1POSTSCREEN(8) System Manager's Manual POSTSCREEN(8)
2
6 postscreen - Postfix zombie blocker
7
9 postscreen [generic Postfix daemon options]
10
12 The Postfix postscreen(8) server provides additional protection against
13 mail server overload. One postscreen(8) process handles multiple in‐
14 bound SMTP connections, and decides which clients may talk to a Postfix
15 SMTP server process. By keeping spambots away, postscreen(8) leaves
16 more SMTP server processes available for legitimate clients, and delays
17 the onset of server overload conditions.
18 This program should not be used on SMTP ports that receive mail from
20 end-user clients (MUAs). In a typical deployment, postscreen(8) handles
21 the MX service on TCP port 25, and smtpd(8) receives mail from MUAs on
22 the submission service (TCP port 587) which requires client authentica‐
23 tion. Alternatively, a site could set up a dedicated, non-postscreen,
24 "port 25" server that provides submission service and client authenti‐
25 cation, but no MX service.
26 postscreen(8) maintains a temporary allowlist for clients that have
28 passed a number of tests. When an SMTP client IP address is al‐
29 lowlisted, postscreen(8) hands off the connection immediately to a
30 Postfix SMTP server process. This minimizes the overhead for legitimate
31 mail.
32 By default, postscreen(8) logs statistics and hands off each connection
34 to a Postfix SMTP server process, while excluding clients in mynetworks
35 from all tests (primarily, to avoid problems with non-standard SMTP im‐
36 plementations in network appliances). This default mode blocks no
37 clients, and is useful for non-destructive testing.
38 In a typical production setting, postscreen(8) is configured to reject
40 mail from clients that fail one or more tests. postscreen(8) logs re‐
41 jected mail with the client address, helo, sender and recipient infor‐
42 mation.
43 postscreen(8) is not an SMTP proxy; this is intentional. The purpose
45 is to keep spambots away from Postfix SMTP server processes, while min‐
46 imizing overhead for legitimate traffic.
47
49 The postscreen(8) server is moderately security-sensitive. It talks to
50 untrusted clients on the network. The process can be run chrooted at
51 fixed low privilege.
52
54 RFC 821 (SMTP protocol)
55 RFC 1123 (Host requirements)
56 RFC 1652 (8bit-MIME transport)
57 RFC 1869 (SMTP service extensions)
58 RFC 1870 (Message Size Declaration)
59 RFC 1985 (ETRN command)
60 RFC 2034 (SMTP Enhanced Status Codes)
61 RFC 2821 (SMTP protocol)
62 Not: RFC 2920 (SMTP Pipelining)
63 RFC 3030 (CHUNKING without BINARYMIME)
64 RFC 3207 (STARTTLS command)
65 RFC 3461 (SMTP DSN Extension)
66 RFC 3463 (Enhanced Status Codes)
67 RFC 5321 (SMTP protocol, including multi-line 220 banners)
68
70 Problems and transactions are logged to syslogd(8) or postlogd(8).
71
73 The postscreen(8) built-in SMTP protocol engine currently does not an‐
74 nounce support for AUTH, XCLIENT or XFORWARD. If you need to make
75 these services available on port 25, then do not enable the optional
76 "after 220 server greeting" tests.
77 The optional "after 220 server greeting" tests may result in unexpected
79 delivery delays from senders that retry email delivery from a different
80 IP address. Reason: after passing these tests a new client must dis‐
81 connect, and reconnect from the same IP address before it can deliver
82 mail. See POSTSCREEN_README, section "Tests after the 220 SMTP server
83 greeting", for a discussion.
84
86 Changes to main.cf are not picked up automatically, as postscreen(8)
87 processes may run for several hours. Use the command "postfix reload"
88 after a configuration change.
89 The text below provides only a parameter summary. See postconf(5) for
91 more details including examples.
92 NOTE: Some postscreen(8) parameters implement stress-dependent behav‐
94 ior. This is supported only when the default parameter value is
95 stress-dependent (that is, it looks like ${stress?{X}:{Y}}, or it is
96 the $name of an smtpd parameter with a stress-dependent default).
97 Other parameters always evaluate as if the stress parameter value is
98 the empty string.
99
101 postscreen_command_filter ($smtpd_command_filter)
102 A mechanism to transform commands from remote SMTP clients.
103 postscreen_discard_ehlo_keyword_address_maps ($smtpd_discard_ehlo_key‐
105 word_address_maps)
106 Lookup tables, indexed by the remote SMTP client address, with
107 case insensitive lists of EHLO keywords (pipelining, starttls,
108 auth, etc.) that the postscreen(8) server will not send in the
109 EHLO response to a remote SMTP client.
110 postscreen_discard_ehlo_keywords ($smtpd_discard_ehlo_keywords)
112 A case insensitive list of EHLO keywords (pipelining, starttls,
113 auth, etc.) that the postscreen(8) server will not send in the
114 EHLO response to a remote SMTP client.
115 Available in Postfix version 3.1 and later:
117 dns_ncache_ttl_fix_enable (no)
119 Enable a workaround for future libc incompatibility.
120 Available in Postfix version 3.4 and later:
122 postscreen_reject_footer_maps ($smtpd_reject_footer_maps)
124 Optional lookup table for information that is appended after a
125 4XX or 5XX postscreen(8) server response.
126 Available in Postfix 3.6 and later:
128 respectful_logging (see 'postconf -d' output)
130 Avoid logging that implies white is better than black.
131
133 postscreen_expansion_filter (see 'postconf -d' output)
134 List of characters that are permitted in postscreen_re‐
135 ject_footer attribute expansions.
136 postscreen_reject_footer ($smtpd_reject_footer)
138 Optional information that is appended after a 4XX or 5XX
139 postscreen(8) server response.
140 soft_bounce (no)
142 Safety net to keep mail queued that would otherwise be returned
143 to the sender.
144
146 Available in Postfix version 2.10 and later:
147 postscreen_upstream_proxy_protocol (empty)
149 The name of the proxy protocol used by an optional be‐
150 fore-postscreen proxy agent.
151 postscreen_upstream_proxy_timeout (5s)
153 The time limit for the proxy protocol specified with the
154 postscreen_upstream_proxy_protocol parameter.
155
157 This test is executed immediately after a remote SMTP client connects.
158 If a client is permanently allowlisted, the client will be handed off
159 immediately to a Postfix SMTP server process.
160 postscreen_access_list (permit_mynetworks)
162 Permanent allow/denylist for remote SMTP client IP addresses.
163 postscreen_blacklist_action (ignore)
165 Renamed to postscreen_denylist_action in Postfix 3.6.
166
168 When postscreen(8) is configured to monitor all primary and backup MX
169 addresses, it can refuse to allowlist clients that connect to a backup
170 MX address only. For small sites, this requires configuring primary and
171 backup MX addresses on the same MTA. Larger sites would have to share
172 the postscreen(8) cache between primary and backup MTAs, which would
173 introduce a common point of failure.
174 postscreen_whitelist_interfaces (static:all)
176 Renamed to postscreen_allowlist_interfaces in Postfix 3.6.
177
179 These tests are executed before the remote SMTP client receives the
180 "220 servername" greeting. If no tests remain after the successful com‐
181 pletion of this phase, the client will be handed off immediately to a
182 Postfix SMTP server process.
183 dnsblog_service_name (dnsblog)
185 The name of the dnsblog(8) service entry in master.cf.
186 postscreen_dnsbl_action (ignore)
188 The action that postscreen(8) takes when a remote SMTP client's
189 combined DNSBL score is equal to or greater than a threshold (as
190 defined with the postscreen_dnsbl_sites and
191 postscreen_dnsbl_threshold parameters).
192 postscreen_dnsbl_reply_map (empty)
194 A mapping from actual DNSBL domain name which includes a secret
195 password, to the DNSBL domain name that postscreen will reply
196 with when it rejects mail.
197 postscreen_dnsbl_sites (empty)
199 Optional list of DNS allow/denylist domains, filters and weight
200 factors.
201 postscreen_dnsbl_threshold (1)
203 The inclusive lower bound for blocking a remote SMTP client,
204 based on its combined DNSBL score as defined with the
205 postscreen_dnsbl_sites parameter.
206 postscreen_greet_action (ignore)
208 The action that postscreen(8) takes when a remote SMTP client
209 speaks before its turn within the time specified with the
210 postscreen_greet_wait parameter.
211 postscreen_greet_banner ($smtpd_banner)
213 The text in the optional "220-text..." server response that
214 postscreen(8) sends ahead of the real Postfix SMTP server's "220
215 text..." response, in an attempt to confuse bad SMTP clients so
216 that they speak before their turn (pre-greet).
217 postscreen_greet_wait (normal: 6s, overload: 2s)
219 The amount of time that postscreen(8) will wait for an SMTP
220 client to send a command before its turn, and for DNS blocklist
221 lookup results to arrive (default: up to 2 seconds under stress,
222 up to 6 seconds otherwise).
223 smtpd_service_name (smtpd)
225 The internal service that postscreen(8) hands off allowed con‐
226 nections to.
227 Available in Postfix version 2.11 and later:
229 postscreen_dnsbl_whitelist_threshold (0)
231 Renamed to postscreen_dnsbl_allowlist_threshold in Postfix 3.6.
232 Available in Postfix version 3.0 and later:
234 postscreen_dnsbl_timeout (10s)
236 The time limit for DNSBL or DNSWL lookups.
237 Available in Postfix version 3.6 and later:
239 postscreen_denylist_action (ignore)
241 The action that postscreen(8) takes when a remote SMTP client is
242 permanently denylisted with the postscreen_access_list parame‐
243 ter.
244 postscreen_allowlist_interfaces (static:all)
246 A list of local postscreen(8) server IP addresses where a
247 non-allowlisted remote SMTP client can obtain postscreen(8)'s
248 temporary allowlist status.
249 postscreen_dnsbl_allowlist_threshold (0)
251 Allow a remote SMTP client to skip "before" and "after 220
252 greeting" protocol tests, based on its combined DNSBL score as
253 defined with the postscreen_dnsbl_sites parameter.
254
256 These tests are executed after the remote SMTP client receives the "220
257 servername" greeting. If a client passes all tests during this phase,
258 it will receive a 4XX response to all RCPT TO commands. After the
259 client reconnects, it will be allowed to talk directly to a Postfix
260 SMTP server process.
261 postscreen_bare_newline_action (ignore)
263 The action that postscreen(8) takes when a remote SMTP client
264 sends a bare newline character, that is, a newline not preceded
265 by carriage return.
266 postscreen_bare_newline_enable (no)
268 Enable "bare newline" SMTP protocol tests in the postscreen(8)
269 server.
270 postscreen_disable_vrfy_command ($disable_vrfy_command)
272 Disable the SMTP VRFY command in the postscreen(8) daemon.
273 postscreen_forbidden_commands ($smtpd_forbidden_commands)
275 List of commands that the postscreen(8) server considers in vio‐
276 lation of the SMTP protocol.
277 postscreen_helo_required ($smtpd_helo_required)
279 Require that a remote SMTP client sends HELO or EHLO before com‐
280 mencing a MAIL transaction.
281 postscreen_non_smtp_command_action (drop)
283 The action that postscreen(8) takes when a remote SMTP client
284 sends non-SMTP commands as specified with the postscreen_forbid‐
285 den_commands parameter.
286 postscreen_non_smtp_command_enable (no)
288 Enable "non-SMTP command" tests in the postscreen(8) server.
289 postscreen_pipelining_action (enforce)
291 The action that postscreen(8) takes when a remote SMTP client
292 sends multiple commands instead of sending one command and wait‐
293 ing for the server to respond.
294 postscreen_pipelining_enable (no)
296 Enable "pipelining" SMTP protocol tests in the postscreen(8)
297 server.
298
300 postscreen_cache_cleanup_interval (12h)
301 The amount of time between postscreen(8) cache cleanup runs.
302 postscreen_cache_map (btree:$data_directory/postscreen_cache)
304 Persistent storage for the postscreen(8) server decisions.
305 postscreen_cache_retention_time (7d)
307 The amount of time that postscreen(8) will cache an expired tem‐
308 porary allowlist entry before it is removed.
309 postscreen_bare_newline_ttl (30d)
311 The amount of time that postscreen(8) will use the result from a
312 successful "bare newline" SMTP protocol test.
313 postscreen_dnsbl_max_ttl
315 (${postscreen_dnsbl_ttl?{$postscreen_dnsbl_ttl}:{1}}h)
316 The maximum amount of time that postscreen(8) will use the re‐
317 sult from a successful DNS-based reputation test before a client
318 IP address is required to pass that test again.
319 postscreen_dnsbl_min_ttl (60s)
321 The minimum amount of time that postscreen(8) will use the re‐
322 sult from a successful DNS-based reputation test before a client
323 IP address is required to pass that test again.
324 postscreen_greet_ttl (1d)
326 The amount of time that postscreen(8) will use the result from a
327 successful PREGREET test.
328 postscreen_non_smtp_command_ttl (30d)
330 The amount of time that postscreen(8) will use the result from a
331 successful "non_smtp_command" SMTP protocol test.
332 postscreen_pipelining_ttl (30d)
334 The amount of time that postscreen(8) will use the result from a
335 successful "pipelining" SMTP protocol test.
336
338 line_length_limit (2048)
339 Upon input, long lines are chopped up into pieces of at most
340 this length; upon delivery, long lines are reconstructed.
341 postscreen_client_connection_count_limit ($smtpd_client_connec‐
343 tion_count_limit)
344 How many simultaneous connections any remote SMTP client is al‐
345 lowed to have with the postscreen(8) daemon.
346 postscreen_command_count_limit (20)
348 The limit on the total number of commands per SMTP session for
349 postscreen(8)'s built-in SMTP protocol engine.
350 postscreen_command_time_limit (normal: 300s, overload: 10s)
352 The time limit to read an entire command line with
353 postscreen(8)'s built-in SMTP protocol engine.
354 postscreen_post_queue_limit ($default_process_limit)
356 The number of clients that can be waiting for service from a
357 real Postfix SMTP server process.
358 postscreen_pre_queue_limit ($default_process_limit)
360 The number of non-allowlisted clients that can be waiting for a
361 decision whether they will receive service from a real Postfix
362 SMTP server process.
363 postscreen_watchdog_timeout (10s)
365 How much time a postscreen(8) process may take to respond to a
366 remote SMTP client command or to perform a cache operation be‐
367 fore it is terminated by a built-in watchdog timer.
368
370 postscreen_tls_security_level ($smtpd_tls_security_level)
371 The SMTP TLS security level for the postscreen(8) server; when a
372 non-empty value is specified, this overrides the obsolete param‐
373 eters postscreen_use_tls and postscreen_enforce_tls.
374 tlsproxy_service_name (tlsproxy)
376 The name of the tlsproxy(8) service entry in master.cf.
377
379 These parameters are supported for compatibility with smtpd(8) legacy
380 parameters.
381 postscreen_use_tls ($smtpd_use_tls)
383 Opportunistic TLS: announce STARTTLS support to remote SMTP
384 clients, but do not require that clients use TLS encryption.
385 postscreen_enforce_tls ($smtpd_enforce_tls)
387 Mandatory TLS: announce STARTTLS support to remote SMTP clients,
388 and require that clients use TLS encryption.
389
391 config_directory (see 'postconf -d' output)
392 The default location of the Postfix main.cf and master.cf con‐
393 figuration files.
394 delay_logging_resolution_limit (2)
396 The maximal number of digits after the decimal point when log‐
397 ging sub-second delay values.
398 command_directory (see 'postconf -d' output)
400 The location of all postfix administrative commands.
401 max_idle (100s)
403 The maximum amount of time that an idle Postfix daemon process
404 waits for an incoming connection before terminating voluntarily.
405 process_id (read-only)
407 The process ID of a Postfix command or daemon process.
408 process_name (read-only)
410 The process name of a Postfix command or daemon process.
411 syslog_facility (mail)
413 The syslog facility of Postfix logging.
414 syslog_name (see 'postconf -d' output)
416 A prefix that is prepended to the process name in syslog
417 records, so that, for example, "smtpd" becomes "prefix/smtpd".
418 Available in Postfix 3.3 and later:
420 service_name (read-only)
422 The master.cf service name of a Postfix daemon process.
423 Available in Postfix 3.5 and later:
425 info_log_address_format (external)
427 The email address form that will be used in non-debug logging
428 (info, warning, etc.).
429
431 smtpd(8), Postfix SMTP server
432 tlsproxy(8), Postfix TLS proxy server
433 dnsblog(8), DNS allow/denylist logger
434 postlogd(8), Postfix logging
435 syslogd(8), system logging
436
438 Use "postconf readme_directory" or "postconf html_directory" to locate
439 this information.
440 POSTSCREEN_README, Postfix Postscreen Howto
441
443 The Secure Mailer license must be distributed with this software.
444
446 This service was introduced with Postfix version 2.8.
447 Many ideas in postscreen(8) were explored in earlier work by Michael
449 Tokarev, in OpenBSD spamd, and in MailChannels Traffic Control.
450
452 Wietse Venema
453 IBM T.J. Watson Research
454 P.O. Box 704
455 Yorktown Heights, NY 10598, USA
456 Wietse Venema
458 Google, Inc.
459 111 8th Avenue
460 New York, NY 10011, USA
461 POSTSCREEN(8)