docs/diploma

annotate thesis/tex/5-Improvements.tex @ 296:a014ce012053

further work on discussion in ch04
author meillo@marmaro.de
date Sun, 18 Jan 2009 11:54:04 +0100
parents 6cf649e62d42
children 0d88bf21e152
rev   line source
meillo@246 1 \chapter{Improvement plans}
meillo@89 2
meillo@246 3 << what to implement how in future >>
meillo@109 4
meillo@249 5 short time goals on current code: first part.
meillo@249 6
meillo@249 7 long time goal, a new architecture: second part.
meillo@89 8
meillo@184 9
meillo@184 10
meillo@184 11
meillo@287 12 \section{Based on current code}
meillo@125 13
meillo@249 14 The first three \TODO{}s are implementable by improving the current code or by adding wrappers or interposition filters. The following sections describe solution approaches to do that work.
meillo@184 15
meillo@184 16
meillo@184 17
meillo@246 18 \subsection{Encryption}
meillo@246 19
meillo@246 20 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@246 21
meillo@246 22 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@246 23
meillo@246 24 \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@246 25
meillo@246 26 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@246 27
meillo@246 28 For incoming connections, \NAME{STARTTLS}---defined in \RFC2487---is what \mta{}s implement.
meillo@246 29
meillo@246 30 \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@246 31
meillo@246 32 %TLS/SSL prevents attackers to listen on the cable
meillo@246 33 %but it does not prevent man-in-the-middle attacks
meillo@246 34 %signed certificates help here
meillo@246 35 % or PGP encryption
meillo@246 36
meillo@246 37
meillo@246 38 %do not use stunnel wit SMTP:
meillo@246 39 %because all incoming mail would be from 127.0.0.1 !!
meillo@246 40 %use STARTTLS instead
meillo@246 41
meillo@246 42 %postfix: main.cf
meillo@246 43 %\begin{verbatim}
meillo@246 44 % smtpd_use_tls = yes
meillo@246 45 % smtpd_tls_received_header = no (does not log in received headers)
meillo@246 46 %
meillo@246 47 % smtpd_tls_key_file = /etc/postfix/key.pem
meillo@246 48 % smtpd_tls_cert_file = /etc/postfix/cert.pem
meillo@246 49 % smtpd_tls_CA_file = /etc/postfix/CAcert.pem
meillo@246 50 %
meillo@246 51 % smtp_use_tls = yes (use TLS for sending)
meillo@246 52 % smtp_tls_key_file = /etc/postfix/key.pem
meillo@246 53 % smtp_tls_cert_file = /etc/postfix/cert.pem
meillo@246 54 % smtp_tls_CA_file = /etc/postfix/CAcert.pem
meillo@246 55 %\end{verbatim}
meillo@246 56
meillo@246 57
meillo@246 58
meillo@184 59
meillo@249 60 \subsection{Authentication}
meillo@125 61
meillo@184 62 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@184 63
meillo@184 64 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@277 65
meillo@184 66 \begin{enumerate}
meillo@184 67 \item \SMTP-after-\NAME{POP}: uses authenication on the \NAME{POP} protocol to permit incoming \SMTP\ connections for a limited time afterwards.
meillo@287 68 \item \SMTP\ authentication: is an extension to \SMTP. Authentication can be requested before mail is accepted.
meillo@184 69 \item Certificates: confirm the identity of someone.
meillo@184 70 \end{enumerate}
meillo@277 71
meillo@184 72 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@184 73
meillo@184 74 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@184 75
meillo@184 76 \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@184 77 \begin{quote}
meillo@184 78 %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@184 79 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@218 80 \hfill\cite[page 44]{dent04}
meillo@184 81 \end{quote}
meillo@184 82
meillo@277 83
meillo@277 84 Compare static with dynamic authentication: pros and cons; usecases: when to use what; how could this be covered by architecture (e.g. smtp submission).
meillo@184 85 %either by
meillo@184 86 %- network/ip address
meillo@184 87 % easiest: restricting by static IP addresses (Access control via hosts.allow/hosts.deny)
meillo@184 88 %or
meillo@184 89 %- some kind of auth (for dynamic remote hosts)
meillo@184 90 % adds complexity
meillo@184 91 % - SASL
meillo@184 92 % - POP/IMAP: pop-before-smtp, DRAC, WHOSON
meillo@184 93 % - TLS (certificates)
meillo@184 94
meillo@184 95
meillo@184 96
meillo@246 97 \subsection{Security}
meillo@184 98
meillo@246 99 by using wrappers and interposition filters
meillo@184 100
meillo@246 101 split masqmail into two instances
meillo@184 102
meillo@277 103 \begin{verbatim}
meillo@277 104 +--------+ ext ||||| int +--------+
meillo@277 105 ---> |stripped|---> inter --->|normal |
meillo@277 106 |masqmail| pos |masqmail|
meillo@277 107 +--------+ ||||| +--------+
meillo@277 108 \end{verbatim}
meillo@184 109
meillo@246 110
meillo@288 111 \subsection{Reliability}
meillo@288 112
meillo@288 113 discuss persistence through using databases
meillo@288 114
meillo@288 115
meillo@246 116
meillo@277 117 \subsection{Spam and malware handling}
meillo@277 118
meillo@277 119 discuss the MTA->scanner->MTA approach
meillo@277 120
meillo@277 121
meillo@246 122
meillo@246 123 \subsection{Bug fixes}
meillo@246 124
meillo@246 125 already fixed bugs
meillo@246 126
meillo@246 127
meillo@246 128
meillo@246 129
meillo@246 130
meillo@246 131
meillo@246 132
meillo@246 133
meillo@246 134
meillo@246 135
meillo@246 136
meillo@246 137
meillo@246 138
meillo@246 139
meillo@285 140 \section{A new design}
meillo@246 141
meillo@249 142 The last chapter identified the requirements for a modern and securt \masqmail. Now the various jobs of an \MTA\ get assigned to modules, of which the new architecture is created. It is inspired by existing \MTA{}s and driven by the identified requirements.
meillo@246 143
meillo@249 144 One wise experience was kept in mind during the design: ``Many times in life, getting off to the right start makes all the difference.'' \cite[page~32]{graff03}.
meillo@246 145
meillo@246 146
meillo@246 147
meillo@246 148 \subsection{Design decisions}
meillo@246 149
meillo@246 150 One major design idea of the design were:
meillo@246 151 \begin{itemize}
meillo@246 152 \item free the internal system from in and out channels
meillo@246 153 \item arbitrary protocol handlers have to be addable afterwards
meillo@246 154 \item a single facility for scanning (all mail goes through it)
meillo@246 155 \item concentrate on mail transfer
meillo@246 156 \end{itemize}
meillo@246 157
meillo@246 158
meillo@246 159 \subsubsection*{Incoming channels}
meillo@246 160
meillo@246 161 \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@246 162
meillo@246 163 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@248 164 \person{Finch} proposes a similar approach. He wants the \texttt{sendmail} command to be a simple \SMTP\ client that contacts the \SMTP\ daemon of the \MTA\ like it is done by connections from remote. The advantage here is one single module where all \SMTP\ dialog with submitters is done. Hence one single point to accept or refuse incoming mail. Additionally does the module to put mail into the queue not need to be \name{setuid} or \name{setgid} because it is only invoked from the \SMTP\ daemon. The \MTA's architecture would become simpler and common tasks are not duplicated in modules that do similar jobs.
meillo@246 165
meillo@246 166 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@246 167
meillo@246 168 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@246 169
meillo@246 170
meillo@246 171 \subsubsection*{Outgoing channels}
meillo@246 172
meillo@246 173 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@246 174
meillo@246 175 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@246 176
meillo@246 177 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@246 178
meillo@246 179 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@246 180
meillo@246 181
meillo@246 182
meillo@246 183 \subsubsection*{Mail queue}
meillo@246 184
meillo@246 185 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@246 186
meillo@246 187 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@246 188
meillo@246 189 \sendmail, \exim, \qmail, \name{sendmail X}, and \masqmail\ feature one single mail queue. \postfix\ has more of them.
meillo@246 190
meillo@246 191 \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@246 192
meillo@246 193 \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@246 194 %fixme: check, cite, and think about
meillo@246 195
meillo@277 196 %fixme: discuss: filesystem vs. database
meillo@277 197
meillo@277 198 %fixme: what about the ``rule of repair''?
meillo@246 199
meillo@246 200
meillo@246 201 \subsubsection*{Sanitize mail}
meillo@246 202
meillo@246 203 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@246 204
meillo@246 205 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@246 206
meillo@246 207 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@246 208
meillo@246 209
meillo@246 210
meillo@246 211 \subsubsection*{Aliasing}
meillo@246 212
meillo@246 213 Where should aliases get expanded? They appear in different kind. Important are the ones available in the \path{aliases} file. Aliases can be:
meillo@285 214 \begin{enumerate}
meillo@246 215 \item a different local user (e.g.\ ``\texttt{bob: alice}'')
meillo@246 216 \item a remote user (e.g.\ ``\texttt{bob: john@example.com}'')
meillo@246 217 \item a list of users (e.g.\ ``\texttt{bob: alice, john@example.com}'')
meillo@246 218 \item a command (e.g.\ ``\texttt{bob: |foo}'')
meillo@285 219 \end{enumerate}
meillo@287 220 Addresses expanding to lists of users lead to more envelopes. Aliases changing the reciptients domain part may make the message unsuitable for a specific online route.
meillo@246 221
meillo@246 222 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@246 223
meillo@246 224
meillo@246 225
meillo@287 226 \subsubsection*{Route management}
meillo@246 227
meillo@287 228 %fixme: rework!!
meillo@246 229 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@246 230
meillo@246 231 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@246 232
meillo@246 233 \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@246 234 %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@246 235
meillo@246 236
meillo@246 237
meillo@246 238 \subsubsection*{Authentication}
meillo@246 239
meillo@246 240 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@246 241
meillo@246 242 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@246 243
meillo@246 244 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@246 245 \begin{enumerate}
meillo@246 246 \item \SMTP-after-\NAME{POP}: uses authenication on the \NAME{POP} protocol to permit incoming \SMTP\ connections for a limited time afterwards.
meillo@246 247 \item \SMTP authentication: is an extension to \SMTP. Authentication can be requested before mail is accepted.
meillo@246 248 \item Certificates: confirm the identity of someone.
meillo@246 249 \end{enumerate}
meillo@246 250
meillo@246 251
meillo@246 252
meillo@246 253 \subsubsection*{Encryption}
meillo@129 254
meillo@184 255 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@184 256
meillo@184 257 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@184 258
meillo@184 259 \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@184 260
meillo@184 261 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@184 262
meillo@184 263 For incoming connections, \NAME{STARTTLS}---defined in \RFC2487---is what \mta{}s implement.
meillo@184 264
meillo@184 265 \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@184 266
meillo@184 267
meillo@184 268
meillo@184 269
meillo@246 270
meillo@246 271 \subsubsection*{Spam prevention}
meillo@246 272
meillo@246 273 ---
meillo@287 274
meillo@287 275 Spam is a major threat nowadays and the goal is to reduce it to a bearable level (see section \ref{sec:swot-analysis}). Spam fighting is a war in which the good guys tend to lose. Putting too much effort there will result in few gain. Real success will only be possible with new---better---protocols and abandonning the weak legacy technologies. Hence \masqmail\ should be able to provide state-of-the-art spam protection, but not more.
meillo@287 276
meillo@246 277 ---
meillo@246 278
meillo@246 279 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@246 280
meillo@287 281 As spam is not just a nuisance for end users but also for the infrastructure---the \mta{}s---by increasing the amount of mail messages. Thus \MTA{}s need to protect themself. Two different approaches are used:
meillo@246 282
meillo@287 283 \begin{enumerate}
meillo@287 284 \item Refusing spam during the \SMTP\ dialog. This is the way it was meant by the designers of the \SMTP\ protocol. They thought checking the sender and reciptient mail addresses would be enough, but as they are forgable it is not. More and more complex checks need to be done. Checking needs time, but \SMTP\ dialogs time out if it takes too long. Thus only limited time can be used, during the \SMTP\ dialog, for checking if a message seems to be spam. The advantage is that acceptance of bad messages can be simply refused---no responsibility for the message is taken and no further system load is added. See \RFC2505 (especially section 1.5) for detail.
meillo@246 285
meillo@287 286 \item
meillo@287 287 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@287 288 \end{enumerate}
meillo@246 289
meillo@246 290 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@246 291
meillo@246 292 \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@246 293
meillo@246 294
meillo@246 295
meillo@246 296
meillo@246 297 \subsubsection*{Virus checking}
meillo@246 298
meillo@246 299 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@246 300
meillo@246 301 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@246 302
meillo@246 303 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@246 304
meillo@246 305 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@246 306
meillo@246 307
meillo@246 308 %AMaViS (amavisd-new): email filter framework to integrate spam and virus scanner
meillo@184 309 %\begin{verbatim}
meillo@246 310 %internet -->25 MTA -->10024 amavis -->10025 MTA --> reciptient
meillo@246 311 %| |
meillo@246 312 %+----------------------------+
meillo@246 313 %\end{verbatim}
meillo@184 314 %
meillo@246 315 %postfix and exim can habe both mta servises in the same instance, sendmail needs two instances running.
meillo@184 316 %
meillo@246 317 %MailScanner:
meillo@246 318 %incoming queue --> MailScanner --> outgoing queue
meillo@246 319 %
meillo@246 320 %postfix: with one instance possible, exim and sendmail need two instances running
meillo@184 321
meillo@184 322
meillo@246 323 %message body <-> envelope, header
meillo@246 324 %
meillo@246 325 %anti-virus: clamav
meillo@246 326 %postfix: via amavis
meillo@246 327 %exim: via content-scanning-feature called from acl
meillo@246 328 %sendmail: with milter
meillo@246 329 %procmail
meillo@246 330 %
meillo@246 331 %virus scanner work on file level
meillo@246 332 %amavis receives mail via smtp or pipe, splits it in its parts (MIME) and extracks archives, the come the virus scanners
meillo@246 333 %if the mail is okay, it goes via smtp to a second mta
meillo@184 334
meillo@246 335 %what amavis recognizes:
meillo@246 336 %- invalid headers
meillo@246 337 %- banned files
meillo@246 338 %- viruses
meillo@246 339 %- spam (using spam assassin)
meillo@246 340 %
meillo@246 341 %mimedefang: uses milter interface with sendmail
meillo@184 342
meillo@184 343
meillo@89 344
meillo@246 345 \subsubsection*{Archiving}
meillo@89 346
meillo@246 347 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@194 348
meillo@246 349 \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@194 350
meillo@194 351
meillo@194 352
meillo@194 353
meillo@89 354
meillo@89 355
meillo@175 356
meillo@246 357
meillo@246 358 \subsection{The resulting architecture}
meillo@246 359
meillo@246 360 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@246 361
meillo@246 362 \begin{figure}
meillo@246 363 \begin{center}
meillo@246 364 \includegraphics[width=\textwidth]{img/masqmail-arch-new.eps}
meillo@246 365 \end{center}
meillo@246 366 \caption{A new designed architecture for \masqmail}
meillo@246 367 \label{fig:masqmail-arch-new}
meillo@246 368 \end{figure}
meillo@246 369
meillo@246 370 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@246 371
meillo@246 372 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@246 373 %fixme: do i need all this ``quesses''??
meillo@246 374
meillo@246 375
meillo@246 376 \subsubsection*{Modules and queues}
meillo@246 377
meillo@246 378 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@246 379
meillo@246 380
meillo@246 381 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@246 382 %fixme: should be no daemon
meillo@246 383
meillo@246 384
meillo@246 385 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@246 386
meillo@246 387
meillo@246 388 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@287 389 %fixme: rework route selection
meillo@246 390
meillo@246 391
meillo@246 392 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@246 393
meillo@246 394
meillo@246 395 The \name{outgoing} queue contains processed messages. The header and envelope information is complete and in valid form.
meillo@246 396
meillo@246 397 \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@282 398 %todo: get invoked by inetd, or better ucspi-tcp (by bernstein) which can limit max number of concurrent connections. and includes tcp-wrappers functionality.
meillo@282 399
meillo@246 400
meillo@246 401 \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@246 402 Thus a \name{mail delivery agent} (like \name{procmail}) is to be used with the \name{pipe} module.
meillo@246 403
meillo@246 404
meillo@246 405
meillo@246 406 \subsubsection*{Inter-module communication}
meillo@246 407
meillo@246 408 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@246 409
meillo@246 410 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@246 411
meillo@246 412 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@246 413
meillo@246 414 \begin{figure}
meillo@246 415 \begin{center}
meillo@273 416 \codeinput{input/ipc-protocol.txt}
meillo@273 417 %\includegraphics[scale=0.75]{img/ipc-protocol.eps}
meillo@246 418 \end{center}
meillo@246 419 \caption{State diagram of the protocol used for \NAME{IPC}}
meillo@246 420 \label{fig:ipc-protocol}
meillo@246 421 \end{figure}
meillo@246 422
meillo@246 423 % timing
meillo@246 424 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@246 425 %fixme: split between header and data
meillo@246 426
meillo@246 427 % semantics
meillo@246 428 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@246 429
meillo@246 430 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@246 431 %fixme: split between header and data
meillo@246 432
meillo@246 433 % syntax
meillo@246 434 Data transfer is done sending plain text data. %fixme: utf8 ?
meillo@246 435 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@246 436 %fixme: split between header and data
meillo@246 437
meillo@246 438 Figure \ref{fig:ipc-protocol} is a state diagram for the protocol.
meillo@246 439
meillo@246 440
meillo@246 441
meillo@246 442 \subsubsection*{Spool file format}
meillo@246 443
meillo@246 444 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@246 445
meillo@246 446 \begin{figure}
meillo@246 447 \begin{center}
meillo@273 448 %\input{img/queue-data-flow.eps}
meillo@246 449 \end{center}
meillo@246 450 \caption{Data flow of messages in the queue}
meillo@246 451 \label{fig:queue-data-flow}
meillo@246 452 \end{figure}
meillo@246 453
meillo@246 454 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@246 455
meillo@246 456 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@246 457
meillo@246 458 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@246 459
meillo@246 460 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@249 461 %fixme: why plain text and not db? -> simplicity
meillo@246 462
meillo@249 463 Mark spooled mail messages when processing of the writing module is finished: Either by setting the executable bit (like \postfix\ does), or by changing the owner (an approach for multiple masqmail users).
meillo@246 464
meillo@246 465
meillo@249 466 A sample header file. With comments in paranthesis.
meillo@246 467
meillo@261 468 \begin{quote}\footnotesize
meillo@246 469 \begin{verbatim}
meillo@246 470 1LGtYh-0ut-00 (backup copy of the file name)
meillo@246 471 MF:<meillo@dream> (envelope: sender)
meillo@246 472 RT: <user@example.org> (envelope: recipient)
meillo@246 473 PR:local (meta info: protocol)
meillo@246 474 ID:meillo (meta info: id/user/ip)
meillo@246 475 DS: 18 (meta info: size)
meillo@246 476 TR: 1230462707 (meta info: timestamp)
meillo@246 477 (following: headers)
meillo@246 478 HD:Received: from meillo by dream with local (masqmail 0.2.21) id
meillo@246 479 1LGtYh-0ut-00 for <user@example.org>; Sun, 28 Dec 2008 12:11:47 +0100
meillo@246 480 HD:To: user@example.org
meillo@246 481 HD:Subject: test mail
meillo@246 482 HD:From: <meillo@dream>
meillo@246 483 HD:Date: Sun, 28 Dec 2008 12:11:47 +0100
meillo@246 484 HD:Message-ID: <1LGtYh-0ut-00@dream>
meillo@246 485 \end{verbatim}
meillo@261 486 \end{quote}
meillo@246 487
meillo@246 488
meillo@246 489
meillo@246 490
meillo@246 491 \subsubsection*{Rights and permission}
meillo@246 492
meillo@246 493 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@246 494
meillo@273 495 Table \ref{tab:new-masqmail-permissions} shows the suggested ownership and permissions of the modules.
meillo@246 496
meillo@246 497 \begin{table}
meillo@246 498 \begin{center}
meillo@271 499 \input{tbl/new-masqmail-permissions.tbl}
meillo@246 500 \end{center}
meillo@246 501 \caption{Ownership and permissions of the modules}
meillo@246 502 \label{tab:new-masqmail-permission}
meillo@246 503 \end{table}
meillo@246 504
meillo@273 505 These are the permissions and ownership used for the queue:
meillo@273 506 \codeinput{input/new-masqmail-queue.txt}
meillo@246 507
meillo@246 508
meillo@246 509
meillo@246 510
meillo@246 511
meillo@246 512 setuid/setgid or not?
meillo@246 513
meillo@246 514 what can crash if an attacker succeeds?
meillo@246 515
meillo@246 516 where to drop privelege?
meillo@246 517
meillo@246 518 how is which process invoked?
meillo@246 519
meillo@246 520 master process? needed, or wanted?
meillo@246 521
meillo@246 522 which are the daemon processes?
meillo@246 523
meillo@246 524
meillo@246 525
meillo@246 526
meillo@246 527
meillo@246 528
meillo@246 529
meillo@246 530 http://fanf.livejournal.com/50917.html %how not to design an mta - the sendmail command
meillo@246 531 http://fanf.livejournal.com/51349.html %how not to design an mta - partitioning for security
meillo@246 532 http://fanf.livejournal.com/61132.html %how not to design an mta - local delivery
meillo@246 533 http://fanf.livejournal.com/64941.html %how not to design an mta - spool file format
meillo@246 534 http://fanf.livejournal.com/65203.html %how not to design an mta - spool file logistics
meillo@246 535 http://fanf.livejournal.com/65911.html %how not to design an mta - more about log-structured MTA queues
meillo@246 536 http://fanf.livejournal.com/67297.html %how not to design an mta - more log-structured MTA queues
meillo@246 537 http://fanf.livejournal.com/70432.html %how not to design an mta - address verification
meillo@246 538 http://fanf.livejournal.com/72258.html %how not to design an mta - content scanning
meillo@246 539
meillo@246 540
meillo@246 541