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view thesis/tex/4-MasqmailsFuture.tex @ 164:0e208e23aac3
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| author | meillo@marmaro.de |
|---|---|
| date | Thu, 18 Dec 2008 13:47:27 +0100 |
| parents | 18b7b517e2dd |
| children | a7fd6d974d3c |
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1 \chapter{\masqmail's present and future}
3 \section{Existing code base}
4 Here regarded is version 0.2.21 of \masqmail. This is the last version released by Oliver \person{Kurth}, and the basis for my thesis.
7 \subsubsection*{Features}
9 \masqmail\ accepts mail on the command line and via \SMTP. Mail queueing and alias expansion is supported. \masqmail\ is able to deliver mail to local mailboxes (in \name{mbox} or \name{maildir} format) or pass it to a \name{mail delivery agent} (like \name{procmail}). Mail destinated to remote locations is sent using \SMTP\ or can be piped to commands, being gatesways to \NAME{UUCP} or \NAME{FAX} for example.
11 Outgoing \SMTP\ connections feature \SMTP-\NAME{AUTH} and \SMTP-after-\NAME{POP} authentication, but incoming connections do not. Using wrappers for outgoing connections is supported. This offers a two way communication through a wrapper application like \name{openssl}.
12 %todo: what about SSL/TLS encryption?
14 \masqmail\ focuses on non-permanent online connections, thus a concept of online routes is used. One may configure any amount of routes to send mail. Each route can have criterias, like matching \texttt{From:} or \texttt{To:} headers, to determine if mail is allowed to be sent using it. Mail to destinations outside the local net gets queued until \masqmail\ is informed about the existance of a online connection.
16 The \masqmail\ executable can be called under various names for sendmail-compatibility reasons. This is organized by symbolic links with different names pointing to the \masqmail\ executable. The \sendmail\ names are \path{/usr/lib/sendmail} and \path{/usr/sbin/sendmail} because many programs expect the \mta\ to be located there. Further more \sendmail\ supports calling it with a different name instead of supplying command line arguments. The best known of this shortcuts is \path{mailq}, which is equivilent to calling it with the argument \verb+-bq+. \masqmail\ recognizes the names \path{mailq}, \path{smtpd}, \path{mailrm}, \path{runq}, \path{rmail}, and \path{in.smtpd}. The first two are inspired by \sendmail. Not implemented is the name \path{newaliases} because \masqmail\ does not generate binary representations of the alias file.\footnote{A shell script located named \path{newaliases}, that invokes \texttt{masqmail -bi}, can provide the command to satisfy other software needing it.} \path{hoststat} and \path{purgestat} are missing for sendmail-compatibility.
17 %masqmail: mailq, mailrm, runq, rmail, smtpd/in.smtpd
18 %sendmail: hoststat, mailq, newaliases, purgestat, smtpd
20 Additional to the \mta\ job, \masqmail\ also offers mail retrieval services with being a \NAME{POP3} client. It can fetch mail from different remote locations, dependent on the active online route.
24 \subsubsection*{The code}
26 \masqmail\ is written in the C programming language. The program, as of version 0.2.21, consists of 34 source code and eight header files, containing about 9,000 lines of code\footnote{Measured with \name{sloccount} by David A.\ Wheeler.}. Additionally, it includes a \name{base64} implementation (about 300 lines) and \name{md5} code (about 150 lines). For systems that do not provide \name{libident}, this library is distributed as well (circa 600 lines); an available shared library however has higher precedence in linking.
28 The only mandatory dependency is \name{glib}---a cross-platform software utility library, originated in the \NAME{GTK+} project. It provides safer replacements for many standard library functions. It also offers handy data containers, easy-to-use implementations of data structures, and much more.
31 With \masqmail\ comes the small tool \path{mservdetect}; it helps setting up a configuration that uses the \name{mserver} system to detect the online state. Two other binaries get compiled for testing purposes: \path{readtest} and \path{smtpsend}. All three programms use \masqmail\ source code; they only add a file with a \verb+main()+ function each.
34 \masqmail\ does not provide an interface to plug in modules with additional functionality. There exists no add-on or module system. The code is only separated by function to the various source files. Some functional parts can be included or excluded by defining symbols. Adding maildir support at compile time, means giving the option \verb+--enable-maildir+ to the \path{configure} call. This preserves the concerning code to get removed by the preprocessor. Unfortunately the \verb+#ifdef+s are scattered through all the source, leading to a FIXME(holperig) code base.
41 \section{Requirements}
43 Following is a list of current and future requirements to make \masqmail\ ready for the future.
46 \subsubsection*{Large message handling}
47 Trends in the market for electronic communication go towards consolidated communication, hence email will be used more to transfer voice and video messages. This leads to larger messages. The store-and-forward transport of email is not good suited for large data. Thus new protocols, like \NAME{QMTP} (described in section %\ref{FIXME}
48 ), may become popular.
51 \subsubsection*{Ressource friendly software}
52 The merge of communication hardware and the move of email services from providers to homes, demands smaller and more resource-friendly software. The amount of mail will be lower, even if much more mail will be sent. More important will be the energy consumption and heat emission. These topics increased in relevance during the past years and they are expected to become more central. \masqmail\ is not a program to be used on large servers, but to be used on small devices. Thus focusing on energy and heat, not on performance, is the direction to go.
55 \subsubsection*{New mail transfer protocols}
56 Large messages demand more efficient transport through the net. As well is a final solution needed to defeat the spam problem. New mail transport protocols may be the only good solutions for both problems. They also can improve reliability, authentication, and verification issues. \masqmail\ should be able to support new protocols as they appear and are used.
59 \subsubsection*{Spam handling}
60 Spam is a major threat. According to the \NAME{SWOT} analysis, the goal is to reduce it to a bearable level. Spam fighting is a war are where 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.
63 \subsubsection*{Security}
64 \MTA{}s are critical points for computer security, as they are accessable from external networks. They must be secured with high effort. Properties like high priviledge level, work load influenced from extern, work on unsafe data, and demand for reliability, increase the security needed. Unsecure and unreliable \mta{}s are of no value. \masqmail\ needs to b e secure enough for its target field of operation.
67 \subsubsection*{Easy configuration}
68 Having \mta{}s on many home servers and clients, requires easy and standardized configuration. The common setups should be configurable with single actions by the user. Complex configuration should be possible, but focused must be the most common form of configuration: choosing one of several standard setups.
75 \section{Discussion on architecture}
77 A program's architecture is probably the most influencing design decision, and has the greatest impact on the program's future capabilities. %fixme: search quote ... check if good
79 \masqmail's current artitecture is monolitic like \sendmail's and \exim's. But more than the other two, is it one block of interweaved code. \sendmail\ provides now, with its \name{milter} interface, standardized connection channels to external modules. \exim\ has a highly structured code with many internal interfaces, like the one for supported authentication ``modules''. \masqmail\ has none of them; it is what \sendmail\ was in the beginning: a single large block.
81 Figure \ref{fig:masqmail-arch} is an attempt to depict \masqmail's internal structure.
83 \begin{figure}
84 \begin{center}
85 \input{input/masqmail-arch.tex}
86 \end{center}
87 \caption{Internal architecture of \masqmail}
88 \label{fig:masqmail-arch}
89 \end{figure}
91 \sendmail\ improved its old architecture, for example by adding the milter interface. \exim\ was designed and is carefully maintained with a modular-like code structure in mind. \qmail\ started from scratch with a ``security-first'' approach, \postfix\ improved on it, and \name{sendmail X}/\name{MeTA1} tries to adopt the best of \qmail\ and \postfix, to completely replace the old \sendmail\ architecture. \person{Hafiz} \cite{hafiz05}. describes this evolution of \mta\ architecture very well.
93 Every one of the popular \MTA{}s is more modular, or became more modular over time, than \masqmail\ is. Modern requirements like spam protection and future requirements like the use of new mail transport protocols demand modular designs for keeping the software simple. Simplicity is a key property for security.
95 \person{Hafiz} agrees:
96 \begin{quote}
97 The goal of making software secure can be better achieved by making the design simple and easier to understand and verify. \cite[page64]{hafiz05}
98 \end{quote}
99 He identifies the security of \qmail\ to come from it's \name{compartmentalization}, which goes hand in hand with modularity:
100 \begin{quote}
101 A perfect example is the contrast between the feature envy early \sendmail\ architecture implemented as one process and the simple, modular architecture of \qmail. The security of \qmail\ comes from its compartmentalized simple processes that perform one task only and are therefor testable for security. \cite[page 64]{hafiz05}
102 \end{quote}
104 Modularity is needed for supporting modern \MTA\ requirements, providing a clear interface to add further functionality without increasing the overall complexity much. Modularity is also an enabler for security. Security comes from good design, as \person{Graff} and \person{van Wyk} explain:
105 \begin{quote}
106 Good design is the sword and shield of the security-conscious developer. Sound design defends your application from subversion or misuse, protecting your network and the information on it from internal and external attacks alike. It also provides a safe foundation for future extensions and maintainance of the software.
107 %
108 %Bad design makes life easier for attackers and harder for the good guys, especially if it contributes to a false sends of security while obscuring pertinent failings.
109 \cite[page 55]{graff03}
110 \end{quote}
112 \person{Hafiz} adds: ``The major idea is that security cannot be retrofitted into an architecture.''\cite[page 64]{hafiz05}
114 All this leads to one logical step: The rewrite of \masqmail\ using a modern, modular architecture, to get a modern \MTA\ satisfying nowadays needs.
119 \subsection{Modules needed}
121 This section tries to identify the needed modules for a modern \MTA. They are later the pieces of which the new architecture is built of.
124 \subsubsection*{The simplest MTA}
125 This view of the problem is taken from \person{Hafiz} \cite[pages 3-5]{hafiz05}.
127 The basic job of a \mta\ is to tranport mail from a sender to a recipient. The simplest \MTA\ therefor needs at least a mail receiving facility and a mail sending facility. This basic \MTA---following the definition of an \MTA---is much to abstract. Hence a next step to add some important features is needed, the result is an operational \MTA.
131 \subsubsection*{Mail queue}
133 \person{Hafif} adds a mail queue to make it possible to not deliver at once.
135 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.
138 \subsubsection*{Incoming channels}
140 The second addition \person{Hafiz} made is the split of incoming and outgoing channels into local and remote. The question is, if this is nessesary. It is the way, it was done for a long time, but is this extra complexity needed?
142 The common situation is incoming mail on port 25 using \SMTP\ and via the \texttt{sendmail} command. Outgoing mail is either sent using \SMTP, piped into local commands (for example \texttt{uucp}), or delivered locally by appending to a mailbox.
144 The \MTA's architecture would be simpler if some of these channels could be merged. The reason is, if various modules do similar jobs, common things might need to be duplicated. On the other side is it better to have more independent modules if each one is simpler then.
146 \qmail\ uses \name{qmail-inject} (local message in) and \name{qmail-smtpd} (remote message in), which both handle messages over to \name{qmail-queue} that puts it into the mail queue. \postfix's approach is similar. \name{sendmail X} %fixme: what about meta1 here?
147 used only \NAME{SMTPS}, which is for receiving mail from remote, to communicate with the queue manager \NAME{QMGR}. Mail from local goes over \NAME{SMTPS}.
149 The \name{sendmail X} approach seems to be the simpler one, but does heavily rely on \SMTP\ being the main mail transfer protocol. To \qmail\ and \postfix\ new modules may be added to support other ways of message receival, without any change of other parts of the system.
152 \subsubsection*{Outgoing channels}
154 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.
156 As mail delivery to local users, is \emph{not} included in the basic job of \MTA{}s, why should they care about it? In order to keep the system simple and to have programs do one job well, the local delivery job should be handed over to \NAME{MDA}s. \name{Mail delivery agents} are the tools that are specialized for local delivery. They 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 for it over to a mail delivery agent, like \name{procmail} or \name{maildrop}, seems to be the right way to go.
158 This means outgoing connections, piping mails into local commands needs to be implemented.
161 \subsubsection*{Mail queue (again)}
166 \subsubsection*{Authentication}
168 easiest: restricting by static IP addresses (Access control via hosts.allow/hosts.deny)
169 if dynamic remote hosts need access: some auth is needed
170 - SASL
171 - POP/IMAP: pop-before-smtp, DRAC, WHOSON
172 - TLS (certificates)
174 ``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 thyour 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.'' (Dent: Postfix, page 44, ch04)
177 \subsubsection*{Encryption}
180 \subsubsection*{Spam prevention}
183 where to filter what
186 postfix: after-queue-content-filter (smtp communication)
187 exim: content-scan-feature
188 sendmail: milter (tcp or unix sockets)
190 checks while smtp dialog (pre-queue): in MTA implemented (need to be fast)
191 checks when mail is accepted and queued: external (amavis, spamassassin)
194 AMaViS (amavisd-new): email filter framework to integrate spam and virus scanner
195 internet -->25 MTA -->10024 amavis -->10025 MTA --> reciptient
196 | |
197 +----------------------------+
198 mail scanner:
199 incoming queue --> mail scanner --> outgoing queue
201 mimedefang: uses milter interface with sendmail
204 \subsubsection*{Virus checking}
206 The same for malicious content (\name{malware}) like viruses, worms, trojan horses. They are related to spam, but affect the \MTA less, as they are in the mail body.
208 message body <-> envelope, header
211 anti-virus: clamav
217 \subsubsection*{Archiving}
223 \section{A new architecture}
226 (ssl)
227 -> msg-in (local or remote protocol handlers)
228 -> spam-filter (and more)
229 -> queue
230 -> msg-out (local-delivery by MDA, or remote-protocol-handlers)
231 (ssl)
237 http://fanf.livejournal.com/50917.html %how not to design an mta - the sendmail command
238 http://fanf.livejournal.com/51349.html %how not to design an mta - partitioning for security
239 http://fanf.livejournal.com/61132.html %how not to design an mta - local delivery
240 http://fanf.livejournal.com/64941.html %how not to design an mta - spool file format
241 http://fanf.livejournal.com/65203.html %how not to design an mta - spool file logistics
242 http://fanf.livejournal.com/65911.html %how not to design an mta - more about log-structured MTA queues
243 http://fanf.livejournal.com/67297.html %how not to design an mta - more log-structured MTA queues
244 http://fanf.livejournal.com/70432.html %how not to design an mta - address verification
245 http://fanf.livejournal.com/72258.html %how not to design an mta - content scanning
266 \section{Directions to go}
268 This section discusses about what shapes \masqmail\ could have---which directions the development could go to.
274 \subsubsection*{\masqmail\ in five years}
276 Now how could \masqmail\ be like in, say, five years?
278 ---
280 A design from scratch?
281 << what would be needed (effort) >>
282 But how is the effort of this complete rewrite compared to what is gained afterwards?
284 << would one create it at all? >>
286 ---
288 << plans to get masqmail more popular again (if that is the goal) >>
290 << More users >>
298 \section{Work to do}
300 << short term goals --- long term goals >>
302 do it like sendmail: first do the most needed stuff on the old design to make it still usable. Then design a new version from scratch, for the future.
304 << which parts to take out and do within the thesis >>