1 \chapter{\masqmail's present and future}

2

3 \section{Existing code base}

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.

5

6

7 \subsubsection*{Features}

8

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.

10

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}.

12 %todo: what about SSL/TLS encryption?

13

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.

15

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.

17 %masqmail: mailq, mailrm, runq, rmail, smtpd/in.smtpd

18 %sendmail: hoststat, mailq, newaliases, purgestat, smtpd

19

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.

21

22

23

24 \subsubsection*{The code}

25

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.

27

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.

29

30

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.

32

33

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.

35

36

37

38

39

40

41 \section{Requirements}

42

43 Following is a list of current and future requirements to make \masqmail\ ready for the future.

44

45

46 \subsubsection*{Large message handling}

47 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}

48 ), may become popular.

49

50

51 \subsubsection*{Ressource friendly software}

52 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.

53

54

55 \subsubsection*{New mail transfer protocols}

56 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.

57

58

59 \subsubsection*{Spam handling}

60 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.

61

62

63 \subsubsection*{Security}

64 \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.

65

66

67 \subsubsection*{Easy configuration}

68 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.

69

70

71

72

73

74

75 \section{Discussion on architecture}

76

77 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

78

79 \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.

80

81 Figure \ref{fig:masqmail-arch} is an attempt to depict \masqmail's internal structure.

82

83 \begin{figure}

84 \begin{center}

85 \input{input/masqmail-arch.tex}

86 \end{center}

87 \caption{Internal architecture of \masqmail}

88 \label{fig:masqmail-arch}

89 \end{figure}

90

91 \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.

92

93 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.

94

95 \person{Hafiz} agrees:

96 \begin{quote}

97 The goal of making software secure can be better achieved by making the design simple and easier to understand and verify. \cite[page64]{hafiz05}

98 \end{quote}

99 He identifies the security of \qmail\ to come from it's \name{compartmentalization}, which goes hand in hand with modularity:

100 \begin{quote}

101 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}

102 \end{quote}

103 As well does \person{Dent}: ``The modular architecture of Postfix forms the basis for much of its security.''\cite[page 7]{dent04}

104

105 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:

106 \begin{quote}

107 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.

108 %

109 %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.

110 \cite[page 55]{graff03}

111 \end{quote}

112

113 \person{Hafiz} adds: ``The major idea is that security cannot be retrofitted into an architecture.''\cite[page 64]{hafiz05}

114

115 All this leads to one logical step: The rewrite of \masqmail\ using a modern, modular architecture, to get a modern \MTA\ satisfying nowadays needs.

116

117

118

119

120 \subsection{Modules needed}

121

122 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.

123

124

125 \subsubsection*{The simple view}

126

127 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} start on the design.

128

129 An \MTA\ therefor needs at least a mail receiving facility and a mail sending facility. But both, and probably all \MTA\ developers (excluded the only forwarders), see the need for a mail queue. A mail queue removed the need to deliver at once. They also provide fail-safe storage of mails until they are delivered.

130

131

132

133

134 \subsubsection*{Incoming channels}

135

136 The second addition \person{Hafiz} made is the split of incoming and outgoing channels into local and remote. The question is, if this is nessesary. It is the way, it was done for a long time, but is this extra complexity needed?

137

138 The common situation is incoming mail on port 25 using \SMTP\ and via the \texttt{sendmail} command. Outgoing mail is either sent using \SMTP, piped into local commands (for example \texttt{uucp}), or delivered locally by appending to a mailbox.

139

140 The \MTA's architecture would be simpler if some of these channels could be merged. The reason is, if various modules do similar jobs, common things might need to be duplicated. On the other side is it better to have more independent modules if each one is simpler then.

141

142 \qmail\ uses \name{qmail-inject} (local message in) and \name{qmail-smtpd} (remote message in), which both handle messages over to \name{qmail-queue} that puts it into the mail queue. \postfix's approach is similar. \name{sendmail X} %fixme: what about meta1 here?

143 used only \NAME{SMTPS}, which is for receiving mail from remote, to communicate with the queue manager \NAME{QMGR}. Mail from local goes over \NAME{SMTPS}.

144

145 The \name{sendmail X} approach seems to be the simpler one, but does heavily rely on \SMTP\ being the main mail transfer protocol. To \qmail\ and \postfix\ new modules may be added to support other ways of message receival, without any change of other parts of the system.

146

147

148 \subsubsection*{Outgoing channels}

149

150 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.

151

152 As mail delivery to local users, is \emph{not} included in the basic job of \MTA{}s, why should they care about it? In order to keep the system simple and to have programs do one job well, the local delivery job should be handed over to \NAME{MDA}s. \name{Mail delivery agents} are the tools that are specialized for local delivery. They 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 for it over to a mail delivery agent, like \name{procmail} or \name{maildrop}, seems to be the right way to go.

153

154 This means outgoing connections, piping mails into local commands needs to be implemented.

155

156

157

158 \subsubsection*{Sanitize mail}

159 generate valid headers: add, rewrite

160 ... better before inserting into the queue

161

162 (determine the method to send at that position?)

163

164

165

166

167 \subsubsection*{Aliasing}

168

169 where to expand aliases?

170

171

172

173 \subsubsection*{Mail queue}

174

175 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.

176

177

178

179

180

181

182

183 \subsubsection*{Authentication}

184

185 either by

186 - network/ip address

187 easiest: restricting by static IP addresses (Access control via hosts.allow/hosts.deny)

188 or

189 - some kind of auth (for dynamic remote hosts)

190 adds complexity

191 - SASL

192 - POP/IMAP: pop-before-smtp, DRAC, WHOSON

193 - TLS (certificates)

194

195 \begin{quote}

196 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 thyour 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.

197 \cite[page 44]{dent04}

198 \end{quote}

199

200

201 \subsubsection*{Encryption}

202

203 TLS/SSL prevents attackers to listen on the cable

204 but it does not prevent man-in-the-middle attacks

205 signed certificates help here

206

207

208 ch /usr/share/ssl/misc

209

210 create new CA:

211 \begin{verbatim}

212 CA.pl -newca

213 country: DE

214 state: schwaben

215 city: Ulm

216 company:

217 section:

218 name:

219 emailaddress:

220 \end{verbatim}

221

222 generate ssl key:

223 \begin{verbatim}

224 CA.pl -newreq

225 ... the same questions

226 \end{verbatim}

227

228 sign request with CA:

229 \begin{verbatim}

230 CA.pl -sign

231 \end{verbatim}

232

233 remove passphrase from private key:

234 \begin{verbatim}

235 openssl rsa <newreq.pem >key.pem

236 (to be used by programs automaticly)

237 \end{verbatim}

238

239 secure:

240 \begin{verbatim}

241 chmod 400 *.pem

242 cp newcert.pem /etc/postfix/cert.pem

243 cp key.pem /etc/postfix/key.pem

244 cp demoCA/cacert.pem /etc/postfix/CAcert.pem

245 chmode 400 /etc/postfix/*.pem

246

247 mkdir /etc/stunnel

248 cat newcert.pem key.pem >/etc/stunnel/stunnel.pem

249 chmod 400 /etc/stunnel/stunnel.pem

250 (check /etc/stunnel with `stunnel -V')

251 \end{verbatim}

252

253

254 set up stunnels for POP, etc:

255 \begin{verbatim}

256 nmap localhost

257 stunnel -d pop3s -r localhost:pop3 -p /etc/stunnel/stunnel.pem

258 stunnel -d imaps -r localhost:imap -p /etc/stunnel/stunnel.pem

259 nmap localhost

260 pop3s 995

261 imaps 993

262 \end{verbatim}

263

264 do not use stunnel wit SMTP:

265 because all incoming mail would be from 127.0.0.1 !!

266 use STARTTLS instead

267

268 postfix: main.cf

269 \begin{verbatim}

270 smtpd_use_tls = yes

271 smtpd_tls_received_header = no (does not log in received headers)

272

273 smtpd_tls_key_file = /etc/postfix/key.pem

274 smtpd_tls_cert_file = /etc/postfix/cert.pem

275 smtpd_tls_CA_file = /etc/postfix/CAcert.pem

276

277 smtp_use_tls = yes (use TLS for sending)

278 smtp_tls_key_file = /etc/postfix/key.pem

279 smtp_tls_cert_file = /etc/postfix/cert.pem

280 smtp_tls_CA_file = /etc/postfix/CAcert.pem

281 \end{verbatim}

282

283

284

285

286 \subsubsection*{Spam prevention}

287

288

289 where to filter what

290

291

292 postfix:

293 content-filter: arbitrary programs that talk smtp, can filter, rewrite or delete mail

294 - before-queue-c-f: need to be fast, can prevent system load

295 - after-queue-c-f: need more resources in global, more load

296

297 exim:

298 acls: to filter, what to accept (hook into smtp dialog) (complex)

299 routers: take recipient address and choose a matching transport

300 transports: ways to deliver mail (smtp, local)

301

302

303 postfix: after-queue-content-filter (smtp communication)

304 exim: content-scan-feature (analyses the content: MIME stuff, blacklisted words, virus scanning) (all within smtp dialog)

305 sendmail: milter (tcp or unix sockets)

306

307 checks while smtp dialog (pre-queue): in MTA implemented (need to be fast)

308 checks when mail is accepted and queued: external (amavis, spamassassin)

309

310

311

312

313

314

315 what do do with recognized mail?

316 - reject (only possible if recognized during SMTP dialog)

317 - forward with added header line or changed subject

318 (eisentraut05: page 18--20)

319

320 check incoming and outgoing mail

321 (eisentraut05: page 21)

322

323

324 milter:

325 communication with external daemons via a special protocol

326 at various times in the smtp dialog possible

327 can reject, delete or alter messages

328 http://milter.org

329 (eisentraut05: page 69)

330

331

332 use SA with exim:

333 - with transport: piped into sa

334 - content-scanning-feature: with ACL during smtp dialog

335 - plugin: sa-exim

336 - within amavis

337

338 use SA with sendmail:

339 - with milter

340 - within mimedefang or amavis

341

342 use SA with postfix:

343 - within amavis or mailfilter

344

345

346 (eisentraut05: page 25) ``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.''

347

348

349 DNSBL can contain:

350 - open relays

351 - dynamic IP addresses

352 - verified spam sources

353 - open multistage relays

354 - vulnerable CGI scripts

355 - open proxy servers

356 example: NJABL (http://njabl.org)

357

358 DNSBL in smpt dialog is aggressive and can lead to problems (eisentraut05: page 126)

359

360

361 greylisting:

362 if first contact from that address: temp failure and add to list

363 sender will retry, then accept

364

365 ``Das Greylisting zählt derzeit zu den effektivsten Methoden, um gegen unerwünschte E-Mails vorzugehen. Allein durch Greylisting können derzeit rund 70\% des potenziellen Spam-Aufkommens auf einem Mailserver vollständig geblockt werden. Allerdings ist es auch nur eine Frage der Zeit, bis sich die Gemeinde der Spammer und Virenautoren auf diese Methode der Spam-Bekämpfung eingerichtet und entsprechende Queues in ihre Software eingebaut hat.''(eisentraut05: page 138)

366 Probleme: load balancing using multiple servers with different IPs.

367 postfix: with policy server

368 exim: direct in config

369 sendmail: with greylist milter

370

371

372

373 hashcash

374

375

376 \subsubsection*{Virus checking}

377

378 The same for malicious content (\name{malware}) like viruses, worms, trojan horses. They are related to spam, but affect the \MTA less, as they are in the mail body.

379

380 message body <-> envelope, header

381

382

383 anti-virus: clamav

384 postfix: via amavis

385 exim: via content-scanning-feature called from acl

386 sendmail: with milter

387 procmail

388

389

390 virus scanner work on file level

391 amavis receives mail via smtp or pipe, splits it in its parts (MIME) and extracks archives, the come the virus scanners

392 if the mail is okay, it goes via smtp to a second mta

393

394

395 AMaViS (amavisd-new): email filter framework to integrate spam and virus scanner

396 \begin{verbatim}

397 internet -->25 MTA -->10024 amavis -->10025 MTA --> reciptient

398 | |

399 +----------------------------+

400 \end{verbatim}

401

402 postfix and exim can habe both mta servises in the same instance, sendmail needs two instances running.

403

404 what amavis recognizes:

405 - invalid headers

406 - banned files

407 - viruses

408 - spam (using spam assassin)

409

410

411 mimedefang: uses milter interface with sendmail

412

413

414 MailScanner:

415 incoming queue --> MailScanner --> outgoing queue

416

417 postfix: with one instance possible, exim and sendmail need two instances running

418

419

420

421

422 \subsubsection*{Archiving}

423

424

425 \texttt{always\_bcc} feature of postfix

426

427

428

429 \section{A new architecture}

430

431

432 (ssl)

433 -> msg-in (local or remote protocol handlers)

434 -> spam-filter (and more)

435 -> queue

436 -> msg-out (local-delivery by MDA, or remote-protocol-handlers)

437 (ssl)

438

439

440

441

442

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

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

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

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

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

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

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

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

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

452

453

454

455

456

457

458

459

460

461

462

463

464

465

466

467

468

469

470

471

472 \section{Directions to go}

473

474 This section discusses about what shapes \masqmail\ could have---which directions the development could go to.

475

476

477

478

479

480 \subsubsection*{\masqmail\ in five years}

481

482 Now how could \masqmail\ be like in, say, five years?

483

484 ---

485

486 A design from scratch?

487 << what would be needed (effort) >>

488 But how is the effort of this complete rewrite compared to what is gained afterwards?

489

490 << would one create it at all? >>

491

492 ---

493

494 << plans to get masqmail more popular again (if that is the goal) >>

495

496 << More users >>

497

498

499

500

501

502

503

504 \section{Work to do}

505

506 << short term goals --- long term goals >>

507

508 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.

509

510 << which parts to take out and do within the thesis >>

511

512