docs/diploma

annotate thesis/tex/6-NewDesign.tex @ 216:0fb60ac9610f

new book
author meillo@marmaro.de
date Sun, 04 Jan 2009 22:36:24 +0100
parents b08be036783d
children 6d21f55323b8
rev   line source
meillo@197 1 \chapter{A design from scratch}
meillo@197 2
meillo@197 3 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@197 4
meillo@211 5 ``Many times in life, getting off to the right start makes all the difference.'' \cite[page~32]{graff03}
meillo@197 6
meillo@197 7
meillo@197 8
meillo@197 9 \section{Design decisions}
meillo@197 10
meillo@197 11 One major design idea of the design were:
meillo@197 12 \begin{itemize}
meillo@197 13 \item free the internal system from in and out channels
meillo@197 14 \item arbitrary protocol handlers have to be addable afterwards
meillo@197 15 \item a single facility for scanning (all mail goes through it)
meillo@197 16 \item concentrate on mail transfer
meillo@197 17 \end{itemize}
meillo@197 18
meillo@197 19
meillo@197 20 \subsubsection*{Incoming channels}
meillo@197 21
meillo@197 22 \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@197 23
meillo@197 24 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@197 25 \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@197 26
meillo@197 27 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@197 28
meillo@197 29 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@197 30
meillo@197 31
meillo@197 32 \subsubsection*{Outgoing channels}
meillo@197 33
meillo@197 34 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@197 35
meillo@197 36 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@197 37
meillo@197 38 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@197 39
meillo@197 40 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@197 41
meillo@197 42
meillo@197 43
meillo@197 44 \subsubsection*{Mail queue}
meillo@197 45
meillo@197 46 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@197 47
meillo@197 48 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@197 49
meillo@197 50 \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@197 51
meillo@197 52 \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@197 53
meillo@197 54 \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}.
meillo@197 55 %fixme: check, cite, and think about
meillo@197 56
meillo@197 57
meillo@197 58
meillo@197 59 \subsubsection*{Sanitize mail}
meillo@197 60
meillo@197 61 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@197 62
meillo@197 63 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@197 64
meillo@197 65 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@197 66
meillo@197 67
meillo@197 68
meillo@197 69 \subsubsection*{Aliasing}
meillo@197 70
meillo@197 71 Where should aliases get expanded? They appear in different kind. Important are the ones available in the \path{aliases} file. Aliases can be:
meillo@197 72 \begin{itemize}
meillo@197 73 \item a different local user (e.g.\ ``\texttt{bob: alice}'')
meillo@197 74 \item a remote user (e.g.\ ``\texttt{bob: john@example.com}'')
meillo@197 75 \item a list of users (e.g.\ ``\texttt{bob: alice, john@example.com}'')
meillo@197 76 \item a command (e.g.\ ``\texttt{bob: |foo}'')
meillo@197 77 \end{itemize}
meillo@197 78 Addresses expanding to lists of users lead to more envelopes. Aliases changing the reciptients domain part may require a different route to use.
meillo@197 79
meillo@197 80 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@197 81
meillo@197 82
meillo@197 83
meillo@197 84 \subsubsection*{Choose route to use}
meillo@197 85
meillo@197 86 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@197 87
meillo@197 88 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@197 89
meillo@197 90 \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@197 91 %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@197 92
meillo@197 93
meillo@197 94
meillo@197 95 \subsubsection*{Authentication}
meillo@197 96
meillo@197 97 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@197 98
meillo@197 99 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@197 100
meillo@197 101 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@197 102 \begin{enumerate}
meillo@197 103 \item \SMTP-after-\NAME{POP}: uses authenication on the \NAME{POP} protocol to permit incoming \SMTP\ connections for a limited time afterwards.
meillo@197 104 \item \SMTP authentication: is an extension to \SMTP. Authentication can be requested before mail is accepted.
meillo@197 105 \item Certificates: confirm the identity of someone.
meillo@197 106 \end{enumerate}
meillo@197 107
meillo@197 108
meillo@197 109
meillo@197 110 \subsubsection*{Encryption}
meillo@197 111
meillo@197 112 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@197 113
meillo@197 114 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@197 115
meillo@197 116 \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@197 117
meillo@197 118 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@197 119
meillo@197 120 For incoming connections, \NAME{STARTTLS}---defined in \RFC2487---is what \mta{}s implement.
meillo@197 121
meillo@197 122 \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@197 123
meillo@197 124
meillo@197 125
meillo@197 126
meillo@197 127
meillo@197 128 \subsubsection*{Spam prevention}
meillo@197 129
meillo@197 130 ---
meillo@197 131 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@197 132 ---
meillo@197 133
meillo@197 134 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@197 135
meillo@197 136 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@197 137
meillo@197 138 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@197 139
meillo@197 140 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@197 141
meillo@197 142 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@197 143
meillo@197 144 \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@197 145
meillo@197 146
meillo@197 147
meillo@197 148
meillo@197 149 \subsubsection*{Virus checking}
meillo@197 150
meillo@197 151 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@197 152
meillo@197 153 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@197 154
meillo@197 155 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@197 156
meillo@197 157 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@197 158
meillo@197 159
meillo@197 160 %AMaViS (amavisd-new): email filter framework to integrate spam and virus scanner
meillo@197 161 %\begin{verbatim}
meillo@197 162 %internet -->25 MTA -->10024 amavis -->10025 MTA --> reciptient
meillo@197 163 %| |
meillo@197 164 %+----------------------------+
meillo@197 165 %\end{verbatim}
meillo@197 166 %
meillo@197 167 %postfix and exim can habe both mta servises in the same instance, sendmail needs two instances running.
meillo@197 168 %
meillo@197 169 %MailScanner:
meillo@197 170 %incoming queue --> MailScanner --> outgoing queue
meillo@197 171 %
meillo@197 172 %postfix: with one instance possible, exim and sendmail need two instances running
meillo@197 173
meillo@197 174
meillo@197 175 %message body <-> envelope, header
meillo@197 176 %
meillo@197 177 %anti-virus: clamav
meillo@197 178 %postfix: via amavis
meillo@197 179 %exim: via content-scanning-feature called from acl
meillo@197 180 %sendmail: with milter
meillo@197 181 %procmail
meillo@197 182 %
meillo@197 183 %virus scanner work on file level
meillo@197 184 %amavis receives mail via smtp or pipe, splits it in its parts (MIME) and extracks archives, the come the virus scanners
meillo@197 185 %if the mail is okay, it goes via smtp to a second mta
meillo@197 186
meillo@197 187 %what amavis recognizes:
meillo@197 188 %- invalid headers
meillo@197 189 %- banned files
meillo@197 190 %- viruses
meillo@197 191 %- spam (using spam assassin)
meillo@197 192 %
meillo@197 193 %mimedefang: uses milter interface with sendmail
meillo@197 194
meillo@197 195
meillo@197 196
meillo@197 197 \subsubsection*{Archiving}
meillo@197 198
meillo@197 199 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@197 200
meillo@197 201 \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@197 202
meillo@197 203
meillo@197 204
meillo@197 205
meillo@197 206
meillo@197 207 \section{The resulting architecture}
meillo@197 208
meillo@197 209 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@197 210
meillo@197 211 \begin{figure}
meillo@197 212 \begin{center}
meillo@211 213 \includegraphics[width=\textwidth]{img/masqmail-arch-new.eps}
meillo@197 214 \end{center}
meillo@197 215 \caption{A new designed architecture for \masqmail}
meillo@197 216 \label{fig:masqmail-arch-new}
meillo@197 217 \end{figure}
meillo@197 218
meillo@197 219 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@197 220
meillo@197 221 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@197 222 %fixme: do i need all this ``quesses''??
meillo@197 223
meillo@197 224
meillo@197 225 \subsection{Modules and queues}
meillo@197 226
meillo@197 227 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@197 228
meillo@197 229
meillo@197 230 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@197 231 %fixme: should be no daemon
meillo@197 232
meillo@197 233
meillo@197 234 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@197 235
meillo@197 236
meillo@197 237 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@197 238
meillo@197 239
meillo@197 240 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@197 241
meillo@197 242
meillo@197 243 The \name{outgoing} queue contains processed messages. The header and envelope information is complete and in valid form.
meillo@197 244
meillo@197 245 \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@197 246
meillo@197 247 \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@197 248 Thus a \name{mail delivery agent} (like \name{procmail}) is to be used with the \name{pipe} module.
meillo@197 249
meillo@197 250
meillo@197 251
meillo@197 252 \subsection{Inter-module communication}
meillo@197 253
meillo@197 254 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.
meillo@197 255
meillo@197 256 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.
meillo@197 257
meillo@197 258 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.
meillo@197 259
meillo@197 260 \begin{figure}
meillo@197 261 \begin{center}
meillo@197 262 \input{input/ipc-protocol.tex}
meillo@197 263 \end{center}
meillo@197 264 \caption{State diagram of the protocol used for \NAME{IPC}}
meillo@197 265 \label{fig:ipc-protocol}
meillo@197 266 \end{figure}
meillo@197 267
meillo@197 268 % timing
meillo@197 269 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.
meillo@197 270 %fixme: split between header and data
meillo@197 271
meillo@197 272 % semantics
meillo@197 273 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.
meillo@197 274
meillo@197 275 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
meillo@197 276 %fixme: split between header and data
meillo@197 277
meillo@197 278 % syntax
meillo@197 279 Data transfer is done sending plain text data. %fixme: utf8 ?
meillo@197 280 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?
meillo@197 281 %fixme: split between header and data
meillo@197 282
meillo@197 283 Figure \ref{fig:ipc-protocol} is a state diagram for the protocol.
meillo@197 284
meillo@197 285
meillo@197 286
meillo@197 287 \subsection{Spool file format}
meillo@197 288
meillo@197 289 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}.
meillo@197 290
meillo@197 291 \begin{figure}
meillo@197 292 \begin{center}
meillo@197 293 \input{input/queue-data-flow.tex}
meillo@197 294 \end{center}
meillo@197 295 \caption{Data flow of messages in the queue}
meillo@197 296 \label{fig:queue-data-flow}
meillo@197 297 \end{figure}
meillo@197 298
meillo@197 299 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.
meillo@197 300
meillo@197 301 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.
meillo@197 302
meillo@197 303 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.
meillo@197 304
meillo@197 305 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.
meillo@197 306
meillo@197 307
meillo@197 308 \begin{tabular}[hbt]{ l l }
meillo@197 309
meillo@197 310 \mbox{ queue-in:} & \mbox{
meillo@197 311 \begin{tabular}[hbt]{| c | c | c |}
meillo@197 312 \hline
meillo@197 313 incoming & outgoing & pool \\
meillo@197 314 \hline
meillo@197 315 \hline
meillo@197 316 - & - & - \\
meillo@197 317 \hline
meillo@197 318 0600 & - & - \\
meillo@197 319 \hline
meillo@197 320 0600 & - & 0600 \\
meillo@197 321 \hline
meillo@197 322 0700 & - & 0600 \\
meillo@197 323 \hline
meillo@197 324 \end{tabular}
meillo@197 325 } \\
meillo@197 326
meillo@197 327 \quad & \\
meillo@197 328
meillo@197 329 \mbox{scanning:} & \mbox{
meillo@197 330 \begin{tabular}[hbt]{| c | c | c |}
meillo@197 331 \hline
meillo@197 332 incoming & outgoing & pool \\
meillo@197 333 \hline
meillo@197 334 \hline
meillo@197 335 0700 & - & 0600 \\
meillo@197 336 \hline
meillo@197 337 0700 & 0600 & 0600 \\
meillo@197 338 \hline
meillo@197 339 0700 & 0700 & 0600 \\
meillo@197 340 \hline
meillo@197 341 - & 0700 & 0600 \\
meillo@197 342 \hline
meillo@197 343 \end{tabular}
meillo@197 344 } \\
meillo@197 345
meillo@197 346 \quad & \\
meillo@197 347
meillo@197 348 \mbox{queue-out:} & \mbox{
meillo@197 349 \begin{tabular}[hbt]{| c | c | c |}
meillo@197 350 \hline
meillo@197 351 incoming & outgoing & pool \\
meillo@197 352 \hline
meillo@197 353 \hline
meillo@197 354 - & 0700 & 0600 \\
meillo@197 355 \hline
meillo@197 356 - & 0700 & - \\
meillo@197 357 \hline
meillo@197 358 - & - & - \\
meillo@197 359 \hline
meillo@197 360 \end{tabular}
meillo@197 361 } \\
meillo@197 362
meillo@197 363 \end{tabular}
meillo@197 364
meillo@197 365 A sample header file.
meillo@197 366 \begin{verbatim}
meillo@197 367 1LGtYh-0ut-00 (backup copy of the file name)
meillo@197 368 MF:<meillo@dream> (envelope: sender)
meillo@197 369 RT: <user@example.org> (envelope: recipient)
meillo@197 370 PR:local (meta info: protocol)
meillo@197 371 ID:meillo (meta info: id/user/ip)
meillo@197 372 DS: 18 (meta info: size)
meillo@197 373 TR: 1230462707 (meta info: timestamp)
meillo@197 374 (following: headers)
meillo@197 375 HD:Received: from meillo by dream with local (masqmail 0.2.21) id
meillo@197 376 1LGtYh-0ut-00 for <user@example.org>; Sun, 28 Dec 2008 12:11:47 +0100
meillo@197 377 HD:To: user@example.org
meillo@197 378 HD:Subject: test mail
meillo@197 379 HD:From: <meillo@dream>
meillo@197 380 HD:Date: Sun, 28 Dec 2008 12:11:47 +0100
meillo@197 381 HD:Message-ID: <1LGtYh-0ut-00@dream>
meillo@197 382 \end{verbatim}
meillo@197 383
meillo@197 384
meillo@197 385
meillo@197 386
meillo@197 387 \subsection{Rights and permission}
meillo@197 388
meillo@197 389 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.
meillo@197 390
meillo@197 391 Table \ref{tab:new-masqmail-permissions} shows the suggested ownership and permissions of the modules. Figure \ref{fig:new-masqmail-queue} shows the permissions and ownership used for the queue.
meillo@197 392
meillo@197 393 \begin{table}
meillo@197 394 \begin{center}
meillo@197 395 \input{input/new-masqmail-permissions.tex}
meillo@197 396 \end{center}
meillo@197 397 \caption{Ownership and permissions of the modules}
meillo@197 398 \label{tab:new-masqmail-permission}
meillo@197 399 \end{table}
meillo@197 400
meillo@197 401 \begin{figure}
meillo@197 402 \begin{center}
meillo@197 403 \input{input/new-masqmail-queue.tex}
meillo@197 404 \end{center}
meillo@197 405 \caption{Ownership and permissions of the queue}
meillo@197 406 \label{fig:new-masqmail-queue}
meillo@197 407 \end{figure}
meillo@197 408
meillo@197 409
meillo@197 410
meillo@197 411
meillo@197 412
meillo@197 413 setuid/setgid or not?
meillo@197 414
meillo@197 415 what can crash if an attacker succeeds?
meillo@197 416
meillo@197 417 where to drop privelege?
meillo@197 418
meillo@197 419 how is which process invoked?
meillo@197 420
meillo@197 421 master process? needed, or wanted?
meillo@197 422
meillo@197 423 which are the daemon processes?
meillo@197 424
meillo@197 425
meillo@197 426
meillo@197 427
meillo@197 428
meillo@197 429
meillo@197 430
meillo@197 431 http://fanf.livejournal.com/50917.html %how not to design an mta - the sendmail command
meillo@197 432 http://fanf.livejournal.com/51349.html %how not to design an mta - partitioning for security
meillo@197 433 http://fanf.livejournal.com/61132.html %how not to design an mta - local delivery
meillo@197 434 http://fanf.livejournal.com/64941.html %how not to design an mta - spool file format
meillo@197 435 http://fanf.livejournal.com/65203.html %how not to design an mta - spool file logistics
meillo@197 436 http://fanf.livejournal.com/65911.html %how not to design an mta - more about log-structured MTA queues
meillo@197 437 http://fanf.livejournal.com/67297.html %how not to design an mta - more log-structured MTA queues
meillo@197 438 http://fanf.livejournal.com/70432.html %how not to design an mta - address verification
meillo@197 439 http://fanf.livejournal.com/72258.html %how not to design an mta - content scanning
meillo@197 440
meillo@197 441
meillo@197 442