docs/diploma

annotate thesis/tex/4-MasqmailsFuture.tex @ 176:d4f818a4da04

restructured ch05
author meillo@marmaro.de
date Fri, 26 Dec 2008 16:37:44 +0100
parents 5c873e6478ef
children 7781ad0811f7
rev   line source
meillo@109 1 \chapter{\masqmail's present and future}
meillo@93 2
meillo@137 3 \section{Existing code base}
meillo@142 4 Here regarded is version 0.2.21 of \masqmail. This is the last version released by Oliver \person{Kurth}, and the basis for my thesis.
meillo@142 5
meillo@93 6
meillo@137 7 \subsubsection*{Features}
meillo@93 8
meillo@142 9 \masqmail\ accepts mail on the command line and via \SMTP. Mail queueing and alias expansion is supported. \masqmail\ is able to deliver mail to local mailboxes (in \name{mbox} or \name{maildir} format) or pass it to a \name{mail delivery agent} (like \name{procmail}). Mail destinated to remote locations is sent using \SMTP\ or can be piped to commands, being gatesways to \NAME{UUCP} or \NAME{FAX} for example.
meillo@93 10
meillo@142 11 Outgoing \SMTP\ connections feature \SMTP-\NAME{AUTH} and \SMTP-after-\NAME{POP} authentication, but incoming connections do not. Using wrappers for outgoing connections is supported. This offers a two way communication through a wrapper application like \name{openssl}.
meillo@137 12 %todo: what about SSL/TLS encryption?
meillo@93 13
meillo@142 14 \masqmail\ focuses on non-permanent online connections, thus a concept of online routes is used. One may configure any amount of routes to send mail. Each route can have criterias, like matching \texttt{From:} or \texttt{To:} headers, to determine if mail is allowed to be sent using it. Mail to destinations outside the local net gets queued until \masqmail\ is informed about the existance of a online connection.
meillo@142 15
meillo@137 16 The \masqmail\ executable can be called under various names for sendmail-compatibility reasons. This is organized by symbolic links with different names pointing to the \masqmail\ executable. The \sendmail\ names are \path{/usr/lib/sendmail} and \path{/usr/sbin/sendmail} because many programs expect the \mta\ to be located there. Further more \sendmail\ supports calling it with a different name instead of supplying command line arguments. The best known of this shortcuts is \path{mailq}, which is equivilent to calling it with the argument \verb+-bq+. \masqmail\ recognizes the names \path{mailq}, \path{smtpd}, \path{mailrm}, \path{runq}, \path{rmail}, and \path{in.smtpd}. The first two are inspired by \sendmail. Not implemented is the name \path{newaliases} because \masqmail\ does not generate binary representations of the alias file.\footnote{A shell script located named \path{newaliases}, that invokes \texttt{masqmail -bi}, can provide the command to satisfy other software needing it.} \path{hoststat} and \path{purgestat} are missing for sendmail-compatibility.
meillo@109 17 %masqmail: mailq, mailrm, runq, rmail, smtpd/in.smtpd
meillo@109 18 %sendmail: hoststat, mailq, newaliases, purgestat, smtpd
meillo@109 19
meillo@137 20 Additional to the \mta\ job, \masqmail\ also offers mail retrieval services with being a \NAME{POP3} client. It can fetch mail from different remote locations, dependent on the active online route.
meillo@109 21
meillo@137 22
meillo@137 23
meillo@137 24 \subsubsection*{The code}
meillo@137 25
meillo@137 26 \masqmail\ is written in the C programming language. The program, as of version 0.2.21, consists of 34 source code and eight header files, containing about 9,000 lines of code\footnote{Measured with \name{sloccount} by David A.\ Wheeler.}. Additionally, it includes a \name{base64} implementation (about 300 lines) and \name{md5} code (about 150 lines). For systems that do not provide \name{libident}, this library is distributed as well (circa 600 lines); an available shared library however has higher precedence in linking.
meillo@137 27
meillo@137 28 The only mandatory dependency is \name{glib}---a cross-platform software utility library, originated in the \NAME{GTK+} project. It provides safer replacements for many standard library functions. It also offers handy data containers, easy-to-use implementations of data structures, and much more.
meillo@137 29
meillo@109 30
meillo@109 31 With \masqmail\ comes the small tool \path{mservdetect}; it helps setting up a configuration that uses the \name{mserver} system to detect the online state. Two other binaries get compiled for testing purposes: \path{readtest} and \path{smtpsend}. All three programms use \masqmail\ source code; they only add a file with a \verb+main()+ function each.
meillo@109 32
meillo@93 33
meillo@137 34 \masqmail\ does not provide an interface to plug in modules with additional functionality. There exists no add-on or module system. The code is only separated by function to the various source files. Some functional parts can be included or excluded by defining symbols. Adding maildir support at compile time, means giving the option \verb+--enable-maildir+ to the \path{configure} call. This preserves the concerning code to get removed by the preprocessor. Unfortunately the \verb+#ifdef+s are scattered through all the source, leading to a FIXME(holperig) code base.
meillo@93 35
meillo@132 36
meillo@132 37
meillo@132 38
meillo@132 39
meillo@170 40 \section{\masqmail\ next generation}
meillo@146 41
meillo@170 42 \subsection{Requirements}
meillo@146 43
meillo@146 44 Following is a list of current and future requirements to make \masqmail\ ready for the future.
meillo@146 45
meillo@146 46
meillo@146 47 \subsubsection*{Large message handling}
meillo@155 48 Trends in the market for electronic communication go towards consolidated communication, hence email will be used more to transfer voice and video messages. This leads to larger messages. The store-and-forward transport of email is not good suited for large data. Thus new protocols, like \NAME{QMTP} (described in section %\ref{FIXME}
meillo@155 49 ), may become popular.
meillo@146 50
meillo@146 51
meillo@146 52 \subsubsection*{Ressource friendly software}
meillo@149 53 The merge of communication hardware and the move of email services from providers to homes, demands smaller and more resource-friendly software. The amount of mail will be lower, even if much more mail will be sent. More important will be the energy consumption and heat emission. These topics increased in relevance during the past years and they are expected to become more central. \masqmail\ is not a program to be used on large servers, but to be used on small devices. Thus focusing on energy and heat, not on performance, is the direction to go.
meillo@146 54
meillo@146 55
meillo@146 56 \subsubsection*{New mail transfer protocols}
meillo@149 57 Large messages demand more efficient transport through the net. As well is a final solution needed to defeat the spam problem. New mail transport protocols may be the only good solutions for both problems. They also can improve reliability, authentication, and verification issues. \masqmail\ should be able to support new protocols as they appear and are used.
meillo@146 58
meillo@146 59
meillo@149 60 \subsubsection*{Spam handling}
meillo@149 61 Spam is a major threat. According to the \NAME{SWOT} analysis, the goal is to reduce it to a bearable level. Spam fighting is a war are where the good guys tend to lose. Putting too much effort there will result in few gain. Real success will only be possible with new---better---protocols and abandonning the weak legacy technologies. Hence \masqmail\ should be able to provide state-of-the-art spam protection, but not more.
meillo@146 62
meillo@146 63
meillo@161 64 \subsubsection*{Security}
meillo@161 65 \MTA{}s are critical points for computer security, as they are accessable from external networks. They must be secured with high effort. Properties like high priviledge level, work load influenced from extern, work on unsafe data, and demand for reliability, increase the security needed. Unsecure and unreliable \mta{}s are of no value. \masqmail\ needs to b e secure enough for its target field of operation.
meillo@161 66
meillo@161 67
meillo@146 68 \subsubsection*{Easy configuration}
meillo@149 69 Having \mta{}s on many home servers and clients, requires easy and standardized configuration. The common setups should be configurable with single actions by the user. Complex configuration should be possible, but focused must be the most common form of configuration: choosing one of several standard setups.
meillo@146 70
meillo@146 71
meillo@146 72
meillo@146 73
meillo@146 74
meillo@146 75
meillo@170 76 \subsection{Discussion on architecture}
meillo@146 77
meillo@163 78 A program's architecture is probably the most influencing design decision, and has the greatest impact on the program's future capabilities. %fixme: search quote ... check if good
meillo@132 79
meillo@161 80 \masqmail's current artitecture is monolitic like \sendmail's and \exim's. But more than the other two, is it one block of interweaved code. \sendmail\ provides now, with its \name{milter} interface, standardized connection channels to external modules. \exim\ has a highly structured code with many internal interfaces, like the one for supported authentication ``modules''. \masqmail\ has none of them; it is what \sendmail\ was in the beginning: a single large block.
meillo@161 81
meillo@161 82 Figure \ref{fig:masqmail-arch} is an attempt to depict \masqmail's internal structure.
meillo@161 83
meillo@161 84 \begin{figure}
meillo@161 85 \begin{center}
meillo@161 86 \input{input/masqmail-arch.tex}
meillo@161 87 \end{center}
meillo@161 88 \caption{Internal architecture of \masqmail}
meillo@161 89 \label{fig:masqmail-arch}
meillo@161 90 \end{figure}
meillo@161 91
meillo@163 92 \sendmail\ improved its old architecture, for example by adding the milter interface. \exim\ was designed and is carefully maintained with a modular-like code structure in mind. \qmail\ started from scratch with a ``security-first'' approach, \postfix\ improved on it, and \name{sendmail X}/\name{MeTA1} tries to adopt the best of \qmail\ and \postfix, to completely replace the old \sendmail\ architecture. \person{Hafiz} \cite{hafiz05}. describes this evolution of \mta\ architecture very well.
meillo@161 93
meillo@163 94 Every one of the popular \MTA{}s is more modular, or became more modular over time, than \masqmail\ is. Modern requirements like spam protection and future requirements like the use of new mail transport protocols demand modular designs for keeping the software simple. Simplicity is a key property for security.
meillo@161 95
meillo@163 96 \person{Hafiz} agrees:
meillo@163 97 \begin{quote}
meillo@163 98 The goal of making software secure can be better achieved by making the design simple and easier to understand and verify. \cite[page64]{hafiz05}
meillo@163 99 \end{quote}
meillo@163 100 He identifies the security of \qmail\ to come from it's \name{compartmentalization}, which goes hand in hand with modularity:
meillo@163 101 \begin{quote}
meillo@163 102 A perfect example is the contrast between the feature envy early \sendmail\ architecture implemented as one process and the simple, modular architecture of \qmail. The security of \qmail\ comes from its compartmentalized simple processes that perform one task only and are therefor testable for security. \cite[page 64]{hafiz05}
meillo@163 103 \end{quote}
meillo@165 104 As well does \person{Dent}: ``The modular architecture of Postfix forms the basis for much of its security.''\cite[page 7]{dent04}
meillo@161 105
meillo@163 106 Modularity is needed for supporting modern \MTA\ requirements, providing a clear interface to add further functionality without increasing the overall complexity much. Modularity is also an enabler for security. Security comes from good design, as \person{Graff} and \person{van Wyk} explain:
meillo@163 107 \begin{quote}
meillo@163 108 Good design is the sword and shield of the security-conscious developer. Sound design defends your application from subversion or misuse, protecting your network and the information on it from internal and external attacks alike. It also provides a safe foundation for future extensions and maintainance of the software.
meillo@163 109 %
meillo@163 110 %Bad design makes life easier for attackers and harder for the good guys, especially if it contributes to a false sends of security while obscuring pertinent failings.
meillo@163 111 \cite[page 55]{graff03}
meillo@163 112 \end{quote}
meillo@161 113
meillo@163 114 \person{Hafiz} adds: ``The major idea is that security cannot be retrofitted into an architecture.''\cite[page 64]{hafiz05}
meillo@161 115
meillo@163 116 All this leads to one logical step: The rewrite of \masqmail\ using a modern, modular architecture, to get a modern \MTA\ satisfying nowadays needs.
meillo@161 117
meillo@161 118
meillo@161 119
meillo@161 120
meillo@170 121 \subsection{Jobs of an MTA}
meillo@161 122
meillo@163 123 This section tries to identify the needed modules for a modern \MTA. They are later the pieces of which the new architecture is built of.
meillo@163 124
meillo@170 125 The basic job of a \mta\ is to tranport mail from a sender to a recipient. This is the definition of such a program and this is how \person{Dent}\cite[page 19]{dent04} and \person{Hafiz} \cite[pages 3-5]{hafiz05} generally see its design.
meillo@163 126
meillo@170 127 An \MTA\ therefor needs at least a mail receiving facility and a mail sending facility. Additionally probably all \MTA\ developers (excluded the only forwarders), see the need for a mail queue. A mail queue removes the need to deliver at once a message is received. They also provide fail-safe storage of mails until they are delivered.
meillo@163 128
meillo@163 129
meillo@163 130
meillo@163 131 \subsubsection*{Incoming channels}
meillo@163 132
meillo@170 133 \sendmail-compatible \mta{}s must support at least two incoming channels: mail submitted using the \sendmail\ command, and mail received via the \SMTP\ daemon. It is therefor common to split the incoming channel into local and remote. This is done by \qmail\ and \postfix. The same way is \person{Hafiz}'s view.
meillo@163 134
meillo@170 135 In contrast is \name{sendmail X}: Its locally submitted messages go to the \SMTP\ daemon, which is the only connection towards the mail queue. %fixme: is it a smtp dialog? or a second door?
meillo@170 136 \person{fanf} proposes a similar approach. He wants the \texttt{sendmail} command to be a simple \SMTP\ client that contacts the \SMTP\ daemon of the \MTA\ like it is done by connections from remote. The advantage here is one single module where all \SMTP\ dialog with submitters is done. Hence one single point to accept or refuse incoming mail. Additionally does the module to put mail into the queue not need to be \name{setuid} or \name{setgid} because it is only invoked from the \SMTP\ daemon. The \MTA's architecture would become simpler and common tasks are not duplicated in modules that do similar jobs.
meillo@163 137
meillo@170 138 But merging the input channels in the \SMTP\ daemon makes the \MTA\ heavily dependent on \SMTP\ being the main mail transfer protocol. To \qmail\ and \postfix\ new modules to support other ways of message receival may be added without change of other parts of the system. Also is it better to have more independent modules if each one is simpler then.
meillo@163 139
meillo@170 140 With the increasing need for new protocols in mind, it seems better to have single modules for each incoming channel, although this leads to duplicated acceptance checks.
meillo@163 141
meillo@163 142
meillo@163 143 \subsubsection*{Outgoing channels}
meillo@163 144
meillo@170 145 Outgoing mail is commonly either sent using \SMTP, piped into local commands (for example \texttt{uucp}), or delivered locally by appending to a mailbox.
meillo@170 146
meillo@163 147 Outgoing channels are similar for \qmail, \postfix, and \name{sendmail X}: All of them have a module to send mail using \SMTP, and one for writing into a local mailbox. Local mail delivery is a job that requires root priveledge to be able to switch to any user in order to write to his mailbox. Modular \MTA{}s do not need \name{setuid root}, but the local delivery process (or its parent) needs to run as root.
meillo@163 148
meillo@170 149 As mail delivery to local users, is \emph{not} included in the basic job of an \MTA{}, why should it care about it? In order to keep the system simple and to have programs that do one job well, the local delivery job should be handed over to a specialist: the \name{mail delivery agent}. \NAME{MDA}s know about the various mailbox formats and are aware of the problems of concurrent write access and thelike. Hence handling the message and the responsiblity over to a \NAME{MDA}, like \name{procmail} or \name{maildrop}, seems to be the right way to go.
meillo@163 150
meillo@170 151 This means an outgoing connection that pipes mail into local commands is required. Other outgoing channels, one for each supportet protocol, may be designed like it was done in other \MTA{}s.
meillo@170 152
meillo@170 153
meillo@170 154
meillo@170 155 \subsubsection*{Mail queue}
meillo@170 156
meillo@170 157 Mail queues are probably used in all \mta{}s, excluding the simple forwarders. A mail queue is a essential requirement for \masqmail, as it is to be used for non-permanent online connections. This means, mail must be queued until a online connection is available to send the message.
meillo@170 158
meillo@170 159 The mail queue and the module to manage it are the central part of the whole system. This demands especially for robustness and reliability, as a failure here can lead to loosing mail. An \MTA\ takes over responsibility for mail in accepting it, hence loosing mail messages is absolutely to avoid. This covers any kind of crash situation too. The worst thing acceptable to happen is a mail to be sent twice.
meillo@170 160
meillo@170 161 \sendmail, \exim, \qmail, \name{sendmail X}, and \masqmail\ feature one single mail queue. \postfix\ has three of them: \name{incoming}, \name{active}, and \name{deferred}. (The \name{maildrop} queue is excluded, as it is only used for the \texttt{sendmail} command.)
meillo@170 162
meillo@170 163 \MTA\ setups that include content scanning tend to require two separate queues. To use \sendmail\ in such setups requires two independent instances, with two separate queues, running. \exim\ can handle it with special \name{router} and \name{transport} rules, but the data flow gets complicated. Hence an idea is to use two queues, \name{incoming} and \name{active} in \postfix's terminology, with the content scanning within the move from \name{incoming} to \name{active}.
meillo@163 164
meillo@163 165
meillo@165 166
meillo@165 167 \subsubsection*{Sanitize mail}
meillo@165 168
meillo@170 169 Mail coming into the system often lacks important header lines. At least the required ones must be added from the \MTA. A good example is the \texttt{Message-Id:} header.
meillo@165 170
meillo@170 171 In \postfix, this is done by the \name{cleanup} module, which invokes \name{rewrite}. The position in the message flow is after coming from one of the several incoming channels and before the message is stored into the \name{incoming} queue. Modules that handle incoming channels may also add headers, for example the \texttt{From:} and \texttt{Date:} headers. \name{cleanup}, however, does a complete check to make the mail header complete and valid.
meillo@170 172
meillo@170 173 Apart from deciding where to sanitize the mail header, is the question where to generate the envelope. The envelope specifies the actual recipient of the mail, no matter what the \texttt{To:}, \texttt{Cc:}, and \texttt{Bcc:} headers tell. Multiple reciptients lead to multiple different envelopes, containing all the same mail message.
meillo@170 174
meillo@170 175
meillo@170 176
meillo@170 177 \subsubsection*{Choose route to use}
meillo@170 178
meillo@170 179 One key feature of \masqmail\ is its ability to send mail out in different ways. The decision is based on the current online state and whether a route may be used for a message or not. The online state can be retrieved in tree ways, explained in \ref{sec:fixme}. A route to send is found by checking every available route for being able to transfer the current message, until one matches.
meillo@170 180
meillo@170 181 This functionality should be implemented in the module that is responsible to invoke one of the outgoing channel modules (for example the one for \SMTP\ or the pipe module).
meillo@170 182
meillo@170 183 \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@165 184
meillo@165 185
meillo@165 186
meillo@165 187 \subsubsection*{Aliasing}
meillo@165 188
meillo@170 189 Where should aliases get expanded? They appear in different kind. Important are the ones available in the \path{aliases} file. Aliases can be:
meillo@170 190 \begin{itemize}
meillo@170 191 \item a different local user (e.g.\ ``\texttt{bob: alice}'')
meillo@170 192 \item a remote user (e.g.\ ``\texttt{bob: john@example.com}'')
meillo@170 193 \item a list of users (e.g.\ ``\texttt{bob: alice, john@example.com}'')
meillo@170 194 \item a command (e.g.\ ``\texttt{bob: |foo}'')
meillo@170 195 \end{itemize}
meillo@170 196 Addresses expanding to lists of users lead to more envelopes. Aliases changing the reciptients domain part may require a different route to use.
meillo@165 197
meillo@172 198 Aliasing is often handled in expanding the alias and reinjecting the mail into the system. Unfortunately, the mail is processed twice then; additionally does the system have to handle more mail this way. If it is wanted to check the new recipient address for acceptance and do all processing again, then reinjecting it is the best choice.
meillo@163 199
meillo@163 200
meillo@163 201
meillo@163 202 \subsubsection*{Authentication}
meillo@163 203
meillo@170 204 One thing to avoid is being an \name{open relay}. Open relays allow to relay mail from everywhere to everywhere. This is a major source of spam. The solution is restricting relay\footnote{Relaying is passing mail, that is not from and not for the own system, through it.} access.
meillo@163 205
meillo@170 206 Several ways to restrict access are available. The most simple one is restrictiction by the \NAME{IP} address. No extra complexity is added this way, but static \NAME{IP} addresses are mandatory. This kind of restriction may be enabled using the operating system's \path{hosts.allow} and \path{hosts.deny} files. To allow only connections to port 25 from localhost or the local network \texttt{192.168.100.0/24} insert the line ``\texttt{25: ALL}'' into \path{hosts.deny} and ``\texttt{25: 127.0.0.1, 192.168.100.}'' into \path{hosts.allow}.
meillo@170 207
meillo@170 208 If static access restriction is not possible, for example if mail from locations with changing \NAME{IP} addresses wants to be accepted, some kind of authentication mechanism is required. Three common kinds exist:
meillo@170 209 \begin{enumerate}
meillo@170 210 \item \SMTP-after-\NAME{POP}: uses authenication on the \NAME{POP} protocol to permit incoming \SMTP\ connections for a limited time afterwards.
meillo@170 211 \item \SMTP authentication: is an extension to \SMTP. Authentication can be requested before mail is accepted.
meillo@170 212 \item Certificates: confirm the identity of someone.
meillo@170 213 \end{enumerate}
meillo@170 214 The first mechanism requires a \NAME{POP} (or \NAME{IMAP}) server running on the same host (or a trusted one), to enable the \SMTP\ server to use the login dates on the \NAME{POP} server. This is a common practice used by mail service providers, but is not adequate for the environments \masqmail\ is designed for.
meillo@170 215
meillo@170 216 Certificate based authentication, like provided by \NAME{TLS}, suffers from the overhead of certificate management. But \NAME{TLS} provides encryption too, so is useful anyway.
meillo@170 217
meillo@170 218 \SMTP\ authentication (also refered to as \NAME{SMTP-AUTH}) suppoert is easiest received by using a \name{Simple Authentication and Security Layer} implementation. \person{Dent} sees in \NAME{SASL} the best solution for authenticating dynamic users:
meillo@165 219 \begin{quote}
meillo@170 220 %None of these add-ons is an ideal solution. They require additional code compiled into your existing daemons that may then require special write accesss to system files. They also require additional work for busy system administrators. If you cannot use any of the nonauthenticating alternatives mentioned earlier, or your business requirements demand that all of your users' mail pass through your system no matter where they are on the Internet, SASL is probably the solution that offers the most reliable and scalable method to authenticate users.
meillo@170 221 None of these [authentication methods] is an ideal solution. They require additional code compiled into your existing daemons that may then require special write accesss to system files. They also require additional work for busy system administrators. If you cannot use any of the nonauthenticating alternatives mentioned earlier, or your business requirements demand that all of your users' mail pass through your system no matter where they are on the Internet, \NAME{SASL} is probably the solution that offers the most reliable and scalable method to authenticate users.
meillo@165 222 \cite[page 44]{dent04}
meillo@165 223 \end{quote}
meillo@163 224
meillo@170 225 %either by
meillo@170 226 %- network/ip address
meillo@170 227 % easiest: restricting by static IP addresses (Access control via hosts.allow/hosts.deny)
meillo@170 228 %or
meillo@170 229 %- some kind of auth (for dynamic remote hosts)
meillo@170 230 % adds complexity
meillo@170 231 % - SASL
meillo@170 232 % - POP/IMAP: pop-before-smtp, DRAC, WHOSON
meillo@170 233 % - TLS (certificates)
meillo@170 234
meillo@170 235
meillo@163 236
meillo@163 237 \subsubsection*{Encryption}
meillo@163 238
meillo@172 239 Electronic mail is very weak to sniffing attacks, because all data transfer is unencrypted. This concerns the message's content, as well as the email addresses in header and envelope, but also authentication dialogs that may transfer plain text passwords (\NAME{PLAIN} and \NAME{LOGIN} are examples). Adding encryption is therefor wanted.
meillo@165 240
meillo@172 241 The common way to encrypt \SMTP\ dialogs is using \name{Transport Layer Security} (short: \TLS, successor of \NAME{SSL}). \TLS\ encrypts the datagrams of the \name{transport layer}. This means it works below the application protocols and can be used by any of them\citeweb{wikipedia:tls}.
meillo@165 242
meillo@172 243 \TLS\ allows to create secure tunnels through which arbitrary programs can communicate. Hence one can add secure communication afterwards to programs without changing them. \name{OpenSSL} for example---a free implementation---allows traffic to be piped into a command; a secure tunnel is created and the traffic is forwarded through it. Or a secure tunnel can be set up between a local and a remote port; this tunnel can then be used by any application.
meillo@165 244
meillo@172 245 The \NAME{POP} protocol, for example, is good suited for such tunneling, but \SMTP\ is is not generally. Outgoing \SMTP\ client connections can be tunneled without problem---\masqmail\ already provides a configure option called \texttt{wrapper} to do so. Tunneling incomming connections to a server leads to problems with \SMTP. As data comes encrypted through the tunnel to the receiving host and gets then decrypted and forwarded on local to the port the application listens on. From the \MTA's view, this makes all connections appear to come from localhost, unfortunately. Figure \ref{fig:stunnel} depicts the data flow.
meillo@165 246
meillo@172 247 \begin{figure}
meillo@172 248 \begin{center}
meillo@172 249 \input{input/stunnel.tex}
meillo@172 250 \end{center}
meillo@172 251 \caption{Data flow using \name{stunnel}}
meillo@172 252 \label{fig:stunnel}
meillo@172 253 \end{figure}
meillo@165 254
meillo@172 255 For incoming connections, \NAME{STARTTLS}---defined in \RFC2487---is what \mta{}s implement.
meillo@165 256
meillo@172 257 \masqmail\ is already able to encrypt outgoing connections, but encryption of incoming connections, using \NAME{STARTTLS} should be implemented. This only affects the \SMTP\ server module.
meillo@165 258
meillo@172 259 %TLS/SSL prevents attackers to listen on the cable
meillo@172 260 %but it does not prevent man-in-the-middle attacks
meillo@172 261 %signed certificates help here
meillo@172 262 % or PGP encryption
meillo@165 263
meillo@165 264
meillo@172 265 %do not use stunnel wit SMTP:
meillo@172 266 %because all incoming mail would be from 127.0.0.1 !!
meillo@172 267 %use STARTTLS instead
meillo@165 268
meillo@172 269 %postfix: main.cf
meillo@172 270 %\begin{verbatim}
meillo@172 271 % smtpd_use_tls = yes
meillo@172 272 % smtpd_tls_received_header = no (does not log in received headers)
meillo@172 273 %
meillo@172 274 % smtpd_tls_key_file = /etc/postfix/key.pem
meillo@172 275 % smtpd_tls_cert_file = /etc/postfix/cert.pem
meillo@172 276 % smtpd_tls_CA_file = /etc/postfix/CAcert.pem
meillo@172 277 %
meillo@172 278 % smtp_use_tls = yes (use TLS for sending)
meillo@172 279 % smtp_tls_key_file = /etc/postfix/key.pem
meillo@172 280 % smtp_tls_cert_file = /etc/postfix/cert.pem
meillo@172 281 % smtp_tls_CA_file = /etc/postfix/CAcert.pem
meillo@172 282 %\end{verbatim}
meillo@165 283
meillo@165 284
meillo@165 285
meillo@163 286
meillo@163 287 \subsubsection*{Spam prevention}
meillo@163 288
meillo@173 289 Spam is a major threat to email, as described in section \ref{sec:swot-analysis}. The two main problems are forgable sender addresses and that it is cheap to send hundreds of thousands of messages. Hence, spam senders can operate in disguise and have minimal cost.
meillo@163 290
meillo@173 291 As spam is not just a nuisance for end users, but also for the infrastructure---the \mta{}s---by increasing the amount of mail messages, \MTA{}s need to protect themself. Two approaches are used.
meillo@163 292
meillo@173 293 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.
meillo@163 294
meillo@173 295 Second checking for spam after the mail was accepted and queued. Here more processing time can be invested, so more detailed checks can be done. But, as responsibility for messages was taken by accepting them, it is no choice to simply delete spam mail. Checks for spam do not lead to sure results, they just indicate the possibility the message is unwanted mail. \person{Eisentraut} indicates actions to take after a message is recognized as probably spam \cite[pages 18--20]{eisentraut05}. The only acceptable one, for mail the \MTA\ is responsible for, is adding further or rewriting existent header lines. Thus all further work on the message is the same as for non-spam messages.
meillo@165 296
meillo@173 297 Modern \MTA{}s use both techniques in combination. Checks during the \SMTP\ dialog tend to be implemented in the \mta\ to make it fast; checks after the message was queued are often done using external programs (\name{spamassassin} is a well known one). \person{Eisentraut} sees the checks during the \SMTP\ dialog to be essentiell: ``Ganz ohne Analyse während der SMTP-Phase kommt sowieso kein MTA aus, und es ist eine Frage der Einschätzung, wie weit man diese Phase belasten möchte.''\cite[page 25]{eisentraut05} (translated: ``No \MTA\ can go without analysis during the \SMTP\ dialog, anyway, and it is a question of estimation how much to stress this period.'')
meillo@165 298
meillo@173 299 \NAME{DNS} blacklists (short: \NAME{DNSBL}) and \name{greylisting} are checks to be done before accepting the message. Invoking \name{spamassassin}, to add headers containing the estimated spam probability, is best to be invoked after the message is queued.
meillo@165 300
meillo@163 301
meillo@163 302
meillo@163 303
meillo@163 304 \subsubsection*{Virus checking}
meillo@163 305
meillo@173 306 Related to spam is malicous content (short: \name{malware}) like viruses, worms, trojan horses. They, in contrast to spam, do not affect the \MTA\ itself, as they are in the mail body. The same situation in the real world is post offices opening letters to check if they contain something that could harm the recipient. This is not a mail transport concern. Apart of not being the right program to do the job, the \MTA\---the one which is responsible for the recipient---is at a good position to do this work.
meillo@163 307
meillo@173 308 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}, appear to be better suited locations to invoke content scanners.
meillo@163 309
meillo@163 310
meillo@165 311
meillo@165 312 AMaViS (amavisd-new): email filter framework to integrate spam and virus scanner
meillo@165 313 \begin{verbatim}
meillo@165 314 internet -->25 MTA -->10024 amavis -->10025 MTA --> reciptient
meillo@165 315 | |
meillo@165 316 +----------------------------+
meillo@165 317 \end{verbatim}
meillo@165 318
meillo@165 319 postfix and exim can habe both mta servises in the same instance, sendmail needs two instances running.
meillo@165 320
meillo@165 321 MailScanner:
meillo@165 322 incoming queue --> MailScanner --> outgoing queue
meillo@165 323
meillo@165 324 postfix: with one instance possible, exim and sendmail need two instances running
meillo@165 325
meillo@163 326
meillo@173 327 %message body <-> envelope, header
meillo@173 328 %
meillo@173 329 %anti-virus: clamav
meillo@173 330 %postfix: via amavis
meillo@173 331 %exim: via content-scanning-feature called from acl
meillo@173 332 %sendmail: with milter
meillo@173 333 %procmail
meillo@173 334 %
meillo@173 335 %virus scanner work on file level
meillo@173 336 %amavis receives mail via smtp or pipe, splits it in its parts (MIME) and extracks archives, the come the virus scanners
meillo@173 337 %if the mail is okay, it goes via smtp to a second mta
meillo@173 338
meillo@173 339 %what amavis recognizes:
meillo@173 340 %- invalid headers
meillo@173 341 %- banned files
meillo@173 342 %- viruses
meillo@173 343 %- spam (using spam assassin)
meillo@173 344 %
meillo@173 345 %mimedefang: uses milter interface with sendmail
meillo@173 346
meillo@163 347
meillo@163 348
meillo@163 349 \subsubsection*{Archiving}
meillo@163 350
meillo@163 351
meillo@165 352 \texttt{always\_bcc} feature of postfix
meillo@163 353
meillo@163 354
meillo@163 355
meillo@163 356 \section{A new architecture}
meillo@161 357
meillo@161 358
meillo@161 359 (ssl)
meillo@161 360 -> msg-in (local or remote protocol handlers)
meillo@161 361 -> spam-filter (and more)
meillo@161 362 -> queue
meillo@161 363 -> msg-out (local-delivery by MDA, or remote-protocol-handlers)
meillo@161 364 (ssl)
meillo@161 365
meillo@161 366
meillo@161 367
meillo@161 368
meillo@161 369
meillo@161 370 http://fanf.livejournal.com/50917.html %how not to design an mta - the sendmail command
meillo@161 371 http://fanf.livejournal.com/51349.html %how not to design an mta - partitioning for security
meillo@161 372 http://fanf.livejournal.com/61132.html %how not to design an mta - local delivery
meillo@161 373 http://fanf.livejournal.com/64941.html %how not to design an mta - spool file format
meillo@161 374 http://fanf.livejournal.com/65203.html %how not to design an mta - spool file logistics
meillo@161 375 http://fanf.livejournal.com/65911.html %how not to design an mta - more about log-structured MTA queues
meillo@161 376 http://fanf.livejournal.com/67297.html %how not to design an mta - more log-structured MTA queues
meillo@161 377 http://fanf.livejournal.com/70432.html %how not to design an mta - address verification
meillo@161 378 http://fanf.livejournal.com/72258.html %how not to design an mta - content scanning
meillo@161 379
meillo@161 380
meillo@161 381
meillo@132 382
meillo@132 383
meillo@137 384
meillo@137 385
meillo@149 386
meillo@149 387
meillo@149 388
meillo@149 389
meillo@149 390
meillo@149 391
meillo@149 392
meillo@149 393
meillo@93 394
meillo@93 395
meillo@99 396
meillo@93 397
meillo@93 398
meillo@161 399 \section{Directions to go}
meillo@161 400
meillo@161 401 This section discusses about what shapes \masqmail\ could have---which directions the development could go to.
meillo@161 402
meillo@93 403
meillo@146 404
meillo@146 405
meillo@146 406
meillo@146 407 \subsubsection*{\masqmail\ in five years}
meillo@146 408
meillo@146 409 Now how could \masqmail\ be like in, say, five years?
meillo@146 410
meillo@163 411 ---
meillo@163 412
meillo@163 413 A design from scratch?
meillo@163 414 << what would be needed (effort) >>
meillo@163 415 But how is the effort of this complete rewrite compared to what is gained afterwards?
meillo@163 416
meillo@163 417 << would one create it at all? >>
meillo@163 418
meillo@163 419 ---
meillo@163 420
meillo@146 421 << plans to get masqmail more popular again (if that is the goal) >>
meillo@146 422
meillo@146 423 << More users >>
meillo@146 424
meillo@146 425
meillo@146 426
meillo@146 427
meillo@163 428
meillo@163 429
meillo@163 430
meillo@93 431 \section{Work to do}
meillo@93 432
meillo@146 433 << short term goals --- long term goals >>
meillo@146 434
meillo@163 435 do it like sendmail: first do the most needed stuff on the old design to make it still usable. Then design a new version from scratch, for the future.
meillo@163 436
meillo@140 437 << which parts to take out and do within the thesis >>
meillo@93 438
meillo@167 439