Zimbra Collaboration Postscreen
Zimbra Collaboration Postscreen
Starting with Zimbra Collaboration 8.7 and above, Zimbra introduces Postscreen like an additional Anti-SPAM strategy. Zimbra Postscreen provides additional protection against mail server overload. One postscreen process handles multiple inbound SMTP connections, and decides which clients may talk to a Post-fix SMTP server process. By keeping spambots away, postscreen leaves more SMTP server processes available for legitimate clients, and delays the onset of server overload conditions.
Zimbra Collaboration Postscreen should not be used on SMTP ports that receive mail from end-user clients (MUAs). In a typical deployment, postscreen handles the MX service on TCP port 25, while MUA clients submit mail via the submission service on TCP port 587 which requires client authentication. Alternatively, a site could set up a dedicated, non-postscreen, "port 25" server that provides submission service and client authentication, but no MX service.
Zimbra Collaboration Postscreen maintains a temporary white-list for clients that have passed a number of tests. When an SMTP client IP address iswhitelisted, postscreen hands off the connection immediately to a Postfix SMTP server process. This minimizes the overhead for legitimate mail.
In a typical production setting, postscreen is configured to reject mail from clients that fail one or more tests. Zimbra Collaboration Postscreen logs rejected mail with the client address, helo, sender and recipient information.
Zimbra Collaboration Postscreen is not an SMTP proxy; this is intentional. The purpose is to keep spambots away from Postfix SMTP server processes, while minimizing overhead for legitimate traffic.
How it works
Scenario without Postscreen
A typical scenario without Postscreen, and without other Anti-SPAM security, will suffer of this common Problem, where bot and zombies talks with all the smtpd listeners that Zimbra is offering.
In this scenario, the good connections, or called other in this diagram, must wait until the bot or zombie finishes the communication, which sometimes can create a Timeout Error on Postfix for the good connections:
Mar 01 19:29:54 zimbrauk postfix/smtpd[24266]: timeout after RCPT from mail.example.com[60.60.60.70]
Scenario with Postscreen
A typical scenario with Postscreen, where bot and zombies talks with Postscreen, who do all the basic checks, and who can deny the connection if the message is clearly from a bot or zombie, if the connection is not in the temporary whitelist, Postscreen will pass the Email to the local Anti-SPAM and Anti-Virus engines, who can accept it or deny it as usual. You can see how is the Mail Flow in Postscreen on the section below.
In this scenario, the good connections, or called other in this diagram, pass the Postscreen security and talks directly with the smtp daemon, who will scan the Email as usual with the AS/AV. All the bot or zombie are rejected by default.
Postscreen workflow
See attached the workflow for Zimbra Collaboration Postscreen
Zimbra attributes for Postscreen
Here you can find all the new attributes for Postscreen, and the link to the original Postfix description help per attribute.
Please note the difference between the ignore, enforce and drop for certain attributes:
- ignore (default) - Ignore this result. Allow other tests to complete. Repeat this test the next time the client connects. This option is useful for testing and collecting statistics without blocking mail.
- enforce - Allow other tests to complete. Reject attempts to deliver mail with a 550 SMTP reply, and log the helo/sender/recipient information. Repeat this test the next time the client connects.
- drop - Drop the connection immediately with a 521 SMTP reply. Repeat this test the next time the client connects.
Name | Description | Type | Optional in | Default value | Options |
---|---|---|---|---|---|
zimbraMtaPostscreenAccessList | Value for postconf postscreen_access_list. Single valued, commas,separated list. | string | server,globalConfig | permit_mynetworks | |
zimbraMtaPostscreenBareNewlineAction | Value for postconf postscreen_bare_newline_action. | enum | server,globalConfig | ignore | ignore,enforce,drop |
zimbraMtaPostscreenBareNewlineEnable | Value for postconf postscreen_bare_newline_enable. | enum | server,globalConfig | no | yes,no |
zimbraMtaPostscreenBareNewlineTTL | Value for postconf postscreen_bare_newline_ttl. | string | server,globalConfig | 30d | |
zimbraMtaPostscreenBlacklistAction | Value for postconf postscreen_blacklist_action. | enum | server,globalConfig | ignore | ignore,enforce,drop |
zimbraMtaPostscreenCacheCleanupInterval | Value for postconf postscreen_cache_cleanup_interval. | string | server,globalConfig | 12h | |
zimbraMtaPostscreenCacheRetentionTime | Value for postconf postscreen_cache_retention_time. | string | server,globalConfig | 7d | |
zimbraMtaPostscreenCommandCountLimit | Value for postconf postscreen_command_count_limit. | integer | server,globalConfig | 20 | |
zimbraMtaPostscreenDnsblAction | Value for postconf postscreen_dnsbl_action. | enum | server,globalConfig | ignore | ignore,enforce,drop |
zimbraMtaPostscreenDnsblSites | Value for postconf postscreen_dnsbl_sites. Multi valued, one DNSBL,value pair per attribute value. | string | server,globalConfig | ||
zimbraMtaPostscreenDnsblThreshold | Value for postconf postscreen_dnsbl_threshold. | integer | server,globalConfig | 1 | |
zimbraMtaPostscreenDnsblTTL | Value for postconf postscreen_dnsbl_ttl. | string | server,globalConfig | 1h | |
zimbraMtaPostscreenDnsblWhitelistThreshold | Value for postconf postscreen_dnsbl_whitelist_threshold. | integer | server,globalConfig | 0 | |
zimbraMtaPostscreenGreetAction | Value for postconf postscreen_greet_action. | enum | server,globalConfig | ignore | ignore,enforce,drop |
zimbraMtaPostscreenGreetTTL | Value for postconf postscreen_greet_ttl. | string | server,globalConfig | 1d | |
zimbraMtaPostscreenNonSmtpCommandAction | Value for postconf postscreen_non_smtp_command_action. | enum | server,globalConfig | drop | ignore,enforce,drop |
zimbraMtaPostscreenNonSmtpCommandEnable | Value for postconf postscreen_non_smtp_command_enable. | enum | server,globalConfig | no | yes,no |
zimbraMtaPostscreenNonSmtpCommandTTL | Value for postconf postscreen_non_smtp_command_ttl. | string | server,globalConfig | 30d | |
zimbraMtaPostscreenPipeliningAction | Value for postconf postscreen_pipelining_action. | enum | server,globalConfig | enforce | ignore,enforce,drop |
zimbraMtaPostscreenPipeliningEnable | Value for postconf postscreen_pipelining_enable. | enum | server,globalConfig | no | yes,no |
zimbraMtaPostscreenPipeliningTTL | Value for postconf postscreen_pipelining_ttl. | string | server,globalConfig | 30d | |
zimbraMtaPostscreenWatchdogTimeout | Value for postconf postscreen_watchdog_timeout. | string | server,globalConfig | 10s | |
zimbraMtaPostscreenWhitelistInterfaces | Value for postconf postscreen_whitelist_interfaces. Single valued,,comma separated list. | string | server,globalConfig | static:all |
How to enable it
Zimbra Collaboration Postscreen comes enabled by default in ZCS 8.7 or above, take a look to the previous Table where find all the defaults values per each Postscreen attribute.
Quick Example configuring Postscreen
Each scenario can be different, so please tune the next values according to your own Environment, in this case all values are set at GlobalConfig level: This configuration is medium/high level, enforcing a few attributes instead of drop, change them to drop for higher level of security
zmprov mcf zimbraMtaPostscreenAccessList permit_mynetworks zmprov mcf zimbraMtaPostscreenBareNewlineAction ignore zmprov mcf zimbraMtaPostscreenBareNewlineEnable no zmprov mcf zimbraMtaPostscreenBareNewlineTTL 30d zmprov mcf zimbraMtaPostscreenBlacklistAction ignore zmprov mcf zimbraMtaPostscreenCacheCleanupInterval 12h zmprov mcf zimbraMtaPostscreenCacheRetentionTime 7d zmprov mcf zimbraMtaPostscreenCommandCountLimit 20 zmprov mcf zimbraMtaPostscreenDnsblAction enforce zmprov mcf zimbraMtaPostscreenDnsblSites 'b.barracudacentral.org=127.0.0.2*7' zimbraMtaPostscreenDnsblSites 'dnsbl.inps.de=127.0.0.2*7' zimbraMtaPostscreenDnsblSites 'zen.spamhaus.org=127.0.0.[10;11]*8' zimbraMtaPostscreenDnsblSites 'zen.spamhaus.org=127.0.0.[4..7]*6' zimbraMtaPostscreenDnsblSites 'zen.spamhaus.org=127.0.0.3*4' zimbraMtaPostscreenDnsblSites 'zen.spamhaus.org=127.0.0.2*3' zimbraMtaPostscreenDnsblSites 'list.dnswl.org=127.0.[0..255].0*-2' zimbraMtaPostscreenDnsblSites 'list.dnswl.org=127.0.[0..255].1*-3' zimbraMtaPostscreenDnsblSites 'list.dnswl.org=127.0.[0..255].2*-4' zimbraMtaPostscreenDnsblSites 'list.dnswl.org=127.0.[0..255].3*-5' zimbraMtaPostscreenDnsblSites 'bl.mailspike.net=127.0.0.2*5' zimbraMtaPostscreenDnsblSites 'bl.mailspike.net=127.0.0.[10;11;12]*4' zimbraMtaPostscreenDnsblSites 'wl.mailspike.net=127.0.0.[18;19;20]*-2' zimbraMtaPostscreenDnsblSites 'dnsbl.sorbs.net=127.0.0.10*8' zimbraMtaPostscreenDnsblSites 'dnsbl.sorbs.net=127.0.0.5*6' zimbraMtaPostscreenDnsblSites 'dnsbl.sorbs.net=127.0.0.7*3' zimbraMtaPostscreenDnsblSites 'dnsbl.sorbs.net=127.0.0.8*2' zimbraMtaPostscreenDnsblSites 'dnsbl.sorbs.net=127.0.0.6*2' zimbraMtaPostscreenDnsblSites 'dnsbl.sorbs.net=127.0.0.9*2' zmprov mcf zimbraMtaPostscreenDnsblTTL 5m zmprov mcf zimbraMtaPostscreenDnsblThreshold 8 zmprov mcf zimbraMtaPostscreenDnsblTimeout 10s zmprov mcf zimbraMtaPostscreenDnsblWhitelistThreshold 0 zmprov mcf zimbraMtaPostscreenGreetAction enforce zmprov mcf zimbraMtaPostscreenGreetTTL 1d zmprov mcf zimbraMtaPostscreenNonSmtpCommandAction drop zmprov mcf zimbraMtaPostscreenNonSmtpCommandEnable no zmprov mcf zimbraMtaPostscreenNonSmtpCommandTTL 30d zmprov mcf zimbraMtaPostscreenPipeliningAction enforce zmprov mcf zimbraMtaPostscreenPipeliningEnable no zmprov mcf zimbraMtaPostscreenPipeliningTTL 30d zmprov mcf zimbraMtaPostscreenWatchdogTimeout 10s zmprov mcf zimbraMtaPostscreenWhitelistInterfaces static:all
Quick Example configuring Postscreen (high security)
Each scenario can be different, so please tune the next values according to your own Environment, in this case all values are set at GlobalConfig level: This configuration is medium level, enforcing a few attributes instead of drop, change them to drop for higher level of security
zmprov mcf zimbraMtaPostscreenAccessList permit_mynetworks zmprov mcf zimbraMtaPostscreenBareNewlineAction drop zmprov mcf zimbraMtaPostscreenBareNewlineEnable yes zmprov mcf zimbraMtaPostscreenBareNewlineTTL 20d zmprov mcf zimbraMtaPostscreenBlacklistAction drop zmprov mcf zimbraMtaPostscreenCacheCleanupInterval 12h zmprov mcf zimbraMtaPostscreenCacheRetentionTime 10d zmprov mcf zimbraMtaPostscreenCommandCountLimit 15 zmprov mcf zimbraMtaPostscreenDnsblAction drop zmprov mcf zimbraMtaPostscreenDnsblSites "domaintoblock.com*2, domaintoblock.com*2" zmprov mcf zimbraMtaPostscreenDnsblThreshold 2 zmprov mcf zimbraMtaPostscreenDnsblTTL 2h zmprov mcf zimbraMtaPostscreenGreetAction drop zmprov mcf zimbraMtaPostscreenGreetTTL 5d zmprov mcf zimbraMtaPostscreenNonSmtpCommandAction drop zmprov mcf zimbraMtaPostscreenNonSmtpCommandEnable yes zmprov mcf zimbraMtaPostscreenNonSmtpCommandTTL 20d zmprov mcf zimbraMtaPostscreenPipeliningAction drop zmprov mcf zimbraMtaPostscreenPipeliningEnable yes zmprov mcf zimbraMtaPostscreenPipeliningTTL 20d zmprov mcf zimbraMtaPostscreenWatchdogTimeout 30s zmprov mcf zimbraMtaPostscreenWhitelistInterfaces "1.2.3.4 static:all"
Testing the Zimbra Collaboration Postscreen
Here is an example coming from GMail to Zimbra with a Postscreen set to medium (enforce), see the new postfix/postscreen checks
Mar 1 17:22:57 zimbra86 postfix/postscreen[12538]: CONNECT from [209.85.213.181]:33183 to [46.101.92.202]:25 Mar 1 17:23:03 zimbra86 postfix/tlsproxy[12682]: CONNECT from [209.85.213.181]:33183 Mar 1 17:23:03 zimbra86 postfix/tlsproxy[12682]: Anonymous TLS connection established from [209.85.213.181]:33183: TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits) Mar 1 17:23:03 zimbra86 postfix/postscreen[12538]: NOQUEUE: reject: RCPT from [209.85.213.181]:33183: 450 4.3.2 Service currently unavailable; from=<testzimbra@gmail.com>, to=<admin@example.com>, proto=ESMTP, helo=<mail-ig0-f181.google.com> Mar 1 17:23:03 zimbra86 postfix/tlsproxy[12682]: DISCONNECT [209.85.213.181]:33183 Mar 1 17:23:03 zimbra86 postfix/postscreen[12538]: HANGUP after 0.82 from [209.85.213.181]:33183 in tests after SMTP handshake Mar 1 17:23:03 zimbra86 postfix/postscreen[12538]: PASS NEW [209.85.213.181]:33183 Mar 1 17:23:03 zimbra86 postfix/postscreen[12538]: DISCONNECT [209.85.213.181]:33183
Additonal Content
- See the Official Postfix Postscreen page
- Rob0's Postscreen Configuration A non-official but real-world example
Identified Support Issues
- No Support issues reported yet.