docs/diploma

annotate thesis/tex/5-Improvements.tex @ 273:92578f124df6

reorganized the input files and sorted old ones out
author meillo@marmaro.de
date Thu, 15 Jan 2009 12:29:54 +0100
parents c80b6b6fb798
children 8a25b6262497
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@246 12 \section{On base of 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@184 65 \begin{enumerate}
meillo@184 66 \item \SMTP-after-\NAME{POP}: uses authenication on the \NAME{POP} protocol to permit incoming \SMTP\ connections for a limited time afterwards.
meillo@184 67 \item \SMTP authentication: is an extension to \SMTP. Authentication can be requested before mail is accepted.
meillo@184 68 \item Certificates: confirm the identity of someone.
meillo@184 69 \end{enumerate}
meillo@184 70 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 71
meillo@184 72 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 73
meillo@184 74 \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 75 \begin{quote}
meillo@184 76 %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 77 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 78 \hfill\cite[page 44]{dent04}
meillo@184 79 \end{quote}
meillo@184 80
meillo@184 81 %either by
meillo@184 82 %- network/ip address
meillo@184 83 % easiest: restricting by static IP addresses (Access control via hosts.allow/hosts.deny)
meillo@184 84 %or
meillo@184 85 %- some kind of auth (for dynamic remote hosts)
meillo@184 86 % adds complexity
meillo@184 87 % - SASL
meillo@184 88 % - POP/IMAP: pop-before-smtp, DRAC, WHOSON
meillo@184 89 % - TLS (certificates)
meillo@184 90
meillo@184 91
meillo@184 92
meillo@246 93 \subsection{Security}
meillo@184 94
meillo@246 95 by using wrappers and interposition filters
meillo@184 96
meillo@246 97 split masqmail into two instances
meillo@184 98
meillo@184 99
meillo@246 100
meillo@246 101
meillo@246 102
meillo@246 103 \subsection{Bug fixes}
meillo@246 104
meillo@246 105 already fixed bugs
meillo@246 106
meillo@246 107
meillo@246 108
meillo@246 109
meillo@246 110
meillo@246 111
meillo@246 112
meillo@246 113
meillo@246 114
meillo@246 115
meillo@246 116
meillo@246 117
meillo@246 118
meillo@246 119
meillo@246 120 \section{The new design}
meillo@246 121
meillo@249 122 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@246 123
meillo@249 124 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@246 125
meillo@246 126
meillo@246 127
meillo@246 128 \subsection{Design decisions}
meillo@246 129
meillo@246 130 One major design idea of the design were:
meillo@246 131 \begin{itemize}
meillo@246 132 \item free the internal system from in and out channels
meillo@246 133 \item arbitrary protocol handlers have to be addable afterwards
meillo@246 134 \item a single facility for scanning (all mail goes through it)
meillo@246 135 \item concentrate on mail transfer
meillo@246 136 \end{itemize}
meillo@246 137
meillo@246 138
meillo@246 139 \subsubsection*{Incoming channels}
meillo@246 140
meillo@246 141 \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@246 142
meillo@246 143 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@248 144 \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@246 145
meillo@246 146 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@246 147
meillo@246 148 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@246 149
meillo@246 150
meillo@246 151 \subsubsection*{Outgoing channels}
meillo@246 152
meillo@246 153 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@246 154
meillo@246 155 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@246 156
meillo@246 157 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@246 158
meillo@246 159 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@246 160
meillo@246 161
meillo@246 162
meillo@246 163 \subsubsection*{Mail queue}
meillo@246 164
meillo@246 165 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@246 166
meillo@246 167 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@246 168
meillo@246 169 \sendmail, \exim, \qmail, \name{sendmail X}, and \masqmail\ feature one single mail queue. \postfix\ has more of them.
meillo@246 170
meillo@246 171 \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@246 172
meillo@246 173 \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@246 174 %fixme: check, cite, and think about
meillo@246 175
meillo@246 176
meillo@246 177
meillo@246 178 \subsubsection*{Sanitize mail}
meillo@246 179
meillo@246 180 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@246 181
meillo@246 182 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@246 183
meillo@246 184 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@246 185
meillo@246 186
meillo@246 187
meillo@246 188 \subsubsection*{Aliasing}
meillo@246 189
meillo@246 190 Where should aliases get expanded? They appear in different kind. Important are the ones available in the \path{aliases} file. Aliases can be:
meillo@246 191 \begin{itemize}
meillo@246 192 \item a different local user (e.g.\ ``\texttt{bob: alice}'')
meillo@246 193 \item a remote user (e.g.\ ``\texttt{bob: john@example.com}'')
meillo@246 194 \item a list of users (e.g.\ ``\texttt{bob: alice, john@example.com}'')
meillo@246 195 \item a command (e.g.\ ``\texttt{bob: |foo}'')
meillo@246 196 \end{itemize}
meillo@246 197 Addresses expanding to lists of users lead to more envelopes. Aliases changing the reciptients domain part may require a different route to use.
meillo@246 198
meillo@246 199 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@246 200
meillo@246 201
meillo@246 202
meillo@246 203 \subsubsection*{Choose route to use}
meillo@246 204
meillo@246 205 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@246 206
meillo@246 207 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@246 208
meillo@246 209 \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@246 210 %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@246 211
meillo@246 212
meillo@246 213
meillo@246 214 \subsubsection*{Authentication}
meillo@246 215
meillo@246 216 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@246 217
meillo@246 218 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@246 219
meillo@246 220 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@246 221 \begin{enumerate}
meillo@246 222 \item \SMTP-after-\NAME{POP}: uses authenication on the \NAME{POP} protocol to permit incoming \SMTP\ connections for a limited time afterwards.
meillo@246 223 \item \SMTP authentication: is an extension to \SMTP. Authentication can be requested before mail is accepted.
meillo@246 224 \item Certificates: confirm the identity of someone.
meillo@246 225 \end{enumerate}
meillo@246 226
meillo@246 227
meillo@246 228
meillo@246 229 \subsubsection*{Encryption}
meillo@129 230
meillo@184 231 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@184 232
meillo@184 233 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@184 234
meillo@184 235 \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@184 236
meillo@184 237 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@184 238
meillo@184 239 For incoming connections, \NAME{STARTTLS}---defined in \RFC2487---is what \mta{}s implement.
meillo@184 240
meillo@184 241 \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@184 242
meillo@184 243
meillo@184 244
meillo@184 245
meillo@246 246
meillo@246 247 \subsubsection*{Spam prevention}
meillo@246 248
meillo@246 249 ---
meillo@246 250 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@246 251 ---
meillo@246 252
meillo@246 253 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@246 254
meillo@246 255 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@246 256
meillo@246 257 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@246 258
meillo@246 259 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@246 260
meillo@246 261 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@246 262
meillo@246 263 \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@246 264
meillo@246 265
meillo@246 266
meillo@246 267
meillo@246 268 \subsubsection*{Virus checking}
meillo@246 269
meillo@246 270 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@246 271
meillo@246 272 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@246 273
meillo@246 274 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@246 275
meillo@246 276 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@246 277
meillo@246 278
meillo@246 279 %AMaViS (amavisd-new): email filter framework to integrate spam and virus scanner
meillo@184 280 %\begin{verbatim}
meillo@246 281 %internet -->25 MTA -->10024 amavis -->10025 MTA --> reciptient
meillo@246 282 %| |
meillo@246 283 %+----------------------------+
meillo@246 284 %\end{verbatim}
meillo@184 285 %
meillo@246 286 %postfix and exim can habe both mta servises in the same instance, sendmail needs two instances running.
meillo@184 287 %
meillo@246 288 %MailScanner:
meillo@246 289 %incoming queue --> MailScanner --> outgoing queue
meillo@246 290 %
meillo@246 291 %postfix: with one instance possible, exim and sendmail need two instances running
meillo@184 292
meillo@184 293
meillo@246 294 %message body <-> envelope, header
meillo@246 295 %
meillo@246 296 %anti-virus: clamav
meillo@246 297 %postfix: via amavis
meillo@246 298 %exim: via content-scanning-feature called from acl
meillo@246 299 %sendmail: with milter
meillo@246 300 %procmail
meillo@246 301 %
meillo@246 302 %virus scanner work on file level
meillo@246 303 %amavis receives mail via smtp or pipe, splits it in its parts (MIME) and extracks archives, the come the virus scanners
meillo@246 304 %if the mail is okay, it goes via smtp to a second mta
meillo@184 305
meillo@246 306 %what amavis recognizes:
meillo@246 307 %- invalid headers
meillo@246 308 %- banned files
meillo@246 309 %- viruses
meillo@246 310 %- spam (using spam assassin)
meillo@246 311 %
meillo@246 312 %mimedefang: uses milter interface with sendmail
meillo@184 313
meillo@184 314
meillo@89 315
meillo@246 316 \subsubsection*{Archiving}
meillo@89 317
meillo@246 318 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@194 319
meillo@246 320 \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@194 321
meillo@194 322
meillo@194 323
meillo@194 324
meillo@89 325
meillo@89 326
meillo@175 327
meillo@246 328
meillo@246 329 \subsection{The resulting architecture}
meillo@246 330
meillo@246 331 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@246 332
meillo@246 333 \begin{figure}
meillo@246 334 \begin{center}
meillo@246 335 \includegraphics[width=\textwidth]{img/masqmail-arch-new.eps}
meillo@246 336 \end{center}
meillo@246 337 \caption{A new designed architecture for \masqmail}
meillo@246 338 \label{fig:masqmail-arch-new}
meillo@246 339 \end{figure}
meillo@246 340
meillo@246 341 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@246 342
meillo@246 343 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@246 344 %fixme: do i need all this ``quesses''??
meillo@246 345
meillo@246 346
meillo@246 347 \subsubsection*{Modules and queues}
meillo@246 348
meillo@246 349 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@246 350
meillo@246 351
meillo@246 352 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@246 353 %fixme: should be no daemon
meillo@246 354
meillo@246 355
meillo@246 356 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@246 357
meillo@246 358
meillo@246 359 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@246 360
meillo@246 361
meillo@246 362 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@246 363
meillo@246 364
meillo@246 365 The \name{outgoing} queue contains processed messages. The header and envelope information is complete and in valid form.
meillo@246 366
meillo@246 367 \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@246 368
meillo@246 369 \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@246 370 Thus a \name{mail delivery agent} (like \name{procmail}) is to be used with the \name{pipe} module.
meillo@246 371
meillo@246 372
meillo@246 373
meillo@246 374 \subsubsection*{Inter-module communication}
meillo@246 375
meillo@246 376 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@246 377
meillo@246 378 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@246 379
meillo@246 380 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@246 381
meillo@246 382 \begin{figure}
meillo@246 383 \begin{center}
meillo@273 384 \codeinput{input/ipc-protocol.txt}
meillo@273 385 %\includegraphics[scale=0.75]{img/ipc-protocol.eps}
meillo@246 386 \end{center}
meillo@246 387 \caption{State diagram of the protocol used for \NAME{IPC}}
meillo@246 388 \label{fig:ipc-protocol}
meillo@246 389 \end{figure}
meillo@246 390
meillo@246 391 % timing
meillo@246 392 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@246 393 %fixme: split between header and data
meillo@246 394
meillo@246 395 % semantics
meillo@246 396 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@246 397
meillo@246 398 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@246 399 %fixme: split between header and data
meillo@246 400
meillo@246 401 % syntax
meillo@246 402 Data transfer is done sending plain text data. %fixme: utf8 ?
meillo@246 403 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@246 404 %fixme: split between header and data
meillo@246 405
meillo@246 406 Figure \ref{fig:ipc-protocol} is a state diagram for the protocol.
meillo@246 407
meillo@246 408
meillo@246 409
meillo@246 410 \subsubsection*{Spool file format}
meillo@246 411
meillo@246 412 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@246 413
meillo@246 414 \begin{figure}
meillo@246 415 \begin{center}
meillo@273 416 %\input{img/queue-data-flow.eps}
meillo@246 417 \end{center}
meillo@246 418 \caption{Data flow of messages in the queue}
meillo@246 419 \label{fig:queue-data-flow}
meillo@246 420 \end{figure}
meillo@246 421
meillo@246 422 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@246 423
meillo@246 424 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@246 425
meillo@246 426 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@246 427
meillo@246 428 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@249 429 %fixme: why plain text and not db? -> simplicity
meillo@246 430
meillo@249 431 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@246 432
meillo@246 433
meillo@249 434 A sample header file. With comments in paranthesis.
meillo@246 435
meillo@261 436 \begin{quote}\footnotesize
meillo@246 437 \begin{verbatim}
meillo@246 438 1LGtYh-0ut-00 (backup copy of the file name)
meillo@246 439 MF:<meillo@dream> (envelope: sender)
meillo@246 440 RT: <user@example.org> (envelope: recipient)
meillo@246 441 PR:local (meta info: protocol)
meillo@246 442 ID:meillo (meta info: id/user/ip)
meillo@246 443 DS: 18 (meta info: size)
meillo@246 444 TR: 1230462707 (meta info: timestamp)
meillo@246 445 (following: headers)
meillo@246 446 HD:Received: from meillo by dream with local (masqmail 0.2.21) id
meillo@246 447 1LGtYh-0ut-00 for <user@example.org>; Sun, 28 Dec 2008 12:11:47 +0100
meillo@246 448 HD:To: user@example.org
meillo@246 449 HD:Subject: test mail
meillo@246 450 HD:From: <meillo@dream>
meillo@246 451 HD:Date: Sun, 28 Dec 2008 12:11:47 +0100
meillo@246 452 HD:Message-ID: <1LGtYh-0ut-00@dream>
meillo@246 453 \end{verbatim}
meillo@261 454 \end{quote}
meillo@246 455
meillo@246 456
meillo@246 457
meillo@246 458
meillo@246 459 \subsubsection*{Rights and permission}
meillo@246 460
meillo@246 461 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@246 462
meillo@273 463 Table \ref{tab:new-masqmail-permissions} shows the suggested ownership and permissions of the modules.
meillo@246 464
meillo@246 465 \begin{table}
meillo@246 466 \begin{center}
meillo@271 467 \input{tbl/new-masqmail-permissions.tbl}
meillo@246 468 \end{center}
meillo@246 469 \caption{Ownership and permissions of the modules}
meillo@246 470 \label{tab:new-masqmail-permission}
meillo@246 471 \end{table}
meillo@246 472
meillo@273 473 These are the permissions and ownership used for the queue:
meillo@273 474 \codeinput{input/new-masqmail-queue.txt}
meillo@246 475
meillo@246 476
meillo@246 477
meillo@246 478
meillo@246 479
meillo@246 480 setuid/setgid or not?
meillo@246 481
meillo@246 482 what can crash if an attacker succeeds?
meillo@246 483
meillo@246 484 where to drop privelege?
meillo@246 485
meillo@246 486 how is which process invoked?
meillo@246 487
meillo@246 488 master process? needed, or wanted?
meillo@246 489
meillo@246 490 which are the daemon processes?
meillo@246 491
meillo@246 492
meillo@246 493
meillo@246 494
meillo@246 495
meillo@246 496
meillo@246 497
meillo@246 498 http://fanf.livejournal.com/50917.html %how not to design an mta - the sendmail command
meillo@246 499 http://fanf.livejournal.com/51349.html %how not to design an mta - partitioning for security
meillo@246 500 http://fanf.livejournal.com/61132.html %how not to design an mta - local delivery
meillo@246 501 http://fanf.livejournal.com/64941.html %how not to design an mta - spool file format
meillo@246 502 http://fanf.livejournal.com/65203.html %how not to design an mta - spool file logistics
meillo@246 503 http://fanf.livejournal.com/65911.html %how not to design an mta - more about log-structured MTA queues
meillo@246 504 http://fanf.livejournal.com/67297.html %how not to design an mta - more log-structured MTA queues
meillo@246 505 http://fanf.livejournal.com/70432.html %how not to design an mta - address verification
meillo@246 506 http://fanf.livejournal.com/72258.html %how not to design an mta - content scanning
meillo@246 507
meillo@246 508
meillo@246 509