 330 %
meillo@163 331 %Bad design makes life easier for attackers and harder for the good guys, especially if it contributes to a false sends of security while obscuring pertinent failings.
meillo@163 332 \cite[page 55]{graff03}
meillo@163 333 \end{quote}
meillo@161 334
meillo@161 335
meillo@200 336 All this leads to the wish of a rewrite of \masqmail, using a modern, modular architecture, \emph{if} further features need to be added---ones that require changes in \masqmail's structure. But a rewrite is also needed, if \masqmail\ should become a modern \MTA, with good quality properties.
meillo@161 337
meillo@200 338 But redesigning and rewriting a software from scratch is hard. It takes time to design a new architecture, which must prove it is secure and reliable. As well is much time and work needed to implement the design, test it, fix bugs, and so on. Thus the gain of a new design must overweight the effort needed.
meillo@161 339
meillo@188 340 \person{Wheeler}'s program \name{sloccount} calculates following estimations for \masqmail's code base as of version 0.2.21 (excluding library code):
meillo@188 341 {\small
meillo@188 342 \begin{verbatim}
meillo@188 343 Total Physical Source Lines of Code (SLOC) = 9,041
meillo@188 344 Development Effort Estimate, Person-Years (Person-Months) = 2.02 (24.22)
meillo@188 345 (Basic COCOMO model, Person-Months = 2.4 * (KSLOC**1.05))
meillo@188 346 Schedule Estimate, Years (Months) = 0.70 (8.39)
meillo@188 347 (Basic COCOMO model, Months = 2.5 * (person-months**0.38))
meillo@188 348 Estimated Average Number of Developers (Effort/Schedule) = 2.89
meillo@188 349 Total Estimated Cost to Develop = $ 272,690
meillo@188 350 (average salary = $56,286/year, overhead = 2.40).
meillo@188 351 SLOCCount, Copyright (C) 2001-2004 David A. Wheeler
meillo@188 352 \end{verbatim}
meillo@188 353 }
meillo@200 354 The development cost is not relevant for a \freesw\ project of volunteer developers, but the development time is. About 24 man-months are estimated. The current code base was written almost completely by Oliver \person{Kurth} within four years, in his spare time. This means he needed around twice as much time.
meillo@161 355
meillo@188 356 Given the asumptions that an equal amount of code is to be produced, a third of existing code can be reused plus concepts and experience, and development speed is like \person{Kurth}'s. Then it would take about two years to have a redesigned new \masqmail\ with the same features. Less time could be needed if a simpler architecture allows faster develpement, better testing, and less bugs.
meillo@177 357
meillo@177 358
meillo@177 359
meillo@177 360
meillo@200 361
meillo@177 362
meillo@177 363
meillo@177 364
meillo@177 365
meillo@196 366
meillo@196 367
meillo@196 368
meillo@196 369
meillo@196 370
meillo@196 371 \section{Result}
meillo@196 372
meillo@200 373 The most needed features, authentication and encryption, can be added to the current code base with changes only some areas. These changes should be made soon. Archiving of mail is another feature to add then. More complete logging coverage, reporting of unsafe environment, and fixing security flaws are quality improvements to do. All this work should be done on basis of the current code.
meillo@196 374
meillo@200 375 The implementation of authentication and encryption are covered in the next chapter within this thesis.
meillo@196 376
meillo@200 377 All other work depends on how \masqmail's future plans look like.
meillo@196 378
meillo@200 379 Now, how could \masqmail\ be like, in five years, for instance? Two ways of further development come to mind.
meillo@196 380
meillo@200 381 First, stick to the old architecture and try to add features as possible. This approach needs less effort to be spent, because a working code is alreads present. Further development is only adding small increments to a exiting code base. But the further development goes, the larger is the work needed to add more functionality, and the more bugs will appear, caused by the increasing complexity. Quality of the software will decrease, because lacking of clear internal structure encourages further work to be quick fixes instead of requiring good solutions by design.
meillo@196 382
meillo@200 383 Second, designing \masqmail\ from scratch and rebuilding it. A lot of time and work is required to do so. Additionally, a new design from scratch introduces new risks: Is the design really better? Was thought of everything? Will there come problems not seeable now? Starting from scratch also means a step back. Against these disadvantages stands the gain of the new design: Further development will be easier and probably faster, overall quality will be better and easier to keep up, and dead ends for further development can be avoided.
meillo@196 384
meillo@200 385 Finally, the decision for one of the ways depends on the question whether \masqmail\ should remain what it is, then the first option seems to be the right one to choose. Or \masqmail\ should become a modern \mta\ which is able to expand to include new functionality, then the second option is to choose.
meillo@196 386
meillo@200 387 Security, extendability, and the other quality properties appear to have crucial importance in this decision. If they are important for future versions of \masqmail, a new design is a must.
meillo@196 388
meillo@211 389 \person{Graff} and \person{van Wyk} describe the situation best: ``[I]n today's world, your software is likely to have to operate in a very hostile security environment.'' \cite{graff03} An old-fashioned \mta\ depends, for sure, on a dieing branch, called \name{trusted envrironments}. And nothing other than a fresh and better design will help to survive.
meillo@196 390
meillo@200 391 The suggested further development on \masqmail\ is: First add the most needed features, being authentication and encryption, to the current code base. This is the short time goal. Second---and this is the long time goal---design a new architecture that satisfies the requirements identified, especially the quality requirements. The implementation of this design shall then, after being usable and throughoutly tested, superseed the old \masqmail.
meillo@196 392
meillo@200 393 This action is similar to the change from \sendmail\ to \name{sendmail X}/\name{MeTA1}, except the \sendmail\ change was too late. The plan is to first do the most needed stuff on the old design to make it still usable; then design a new version from scratch, for the future.
meillo@196 394
meillo@200 395 The following chapter \ref{chap:improvements} is about the work on the current code base, to reach the short time goals. The chapter afterwards, numbered \ref{chap:new-design}, then introduces a new, modern design for future versions of \masqmail.
meillo@196 396