docs/diploma

annotate thesis/tex/4-MasqmailsFuture.tex @ 178:b426a663d5f0

wrote about modules and queues
author meillo@marmaro.de
date Sat, 27 Dec 2008 12:37:27 +0100
parents 7781ad0811f7
children d75bd7a72d8b
rev   line source
meillo@109 1 \chapter{\masqmail's present and future}
meillo@93 2
meillo@137 3 \section{Existing code base}
meillo@142 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.
meillo@142 5
meillo@93 6
meillo@137 7 \subsubsection*{Features}
meillo@93 8
meillo@142 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.
meillo@93 10
meillo@142 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}.
meillo@137 12 %todo: what about SSL/TLS encryption?
meillo@93 13
meillo@142 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.
meillo@142 15
meillo@137 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.
meillo@109 17 %masqmail: mailq, mailrm, runq, rmail, smtpd/in.smtpd
meillo@109 18 %sendmail: hoststat, mailq, newaliases, purgestat, smtpd
meillo@109 19
meillo@137 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.
meillo@109 21
meillo@137 22
meillo@137 23 \subsubsection*{The code}
meillo@137 24
meillo@137 25 \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.
meillo@137 26
meillo@137 27 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.
meillo@137 28
meillo@109 29
meillo@109 30 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.
meillo@109 31
meillo@93 32
meillo@137 33 \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.
meillo@93 34
meillo@132 35
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meillo@132 38
meillo@177 39 \section{Requirements}
meillo@146 40
meillo@177 41 This section identifies the requirements for future version of \masqmail. Most of them will apply to modern \MTA{}s in general.
meillo@177 42
meillo@177 43 \subsection{General requirements}
meillo@146 44
meillo@146 45 Following is a list of current and future requirements to make \masqmail\ ready for the future.
meillo@146 46
meillo@146 47
meillo@177 48 \subsubsection*{Security}
meillo@177 49 \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.
meillo@177 50
meillo@177 51
meillo@177 52 \subsubsection*{Reliability}
meillo@177 53 << crash only software >>
meillo@177 54
meillo@177 55 << dont lose mail >>
meillo@177 56
meillo@177 57
meillo@177 58 \subsubsection*{Extendability}
meillo@177 59 Modern needs like large messages demand for more efficient mail transport through the net. Aswell is a final solution needed to defeat the spam problem. New mail transport protocols seem to be the only good solutions for both problems. They also can improve reliability, authentication, and verification issues. \masqmail\ should be able to support new mail transfer protocols as they appear and are used.
meillo@177 60 %fixme: like old sendmail, but not too much like it
meillo@146 61
meillo@146 62
meillo@146 63 \subsubsection*{Ressource friendly software}
meillo@149 64 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.
meillo@146 65
meillo@146 66
meillo@146 67 \subsubsection*{Easy configuration}
meillo@149 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.
meillo@146 69
meillo@146 70
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meillo@146 73
meillo@146 74
meillo@170 75 \subsection{Discussion on architecture}
meillo@146 76
meillo@163 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
meillo@132 78
meillo@161 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.
meillo@161 80
meillo@161 81 Figure \ref{fig:masqmail-arch} is an attempt to depict \masqmail's internal structure.
meillo@161 82
meillo@161 83 \begin{figure}
meillo@161 84 \begin{center}
meillo@161 85 \input{input/masqmail-arch.tex}
meillo@161 86 \end{center}
meillo@161 87 \caption{Internal architecture of \masqmail}
meillo@161 88 \label{fig:masqmail-arch}
meillo@161 89 \end{figure}
meillo@161 90
meillo@163 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.
meillo@161 92
meillo@163 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.
meillo@161 94
meillo@163 95 \person{Hafiz} agrees:
meillo@163 96 \begin{quote}
meillo@163 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}
meillo@163 98 \end{quote}
meillo@163 99 He identifies the security of \qmail\ to come from it's \name{compartmentalization}, which goes hand in hand with modularity:
meillo@163 100 \begin{quote}
meillo@163 101 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}
meillo@163 102 \end{quote}
meillo@165 103 As well does \person{Dent}: ``The modular architecture of Postfix forms the basis for much of its security.''\cite[page 7]{dent04}
meillo@161 104
meillo@163 105 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:
meillo@163 106 \begin{quote}
meillo@163 107 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.
meillo@163 108 %
meillo@163 109 %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.
meillo@163 110 \cite[page 55]{graff03}
meillo@163 111 \end{quote}
meillo@161 112
meillo@163 113 \person{Hafiz} adds: ``The major idea is that security cannot be retrofitted into an architecture.''\cite[page 64]{hafiz05}
meillo@161 114
meillo@163 115 All this leads to one logical step: The rewrite of \masqmail\ using a modern, modular architecture, to get a modern \MTA\ satisfying nowadays needs.
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meillo@177 120
meillo@177 121
meillo@177 122 http://fanf.livejournal.com/50917.html %how not to design an mta - the sendmail command
meillo@177 123 http://fanf.livejournal.com/51349.html %how not to design an mta - partitioning for security
meillo@177 124 http://fanf.livejournal.com/61132.html %how not to design an mta - local delivery
meillo@177 125 http://fanf.livejournal.com/64941.html %how not to design an mta - spool file format
meillo@177 126 http://fanf.livejournal.com/65203.html %how not to design an mta - spool file logistics
meillo@177 127 http://fanf.livejournal.com/65911.html %how not to design an mta - more about log-structured MTA queues
meillo@177 128 http://fanf.livejournal.com/67297.html %how not to design an mta - more log-structured MTA queues
meillo@177 129 http://fanf.livejournal.com/70432.html %how not to design an mta - address verification
meillo@177 130 http://fanf.livejournal.com/72258.html %how not to design an mta - content scanning
meillo@177 131
meillo@177 132
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meillo@177 134
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meillo@177 137
meillo@170 138 \subsection{Jobs of an MTA}
meillo@161 139
meillo@163 140 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.
meillo@163 141
meillo@170 142 The basic job of a \mta\ is to tranport mail from a sender to a recipient. This is the definition of such a program and this is how \person{Dent}\cite[page 19]{dent04} and \person{Hafiz} \cite[pages 3-5]{hafiz05} generally see its design.
meillo@163 143
meillo@170 144 An \MTA\ therefor needs at least a mail receiving facility and a mail sending facility. Additionally probably all \MTA\ developers (excluded the only forwarders), see the need for a mail queue. A mail queue removes the need to deliver at once a message is received. They also provide fail-safe storage of mails until they are delivered.
meillo@163 145
meillo@163 146
meillo@163 147
meillo@163 148 \subsubsection*{Incoming channels}
meillo@163 149
meillo@170 150 \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.
meillo@163 151
meillo@170 152 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?
meillo@170 153 \person{fanf} 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.
meillo@163 154
meillo@170 155 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.
meillo@163 156
meillo@170 157 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.
meillo@163 158
meillo@163 159
meillo@163 160 \subsubsection*{Outgoing channels}
meillo@163 161
meillo@170 162 Outgoing mail is commonly either sent using \SMTP, piped into local commands (for example \texttt{uucp}), or delivered locally by appending to a mailbox.
meillo@170 163
meillo@163 164 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.
meillo@163 165
meillo@170 166 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.
meillo@163 167
meillo@170 168 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.
meillo@170 169
meillo@170 170
meillo@170 171
meillo@170 172 \subsubsection*{Mail queue}
meillo@170 173
meillo@170 174 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.
meillo@170 175
meillo@170 176 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.
meillo@170 177
meillo@170 178 \sendmail, \exim, \qmail, \name{sendmail X}, and \masqmail\ feature one single mail queue. \postfix\ has three of them: \name{incoming}, \name{active}, and \name{deferred}. (The \name{maildrop} queue is excluded, as it is only used for the \texttt{sendmail} command.)
meillo@170 179
meillo@170 180 \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}.
meillo@163 181
meillo@163 182
meillo@165 183
meillo@165 184 \subsubsection*{Sanitize mail}
meillo@165 185
meillo@170 186 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.
meillo@165 187
meillo@170 188 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.
meillo@170 189
meillo@170 190 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.
meillo@170 191
meillo@170 192
meillo@170 193
meillo@170 194 \subsubsection*{Choose route to use}
meillo@170 195
meillo@170 196 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.
meillo@170 197
meillo@170 198 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).
meillo@170 199
meillo@170 200 \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.
meillo@178 201 %fixme: see hafiz05 page 57: maybe put the rewriting into the sending module (like smx, exim, courier) (problem with archiving of all outgoing mail?)
meillo@165 202
meillo@165 203
meillo@165 204
meillo@165 205 \subsubsection*{Aliasing}
meillo@165 206
meillo@170 207 Where should aliases get expanded? They appear in different kind. Important are the ones available in the \path{aliases} file. Aliases can be:
meillo@170 208 \begin{itemize}
meillo@170 209 \item a different local user (e.g.\ ``\texttt{bob: alice}'')
meillo@170 210 \item a remote user (e.g.\ ``\texttt{bob: john@example.com}'')
meillo@170 211 \item a list of users (e.g.\ ``\texttt{bob: alice, john@example.com}'')
meillo@170 212 \item a command (e.g.\ ``\texttt{bob: |foo}'')
meillo@170 213 \end{itemize}
meillo@170 214 Addresses expanding to lists of users lead to more envelopes. Aliases changing the reciptients domain part may require a different route to use.
meillo@165 215
meillo@172 216 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.
meillo@163 217
meillo@163 218
meillo@163 219
meillo@163 220 \subsubsection*{Authentication}
meillo@163 221
meillo@170 222 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.
meillo@163 223
meillo@170 224 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}.
meillo@170 225
meillo@170 226 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:
meillo@170 227 \begin{enumerate}
meillo@170 228 \item \SMTP-after-\NAME{POP}: uses authenication on the \NAME{POP} protocol to permit incoming \SMTP\ connections for a limited time afterwards.
meillo@170 229 \item \SMTP authentication: is an extension to \SMTP. Authentication can be requested before mail is accepted.
meillo@170 230 \item Certificates: confirm the identity of someone.
meillo@170 231 \end{enumerate}
meillo@170 232 The first mechanism requires a \NAME{POP} (or \NAME{IMAP}) server running on the same host (or a trusted one), to enable the \SMTP\ server to use the login dates on the \NAME{POP} server. This is a common practice used by mail service providers, but is not adequate for the environments \masqmail\ is designed for.
meillo@170 233
meillo@170 234 Certificate based authentication, like provided by \NAME{TLS}, suffers from the overhead of certificate management. But \NAME{TLS} provides encryption too, so is useful anyway.
meillo@170 235
meillo@170 236 \SMTP\ authentication (also refered to as \NAME{SMTP-AUTH}) suppoert is easiest received by using a \name{Simple Authentication and Security Layer} implementation. \person{Dent} sees in \NAME{SASL} the best solution for authenticating dynamic users:
meillo@165 237 \begin{quote}
meillo@170 238 %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.
meillo@170 239 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.
meillo@165 240 \cite[page 44]{dent04}
meillo@165 241 \end{quote}
meillo@163 242
meillo@170 243 %either by
meillo@170 244 %- network/ip address
meillo@170 245 % easiest: restricting by static IP addresses (Access control via hosts.allow/hosts.deny)
meillo@170 246 %or
meillo@170 247 %- some kind of auth (for dynamic remote hosts)
meillo@170 248 % adds complexity
meillo@170 249 % - SASL
meillo@170 250 % - POP/IMAP: pop-before-smtp, DRAC, WHOSON
meillo@170 251 % - TLS (certificates)
meillo@170 252
meillo@170 253
meillo@163 254
meillo@163 255 \subsubsection*{Encryption}
meillo@163 256
meillo@172 257 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.
meillo@165 258
meillo@172 259 The common way to encrypt \SMTP\ dialogs is using \name{Transport Layer Security} (short: \TLS, successor of \NAME{SSL}). \TLS\ encrypts the datagrams of the \name{transport layer}. This means it works below the application protocols and can be used by any of them\citeweb{wikipedia:tls}.
meillo@165 260
meillo@172 261 \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.
meillo@165 262
meillo@172 263 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.
meillo@165 264
meillo@172 265 \begin{figure}
meillo@172 266 \begin{center}
meillo@172 267 \input{input/stunnel.tex}
meillo@172 268 \end{center}
meillo@172 269 \caption{Data flow using \name{stunnel}}
meillo@172 270 \label{fig:stunnel}
meillo@172 271 \end{figure}
meillo@165 272
meillo@172 273 For incoming connections, \NAME{STARTTLS}---defined in \RFC2487---is what \mta{}s implement.
meillo@165 274
meillo@172 275 \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.
meillo@165 276
meillo@172 277 %TLS/SSL prevents attackers to listen on the cable
meillo@172 278 %but it does not prevent man-in-the-middle attacks
meillo@172 279 %signed certificates help here
meillo@172 280 % or PGP encryption
meillo@165 281
meillo@165 282
meillo@172 283 %do not use stunnel wit SMTP:
meillo@172 284 %because all incoming mail would be from 127.0.0.1 !!
meillo@172 285 %use STARTTLS instead
meillo@165 286
meillo@172 287 %postfix: main.cf
meillo@172 288 %\begin{verbatim}
meillo@172 289 % smtpd_use_tls = yes
meillo@172 290 % smtpd_tls_received_header = no (does not log in received headers)
meillo@172 291 %
meillo@172 292 % smtpd_tls_key_file = /etc/postfix/key.pem
meillo@172 293 % smtpd_tls_cert_file = /etc/postfix/cert.pem
meillo@172 294 % smtpd_tls_CA_file = /etc/postfix/CAcert.pem
meillo@172 295 %
meillo@172 296 % smtp_use_tls = yes (use TLS for sending)
meillo@172 297 % smtp_tls_key_file = /etc/postfix/key.pem
meillo@172 298 % smtp_tls_cert_file = /etc/postfix/cert.pem
meillo@172 299 % smtp_tls_CA_file = /etc/postfix/CAcert.pem
meillo@172 300 %\end{verbatim}
meillo@165 301
meillo@165 302
meillo@165 303
meillo@163 304
meillo@163 305 \subsubsection*{Spam prevention}
meillo@163 306
meillo@177 307 ---
meillo@177 308 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 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.
meillo@177 309 ---
meillo@177 310
meillo@173 311 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.
meillo@163 312
meillo@173 313 As spam is not just a nuisance for end users, but also for the infrastructure---the \mta{}s---by increasing the amount of mail messages, \MTA{}s need to protect themself. Two approaches are used.
meillo@163 314
meillo@177 315 First 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 takes and no further system load is added. See \RFC2505 (especially section 1.5) for detail.
meillo@163 316
meillo@173 317 Second 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.
meillo@165 318
meillo@173 319 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 während der SMTP-Phase kommt sowieso kein MTA aus, und es ist eine Frage der Einschätzung, wie weit man diese Phase belasten möchte.''\cite[page 25]{eisentraut05} (translated: ``No \MTA\ can go without analysis during the \SMTP\ dialog, anyway, and it is a question of estimation how much to stress this period.'')
meillo@165 320
meillo@173 321 \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.
meillo@165 322
meillo@163 323
meillo@163 324
meillo@163 325
meillo@163 326 \subsubsection*{Virus checking}
meillo@163 327
meillo@173 328 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.
meillo@163 329
meillo@177 330 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.
meillo@163 331
meillo@177 332 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.
meillo@163 333
meillo@177 334 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.
meillo@165 335
meillo@165 336
meillo@177 337 %AMaViS (amavisd-new): email filter framework to integrate spam and virus scanner
meillo@177 338 %\begin{verbatim}
meillo@177 339 %internet -->25 MTA -->10024 amavis -->10025 MTA --> reciptient
meillo@177 340 %| |
meillo@177 341 %+----------------------------+
meillo@177 342 %\end{verbatim}
meillo@177 343 %
meillo@177 344 %postfix and exim can habe both mta servises in the same instance, sendmail needs two instances running.
meillo@177 345 %
meillo@177 346 %MailScanner:
meillo@177 347 %incoming queue --> MailScanner --> outgoing queue
meillo@177 348 %
meillo@177 349 %postfix: with one instance possible, exim and sendmail need two instances running
meillo@165 350
meillo@163 351
meillo@173 352 %message body <-> envelope, header
meillo@173 353 %
meillo@173 354 %anti-virus: clamav
meillo@173 355 %postfix: via amavis
meillo@173 356 %exim: via content-scanning-feature called from acl
meillo@173 357 %sendmail: with milter
meillo@173 358 %procmail
meillo@173 359 %
meillo@173 360 %virus scanner work on file level
meillo@173 361 %amavis receives mail via smtp or pipe, splits it in its parts (MIME) and extracks archives, the come the virus scanners
meillo@173 362 %if the mail is okay, it goes via smtp to a second mta
meillo@173 363
meillo@173 364 %what amavis recognizes:
meillo@173 365 %- invalid headers
meillo@173 366 %- banned files
meillo@173 367 %- viruses
meillo@173 368 %- spam (using spam assassin)
meillo@173 369 %
meillo@173 370 %mimedefang: uses milter interface with sendmail
meillo@173 371
meillo@163 372
meillo@163 373
meillo@163 374 \subsubsection*{Archiving}
meillo@163 375
meillo@177 376 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.
meillo@163 377
meillo@177 378 \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.
meillo@163 379
meillo@163 380
meillo@163 381
meillo@177 382 \section{Merging the parts}
meillo@161 383
meillo@177 384 The last sections identified the jobs that need to be done by a modern \MTA; problems and prefered choices were mentioned too. Now the various jobs are assigned to modules, of which an architecture is created. It is inpired by existing ones and driven by the identified jobs and requirements.
meillo@161 385
meillo@177 386 One major design idea of the design were:
meillo@177 387 \begin{itemize}
meillo@177 388 \item free the internal system from in and out channels
meillo@177 389 \item arbitrary protocol handlers have to be addable afterwards
meillo@177 390 \item a single facility for scanning (all mail goes through it)
meillo@177 391 \item concentrate on mail transfer
meillo@177 392 \end{itemize}
meillo@161 393
meillo@178 394 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.
meillo@161 395
meillo@177 396 \begin{figure}
meillo@177 397 \begin{center}
meillo@177 398 \input{input/masqmail-arch-new.tex}
meillo@177 399 \end{center}
meillo@177 400 \caption{A new designed architecture for \masqmail}
meillo@177 401 \label{fig:masqmail-arch-new}
meillo@177 402 \end{figure}
meillo@161 403
meillo@178 404 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.
meillo@178 405
meillo@178 406 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.
meillo@178 407 %fixme: do i need all this ``quesses''??
meillo@161 408
meillo@161 409
meillo@177 410 \subsection{Modules and queues}
meillo@161 411
meillo@178 412 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.
meillo@161 413
meillo@161 414
meillo@178 415 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.
meillo@178 416
meillo@178 417
meillo@178 418 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.
meillo@178 419
meillo@178 420
meillo@178 421 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.
meillo@178 422
meillo@178 423
meillo@178 424 The \name{incoming} queue stores messages received via one of the incoming channels. The messages are in unprocessed form; only envelope data is prepended.
meillo@178 425
meillo@178 426
meillo@178 427 The \name{outgoing} queue contains processed messages. The header and envelope information is complete and in valid form.
meillo@178 428
meillo@178 429 \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.
meillo@178 430
meillo@178 431 \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
meillo@178 432 Thus a \name{mail delivery agent} (like \name{procmail}) is to be used with the \name{pipe} module.
meillo@132 433
meillo@132 434
meillo@137 435
meillo@177 436 \subsection{Intermodule communication}
meillo@137 437
meillo@149 438
meillo@149 439
meillo@177 440 \subsection{Spool file format}
meillo@177 441
meillo@177 442
meillo@177 443 \subsection{Rights and permission}
meillo@177 444
meillo@149 445
meillo@149 446
meillo@149 447
meillo@149 448
meillo@149 449
meillo@149 450
meillo@93 451
meillo@93 452
meillo@99 453
meillo@93 454
meillo@93 455
meillo@161 456 \section{Directions to go}
meillo@161 457
meillo@161 458 This section discusses about what shapes \masqmail\ could have---which directions the development could go to.
meillo@161 459
meillo@93 460
meillo@146 461 \subsubsection*{\masqmail\ in five years}
meillo@146 462
meillo@146 463 Now how could \masqmail\ be like in, say, five years?
meillo@146 464
meillo@163 465 ---
meillo@163 466
meillo@163 467 A design from scratch?
meillo@163 468 << what would be needed (effort) >>
meillo@163 469 But how is the effort of this complete rewrite compared to what is gained afterwards?
meillo@163 470
meillo@163 471 << would one create it at all? >>
meillo@163 472
meillo@163 473 ---
meillo@163 474
meillo@146 475
meillo@146 476
meillo@177 477 \subsubsection*{Work to do}
meillo@93 478
meillo@146 479 << short term goals --- long term goals >>
meillo@146 480
meillo@163 481 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.
meillo@163 482
meillo@140 483 << which parts to take out and do within the thesis >>
meillo@93 484
meillo@167 485