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

comparison thesis/tex/5-Improvements.tex @ 402:e57129f57faa

finished the indexing in a huge last effort

author	meillo@marmaro.de
date	Sun, 08 Feb 2009 22:51:42 +0100
parents	13e630c5a44d
children	1d527ad76c97

comparison

equal deleted inserted replaced

-:d6ff5728dcd1
+:e57129f57faa
 \subsection{Encryption}
 \index{enc}
 Encryption (\TODO\,1) should be the first functionality to be added to the current code. The requirement was already discussed on page~\pageref{requirement-encryption}. As explained there, \NAME{STARTTLS} encryption---defined in \RFC\,2487---should be added to \masqmail.
 \index{starttls}
+\index{rfc}
 This work requires changes mainly in three source files: \path{smtp_in.c}, \path{smtp_out.c}, and \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
 \index{smtp}
 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 over encrypted channels only.
 The third file controls the configuration files. New configuration options 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.
+\index{configuration}
 \subsubsection*{Depencencies}
 \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. \name{openssl} or a substitute like \name{gnutls} does then become a dependency for \masqmail. \name{gnutls} seems to be the better choice because the \name{openssl} license is incompatible to the \NAME{GPL}, under which \masqmail\ and \name{gnutls} are covered.
 \index{tls}
 \subsubsection*{Existing code}
 \index{existing code}
 \person{Frederik Vermeulen} wrote an encryption patch for \qmail\ which adds \NAME{STARTTLS} support \citeweb{qmail:tls-patch}. This patch includes about 500 lines of code.
 \index{qmail}
+\index{starttls}
 Adding this code in a similar form to \masqmail\ will be fairly easy. It will save a lot of work as it is not necessary to write the code completely from scratch.
 \subsection{Authentication}
 \index{auth}
 Authentication (\TODO\,2) is the second function to be added. It is important to restrict the access to \masqmail, especially for mail relay. The requirements for authentication where identified on page~\pageref{requirement-authentication}.
-Static access restriction, based on the \NAME{IP} address is already possible by using \NAME{TCP} \name{Wrappers}. This makes it easy to refuse all connections from outside the local network for example, which is a good prevention against being an open relay. More detailed static restrictions, like splitting between mail for users on the system and mail for relay, should \emph{not} be added to the current code. This is a concern for the new design.
+Static access restriction, based on the \NAME{IP} address is already possible by using \NAME{TCP} \name{Wrapper}. This makes it easy to refuse all connections from outside the local network for example, which is a good prevention against being an open relay. More detailed static restrictions, like splitting between mail for users on the system and mail for relay, should \emph{not} be added to the current code. This is a concern for the new design.
-\index{tcp wrappers}
+\index{access restriction}
+\index{tcp wrapper}
+\index{open relay}
 \subsubsection*{One of the dynamic methods}
 Of the three 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.
 \index{auth!methods}
 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 referred to as \NAME{SMTP-AUTH}) support is easiest attained 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:
-\index{smtp-auth}
+\index{auth!smtp-auth}
 \index{sasl}
 \begin{quote}
 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 \NAME{SMTP-AUTH}---defined in \RFC\,2554---is supported by almost all email clients. If encryption is used then even insecure authentication methods like \NAME{PLAIN} and \NAME{LOGIN} become secure.
+\index{rfc}
 \subsubsection*{Simple Authentication and Security Layer}
 \index{sasl}
 \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 the \path{passwd} file 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 with the decision for a method and with 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, for instance, by \name{msmtp}. It seems best to give both concepts a try and decide then which one to use.
+\index{sendmail}
+\index{postfix}
 \index{cyrus sasl}
 \index{pam}
 \index{gsasl}
 Currently, outgoing connections already feature \SMTP-\NAME{AUTH} but only in a hand-coded way. It is to decide whether this should remains as it is or should get replaced by the \NAME{SASL} approach that will be used for incoming connections. The decision should be influenced by the estimated time until the new design is usable.
 Authentication needs code changes in 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.
+\index{configuration}
+\index{plain text}
 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.
 \subsubsection*{Authentication backend}
 Because the \MTA\ does not listen for connections from outside now, it is not directly vulnerable. Unfortunately, the \MTA\ can not react on relaying and spam by 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 of mail security layers is that the \MTA\ itself needs not to bother much with untrusted environments. The proxy cares for this.
 \name{smap} is non-free software and thus no general choice for \masqmail. A way to achieve a similar setup is to copy \masqmail\ and strip one copy to the bare minimum of 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.
+\index{Free Software}
 \index{inetd}
 \index{proxy}
+\index{root privilege}
+\index{setuid}
 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.
 \index{policy}
 The here described setup comes close to the structure of the incoming channels in the new design which is described in section~\ref{sec:new-design}. This shows the capabilities of the here chosen approach.
 \subsubsection*{A concrete setup}
 A stripped down proxy needs to be created. It should only be able to receive mail via \SMTP, encrypt the communication, authenticate clients, and send mail out via \SMTP\ to an internal socket (named ``X'' in the figure). This is a straight forward task. The normal \masqmail\ instance runs on the system, too. It takes input from \name{stdin} (when the \path{sendmail} command is invoked) and via \SMTP\ where it listens on an internal socket (named ``X'' in the figure). Outgoing mail is handled without difference to a regular setup. Figure~\ref{fig:proxy-setup} depicts the setup.
-\index{auth}
+\index{sendmail!command}
-\index{enc}
 \begin{figure}
 	\begin{center}
 		\includegraphics[scale=0.75]{fig/proxy-setup.eps}
 	\end{center}
 	\caption{A setup with a proxy}
-	\index{figure!A setup with a proxy}
 	\label{fig:proxy-setup}
 \end{figure}
 \subsubsection*{Spam and malware handling}
 The functional requirements for incoming channels were already discussed as \RF\,1 on page~\pageref{rf1}. Two required incoming channels were identified: the \path{sendmail} command for local mail submission and the \SMTP\ daemon for remote connections.
 \index{sendmail!command}
 A bit different is the structure of \name{sendmail~X} at that point: Locally submitted messages go also to the \SMTP\ daemon, which is the only connection to the mail queue. \person{Finch} proposes a similar approach \cite{finch-sendmail}: He wants the \path{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 to have 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.
-\index{sendmailx}
+\index{sendmail!sendmailx}
 \index{smtp}
 \index{setuid}
 But merging the input channels in the \SMTP\ daemon makes the \MTA\ heavily dependent on \SMTP. To \qmail\ and \postfix\ new protocol handlers may be added without change in other parts of the system. The \SMTP\ modules can even get removed if it is not needed. It is better to have a larger number of independent modules if each one is simpler then. The need to implement \SMTP\ clients in every module for internal communication makes them more complicated.
 \index{qmail}
 \subsubsection*{Outgoing channels}
 \index{outgoing channels}
 Outgoing mail is commonly either sent using \SMTP, piped into local commands (for example \path{uucp}), or delivered locally by appending to a mailbox. The requirements were identified on page~\pageref{rf1}.
 \index{uucp}
+\index{pipe}
 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 should have root privilege to be able to switch to any user in order to write to his mailbox. Modular \MTA{}s do not require \name{setuid root} but the local delivery process (or its parent) should run as root. root privilege is not a mandatory requirement but any other approach has some disadvantages thus commonly root privilege is used.
+\index{postfix}
+\index{qmail}
+\index{sendmail!sendmailx}
 \index{setuid}
+\index{root privilege}
 Local mail delivery should not be done by the \MTA, but by an \NAME{MDA} instead. 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.
 \index{local delivery}
+\index{mda}
+\index{pipe}
 Other outgoing channels, one for each supported protocol, should be designed like it was done in other \MTA{}s.
 \subsubsection*{Mail queuing}
 \index{mail queue}
 The mail queue is the central part of an \MTA. This fact demands especially for robustness and reliability as a failure here can lead to mail loss. (See \RF\,2 on page~\pageref{rf2}.)
+\index{mail loss}
 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 one own queue each. \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) and have the content scanning within the move from the one to the other.
+\index{sendmail}
 \index{exim}
 \index{postfix}
 \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: The whole queue should be stored in one single file with pointers to separating positions \cite{finch-queue}.
 All of the presented \MTA{}s use the file system to hold the queue; none uses a database to hold it. A database could improve the reliability of the queue through better persistence. This might be a choice for larger \MTA{}s but is none for \masqmail\ which should be kept small and simple. A running database system does likely require much more resources than \masqmail\ itself does. And as the queue's job is more storing data, than running data selection queries, a database does not gain enough to outweigh its costs.
 \index{database system}
 Hence the choice here is having a directory with simple text files in it. This is straight forward, simple, clear, and general \dots\ and thus a good basis for reliability. It is additionally always an advantage if data is stored in the operating system's natural form, which is plain text in the Unix' case.
+\index{Unix}
+\index{plain text}
 Robustness of the queue is covered in the next section.
 The mail body will never get modified, except for removing and adding transfer protocol specific requirements like dot stuffing or special line ending characters. These translations are only done in receiving and sending modules.
 \person{Jon Postel}'s robustness principle\footnote{``Be liberal in what you accept, and conservative in what you send.''. In this wording in \RFC\,1122 and in different wordings in numerous \RFC{}s} should be respected in the \name{scanning} module. The module should parse the given input in a liberal way and generate clean output. \person{Raymond}'s \name{Rule of Repair}\footnote{``Repair what you can -- but when you must fail, fail noisily and as soon as possible.'' \cite[page~18]{raymond03}} can be applied, too. But it is important to repair only obvious problems, because repairing functionality is likely a target for attacks.
 \index{robustness!principle of}
+\index{repair!rule of}
+\index{rfc}
 \subsubsection*{Aliasing}
-\index{aliases}
+\index{alias expansion}
 The functional requirements were identified under \RF\,4 on page~\pageref{rf4}. From the architectural point of view, the main question about aliasing is: Where should aliases get expanded?
 Two facts are important to consider: (1) Addresses that expand to a list of users lead to more envelopes. (2) Aliases that change the recipient's domain part may make the message unsuitable for a specific online route.
 Aliasing is often handled by 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, though the replacement address was set inside the system. This seems not to be wanted.
+\index{smtp!rejecting}
 Doing the alias expansion in the \name{scanning} module appears to be the best solution. Unfortunately, a second alias expansion must be made on delivery, because only then is clear which route is used for the message. This compromise should get accepted.
 \index{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 of outgoing mail. But the changes that are made by the receiving modules (adding further headers) and sending modules (address rewrites) are not respected with this approach.
 \qmail\ has the ability to log complete \SMTP\ dialogs. Logging the complete data transaction into and out of the system is a great feature which should be implemented into each receiving and sending module. Though, as this will produce a huge amount of output, it should be disabled by default.
+\index{qmail}
 \index{smtp!dialog}
 Archiving's functional requirements were described as \RF\,10 on page~\pageref{rf10}.
 The two approaches for spam handling were already presented to the reader in section~\ref{sec:functional-requirements} as \RF\,8 and \RF\,9. 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's and recipient's mail addresses would be enough, but as they are forgeable, it is not. More and more complex checks are needed to be done. Checking needs time, but \SMTP\ dialogs time out if it takes too long. Thus during the \SMTP\ dialog, only limited time can be used for checking if a message seems to be spam. The advantage of this approach is that bad messages can simply get refused---no responsibility for them is taken and no further system load is added. See \RFC\,2505 (especially section 1.5) for detail.
+\index{smtp!responsibility}
 \index{smtp!dialog}
+\index{rfc}
 	\item Checking for spam after the mail was accepted and queued: Here it is possible to invest more processing time, thus more detailed checks can be done. But, as responsibility for messages was taken, 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} lists actions to take after a message is recognized as probably spam \cite[pages 18--20]{eisentraut05}. For mail the \MTA\ is responsible for, the only acceptable action is adding further or rewriting existing header lines. Thus all further work on the spam messages 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 them 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, (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.'')]{eisentraut05}
+\index{spamassassin}
 Checks before a message is accepted, like \NAME{DNS} blacklists and \name{greylisting}, need to be invoked from within the receiving modules. Like for authentication and encryption, the implementation of this functionality should be provided by a central source.
 \index{dns blacklist}
 \index{greylisting}
 \begin{figure}
 	\begin{center}
 		\includegraphics[width=\textwidth]{fig/masqmail-arch-new.eps}
 	\end{center}
 	\caption{The new designed architecture for \masqmail}
-	\index{figure!The new designed architecture for \masqmail}
 	\label{fig:masqmail-arch-new}
 \end{figure}
 This architecture is heavily influenced by the ones of \qmail\ and \postfix. Both have different incoming channels which merge in the module that puts mail into the queue; central is the queue (or more of them); and one module takes mail from the queue and passes it to one of the outgoing channels. But mail processing is built into the architecture in a more explicit way in this design than it was done in \qmail\ and \postfix.
 \index{qmail}
 \paragraph{Receiver modules}
 \index{incoming channels}
 They are the communication interface between external senders and the \name{queue-in} module. Each protocol needs a corresponding \name{receiver module} to be supported. Most popular is the \name{sendmail} module, which is a command to be called from the local host, and the \name{smtpd} module which usually listens on port 25. Other modules to support other protocols may be added as needed. Receiving modules that need to listen on ports should get invoked by \name{inetd}, or by \person{Bernstein}'s more secure \name{ucspi-tcp}. This makes it possible to run them with least privilege.
-\index{least privilege}
+\index{inetd}
+\index{inetd!ucspi-tcp}
+\index{least privilege, principle of}
 \paragraph{The \name{queue-in} module}
 \index{mail queue}
 Its job is to store new messages into the queue. When one of the receiving modules has a new message, it invokes the \name{queue-in} module which creates a spool file in the \name{incoming} queue and a data file in the \name{pool}. The receiver module then sends the envelope, the message header, and the message body. The \name{queue-in} modules writes the first two into the spool file, the latter one into the \name{pool}.
 \paragraph{Transport modules}
 \index{outgoing channels}
 These modules send outgoing mail; they are the interface between \name{queue-out} and remote hosts or local commands. The most popular modules of this kind are the \name{smtp} module which acts as an \SMTP\ client and the \name{pipe} module to interface gateways to other systems or networks like \NAME{FAX} and \NAME{UUCP}. A module for local delivery is not included; \masqmail\ passes this job to an \NAME{MDA} which gets invoked through the \name{pipe} module. (See section~\ref{sec:functional-requirements} for reasons.)
+\index{fax}
+\index{gateway}
+\index{mda}
+\index{pipe}
 \subsubsection*{The queue}
 \index{mail queue}
 The queuing system consists of two queues and a message pool. The queues store the spool files---in unprocessed form in \name{incoming} and in complete and valid form in \name{outgoing}. The \name{pool} is the storage of the data files. On disk, the three parts of the queuing system are represented by three directories within the queue path.
 The representation of queued messages on disk is basically the same as in current \masqmail: One file for the envelope and message header information (the ``spool file'') and a second file for the message body (the ``data file'').
-\index{spool file}
-\index{data file}
 The currently used internal structure of the spool files can remain. Following is a sample spool file from current \masqmail. The first part is the envelope and meta information. The annotations in parenthesis are only added to ease the understanding. The second part, after the empty line, is the message header.
 \codeinput{input/sample-spool-file.txt}
 The spool file owner's executable bit shows if a file is ready for further processing: The module that writes the file into the queue sets the bit as last action. Modules that read from the queue can process messages that have the bit set. This approach is derived from \postfix.
+\index{executable bit}
+\index{postfix}
 The data file is stored into the \name{pool} by \name{queue-in}; it never gets modified until it is deleted by \name{queue-out}. They consist of data in local default text format.
 \subsubsection*{Inter-module communication}
 \index{ipc}
 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.
+\index{ipc}
 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 signals might be useful to trigger runs. Communication between receiver and transport modules and the outside world is organized by their specific protocol (e.g.\ \SMTP).
 Left is only the communication between the receiver modules and \name{queue-in}, and between \name{queue-out} and the transport modules. Suggested for this communication is a simple protocol with data exchange through Unix pipes. Figure~\ref{fig:ipc-protocol} shows a state diagram for the protocol.
+\index{pipe}
 The protocol is described in more detail now:
-\index{protocol}
 \paragraph{Timing}
 One dialog consists of exactly three phases: (1) The connection attempt, (2) The envelope and header transfer, and (3) The 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 failure reply. Timeouts for each phase need to be implemented.
 \begin{figure}
 	\begin{center}
 		\includegraphics[scale=0.75]{fig/ipc-protocol.eps}
 	\end{center}
 	\caption{State diagram of the \NAME{IPC} protocol. (Solid lines indicate client actions, dashed lines indicate server responses.)}
-	\index{figure!State diagram of the \NAME{IPC} protocol.}
 	\label{fig:ipc-protocol}
 \end{figure}
 \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 the envelope and the message header. If the server again sends a positive reply, the message data is transferred. A last server reply ends the dialog.
 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. The server then needs to send its reply. The server process takes responsibility for 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} (`\texttt{\textbackslash{}n}')---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}') indicating failure.
+\index{Line Feed}
+\index{ascii}
+\index{Unix}
+\index{plain text}
 \subsubsection*{Rights and permissions}
 \index{permission}
 The set of system users that is required for \qmail\ seems to be too complex for \masqmail. One system user, like \postfix\ uses, is more appropriate. \name{root} privilege and \name{setuid} permission should to be avoided if feasible.
+\index{system user management}
+\index{postfix}
+\index{qmail}
+\index{root privilege}
 The \name{queue-in} module is the part of the system that is most critical about permission. It either needs to run as deamon or be \name{setuid} or \name{setgid} in order to avoid a world-writable queue. \person{Ian~R.\ Justman} recommends to use \name{setgid} in this situation:
 \index{setuid}
 \begin{quote}
 \end{quote}
 \person{Bernstein} chose \name{setuid} for the \name{qmail-queue} module, \person{Venema} uses \name{setgid} in \postfix, yet the differences are small. Better than running the module as a deamon is each of them. A deamon needs more resources and therefore becomes inefficient on systems with low mail amount, like the ones \masqmail\ will probably run on. Short running processes are additionally higher obstacles for intruders, because a process will die soon if an intruder managed to take one over.
 \index{qmail}
 \index{postfix}
+\index{setuid}
 The modules \name{scanning} and \name{queue-out} are candidates for all-time running daemon processes. Alternatively they could be started by \name{cron} to do single runs.
+\index{cron}
 Another possibility is to run a master process as daemon which starts and restarts the system parts. \postfix\ has such a master process, \qmail\ lacks it. The jobs of a master process can be done by other tools of the operating system too, thus making a master process abdicable. \masqmail\ does probably better go without a master process, because it aims to save resources, not to get the best performance.
+\index{qmail}
 \index{master process}
 A sane permission management is very important for secure software in general. The \name{principle of least privilege} \cite[section~I.A.3.f]{saltzer75}, as it is often called, should be respected. If it is possible to use lower privilege then it should be done. An example for doing so is the \name{smtpd} module. It is a server module which listens on a port. One way is to start it as root and let it bind to the port and drop all privilege before it does any other work. But root privilege is avoidable completely if \name{inetd}, or one of its substitutes, listens on the port instead of the \name{smtpd} module. \name{inetd} will then launch the \name{smtpd} module to handle the connection whenever a connection attempt to the port is made. The \name{smtpd} module needs no privilege at all this way.
+\index{least privilege, principle of}
+\index{inetd}
+\index{root privilege}

Mercurial > docs > diploma

comparison thesis/tex/5-Improvements.tex @ 402:e57129f57faa