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annotate thesis/tex/4-MasqmailsFuture.tex @ 239:a6be202e1b49

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reworked nearly everything in ch04!
author meillo@marmaro.de
date Sat, 10 Jan 2009 22:27:06 +0100
parents 1afdb3f85e69
children 2c56f26758eb
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meillo@109 1 \chapter{\masqmail's present and future}
meillo@93 2
meillo@185 3 This chapter \dots %fixme write text here
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meillo@177 11 \section{Requirements}
meillo@219 12 \label{sec:mta-requirements}
meillo@146 13
meillo@185 14 This section identifies the requirements for a modern \masqmail. Most of them will apply to modern \MTA{}s in general.
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meillo@177 18
meillo@219 19 \subsection{Functional requirements}
meillo@146 20
meillo@232 21 Functional requirements are about the function of the software. They define what the program can do and in what way.
meillo@232 22 %fixme: add ref
meillo@239 23 The requirements are named ``\NAME{RF}'' for ``requirement, functional''.
meillo@219 24
meillo@219 25
meillo@239 26 \paragraph{\RF1: Incoming and outgoing channels}
meillo@232 27 \sendmail-compatible \mta{}s must support at least two incoming channels: mail submitted using the \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 28
meillo@232 29 \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 leads to multiple remote channels, one for each supported protocol as it was done in other \MTA{}s. Best would be interfaces to add further protocols as modules.
meillo@219 30
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meillo@232 32 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 33
meillo@232 34 %todo: is the def of MTA: transfer between machines, or transfer between users?
meillo@232 35 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 it. 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 36
meillo@232 37 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 38
meillo@232 39 \begin{figure}
meillo@232 40 \begin{center}
meillo@232 41 \includegraphics[scale=0.75]{img/mta-channels.eps}
meillo@232 42 \end{center}
meillo@232 43 \caption{Incoming and outgoing channels required}
meillo@232 44 \label{fig:mta-channels}
meillo@232 45 \end{figure}
meillo@219 46
meillo@232 47 An overview on in and outgoing channels required for an \MTA, gives figure \ref{fig:mta-channels}.
meillo@219 48
meillo@239 49 %fixme: write about submission (port 587)
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meillo@219 53
meillo@239 54 \paragraph{\RF2: Mail queuing}
meillo@232 55 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, excluding the simple forwarders. The mail queue is essential for \masqmail, as \masqmail\ is used 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 56
meillo@219 57 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 loosing mail. An \MTA\ takes over responsibility for mail in accepting it, hence loosing mail messages is absolutely to avoid. This covers any kind of crash situation too. The worst thing acceptable to happen is an already sent mail to be sent again.
meillo@219 58
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meillo@239 62 \paragraph{\RF3: Header sanitizing}
meillo@219 63 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 64
meillo@225 65 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, containing all the same mail message.
meillo@219 66
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meillo@239 70 \paragraph{\RF4: Aliasing}
meillo@225 71 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 containing local and remote users, or a command. Most important are the aliases in the \path{aliases} file, usually located at \path{/etc/aliases}. Addresses expanding to lists of users lead to more envelopes. Aliases changing the recipient's domain part may require a different route to be used.
meillo@219 72
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meillo@219 75
meillo@239 76 \paragraph{\RF5: Selecting a route}
meillo@232 77 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 82 \paragraph{\RF6: Authentication}
meillo@232 83 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 be also wanted to refuse all connections to the \MTA\ except ones from a specific set of hosts.
meillo@219 84
meillo@232 85 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 have 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 maintainence work needed. This kind of access restriction is important to be implemented.
meillo@219 86
meillo@232 87 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 subnet, need access. Then a authentication mechanism based on some \emph{secret} is required. Three common approaches exist:
meillo@219 88 \begin{itemize}
meillo@232 89 \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 90 \item \SMTP\ authentication: An extension to \SMTP. It allows to request authentication before mail is accepted. Here no helper protocols are needed.
meillo@232 91 \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: \NAME{SSL} tunnels encrypt the data transmission and allow to identify the remote user/host by his certificate.
meillo@219 92 \end{itemize}
meillo@232 93 At least one of the secret-based mechanisms should be supported.
meillo@219 94
meillo@219 95
meillo@219 96
meillo@239 97 \paragraph{\RF7: Encryption}
meillo@232 98 Electronic mail is vulnerable to sniffing attacks, because in generic \SMTP\ all data transfer is unencrypted. Unencrypted is the message's content, the email addresses in header and envelope, but also authentication dialogs that transfer plain text passwords (e.g.\ \NAME{PLAIN} and \NAME{LOGIN}). Hence encryption is important.
meillo@219 99
meillo@219 100 The common way to encrypt \SMTP\ dialogs is using \name{Transport Layer Security} (short: \TLS, successor of \NAME{SSL}). \TLS\ encrypts the datagrams of the \name{transport layer}. This means it works below the application protocols and can be used by any of them \citeweb{wikipedia:tls}.
meillo@219 101
meillo@239 102 Using secure tunnels, that are provided by external applications, is prefered because the \MTA\ 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 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 that is the information the \MTA\ gets. Authentication based on \NAME{IP} addresses and many spam prevention methods are useless then.
meillo@219 103
meillo@232 104 \begin{figure}
meillo@232 105 \begin{center}
meillo@232 106 \includegraphics[scale=0.75]{img/stunnel.eps}
meillo@232 107 \end{center}
meillo@232 108 \caption{Using \name{stunnel} for incoming connections}
meillo@232 109 \label{fig:stunnel}
meillo@232 110 \end{figure}
meillo@219 111
meillo@239 112 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}. \RFC3207 ``\SMTP\ Service Extension for Secure \SMTP\ over Transport Layer Security'' shows this in not mentioning \NAME{SMTPS} on port 465. Also port 465 is not even reserved for \NAME{SMTPS} anymore \citeweb{iana:port-numbers}.
meillo@219 113
meillo@239 114 \NAME{STARTTLS}---defined in \RFC2487---is what \RFC3207 recommends to use for secure \SMTP. The connection then goes over port 25 (or the submission port 587), but gets encrypted as the \NAME{STARTTLS} keyword is issued.
meillo@232 115
meillo@239 116 \NAME{STARTTLS} encryption should be supported.
meillo@232 117
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meillo@232 119
meillo@239 120 \paragraph{\RF8: Spam prevention}
meillo@219 121 Spam is a major threat nowadays, but it is a war that is hard to win. The goal is to provide state-of-the-art spam protection, but not more (see section \ref{sec:swot-analysis}).
meillo@219 122
meillo@239 123 As spam is not just a nuisance for end users, but also for the infrastructure---the \mta{}s---by increasing the amount of mail messages, \MTA{}s need to protect themselves.
meillo@219 124
meillo@239 125 Filtering spam can be done in two ways: Refusing spam during the \SMTP\ dialog or checking for spam after the mail was accepted and queued. Both ways have advantages and disadvantages, so modern \MTA{}s use them in combination. Spam is identified by the results of a set of checks. Static rules, querying databases (\NAME{DNS} blacklists \cite{cole07} \cite{levine08}), requesting special client behavior (\name{greylisting} \cite{harris03}, \name{hashcash} \cite{back02}), or statistical analysis (\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@219 126
meillo@239 127 Doing some basic checks during the \SMTP\ dialog seems to be a must \cite[page~25]{eisentraut05}. They should best be included into the \MTA, because they need to be fast to avoid \SMTP\ dialog timeouts. Internal interfaces to specialized modules seem to be best.
meillo@219 128
meillo@239 129 More detailed checks after the message is queued should be done using 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@239 135 \paragraph{\RF9: Virus checking}
meillo@225 136 Related to spam is malicious content (short: \name{malware}) like viruses, worms, trojan horses. They, in contrast to spam, do not affect the \MTA\ itself, as they are in the mail's body. \MTA{}s searching for malware is equal to real world's post offices opening letters to check if they contain something that could harm the recipient. This is not a mail transport job. But the \MTA\ responsible for the recipient seems to be at a good position to do this work, so it is often done there.
meillo@219 137
meillo@219 138 In any way should malware checking be performed by external programs that may be invoked by the \mta. But using mail deliver agents, like \name{procmail}, are better suited locations to invoke content scanners.
meillo@219 139
meillo@219 140 A popular email filter framework is \name{amavis} which integrates various spam and virus scanners. The common setup includes a receiving \MTA\ which sends it to \name{amavis} using \SMTP, \name{amavis} processes the mail and sends it then to a second \MTA\ that does the outgoing transfer. Having interfaces to such scanners is nice to have, though.
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meillo@239 144 \paragraph{\RF10: Archiving}
meillo@219 145 Mail archiving and auditability become more important as email establishes as technology for serious business communication. The ability to archive verbatim copies of every mail coming into and every mail going out of the system, with relation between them, appears to be a goal to achieve.
meillo@219 146
meillo@225 147 \postfix\ for example has a \texttt{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 is preferable.
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meillo@219 153 \subsection{Non-functional requirements}
meillo@219 154
meillo@225 155 Here follows a list of non-functional requirements for \masqmail. These requirements specify the quality properties of 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@185 156 %fixme: refer to ch01 and ch02
meillo@239 157 These non-functional requirements are named ``\NAME{RG}'' for ``requirement, general''.
meillo@146 158
meillo@146 159
meillo@239 160 \paragraph{\RG1: Security}
meillo@239 161 \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{compartementalization}, as described in section \ref{sec:discussion-mta-arch}. \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 break \masqmail.
meillo@177 162
meillo@177 163
meillo@239 164 \paragraph{\RG2: Reliability}
meillo@239 165 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 are distributed systems, one of the communication partners or the transport medium may crash at any time during mail tranfer. Thus reliability is needed for mail transfer communication too.
meillo@177 166
meillo@239 167 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 now 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 acknowledgement, then handles the transfer. (2) The client reissues the transfer only if no acknowledgement was received; the server first handles the transfer and sends the acknowledgement afterwards. The first strategy does not need acknowledgements at all, however, it will lose mail if the second transfer fails too.
meillo@189 168
meillo@239 169 Hence, mail transfer between two processes must use the strategy: The client reissues if it receives no acknowledgement; the server first handles the message and then sends the acknowledgement. This strategy only leads to duplicates if a crash happens in the time between the message is fully transfered to the server and the acknowlegement is received by the client. No mail will get lost.
meillo@239 170
meillo@239 171
meillo@239 172 \paragraph{\RG3: Robustness}
meillo@219 173 Being robust means handling errors properly. Small errors may get corrected, large errors may kill a process. Killed processes should 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@177 174
meillo@177 175
meillo@239 176 \paragraph{\RG4: Extendability}
meillo@219 177 \masqmail's architecture needs to be extendable, to allow new features to be added afterwards. The reason for this need are changing requirements. New requirements 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@196 178
meillo@146 179
meillo@239 180 \paragraph{\RG5: Maintainability}
meillo@219 181 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 work afterwards.
meillo@146 182
meillo@189 183
meillo@239 184 \paragraph{\RG6: Testability}
meillo@225 185 Good testability make maintenance easier too, because functionality is directly verifiable when changes are done, thus removing 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@189 186
meillo@189 187
meillo@239 188 \paragraph{\RG7: Performance}
meillo@196 189 Also called ``efficiency''. Efficient software requires few time and few resources. The merge of communication hardware and its move from service providers to homes and to mobile devices, demand smaller and more resource-friendly software. The amount of mail will be lower, even if much more mail will be sent. More important will be the energy consumption and heat emission. These topics increased in relevance during the past years and they are expected to become more central.
meillo@146 190
meillo@146 191
meillo@239 192 \paragraph{\RG8: Availability}
meillo@225 193 Availability is important for server programs. They must stay operational by blocking \name{denial of service} attacks and the like.
meillo@146 194
meillo@146 195
meillo@239 196 \paragraph{\RG9: Portability}
meillo@225 197 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.
meillo@189 198
meillo@189 199
meillo@239 200 \paragraph{\RG10: Usability}
meillo@219 201 Usability, not mentioned by \person{Hafiz} (he focuses on architecture) but by \person{Spinellis} and \person{Kan}, is a property very important from the user's point of view. Software with bad usability is rarely used, no matter how good it is. If substitutes with better usability exist, the user will switch to one of them. Here, usability includes setting up and configuring; and 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 focused must be the most common form of configuration: choosing one of several common setups.
meillo@185 202
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meillo@185 205
meillo@239 206 \subsection{Architectural requirements}
meillo@239 207 \label{sec:discussion-mta-arch}
meillo@187 208
meillo@225 209 \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 one for authentication ``modules''. %fixme: add ref
meillo@219 210 \sendmail\ provides now, with its \name{milter} interface, standardized connection channels to external modules.
meillo@188 211 \masqmail\ has none of them; it is what \sendmail\ was in the beginning: a single large block.
meillo@161 212
meillo@239 213 Figure \ref{fig:masqmail-arch} is a call graph generated from \masqmail's source code, excluding logging functions. It gives a impression of how interweaved the internals are. There are no compartments existent.
meillo@219 214 %fixme: what is included, what not?
meillo@161 215
meillo@161 216 \begin{figure}
meillo@161 217 \begin{center}
meillo@219 218 \vspace*{2ex}
meillo@211 219 \includegraphics[scale=0.75]{img/callgraph.eps}
meillo@161 220 \end{center}
meillo@200 221 \caption{Call graph of \masqmail\ to show its internal structure}
meillo@161 222 \label{fig:masqmail-arch}
meillo@161 223 \end{figure}
meillo@161 224
meillo@225 225 \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@161 226
meillo@239 227 Every one of these programs is more modular, or became more modular over time, than \masqmail\ is. Modern requirements like spam protection and future requirements like---probably---the use of new mail transport protocols demand for modular designs in order to keep the software simple. Simplicity is a key property for security. ``the essence of security engineering is to build systems that are as simple as possible.'' \cite[page 45]{graff03}.
meillo@161 228
meillo@219 229 \person{Hafiz} agrees: ``The goal of making software secure can be better achieved by making the design simple and easier to understand and verify.'' \cite[page 64]{hafiz05}. He identifies the security of \qmail\ to come from it's \name{compartmentalization}, which goes hand in hand with modularity:
meillo@163 230 \begin{quote}
meillo@218 231 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@218 232 \hfill\cite[page 64]{hafiz05}
meillo@163 233 \end{quote}
meillo@219 234 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@200 235
meillo@200 236 Modularity is also needed to satisfy modern \MTA\ requirements, in providing a clear interface to add functionality without increasing the overall complexity much.
meillo@161 237
meillo@239 238 Modularity is a goal that, if achieved, has positive influence on other important properties like security, testability, extendability, maintainability, and not least simplicity. These quality properties then, on their part, make achieving the functional requirements easier.
meillo@239 239
meillo@239 240 Hence, aspiration for modularity, by compartmentalization, leads to improvement of the overall quality of the software. It is an architectural requirement for a secure and modern \MTA.
meillo@239 241
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meillo@239 244
meillo@239 245
meillo@239 246 \section{Fulfilled requirements}
meillo@239 247 \label{sec:fulfilled-requirements}
meillo@239 248
meillo@239 249 Here follows a description of how far the requirements are already fulfilled by \masqmail.
meillo@239 250
meillo@239 251
meillo@239 252 \paragraph{\RF1: In/out channels}
meillo@239 253 \masqmail's incoming and outgoing channels are the ones required for an \MTA{}s at the moment. They are depicted in figure \ref{fig:masqmail-in-out} on page \pageref{fig:masqmail-in-out}. This is all what is currently needed. But new protocols and mailing concepts are likely to appear (see section \ref{sec:electronic-mail}). \masqmail\ has no support for adding further protocols. Thus modifications at many places in the source are needed to add them though. Today, support for further protocols is not needed, so \masqmail\ is regarded to fulfill \RF1, but the probable future need should be kept in mind.
meillo@239 254
meillo@239 255 \paragraph{\RF2: Queueing}
meillo@239 256 One single mail queue is used in \masqmail; it satisfies all current requirements.
meillo@239 257
meillo@239 258 \paragraph{\RF3: Header sanitizing}
meillo@239 259 The envelope and mail headers are generated when the mail is put into the queue. The requirements are fulfilled.
meillo@239 260
meillo@239 261 \paragraph{\RF4: Aliasing}
meillo@239 262 Aliasing is done on delivery. All common kinds of aliases in the global aliases file are supported. \name{.forward} aliasing is not, but this is less common and seldom used.
meillo@239 263
meillo@239 264 \paragraph{\RF5: Select route}
meillo@239 265 Setting of the route to use is done on delivery. Headers can get rewritten a second time then. This part does provide all the functionality required.
meillo@239 266
meillo@239 267 \paragraph{\RF6: Authentication}
meillo@239 268 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@239 269
meillo@239 270 \paragraph{\RF7: Encryption}
meillo@239 271 Similar is the situation for encryption which is also only available for outgoing channels; here a wrapper application like \name{openssl} is needed. This creates a secure tunnel to send mail trough, but state-of-the-art is using \NAME{STARTTLS}, which 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 translate between the secure connection to the remote host and the \MTA. Unfortunately, this suffers from the problem explained in section \ref{sec:FIXME} and figure \ref{fig:stunnel}. Anyway, this would still be no \NAME{STARTTLS} support.
meillo@239 272
meillo@239 273 \paragraph{\RF8: Spam handling}
meillo@239 274 \masqmail\ nowadays 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 inbetween. 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 a good concept in principle to separate work with clear interfaces. But the need of two instances of the same \MTA (each for only half of the job) with doubled setup, is more a work-around. Best is to have this data flow respected in the \MTA\ design, like in \postfix. But the more important part of spam handling, for sure, is done during the \SMTP\ dialog in completely refusing unwanted mail.
meillo@239 275
meillo@239 276 \paragraph{\RF9: Malware handling}
meillo@239 277 For malware handling applies nearly the same, except all checks are done after mail is accepted. So the possible setup is the same with the two \MTA\ instances and the filter inbetween. \masqmail\ does support such a setup, but not in a nice way.
meillo@239 278
meillo@239 279 \paragraph{\RF10: Archiving}
meillo@239 280 There is currently no way of archiving every message going through \masqmail.
meillo@239 281
meillo@239 282
meillo@239 283 %Non-functional requirements are not so easy to be marked as fulfilled or not. Instead they are discussed here.
meillo@239 284
meillo@239 285 \paragraph{\RG1: Security}
meillo@239 286 \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 attackers. In environments where untrusted components or persons have access to \masqmail, its security is too low. In any way, is a security report missing that confirms \masqmail's security level.
meillo@239 287
meillo@239 288 \paragraph{\RG2: Reliability}
meillo@239 289 Similar is its reliability not good enough. Situations where only one part of sent message was removed from the queue, and the other part remained as garbage, showed off \citeweb{debian:bug245882}. Problems with large mail and small bandwidth were also reported \citeweb{debian:bug216226}. Fortunately, lost email was no big problem yet, but \person{Kurth} warns:
meillo@163 290 \begin{quote}
meillo@239 291 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@239 292 \hfill\citeweb{masqmail:homepage2}
meillo@163 293 \end{quote}
meillo@239 294 In summary: Current reliability needs to be improved.
meillo@239 295 %fixme: state machine
meillo@161 296
meillo@239 297 \paragraph{\RG3: Robustness}
meillo@239 298 The logging behavior of \masqmail\ is good, although it does not cover all problem situations. 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@239 299
meillo@239 300 \paragraph{\RG4: Extendability}
meillo@239 301 \masqmail's extendability is very poor. This is a general problem of monolithic software, but can thus be provided with high effort. \exim\ is an example for good extendability in a monolithic program.
meillo@239 302
meillo@239 303 \paragraph{\RG5: Maintainability}
meillo@239 304 The maintainability of \masqmail\ is equivalent to other software of similar kind. Missing modularity and therefore more complexity makes the maintainer's work harder. In summary is \masqmail's maintainability bearable, like in average Free Software projects.
meillo@239 305
meillo@239 306 \paragraph{\RG6: Testability}
meillo@239 307 The testability suffers from missing modularity. Testing program parts is hard to do. Nevertheless, it is done by compiling parts of the source to special test programs.
meillo@239 308
meillo@239 309 \paragraph{\RG7: Performance}
meillo@239 310 The performance---efficiency---of \masqmail\ is good enough for its target field of operation, where this is a minor goal.
meillo@239 311
meillo@239 312 \paragraph{\RG8: Availability}
meillo@239 313 This applies equal to availability. Hence no further work needs to be done her.
meillo@239 314
meillo@239 315 \paragraph{\RG9: Portability}
meillo@239 316 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. Therefore, the portability is already good.
meillo@239 317
meillo@239 318 \paragraph{\RG10: Usability}
meillo@239 319 The usability, from the administrator's point of view, is very good. \masqmail\ was developed to suite a specific, limited job---its configuration does perfect match. The user's view does not reach to the \MTA, as it is hidden behind the \name{mail user agent}.
meillo@239 320
meillo@239 321
meillo@239 322
meillo@239 323
meillo@239 324
meillo@239 325
meillo@239 326 \section{Work to do}
meillo@239 327
meillo@239 328 After the requirements for modern \mta{}s were identified in section \ref{sec:mta-requirements} and \masqmail's features were set against them in section \ref{sec:fulfilled-requirements}, here the the work that is left to do is identified. Table \ref{tab:requirements} lists all requirements with importance and the work needed to achieve them. The attention a work task should receive---the focus---depends on its importance and the amount of work it includes.
meillo@239 329
meillo@239 330 \begin{table}
meillo@239 331 \begin{center}
meillo@239 332 \input{input/requirements.tex}
meillo@239 333 \end{center}
meillo@239 334 \caption{Importance of and pending work for requirements}
meillo@239 335 \label{tab:requirents}
meillo@239 336 \end{table}
meillo@239 337
meillo@239 338 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 are in focus. In contrast should small low importance work receive few attention. Here the attention/focus a task should get 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@239 339
meillo@239 340 The functional requirements that receive highest attention are \RF6: authentication, \RF7: encryption, and \RF8: spam handling. Of the non-functional requirements, \RG1: security, \RG2: reliability, and \RG4: Extendability, rank highest.
meillo@239 341
meillo@239 342 These tasks to do are presented in more detail now.
meillo@239 343
meillo@239 344
meillo@239 345
meillo@239 346 \subsubsection*{\TODO1: Encryption (\RF7)}
meillo@239 347 Encryption by using \NAME{STARTTLS} is definitely needed and should be added soon. This feature is essential for providing privacy.
meillo@239 348
meillo@239 349 \subsubsection*{\TODO2: Spam handling (\RF8)}
meillo@239 350 \subsubsection*{\TODO6: Authentication (\RF6)}
meillo@239 351 Authentication of incoming \SMTP\ connections is definitely needed and should be added soon. It is essential for restricting relaying.
meillo@239 352
meillo@239 353 As authentication can be a guard against spam, filter facilities have lower priority. But basic spam filtering and interfaces for external tools should be implemented in future. Content checking should be left over to the \NAME{MDA}, to deal with it during local delivery.
meillo@239 354
meillo@239 355 \subsubsection*{\TODO3: Security (\RG1)}
meillo@239 356 \masqmail's security is bad, thus the program is forced into a limited field of operation. The field of operation even shrinks, as security becomes more important and networking and interaction increases. Save and trusted environment become rare. Improving security is an important thing to do.
meillo@239 357
meillo@239 358
meillo@239 359 \subsubsection*{\TODO4: Reliability (\RG2)}
meillo@239 360 Reliability is also to improve. It is a key quality property for an \MTA, and not good enough in \masqmail. Additionally, the program is lacking robustness. Checking the environment and reporting bad characteristics is wanted. Especially improving robustness in relation to the queue is favorable; applying ideas of \name{crash-only software}\cite{candea03} will be a good step.
meillo@239 361
meillo@239 362
meillo@239 363 \subsubsection*{\TODO5: Extendability (\RG4)}
meillo@239 364 Extendability, maintainability, and testability do all suffer from the monolithic architecture and are nearly impossible to improve without changing the programs structure. These properties can hardly be retrofitted into software. Extendability might become important in the future. The other two ease all further work on the software, and also improve security and reliability.
meillo@239 365
meillo@239 366
meillo@239 367
meillo@239 368 \subsubsection*{Further \NAME{TODO}s}
meillo@239 369
meillo@239 370 Support for other protocols than \SMTP\ seems not to be necessary at the moment. Adding such support will need lots of work in many parts of \masqmail. Hence delaying this work until the support becomes mandatory, appears to be the best strategy. This way work can be saved if some protocols never become popular.
meillo@239 371
meillo@239 372 Archiving again is preferred to be implemented soon. It does not require much work, but enables all kinds of statistical analysis.
meillo@239 373
meillo@239 374 Performance is a property that is nice to have. But 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}. \masqmail\ is not a program to be used on large servers, but on small devices. Thus important for \masqmail\ could be energy and heat saving, maybe also system resources, but not performance. Anyway, simplicity and clearness are of higher value.
meillo@239 375
meillo@239 376 Portability among the various flavors of \unix\ systems is a goal, because these systems are the ones \MTA{}s run on usually. Portability problems with non-\unix\ platforms are primary expected to come from file systems lacking required features. But no special care should be taken here.
meillo@239 377
meillo@239 378 Configuration could be eased more, by providing configuration generators to be able to use \masqmail\ 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@239 379
meillo@239 380
meillo@239 381
meillo@239 382
meillo@239 383
meillo@239 384
meillo@239 385 \section{Ways for further development}
meillo@239 386
meillo@239 387
meillo@239 388 \subsubsection*{The need for structural changes}
meillo@239 389
meillo@239 390 Adding authentication and encryption support, for example, is limited to a narrow region in the code. Such features are addable to the current code base without much problem. In contrast does adding support for new protocols or mail processing interfaces to external programs require a lot of effort. Changes in many parts of the source code are required. It is a bad idea to implement large retro-fitted features into software that is critical about security and reliability, like \MTA{}s. Worse if these features need changes in the program's structure, like adding mail scanning interfaces would do.
meillo@239 391
meillo@239 392 If such large features are needed, it is best to redesign the program's structure and rebuild it. A program's structure is primary its architecture. Which is the most influencing design decision, and has the greatest impact on the program's future capabilities. The architecture defines what the program can do, and how it can be used. If the architecture does not fit to the requirements, development will reach a dead end \dots\ further work then will make everything worse. The only good solution is to change the architecture, which, sadly but most likely, means a redesign from scratch.
meillo@239 393
meillo@239 394 Quality properties, like security and reliability, as well as extendability and maintainability, can hardly be added afterwards---if at all. Only structural changes will improve them. Hence, if security, reliability, extendability (to add support for future mail transfer protocols), or maintainability shall be improved, a redesign of \masqmail\ is the only sane way to go.
meillo@239 395
meillo@239 396 %\person{Hafiz} adds: ``The major idea is that security cannot be retrofitted into an architecture.''\cite[page 64]{hafiz05}
meillo@239 397
meillo@239 398
meillo@239 399
meillo@239 400 The most needed features---authentication and encryption---can be added to the current code base with changes in only few parts of the source. These changes should be made soon. Archiving of mail is another feature to add then. More complete logging coverage, reporting of unsafe environment, and fixing high risk security flaws are quality improvements to do. All this work should be done on basis of the current code.
meillo@239 401
meillo@239 402 All other work depends on how the plans for \masqmail's future look like.
meillo@239 403
meillo@239 404 What shall \masqmail\ be like, in, for instance, five years?
meillo@239 405
meillo@239 406 Two ways of further development come to mind.
meillo@239 407
meillo@239 408 First, stick to the old architecture and try to add features as possible. This approach needs less effort to be spent, because a working code is already present. Further development is only adding small increments to a exiting code base. But the further development goes, the larger is the work needed to add more functionality, and the more bugs will appear, caused by the increasing complexity. Quality of the software will decrease, because lacking of clear internal structure encourages further work to be quick fixes rather than good solutions.
meillo@239 409
meillo@239 410 Second, the way of designing \masqmail\ from scratch and rebuilding it. A lot of time and work is required to do this. Additionally, a new design from scratch introduces new risks: Is the design really better? Was thought of everything? Will there come problems not foreseeable now? Starting from scratch also means a step back. Against these disadvantages stands the gain from the new design: Further development will be easier and probably faster, overall quality will be better and easier to keep up, and dead ends for further development are better avoidable.
meillo@239 411
meillo@239 412 Essentially, the decision for one of the ways depends on the question whether \masqmail\ should remain what it is, then the first option seems to be the right one to choose. Or whether \masqmail\ should become a modern \mta\ which is able to expand to include new functionality, then the second option is to choose.
meillo@239 413
meillo@239 414 Security, extendability, and the other quality properties appear to have also crucial importance in this decision. If they are required for future versions of \masqmail, then a new design is a must.
meillo@239 415
meillo@239 416 \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{graff03}. An old-fashioned \mta\ depends, for sure, on a dieing branch, called \name{trusted environments}. And nothing other than a fresh and better design will help to survive.
meillo@239 417
meillo@239 418
meillo@239 419
meillo@239 420 1) on current code base
meillo@239 421
meillo@239 422 2) wrappers, interposition
meillo@239 423
meillo@239 424 3) new design
meillo@239 425
meillo@239 426
meillo@239 427
meillo@239 428
meillo@239 429
meillo@239 430 \subsubsection*{A redesign from scratch}
meillo@161 431
meillo@219 432 All this leads to the wish of a rewrite of \masqmail, using a modern, modular architecture, \emph{if} further features need to be added---features that require changes in \masqmail's structure. But a rewrite is also mandatory, if \masqmail\ should become a modern \MTA, with good quality properties.
meillo@161 433
meillo@225 434 But redesigning and rewriting a software from scratch is hard. It takes time to design a new architecture, which then must prove it is secure and reliable. As well is much time and work needed to implement the design, test it, fix bugs, and so on. If flaws in the design appear during prototype implementation, it is necessary to start again. Thus the gain of a new design must overweight the effort needed.
meillo@161 435
meillo@188 436 \person{Wheeler}'s program \name{sloccount} calculates following estimations for \masqmail's code base as of version 0.2.21 (excluding library code):
meillo@188 437 {\small
meillo@188 438 \begin{verbatim}
meillo@188 439 Total Physical Source Lines of Code (SLOC) = 9,041
meillo@188 440 Development Effort Estimate, Person-Years (Person-Months) = 2.02 (24.22)
meillo@188 441 (Basic COCOMO model, Person-Months = 2.4 * (KSLOC**1.05))
meillo@188 442 Schedule Estimate, Years (Months) = 0.70 (8.39)
meillo@188 443 (Basic COCOMO model, Months = 2.5 * (person-months**0.38))
meillo@188 444 Estimated Average Number of Developers (Effort/Schedule) = 2.89
meillo@188 445 Total Estimated Cost to Develop = $ 272,690
meillo@188 446 (average salary = $56,286/year, overhead = 2.40).
meillo@188 447 SLOCCount, Copyright (C) 2001-2004 David A. Wheeler
meillo@188 448 \end{verbatim}
meillo@188 449 }
meillo@219 450 The development cost is 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 Oliver \person{Kurth} within four years, in his spare time. This means he needed around twice as much time. Of course, he programmed as volunteer developer, not as employee with eight work-hours per day.
meillo@161 451
meillo@225 452 Given the assumptions that (1) an equal amount of code needs to be produced for a new \masqmail, (2) a third of 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 to have 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.
meillo@200 453
meillo@177 454
meillo@177 455
meillo@177 456
meillo@177 457
meillo@196 458
meillo@196 459
meillo@196 460
meillo@196 461
meillo@196 462
meillo@196 463 \section{Result}
meillo@196 464
meillo@219 465 The suggested further development plan for \masqmail\ is:
meillo@219 466 \begin{enumerate}
meillo@225 467 \item The short time goal: Add the most needed features, being authentication and encryption, to the current code base. \item The long time goal: Design a new architecture that satisfies the requirements identified, especially the quality requirements. The implementation of this design shall then, after being usable and throughout tested, supersede the old \masqmail.
meillo@219 468 \end{enumerate}
meillo@196 469
meillo@219 470 This plan is similar to the change from \sendmail\ to \name{sendmail X}/\name{MeTA1}, except the \sendmail\ change was much too late.
meillo@196 471
meillo@219 472 The following chapter is about the work on the current code base, to reach the short time goals. The chapter afterwards then introduces a new, modern design for future versions of \masqmail.
meillo@196 473
meillo@219 474
meillo@219 475 %The plan is to first do the most needed stuff on the old design to make it still usable; then design a new version from scratch, for the future.