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view thesis/tex/5-Improvements.tex @ 327:d13bddf0f994
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| author | meillo@marmaro.de |
|---|---|
| date | Fri, 23 Jan 2009 12:58:52 +0100 |
| parents | 8671d9c0f29a |
| children | 2b1da14922f7 |
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1 \chapter{Improvement plans}
3 The last chapter came to the result that further development is best done in a double-strategy. First the existing code base should be improved to satisfy the most important needs in order to make it usable for some more time. Then \masqmail\ should get redesigned from scratch and rebuild to gain a secure and modern \MTA\ architecture for the future.
5 This chapter finally gives concrete suggestions \emph{how} to realize these plans.
7 The first part covers the short-time goals which base on current code. The second part deals with the long-time goal---the redesign.
12 \section{Based on current code}
14 The first three \TODO{}s are implementable by improving the current code or by adding wrappers or interposition filters. The following sections describe solution approaches to do that work.
18 \subsection{Encryption}
20 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.
22 Adding encryption requires changes mainly in three source files: \path{smtp_in.c}, \path{smtp_out.c}, and in \path{conf.c}.
24 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.
26 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.
28 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.
30 \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.
32 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.
35 << TLS patch of qmail >>
37 %postfix: main.cf
38 %
39 % smtpd_use_tls = yes
40 % smtpd_tls_received_header = no (does not log in received headers)
41 %
42 % smtpd_tls_key_file = /etc/postfix/key.pem
43 % smtpd_tls_cert_file = /etc/postfix/cert.pem
44 % smtpd_tls_CA_file = /etc/postfix/CAcert.pem
45 %
46 % smtp_use_tls = yes (use TLS for sending)
47 % smtp_tls_key_file = /etc/postfix/key.pem
48 % smtp_tls_cert_file = /etc/postfix/cert.pem
49 % smtp_tls_CA_file = /etc/postfix/CAcert.pem
54 \subsection{Authentication}
56 Authentication is the second function to add; it is important to restrict the access to \masqmail, especially for mail relay. The requirements for authentication where identified on page \pageref{requirement-authentication}.
58 Static access restriction, based on the \NAME{IP} address is already possible by using \name{TCP Wrappers}. This makes it easy to refuse all connections from outside the local net for example, which is a good prevention of being an open relay. More detailed static restrictions, like splitting between mail for the system and mail to relay, should not be added to the current code. This may be a concern for the new design.
60 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.
62 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.
64 \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:
65 \begin{quote}
66 %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.
67 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.
68 \hfill\cite[page 44]{dent04}
69 \end{quote}
71 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.
73 \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.
75 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.
77 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}.
79 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.
81 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.
84 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.
90 << Compare static with dynamic authentication: pros and cons; usecases: when to use what; >>
92 << how could this be covered by architecture (e.g. smtp submission). >>
98 \subsection{Security}
100 Improvements to \masqmail's security are an important requirement and are the third task to work on. Retrofitting security \emph{into} \masqmail\ is not or hardly possible as it was explained in section \ref{sec:discussion-further-devel}. But adding wrappers and interposition filters can be a large step towards security.
102 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.
104 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.
106 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.
108 The advantage is that the \MTA\ itself needs not to bother much with untrusted environments. And a small proxy cares only about that work.
110 \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.
112 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.
114 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
117 \subsubsection*{A concrete setup}
119 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} (by calling the \path{sendmail} command) 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.
121 \begin{figure}
122 \begin{center}
123 \includegraphics[scale=0.75]{img/proxy-setup.eps}
124 \end{center}
125 \caption{A setup with a proxy}
126 \label{fig:proxy-setup}
127 \end{figure}
130 \subsubsection*{Spam and malware handling}
132 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''.
137 \subsubsection*{Conditional compilation}
138 << conditional compilation >>
156 \section{A new design}
157 \label{sec:new-design}
159 The last chapter identified the requirements for a modern and securt \masqmail. Now the various jobs of an \MTA\ get assigned to modules, of which the new architecture is created. It is inspired by existing \MTA{}s and driven by the identified requirements.
161 One wise experience was kept in mind during the design: ``Many times in life, getting off to the right start makes all the difference.'' \cite[page~32]{graff03}.
164 Major design ideas of the design were:
165 \begin{itemize}
166 \item free the internal system from in and out channels
167 \item arbitrary protocol handlers have to be addable afterwards
168 \item a single facility for scanning (all mail goes through it)
169 \item concentrate on mail transfer
170 \end{itemize}
174 \subsection{Architectural design}
176 \subsubsection*{Incoming channels}
178 \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 therefor 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.
180 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 second door?
181 \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 to put 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.
183 But merging the input channels in the \SMTP\ daemon makes the \MTA\ heavily dependent on \SMTP\ being the main mail transfer protocol. 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 is it better to have more independent modules if each one is simpler then.
185 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.
188 \subsubsection*{Outgoing channels}
190 Outgoing mail is commonly either sent using \SMTP, piped into local commands (for example \texttt{uucp}), or delivered locally by appending to a mailbox.
192 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 priveledge 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.
194 As mail delivery to local users, is \emph{not} included in the basic job of an \MTA{}, why should it care about it? In order to keep the system simple and to have programs that do one job well, the local delivery job should be handed over to a specialist: the \name{mail delivery agent}. \NAME{MDA}s know about the various mailbox formats and are aware of the problems of concurrent write access and thelike. Hence handling the message and the responsiblity over to a \NAME{MDA}, like \name{procmail} or \name{maildrop}, seems to be the right way to go.
196 This means an outgoing connection that pipes mail into local commands is required. Other outgoing channels, one for each supportet protocol, may be designed like it was done in other \MTA{}s.
200 \subsubsection*{Mail queue}
202 Mail queues are probably used in all \mta{}s, excluding the simple forwarders. A mail queue is a essential requirement for \masqmail, as it is to be used for non-permanent online connections. This means, mail must be queued until a online connection is available to send the message.
204 The mail queue and the module 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 a mail to be sent twice.
206 \sendmail, \exim, \qmail, \name{sendmail X}, and \masqmail\ feature one single mail queue. \postfix\ has more of them.
208 \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, running. \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}.
210 \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} takes yet another different approach in suggesting to store the whole queue in one single file with pointers to separating positions \cite{finchFIXME}.
211 %fixme: check, cite, and think about
213 %fixme: discuss: filesystem vs. database
214 << \masqmail\ uses the filesytem to store the queue, storing the queue in a databases might improve the reliability through better persistence. >> %fixme
216 %fixme: what about the ``rule of repair''?
219 \subsection{Functional design}
222 \subsubsection*{Aliasing}
224 Where should aliases get expanded? They appear in different kind. Important are the ones available in the \path{aliases} file. Aliases can be:
225 \begin{enumerate}
226 \item a different local user (e.g.\ ``\texttt{bob: alice}'')
227 \item a remote user (e.g.\ ``\texttt{bob: john@example.com}'')
228 \item a list of users (e.g.\ ``\texttt{bob: alice, john@example.com}'')
229 \item a command (e.g.\ ``\texttt{bob: |foo}'')
230 \end{enumerate}
231 Addresses expanding to lists of users lead to more envelopes. Aliases changing the reciptients domain part may make the message unsuitable for a specific online route.
233 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.
237 \subsubsection*{Route management}
239 %fixme: rework!!
240 One key feature of \masqmail\ is its ability to send mail out in different ways. The decision is based on the current online state and whether a route may be used for a message or not. The online state can be retrieved in tree ways, explained in \ref{sec:fixme}. A route to send is found by checking every available route for being able to transfer the current message, until one matches.
242 This functionality should be implemented in the module that is responsible to invoke one of the outgoing channel modules (for example the one for \SMTP\ or the pipe module).
244 \masqmail\ can rewrite the envelope's from address and the \texttt{From:} header, dependent on the outgoing route to use. This rewrite must be done \emph{after} it is clear which route a mail will take, of course, so this may be not the module where other header editing is done.
245 %fixme: see hafiz05 page 57: maybe put the rewriting into the sending module (like smx, exim, courier) (problem with archiving of all outgoing mail?)
249 \subsubsection*{Authentication}
251 One thing to avoid is being an \name{open relay}. Open relays allow to relay mail from everywhere to everywhere. This is a major 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.
253 Several ways to restrict access are available. The most simple one is restrictiction by the \NAME{IP} address. No extra complexity is added this way, but static \NAME{IP} addresses are mandatory. This kind of restriction may be enabled using the operating system's \path{hosts.allow} and \path{hosts.deny} files. To allow only connections to port 25 from localhost or the local network \texttt{192.168.100.0/24} insert the line ``\texttt{25: ALL}'' into \path{hosts.deny} and ``\texttt{25: 127.0.0.1, 192.168.100.}'' into \path{hosts.allow}.
255 If static access restriction is not possible, for example if mail from locations with changing \NAME{IP} addresses wants to be accepted, some kind of authentication mechanism is required. Three common kinds exist:
256 \begin{enumerate}
257 \item \SMTP-after-\NAME{POP}: uses authenication on the \NAME{POP} protocol to permit incoming \SMTP\ connections for a limited time afterwards.
258 \item \SMTP authentication: is an extension to \SMTP. Authentication can be requested before mail is accepted.
259 \item Certificates: confirm the identity of someone.
260 \end{enumerate}
264 \subsubsection*{Encryption}
266 Electronic mail is very weak to sniffing attacks, because all data transfer is unencrypted. This concerns the message's content, as well as the email addresses in header and envelope, but also authentication dialogs that may transfer plain text passwords (\NAME{PLAIN} and \NAME{LOGIN} are examples). Adding encryption is therefor wanted.
268 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}.
270 \TLS\ allows to create secure tunnels through which arbitrary programs can communicate. Hence one can add secure communication afterwards to programs without changing them. \name{OpenSSL} for example---a free implementation---allows traffic to be piped into a command; a secure tunnel is created and the traffic is forwarded through it. Or a secure tunnel can be set up between a local and a remote port; this tunnel can then be used by any application.
272 The \NAME{POP} protocol, for example, is good suited for such tunneling, but \SMTP\ is is not generally. Outgoing \SMTP\ client connections can be tunneled without problem---\masqmail\ already provides a configure option called \texttt{wrapper} to do so. Tunneling incomming connections to a server leads to problems with \SMTP. As data comes encrypted through the tunnel to the receiving host and gets then decrypted and forwarded on local to the port the application listens on. From the \MTA's view, this makes all connections appear to come from localhost, unfortunately. Figure \ref{fig:stunnel} depicts the data flow.
274 For incoming connections, \NAME{STARTTLS}---defined in \RFC2487---is what \mta{}s implement.
276 \masqmail\ is already able to encrypt outgoing connections, but encryption of incoming connections, using \NAME{STARTTLS} should be implemented. This only affects the \SMTP\ server module.
282 \subsubsection*{Spam prevention}
284 ---
286 Spam is a major threat nowadays and the goal is to reduce it to a bearable level (see section \ref{sec:swot-analysis}). Spam fighting is a war in which the good guys tend to lose. Putting too much effort there will result in few gain. Real success will only be possible with new---better---protocols and abandonning the weak legacy technologies. Hence \masqmail\ should be able to provide state-of-the-art spam protection, but not more.
288 ---
290 Spam is a major threat to email, as described in section \ref{sec:swot-analysis}. The two main problems are forgable sender addresses and that it is cheap to send hundreds of thousands of messages. Hence, spam senders can operate in disguise and have minimal cost.
292 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. Thus \MTA{}s need to protect themself. Two different approaches are used:
294 \begin{enumerate}
295 \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.
297 \item
298 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.
299 \end{enumerate}
301 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.'')
303 \NAME{DNS} blacklists (short: \NAME{DNSBL}) and \name{greylisting} are checks to be done before accepting the message. Invoking \name{spamassassin}, to add headers containing the estimated spam probability, is best to be invoked after the message is queued.
308 \subsubsection*{Virus checking}
310 Related to spam is malicous 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 body. The same situation in the real world is post offices opening letters to check if they contain something that could harm the recipient. This is not a mail transport concern. Apart of not being the right program to do the job, the \MTA\---the one which is responsible for the recipient---is at a good position to do this work.
312 In any way should malware checking be done by external programs that may be invoked by the \mta. But using mail deliver and processing agents, like \name{procmail}, seem to be better suited locations to invoke content scanners.
314 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. \postfix\ and \exim\ can be configured so that one instance can work as both, the \MTA\ for incoming and outgoing transfer. A setup with \sendmail\ needs two separate instances running. It must be quarateed that all mail flows through the scanner.
316 A future \masqmail\ would do good to have a single point, where all traffic flows through, that is able to invoke external programs to do mail processing of any kind.
319 %AMaViS (amavisd-new): email filter framework to integrate spam and virus scanner
320 %\begin{verbatim}
321 %internet -->25 MTA -->10024 amavis -->10025 MTA --> reciptient
322 %| |
323 %+----------------------------+
324 %\end{verbatim}
325 %
326 %postfix and exim can habe both mta servises in the same instance, sendmail needs two instances running.
327 %
328 %MailScanner:
329 %incoming queue --> MailScanner --> outgoing queue
330 %
331 %postfix: with one instance possible, exim and sendmail need two instances running
334 %message body <-> envelope, header
335 %
336 %anti-virus: clamav
337 %postfix: via amavis
338 %exim: via content-scanning-feature called from acl
339 %sendmail: with milter
340 %procmail
341 %
342 %virus scanner work on file level
343 %amavis receives mail via smtp or pipe, splits it in its parts (MIME) and extracks archives, the come the virus scanners
344 %if the mail is okay, it goes via smtp to a second mta
346 %what amavis recognizes:
347 %- invalid headers
348 %- banned files
349 %- viruses
350 %- spam (using spam assassin)
351 %
352 %mimedefang: uses milter interface with sendmail
356 \subsubsection*{Archiving}
358 Mail archiving and auditability become more important as electronic mail becomes more important. 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.
360 \postfix\ for example has a \texttt{always\_bcc} feature, to send a copy of every mail to a definable reciptient. At least this funtionality should be given, although a more complete approach is preferable.
368 \subsection{Security design}
370 \subsubsection*{Sanitize mail}
372 Mail coming into the system often lacks important header lines. At least the required ones must be added from the \MTA. A good example is the \texttt{Message-Id:} header.
374 In \postfix, this is done by the \name{cleanup} module, which invokes \name{rewrite}. The position in the message flow is after coming from one of the several incoming channels and before the message is stored into the \name{incoming} queue. Modules that handle incoming channels may also add headers, for example the \texttt{From:} and \texttt{Date:} headers. \name{cleanup}, however, does a complete check to make the mail header complete and valid.
376 Apart from deciding where to sanitize the mail header, is the question where to generate the envelope. The envelope specifies the actual recipient of the mail, no matter what the \texttt{To:}, \texttt{Cc:}, and \texttt{Bcc:} headers tell. Multiple reciptients lead to multiple different envelopes, containing all the same mail message.
387 \subsection{The resulting architecture}
389 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.
391 \begin{figure}
392 \begin{center}
393 \includegraphics[width=\textwidth]{img/masqmail-arch-new.eps}
394 \end{center}
395 \caption{A new designed architecture for \masqmail}
396 \label{fig:masqmail-arch-new}
397 \end{figure}
399 This architecture is heavily influenced by the ones of \qmail\ and \postfix. Both have different incoming channels that 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. Mail processing, in any way, is build in in a more explicit way than done in the other two. It is more similar to the \NAME{AR} module of \name{sendmail X}, which is the central point for spam checking.
401 Special regard was put on addable support for further mail transfer protocols. This appears to be most similar to \qmail, which was designed to handle multiple protocols.
402 %fixme: do i need all this ``quesses''??
405 \subsubsection*{Modules and queues}
407 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.
410 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.
411 %fixme: should be no daemon
414 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.
417 The \name{queue-out} module takes messages from the \name{outgoing} queue, queries information about the online connection, and then selects matching routes, 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.
418 %fixme: rework route selection
421 The \name{incoming} queue stores messages received via one of the incoming channels. The messages are in unprocessed form; only envelope data is prepended.
424 The \name{outgoing} queue contains processed messages. The header and envelope information is complete and in valid form.
426 \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.
427 %fixme: get invoked by inetd, or better ucspi-tcp (by bernstein) which can limit max number of concurrent connections. and includes tcp-wrappers functionality.
430 \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, as it is in most other \MTA{}s; the reasons are described in FIXME.%fixme
431 Thus a \name{mail delivery agent} (like \name{procmail}) is to be used with the \name{pipe} module.
435 \subsubsection*{Inter-module communication}
437 Communication between modules is required to exchange data and status information. It is also called ``Inter-process communication'' (short: \NAME{IPC}), as modules are programs being part of a larger system, and processes are generally seen as programs in execution.
439 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 instant runs may be useful. Communication between receiving and transport modules and the outside world are done using the specific protocol they do handle.
441 Left is only communication between the receiver modules and \name{queue-in}, and between \name{queue-out} and the transport modules. Data is exchanged done using \unix\ pipes and a simple protocol is used.
443 \begin{figure}
444 \begin{center}
445 \codeinput{input/ipc-protocol.txt}
446 %\includegraphics[scale=0.75]{img/ipc-protocol.eps}
447 \end{center}
448 \caption{State diagram of the protocol used for \NAME{IPC}}
449 \label{fig:ipc-protocol}
450 \end{figure}
452 % timing
453 One dialog consists of the four phases: connection attempt, acceptance reply, data transfer, success reply. The order is always the same. The connection attempt and data transfer are sent by the client process; replies are sent by the server process.
454 %fixme: split between header and data
456 % semantics
457 The connection attempt is simply opening the connection. This starts the dialog. A positive reply by the server leads to the data transfer, but a negative reply refuses the connection and resets both client and server to the state before the connection attempt. If the connection attempt was accepted, the client sends the data ending with a terminator sequence. When this terminator appears, the server process knows the complete data was transfered. The server process takes responsibility of the data in sending a positive success reply. A negative success reply resets both client and server to the state before the connection attempt.
459 The data transfered needs to be of specific format. Used is the same format in which messages are spooled in the mail queues. See the following section for details. %fixme: check if it is the following section
460 %fixme: split between header and data
462 % syntax
463 Data transfer is done sending plain text data. %fixme: utf8 ?
464 The terminator sequence used to indicate the end of the data transfer is a single dot on a line on its own. Line separators are the combination of \name{Carriage Return} and \name{Line Feed}, as it is used in various Internet protocols like \SMTP. Replys are one-digit numbers with \texttt{0} meaning success and any other number (\texttt{1}--\texttt{9}) indicate failure. %fixme: What are the octal values?
465 %fixme: split between header and data
467 Figure \ref{fig:ipc-protocol} is a state diagram for the protocol.
471 \subsubsection*{Spool file format}
473 The spool file format is basically the same as the one in current \masqmail: one file for the message body, the other for envelope and header information. The data file is stored in a separate data pool. It is written by \name{queue-in}, \name{scanning} can read it if necessary, \name{queue-out} reads it to generate the outgoing message, and deletes it after successful transfer. The header file (including the envelope) is written into the \name{incoming} queue. The \name{scanning} modules reads it, processes it, and writes a modified copy into the \name{outgoing} queue; the file in \name{incoming} is deleted then. \name{queue-out} finally takes the header file from \name{outgoing} to generate the resulting message. This data flow is shown in figure \ref{fig:queue-data-flow}.
475 \begin{figure}
476 \begin{center}
477 %\input{img/queue-data-flow.eps}
478 \end{center}
479 \caption{Data flow of messages in the queue}
480 \label{fig:queue-data-flow}
481 \end{figure}
483 The queue consists of three directories within the queue path. Two, named \name{incoming} and \name{outgoing}, for storing the header files; one, called \name{pool}, to store the message bodies. The files being part of one message share the same unique name. The header files internal structure can be the same as the one of current \masqmail.
485 Messages in queues are a header file in \name{incoming} or \name{outgoing} and a data file in \name{pool}. The header file owner's executable bit indicates if the 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 with the bit set.
487 No spool files are modified after they are written to disk. Modifications to header files can be made by the \name{scanning} module in the ``move'' from \name{incoming} to \name{outgoing}---it is a create and remove, actually. Further rewriting can happen in \name{queue-out}, as well without altering the file.
489 Data files do not change at all within the system. They are written in default local plain text format. Required translation is done in the receiver and transport modules.
490 %fixme: why plain text and not db? -> simplicity
492 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).
495 A sample header file. With comments in paranthesis.
497 \begin{quote}\footnotesize
498 \begin{verbatim}
499 1LGtYh-0ut-00 (backup copy of the file name)
500 MF:<meillo@dream> (envelope: sender)
501 RT: <user@example.org> (envelope: recipient)
502 PR:local (meta info: protocol)
503 ID:meillo (meta info: id/user/ip)
504 DS: 18 (meta info: size)
505 TR: 1230462707 (meta info: timestamp)
506 (following: headers)
507 HD:Received: from meillo by dream with local (masqmail 0.2.21) id
508 1LGtYh-0ut-00 for <user@example.org>; Sun, 28 Dec 2008 12:11:47 +0100
509 HD:To: user@example.org
510 HD:Subject: test mail
511 HD:From: <meillo@dream>
512 HD:Date: Sun, 28 Dec 2008 12:11:47 +0100
513 HD:Message-ID: <1LGtYh-0ut-00@dream>
514 \end{verbatim}
515 \end{quote}
520 \subsubsection*{Rights and permission}
522 The user set required for \qmail\ seems to be too complex. One special user, like \postfix\ uses, is more appropriate. \name{root} privilege and \name{setuid} permission is avoided as much as possible.
524 Table \ref{tab:new-masqmail-permissions} shows the suggested ownership and permissions of the modules.
526 \begin{table}
527 \begin{center}
528 \input{tbl/new-masqmail-permissions.tbl}
529 \end{center}
530 \caption{Ownership and permissions of the modules}
531 \label{tab:new-masqmail-permission}
532 \end{table}
534 These are the permissions and ownership used for the queue:
535 \codeinput{input/new-masqmail-queue.txt}
541 setuid/setgid or not?
543 what can crash if an attacker succeeds?
545 where to drop privelege?
547 how is which process invoked?
549 master process? needed, or wanted?
551 which are the daemon processes?
559 http://fanf.livejournal.com/50917.html %how not to design an mta - the sendmail command
560 http://fanf.livejournal.com/51349.html %how not to design an mta - partitioning for security
561 http://fanf.livejournal.com/61132.html %how not to design an mta - local delivery
562 http://fanf.livejournal.com/64941.html %how not to design an mta - spool file format
563 http://fanf.livejournal.com/65203.html %how not to design an mta - spool file logistics
564 http://fanf.livejournal.com/65911.html %how not to design an mta - more about log-structured MTA queues
565 http://fanf.livejournal.com/67297.html %how not to design an mta - more log-structured MTA queues
566 http://fanf.livejournal.com/70432.html %how not to design an mta - address verification
567 http://fanf.livejournal.com/72258.html %how not to design an mta - content scanning