postfix-master/STRESS_README.html

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  16 <h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix
  17 Stress-Dependent Configuration</h1>
  18
  19 <hr>
  20
  21 <h2>Overview </h2>
  22
  23 <p> This document describes the symptoms of Postfix SMTP server
  24 overload. It presents permanent <a href="postconf.5.html">main.cf</a> changes to avoid overload
  25 during normal operation, and temporary <a href="postconf.5.html">main.cf</a> changes to cope with
  26 an unexpected burst of mail. This document makes specific suggestions
  27 for Postfix 2.5 and later which support stress-adaptive behavior,
  28 and for earlier Postfix versions that don't.  </p>
  29
  30 <p> Topics covered in this document: </p>
  31
  32 <ul>
  33
  34 <li><a href="#overload"> Symptoms of Postfix SMTP server overload </a>
  35
  36 <li><a href="#concurrency"> Service more SMTP clients at the same time </a>
  37
  38 <li><a href="#time"> Spend less time per SMTP client </a>
  39
  40 <li><a href="#hangup"> Disconnect suspicious SMTP clients </a>
  41
  42 <li><a href="#legacy"> Temporary measures for older Postfix releases </a>
  43
  44 <li><a href="#adapt"> Automatic stress-adaptive behavior </a>
  45
  46 <li><a href="#feature"> Detecting support for stress-adaptive behavior </a>
  47
  48 <li><a href="#forcing"> Forcing stress-adaptive behavior on or off </a>
  49
  50 <li><a href="#other"> Other measures to off-load zombies </a>
  51
  52 <li><a href="#credits"> Credits </a>
  53
  54 </ul>
  55
  56 <h2><a name="overload"> Symptoms of Postfix SMTP server overload </a></h2>
  57
  58 <p> Under normal conditions, the Postfix SMTP server responds
  59 immediately when an SMTP client connects to it; the time to deliver
  60 mail is noticeable only with large messages.  Performance degrades
  61 dramatically when the number of SMTP clients exceeds the number of
  62 Postfix SMTP server processes.  When an SMTP client connects while
  63 all Postfix SMTP server processes are busy, the client must wait
  64 until a server process becomes available. </p>
  65
  66 <p> SMTP server overload may be caused by a surge of legitimate
  67 mail (example: a DNS registrar opens a new zone for registrations),
  68 by mistake (mail explosion caused by a forwarding loop) or by malice
  69 (worm outbreak, botnet, or other illegitimate activity).  </p>
  70
  71 <p> Symptoms of Postfix SMTP server overload are: </p>
  72
  73 <ul>
  74
  75 <li> <p> Remote SMTP clients experience a long delay before Postfix
  76 sends the "220 hostname.example.com ESMTP Postfix" greeting. </p>
  77
  78 <ul>
  79
  80 <li> <p> NOTE: Broken DNS configurations can also cause lengthy
  81 delays before Postfix sends "220 hostname.example.com ...". These
  82 delays also exist when Postfix is NOT overloaded.  </p>
  83
  84 <li> <p> NOTE:  To avoid "overload" delays for end-user mail
  85 clients, enable the "submission" service entry in <a href="master.5.html">master.cf</a> (present
  86 since Postfix 2.1), and tell users to connect to this instead of
  87 the public SMTP service. </p>
  88
  89 </ul>
  90
  91 <li> <p> The Postfix SMTP server logs an increased number of "lost
  92 connection after CONNECT" events. This happens because remote SMTP
  93 clients disconnect before Postfix answers the connection. </p>
  94
  95 <ul>
  96
  97 <li> <p> NOTE: A portscan for open SMTP ports can also result in
  98 "lost connection ..." logfile messages. </p>
  99
 100 </ul>
 101
 102 <li> <p> Postfix 2.3 and later logs a warning that all server ports
 103 are busy: </p>
 104
 105 <pre>
 106 Oct  3 20:39:27 spike postfix/master[28905]: warning: service "smtp"
 107  (25) has reached its process limit "30": new clients may experience
 108  noticeable delays
 109 Oct  3 20:39:27 spike postfix/master[28905]: warning: to avoid this
 110  condition, increase the process count in <a href="master.5.html">master.cf</a> or reduce the
 111  service time per client
 112 </pre>
 113
 114 </ul>
 115
 116 <p> Legitimate mail that doesn't get through during an episode of
 117 Postfix SMTP server overload is not necessarily lost. It should
 118 still arrive once the situation returns to normal, as long as the
 119 overload condition is temporary.  </p>
 120
 121 <h2><a name="concurrency"> Service more SMTP clients at the same time </a> </h2>
 122
 123 <p> One measure to avoid the "all server processes busy" condition
 124 is to service more SMTP clients simultaneously. For this you need
 125 to increase the number of Postfix SMTP server processes. This will
 126 improve the
 127 responsiveness for remote SMTP clients, as long as the server machine
 128 has enough hardware and software resources to run the additional
 129 processes, and as long as the file system can keep up with the
 130 additional load. </p>
 131
 132 <ul>
 133
 134 <li> <p> You increase the number of SMTP server processes either
 135 by increasing the <a href="postconf.5.html#default_process_limit">default_process_limit</a> in <a href="postconf.5.html">main.cf</a> (line 3 below),
 136 or by increasing the SMTP server's "maxproc" field in <a href="master.5.html">master.cf</a>
 137 (line 10 below).  Either way, you need to issue a "postfix reload"
 138 command to make the change effective.  </p>
 139
 140 <li> <p> Process limits above 1000 require Postfix version 2.4 or
 141 later, and an operating system that supports kernel-based event
 142 filters (BSD kqueue(2), Linux epoll(4), or Solaris /dev/poll).
 143 </p>
 144
 145 <li> <p> More processes use more memory. You can reduce the Postfix
 146 memory footprint by using <a href="CDB_README.html">cdb</a>:
 147 lookup tables instead of Berkeley DB's hash: or btree: tables. </p>
 148
 149 <pre>
 150  1 /etc/postfix/<a href="postconf.5.html">main.cf</a>:
 151  2     # Raise the global process limit, 100 since Postfix 2.0.
 152  3     <a href="postconf.5.html#default_process_limit">default_process_limit</a> = 200
 153  4
 154  5 /etc/postfix/<a href="master.5.html">master.cf</a>:
 155  6     # =============================================================
 156  7     # service type  private unpriv  chroot  wakeup  maxproc command
 157  8     # =============================================================
 158  9     # Raise the SMTP service process limit only.
 159 10     smtp      inet  n       -       n       -       200     smtpd
 160 </pre>
 161
 162 <li> <p> NOTE: older versions of the <a href="SMTPD_POLICY_README.html">SMTPD_POLICY_README</a> document
 163 contain a mistake: they configure a fixed number of policy daemon
 164 processes.  When you raise the SMTP server's "maxproc" field in
 165 <a href="master.5.html">master.cf</a>, SMTP server processes will report problems when connecting
 166 to policy server processes, because there aren't enough of them.
 167 Examples of errors are "connection refused" or "operation timed
 168 out".  </p>
 169
 170 <p> To fix, edit <a href="master.5.html">master.cf</a> and specify a zero "maxproc" field
 171 in all policy server entries; see line 6 in the example below.
 172 Issue a "postfix reload" command to make the change effective.  </p>
 173
 174 <pre>
 175 1 /etc/postfix/<a href="master.5.html">master.cf</a>:
 176 2     # =============================================================
 177 3     # service type  private unpriv  chroot  wakeup  maxproc command
 178 4     # =============================================================
 179 5     # Disable the policy service process limit.
 180 6     policy    unix  -       n       n       -       0       spawn
 181 7         user=nobody argv=/some/where/policy-server
 182 </pre>
 183
 184 </ul>
 185
 186 <h2><a name="time"> Spend less time per SMTP client </a></h2>
 187
 188 <p> When increasing the number of SMTP server processes is not
 189 practical, you can improve Postfix server responsiveness by eliminating
 190 delays.  When Postfix spends less time per SMTP session, the same
 191 number of SMTP server processes can service more clients in a given
 192 amount of time. </p>
 193
 194 <ul>
 195
 196 <li> <p> Eliminate non-functional RBL lookups (blocklists that are
 197 no longer in operation). These lookups can degrade performance.
 198 Postfix logs a warning when an RBL server does not respond. </p>
 199
 200 <li> <p> Eliminate redundant RBL lookups (people often use multiple
 201 Spamhaus RBLs that include each other).  To find out whether RBLs
 202 include other RBLs, look up the websites that document the RBL's
 203 policies. </p>
 204
 205 <li> <p> Eliminate <a href="postconf.5.html#header_checks">header_checks</a> and <a href="postconf.5.html#body_checks">body_checks</a>, and keep just a few
 206 emergency patterns to block the latest worm explosion or backscatter
 207 mail.  See <a href="BACKSCATTER_README.html">BACKSCATTER_README</a> for examples of the latter.
 208
 209 <li> <p> Group your <a href="postconf.5.html#header_checks">header_checks</a> and <a href="postconf.5.html#body_checks">body_checks</a> patterns to avoid
 210 unnecessary pattern matching operations:
 211
 212 <pre>
 213  1  /etc/postfix/header_checks:
 214  2      if /^Subject:/
 215  3      /^Subject: virus found in mail from you/ reject
 216  4      /^Subject: ..other../ reject
 217  5      endif
 218  6
 219  7      if /^Received:/
 220  8      /^Received: from (postfix\.org) / reject forged client name in received header: $1
 221  9      /^Received: from ..other../ reject ....
 222 10      endif
 223 </pre>
 224
 225 </ul>
 226
 227 <h2><a name="hangup"> Disconnect suspicious SMTP clients </a></h2>
 228
 229 <p> Under conditions of overload you can improve Postfix SMTP server
 230 responsiveness by hanging up on suspicious clients, so that other
 231 clients get a chance to talk to Postfix.  </p>
 232
 233 <ul>
 234
 235 <li> <p> Use "521" SMTP reply codes (Postfix 2.6 and later) or "421"
 236 (Postfix 2.3-2.5) to hang up on clients that that match botnet-related
 237 RBLs (see next bullet) or that match selected non-RBL restrictions
 238 such as SMTP access maps.  The Postfix SMTP server will reject mail
 239 and disconnect without waiting for the remote SMTP client to send
 240 a QUIT command.  </p>
 241
 242 <li> <p> To hang up connections from blacklisted zombies, you can
 243 set specific Postfix SMTP server reject codes for specific RBLs,
 244 and for individual responses from specific RBLs. We'll use
 245 zen.spamhaus.org as an example; by the time you read this document,
 246 details may have changed.  Right now, their documents say that a
 247 response of 127.0.0.10 or 127.0.0.11 indicates a dynamic client IP
 248 address, which means that the machine is probably running a bot of
 249 some kind.  To give a 521 response instead of the default 554
 250 response, use something like: </p>
 251
 252 <pre>
 253  1  /etc/postfix/<a href="postconf.5.html">main.cf</a>:
 254  2      <a href="postconf.5.html#smtpd_client_restrictions">smtpd_client_restrictions</a> =
 255  3         <a href="postconf.5.html#permit_mynetworks">permit_mynetworks</a>
 256  4         <a href="postconf.5.html#reject_rbl_client">reject_rbl_client</a> zen.spamhaus.org=127.0.0.10
 257  5         <a href="postconf.5.html#reject_rbl_client">reject_rbl_client</a> zen.spamhaus.org=127.0.0.11
 258  6         <a href="postconf.5.html#reject_rbl_client">reject_rbl_client</a> zen.spamhaus.org
 259  7
 260  8      <a href="postconf.5.html#rbl_reply_maps">rbl_reply_maps</a> = hash:/etc/postfix/rbl_reply_maps
 261  9
 262 10  /etc/postfix/rbl_reply_maps:
 263 11      # With Postfix 2.3-2.5 use "421" to hang up connections.
 264 12      zen.spamhaus.org=127.0.0.10 521 4.7.1 Service unavailable;
 265 13       $rbl_class [$rbl_what] blocked using
 266 14       $rbl_domain${rbl_reason?; $rbl_reason}
 267 15
 268 16      zen.spamhaus.org=127.0.0.11 521 4.7.1 Service unavailable;
 269 17       $rbl_class [$rbl_what] blocked using
 270 18       $rbl_domain${rbl_reason?; $rbl_reason}
 271 </pre>
 272
 273 <p> Although the above example shows three RBL lookups (lines 4-6),
 274 Postfix will only do a single DNS query, so it does not affect the
 275 performance. </p>
 276
 277 <li> <p> With Postfix 2.3-2.5, use reply code 421 (521 will not
 278 cause Postfix to disconnect). The down-side of replying with 421
 279 is that it works only for zombies and other malware. If the client
 280 is running a real MTA, then it may connect again several times until
 281 the mail expires in its queue. When this is a problem, stick with
 282 the default 554 reply, and use "<a href="postconf.5.html#smtpd_hard_error_limit">smtpd_hard_error_limit</a> = 1" as
 283 described below.  </p>
 284
 285 <li> <p> You can automatically turn on the above overload measure
 286 with Postfix 2.5 and later, or with earlier releases that contain
 287 the stress-adaptive behavior source code patch from the mirrors
 288 listed at <a href="http://www.postfix.org/download.html">http://www.postfix.org/download.html</a>. Simply replace line
 289 above 8 with: </p>
 290
 291 <pre>
 292  8      <a href="postconf.5.html#rbl_reply_maps">rbl_reply_maps</a> = ${stress?hash:/etc/postfix/rbl_reply_maps}
 293 </pre>
 294
 295 </ul>
 296
 297 <p> More information about automatic stress-adaptive behavior is
 298 in section "<a href="#adapt">Automatic stress-adaptive behavior</a>".
 299 </p>
 300
 301 <h2><a name="legacy"> Temporary measures for older Postfix releases </a></h2>
 302
 303 <p> See the next section, "<a href="#adapt">Automatic stress-adaptive
 304 behavior</a>", if you are running Postfix version 2.5 or later, or
 305 if you have applied the source code patch for stress-adaptive
 306 behavior from the mirrors listed at <a href="http://www.postfix.org/download.html">http://www.postfix.org/download.html</a>.
 307 </p>
 308
 309 <p> The following measures can be applied temporarily during overload.
 310 They still allow <b>most</b> legitimate clients to connect and send
 311 mail, but may affect some legitimate clients. </p>
 312
 313 <ul>
 314
 315 <li> <p> Reduce <a href="postconf.5.html#smtpd_timeout">smtpd_timeout</a> (default: 300s). Experience on the
 316 postfix-users list from a variety of sysadmins shows that reducing
 317 the "normal" <a href="postconf.5.html#smtpd_timeout">smtpd_timeout</a> to 60s is unlikely to affect legitimate
 318 clients. However, it is unlikely to become the Postfix default
 319 because it's not RFC compliant. Setting <a href="postconf.5.html#smtpd_timeout">smtpd_timeout</a> to 10s (line
 320 2 below) or even 5s under stress will still allow <b>most</b>
 321 legitimate clients to connect and send mail, but may delay mail
 322 from some clients.  No mail should be lost, as long as this measure
 323 is used only temporarily.  </p>
 324
 325 <li> <p> Reduce <a href="postconf.5.html#smtpd_hard_error_limit">smtpd_hard_error_limit</a> (default: 20). Setting this
 326 to 1 under stress (line 3 below) helps by disconnecting clients
 327 after a single error, giving other clients a chance to connect.
 328 However, this may cause significant delays with legitimate mail,
 329 such as a mailing list that contains a few no-longer-active user
 330 names that didn't bother to unsubscribe. No mail should be lost,
 331 as long as this measure is used only temporarily. </p>
 332
 333 <li> <p> Use an <a href="postconf.5.html#smtpd_junk_command_limit">smtpd_junk_command_limit</a> of 1 instead of the default
 334 100. This prevents clients from keeping idle connections open by
 335 repeatedly sending NOOP or RSET commands. </p>
 336
 337 </ul>
 338
 339 <blockquote>
 340 <pre>
 341 1  /etc/postfix/<a href="postconf.5.html">main.cf</a>:
 342 2      <a href="postconf.5.html#smtpd_timeout">smtpd_timeout</a> = 10
 343 3      <a href="postconf.5.html#smtpd_hard_error_limit">smtpd_hard_error_limit</a> = 1
 344 4      <a href="postconf.5.html#smtpd_junk_command_limit">smtpd_junk_command_limit</a> = 1
 345 </pre>
 346 </blockquote>
 347
 348 <p> With these measures, no mail should be lost, as long
 349 as these measures are used only temporarily. The next section of
 350 this document introduces a way to automate this process. </p>
 351
 352 <h2><a name="adapt"> Automatic stress-adaptive behavior </a></h2>
 353
 354 <p> Postfix version 2.5 introduces automatic stress-adaptive behavior.
 355 This is also available as a source code patch for Postfix versions
 356 2.4 and 2.3 from the mirrors listed at
 357 <a href="http://www.postfix.org/download.html">http://www.postfix.org/download.html</a>.  </p>
 358
 359 <p> It works as follows. When a "public" network service such as
 360 the SMTP server runs into an "all server ports are busy" condition,
 361 the Postfix <a href="master.8.html">master(8)</a> daemon logs a warning, restarts the service
 362 (without interrupting existing network sessions), and runs the
 363 service with "-o stress=yes" on the server process command line:
 364 </p>
 365
 366 <blockquote>
 367 <pre>
 368 80821  ??  S      0:00.24 smtpd -n smtp -t inet -u -c -o stress=yes
 369 </pre>
 370 </blockquote>
 371
 372 <p> Normally, the Postfix <a href="master.8.html">master(8)</a> daemon runs such a service with
 373 "-o stress=" on the command line (i.e.  with an empty parameter
 374 value):  </p>
 375
 376 <blockquote>
 377 <pre>
 378 83326  ??  S      0:00.28 smtpd -n smtp -t inet -u -c -o stress=
 379 </pre>
 380 </blockquote>
 381
 382 <p> Services that have local access only never have "-o stress"
 383 parameters on the command line. This includes services internal to
 384 Postfix such as the queue manager, and services that listen on a
 385 loopback interface only, such as after-filter SMTP services.  </p>
 386
 387 <p> The "stress" parameter value is the key to making <a href="postconf.5.html">main.cf</a>
 388 parameter settings stress adaptive. The following settings are the
 389 default with Postfix 2.6 and later. With earlier Postfix versions
 390 that have stress-adaptive support, append the lines below to the
 391 <a href="postconf.5.html">main.cf</a> file and issue a "postfix reload" command: </p>
 392
 393 <blockquote>
 394 <pre>
 395 1 <a href="postconf.5.html#smtpd_timeout">smtpd_timeout</a> = ${stress?10}${stress:300}s
 396 2 <a href="postconf.5.html#smtpd_hard_error_limit">smtpd_hard_error_limit</a> = ${stress?1}${stress:20}
 397 3 <a href="postconf.5.html#smtpd_junk_command_limit">smtpd_junk_command_limit</a> = ${stress?1}${stress:100}
 398 </pre>
 399 </blockquote>
 400
 401 <p> Translation: <p>
 402
 403 <ul>
 404
 405 <li> <p> Line 1: under conditions of stress, use an <a href="postconf.5.html#smtpd_timeout">smtpd_timeout</a>
 406 value of 10 seconds instead of the default 300 seconds. Experience
 407 on the postfix-users list from a variety of sysadmins shows that
 408 reducing the "normal" <a href="postconf.5.html#smtpd_timeout">smtpd_timeout</a> to 60s is unlikely to affect
 409 legitimate clients. However, it is unlikely to become the Postfix
 410 default because it's not RFC compliant. Setting <a href="postconf.5.html#smtpd_timeout">smtpd_timeout</a> to
 411 10s (line 2 below) or even 5s under stress will still allow most
 412 legitimate clients to connect and send mail, but may delay mail
 413 from some clients. No mail should be lost, as long as this measure
 414 is used only temporarily. </p>
 415
 416 <li> <p> Line 2: under conditions of stress, use an <a href="postconf.5.html#smtpd_hard_error_limit">smtpd_hard_error_limit</a>
 417 of 1 instead of the default 20. This helps by disconnecting clients
 418 after a single error, giving other clients a chance to connect.
 419 However, this may cause significant delays with legitimate mail,
 420 such as a mailing list that contains a few no-longer-active user
 421 names that didn't bother to unsubscribe. No mail should be lost,
 422 as long as this measure is used only temporarily. </p>
 423
 424 <li> <p> Line 3: under conditions of stress, use an
 425 <a href="postconf.5.html#smtpd_junk_command_limit">smtpd_junk_command_limit</a> of 1 instead of the default 100. This
 426 prevents clients from keeping idle connections open by repeatedly
 427 sending NOOP or RSET commands. </p>
 428
 429 </ul>
 430
 431 <p> The syntax of ${name?value} and ${name:value} is explained at
 432 the beginning of the <a href="postconf.5.html">postconf(5)</a> manual page. </p>
 433
 434 <p> NOTE: Please keep in mind that the stress-adaptive feature is
 435 a fairly desperate measure to keep <b>some</b> legitimate mail
 436 flowing under overload conditions.  If a site is reaching the SMTP
 437 server process limit when there isn't an attack or bot flood
 438 occurring, then either the process limit needs to be raised or more
 439 hardware needs to be added.  </p>
 440
 441 <h2><a name="feature"> Detecting support for stress-adaptive behavior </a></h2>
 442
 443 <p> To find out if your Postfix installation supports stress-adaptive
 444 behavior, use the "ps" command, and look for the smtpd processes.
 445 Postfix has stress-adaptive support when you see "-o stress=" or
 446 "-o stress=yes" command-line options. Remember that Postfix never
 447 enables stress-adaptive behavior on servers that listen on local
 448 addresses only. </p>
 449
 450 <p> The following example is for FreeBSD or Linux. On Solaris, HP-UX
 451 and other System-V flavors, use "ps -ef" instead of "ps ax". </p>
 452
 453 <blockquote>
 454 <pre>
 455 $ ps ax|grep smtpd
 456 83326  ??  S      0:00.28 smtpd -n smtp -t inet -u -c -o stress=
 457 84345  ??  Ss     0:00.11 /usr/bin/perl /usr/libexec/postfix/smtpd-policy.pl
 458 </pre>
 459 </blockquote>
 460
 461 <p> You can't use <a href="postconf.1.html">postconf(1)</a> to detect stress-adaptive support.
 462 The <a href="postconf.1.html">postconf(1)</a> command ignores the existence of the stress parameter
 463 in <a href="postconf.5.html">main.cf</a>, because the parameter has no effect there.  Command-line
 464 "-o parameter" settings always take precedence over <a href="postconf.5.html">main.cf</a> parameter
 465 settings.  <p>
 466
 467 <p> If you configure stress-adaptive behavior in <a href="postconf.5.html">main.cf</a> when it
 468 isn't supported, nothing bad will happen.  The processes will run
 469 as if the stress parameter always has an empty value. </p>
 470
 471 <h2><a name="forcing"> Forcing stress-adaptive behavior on or off </a></h2>
 472
 473 <p> You can manually force stress-adaptive behavior on, by adding
 474 a "-o stress=yes" command-line option in <a href="master.5.html">master.cf</a>. This can be
 475 useful for testing overrides on the SMTP service. Issue "postfix
 476 reload" to make the change effective.  </p>
 477
 478 <p> Note: setting the stress parameter in <a href="postconf.5.html">main.cf</a> has no effect for
 479 services that accept remote connections. </p>
 480
 481 <blockquote>
 482 <pre>
 483 1 /etc/postfix/<a href="master.5.html">master.cf</a>:
 484 2     # =============================================================
 485 3     # service type  private unpriv  chroot  wakeup  maxproc command
 486 4     # =============================================================
 487 5     #
 488 6     smtp      inet  n       -       n       -       -       smtpd
 489 7         -o stress=yes
 490 8         -o . . .
 491 </pre>
 492 </blockquote>
 493
 494 <p> To permanently force stress-adaptive behavior off with a specific
 495 service, specify "-o stress=" on its <a href="master.5.html">master.cf</a> command line.  This
 496 may be desirable for the "submission" service. Issue "postfix reload"
 497 to make the change effective.  </p>
 498
 499 <p> Note: setting the stress parameter in <a href="postconf.5.html">main.cf</a> has no effect for
 500 services that accept remote connections. </p>
 501
 502 <blockquote>
 503 <pre>
 504 1 /etc/postfix/<a href="master.5.html">master.cf</a>:
 505 2     # =============================================================
 506 3     # service type  private unpriv  chroot  wakeup  maxproc command
 507 4     # =============================================================
 508 5     #
 509 6     submission inet n       -       n       -       -       smtpd
 510 7         -o stress=
 511 8         -o . . .
 512 </pre>
 513 </blockquote>
 514
 515 <h2><a name="other"> Other measures to off-load zombies </a> </h2>
 516
 517 <p> OpenBSD <a href="http://www.openbsd.org/spamd/">spamd</a>
 518 implements a daemon that handles all connections from "new" clients.
 519 Only well-behaved mail clients are allowed to talk to the mail
 520 server. Other clients are tarpitted, and will never get a chance
 521 to affect mail server performance. </p>
 522
 523 <p> At some point in the future, Postfix may come with a simple
 524 front-end daemon that does basic greylisting and pipelining detection
 525 to keep zombies and other ratware away from Postfix itself. This
 526 would use the "pass" service type which has been available in
 527 stable Postfix releases since Postfix 2.5. </p>
 528
 529 <h2><a name="credits"> Credits </a></h2>
 530
 531 <ul>
 532
 533 <li>  Thanks to the postfix-users mailing list members for sharing
 534 early experiences with the stress-adaptive feature.
 535
 536 <li>  The RBL example and several other paragraphs of text were
 537 adapted from postfix-users postings by Noel Jones.
 538
 539 <li>  Wietse implemented stress-adaptive behavior as the smallest
 540 possible patch while he should be working on other things.
 541
 542 </ul>
 543
 544 </body> </html>