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author meillo@marmaro.de
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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.
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meillo@267 7 \section{The goal}
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meillo@293 9 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@185 10
meillo@293 11 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?
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meillo@381 13 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@256 14
meillo@381 15 \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 16
meillo@381 17 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.
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meillo@293 19 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.
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meillo@177 26 \section{Requirements}
meillo@219 27 \label{sec:mta-requirements}
meillo@146 28
meillo@267 29 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.
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meillo@219 33 \subsection{Functional requirements}
meillo@327 34 \label{sec:functional-requirements}
meillo@146 35
meillo@232 36 Functional requirements are about the function of the software. They define what the program can do and in what way.
meillo@232 37 %fixme: add ref
meillo@239 38 The requirements are named ``\NAME{RF}'' for ``requirement, functional''.
meillo@219 39
meillo@219 40
meillo@239 41 \paragraph{\RF1: Incoming and outgoing channels}
meillo@339 42 \label{rf1}
meillo@381 43 \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@219 44
meillo@381 45 \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.
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meillo@378 48 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@219 49
meillo@318 50 %fixme: is the def of MTA: transfer between machines, or transfer between users?
meillo@381 51 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@219 52
meillo@232 53 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 54
meillo@232 55 \begin{figure}
meillo@232 56 \begin{center}
meillo@232 57 \includegraphics[scale=0.75]{img/mta-channels.eps}
meillo@232 58 \end{center}
meillo@277 59 \caption{Required incoming and outgoing channels}
meillo@232 60 \label{fig:mta-channels}
meillo@232 61 \end{figure}
meillo@219 62
meillo@381 63 An overview on incoming and outgoing channels which are required for an \MTA, gives figure~\ref{fig:mta-channels}.
meillo@219 64
meillo@239 65 %fixme: write about submission (port 587)
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meillo@219 69
meillo@239 70 \paragraph{\RF2: Mail queuing}
meillo@339 71 \label{rf2}
meillo@381 72 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@219 73
meillo@381 74 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.
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meillo@239 79 \paragraph{\RF3: Header sanitizing}
meillo@339 80 \label{rf3}
meillo@219 81 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@219 82
meillo@381 83 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.
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meillo@239 88 \paragraph{\RF4: Aliasing}
meillo@339 89 \label{rf4}
meillo@381 90 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.
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meillo@287 95 \paragraph{\RF5: Route management}
meillo@339 96 \label{rf5}
meillo@378 97 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}.
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meillo@239 102 \paragraph{\RF6: Authentication}
meillo@339 103 \label{rf6}
meillo@317 104 \label{requirement-authentication}
meillo@317 105 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@219 106
meillo@317 107 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@219 108
meillo@316 109 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@334 110
meillo@285 111 \begin{enumerate}
meillo@232 112 \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@232 113 \item \SMTP\ authentication: An extension to \SMTP. It allows to request authentication before mail is accepted. Here no helper protocols are needed.
meillo@381 114 \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@285 115 \end{enumerate}
meillo@334 116
meillo@381 117 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.
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meillo@381 119 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.
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meillo@381 121 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.
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meillo@381 123 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.
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meillo@239 127 \paragraph{\RF7: Encryption}
meillo@339 128 \label{rf7}
meillo@314 129 \label{requirement-encryption}
meillo@381 130 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@219 131
meillo@288 132 The common way to encrypt \SMTP\ dialogs is using \name{Transport Layer Security} (short: \TLS, the successor of \NAME{SSL}). \TLS\ encrypts the datagrams of the \name{transport layer}. This means it works below the application protocols and can be used with any of them \citeweb{wikipedia:tls}.
meillo@219 133
meillo@381 134 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@219 135
meillo@232 136 \begin{figure}
meillo@232 137 \begin{center}
meillo@232 138 \includegraphics[scale=0.75]{img/stunnel.eps}
meillo@232 139 \end{center}
meillo@232 140 \caption{Using \name{stunnel} for incoming connections}
meillo@232 141 \label{fig:stunnel}
meillo@232 142 \end{figure}
meillo@219 143
meillo@366 144 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@219 145
meillo@381 146 \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.
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meillo@288 150 \paragraph{\RF8: Spam handling}
meillo@339 151 \label{rf8}
meillo@381 152 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}.)
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meillo@381 154 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.
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meillo@381 156 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.
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meillo@381 158 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@288 159
meillo@381 160 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}.
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meillo@381 162 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.)
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meillo@287 168 \paragraph{\RF9: Malware handling}
meillo@339 169 \label{rf9}
meillo@381 170 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.
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meillo@381 172 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.
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meillo@381 174 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}.)
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meillo@239 178 \paragraph{\RF10: Archiving}
meillo@339 179 \label{rf10}
meillo@381 180 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@334 181
meillo@381 182 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@219 183
meillo@381 184 \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}.
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meillo@381 186 But if archiving is of high importance, a dedicated archiving solution is advisable, anyway.
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meillo@219 190
meillo@219 191 \subsection{Non-functional requirements}
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meillo@381 193 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@185 194 %fixme: refer to ch01 and ch02
meillo@239 195 These non-functional requirements are named ``\NAME{RG}'' for ``requirement, general''.
meillo@146 196
meillo@146 197
meillo@239 198 \paragraph{\RG1: Security}
meillo@381 199 \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@293 200
meillo@381 201 \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@177 202
meillo@259 203
meillo@239 204 \paragraph{\RG2: Reliability}
meillo@381 205 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@177 206
meillo@381 207 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@189 208
meillo@381 209 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@239 210
meillo@239 211
meillo@239 212 \paragraph{\RG3: Robustness}
meillo@381 213 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@177 214
meillo@177 215
meillo@239 216 \paragraph{\RG4: Extendability}
meillo@381 217 \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 for spam problem. Extendability is the ability of software to include new function with little work.
meillo@196 218
meillo@146 219
meillo@239 220 \paragraph{\RG5: Maintainability}
meillo@381 221 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@146 222
meillo@189 223
meillo@239 224 \paragraph{\RG6: Testability}
meillo@381 225 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@189 226
meillo@189 227
meillo@239 228 \paragraph{\RG7: Performance}
meillo@381 229 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@293 230
meillo@381 231 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.
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meillo@146 234
meillo@239 235 \paragraph{\RG8: Availability}
meillo@381 236 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@146 237
meillo@146 238
meillo@239 239 \paragraph{\RG9: Portability}
meillo@381 240 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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meillo@189 243
meillo@239 244 \paragraph{\RG10: Usability}
meillo@381 245 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@185 246
meillo@298 247 %fixme: << masqmail as portable app? >>
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meillo@185 250
meillo@293 251 \subsection{Architecture}
meillo@239 252 \label{sec:discussion-mta-arch}
meillo@187 253
meillo@318 254 %fixme: what's this section to do with requirements?
meillo@242 255
meillo@381 256 \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@381 257 \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@161 258
meillo@381 259 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@161 260
meillo@161 261 \begin{figure}
meillo@161 262 \begin{center}
meillo@219 263 \vspace*{2ex}
meillo@256 264 %\includegraphics[scale=0.75]{img/callgraph.eps}
meillo@339 265 %\includegraphics[scale=0.75]{img/masqmail-3-omitlog5.eps}
meillo@339 266 \includegraphics[scale=0.75]{img/bb.eps}
meillo@161 267 \end{center}
meillo@339 268 \caption{Internal structure of \masqmail, showed by a call graph. (Logging functions are ignored; test and \NAME{POP3} code is excluded.)}
meillo@161 269 \label{fig:masqmail-arch}
meillo@161 270 \end{figure}
meillo@161 271
meillo@378 272 \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@161 273
meillo@381 274 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@161 275
meillo@219 276 \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@381 277
meillo@163 278 \begin{quote}
meillo@218 279 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@218 280 \hfill\cite[page 64]{hafiz05}
meillo@163 281 \end{quote}
meillo@200 282
meillo@381 283 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@161 284
meillo@381 285 Modularity is also needed to satisfy modern \MTA\ requirements in providing a clear interface to add functionality without increasing the overall complexity much.
meillo@381 286
meillo@381 287 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@239 288
meillo@242 289 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@239 290
meillo@277 291 %fixme: explain: why are compartments and interfaces so good?
meillo@239 292
meillo@239 293
meillo@239 294
meillo@288 295
meillo@288 296
meillo@288 297
meillo@288 298
meillo@288 299
meillo@288 300
meillo@239 301 \section{Fulfilled requirements}
meillo@239 302 \label{sec:fulfilled-requirements}
meillo@239 303
meillo@239 304 Here follows a description of how far the requirements are already fulfilled by \masqmail.
meillo@239 305
meillo@239 306
meillo@239 307 \paragraph{\RF1: In/out channels}
meillo@381 308 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.
meillo@239 309
meillo@339 310 %fixme: << smtp submission >> %fixme
meillo@287 311
meillo@316 312 \paragraph{\RF2: Queuing}
meillo@381 313 One single mail queue is used in \masqmail. It satisfies all current requirements.
meillo@239 314
meillo@239 315 \paragraph{\RF3: Header sanitizing}
meillo@239 316 The envelope and mail headers are generated when the mail is put into the queue. The requirements are fulfilled.
meillo@239 317
meillo@239 318 \paragraph{\RF4: Aliasing}
meillo@381 319 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@239 320
meillo@287 321 \paragraph{\RF5: Route management}
meillo@381 322 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@239 323
meillo@239 324 \paragraph{\RF6: Authentication}
meillo@381 325 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 host 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@239 326
meillo@239 327 \paragraph{\RF7: Encryption}
meillo@381 328 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@239 329
meillo@239 330 \paragraph{\RF8: Spam handling}
meillo@381 331 \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@239 332
meillo@239 333 \paragraph{\RF9: Malware handling}
meillo@381 334 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@239 335
meillo@239 336 \paragraph{\RF10: Archiving}
meillo@381 337 There is currently no way for archiving every message that does through \masqmail.
meillo@239 338
meillo@239 339
meillo@239 340
meillo@239 341 \paragraph{\RG1: Security}
meillo@381 342 \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@259 343
meillo@316 344 \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@239 345
meillo@239 346 \paragraph{\RG2: Reliability}
meillo@381 347 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@381 348
meillo@163 349 \begin{quote}
meillo@239 350 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@239 351 \hfill\citeweb{masqmail:homepage2}
meillo@163 352 \end{quote}
meillo@381 353
meillo@239 354 In summary: Current reliability needs to be improved.
meillo@239 355 %fixme: state machine
meillo@161 356
meillo@239 357 \paragraph{\RG3: Robustness}
meillo@381 358 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@318 359 %fixme: rule of robustness, rule of repair
meillo@239 360
meillo@239 361 \paragraph{\RG4: Extendability}
meillo@381 362 \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@239 363
meillo@239 364 \paragraph{\RG5: Maintainability}
meillo@381 365 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@239 366
meillo@288 367
meillo@287 368
meillo@239 369 \paragraph{\RG6: Testability}
meillo@381 370 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@239 371
meillo@381 372 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@339 373
meillo@339 374 %fixme: think about clean-room testing
meillo@293 375
meillo@239 376 \paragraph{\RG7: Performance}
meillo@239 377 The performance---efficiency---of \masqmail\ is good enough for its target field of operation, where this is a minor goal.
meillo@239 378
meillo@239 379 \paragraph{\RG8: Availability}
meillo@239 380 This applies equal to availability. Hence no further work needs to be done her.
meillo@239 381
meillo@239 382 \paragraph{\RG9: Portability}
meillo@381 383 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@239 384
meillo@293 385
meillo@239 386 \paragraph{\RG10: Usability}
meillo@381 387 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@239 388
meillo@239 389
meillo@239 390
meillo@239 391
meillo@239 392
meillo@239 393 \section{Work to do}
meillo@239 394
meillo@381 395 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@239 396
meillo@239 397 \begin{table}
meillo@239 398 \begin{center}
meillo@271 399 \input{tbl/requirements.tbl}
meillo@239 400 \end{center}
meillo@239 401 \caption{Importance of and pending work for requirements}
meillo@242 402 \label{tab:requirements}
meillo@239 403 \end{table}
meillo@239 404
meillo@381 405 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@239 406
meillo@381 407 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@239 408
meillo@381 409 These tasks are presented in more detail in a todo list, now. The list is sorted by focus and then by importance.
meillo@239 410
meillo@239 411
meillo@239 412 \subsubsection*{\TODO1: Encryption (\RF7)}
meillo@381 413 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@288 414
meillo@239 415
meillo@241 416 \subsubsection*{\TODO2: Authentication (\RF6)}
meillo@381 417 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@288 418
meillo@239 419
meillo@239 420 \subsubsection*{\TODO3: Security (\RG1)}
meillo@381 421 \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}.) Further more should \masqmail's security be tested throughout to get a definitive view how good it really is and where the weak spots are.
meillo@239 422
meillo@239 423
meillo@239 424 \subsubsection*{\TODO4: Reliability (\RG2)}
meillo@381 425 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@288 426
meillo@239 427
meillo@241 428 \subsubsection*{\TODO5: Spam handling (\RF8)}
meillo@381 429 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@239 430
meillo@241 431
meillo@241 432 \subsubsection*{\TODO6: Extendability (\RG4)}
meillo@381 433 \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@254 434
meillo@239 435
meillo@239 436
meillo@239 437
meillo@239 438
meillo@239 439
meillo@239 440
meillo@239 441
meillo@239 442
meillo@239 443
meillo@239 444 \section{Ways for further development}
meillo@239 445
meillo@293 446 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@293 447
meillo@293 448
meillo@293 449 \subsection{Possibilities}
meillo@293 450
meillo@316 451 Further development of software can always go three different ways:
meillo@241 452 \begin{enumerate}
meillo@381 453 \item Improve the current code base. (S\,1)
meillo@381 454 \item Add wrappers or interposition filters. (S\,2)
meillo@381 455 \item Redesign the software from scratch and rebuild it. (S\,3)
meillo@241 456 \end{enumerate}
meillo@239 457
meillo@381 458 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@239 459
meillo@241 460
meillo@381 461 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.
meillo@293 462
meillo@381 463 This linking of requirements to the strategies is shown in table~\ref{tab:strategies}. The requirements are ordered by their focus.
meillo@241 464
meillo@241 465 \begin{table}
meillo@241 466 \begin{center}
meillo@271 467 \input{tbl/strategies.tbl}
meillo@241 468 \end{center}
meillo@241 469 \caption{Development strategies and their suitability for requirements}
meillo@241 470 \label{tab:strategies}
meillo@241 471 \end{table}
meillo@241 472
meillo@239 473
meillo@381 474 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@267 475
meillo@381 476 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@267 477
meillo@381 478 This leads to the conclusion that S\,3 (A new design) is probably the best strategy for further development.
meillo@381 479
meillo@381 480 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@267 481
meillo@267 482
meillo@267 483
meillo@267 484
meillo@267 485
meillo@267 486
meillo@267 487
meillo@267 488
meillo@293 489 \subsection{Discussion}
meillo@323 490 \label{sec:discussion-further-devel}
meillo@267 491
meillo@267 492
meillo@296 493 \subsubsection*{Quality improvements}
meillo@239 494
meillo@381 495 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@288 496
meillo@288 497
meillo@241 498
meillo@293 499 \subsubsection*{Security}
meillo@261 500
meillo@293 501 Similar is the situation for security. Security comes from good design, explain \person{Graff} and \person{van Wyk}:
meillo@381 502
meillo@241 503 \begin{quote}
meillo@241 504 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@241 505 %
meillo@241 506 %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@241 507 \hfill\cite[page 55]{graff03}
meillo@241 508 \end{quote}
meillo@241 509
meillo@296 510 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@241 511
meillo@381 512 \person{Hafiz} adds: ``The major idea is that security cannot be retrofitted \emph{into} an architecture.'' \cite[page 64, (emphasis added)]{hafiz05}.
meillo@241 513
meillo@241 514
meillo@241 515
meillo@241 516
meillo@293 517 \subsubsection*{Effort estimation}
meillo@293 518
meillo@293 519 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@293 520
meillo@293 521 \person{Wheeler}'s program \name{sloccount} calculates following estimations for \masqmail's code base as of version 0.2.21 (excluding library code):
meillo@293 522
meillo@293 523 \codeinput{input/masqmail-sloccount.txt}
meillo@293 524
meillo@296 525 The development costs in money are not relevant for a \freesw\ 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@293 526
meillo@381 527 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@293 528
meillo@293 529
meillo@293 530
meillo@293 531
meillo@293 532 \subsubsection*{Risks}
meillo@293 533
meillo@381 534 The gained result of a new design might still outweigh the development effort. But risks are something more to consider.
meillo@293 535
meillo@293 536 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@293 537
meillo@381 538 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@293 539
meillo@296 540 One thing is clear: Doing a redesign and rebuild \emph{is} a risky decision.
meillo@293 541
meillo@293 542
meillo@293 543
meillo@293 544 \subsubsection*{Existing code is precious}
meillo@293 545
meillo@296 546 If a new design needs much effort and additionally is a risk, what about the existing code base then?
meillo@293 547
meillo@381 548 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@293 549
meillo@381 550 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@293 551
meillo@296 552 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@293 553
meillo@293 554
meillo@296 555
meillo@293 556
meillo@293 557 \subsubsection*{Repairing}
meillo@293 558
meillo@381 559 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@293 560
meillo@381 561 \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@293 562
meillo@381 563 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@381 564
meillo@381 565 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@293 566
meillo@296 567 %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@296 568
meillo@381 569 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@293 570
meillo@293 571
meillo@293 572
meillo@293 573
meillo@293 574
meillo@293 575 \subsubsection*{A guard against dead ends}
meillo@293 576
meillo@296 577 A new design does protect against such dead ends.
meillo@293 578
meillo@381 579 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.
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meillo@381 581 Redesigning a software as requirements change helps keeping it alive. % fixme: add quote: ``one thing surely remains: change'' (something like that)
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meillo@381 583 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.
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meillo@293 588
meillo@293 589 \subsubsection*{Modularity}
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meillo@381 591 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.
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meillo@381 593 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.
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meillo@381 595 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.
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meillo@293 601
meillo@296 602 \subsubsection*{Function versus quality}
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meillo@381 604 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.
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meillo@381 606 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.
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meillo@381 608 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.
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meillo@293 614
meillo@297 615 \subsubsection*{Break Even}
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meillo@381 617 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!
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meillo@381 619 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.
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meillo@381 621 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.
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meillo@318 623 %fixme: define exactly, be clear: what does ``break even'' here mean
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meillo@293 627 \subsubsection*{The problem with ``good enough''}
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meillo@381 629 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.
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meillo@381 631 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.
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meillo@381 633 Also, if some design is bad one should never hesitate to erase it and rebuild it in a sane way.
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meillo@381 635 Again \person{Doug McIlroy} gives valuable advice: ``Don't hesitate to throw away the clumsy parts and rebuild them.'' \cite{mcilroy78}.
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meillo@381 637 However, making such a cut is hard, especially if the bad design is still \dots\ ``good enough''.
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meillo@297 643 \subsubsection*{Good software, good feelings}
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meillo@381 645 One last argument shall be added. This one is more common to Free Software but can also be found in non-free software.
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meillo@381 647 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.
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meillo@381 649 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.
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meillo@381 651 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.
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meillo@241 660 \section{Result}
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meillo@381 662 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.
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meillo@381 664 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.
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meillo@381 666 The discussion afterwards did generally support the new design strategy. But some arguments stood against it. These were:
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meillo@293 668 \begin{enumerate}
meillo@297 669 \item The development time and effort
meillo@297 670 \item The time delay until new features can be added
meillo@381 671 \item The risk of failure
meillo@293 672 \end{enumerate}
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meillo@297 674 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.
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meillo@381 676 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.
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meillo@316 679 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:
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meillo@219 681 \begin{enumerate}
meillo@381 682 \item The short-time plan: Add the most needed features, namely encryption, authentication, and security wrappers, to the current code base.
meillo@381 683 \item The long-time plan: Design a new architecture that satisfies the modern requirements, especially the quality requirements.
meillo@219 684 \end{enumerate}
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meillo@381 686 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.
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meillo@381 688 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.
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meillo@381 690 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.
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