1 \chapter{Improvement plans}

2

3 The last chapter came to the result that further development is best done in a double-strategy. First the existing code base should be improved to satisfy the most important needs in order to make it usable for some more time. Then \masqmail\ should get redesigned from scratch and rebuild to gain a secure and modern \MTA\ architecture for the future.

4

5 This chapter finally gives concrete suggestions \emph{how} to realize these plans.

6

7 The first part covers the short-time goals which base on current code. The second part deals with the long-time goal---the redesign.

8

9

10

11

12 \section{Based on current code}

13

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.

15

16

17

18 \subsection{Encryption}

19

20 Encryption should be the first functionality to add to the current code. This requirement was already discussed on page \pageref{requirement-encryption}. As explained there, \NAME{STARTTLS} encryption---as defined in \RFC\,2487---should be added to \masqmail.

21

22 Adding encryption requires changes mainly in three source files: \path{smtp_in.c}, \path{smtp_out.c}, and in \path{conf.c}.

23

24 The first file includes the functionality for the \SMTP\ server. It needs to offer \NAME{STARTTLS} support to clients and needs to initiate the encryption when the client requests it. Additionally, the server should be able to insist on encryption before it accepts any message.

25

26 The second file includes the functionality for the \SMTP\ client. It should start the encryption by issuing the \NAME{STARTTLS} keyword if the server supports it. It should be possible to send messages only if encryption is possible.

27

28 The third file controls the configuration files. New configuration option need to be added. The encryption policy for incoming connections needs to be defined. Three choices seem necessary: no encryption, offer encryption, insist on encryption. The encryption policy for outgoing connections should be part of each route setup. The options are the same: never encrypt, encrypt if possible, insist on encryption.

29

30 \NAME{STARTTLS} uses \NAME{TLS} encryption which is based on certificates. Thus the \MTA\ needs its own certificate. This should be generated during installation. A third party application like \name{openssl} should be taken for this job. The encryption itself should also be done using an available library. Open\NAME{SSL} or a substitute like Gnu\NAME{TLS} does then become a dependency for \masqmail. Gnu\NAME{TLS} seems to be the better choice because the Open\NAME{SSL} license is incompatible to the \NAME{GPL}, under which \masqmail\ and Gnu\NAME{TLS} are covered.

31

32 User definable paths to \masqmail's secret key, \masqmail's certificate, and the public certificates of trusted \name{Certificate Authorities} (short: \NAME{CA}s) are also nice to have.

33

34

35 << TLS patch of qmail >>

36

37 %postfix: main.cf

38 %

39 % smtpd_use_tls = yes

40 % smtpd_tls_received_header = no (does not log in received headers)

41 %

42 % smtpd_tls_key_file = /etc/postfix/key.pem

43 % smtpd_tls_cert_file = /etc/postfix/cert.pem

44 % smtpd_tls_CA_file = /etc/postfix/CAcert.pem

45 %

46 % smtp_use_tls = yes (use TLS for sending)

47 % smtp_tls_key_file = /etc/postfix/key.pem

48 % smtp_tls_cert_file = /etc/postfix/cert.pem

49 % smtp_tls_CA_file = /etc/postfix/CAcert.pem

50

51

52

53

54 \subsection{Authentication}

55

56 Authentication is the second function to add; it is important to restrict the access to \masqmail, especially for mail relay. The requirements for authentication where identified on page \pageref{requirement-authentication}.

57

58 Static access restriction, based on the \NAME{IP} address is already possible by using \name{TCP Wrappers}. This makes it easy to refuse all connections from outside the local net for example, which is a good prevention of being an open relay. More detailed static restrictions, like splitting between mail for the system and mail to relay, should not be added to the current code. This may be a concern for the new design.

59

60 Of the dynamic, secret based, authentication methods (\SMTP-after-\NAME{POP}, \SMTP\ authentication, and certificates) the first one drops out as it requires a \NAME{POP} server running on the same or a trusted host. \NAME{POP} servers are rare on workstations and home servers do also not regularly include them. Thus it is no option for \masqmail.

61

62 Authentication based on certificates does suffer from the certificate infrastructure that is required. Although certificates are already used for encryption, its management overhead prevented wide spread usage for authentication.

63

64 \SMTP\ authentication (also referred to as \NAME{SMTP-AUTH}) support is easiest received by using a \name{Simple Authentication and Security Layer} (short: \NAME{SASL}) implementation. \person{Dent} sees in \NAME{SASL} the best solution for dynamic authentication of users:

65 \begin{quote}

66 %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.

67 None of these [authentication methods] is an ideal solution. They require additional code compiled into your existing daemons that may then require special write access 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.

68 \hfill\cite[page 44]{dent04}

69 \end{quote}

70

71 These days is \NAME{SMTP-AUTH}, which is defined in \RFC\,2554, supported by most email clients. If encryption is used then even insecure authentication methods like \NAME{PLAIN} and \NAME{LOGIN} become secure.

72

73 \masqmail\ best uses an available \NAME{SASL} library. \name{Cyrus} \NAME{SASL} is used by \postfix\ and \sendmail. It is a complete framework that makes use of existing authentication concepts like \path{/etc/passwd} or \NAME{PAM}. As advantage it can be included in existing user data bases. \name{gsasl} is an alternative. It comes as a library which helps on deciding for a method and on generating the appropriate dialog data; the actual transmission of the data and the authentication against some database is left open to the programmer. \name{gsasl} is used by \name{msmtp} for example. It seems best to give both concepts a try and decide then which one to use.

74

75 Currently, outgoing connections already feature \SMTP-\NAME{AUTH} but only in a hand-coded way. It is to decide whether it remains as it is or gets replaced by the \NAME{SASL} approach, that is used for incoming connections. The decision should be based on the estimated time until the new design is usable.

76

77 Authentication needs code changes at the same places as encryption. The relevant code files are \path{smtp_in.c}, \path{smtp_out.c}, and \path{conf.c}.

78

79 The server code, to authenticate clients, must be added to \path{smtp_in.c} and the configuration options to \path{conf.c}. Several configuration options should be provided: the authentication policy (no authentication, offer authentication, insist on authentication), the authentication backend (if several are supported), an option to refuse plain text methods (\NAME{PLAIN} and \NAME{LOGIN}), and one to require encryption before authentication.

80

81 If the authentication code for outgoing connects shall be changed too, it must be done in \path{smtp_out.c}. The configuration options are already present.

82

83

84 About the authentication backend. For a small \MTA\ like \masqmail, it seems preferable to store the login data in a text file under \masqmail's control. This is the most simple choice for many usage scenarios. But using a central authentication facility has advantages in larger setups too. \name{Cyrus} \NAME{SASL} supports both, so there is no problem. If \name{gsasl} is chosen, it seems best to start with an authentication file under \masqmail's control.

85

86

87

88

89

90 << Compare static with dynamic authentication: pros and cons; usecases: when to use what; >>

91

92 << how could this be covered by architecture (e.g. smtp submission). >>

93

94

95

96

97

98 \subsection{Security}

99

100 Improvements to \masqmail's security are an important requirement and are the third task to work on. Retrofitting security \emph{into} \masqmail\ is not or hardly possible as it was explained in section \ref{sec:discussion-further-devel}. But adding wrappers and interposition filters can be a large step towards security.

101

102 At first mail security layers like \name{smap} come to mind. The market share analysis in section \ref{sec:market-share} identified such software. This is an interposition filter that stands between the untrusted network and the \MTA. It accepts mail in replacement for the \MTA\ (also called \name{proxy}) in order to separate the \MTA\ from the untrusted network.

103

104 The work \name{smap} does is described in \cite{cabral01}: \name{smap} accepts messages as proxy for the \MTA\ and puts it into a queue. \name{smapd} a brother program runs as daemon and watches for new messages in the queue which it submits into the \MTA\ then.

105

106 Because the \MTA\ does not listen for connections from outside now, it is not directly vulnerable. But the \MTA\ can not react on relaying and spam on itself anymore because it has no direct connection to the mail sender. This job needs to be covered by the proxy now. Similar is the situation for encryption and authentication. However, care must be taken that the proxy stays small and simple as its own security will suffer otherwise.

107

108 The advantage is that the \MTA\ itself needs not to bother much with untrusted environments. And a small proxy cares only about that work.

109

110 \name{smap} is non-free software and thus no general choice for \masqmail. A way to achieve a similar setup would be to copy \masqmail\ and strip one copy to the bare minimum what is needed for the proxy job. \name{setuid} could be removed and root privilege too if \name{inetd} is used. This hardens the proxy instance.

111

112 Mail from outside would then come through the proxy into the system. Mail from the local host and from the local network could be directly accepted by the normal \masqmail, if those locations are considered trusted. But it seems better to have them use the proxy too, or maybe a second proxy instance with different policy.

113

114 The here described setup comes close to the structure of the incoming channels in the new design which is described in \ref{sec:new-design}. This shows the possibilities of the here chosen approach. %fixme: rethink this sentence

115

116

117 \subsubsection*{A concrete setup}

118

119 A stripped down proxy needs to be created. It should only be able to receive mail via \SMTP, encrypt the communication, authenticate clients, and send mail out via \SMTP\ to an internal socket (named ``X'' in the figure). This is a straight forward task. The normal \masqmail\ instance runs on the system too. It takes input from \name{stdin} (by calling the \path{sendmail} command) and via \SMTP\ where it listens on an internal socket (named ``X'' in the figure). Outgoing mail is handled without difference to a regular setup. Figure \ref{fig:proxy-setup} depicts the setup.

120

121 \begin{figure}

122 \begin{center}

123 \includegraphics[scale=0.75]{img/proxy-setup.eps}

124 \end{center}

125 \caption{A setup with a proxy}

126 \label{fig:proxy-setup}

127 \end{figure}

128

129

130 \subsubsection*{Spam and malware handling}

131

132 The presented setup is the same as the one with two \MTA\ instances and a scanner application in between, which was suggested to add spam and malware scanner afterwards to an \MTA. This is a fortunate coincidence, because a scanner like \name{amavis} can simply be put in replace for the internal socket ``X''.

133

134

135

136

137 \subsubsection*{Conditional compilation}

138 << conditional compilation >>

139

140

141

142

143

144

145

146

147

148

149

150

151

152

153

154

155

156 \section{A new design}

157 \label{sec:new-design}

158

159 The last chapter identified the requirements for a modern and secure \masqmail. Now the various jobs of an \MTA\ get assigned to modules of which a new architecture is created. It is inspired by existing \MTA{}s and driven by the identified requirements.

160

161 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}.

162

163

164

165 \subsection{Design decisions}

166

167 This section describes and discusses architectural decision that were made for the new design. At some points function is of matter too, but it is mostly about architecture.

168

169 A number of major design ideas lead the development of the new architecture:

170 \begin{enumerate}

171 \item compartmentalization throughout

172 \item free the internal system from the in and out channels

173 \item provide interfaces to add arbitrary protocol handlers afterwards

174 \item have a single point where all mail goes through for scanning

175 \item concentrate on the mail transfer job; use specialized external programs for other jobs

176 \item keep it simple, clear, and general

177 \end{enumerate}

178

179

180

181 \subsubsection*{Incoming channels}

182

183 \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 therefore 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 \cite{hafiz05}. %fixme: specify page

184

185 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 back door?

186 \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 which puts 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.

187

188 But merging the input channels in the \SMTP\ daemon makes the \MTA\ heavily dependent on \SMTP. To \qmail\ and \postfix\ new modules to support other ways of message reception may be added without change of other parts of the system. Also the \SMTP\ modules can be removed if it is not needed. And it is better to have more independent modules if each one is simpler then---it makes the modules more complicated if each one needs to implement an \SMTP\ client.

189

190 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. Independent checks in different modules, however, have also the advantage to simply apply different policies. Thus it is possible to run two \SMTP\ modules that listen on different ports; one accessible from the Internet but requires authentication, the other only accessible from the local network but does not require authentication.

191

192 The approach of simple independent modules, one for each incoming channel, should be taken.

193

194 A module which is a \NAME{POP} or \NAME{IMAP} client to import contents of other mail boxes into the system may be added afterwards as it is desired.

195

196

197

198 \subsubsection*{Outgoing channels}

199

200 Outgoing mail is commonly either sent using \SMTP, piped into local commands (for example \path{uucp}), or delivered locally by appending to a mailbox.

201

202 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 privilege 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\footnote{root privilege is actually not a mandatory requirement, but any other approach has some disadvantages, so commonly root privilege is used.}.

203

204 Local mail delivery should not be done by the \MTA, but by an \NAME{MDA}. This decision was discussed in section \ref{sec:functional-requirements}. This means only an outgoing channel that pipes mail into a local command is required for local delivery.

205

206 Other outgoing channels, one for each supported protocol, may be designed like it was done in other \MTA{}s.

207

208

209

210 \subsubsection*{The mail queue}

211

212 The mail queue is the central part of an \MTA. This demands especially for robustness and reliability as a failure here can lead to loosing mail.

213

214 %\sendmail, \exim, \qmail, \name{sendmail X}, and \masqmail\ feature one single mail queue. \postfix\ has more of them.

215 Common \MTA{}s feature one or more mail queues, they sometimes have effectively several queues within one physical representation.

216

217 \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. \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}.

218

219 \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} suggest yet another approach: storing the whole queue in one single file with pointers to separating positions \cite{finchFIXME}. %fixme: check, cite, and think about

220

221 All of the presented \MTA{}s use the file system to hold the queue; none uses a database to hold it. A database could improve the reliability of the queue through better persistence. This might be a choice for larger \MTA{}s but is none for \masqmail\ which should be kept small and simple. A running database system does likely require much more resources than \masqmail\ itself does. And as the queue's job is more storing data than running data selection queries, a database does not gain so much that it outweighs its costs.

222

223 Hence here the choice is having a directory with simple text files in it. This is straight forward, simple, clear, and general \dots\ and thus a good basis for reliability. It is additionally always of advantage if data is stored in the operation system's natural form, which in the case of \unix\ is plain text.

224

225 Robustness for the queue is covered in the next section. %fixme: ist this sentence neccessary? Is it still correct.

226

227

228

229 \subsubsection*{Mail sanitizing}

230

231 Mail coming into the system may be may be malformed, lacking headers, or be an attempt to exploit the system. Care must be taken.

232

233 In \postfix, this is done by the \name{cleanup} module, which invokes \name{rewrite}. The position in the message flow is after the message comes from one of the several incoming channels and before the message is stored into the \name{incoming} queue. \name{cleanup} does a complete check to make the mail header complete and valid.

234

235 \qmail\ has the principle of ``don't parse'' which propagates the avoidance of parsing as possible in the system. The reason is that parsing is a highly complex task which often makes code exploitable.

236

237 Mail should be stored into the queue as it is in \masqmail's new design. A scanning module should then parse the message with high care. It seems best to use a \name{parser generator} for this work. The parsed data should then be modified if needed and written into a second queue. This approach has several advantages. First, the receiving parts of the system do not bother about content, they simply store it into the queue. Second, one single modules does the parsing and generates new messages that contain only valid data. Third, the sending parts of the system will only work on messages that consist of valid data. Of course it must be ensured that each message passes through the \name{scanning} module, but this is required for spam and malware scanning too.

238

239 The mail body will never get modified, except of removing and adding transfer protocol specific requirements like dot stuffing or special line ending characters.

240

241 \person{Jon Postel}'s robustness principle ``Be liberal in what you accept, and conservative in what you send.'', which can be found in this wording in \RFC\,1122 and in different wordings in numerous \RFC{}s, is respected in the \name{scanning} module. It parses the given input in some liberal way and generates clean output. \person{Raymond}'s \name{Rule of Repair} ``Repair what you can -- but when you must fail, fail noisily and as soon as possible.'' can be applied too. But it is important to repair only obvious problems, because repairing functionality is likely a target of attacks.

242

243

244

245

246 \subsubsection*{Aliasing}

247

248 The main question about aliasing is: Where should aliases get expanded?

249

250 Two facts are important to consider: Addresses expanding to lists of users lead to more envelopes. And aliases changing the recipient's domain part may make the message unsuitable for a specific online route.

251

252 Aliasing is often handled in expanding the alias and re-injecting 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 re-injecting it is the best choice. But already accepted messages may get rejected in the second go, because of an replacement address from within the system. This seems not to be wanted.

253

254 Doing the alias expansion in the scanning module appears to be the best solution. Unfortunately a second alias expansion must be made on delivery, because only at that point in time is clear which route is used for the message. This compromise is accepted.

255

256

257

258 \subsubsection*{Route management}

259

260 The online state is only important for the sending modules of the system, thus it should be queried in the \name{queue-out} module which selects ready messages from the \name{outgoing} queue and transfers them to the appropriate sending module. Route-based aliasing, which was described in the last section, %fixme: is this still true?

261 should to be done in the same go.

262

263

264

265 \subsubsection*{Archiving}

266

267 The best point to archive copies of every incoming mail is the \name{queue-in} module, respectively the \name{queue-out} module for copies outgoing mail. But not respected with this approach are the changes that are made by the receiving modules (adding further headers) and sending modules (address rewrites).

268

269 \qmail\ has the ability to log complete \SMTP\ dialogs. Logging the complete data transaction into and out of the system into a separate log file is a great feature which should be implemented into each receiving and sending module. But as it will produce a huge amount of output, it should be cared to disabled it by default.

270

271

272

273

274

275 \subsubsection*{Authentication and Encryption}

276

277 Both topics were discussed several time throughout this thesis, among other places on page \pageref{} and \pageref{}.

278

279 Authentication should be done within the receiving modules. Similar should authentication for outgoing connections be handled by the sending modules. To encryption applies the same as to authentication here. Only receiving and sending modules should come in contact with it.

280

281 In order to avoid code duplicates, the actual implementation of both functions should be provided by a central source which gets invoked by the various modules.

282

283

284

285

286

287

288 \subsubsection*{Spam and malware handling}

289

290 The two approaches for spam handling were already presented to the reader in section \ref{}. Here they are described in more detail:

291

292 \begin{enumerate}

293 \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 recipient mail addresses would be enough, but as they are forgeable 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.

294

295 \item 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.

296 \end{enumerate}

297

298 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 essential: ``Ganz ohne Analyse w\"ahrend der \SMTP-Phase kommt sowieso kein \MTA\ aus, und es ist eine Frage der Einsch\"atzung, wie weit man diese Phase belasten m\"ochte.'' \cite[page 25]{eisentraut05} (translated: ``No \MTA\ can go without analysis during the \SMTP\ phase anyway, but the amount of stress one likes to put on this phase is left to his discretion.'')

299

300 Checking before a message is accepted, like \NAME{DNS} blacklists and \name{greylisting}, needs to be invoked from within the receiving modules. Like for authentication and encryption, the implementation of the functionality should be provided by a central source.

301

302 All checking after the message was queued should be done by pushing the message through external scanners like \name{spamassassin}. The \name{scanning} module is the best place to handle this. Hence this module needs interfaces to external scanners.

303

304

305 Malware scanning is similar like the second type of spam scanning. The \name{amavis} framework is a popular mail scanning framework that includes all kinds of malware and also spam scanners; it communicates by using \SMTP.

306

307 Providing \SMTP\ in and out channels from the \name{scanning} module to external scanner applications seems to be a desired goal. Using further instances of the already available \name{smtp} and \name{smtpd} modules therefore appears to be the best solution.

308

309

310

311 \subsubsection*{The scanning module}

312

313 A lot of work was put onto the \name{scanning} module. This is not what is desired. Thus splitting it up into single parts appears to be necessary. But the decision how to split is left up to the time of prototyping.

314

315 << fixme >> %fixme

316

317

318

319

320

321

322

323

324

325

326

327

328

329 \subsection{The resulting architecture}

330

331 The result is a symmetric 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.

332

333 \begin{figure}

334 \begin{center}

335 \includegraphics[width=\textwidth]{img/masqmail-arch-new.eps}

336 \end{center}

337 \caption{A new designed architecture for \masqmail}

338 \label{fig:masqmail-arch-new}

339 \end{figure}

340

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.

342

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.

344 %fixme: do i need all this ``quesses''??

345

346

347 \subsubsection*{Modules and queues}

348

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.

350

351

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 returned.

353 %fixme: should be no daemon

354

355

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 intervals 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.

357

358

359 The \name{queue-out} module takes messages from the \name{outgoing} queue, queries information about the online state which specifies the route to use, creates envelopes for each recipient, and passes the messages to the correct transport module. Successfully transferred messages are removed from the \name{outgoing} queue. This module includes some tasks specific to \masqmail.

360

361

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.

363

364

365 The \name{outgoing} queue contains processed messages. The header and envelope information is complete and in valid form.

366

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.

368 %fixme: get invoked by inetd, or better ucspi-tcp (by bernstein) which can limit max number of concurrent connections. and includes tcp-wrappers functionality.

369

370

371 \name{Transport modules}, on the opposite 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, \masqmail\ passes this job to the \NAME{MDA} (see section \ref{sec:functional-requirements} for reasons). Thus a \name{mail delivery agent} (like \name{procmail}) is to be used with the \name{pipe} module.

372

373

374

375 \subsubsection*{Inter-module communication}

376

377 Communication between modules is required to exchange data and status information. This is also called ``Inter-process communication'' (short: \NAME{IPC}) because the modules are independent programs in this case and processes are programs in execution.

378

379 The connections between \name{queue-in} and \name{scanning}, as well as between \name{scanning} and \name{queue-out} is provided by the queues, only sending signals to trigger runs may be useful. Communication between receiving and transport modules and the outside world are done using the specific protocol they do handle.

380

381 Left is only communication between the receiver modules and \name{queue-in}, and between \name{queue-out} and the transport modules. Data is exchanged using \unix\ pipes and a simple protocol. Figure \ref{fig:ipc-protocol} shows a state diagram for the protocol. Solid lines indicate client actions, dashed lines indicate server responses.

382

383 \begin{figure}

384 \begin{center}

385 \includegraphics[scale=0.75]{img/ipc-protocol.eps}

386 \end{center}

387 \caption{State diagram of the \NAME{IPC} protocol}

388 \label{fig:ipc-protocol}

389 \end{figure}

390

391 \paragraph{Timing}

392 One dialog consists of exactly three phases: connection attempt, envelope and header transfer, and transfer of the message body. The order is always the same. The three phases are all initiated by the client process; after each phase the server process sends a success or error reply. Timeouts for each phase need to be implemented.

393

394 \paragraph{Semantics}

395 The connection attempt is simply opening the connection. This starts the dialog. A positive reply by the server leads to the transfer of envelope and message header. If the server again sends a positive reply, the message data is transferred too. A last server reply ends the dialog.

396

397 The client indicates the end of each data transfer with a special terminator sequence. The appearance of this terminator sequence tells the server process that the data transfer is complete and makes the server send a reply. The server process takes responsibility of the data in sending a success reply. A failure reply immediately stops the dialog and resets both client and server to the state before the connection attempt.

398

399 \paragraph{Syntax}

400 Data transfer is done by sending plain text data. \name{Line Feed}---the native line separator on \unix---is used as line separator. The terminator sequence used to indicate the end of the data transfer is the \NAME{ASCII} \name{null} character (``\texttt{\textbackslash0}''). Replies are one-digit numbers with \texttt{0} meaning success and any other number (\texttt{1}--\texttt{9}) indicate failure.

401

402

403

404 \subsubsection*{Spool file format}

405

406 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}.

407

408 \begin{figure}

409 \begin{center}

410 %\input{img/queue-data-flow.eps}

411 \end{center}

412 \caption{Data flow of messages in the queue}

413 \label{fig:queue-data-flow}

414 \end{figure}

415

416 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.

417

418 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.

419

420 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.

421

422 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.

423 %fixme: why plain text and not db? -> simplicity

424

425 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).

426

427

428 A sample header file. With comments in parenthesis.

429

430 \begin{quote}\footnotesize

431 \begin{verbatim}

432 1LGtYh-0ut-00 (backup copy of the file name)

433 MF:<meillo@dream> (envelope: sender)

434 RT: <user@example.org> (envelope: recipient)

435 PR:local (meta info: protocol)

436 ID:meillo (meta info: id/user/ip)

437 DS: 18 (meta info: size)

438 TR: 1230462707 (meta info: timestamp)

439 (following: headers)

440 HD:Received: from meillo by dream with local (masqmail 0.2.21) id

441 1LGtYh-0ut-00 for <user@example.org>; Sun, 28 Dec 2008 12:11:47 +0100

442 HD:To: user@example.org

443 HD:Subject: test mail

444 HD:From: <meillo@dream>

445 HD:Date: Sun, 28 Dec 2008 12:11:47 +0100

446 HD:Message-ID: <1LGtYh-0ut-00@dream>

447 \end{verbatim}

448 \end{quote}

449

450

451

452

453 \subsubsection*{Rights and permission}

454

455 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.

456

457 Table \ref{tab:new-masqmail-permissions} shows the suggested ownership and permissions of the modules.

458

459 \begin{table}

460 \begin{center}

461 \input{tbl/new-masqmail-permissions.tbl}

462 \end{center}

463 \caption{Ownership and permissions of the modules}

464 \label{tab:new-masqmail-permission}

465 \end{table}

466

467 These are the permissions and ownership used for the queue:

468 \codeinput{input/new-masqmail-queue.txt}

469

470

471

472

473

474 setuid/setgid or not?

475

476 what can crash if an attacker succeeds?

477

478 where to drop privilege?

479

480 how is which process invoked?

481

482 master process? needed, or wanted?

483

484 which are the daemon processes?

485

486

487

488

489

490

491

492 http://fanf.livejournal.com/50917.html %how not to design an mta - the sendmail command

493 http://fanf.livejournal.com/51349.html %how not to design an mta - partitioning for security

494 http://fanf.livejournal.com/61132.html %how not to design an mta - local delivery

495 http://fanf.livejournal.com/64941.html %how not to design an mta - spool file format

496 http://fanf.livejournal.com/65203.html %how not to design an mta - spool file logistics

497 http://fanf.livejournal.com/65911.html %how not to design an mta - more about log-structured MTA queues

498 http://fanf.livejournal.com/67297.html %how not to design an mta - more log-structured MTA queues

499 http://fanf.livejournal.com/70432.html %how not to design an mta - address verification

500 http://fanf.livejournal.com/72258.html %how not to design an mta - content scanning

501

502

503