Zimbra Collaboration Postscreen

Revision as of 23:28, 1 March 2016 by Jorge de la Cruz (talk | contribs) (Quick Example configuring Postscreen (high security))

Zimbra Collaboration Postscreen

   KB 22511        Last updated on 2016-03-1  




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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]

Postscreen-001.png

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-002.png

Postscreen workflow

See attached the workflow for Zimbra Collaboration Postscreen

Postscreen-003.png

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.

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 (medium 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 enforce
zmprov mcf zimbraMtaPostscreenBareNewlineEnable yes
zmprov mcf zimbraMtaPostscreenBareNewlineTTL 20d
zmprov mcf zimbraMtaPostscreenBlacklistAction enforce
zmprov mcf zimbraMtaPostscreenCacheCleanupInterval 12h
zmprov mcf zimbraMtaPostscreenCacheRetentionTime 10d 
zmprov mcf zimbraMtaPostscreenCommandCountLimit 15
zmprov mcf zimbraMtaPostscreenDnsblAction enforce
zmprov mcf zimbraMtaPostscreenDnsblSites "domaintoblock.com*5, domaintoblock.com*5"
zmprov mcf zimbraMtaPostscreenDnsblThreshold 5
zmprov mcf zimbraMtaPostscreenDnsblTTL 2h
zmprov mcf zimbraMtaPostscreenDnsblWhitelistThreshold 2
zmprov mcf zimbraMtaPostscreenGreetAction enforce
zmprov mcf zimbraMtaPostscreenGreetTTL 5d
zmprov mcf zimbraMtaPostscreenNonSmtpCommandAction enforce
zmprov mcf zimbraMtaPostscreenNonSmtpCommandEnable yes
zmprov mcf zimbraMtaPostscreenNonSmtpCommandTTL 20d
zmprov mcf zimbraMtaPostscreenPipeliningAction ignore
zmprov mcf zimbraMtaPostscreenPipeliningEnable yes
zmprov mcf zimbraMtaPostscreenPipeliningTTL 20d
zmprov mcf zimbraMtaPostscreenWatchdogTimeout 30s
zmprov mcf zimbraMtaPostscreenWhitelistInterfaces "1.2.3.4 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 zimbraMtaPostscreenDnsblWhitelistThreshold 5
zmprov mcf zimbraMtaPostscreenGreetAction drop
zmprov mcf zimbraMtaPostscreenGreetTTL 5d
zmprov mcf zimbraMtaPostscreenNonSmtpCommandAction drop
zmprov mcf zimbraMtaPostscreenNonSmtpCommandEnable yes
zmprov mcf zimbraMtaPostscreenNonSmtpCommandTTL 20d
zmprov mcf zimbraMtaPostscreenPipeliningAction ignore
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

Domain white/blacklist

Postscreen is really easy to configure, you can white or blacklist domains easily.

First you need to configure the DnsblThreshold to the score you want your Customers needs to reach before being rejected, for example we can set it to two:

zimbra@zimbra86:~$ zmprov mcf zimbraMtaPostscreenDnsblThreshold 2

Now that the DnsblThreshold is set up to two, we will add the domain we want to block, giving score 2 which means Postscreen will reject it, even before talk to our Zimbra MTA SMTP

zimbra@zimbra86:~$ zmprov mcf zimbraMtaPostscreenDnsblSites "gmail.com*2"

Restart the MTA services

zimbra@zimbra86:~$ zmmtactl restart
Rewriting configuration files...done.
Stopping saslauthd...done.
Starting saslauthd...done.
/postfix-script: refreshing the Postfix mail system

And then test to send an email from GMail to our Zimbra with Postscreen:

Mar  1 18:03:26 zimbra86 postfix/postscreen[6364]: CONNECT from [209.85.213.177]:34931 to [46.101.92.202]:25
Mar  1 18:03:33 zimbra86 postfix/tlsproxy[6374]: CONNECT from [209.85.213.177]:34931
Mar  1 18:03:33 zimbra86 postfix/tlsproxy[6374]: Anonymous TLS connection established from [209.85.213.177]:34931: TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)
Mar  1 18:03:33 zimbra86 postfix/postscreen[6364]: NOQUEUE: reject: RCPT from [209.85.213.177]:34931: 450 4.3.2 Service currently unavailable; from=<testzimbra@gmail.com>, to=<admin@example.com>, proto=ESMTP, helo=<mail-ig0-f177.google.com>
Mar  1 18:03:33 zimbra86 postfix/postscreen[6364]: HANGUP after 0.82 from [209.85.213.177]:34931 in tests after SMTP handshake
Mar  1 18:03:33 zimbra86 postfix/postscreen[6364]: PASS NEW [209.85.213.177]:34931
Mar  1 18:03:33 zimbra86 postfix/tlsproxy[6374]: DISCONNECT [209.85.213.177]:34931
Mar  1 18:03:33 zimbra86 postfix/postscreen[6364]: DISCONNECT [209.85.213.177]:34931

To whitelist a domain just use a negative number, for example to block gmail.com but allow example.net:

zimbra@zimbra86:~$ zmprov mcf zimbraMtaPostscreenDnsblSites "gmail.com*2, example.net*-1"

Restart the MTA services

zimbra@zimbra86:~$ zmmtactl restart
Rewriting configuration files...done.
Stopping saslauthd...done.
Starting saslauthd...done.
/postfix-script: refreshing the Postfix mail system

Additonal Content

Identified Support Issues

  • No Support issues reported yet.
Verified Against: Zimbra Collaboration Suite 8.7 Date Created: 01/03/2016
Article ID: https://wiki.zimbra.com/index.php?title=Zimbra_Collaboration_Postscreen Date Modified: 2016-03-01



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Wiki/KB reviewed by SME1 SME2 Copyeditor Last edit by Jorge de la Cruz
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