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author meillo@marmaro.de
date Sat, 07 Feb 2009 12:00:11 +0100
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42 Functional requirements are about the function of the software. They define what the program can do and in what way. 42 Functional requirements are about the function of the software. They define what the program can do and in what way.
43 %fixme: add ref 43 %fixme: add ref
44 The requirements are named ``\NAME{RF}'' for ``requirement, functional''. 44 The requirements are named ``\NAME{RF}'' for ``requirement, functional''.
45 45
46 46
47 \paragraph{\RF1: Incoming and outgoing channels} 47 \paragraph{\RF\,1: Incoming and outgoing channels}
48 \label{rf1} 48 \label{rf1}
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}. 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}.
50 \index{incoming channels} 50 \index{incoming channels}
51 \index{sendmail!command} 51 \index{sendmail!command}
52 52
78 %fixme: write about submission (port 587) 78 %fixme: write about submission (port 587)
79 79
80 80
81 81
82 82
83 \paragraph{\RF2: Mail queuing} 83 \paragraph{\RF\,2: Mail queuing}
84 \label{rf2} 84 \label{rf2}
85 \index{mail queue} 85 \index{mail queue}
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. 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.
87 \index{forwarder} 87 \index{forwarder}
88 \index{non-permanent} 88 \index{non-permanent}
91 \index{reliability} 91 \index{reliability}
92 92
93 93
94 94
95 95
96 \paragraph{\RF3: Header sanitizing} 96 \paragraph{\RF\,3: Header sanitizing}
97 \label{rf3} 97 \label{rf3}
98 \index{header sanitizing} 98 \index{header sanitizing}
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. 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.
100 \index{masqmail!online routes} 100 \index{masqmail!online routes}
101 101
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. 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.
103 103
104 104
105 105
106 106
107 \paragraph{\RF4: Aliasing} 107 \paragraph{\RF\,4: Aliasing}
108 \label{rf4} 108 \label{rf4}
109 \index{aliases} 109 \index{aliases}
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. 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.
111 111
112 112
113 113
114 114
115 \paragraph{\RF5: Route management} 115 \paragraph{\RF\,5: Route management}
116 \label{rf5} 116 \label{rf5}
117 \index{online routes} 117 \index{online routes}
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}. 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}.
119 119
120 120
121 121
122 122
123 \paragraph{\RF6: Authentication} 123 \paragraph{\RF\,6: Authentication}
124 \label{rf6} 124 \label{rf6}
125 \label{requirement-authentication} 125 \label{requirement-authentication}
126 \index{auth} 126 \index{auth}
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. 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.
128 \index{open relay} 128 \index{open relay}
151 151
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. 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.
153 153
154 154
155 155
156 \paragraph{\RF7: Encryption} 156 \paragraph{\RF\,7: Encryption}
157 \label{rf7} 157 \label{rf7}
158 \label{requirement-encryption} 158 \label{requirement-encryption}
159 \index{enc} 159 \index{enc}
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. 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.
161 \index{auth} 161 \index{auth}
183 183
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. 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.
185 185
186 186
187 187
188 \paragraph{\RF8: Spam handling} 188 \paragraph{\RF\,8: Spam handling}
189 \label{rf8} 189 \label{rf8}
190 \index{spam} 190 \index{spam}
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}.) 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}.)
192 192
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. 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.
209 209
210 210
211 211
212 212
213 213
214 \paragraph{\RF9: Malware handling} 214 \paragraph{\RF\,9: Malware handling}
215 \label{rf9} 215 \label{rf9}
216 \index{malware} 216 \index{malware}
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. 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.
218 218
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. 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.
221 221
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}.) 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}.)
223 223
224 224
225 225
226 \paragraph{\RF10: Archiving} 226 \paragraph{\RF\,10: Archiving}
227 \label{rf10} 227 \label{rf10}
228 \index{archiving} 228 \index{archiving}
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. 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.
230 230
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. 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.
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}. 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}.
245 %fixme: refer to ch01 and ch02 245 %fixme: refer to ch01 and ch02
246 These non-functional requirements are named ``\NAME{RG}'' for ``requirement, general''. 246 These non-functional requirements are named ``\NAME{RG}'' for ``requirement, general''.
247 247
248 248
249 \paragraph{\RG1: Security} 249 \paragraph{\RG\,1: Security}
250 \index{security} 250 \index{security}
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}. 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}.
252 \index{compartmentalization} 252 \index{compartmentalization}
253 253
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. 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.
255 \index{masqmail!security} 255 \index{masqmail!security}
256 256
257 257
258 \paragraph{\RG2: Reliability} 258 \paragraph{\RG\,2: Reliability}
259 \index{reliability} 259 \index{reliability}
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. 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.
261 \index{mail loss} 261 \index{mail loss}
262 262
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. 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.
264 264
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. 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.
266 \index{duplicates} 266 \index{duplicates}
267 267
268 268
269 \paragraph{\RG3: Robustness} 269 \paragraph{\RG\,3: Robustness}
270 \index{robustness} 270 \index{robustness}
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}. 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}.
272 272
273 273
274 \paragraph{\RG4: Extendability} 274 \paragraph{\RG\,4: Extendability}
275 \index{extendability} 275 \index{extendability}
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. 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.
277 277
278 278
279 \paragraph{\RG5: Maintainability} 279 \paragraph{\RG\,5: Maintainability}
280 \index{maintainability} 280 \index{maintainability}
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. 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.
282 282
283 283
284 \paragraph{\RG6: Testability} 284 \paragraph{\RG\,6: Testability}
285 \index{testability} 285 \index{testability}
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. 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.
287 287
288 288
289 \paragraph{\RG7: Performance} 289 \paragraph{\RG\,7: Performance}
290 \index{performance} 290 \index{performance}
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. 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.
292 292
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. 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.
294 294
295 295
296 296
297 \paragraph{\RG8: Availability} 297 \paragraph{\RG\,8: Availability}
298 \index{availability} 298 \index{availability}
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. 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.
300 300
301 301
302 \paragraph{\RG9: Portability} 302 \paragraph{\RG\,9: Portability}
303 \index{portability} 303 \index{portability}
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. 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.
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306 306
307 307
308 \paragraph{\RG10: Usability} 308 \paragraph{\RG\,10: Usability}
309 \index{usability} 309 \index{usability}
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. 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.
311 311
312 %fixme: << masqmail as portable app? >> 312 %fixme: << masqmail as portable app? >>
313 313
378 \label{sec:fulfilled-requirements} 378 \label{sec:fulfilled-requirements}
379 379
380 Here follows a description of how far the requirements are already fulfilled by \masqmail. 380 Here follows a description of how far the requirements are already fulfilled by \masqmail.
381 381
382 382
383 \paragraph{\RF1: In/out channels} 383 \paragraph{\RF\,1: In/out channels}
384 \index{incoming channels} 384 \index{incoming channels}
385 \index{outgoing channels} 385 \index{outgoing channels}
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 \RF1. Without any support in \masqmail\ for adding further protocols, the best strategy is to delaying such work until the functionality is essential, anyway. 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.
387 387
388 %fixme: << smtp submission >> %fixme 388 %fixme: << smtp submission >> %fixme
389 389
390 \paragraph{\RF2: Queuing} 390 \paragraph{\RF\,2: Queuing}
391 \index{mail queue} 391 \index{mail queue}
392 One single mail queue is used in \masqmail. It satisfies all current requirements. 392 One single mail queue is used in \masqmail. It satisfies all current requirements.
393 393
394 \paragraph{\RF3: Header sanitizing} 394 \paragraph{\RF\,3: Header sanitizing}
395 \index{header sanitizing} 395 \index{header sanitizing}
396 The envelope and mail headers are generated when the mail is put into the queue. The requirements are fulfilled. 396 The envelope and mail headers are generated when the mail is put into the queue. The requirements are fulfilled.
397 397
398 \paragraph{\RF4: Aliasing} 398 \paragraph{\RF\,4: Aliasing}
399 \index{aliases} 399 \index{aliases}
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. 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.
401 401
402 \paragraph{\RF5: Route management} 402 \paragraph{\RF\,5: Route management}
403 \index{online routes} 403 \index{online routes}
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. 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.
405 405
406 \paragraph{\RF6: Authentication} 406 \paragraph{\RF\,6: Authentication}
407 \index{auth} 407 \index{auth}
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. 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.
409 \index{auth!smtp-after-pop} 409 \index{auth!smtp-after-pop}
410 \index{auth!smtp-auth} 410 \index{auth!smtp-auth}
411 411
412 \paragraph{\RF7: Encryption} 412 \paragraph{\RF\,7: Encryption}
413 \index{enc} 413 \index{enc}
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. 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.
415 \index{secure tunnel} 415 \index{secure tunnel}
416 416
417 \paragraph{\RF8: Spam handling} 417 \paragraph{\RF\,8: Spam handling}
418 \index{spam!handling} 418 \index{spam!handling}
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. 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.
420 420
421 \paragraph{\RF9: Malware handling} 421 \paragraph{\RF\,9: Malware handling}
422 \index{malware!handling} 422 \index{malware!handling}
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. 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.
424 424
425 \paragraph{\RF10: Archiving} 425 \paragraph{\RF\,10: Archiving}
426 \index{archiving} 426 \index{archiving}
427 There is currently no way for archiving every message that does through \masqmail. 427 There is currently no way for archiving every message that does through \masqmail.
428 428
429 429
430 430
431 \paragraph{\RG1: Security} 431 \paragraph{\RG\,1: Security}
432 \index{security} 432 \index{security}
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. 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.
434 \index{masqmail!security} 434 \index{masqmail!security}
435 435
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. 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.
437 \index{conditional compilation} 437 \index{conditional compilation}
438 438
439 \paragraph{\RG2: Reliability} 439 \paragraph{\RG\,2: Reliability}
440 \index{reliability} 440 \index{reliability}
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: 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:
442 \index{masqmail!bugs} 442 \index{masqmail!bugs}
443 443
444 \begin{quote} 444 \begin{quote}
447 \end{quote} 447 \end{quote}
448 448
449 In summary: Current reliability needs to be improved. 449 In summary: Current reliability needs to be improved.
450 %fixme: state machine 450 %fixme: state machine
451 451
452 \paragraph{\RG3: Robustness} 452 \paragraph{\RG\,3: Robustness}
453 \index{robustness} 453 \index{robustness}
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. 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.
455 %fixme: rule of robustness, rule of repair 455 %fixme: rule of robustness, rule of repair
456 456
457 \paragraph{\RG4: Extendability} 457 \paragraph{\RG\,4: Extendability}
458 \index{extendability} 458 \index{extendability}
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. 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.
460 460
461 \paragraph{\RG5: Maintainability} 461 \paragraph{\RG\,5: Maintainability}
462 \index{maintainability} 462 \index{maintainability}
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. 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.
464 464
465 465
466 466
467 \paragraph{\RG6: Testability} 467 \paragraph{\RG\,6: Testability}
468 \index{testability} 468 \index{testability}
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. 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.
470 470
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. 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.
472 \index{test program} 472 \index{test program}
473 473
474 %fixme: think about clean-room testing 474 %fixme: think about clean-room testing
475 475
476 \paragraph{\RG7: Performance} 476 \paragraph{\RG\,7: Performance}
477 \index{performance} 477 \index{performance}
478 The performance---efficiency---of \masqmail\ is good enough for its target field of operation, where this is a minor goal. 478 The performance---efficiency---of \masqmail\ is good enough for its target field of operation, where this is a minor goal.
479 479
480 \paragraph{\RG8: Availability} 480 \paragraph{\RG\,8: Availability}
481 \index{availability} 481 \index{availability}
482 This applies equal to availability. Hence no further work needs to be done her. 482 This applies equal to availability. Hence no further work needs to be done her.
483 483
484 \paragraph{\RG9: Portability} 484 \paragraph{\RG\,9: Portability}
485 \index{portability} 485 \index{portability}
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. 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.
487 \index{masqmail!supported systems} 487 \index{masqmail!supported systems}
488 488
489 489
490 \paragraph{\RG10: Usability} 490 \paragraph{\RG\,10: Usability}
491 \index{usability} 491 \index{usability}
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. 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.
493 \index{out-of-the-box usage} 493 \index{out-of-the-box usage}
494 494
495 495
515 \index{requirements!ranking} 515 \index{requirements!ranking}
516 516
517 These tasks are presented in more detail in a todo list, now. The list is sorted by focus and then by importance. 517 These tasks are presented in more detail in a todo list, now. The list is sorted by focus and then by importance.
518 518
519 519
520 \subsubsection*{\TODO1: Encryption (\RF7)} 520 \subsubsection*{\TODO\,1: Encryption (\RF\,7)}
521 \index{enc} 521 \index{enc}
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. 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.
523 523
524 524
525 \subsubsection*{\TODO2: Authentication (\RF6)} 525 \subsubsection*{\TODO\,2: Authentication (\RF\,6)}
526 \index{auth} 526 \index{auth}
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. 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.
528 528
529 529
530 \subsubsection*{\TODO3: Security (\RG1)} 530 \subsubsection*{\TODO\,3: Security (\RG\,1)}
531 \index{security} 531 \index{security}
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. 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.
533 \index{modularity} 533 \index{modularity}
534 534
535 535
536 \subsubsection*{\TODO4: Reliability (\RG2)} 536 \subsubsection*{\TODO\,4: Reliability (\RG\,2)}
537 \index{reliability} 537 \index{reliability}
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. 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.
539 \index{crash-only software} 539 \index{crash-only software}
540 540
541 541
542 \subsubsection*{\TODO5: Spam handling (\RF8)} 542 \subsubsection*{\TODO\,5: Spam handling (\RF\,8)}
543 \index{spam!handling} 543 \index{spam!handling}
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. 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.
545 545
546 546
547 \subsubsection*{\TODO6: Extendability (\RG4)} 547 \subsubsection*{\TODO\,6: Extendability (\RG\,4)}
548 \index{extendability} 548 \index{extendability}
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. 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.
550 \index{conditional compilation} 550 \index{conditional compilation}
551 551
552 552
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. 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.
577 \index{wrapper} 577 \index{wrapper}
578 \index{interposition filter} 578 \index{interposition filter}
579 579
580 580
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 (\TODO1) and authentication (\TODO2), 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 (\TODO4), extendability (\TODO6), and maintainability hardly be added to code afterwards---if at all. Security (\TODO3) is improvable in a new design, of course, but also with wrappers or interposition filters. 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.
582 582
583 This linking of requirements to the strategies is shown in table~\ref{tab:strategies}. The requirements are ordered by their focus. 583 This linking of requirements to the strategies is shown in table~\ref{tab:strategies}. The requirements are ordered by their focus.
584 584
585 \begin{table} 585 \begin{table}
586 \begin{center} 586 \begin{center}
718 \index{sendmail} 718 \index{sendmail}
719 719
720 Redesigning a software as requirements change helps keeping it alive. % fixme: add quote: ``one thing surely remains: change'' (something like that) 720 Redesigning a software as requirements change helps keeping it alive. % fixme: add quote: ``one thing surely remains: change'' (something like that)
721 \index{redesign} 721 \index{redesign}
722 722
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. 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.
724 \index{suckless} 724 \index{suckless}
725 725
726 726
727 727
728 728