docs/diploma: thesis/tex/5-Improvements.tex comparison

comparison thesis/tex/5-Improvements.tex @ 332:4d705f7a956a

spell checked

author	meillo@marmaro.de
date	Sat, 24 Jan 2009 12:32:20 +0100
parents	462e9b23d125
children	5f416c27e932

comparison

equal deleted inserted replaced

-:462e9b23d125
+:4d705f7a956a
 \subsection{Encryption}
-Encryption should be the first funtionality to add to the current code. This requirement was already discussed on page \pageref{requirement-encryption}. As explained there, \NAME{STARTTLS} encryption---as defined in \RFC\,2487---should be added to \masqmail.
+Encryption should be the first functionality to add to the current code. This requirement was already discussed on page \pageref{requirement-encryption}. As explained there, \NAME{STARTTLS} encryption---as defined in \RFC\,2487---should be added to \masqmail.
 Adding encryption requires changes mainly in three source files: \path{smtp_in.c}, \path{smtp_out.c}, and in \path{conf.c}.
 The first file includes the functionality for the \SMTP\ server. It needs to offer \NAME{STARTTLS} support to clients and needs to initiate the encryption when the client requests it. Additionally, the server should be able to insist on encryption before it accepts any message.
 The second file includes the functionality for the \SMTP\ client. It should start the encryption by issuing the \NAME{STARTTLS} keyword if the server supports it. It should be possible to send messages only if encryption is possible.
-The third file controls the configuration files. New configuration option need to be added. The encryption policy for incoming connections needs to be defined. Three choises seem necessary: no encryption, offer encryption, insist on encryption. The encryption policy for outgoing connections should be part of each route setup. The options are the same: never encrypt, encrypt if possible, insist on encryption.
+The third file controls the configuration files. New configuration option need to be added. The encryption policy for incoming connections needs to be defined. Three choices seem necessary: no encryption, offer encryption, insist on encryption. The encryption policy for outgoing connections should be part of each route setup. The options are the same: never encrypt, encrypt if possible, insist on encryption.
 \NAME{STARTTLS} uses \NAME{TLS} encryption which is based on certificates. Thus the \MTA\ needs its own certificate. This should be generated during installation. A third party application like \name{openssl} should be taken for this job. The encryption itself should also be done using an available library. Open\NAME{SSL} or a substitute like Gnu\NAME{TLS} does then become a dependency for \masqmail. Gnu\NAME{TLS} seems to be the better choice because the Open\NAME{SSL} license is incompatible to the \NAME{GPL}, under which \masqmail\ and Gnu\NAME{TLS} are covered.
 User definable paths to \masqmail's secret key, \masqmail's certificate, and the public certificates of trusted \name{Certificate Authorities} (short: \NAME{CA}s) are also nice to have.
 Of the dynamic, secret based, authentication methods (\SMTP-after-\NAME{POP}, \SMTP\ authentication, and certificates) the first one drops out as it requires a \NAME{POP} server running on the same or a trusted host. \NAME{POP} servers are rare on workstations and home servers do also not regularly include them. Thus it is no option for \masqmail.
 Authentication based on certificates does suffer from the certificate infrastructure that is required. Although certificates are already used for encryption, its management overhead prevented wide spread usage for authentication.
-\SMTP\ authentication (also refered to as \NAME{SMTP-AUTH}) support is easiest received by using a \name{Simple Authentication and Security Layer} (short: \NAME{SASL}) implementation. \person{Dent} sees in \NAME{SASL} the best solution for dynamic authentication of users:
+\SMTP\ authentication (also referred to as \NAME{SMTP-AUTH}) support is easiest received by using a \name{Simple Authentication and Security Layer} (short: \NAME{SASL}) implementation. \person{Dent} sees in \NAME{SASL} the best solution for dynamic authentication of users:
 \begin{quote}
 %None of these add-ons is an ideal solution. They require additional code compiled into your existing daemons that may then require special write accesss to system files. They also require additional work for busy system administrators. If you cannot use any of the nonauthenticating alternatives mentioned earlier, or your business requirements demand that all of your users' mail pass through your system no matter where they are on the Internet, SASL is probably the solution that offers the most reliable and scalable method to authenticate users.
-None of these [authentication methods] is an ideal solution. They require additional code compiled into your existing daemons that may then require special write accesss to system files. They also require additional work for busy system administrators. If you cannot use any of the nonauthenticating alternatives mentioned earlier, or your business requirements demand that all of your users' mail pass through your system no matter where they are on the Internet, \NAME{SASL} is probably the solution that offers the most reliable and scalable method to authenticate users.
+None of these [authentication methods] is an ideal solution. They require additional code compiled into your existing daemons that may then require special write access to system files. They also require additional work for busy system administrators. If you cannot use any of the nonauthenticating alternatives mentioned earlier, or your business requirements demand that all of your users' mail pass through your system no matter where they are on the Internet, \NAME{SASL} is probably the solution that offers the most reliable and scalable method to authenticate users.
 \hfill\cite[page 44]{dent04}
 \end{quote}
 These days is \NAME{SMTP-AUTH}, which is defined in \RFC\,2554, supported by most email clients. If encryption is used then even insecure authentication methods like \NAME{PLAIN} and \NAME{LOGIN} become secure.
 \masqmail\ best uses an available \NAME{SASL} library. \name{Cyrus} \NAME{SASL} is used by \postfix\ and \sendmail. It is a complete framework that makes use of existing authentication concepts like \path{/etc/passwd} or \NAME{PAM}. As advantage it can be included in existing user data bases. \name{gsasl} is an alternative. It comes as a library which helps on deciding for a method and on generating the appropriate dialog data; the actual transmission of the data and the authentication against some database is left open to the programmer. \name{gsasl} is used by \name{msmtp} for example. It seems best to give both concepts a try and decide then which one to use.
-Currently, outgoing connections already feature \SMTP-\NAME{AUTH} but only in a hand-coded way. It is to decide wether it remains as it is or gets replaced by the \NAME{SASL} approach, that is used for incoming connections. The decision should be based on the estimated time until the new design is usable.
+Currently, outgoing connections already feature \SMTP-\NAME{AUTH} but only in a hand-coded way. It is to decide whether it remains as it is or gets replaced by the \NAME{SASL} approach, that is used for incoming connections. The decision should be based on the estimated time until the new design is usable.
 Authentication needs code changes at the same places as encryption. The relevant code files are \path{smtp_in.c}, \path{smtp_out.c}, and \path{conf.c}.
 The server code, to authenticate clients, must be added to \path{smtp_in.c} and the configuration options to \path{conf.c}. Several configuration options should be provided: the authentication policy (no authentication, offer authentication, insist on authentication), the authentication backend (if several are supported), an option to refuse plain text methods (\NAME{PLAIN} and \NAME{LOGIN}), and one to require encryption before authentication.
 If the authentication code for outgoing connects shall be changed too, it must be done in \path{smtp_out.c}. The configuration options are already present.
-About the authentication backend. For a small \MTA\ like \masqmail, it seems preferrable to store the login data in a text file under \masqmail's control. This is the most simple choice for many usage scenarios. But using a central authentication facility has advantages in larger setups too. \name{Cyrus} \NAME{SASL} supports both, so there is no problem. If \name{gsasl} is chosen, it seems best to start with an authentication file under \masqmail's control.
+About the authentication backend. For a small \MTA\ like \masqmail, it seems preferable to store the login data in a text file under \masqmail's control. This is the most simple choice for many usage scenarios. But using a central authentication facility has advantages in larger setups too. \name{Cyrus} \NAME{SASL} supports both, so there is no problem. If \name{gsasl} is chosen, it seems best to start with an authentication file under \masqmail's control.
 At first mail security layers like \name{smap} come to mind. The market share analysis in section \ref{sec:market-share} identified such software. This is an interposition filter that stands between the untrusted network and the \MTA. It accepts mail in replacement for the \MTA\ (also called \name{proxy}) in order to separate the \MTA\ from the untrusted network.
 The work \name{smap} does is described in \cite{cabral01}: \name{smap} accepts messages as proxy for the \MTA\ and puts it into a queue. \name{smapd} a brother program runs as daemon and watches for new messages in the queue which it submits into the \MTA\ then.
-Because the \MTA\ does not listen for connections from extern now, it is not directly attackable. But the \MTA\ can not react on relaying and spam on itself anymore because it has no direct connection to the mail sender. This job needs to be covered by the proxy now. Similar is the situation for encryption and authentication. However, care must be taken that the proxy stays small and simple as its own security will suffer otherwise.
+Because the \MTA\ does not listen for connections from outside now, it is not directly vulnerable. But the \MTA\ can not react on relaying and spam on itself anymore because it has no direct connection to the mail sender. This job needs to be covered by the proxy now. Similar is the situation for encryption and authentication. However, care must be taken that the proxy stays small and simple as its own security will suffer otherwise.
 The advantage is that the \MTA\ itself needs not to bother much with untrusted environments. And a small proxy cares only about that work.
 \name{smap} is non-free software and thus no general choice for \masqmail. A way to achieve a similar setup would be to copy \masqmail\ and strip one copy to the bare minimum what is needed for the proxy job. \name{setuid} could be removed and root privilege too if \name{inetd} is used. This hardens the proxy instance.
-Mail from extern would then come through the proxy into the system. Mail from the local host and from the local network could be directly accepted by the normal \masqmail, if those locations are considered trusted. But it seems better to have them use the proxy too, or maybe a second proxy instance with different policy.
+Mail from outside would then come through the proxy into the system. Mail from the local host and from the local network could be directly accepted by the normal \masqmail, if those locations are considered trusted. But it seems better to have them use the proxy too, or maybe a second proxy instance with different policy.
 The here described setup comes close to the structure of the incoming channels in the new design which is described in \ref{sec:new-design}. This shows the possibilities of the here chosen approach. %fixme: rethink this sentence
 \subsubsection*{A concrete setup}
 \end{figure}
 \subsubsection*{Spam and malware handling}
-The presented setup is the same as the one with two \MTA\ instances and a scanner application in between, which was suggested to add spam and malware scanner afterwards to an \MTA. This is a fortunate conincidence, because a scanner like \name{amavis} can simply be put in replace for the internal socket ``X''.
+The presented setup is the same as the one with two \MTA\ instances and a scanner application in between, which was suggested to add spam and malware scanner afterwards to an \MTA. This is a fortunate coincidence, because a scanner like \name{amavis} can simply be put in replace for the internal socket ``X''.
 \subsubsection*{Conditional compilation}
 \sendmail-compatible \mta{}s must support at least two incoming channels: mail submitted using the \sendmail\ command, and mail received via the \SMTP\ daemon. It is therefore common to split the incoming channel into local and remote. This is done by \qmail\ and \postfix. The same way is \person{Hafiz}'s view \cite{hafiz05}. %fixme: specify page
 In contrast is \name{sendmail X}: Its locally submitted messages go to the \SMTP\ daemon, which is the only connection towards the mail queue. %fixme: is it a smtp dialog? or a back door?
 \person{Finch} proposes a similar approach. He wants the \texttt{sendmail} command to be a simple \SMTP\ client that contacts the \SMTP\ daemon of the \MTA\ like it is done by connections from remote. The advantage here is one single module where all \SMTP\ dialog with submitters is done. Hence one single point to accept or refuse incoming mail. Additionally does the module which puts mail into the queue not need to be \name{setuid} or \name{setgid} because it is only invoked from the \SMTP\ daemon. The \MTA's architecture would become simpler and common tasks are not duplicated in modules that do similar jobs.
-But merging the input channels in the \SMTP\ daemon makes the \MTA\ heavily dependent on \SMTP. To \qmail\ and \postfix\ new modules to support other ways of message receival may be added without change of other parts of the system. Also the \SMTP\ modules can be removed if it is not needed. And it is better to have more independent modules if each one is simpler then---it makes the modules more complicated if each one needs to implement an \SMTP\ client.
+But merging the input channels in the \SMTP\ daemon makes the \MTA\ heavily dependent on \SMTP. To \qmail\ and \postfix\ new modules to support other ways of message reception may be added without change of other parts of the system. Also the \SMTP\ modules can be removed if it is not needed. And it is better to have more independent modules if each one is simpler then---it makes the modules more complicated if each one needs to implement an \SMTP\ client.
-With the increasing need for new protocols in mind, it seems better to have single modules for each incoming channel, although this leads to duplicated acceptance checks. Independent checks in different modules, however, have also the advantage to simply apply different policies. Thus it is possible to run two \SMTP\ modules that listen on different ports; one accessable from the Internet but requires authentication, the other only accessable from the local network but does not require authentication.
+With the increasing need for new protocols in mind, it seems better to have single modules for each incoming channel, although this leads to duplicated acceptance checks. Independent checks in different modules, however, have also the advantage to simply apply different policies. Thus it is possible to run two \SMTP\ modules that listen on different ports; one accessible from the Internet but requires authentication, the other only accessible from the local network but does not require authentication.
 The approach of simple independent modules, one for each incoming channel, should be taken.
 A module which is a \NAME{POP} or \NAME{IMAP} client to import contents of other mail boxes into the system may be added afterwards as it is desired.
 Outgoing channels are similar for \qmail, \postfix, and \name{sendmail X}: All of them have a module to send mail using \SMTP, and one for writing into a local mailbox. Local mail delivery is a job that requires root privilege to be able to switch to any user in order to write to his mailbox. Modular \MTA{}s do not need \name{setuid root}, but the local delivery process (or its parent) needs to run as root\footnote{root privilege is actually not a mandatory requirement, but any other approach has some disadvantages, so commonly root privilege is used.}.
 Local mail delivery should not be done by the \MTA, but by an \NAME{MDA}. This decision was discussed in section \ref{sec:functional-requirements}. This means only an outgoing channel that pipes mail into a local command is required for local delivery.
-Other outgoing channels, one for each supportet protocol, may be designed like it was done in other \MTA{}s.
+Other outgoing channels, one for each supported protocol, may be designed like it was done in other \MTA{}s.
 \subsubsection*{The mail queue}
 The mail queue is the central part of an \MTA. This demands especially for robustness and reliability as a failure here can lead to loosing mail.
 %\sendmail, \exim, \qmail, \name{sendmail X}, and \masqmail\ feature one single mail queue. \postfix\ has more of them.
-Common \MTA{}s feature one or more mail queues, they sometimes have effectly several queues within one physical representation.
+Common \MTA{}s feature one or more mail queues, they sometimes have effectively several queues within one physical representation.
 \MTA\ setups that include content scanning tend to require two separate queues. To use \sendmail\ in such setups requires two independent instances with two separate queues. \exim\ can handle it with special \name{router} and \name{transport} rules but the data flow gets complicated. Hence an idea is to use two queues, \name{incoming} and \name{active} in \postfix's terminology, with the content scanning within the move from \name{incoming} to \name{active}.
 \sendmail, \exim, \qmail, and \masqmail\ all use at least two files to store one message in the queue: one file contains the message body, another the envelope and header information. The one containing the mail body is not modified at all. \postfix\ takes a different approach in storing queued messages in an internal format within one file. \person{Finch} suggest yet another approach: storing the whole queue in one single file with pointers to separating positions \cite{finchFIXME}. %fixme: check, cite, and think about
 Mail coming into the system may be may be malformed, lacking headers, or be an attempt to exploit the system. Care must be taken.
 In \postfix, this is done by the \name{cleanup} module, which invokes \name{rewrite}. The position in the message flow is after the message comes from one of the several incoming channels and before the message is stored into the \name{incoming} queue. \name{cleanup} does a complete check to make the mail header complete and valid.
-\qmail\ has the principle of ``don't parse'' which propagades the avoidance of parsing as possible in the system. The reason is that parsing is a highly complex task which often makes code exploitable.
+\qmail\ has the principle of ``don't parse'' which propagates the avoidance of parsing as possible in the system. The reason is that parsing is a highly complex task which often makes code exploitable.
 Mail should be stored into the queue as it is in \masqmail's new design. A scanning module should then parse the message with high care. It seems best to use a \name{parser generator} for this work. The parsed data should then be modified if needed and written into a second queue. This approach has several advantages. First, the receiving parts of the system do not bother about content, they simply store it into the queue. Second, one single modules does the parsing and generates new messages that contain only valid data. Third, the sending parts of the system will only work on messages that consist of valid data. Of course it must be ensured that each message passes through the \name{scanning} module, but this is required for spam and malware scanning too.
 The mail body will never get modified, except of removing and adding transfer protocol specific requirements like dot stuffing or special line ending characters.
 \subsubsection*{Aliasing}
 The main question about aliasing is: Where should aliases get expanded?
-Two facts are important to consider: Addresses expanding to lists of users lead to more envelopes. And aliases changing the reciptient's domain part may make the message unsuitable for a specific online route.
+Two facts are important to consider: Addresses expanding to lists of users lead to more envelopes. And aliases changing the recipient's domain part may make the message unsuitable for a specific online route.
-Aliasing is often handled in expanding the alias and reinjecting the mail into the system. Unfortunately, the mail is processed twice then; additionally does the system have to handle more mail this way. If it is wanted to check the new recipient address for acceptance and do all processing again, then reinjecting it is the best choice. But already accepted messages may get rejected in the second go, because of an replacement address from within the system. This seems not to be wanted.
+Aliasing is often handled in expanding the alias and re-injecting the mail into the system. Unfortunately, the mail is processed twice then; additionally does the system have to handle more mail this way. If it is wanted to check the new recipient address for acceptance and do all processing again, then re-injecting it is the best choice. But already accepted messages may get rejected in the second go, because of an replacement address from within the system. This seems not to be wanted.
 Doing the alias expansion in the scanning module appears to be the best solution. Unfortunately a second alias expansion must be made on delivery, because only at that point in time is clear which route is used for the message. This compromise is accepted.
 The online state is only important for the sending modules of the system, thus it should be queried in the \name{queue-out} module which selects ready messages from the \name{outgoing} queue and transfers them to the appropriate sending module. Route-based aliasing, which was described in the last section, %fixme: is this still true?
 should to be done in the same go.
+\subsubsection*{Archiving}
+The best point to archive copies of every incoming mail is the \name{queue-in} module, respectively the \name{queue-out} module for copies outgoing mail. But not respected with this approach are the changes that are made by the receiving modules (adding further headers) and sending modules (address rewrites).
+\qmail\ has the ability to log complete \SMTP\ dialogs. Logging the complete data transaction into and out of the system into a separate log file is a great feature which should be implemented into each receiving and sending module. But as it will produce a huge amount of output, it should be cared to disabled it by default.
 \subsubsection*{Authentication and Encryption}
-Both topics were discussed several time throughout this thesis, amoung other places on page \pageref{} and \pageref{}.
+Both topics were discussed several time throughout this thesis, among other places on page \pageref{} and \pageref{}.
 Authentication should be done within the receiving modules. Similar should authentication for outgoing connections be handled by the sending modules. To encryption applies the same as to authentication here. Only receiving and sending modules should come in contact with it.
-In order to avoid code duplicates, the actual implementation of both functionalities should be provided by a central source which gets invoked by the various modules.
+In order to avoid code duplicates, the actual implementation of both functions should be provided by a central source which gets invoked by the various modules.
 \subsubsection*{Spam and malware handling}
 The two approaches for spam handling were already presented to the reader in section \ref{}. Here they are described in more detail:
 \begin{enumerate}
-\item Refusing spam during the \SMTP\ dialog. This is the way it was meant by the designers of the \SMTP\ protocol. They thought checking the sender and reciptient mail addresses would be enough, but as they are forgable it is not. More and more complex checks need to be done. Checking needs time, but \SMTP\ dialogs time out if it takes too long. Thus only limited time can be used, during the \SMTP\ dialog, for checking if a message seems to be spam. The advantage is that acceptance of bad messages can be simply refused---no responsibility for the message is taken and no further system load is added. See \RFC2505 (especially section 1.5) for detail.
+\item Refusing spam during the \SMTP\ dialog. This is the way it was meant by the designers of the \SMTP\ protocol. They thought checking the sender and recipient mail addresses would be enough, but as they are forgeable it is not. More and more complex checks need to be done. Checking needs time, but \SMTP\ dialogs time out if it takes too long. Thus only limited time can be used, during the \SMTP\ dialog, for checking if a message seems to be spam. The advantage is that acceptance of bad messages can be simply refused---no responsibility for the message is taken and no further system load is added. See \RFC2505 (especially section 1.5) for detail.
 \item Checking for spam after the mail was accepted and queued. Here more processing time can be invested, so more detailed checks can be done. But, as responsibility for messages was taken by accepting them, it is no choice to simply delete spam mail. Checks for spam do not lead to sure results, they just indicate the possibility the message is unwanted mail. \person{Eisentraut} indicates actions to take after a message is recognized as probably spam \cite[pages 18--20]{eisentraut05}. The only acceptable one, for mail the \MTA\ is responsible for, is adding further or rewriting existent header lines. Thus all further work on the message is the same as for non-spam messages.
 \end{enumerate}
-Modern \MTA{}s use both techniques in combination. Checks during the \SMTP\ dialog tend to be implemented in the \mta\ to make it fast; checks after the message was queued are often done using external programs (\name{spamassassin} is a well known one). \person{Eisentraut} sees the checks during the \SMTP\ dialog to be essentiell: ``Ganz ohne Analyse w\"ahrend der \SMTP-Phase kommt sowieso kein \MTA\ aus, und es ist eine Frage der Einsch\"atzung, wie weit man diese Phase belasten m\"ochte.'' \cite[page 25]{eisentraut05} (translated: ``No \MTA\ can go without analysis during the \SMTP\ phase anyway, but the amount of stress one likes to put on this phase is left to his discretion.'')
+Modern \MTA{}s use both techniques in combination. Checks during the \SMTP\ dialog tend to be implemented in the \mta\ to make it fast; checks after the message was queued are often done using external programs (\name{spamassassin} is a well known one). \person{Eisentraut} sees the checks during the \SMTP\ dialog to be essential: ``Ganz ohne Analyse w\"ahrend der \SMTP-Phase kommt sowieso kein \MTA\ aus, und es ist eine Frage der Einsch\"atzung, wie weit man diese Phase belasten m\"ochte.'' \cite[page 25]{eisentraut05} (translated: ``No \MTA\ can go without analysis during the \SMTP\ phase anyway, but the amount of stress one likes to put on this phase is left to his discretion.'')
 Checking before a message is accepted, like \NAME{DNS} blacklists and \name{greylisting}, needs to be invoked from within the receiving modules. Like for authentication and encryption, the implementation of the functionality should be provided by a central source.
 All checking after the message was queued should be done by pushing the message through external scanners like \name{spamassassin}. The \name{scanning} module is the best place to handle this. Hence this module needs interfaces to external scanners.
 \subsubsection*{The scanning module}
-A lot of work was put onto the \name{scanning} module. This is not what is desired. Thus splitting it up into single parts appears to be neccesary. But the decision how to split is left up to the time of prototyping.
+A lot of work was put onto the \name{scanning} module. This is not what is desired. Thus splitting it up into single parts appears to be necessary. But the decision how to split is left up to the time of prototyping.
 << fixme >> %fixme
-\subsubsection*{Archiving}
-The best point to archive copies of every incoming mail is the \name{queue-in} module, respectively the \name{queue-out} module for copies outgoing mail. But not respected with this approach are the changes that are made by the receiving modules (adding further headers) and sending modules (address rewrites).
-\qmail\ has the ability to log complete \SMTP\ dialogs. Logging the complete data transaction into and out of the system into a separate log file is a great feature which should be implemented into each receiving and sending module. But as it will produce a huge amount of output, it should be cared to disabled it by default.
 \subsection{The resulting architecture}
-The result is a symetric design, featuring the following parts: Any number of handlers for incoming connections to receive mail and pass it to the module that stores it into the incoming queue. A central scanning module take mail from the incoming queue, processes it in various ways and puts it afterwards into the outgoing queue. Another module takes it out there and passes it to a matching transport module that transfers it to the destination. In other words, three main modules (queue-in, scanning, queue-out) are connected by the two queues (incoming, outgoing); on each end are more modules to receive and send mail---for each protocol one. Figure \ref{fig:masqmail-arch-new} depicts the new designed architecture.
+The result is a symmetric design, featuring the following parts: Any number of handlers for incoming connections to receive mail and pass it to the module that stores it into the incoming queue. A central scanning module take mail from the incoming queue, processes it in various ways and puts it afterwards into the outgoing queue. Another module takes it out there and passes it to a matching transport module that transfers it to the destination. In other words, three main modules (queue-in, scanning, queue-out) are connected by the two queues (incoming, outgoing); on each end are more modules to receive and send mail---for each protocol one. Figure \ref{fig:masqmail-arch-new} depicts the new designed architecture.
 \begin{figure}
 	\begin{center}
 		\includegraphics[width=\textwidth]{img/masqmail-arch-new.eps}
 	\end{center}
 \subsubsection*{Modules and queues}
 The new architecture consists of several modules and two queues. They are defined in more detail now, and the jobs, identified above, are assigned to them. First the three main modules, then the queues, and afterwards the modules for incoming and outgoing transfer.
-The \name{queue-in} module creates new spool files in the \name{incoming} queue for incoming messages. It is a process running in background, waiting for connections from one of the receiver modules. When one of them requests for a new spool file, the \name{queue-in} module opens one and returns a positive result. The receiver module then sends the envelope and message, which is written into the spool file by \name{queue-in}. If all went well, another positive result is returend.
+The \name{queue-in} module creates new spool files in the \name{incoming} queue for incoming messages. It is a process running in background, waiting for connections from one of the receiver modules. When one of them requests for a new spool file, the \name{queue-in} module opens one and returns a positive result. The receiver module then sends the envelope and message, which is written into the spool file by \name{queue-in}. If all went well, another positive result is returned.
 %fixme: should be no daemon
-The \name{scanning} module is the central part of the system. It takes spooled messages from the \name{incoming} queue, works on them, and writes them to the \name{outgoing} queue afterwards (the message is then removed from the \name{incoming} queue, of course). The main job is the processing done on the message. Headers are fixed and missing ones are added if necessary, aliasing is done, and external processing of any kind is triggered. The \name{scanning} module can run in background and look for new mail in regular intvals or signals may be sent to it by \name{queue-in}. Alternatively it can be called by \name{cron}, for example, to do single runs.
+The \name{scanning} module is the central part of the system. It takes spooled messages from the \name{incoming} queue, works on them, and writes them to the \name{outgoing} queue afterwards (the message is then removed from the \name{incoming} queue, of course). The main job is the processing done on the message. Headers are fixed and missing ones are added if necessary, aliasing is done, and external processing of any kind is triggered. The \name{scanning} module can run in background and look for new mail in regular intervals or signals may be sent to it by \name{queue-in}. Alternatively it can be called by \name{cron}, for example, to do single runs.
-The \name{queue-out} module takes messages from the \name{outgoing} queue, queries information about the online state which specifies the route to use, creates envelopes for each recipient, and passes the messages to the correct transport module. Successfully transfered messages are removed from the \name{outgoing} queue. This module includes some tasks specific to \masqmail.
+The \name{queue-out} module takes messages from the \name{outgoing} queue, queries information about the online state which specifies the route to use, creates envelopes for each recipient, and passes the messages to the correct transport module. Successfully transferred messages are removed from the \name{outgoing} queue. This module includes some tasks specific to \masqmail.
 The \name{incoming} queue stores messages received via one of the incoming channels. The messages are in unprocessed form; only envelope data is prepended.
 \name{Receiver modules} are the communication interface between outside senders and the \name{queue-in} module. Each protocol needs a corresponding \name{receiver module} to be supported. Most popular are the \name{sendmail} module (which is a command to be called from the local host) and the \name{smtpd} module (which listens on port 25). Other modules to support other protocols may be added as needed.
 %fixme: get invoked by inetd, or better ucspi-tcp (by bernstein) which can limit max number of concurrent connections. and includes tcp-wrappers functionality.
-\name{Transport modules}, on the oppersite side of the system, are the modules to send outgoing mail; they are the interface between \name{queue-out} and remote hosts or local commands for further processing. The most popular ones are the \name{smtp} module (which acts as the \SMTP\ client) and the \name{pipe} module (to interface gateways to other systems or networks, like fax or uucp). A module for local delivery is not included, \masqmail\ passes this job to the \NAME{MDA} (see section \ref{sec:functional-requirements} for reasons). Thus a \name{mail delivery agent} (like \name{procmail}) is to be used with the \name{pipe} module.
+\name{Transport modules}, on the opposite side of the system, are the modules to send outgoing mail; they are the interface between \name{queue-out} and remote hosts or local commands for further processing. The most popular ones are the \name{smtp} module (which acts as the \SMTP\ client) and the \name{pipe} module (to interface gateways to other systems or networks, like fax or uucp). A module for local delivery is not included, \masqmail\ passes this job to the \NAME{MDA} (see section \ref{sec:functional-requirements} for reasons). Thus a \name{mail delivery agent} (like \name{procmail}) is to be used with the \name{pipe} module.
 \subsubsection*{Inter-module communication}
 Communication between modules is required to exchange data and status information. This is also called ``Inter-process communication'' (short: \NAME{IPC}) because the modules are independent programs in this case and processes are programs in execution.
-The connections between \name{queue-in} and \name{scanning}, aswell as between \name{scanning} and \name{queue-out} is provided by the queues, only sending signals to trigger runs may be useful. Communication between receiving and transport modules and the outside world are done using the specific protocol they do handle.
+The connections between \name{queue-in} and \name{scanning}, as well as between \name{scanning} and \name{queue-out} is provided by the queues, only sending signals to trigger runs may be useful. Communication between receiving and transport modules and the outside world are done using the specific protocol they do handle.
 Left is only communication between the receiver modules and \name{queue-in}, and between \name{queue-out} and the transport modules. Data is exchanged using \unix\ pipes and a simple protocol. Figure \ref{fig:ipc-protocol} shows a state diagram for the protocol. Solid lines indicate client actions, dashed lines indicate server responses.
 \begin{figure}
 	\begin{center}
 One dialog consists of exactly three phases: connection attempt, envelope and header transfer, and transfer of the message body. The order is always the same. The three phases are all initiated by the client process; after each phase the server process sends a success or error reply. Timeouts for each phase need to be implemented.
 \paragraph{Semantics}
 The connection attempt is simply opening the connection. This starts the dialog. A positive reply by the server leads to the transfer of envelope and message header. If the server again sends a positive reply, the message data is transferred too. A last server reply ends the dialog.
-The client indicates the end of each data transfer with a special terminator sequence. The appearence of this terminator sequence tells the server process that the data transfer is complete and makes the server send a reply. The server process takes responsibility of the data in sending a success reply. A failure reply immediately stops the dialog and resets both client and server to the state before the connection attempt.
+The client indicates the end of each data transfer with a special terminator sequence. The appearance of this terminator sequence tells the server process that the data transfer is complete and makes the server send a reply. The server process takes responsibility of the data in sending a success reply. A failure reply immediately stops the dialog and resets both client and server to the state before the connection attempt.
 \paragraph{Syntax}
-Data transfer is done by sending plain text data. \name{Line Feed}---the native line separator on \unix---is used as line separator. The terminator sequence used to indicate the end of the data transfer is the \NAME{ASCII} \name{null} character (``\texttt{\textbackslash0}''). Replys are one-digit numbers with \texttt{0} meaning success and any other number (\texttt{1}--\texttt{9}) indicate failure.
+Data transfer is done by sending plain text data. \name{Line Feed}---the native line separator on \unix---is used as line separator. The terminator sequence used to indicate the end of the data transfer is the \NAME{ASCII} \name{null} character (``\texttt{\textbackslash0}''). Replies are one-digit numbers with \texttt{0} meaning success and any other number (\texttt{1}--\texttt{9}) indicate failure.
 \subsubsection*{Spool file format}
 %fixme: why plain text and not db? -> simplicity
 Mark spooled mail messages when processing of the writing module is finished: Either by setting the executable bit (like \postfix\ does), or by changing the owner (an approach for multiple masqmail users).
-A sample header file. With comments in paranthesis.
+A sample header file. With comments in parenthesis.
 \begin{quote}\footnotesize
 \begin{verbatim}
 1LGtYh-0ut-00                (backup copy of the file name)
 MF:<meillo@dream>            (envelope: sender)
 setuid/setgid or not?
 what can crash if an attacker succeeds?
-where to drop privelege?
+where to drop privilege?
 how is which process invoked?
 master process? needed, or wanted?

Mercurial > docs > diploma

comparison thesis/tex/5-Improvements.tex @ 332:4d705f7a956a