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author meillo@marmaro.de
date Sat, 07 Feb 2009 12:06:30 +0100
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meillo@109 1 \chapter{\masqmail's present and future}
meillo@339 2 \label{chap:present-and-future}
meillo@93 3
meillo@381 4 This chapter identifies requirements for \masqmail. They are compared against the current code to see what is already fulfilled and what is missing. Then the outstanding work is ordered by relevance and are presented in a list of pending work tasks. The end of this chapter is the evaluation of the best development strategy to get the work done in order to achieve the requirements.
meillo@185 5
meillo@185 6
meillo@267 7 \section{The goal}
meillo@391 8 \index{development goal}
meillo@185 9
meillo@293 10 Before requirements can be identified and further development can be discussed, it is important to clearly specify the goal to achieve. This means: What shall \masqmail\ be like in, for instance, five years?
meillo@391 11 \index{masqmail!in five years}
meillo@185 12
meillo@293 13 Should \masqmail\ become more specific to a more narrow niche or rather become more general and move a bit out of its niche? Or should it even become a totally general \MTA\ like \sendmail, \exim, \qmail, and \postfix?
meillo@256 14
meillo@381 15 Becoming completely general seems to be no choice because the competitors are too many and they are already too strong. It would require a strong base of developers and superior features to establish. There also seems to be no need for another general purpose \MTA\ additional to those four programs. Thus the effort would most likely remain a try. \person{Venema} stated: ``It is becoming less and less likely that someone will write another full-featured Postfix or Sendmail \MTA\ \emph{from scratch} (100 kloc).'' \cite{venema:postfix-growth}. At least \masqmail\ is not going to try that.
meillo@391 16 \index{postfix!no second postfix}
meillo@256 17
meillo@381 18 \masqmail\ was intended to be a small ``real'' \MTA\ which covers the niche of managing the relay over several smart hosts. Small and resource friendly software is still important for workstations, home servers, and especially for embedded computers. Other software that focuses on the same niche is not known. Dial-up connections have become rare but mobile computers that move between different networks are popular. So, the niche is still present.
meillo@256 19
meillo@381 20 What has changed in general is the security that is needed for software. \person{Graff} and \person{van Wyk} describe the situation well: ``[I]n today's world, your software is likely to have to operate in a very hostile security environment.'' \cite[page~33]{graff03}. Additionally they say: ``By definition, mail software processes information from potentially untrusted sources. Therefore, mail software must be written with great care, even when it runs with user privileges and even when it does not talk directly to a network.'' \cite[page~90]{graff03}. As \masqmail\ is mail software and trusted environments become rare, it is best for \masqmail\ to become a secure \MTA.
meillo@391 21 \index{hostile environment}
meillo@391 22 \index{security}
meillo@256 23
meillo@293 24 In summary, the goal for \masqmail\ is to stay in the current niche with respect to modern usage scenarios and to become a secure \MTA.
meillo@256 25
meillo@256 26
meillo@256 27
meillo@132 28
meillo@132 29
meillo@132 30
meillo@177 31 \section{Requirements}
meillo@219 32 \label{sec:mta-requirements}
meillo@146 33
meillo@267 34 This section identifies the requirements for \masqmail\ to reach the above defined goal. Most of the requirements will apply to modern \MTA{}s in general.
meillo@185 35
meillo@218 36
meillo@185 37
meillo@219 38 \subsection{Functional requirements}
meillo@327 39 \label{sec:functional-requirements}
meillo@391 40 \index{functional requirements}
meillo@146 41
meillo@232 42 Functional requirements are about the function of the software. They define what the program can do and in what way.
meillo@232 43 %fixme: add ref
meillo@239 44 The requirements are named ``\NAME{RF}'' for ``requirement, functional''.
meillo@219 45
meillo@219 46
meillo@393 47 \paragraph{\RF\,1: Incoming and outgoing channels}
meillo@339 48 \label{rf1}
meillo@381 49 \sendmail-compatible \MTA{}s must support at least two incoming channels: mail submitted using the \path{sendmail} command, and mail received on a \NAME{TCP} port. Thus it is common to split the incoming channels into local and remote. This is done by \qmail\ and \postfix. The same way is \person{Hafiz}'s view \cite{hafiz05}.
meillo@391 50 \index{incoming channels}
meillo@391 51 \index{sendmail!command}
meillo@219 52
meillo@381 53 \SMTP\ is the primary mail transport protocol today, but with the increasing need for new protocols (see section~\ref{sec:what-will-be-important}) in mind, support for more than just \SMTP\ is good to have. New protocols will show up; maybe multiple protocols need to be supported then. This would lead to multiple remote channels, one for each supported protocol as it was done in other \MTA{}s. Best would be interfaces to add further protocols as modules.
meillo@391 54 \index{smtp}
meillo@219 55
meillo@219 56
meillo@378 57 Outgoing mail is commonly either sent using \SMTP, piped into local commands (for example \path{uucp}), or delivered locally by appending to a mailbox. 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.
meillo@391 58 \index{outgoing channels}
meillo@391 59 \index{uucp}
meillo@219 60
meillo@318 61 %fixme: is the def of MTA: transfer between machines, or transfer between users?
meillo@381 62 Local mail delivery is a job that uses root privilege to be able to switch to any user in order to write to his mailbox. It is possible to deliver without being root privilege, but delivery to user's home folders is not generally possible then. Thus even the modular \MTA{}s \qmail\ and \postfix\ use root privilege for this job. As mail delivery to local users is \emph{not} included in the basic job of an \MTA{} and introduces a lot of new complexity, why should the \MTA\ bother? In order to keep the system simple, reduce privilege, and to have programs that do one job well, the local delivery job should be handed over to a specialist: the \NAME{MDA}. \NAME{MDA}s know about the various mailbox formats and are aware of the problems of concurrent write access and the like. Hence passing the message, and the responsibility for it, over to an \NAME{MDA} seems to be best.
meillo@391 63 \index{local delivery}
meillo@219 64
meillo@232 65 This means an outgoing connection that pipes mail into local commands is required. To other outgoing channels applies what was already said about incoming channels.
meillo@219 66
meillo@232 67 \begin{figure}
meillo@232 68 \begin{center}
meillo@232 69 \includegraphics[scale=0.75]{img/mta-channels.eps}
meillo@232 70 \end{center}
meillo@277 71 \caption{Required incoming and outgoing channels}
meillo@391 72 \index{figure!Required incoming and outgoing channels}
meillo@232 73 \label{fig:mta-channels}
meillo@232 74 \end{figure}
meillo@219 75
meillo@381 76 An overview on incoming and outgoing channels which are required for an \MTA, gives figure~\ref{fig:mta-channels}.
meillo@219 77
meillo@239 78 %fixme: write about submission (port 587)
meillo@219 79
meillo@219 80
meillo@219 81
meillo@219 82
meillo@393 83 \paragraph{\RF\,2: Mail queuing}
meillo@339 84 \label{rf2}
meillo@391 85 \index{mail queue}
meillo@381 86 Mail queuing removes the need to deliver instantly as a message is received. The queue provides fail-safe storage of mails until they are delivered. Mail queues are probably used in all \MTA{}s, even in some simple forwarders. The mail queue is essential for \masqmail, as \masqmail\ is intended for non-permanent online connections. This means, mail must be queued until a online connection is available to send the message. This may be after a reboot. Hence the mail queue must provide persistence.
meillo@391 87 \index{forwarder}
meillo@391 88 \index{non-permanent}
meillo@219 89
meillo@392 90 The mail queue and the module(s) 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 mail loss. An \MTA\ takes over responsibility for mail by 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 an already sent mail to be sent again.
meillo@391 91 \index{reliability}
meillo@219 92
meillo@219 93
meillo@219 94
meillo@219 95
meillo@393 96 \paragraph{\RF\,3: Header sanitizing}
meillo@339 97 \label{rf3}
meillo@391 98 \index{header sanitizing}
meillo@392 99 Mail coming into the system often lacks important header lines. At least the required ones must be added by 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. Changing the locally known domain part of email addresses to globally known ones is an example. \masqmail\ needs to be able to rewrite the domain part dependent on the route used to send the message, to prevent messages to get classified as spam.
meillo@391 100 \index{masqmail!online routes}
meillo@219 101
meillo@381 102 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 contain. Multiple recipients lead to multiple different envelopes, all containing the same mail message.
meillo@219 103
meillo@219 104
meillo@219 105
meillo@219 106
meillo@393 107 \paragraph{\RF\,4: Aliasing}
meillo@339 108 \label{rf4}
meillo@391 109 \index{aliases}
meillo@381 110 Email addresses can have aliases, thus they need to be expanded. Aliases can be of different kind: another local user, a remote user, a list of local and 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 recipient's domain part may require a different route to be used.
meillo@219 111
meillo@219 112
meillo@219 113
meillo@219 114
meillo@393 115 \paragraph{\RF\,5: Route management}
meillo@339 116 \label{rf5}
meillo@391 117 \index{online routes}
meillo@378 118 One key feature of \masqmail\ is its ability to send mail out over different routes. The online state defines the active route to be used. A specific route may not be suited for all messages, thus these messages are hold back until a suiting route is active. For more information on this concept see section~\ref{sec:masqmail-routes}.
meillo@219 119
meillo@219 120
meillo@219 121
meillo@219 122
meillo@393 123 \paragraph{\RF\,6: Authentication}
meillo@339 124 \label{rf6}
meillo@317 125 \label{requirement-authentication}
meillo@391 126 \index{auth}
meillo@317 127 One thing to avoid is being an \name{open relay}. Open relays allow to relay mail from everywhere to everywhere. This is a 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. It may also be wanted to refuse all connections to the \MTA\ except ones from a specific set of hosts.
meillo@391 128 \index{open relay}
meillo@391 129 \index{spam}
meillo@219 130
meillo@317 131 Several ways to restrict access are available. The most simple one is restriction by the \NAME{IP} address. No extra complexity is added this way but the \NAME{IP} addresses need to be static or within known ranges. This approach is often used to allow relaying for local nets. The access check can be done by the \MTA\ or by a guard (e.g.\ \NAME{TCP} \name{Wrappers}) before. The main advantage here is the minimal setup and maintenance work needed. This kind of access restriction is important to be implemented.
meillo@391 132 \index{access restriction}
meillo@219 133
meillo@316 134 This authentication based on \NAME{IP} addresses is impossible in situations where hosts with changing \NAME{IP} addresses, that are not part of a known sub net, need access. Then a authentication mechanism based on some \emph{secret} is required. Three common approaches exist:
meillo@334 135
meillo@285 136 \begin{enumerate}
meillo@392 137 \item \SMTP-after-\NAME{POP}: Uses authentication on the \NAME{POP} protocol to permit incoming \SMTP\ connections for a limited time afterwards. The variant \SMTP-after-\NAME{IMAP} exists, too.
meillo@392 138 \index{auth!smtp-after-pop}
meillo@232 139 \item \SMTP\ authentication: An extension to \SMTP. It allows to request authentication before mail is accepted. Here no helper protocols are needed.
meillo@391 140 \index{auth!smtp-auth}
meillo@381 141 \item Certificates: The identity of a user or a host is confirmed by certificates that are signed by trusted authorities. Certificates are closely related to encryption, they do normally satisfy both needs: encrypt the data transmission and identify the remote user/host.
meillo@391 142 \index{certificates}
meillo@285 143 \end{enumerate}
meillo@334 144
meillo@381 145 Static authentication is the preferred type for authenticating clients. It should be chosen if possible. This means if the \MTA\ resides within a trusted network or it is possible to define trusted network segments on basis of \NAME{IP} addresses, then static authentication is the best choice.
meillo@334 146
meillo@381 147 If the \MTA\ does its job in an untrusted network, if it can be expected that forged \NAME{IP} addresses will appear, or if mobile clients need access, then dynamic authentication should be used.
meillo@391 148 \index{untrusted environment}
meillo@334 149
meillo@392 150 Any combination is possible, too. For example, it is preferred to allow relay access only to authenticated users. Either clients in local networks which are authenticated by their \NAME{IP} addresses or remote clients that authenticate by a secret-based method.
meillo@334 151
meillo@381 152 Static authentication is simpler and requires less administration work but it has limitations. Dynamic authentication should be used if static authentication reaches its limits. At least one of the secret-based mechanisms should be supported.
meillo@219 153
meillo@219 154
meillo@219 155
meillo@393 156 \paragraph{\RF\,7: Encryption}
meillo@339 157 \label{rf7}
meillo@314 158 \label{requirement-encryption}
meillo@391 159 \index{enc}
meillo@381 160 Electronic mail is vulnerable to sniffing attacks, because in generic \SMTP\ all data transfer is unencrypted. The message's body, the header, and the envelope are all unencrypted. But also some authentication dialogs transfer plain text passwords (e.g.\ \NAME{PLAIN} and \NAME{LOGIN}). Hence encryption is throughout important.
meillo@391 161 \index{auth}
meillo@219 162
meillo@394 163 The common way to encrypt \SMTP\ dialogs is using \name{Transport Layer Security} (short: \NAME{TLS}, the successor of \NAME{SSL}). \NAME{TLS} encrypts the datagrams of the \name{transport layer}. This means it works below the application protocols and can be used with any of them \citeweb{wikipedia:tls}.
meillo@391 164 \index{tls}
meillo@391 165 \index{ssl}
meillo@219 166
meillo@381 167 Using secure tunnels that are provided by external programs should be preferred over including encryption into the application, because the application needs not to bother with encryption then. Outgoing \SMTP\ connections can get encrypted using a secure tunnel, created by an external application (like \name{openssl}). But incoming connections can not use external secure tunnels, because the remote \NAME{IP} address is hidden then; all connections would appear to come from localhost instead. Figure~\ref{fig:stunnel} depicts the situation of using an application like \name{stunnel} for incoming connections. The connection to port 25 comes from localhost and this information reaches the \MTA. Authentication based on \NAME{IP} addresses and many spam prevention methods are useless then.
meillo@391 168 \index{secure tunnel}
meillo@391 169 \index{stunnel}
meillo@219 170
meillo@232 171 \begin{figure}
meillo@232 172 \begin{center}
meillo@232 173 \includegraphics[scale=0.75]{img/stunnel.eps}
meillo@232 174 \end{center}
meillo@232 175 \caption{Using \name{stunnel} for incoming connections}
meillo@391 176 \index{figure!Using \name{stunnel} for incoming connections}
meillo@232 177 \label{fig:stunnel}
meillo@232 178 \end{figure}
meillo@219 179
meillo@366 180 To provide encrypted incoming channels, the \MTA\ could implement encryption and listen on a port that is dedicated to encrypted \SMTP\ (\NAME{SMTPS}). This approach would be possible, but it is deprecated in favor for \NAME{STARTTLS}. \RFC\,3207 ``\SMTP\ Service Extension for Secure \SMTP\ over Transport Layer Security'' shows this by not mentioning \NAME{SMTPS} on port 465. Also port 465 is not even reserved for \NAME{SMTPS} anymore \citeweb{iana:port-numbers}.
meillo@391 181 \index{smtps}
meillo@391 182 \index{starttls}
meillo@219 183
meillo@381 184 \NAME{STARTTLS}---defined in \RFC\,2487---is what \RFC\,3207 recommends to use for secure \SMTP. The connection then goes over port 25, but gets encrypted when the \NAME{STARTTLS} keyword is issued. Email depends on compatibility---only encryption methods that client and server support can be used. Hence it is best to act after the recommendations of the \RFC\ documents. This means \NAME{STARTTLS} encryption should be supported for incoming and for outgoing connections.
meillo@232 185
meillo@232 186
meillo@232 187
meillo@393 188 \paragraph{\RF\,8: Spam handling}
meillo@339 189 \label{rf8}
meillo@391 190 \index{spam}
meillo@381 191 Spam is a major threat nowadays, but it is a war that is hard to win. The goal is to provide state-of-the-art spam protection, but not more. (See section~\ref{sec:swot-analysis}.)
meillo@219 192
meillo@381 193 As spam is, by increasing the amount of mail messages, not just a nuisance for end users but also for the infrastructure---the \MTA{}s---they need to protect themselves.
meillo@219 194
meillo@381 195 Filtering spam can be done by either refusing it during the \SMTP\ dialog or by checking for spam after the mail was accepted and queued. Both ways have advantages and disadvantages, so modern \MTA{}s use them in combination.
meillo@391 196 \index{smtp!dialog}
meillo@219 197
meillo@381 198 Spam is usually identified by the results of a set of checks. Static rules, database querying (e.g.\ \NAME{DNS} blacklists \cite{cole07} \cite{levine08}), requesting special client behavior (e.g.\ \name{greylisting} \cite{harris03}, \name{hashcash} \cite{back02}), or statistical analysis (e.g.\ \name{bayesian filters} \cite{graham02}) are checks that may be used. Running more checks leads to better results, but takes more system resources and more time.
meillo@391 199 \index{dns blacklist}
meillo@391 200 \index{greylisting}
meillo@391 201 \index{hashcash}
meillo@391 202 \index{bayesian filter}
meillo@288 203
meillo@381 204 Doing some basic checks during the \SMTP\ dialog seems to be a must \cite[page~25]{eisentraut05}. Including these checks into the \MTA\ makes them fast to avoid \SMTP\ dialog timeouts. For modularity and reusability reasons internal interfaces to specialized modules seem to be best. \person{Raymond} says: ``Modularity (simple parts, clean interfaces) is a way to organize programs to make them simpler.'' \cite[chapter~1]{raymond03}.
meillo@391 205 \index{smtp!dialog}
meillo@391 206 \index{modularity}
meillo@219 207
meillo@381 208 More detailed checks after the message is queued should be done by external scanners. Interfaces to invoke them need to be defined. (See also the remarks about \name{amavis} in the next section.)
meillo@219 209
meillo@219 210
meillo@219 211
meillo@239 212
meillo@239 213
meillo@393 214 \paragraph{\RF\,9: Malware handling}
meillo@339 215 \label{rf9}
meillo@391 216 \index{malware}
meillo@381 217 Related to spam is malicious content (short: \name{malware}) like viruses, worms, and trojan horses. They, in contrast to spam, do not affect the \MTA\ itself, as they are in the mail's body. \MTA{}s that search for malware are equal to post offices that open letters to check if they contain something that could harm the recipient. This is not a mail transport job. But by many people the \MTA\ which is responsible for the recipient is seen to be at a good position to do this work, thus it is often done there. Though, it is nice to have interfaces to such scanners within the \MTA.
meillo@219 218
meillo@381 219 In any way should malware checking be performed by external programs that may be invoked by the \MTA. However, \NAME{MDA}s are better points to invoke content scanners.
meillo@391 220 \index{content scanner}
meillo@219 221
meillo@381 222 A popular email filter framework is \name{amavis} which integrates various spam and malware scanners. The common setup includes a receiving \MTA\ which sends mail to \name{amavis} using \SMTP, \name{amavis} processes the mail and sends it then to a second \MTA\ that does the outgoing transfer. (This setup with two \MTA\ instances is discussed in more detail in section~\ref{sec:current-code-security}.)
meillo@219 223
meillo@219 224
meillo@219 225
meillo@393 226 \paragraph{\RF\,10: Archiving}
meillo@339 227 \label{rf10}
meillo@391 228 \index{archiving}
meillo@381 229 Mail archiving and auditability become more important as email establishes as technology for serious business communication. Archiving is a must for companies in many countries. In the United States, the \name{Sarbanes-Oxley Act} \cite{sox} covers this topic.
meillo@334 230
meillo@381 231 It is a goal to have the ability to archive verbatim copies of every mail coming into and every mail going out of the system, with relation between them.
meillo@219 232
meillo@381 233 \postfix\ for example has a \name{always\_bcc} feature, to send a copy of every outgoing mail to a definable recipient. At least this functionality should be given, although a more complete approach, like \qmail\ provides, is preferable. \qmail\ is able to save copies of all sent and received messages and additionally complete \SMTP\ dialogs \cite[page~12]{sill02}.
meillo@391 234 \index{smtp!dialog}
meillo@281 235
meillo@381 236 But if archiving is of high importance, a dedicated archiving solution is advisable, anyway.
meillo@219 237
meillo@219 238
meillo@219 239
meillo@219 240
meillo@219 241 \subsection{Non-functional requirements}
meillo@391 242 \index{non-functional requirement}
meillo@219 243
meillo@381 244 Now follows a list of non-functional requirements for \masqmail. These requirements specify the quality properties of a software. The list is based on \person{Hafiz} \cite[page~2]{hafiz05}, with inspiration from \person{Spinellis} \cite[page~6]{spinellis06} and \person{Kan} \cite{kan03}.
meillo@185 245 %fixme: refer to ch01 and ch02
meillo@239 246 These non-functional requirements are named ``\NAME{RG}'' for ``requirement, general''.
meillo@146 247
meillo@146 248
meillo@393 249 \paragraph{\RG\,1: Security}
meillo@391 250 \index{security}
meillo@381 251 \MTA{}s are critical points for computer security as they are accessible from external networks. They must be secured with high effort. Properties like the need for high privilege level, from outside influenced work load, work on unsafe data, and demand for reliability, increase the need for security. This is best done by modularization, also called \name{compartmentalization}, as described in section~\ref{sec:discussion-mta-arch}.
meillo@391 252 \index{compartmentalization}
meillo@293 253
meillo@381 254 \masqmail\ needs to be secure enough for its target field of operation. \masqmail\ is targeted to workstations and private networks, with explicit warning to not use it on permanent online hosts \citeweb{masqmail:homepage2}. But as non-permanent online connections and trustable environments become rare, \masqmail's security should be so good that it is usable with permanent online connections and in unsafe environments. For example should mails with bad content not be able to break \masqmail.
meillo@391 255 \index{masqmail!security}
meillo@177 256
meillo@259 257
meillo@393 258 \paragraph{\RG\,2: Reliability}
meillo@391 259 \index{reliability}
meillo@392 260 Reliability is the second essential quality property for an \MTA. Mail for which the \MTA\ took responsibility must never get lost while it is within the \MTA's responsibility. The \MTA\ must not be \emph{the cause} of any mail loss, no matter what happens. Unreliable \MTA{}s are of no value. However, as the mail transport infrastructure is a distributed system, one of the communication partners or the transport medium may crash at any time during mail transfer. Thus reliability is needed for mail transfer communication, too.
meillo@391 261 \index{mail loss}
meillo@177 262
meillo@392 263 The goal is to transfer exactly one copy of the message. \person{Tanenbaum} evaluates the situation and comes to the conclusion that ``in general, there is no way to arrange this.'' \cite[pages~377--379]{tanenbaum02}. Only strategies where no mail gets lost are acceptable; he identifies three of them, but one generates more duplicates than the others, so two strategies remain. (1) The client always reissues the transfer. The server first sends an acknowledgment and then handles the transfer. (2) The client reissues the transfer only if no acknowledgment was received. The server first handles the transfer and sends the acknowledgment afterwards. The first strategy does not need acknowledgments at all, however, it will lose mail if the second transfer fails, too.
meillo@189 264
meillo@381 265 Hence, mail transfer between two processes should use the strategy: The client reissues if it receives no acknowledgment. The server first handles the message and then sends the acknowledgment. This strategy only leads to duplicates if a crash happens in the time between the message is fully transferred to the server and the acknowledgment is received by the client. No mail will get lost.
meillo@391 266 \index{duplicates}
meillo@239 267
meillo@239 268
meillo@393 269 \paragraph{\RG\,3: Robustness}
meillo@391 270 \index{robustness}
meillo@381 271 Being robust means handling errors properly. Small errors may get corrected, large errors may kill a process. Killed processes should get restarted automatically 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 a friendly environment itself. \person{Raymond}'s \name{Rule of Robustness} and his \name{Rule of Repair} are good descriptions \cite[pages~18--21]{raymond03}.
meillo@177 272
meillo@177 273
meillo@393 274 \paragraph{\RG\,4: Extendability}
meillo@391 275 \index{extendability}
meillo@392 276 \masqmail's architecture needs to be extendable to allow new features to be added afterwards. The reasons for this need are the changing requirements. New requirements will appear, like more efficient mail transfer of large messages or a final solution to the spam problem. Extendability is the ability of software to include new function with little work.
meillo@196 277
meillo@146 278
meillo@393 279 \paragraph{\RG\,5: Maintainability}
meillo@391 280 \index{maintainability}
meillo@381 281 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 will ease all further work.
meillo@146 282
meillo@189 283
meillo@393 284 \paragraph{\RG\,6: Testability}
meillo@391 285 \index{testability}
meillo@381 286 Good testability make maintenance easier too, because functionality is directly verifiable when changes are done, thus removing the uncertainty. Modularized software makes testing easier, because parts can be tested without external influences. \person{Spinellis} sees testability as a sub-quality of maintainability.
meillo@189 287
meillo@189 288
meillo@393 289 \paragraph{\RG\,7: Performance}
meillo@391 290 \index{performance}
meillo@381 291 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, thus time performance is less important. \masqmail\ is not a program to be used on large servers, but on small devices. Thus more important for \masqmail\ will be energy and heat saving, maybe also system resources.
meillo@293 292
meillo@381 293 As performance improvements are in contrast to many other quality properties (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}. Simplicity and clearness are of higher value.
meillo@293 294
meillo@146 295
meillo@146 296
meillo@393 297 \paragraph{\RG\,8: Availability}
meillo@391 298 \index{availability}
meillo@381 299 Availability is important for server programs. They must stay operational by blocking \name{denial of service} attacks and the like. Automated restarts into a clean state after fatal errors are also required.
meillo@146 300
meillo@146 301
meillo@393 302 \paragraph{\RG\,9: Portability}
meillo@391 303 \index{portability}
meillo@393 304 Source code that compiles and runs on various operation systems is called portable. Portability can be achieved by using standard features of the programming language and common libraries. Basic rules to achieve portable code are defined by \person{Kernighan} and \person{Pike} \cite{kernighan99}. Portable code lets software spread faster. Portability among the various flavors of Unix systems is a goal for \masqmail, because these systems are the ones \MTA{}s usually run on. No special care needs to be taken for non-Unix platforms.
meillo@293 305
meillo@189 306
meillo@189 307
meillo@393 308 \paragraph{\RG\,10: Usability}
meillo@391 309 \index{usability}
meillo@381 310 Usability, not mentioned by \person{Hafiz} (he focuses on architecture) but by \person{Spinellis} and \person{Kan}, is a property which is very important from the user's point of view. Software with bad usability is rarely used, no matter how good it is. If substitutes with better usability exist, the user will switch to one of them. Here, usability includes setting up and configuring; the term ``users'' includes administrators. Having \MTA{}s on home servers and workstations requires easy and standardized configuration. The common setups should be configurable with little action by the user. Complex configuration should be possible, but the focus should be on the most common form of configuration: choosing one of several common setups.
meillo@185 311
meillo@298 312 %fixme: << masqmail as portable app? >>
meillo@185 313
meillo@185 314
meillo@185 315
meillo@293 316 \subsection{Architecture}
meillo@239 317 \label{sec:discussion-mta-arch}
meillo@391 318 \index{architecture}
meillo@187 319
meillo@318 320 %fixme: what's this section to do with requirements?
meillo@242 321
meillo@381 322 \masqmail's current architecture is monolithic like \sendmail's and \exim's. But more than the other two is it one block of interweaved code. \exim\ has a highly structured code with many internal interfaces, a good example is the interface for authentication ``modules''. %fixme: add ref
meillo@381 323 \sendmail\ provides now, with its \name{milter} interface, standardized connection channels to external modules. \masqmail\ has none of them---it is what \sendmail\ was in the beginning: a single large block.
meillo@391 324 \index{milter}
meillo@391 325 \index{masqmail!architecture}
meillo@161 326
meillo@381 327 Figure~\ref{fig:masqmail-arch} is a call graph generated from \masqmail's source code. It gives an impression of how interweaved the internals are. There are no compartments at all.
meillo@391 328 \index{masqmail!call graph}
meillo@391 329 \index{call graph}
meillo@161 330
meillo@161 331 \begin{figure}
meillo@161 332 \begin{center}
meillo@219 333 \vspace*{2ex}
meillo@256 334 %\includegraphics[scale=0.75]{img/callgraph.eps}
meillo@339 335 %\includegraphics[scale=0.75]{img/masqmail-3-omitlog5.eps}
meillo@339 336 \includegraphics[scale=0.75]{img/bb.eps}
meillo@161 337 \end{center}
meillo@339 338 \caption{Internal structure of \masqmail, showed by a call graph. (Logging functions are ignored; test and \NAME{POP3} code is excluded.)}
meillo@391 339 \index{figure!Internal structure of \masqmail.}
meillo@161 340 \label{fig:masqmail-arch}
meillo@161 341 \end{figure}
meillo@161 342
meillo@378 343 \sendmail\ improved its old architecture by adding the milter interface, to include further functionality by invoking 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} describes this evolution of \MTA\ architecture very well \cite{hafiz05}.
meillo@391 344 \index{security}
meillo@161 345
meillo@381 346 Every one of these programs is more modular, or became more modular over time, than \masqmail\ is. Modern requirements like spam protection and probable future requirements like the use of new mail transport protocols demand for modular designs in order to keep the software simple. Simplicity is a key property for security. ``[T]he essence of security engineering is to build systems that are as simple as possible.'' \cite[page 45]{graff03}.
meillo@391 347 \index{modularity}
meillo@161 348
meillo@219 349 \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@391 350 \index{compartmentalization}
meillo@381 351
meillo@163 352 \begin{quote}
meillo@218 353 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.
meillo@218 354 \hfill\cite[page 64]{hafiz05}
meillo@163 355 \end{quote}
meillo@200 356
meillo@381 357 Equal does \person{Dent} see the situation for \postfix: ``The modular architecture of Postfix forms the basis for much of its security.'' \cite[page 7]{dent04}.
meillo@391 358 \index{modularity}
meillo@161 359
meillo@381 360 Modularity is also needed to satisfy modern \MTA\ requirements in providing a clear interface to add functionality without increasing the overall complexity much.
meillo@381 361
meillo@381 362 Modularity is no direct requirement but a goal that has positive influence on important requirements like security, testability, extendability, maintainability, and not least simplicity. These quality properties then, on their part, make it easier to achieve the functional requirements.
meillo@391 363 \index{security}
meillo@239 364
meillo@242 365 Hence, aspiration for modularity, by compartmentalization, improves the overall quality and function of the software. It can be seen as an architectural requirement for a secure and modern \MTA.
meillo@239 366
meillo@277 367 %fixme: explain: why are compartments and interfaces so good?
meillo@239 368
meillo@239 369
meillo@239 370
meillo@288 371
meillo@288 372
meillo@288 373
meillo@288 374
meillo@288 375
meillo@288 376
meillo@239 377 \section{Fulfilled requirements}
meillo@239 378 \label{sec:fulfilled-requirements}
meillo@239 379
meillo@239 380 Here follows a description of how far the requirements are already fulfilled by \masqmail.
meillo@239 381
meillo@239 382
meillo@393 383 \paragraph{\RF\,1: In/out channels}
meillo@391 384 \index{incoming channels}
meillo@391 385 \index{outgoing channels}
meillo@393 386 The incoming and outgoing channels that \masqmail\ already has (depicted in figure~\ref{fig:masqmail-channels} on page \pageref{fig:masqmail-channels}) are the ones required for an \MTA{}s at the moment. Currently, support for other protocols seems not to be necessary, although new protocols and mailing concepts are likely to appear (see section~\ref{sec:email-trends}). As other protocols are not required today, \masqmail\ is regarded to fulfill \RF\,1. Without any support in \masqmail\ for adding further protocols, the best strategy is to delaying such work until the functionality is essential, anyway.
meillo@239 387
meillo@339 388 %fixme: << smtp submission >> %fixme
meillo@287 389
meillo@393 390 \paragraph{\RF\,2: Queuing}
meillo@391 391 \index{mail queue}
meillo@381 392 One single mail queue is used in \masqmail. It satisfies all current requirements.
meillo@239 393
meillo@393 394 \paragraph{\RF\,3: Header sanitizing}
meillo@391 395 \index{header sanitizing}
meillo@239 396 The envelope and mail headers are generated when the mail is put into the queue. The requirements are fulfilled.
meillo@239 397
meillo@393 398 \paragraph{\RF\,4: Aliasing}
meillo@391 399 \index{aliases}
meillo@381 400 Aliasing is done on delivery. All common kinds of aliases in the global aliases file are supported. So called \name{.forward} aliasing is not supported, but this is less common and seldom used.
meillo@239 401
meillo@393 402 \paragraph{\RF\,5: Route management}
meillo@391 403 \index{online routes}
meillo@381 404 Querying the name of the active route is done on delivery. Headers can get rewritten a second time then. This part does provide all the functionality required.
meillo@239 405
meillo@393 406 \paragraph{\RF\,6: Authentication}
meillo@391 407 \index{auth}
meillo@392 408 Static authentication, based on \NAME{IP} addresses, can be achieved with \person{Venema}'s \NAME{TCP} \name{Wrapper} \cite{venema92}, by editing the \path{hosts.allow} and \path{hosts.deny} files. This is only relevant to authenticate hosts that try to submit mail into the system. Dynamic (secret-based) \SMTP\ authentication is already supported in form of \NAME{SMTP-AUTH} and \SMTP-after-\NAME{POP}, but only for outgoing connections. For incoming connections only address-based authentication is supported.
meillo@391 409 \index{auth!smtp-after-pop}
meillo@391 410 \index{auth!smtp-auth}
meillo@239 411
meillo@393 412 \paragraph{\RF\,7: Encryption}
meillo@391 413 \index{enc}
meillo@381 414 Similar is the situation for encryption which is also only available for outgoing channels; here a tunnel application, like \name{openssl}, is needed. A secure tunnel can be created to send mail trough. State-of-the-art, however, is using \NAME{STARTTLS}, but this is not supported. For incoming channels, no encryption is available. The only possible setup to provide encryption of incoming channels is using an application like \name{stunnel} to crypt between the secure connection to the remote host and the plain connection to the \MTA. Unfortunately, this suffers from the problem explained on page \pageref{fig:stunnel} in figure~\ref{fig:stunnel}. Anyway, it would still be no \NAME{STARTTLS} support.
meillo@391 415 \index{secure tunnel}
meillo@239 416
meillo@393 417 \paragraph{\RF\,8: Spam handling}
meillo@391 418 \index{spam!handling}
meillo@381 419 \masqmail\ does not provide special support for spam filtering. Spam prevention by not accepting spam during the \SMTP\ dialog is not possible at all. Spam filtering is only possible by using two \masqmail\ instances with an external spam filter in between. The mail flow is from the receiving \MTA\ instance, which accepts mail, to the filter application that processes and possible modifies it, to the second \MTA\ which is responsible for further delivery of the mail. This is a concept that works in general, and it is good to separate different work with clear interfaces. But the need of two instances of the same \MTA, with doubled setup, makes it rather a work-around. Better is to have this data flow respected in the \MTA\ design, like it was done in \postfix. Anyway, the more important part of spam handling, for sure, is done during the \SMTP\ dialog by completely refusing unwanted mail.
meillo@239 420
meillo@393 421 \paragraph{\RF\,9: Malware handling}
meillo@391 422 \index{malware!handling}
meillo@381 423 For malware handling applies nearly the same as for spam handling, except that all checks are done after mail is accepted. The possible setup is the same with the two \MTA\ instances and the filter in between. \masqmail\ does support such a setup, but not in a nice way.
meillo@239 424
meillo@393 425 \paragraph{\RF\,10: Archiving}
meillo@391 426 \index{archiving}
meillo@381 427 There is currently no way for archiving every message that does through \masqmail.
meillo@239 428
meillo@239 429
meillo@239 430
meillo@393 431 \paragraph{\RG\,1: Security}
meillo@391 432 \index{security}
meillo@381 433 \masqmail's current security is bad. However, it seems acceptable for using \masqmail\ on workstations and private networks, if the environment is trustable and \masqmail\ is protected against remote attacks. In environments where untrusted components or persons have access to \masqmail, its security is too low. Its author states that \masqmail\ ``is not designed to'' such usage \citeweb{masqmail:homepage2}. This is a clear indicator for being careful. Issues like high memory consumption, low performance, and denial-of-service attacks---things not regarded by design---may cause serious problems. In any way, a security report that confirms \masqmail's security level is missing.
meillo@391 434 \index{masqmail!security}
meillo@259 435
meillo@392 436 \masqmail\ uses conditional compilation to exclude unneeded functionality from the executable at compile time. Excluding code means excluding all bugs and weaknesses within this code, too. Excluding unused code is a good concept to improve security.
meillo@391 437 \index{conditional compilation}
meillo@239 438
meillo@393 439 \paragraph{\RG\,2: Reliability}
meillo@391 440 \index{reliability}
meillo@381 441 Its reliability is also not good enough. Situations where only one part of a sent message was removed from the queue and the other part remained as garbage, showed off \citeweb{debian:bug245882}. Problems with large mail messages in conjunction with small bandwidth were also reported \citeweb{debian:bug216226}. Fortunately, lost email was no big problem yet, but \person{Kurth} warns:
meillo@391 442 \index{masqmail!bugs}
meillo@381 443
meillo@163 444 \begin{quote}
meillo@239 445 There may still be serious bugs in [masqmail], so mail might get lost. But in the nearly two years of its existence so far there was only one time a bug which caused mail retrieved via pop3 to be lost in rare circumstances.
meillo@239 446 \hfill\citeweb{masqmail:homepage2}
meillo@163 447 \end{quote}
meillo@381 448
meillo@239 449 In summary: Current reliability needs to be improved.
meillo@239 450 %fixme: state machine
meillo@161 451
meillo@393 452 \paragraph{\RG\,3: Robustness}
meillo@391 453 \index{robustness}
meillo@381 454 The logging behavior of \masqmail\ is good, although it does not cover the whole code. 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. The origin of this problem, however, is \masqmail's trust in its environment.
meillo@318 455 %fixme: rule of robustness, rule of repair
meillo@239 456
meillo@393 457 \paragraph{\RG\,4: Extendability}
meillo@391 458 \index{extendability}
meillo@381 459 \masqmail's extendability is very poor. This is a general problem of monolithic software, but can though be provided with high effort. \exim\ is an example for good extendability in a monolithic program.
meillo@239 460
meillo@393 461 \paragraph{\RG\,5: Maintainability}
meillo@391 462 \index{maintainability}
meillo@381 463 The maintainability of \masqmail\ is equivalent to other software of similar kind. Missing modularity and therefore more complexity makes the maintainer's work harder. Conditional compilation might be good for security, but \name{ifdef}s scattered throughout the source code is a pain for maintenance. In summary is \masqmail's maintainability bearable, like in average Free Software projects.
meillo@239 464
meillo@288 465
meillo@287 466
meillo@393 467 \paragraph{\RG\,6: Testability}
meillo@391 468 \index{testability}
meillo@381 469 The testability suffers from missing modularity, too. Testing program parts is hard to do. Nevertheless, it is done by compiling parts of the source to two special test programs: One tests reading input from a socket, the other tests constructing messages and sending it directly. Neither is designed for automated testing of source parts, they are rather to help the programmer during development.
meillo@239 470
meillo@381 471 Two additional scripts exist to send a set of mails to differend kinds of recipients. They can be used for automated testing, but both check only the function of the whole system, not its parts.
meillo@391 472 \index{test program}
meillo@339 473
meillo@339 474 %fixme: think about clean-room testing
meillo@293 475
meillo@393 476 \paragraph{\RG\,7: Performance}
meillo@391 477 \index{performance}
meillo@239 478 The performance---efficiency---of \masqmail\ is good enough for its target field of operation, where this is a minor goal.
meillo@239 479
meillo@393 480 \paragraph{\RG\,8: Availability}
meillo@391 481 \index{availability}
meillo@239 482 This applies equal to availability. Hence no further work needs to be done her.
meillo@239 483
meillo@393 484 \paragraph{\RG\,9: Portability}
meillo@391 485 \index{portability}
meillo@391 486 The code's portability is good with view on Unix-like operation systems. At least \name{Debian}, \name{Red Hat}, \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}. Special requirements for the underlying file system are not known. Thus, the portability is already good.
meillo@391 487 \index{masqmail!supported systems}
meillo@239 488
meillo@293 489
meillo@393 490 \paragraph{\RG\,10: Usability}
meillo@391 491 \index{usability}
meillo@381 492 The usability is very good, from the administrator's point of view. \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 \MUA. Configuration could be eased even more by providing configuration generators that enable \masqmail\ to be used right ``out of the box'' after running one of several configuration scripts for common setups. This would improve \masqmail's usability for not technical educated people.
meillo@391 493 \index{out-of-the-box usage}
meillo@239 494
meillo@239 495
meillo@239 496
meillo@239 497
meillo@239 498
meillo@239 499 \section{Work to do}
meillo@239 500
meillo@381 501 After the requirements for modern \MTA{}s were identified in section~\ref{sec:mta-requirements} and \masqmail's features were compared against them in section~\ref{sec:fulfilled-requirements}, here the pending work is identified. Table~\ref{tab:requirements} lists all requirements with importance and the work that is needed to achieve them. The column ``Focus'' shows the attention a work task should get. The focus depends on the task's importance and the amount of work it includes.
meillo@239 502
meillo@239 503 \begin{table}
meillo@239 504 \begin{center}
meillo@271 505 \input{tbl/requirements.tbl}
meillo@239 506 \end{center}
meillo@239 507 \caption{Importance of and pending work for requirements}
meillo@391 508 \index{table!Importance of and pending work for requirements}
meillo@242 509 \label{tab:requirements}
meillo@239 510 \end{table}
meillo@239 511
meillo@381 512 The importance is ranked from `-{}-' (not important) to `++' (very important). The pending work is ranked from `-{}-' (nothing) to `++' (very much). Large work tasks with high importance need to receive much attention, they need to be in focus. In contrast should small, low importance work tasks receive few attention. Here the focus for a task is calculated by summing up the importance and the pending work with equal weight. Normally, tasks with high focus are the ones of high priority and should be done first.
meillo@239 513
meillo@381 514 The functional requirements that receive highest attention are \RF\,6 (authentication), \RF\,7 (encryption), and \RF\,8 (spam handling). Of the non-functional requirements, \RG\,1 (security), \RG\,2 (reliability), and \RG\,4 (extendability), rank highest.
meillo@391 515 \index{requirements!ranking}
meillo@239 516
meillo@381 517 These tasks are presented in more detail in a todo list, now. The list is sorted by focus and then by importance.
meillo@239 518
meillo@239 519
meillo@393 520 \subsubsection*{\TODO\,1: Encryption (\RF\,7)}
meillo@391 521 \index{enc}
meillo@381 522 Encryption is chosen for number one as it is essential to provide privacy. Using \NAME{STARTTLS} for encryption is definitely needed and should be added first; encrypted data transfer is hardly possible without support for it.
meillo@288 523
meillo@239 524
meillo@393 525 \subsubsection*{\TODO\,2: Authentication (\RF\,6)}
meillo@391 526 \index{auth}
meillo@381 527 Authentication of incoming \SMTP\ connections is also highly needed and should be added second. It is important to restrict access and to prevent relaying. For workstations and local networks, this has only medium importance and address-based authentication is sufficient in most times. But secret-based authentication is mandatory to receive mail from the Internet. Additionally it is a guard against spam.
meillo@288 528
meillo@239 529
meillo@393 530 \subsubsection*{\TODO\,3: Security (\RG\,1)}
meillo@391 531 \index{security}
meillo@392 532 \masqmail's security is bad, thus the program is forced into a limited field of operation. This field of operation even shrinks as security becomes more important and networking and interaction increases. Secure and trusted environment become rare, thus improving security is an important thing to do. The focus should be on adding compartments to split \masqmail\ into separate modules. (See section~\ref{sec:discussion-mta-arch}.) Furthermore, \masqmail's security should be tested throughout to get a definitive view how good it really is and where the weak spots are.
meillo@391 533 \index{modularity}
meillo@239 534
meillo@239 535
meillo@393 536 \subsubsection*{\TODO\,4: Reliability (\RG\,2)}
meillo@391 537 \index{reliability}
meillo@381 538 Reliability is also to improve. It is a key quality property for an \MTA, and not good enough in \masqmail. Reliability is strong related to the queue, thus improvements there are favorable. Applying ideas of \name{crash-only software} \cite{candea03} will be a good step. \person{Candea} and \person{Fox} see in killing the process the best way to stop a running program. Doing so inevitably demands for good reliability of the queue, and the start up process inevitably demands for good recovery. Those critical situations for reliability are nothing special anymore, they are common. Hence they are regularly tested and will definitely work.
meillo@391 539 \index{crash-only software}
meillo@288 540
meillo@239 541
meillo@393 542 \subsubsection*{\TODO\,5: Spam handling (\RF\,8)}
meillo@391 543 \index{spam!handling}
meillo@381 544 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. Configuration guides for a setup of two \masqmail\ instances with a spam scanner in between should be written. And at least a basic kind of spam prevention during the \SMTP\ dialog should be implemented.
meillo@239 545
meillo@241 546
meillo@393 547 \subsubsection*{\TODO\,6: Extendability (\RG\,4)}
meillo@391 548 \index{extendability}
meillo@381 549 \masqmail\ lacks an interface to plug in modules with additional functionality. There exists no add-on or module system. The code is only separated by function into various source files. Some functional parts can be included or excluded by conditional compilation. But the \name{ifdef}s are scattered through all the code. This situation needs to be improved by collecting related function into single places that interact through clear interfaces with other parts. Also should these interfaces allow efficient adding of further functionality.
meillo@391 550 \index{conditional compilation}
meillo@254 551
meillo@239 552
meillo@239 553
meillo@239 554
meillo@239 555
meillo@239 556
meillo@239 557
meillo@239 558
meillo@239 559
meillo@239 560
meillo@239 561 \section{Ways for further development}
meillo@391 562 \index{development strategies}
meillo@239 563
meillo@293 564 Knowing what needs to be done is only one part, the other is deciding \emph{how} to do it by focusing on a global development strategy.
meillo@293 565
meillo@293 566
meillo@293 567 \subsection{Possibilities}
meillo@293 568
meillo@316 569 Further development of software can always go three different ways:
meillo@241 570 \begin{enumerate}
meillo@391 571 \item Improve the current code base. (S\,1)
meillo@391 572 \item Add wrappers or interposition filters. (S\,2)
meillo@391 573 \item Redesign the software from scratch and rebuild it. (S\,3)
meillo@241 574 \end{enumerate}
meillo@239 575
meillo@381 576 The first two strategies base on the available source code and can be applied in combination. The third strategy splits from the old code base and starts over again. Wrappers and interposition filters would be outright included into a new architecture; they are a subset of a new design. Of course, parts of existing code can be used in a new design if appropriate.
meillo@391 577 \index{wrapper}
meillo@391 578 \index{interposition filter}
meillo@239 579
meillo@241 580
meillo@393 581 The requirements are now regarded, each on its own, and are linked to the development strategy that is preferred to reach each specific requirement. If some requirement is well achievable by using different strategies then it is linked to all of them. Implementing encryption (\TODO\,1) and authentication (\TODO\,2), for example, are limited to a narrow region in the code. Such features are addable to the current code base without much problem. In contrast can quality properties like reliability (\TODO\,4), extendability (\TODO\,6), and maintainability hardly be added to code afterwards---if at all. Security (\TODO\,3) is improvable in a new design, of course, but also with wrappers or interposition filters.
meillo@293 582
meillo@381 583 This linking of requirements to the strategies is shown in table~\ref{tab:strategies}. The requirements are ordered by their focus.
meillo@241 584
meillo@241 585 \begin{table}
meillo@241 586 \begin{center}
meillo@271 587 \input{tbl/strategies.tbl}
meillo@241 588 \end{center}
meillo@241 589 \caption{Development strategies and their suitability for requirements}
meillo@391 590 \index{figure!Development strategies and their suitability for requirements}
meillo@241 591 \label{tab:strategies}
meillo@241 592 \end{table}
meillo@241 593
meillo@239 594
meillo@381 595 Next, the best strategy for further development needs to be discovered. Therefore a score for each strategy is obtained by summing up the focus points of each requirement for which a strategy is preferred. Only positive focus points are regarded, with each plus symbol counting one. Requirements with negative focus are not regarded because they are already or nearly reached; the view here is on outstanding work.
meillo@267 596
meillo@381 597 Strategy 1 (Improve current code) has a score of 9 points. Strategy 2 (Wrappers and interposition filters) has a score of 7 points. Strategy 3 (A new design) scores on top with 17 points. S\,1 and S\,2 can be used in combination; the combined score is 13 points. Thus strategy 3 ranges first, followed by the combination of strategy 1 and 2.
meillo@267 598
meillo@381 599 This leads to the conclusion that S\,3 (A new design) is probably the best strategy for further development.
meillo@381 600
meillo@392 601 But this result respects only the view on requirements and their relevance. Other factors like development effort and risks are important to think about, too. These issues are discussed in the following sections, comparing S\,3 against the combination S\,1+2.
meillo@267 602
meillo@267 603
meillo@267 604
meillo@267 605
meillo@267 606
meillo@267 607
meillo@267 608
meillo@267 609
meillo@293 610 \subsection{Discussion}
meillo@323 611 \label{sec:discussion-further-devel}
meillo@267 612
meillo@267 613
meillo@296 614 \subsubsection*{Quality improvements}
meillo@391 615 \index{quality improvement}
meillo@239 616
meillo@392 617 Most quality properties can hardly be added afterwards. Hence, if reliability, extendability, or maintainability shall be improved, a redesign of \masqmail\ is the best way to take. The wish to improve quality, inevitably point towards a modular architecture. Modularity with internal and external interfaces is highly preferred from the architectural point of view (see section~\ref{sec:discussion-mta-arch}). The need for further features, especially ones that require changes in \masqmail's structure, support the decision for a new design, too. Hence a rewrite is favored if \masqmail\ should become a modern \MTA\ with good quality properties.
meillo@391 618 \index{modularity}
meillo@288 619
meillo@288 620
meillo@241 621
meillo@293 622 \subsubsection*{Security}
meillo@391 623 \index{security}
meillo@261 624
meillo@293 625 Similar is the situation for security. Security comes from good design, explain \person{Graff} and \person{van Wyk}:
meillo@381 626
meillo@241 627 \begin{quote}
meillo@241 628 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 maintenance of the software.
meillo@241 629 %
meillo@241 630 %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@241 631 \hfill\cite[page 55]{graff03}
meillo@241 632 \end{quote}
meillo@391 633 \index{good design}
meillo@241 634
meillo@296 635 They also suggest to add wrappers and interposition filters \emph{around} applications, but more as repair techniques if it is not possible to design security \emph{into} a software the first way \cite[pages~71--72]{graff03}.
meillo@391 636 \index{wrapper}
meillo@391 637 \index{interposition filter}
meillo@241 638
meillo@381 639 \person{Hafiz} adds: ``The major idea is that security cannot be retrofitted \emph{into} an architecture.'' \cite[page 64, (emphasis added)]{hafiz05}.
meillo@391 640 \index{security!retrofitted}
meillo@241 641
meillo@241 642
meillo@241 643
meillo@241 644
meillo@293 645 \subsubsection*{Effort estimation}
meillo@391 646 \index{effort estimation}
meillo@293 647
meillo@293 648 Although a strategy might lead to the best result, one may choose another one if the required effort is too high. The effort for a redesign and rebuild is estimated now.
meillo@293 649
meillo@293 650 \person{Wheeler}'s program \name{sloccount} calculates following estimations for \masqmail's code base as of version 0.2.21 (excluding library code):
meillo@391 651 \index{masqmail!development effort}
meillo@293 652
meillo@293 653 \codeinput{input/masqmail-sloccount.txt}
meillo@293 654
meillo@391 655 The development costs in money are not relevant for a Free Software project with volunteer developers, but the development time is. About 24 man-months are estimated. The current code base was written almost completely by \person{Oliver Kurth} within four years in his spare time. This means he needed around twice as much time. Of course, he programmed as a volunteer developer not as an employee with eight work-hours per day.
meillo@293 656
meillo@392 657 Given the assumptions that (1) an equal amount of code needs to be produced for a new designed \masqmail, (2) a third of the existing code can be reused plus concepts and knowledge, and (3) development speed is like \person{Kurth}'s, then it would take between two and three years for one programmer to produce a redesigned new \masqmail\ with the same features that \masqmail\ now has. Less time would be needed if a simpler architecture allows faster development, better testing, and less bugs. Of course, more developers would speed it up, too.
meillo@293 658
meillo@293 659
meillo@293 660
meillo@293 661
meillo@293 662 \subsubsection*{Risks}
meillo@391 663 \index{risks}
meillo@293 664
meillo@381 665 The gained result of a new design might still outweigh the development effort. But risks are something more to consider.
meillo@293 666
meillo@293 667 A redesign and rewrite of software from scratch is hard. It takes time to design a new architecture, which then must prove that it is as good as expected. As well is much time and work needed to implement the design, test it, fix bugs, and so on. If flaws in the design appear during prototype implementation, it is necessary to start again.
meillo@293 668
meillo@381 669 Such a redesign can fail at many points and it is long time unclear if the result is really better than the code that already exists. Even if the new code is working like expected, it is still not matured.
meillo@293 670
meillo@296 671 One thing is clear: Doing a redesign and rebuild \emph{is} a risky decision.
meillo@293 672
meillo@293 673
meillo@293 674
meillo@293 675 \subsubsection*{Existing code is precious}
meillo@391 676 \index{existing code}
meillo@293 677
meillo@296 678 If a new design needs much effort and additionally is a risk, what about the existing code base then?
meillo@293 679
meillo@381 680 Adding new functionality to an existing code base seems to be a secure and cheap strategy. The existing code is known to work and features can often be added in small increments. Risks like wasted effort if a new design fails are hardly existent, and the faults in the current design are already made and most probably fixed.
meillo@293 681
meillo@381 682 Functionality that is hard to add incrementally into the application, like support for new protocols, may be addable to the outside. \masqmail\ can be secured to a huge amount by guarding it with wrappers that block attackers. Spam and malware scanners can be included by running two instances of \masqmail. All those methods base on the current code which they can indirectly improve.
meillo@391 683 \index{wrapper}
meillo@391 684 \index{extendability}
meillo@293 685
meillo@296 686 The required effort is probably under one third of a new design and work directly shows results. These are strong arguments against a new design.
meillo@293 687
meillo@293 688
meillo@296 689
meillo@293 690
meillo@293 691 \subsubsection*{Repairing}
meillo@391 692 \index{reparing}
meillo@293 693
meillo@392 694 Besides these advantages of existing code, one must not forget that further work on it is often repair work. Small bug fixes are not the problem, but adding something for which the software originally was not designed, will cause problems. Such work often destroys the clear concepts of the software, especially in interweaved monolithic code.
meillo@293 695
meillo@381 696 \person{Doug McIlroy}, a person with important influence on Unix especially by inventing the Unix pipe, demands: ``To do a new job, build afresh rather than complicate old programs by adding new features.'' \cite{mcilroy78}.
meillo@293 697
meillo@381 698 Repair strategies are useful, but only in the short-time view and in times of trouble. If the future is bright, however, one does best by investing into a software. As shown in section~\ref{sec:market-analysis-conclusion}, the future for \MTA{}s is bright. This means it is time to invest into a redesign with the intension to build up a more modern product.
meillo@381 699
meillo@381 700 In the author's view is \masqmail\ already needing this redesign since about 2003 when the old design was still quite suitable \dots\ it already delayed too long.
meillo@391 701 \index{masqmail!redesign}
meillo@293 702
meillo@296 703 %Clinging to much to existing code will be no help, it is an indicator for fear. Having the courage to through bad code away to make it better, shows the view forward.
meillo@296 704
meillo@392 705 Anyway, further development on base of current code needs to improve the quality properties, too. Some quality requirements can be satisfied by adding wrappers or interposition filters to the outside. For those is the development effort approximately equal to a solution with a new design. But for adding quality requirements like extendability or maintainability which affect the source code throughout, the effort does increase with exponential rate as development proceeds. In case these properties get not improved, development will likely come to a dead end sooner or later.
meillo@391 706 \index{quality improvement}
meillo@293 707
meillo@293 708
meillo@293 709
meillo@293 710
meillo@293 711
meillo@293 712 \subsubsection*{A guard against dead ends}
meillo@391 713 \index{dead ends}
meillo@293 714
meillo@296 715 A new design does protect against such dead ends.
meillo@293 716
meillo@381 717 Changing requirements are one possible dead end if the software does not evolve with them. A famous example is \sendmail, which had an almost monopoly for a long time. But when security became important, \sendmail\ was only repaired instead of the problem sources---its insecure design---would have been removed. Thus security problems reappeared and over the years \sendmail's market share shrank as more secure \MTA{}s became available. \sendmail's reaction to the new requirements, in form of \name{sendmail~X} and \name{MeTA1}, came much to late---the users already switched to other \MTA{}s.
meillo@391 718 \index{sendmail}
meillo@293 719
meillo@381 720 Redesigning a software as requirements change helps keeping it alive. % fixme: add quote: ``one thing surely remains: change'' (something like that)
meillo@391 721 \index{redesign}
meillo@381 722
meillo@393 723 Another danger is the dead end of complexity which is likely to appear by constant work on the same code base. It is even more likely if the code base has a monolithic architecture. A good example for simplicity is \qmail\ which consists of small independent modules, each with only about one thousand lines of code. Such simple code makes it obvious to understand what it does. The \name{suckless} project \citeweb{suckless.org} for example advertises such a philosophy of small and simple software by following the thoughts of the Unix inventors \cite{kernighan84} \cite{kernighan99}. Simple, small, and clear code avoids complexity and is thus also a strong prerequisite for security.
meillo@391 724 \index{suckless}
meillo@293 725
meillo@293 726
meillo@293 727
meillo@293 728
meillo@293 729
meillo@293 730 \subsubsection*{Modularity}
meillo@391 731 \index{modularity}
meillo@293 732
meillo@392 733 The avoidance of dead ends is essential for further development on current code, too. Hence it is mandatory to refactor the existing code base sooner or later. Most important is the intention to modularize it, as modularity improves many quality properties, eases further development, and essentially improves security.
meillo@293 734
meillo@381 735 One example how modular structure makes it easy to add further functionality is described by \person{Sill}: He says that integrating the \name{amavis} filter framework into the \qmail\ system can be done by simply renaming the \path{qmail-queue} module to \path{qmail-queue-real} and renaming the \path{amavis} executable to \path{qmail-queue} \cite[section~12.7.1]{sill02}. Nothing more in the \qmail\ system needs to be changed. This is a very admirable ability which is only possible in a modular system that consists of independent executables.
meillo@391 736 \index{modularity}
meillo@293 737
meillo@381 738 This thesis showed several times that modularity is a key property for good software design. Modularity can hardly be retrofitted into software, hence development on base of current code will need a throughout restructuring too, to modularize the source code. Thus a new design is similar to such a throughout refactoring, except the dependence on current code.
meillo@293 739
meillo@293 740
meillo@293 741
meillo@293 742
meillo@293 743
meillo@293 744
meillo@296 745 \subsubsection*{Function versus quality}
meillo@391 746 \index{quality improvement}
meillo@293 747
meillo@381 748 Remarkable is the distribution of functional and non-functional requirements to the strategies. The strategies for current code (S\,1+2) have a functional to non-functional ratio of 10 to 3. The new design strategy (S\,3) has a ratio of 5 to 12.
meillo@293 749
meillo@381 750 This classifies current code to be better suited for adding functionality, and a new design to be better suited for quality improvements. Both strategies need to improve function as well as quality, however, the focus of the strategy is determined by this difference.
meillo@293 751
meillo@381 752 Easier work is likely to be done earlier in Free Software projects than hard work. Thus, by choosing S\,1+2 volunteer developers tend to implement function first and delay quality improvements, no matter what the suggested order of the work tasks is. S\,3, in contrast, would benefit early quality improvements and later function improvements. This is real-life experience from Free Software development.
meillo@293 753
meillo@293 754
meillo@293 755
meillo@293 756
meillo@293 757
meillo@293 758
meillo@297 759 \subsubsection*{Break Even}
meillo@391 760 \index{Break Even}
meillo@293 761
meillo@381 762 It is important to keep the time dimension in mind. This includes the separation into a short-time and a long-time view. The short-time view shall cover between two and four years, here. The long-time view is the following time. % fixme: find sources!
meillo@293 763
meillo@381 764 In the short-time view, the effort for improving the existing code is much smaller than the effort for a new design plus improvements. But to have similar quality properties at the end of the short-time frame, a version that is based on current code will probably require nearly as much effort as a new designed version will take. For all further development afterwards, the new design will scale well while the old code will require exponential more work.
meillo@391 765 \index{existing code}
meillo@296 766
meillo@381 767 In the long-time view, a restructuring for modularity is necessary anyway. The question is, when it should be done: Right at the start in a new design, or later as restructuring work.
meillo@293 768
meillo@318 769 %fixme: define exactly, be clear: what does ``break even'' here mean
meillo@293 770
meillo@293 771
meillo@293 772
meillo@293 773 \subsubsection*{The problem with ``good enough''}
meillo@391 774 \index{good enough}
meillo@293 775
meillo@381 776 The decision for later restructuring is problematic. Functionality is often more wanted than quality, thus more function is preferred over better quality, as quality is still ``good enough''. But it might be still ``good enough'' the next time, and the time after that one, and so on.
meillo@241 777
meillo@381 778 Quality improvement is no popular work, but it is required to avoid dead ends. As more code increases the work that needs to be done for quality and modularity improvements, it is better to do these improvements early. Afterwards, all further development will profit from it.
meillo@391 779 \index{quality improvement}
meillo@254 780
meillo@381 781 Also, if some design is bad one should never hesitate to erase it and rebuild it in a sane way.
meillo@391 782 %fixme: doubled speech!
meillo@254 783
meillo@381 784 Again \person{Doug McIlroy} gives valuable advice: ``Don't hesitate to throw away the clumsy parts and rebuild them.'' \cite{mcilroy78}.
meillo@254 785
meillo@381 786 However, making such a cut is hard, especially if the bad design is still \dots\ ``good enough''.
meillo@345 787
meillo@254 788
meillo@254 789
meillo@254 790
meillo@254 791
meillo@297 792 \subsubsection*{Good software, good feelings}
meillo@391 793 \index{good feeling}
meillo@254 794
meillo@381 795 One last argument shall be added. This one is more common to Free Software but can also be found in non-free software.
meillo@254 796
meillo@381 797 Free Software ``sells'' if it has a good user base. For example: Although \qmail\ is somehow outdated and its author has not released any new version since about ten years, \qmail\ still has a very strong user base and community.
meillo@391 798 \index{qmail}
meillo@254 799
meillo@381 800 Good concepts, sound design, and a sane philosophy gives users good feelings for the software and faith in it. They become interested in using it and to contribute. In contrast do constant repaire work and reappearance of weaknesses leave a bad feeling.
meillo@254 801
meillo@381 802 The motivation of most volunteer developers is their wish to do good work with the goal to create good software. Projects that follow admirable plans towards a good product will motivate volunteers to help. More helpers can get the 2,5 man-years for a new design in less absolute time done. Additionally is a good developers base the best start for a good user base, and users define a software's value.
meillo@391 803 \index{motivation}
meillo@241 804
meillo@267 805
meillo@267 806
meillo@267 807
meillo@267 808
meillo@267 809
meillo@267 810
meillo@267 811
meillo@241 812 \section{Result}
meillo@241 813
meillo@381 814 This chapter identified the requirements for a modern and secure \masqmail, and the outstanding work to achieve them. Their importance and the required work for them lead to a focus ranking, which resulted in an ordered list of pending work tasks. Afterwards possible development strategies to control the work process were compared and discussed.
meillo@296 815
meillo@381 816 Strategy 3 (A new design) is slightly preferred over the combination of strategy 1 (Improve existing code) and 2 (Add wrappers and interposition filters), from the requirement's point of view.
meillo@296 817
meillo@381 818 The discussion afterwards did generally support the new design strategy. But some arguments stood against it. These were:
meillo@391 819 \index{development strategy}
meillo@239 820
meillo@293 821 \begin{enumerate}
meillo@391 822 \item The development time and effort
meillo@391 823 \item The time delay until new features can be added
meillo@391 824 \item The risk of failure
meillo@293 825 \end{enumerate}
meillo@239 826
meillo@297 827 The first two arguments are only relevant for the short-time view, because both will become \emph{support arguments} for the new design, once the Break Even point is reached.
meillo@239 828
meillo@381 829 The third argument, the risk, remains. There are risks in every investment. Taking no risks means remaining the same, which eventually means, drifting towards a dead end in a world that does change.
meillo@239 830
meillo@239 831
meillo@316 832 With respect to the current situation, the suggested further development plan for \masqmail\ is split into a short-time plan and a long-time plan:
meillo@391 833 \index{development goal}
meillo@196 834
meillo@219 835 \begin{enumerate}
meillo@391 836 \item The short-time plan: Add the most needed features, namely encryption, authentication, and security wrappers, to the current code base.
meillo@391 837 \item The long-time plan: Design a new architecture that satisfies the modern requirements, especially the quality requirements.
meillo@219 838 \end{enumerate}
meillo@196 839
meillo@381 840 The background thought for this development plan is to first do the most needed stuff on the existing code to keep it usable. This satisfies the urgent needs and removes the time pressure from the development of the new design. After this is done, a new designed \masqmail\ should be developed from scratch. This is the work for the future. It shall, after it is usable and throughout tested, supersede the old \masqmail.
meillo@196 841
meillo@381 842 The basics of this development idea can be described as: Recurrent development of a new design from scratch, while the old version is still in use and gets repaired.
meillo@196 843
meillo@381 844 Hence a modern design will inherit an old one in periodic intervals. This is a very future-proof concept that combines the best of short-term and long-term planning. The price to pay is only the increased work, which gets covered by volunteers that \emph{want} to do it.
meillo@391 845 \index{motivation}
meillo@219 846
meillo@297 847