Postwhite - Automatic Postcreen Whitelist & Blacklist Generator

A script for generating a Postscreen whitelist (and optionally a blacklist) based on large and presumably trustworthy senders' SPF records.

Why Postwhite?

Postwhite uses the published SPF records from domains of known webmailers, social networks, ecommerce providers, and compliant bulk senders to generate a list of outbound mailer IP addresses and CIDR ranges to create a whitelist (and optionally a blacklist) for Postfix's Postscreen.

This allows Postscreen to save time and resources by immediately handing off whitelisted connections from these hosts (which we can somewhat safely presume are properly configured) to Postfix's smtpd process for further action. Blacklisted hosts are rejected before they reach Postfix's smtpd process.

Note this does not whitelist (or blacklist) email messages from any of these hosts. A whitelist for Postscreen (which is merely the first line of Postfix's defense) merely allows listed hosts to connect to Postfix without further tests to prove they are properly configured and/or legitimate senders. A Postscreen blacklist does nothing but reject the connection based on the blacklisted host's IP.

If all of the whitelist mailers are selected when Postwhite runs, the resulting whitelist includes over 500 outbound mail servers, all of which have a very high probability of being properly configured.

Warning about Blacklisting

By default, Postwhite has blacklisting turned off. Most users will not need to ever turn it on, but it's there if you really believe you need it. If you choose to enable it, make sure you understand the implications of blacklisting IP addresses based on their hostnames and associated mailers, and re-run Postwhite often via cron to make sure you're not inadvertently blocking legitimate senders.

Requirements

Postwhite runs as a shell script (/bin/sh) and relies on two scripts from the <a target="_blank" href="https://github.com/jsarenik/spf-tools">SPF-Tools project (despf.sh and simplify.sh) to help recursively query SPF records. I recommend cloning or copying the entire SPF-Tools repo to /usr/local/bin/directory on your system, then confirming the spftoolspath value in postwhite.

Please update SPF-Tools whenever you update Postwhite, as both are under continuous development, and sometimes new features of Postwhite depend upon an updated version of SPF-Tools.

Postwhite also assumes that you have Postfix and the appropriate bind-utils package for your Linux distro installed on your system.

Usage

1. Make sure you have SPF-Tools on your system
2. Move the postwhite.conf file to your /etc/ directory
3. Add any custom hosts in postwhite.conf
4. Run ./postwhite from the command line.

You can optionally provide a configuration file via the command line which will override the default configuration file:

./postwhite /path/to/config-file

I recommend cloning both the SPF-Tools and the Postwhite repos into your /usr/local/bin/ directory. Once you're satisfied with its performance, set a daily cron job to pick up any new hosts in the mailers' SPF records like this:

@daily /usr/local/bin/postwhite/postwhite > /dev/null 2>&1 #Update Postscreen Whitelists

I also recommend updating the list of known Yahoo! IP outbound mailers weekly:

@weekly /usr/local/bin/postwhite/scrape_yahoo > /dev/null 2>&1 #Update Yahoo! IPs for Postscreen Whitelists

(Please read more about Yahoo! hosts below)

When executed, Postwhite will generate a file named postscreen_spf_whitelist.cidr, write it to your Postfix directory, then reload Postfix to pick up any changes.

Add the filename of your whitelist (and optionally your blacklist) to the postscreen_access_list option in your Postfix main.cf file, like this:

postscreen_access_list = permit_mynetworks,
...
        cidr:/etc/postfix/postscreen_spf_whitelist.cidr,
        cidr:/etc/postfix/postscreen_spf_blacklist.cidr,
...

IMPORTANT: If you choose to enable blacklisting, list the blacklist file after the whitelist file in main.cf, as shown above. If you misconfigure Postwhite and an IP address inadvertently finds its way onto both lists, the first entry "wins." Listing the whitelist file first in main.cf will assure that whitelisted hosts aren't blacklisted, even if they appear in the blacklist file.

Then do a manual postfix reload or re-run ./postwhite to build a fresh whitelist and automatically reload Postfix.

Options

Options for Postwhite are located in the postwhite.conf file. This file shoud be moved to your system's /etc/ directory before running Postwhite for the first time.

Custom Hosts

By default, Postwhite includes a number of well-known (and presumably trustworthy) mailers in five categories:

• Webmailers
• Ecommerce
• Social Networks
• Bulk Senders
• Miscellaneous

To add your own additional custom hosts, add them to the custom_hosts section of /etc/postwhite.conf separated by a single space:

custom_hosts="aol.com google.com microsoft.com"

Additional trusted mailers are added to the script from time to time, so check back periodically for new versions, or "Watch" this repo to receive update notifications.

Hosts that Don't Publish their Outbound Mailers via SPF Records

Because Postwhite relies on published SPF records to build its whitelist, mailers who refuse to publish outbound mailer IP addresses via SPF are problematic. The largest such host is Yahoo!, which is dealt with separately (see below). For smaller mailhosts without SPF-published mailer lists, the included query_host_ovh file is a working example of a script that queries a range of hostnames for a specific mailer (mail-out.ovh.net in the included example), collects valid IP addresses, and includes them in a custom whitelist. The new custom whitelist may then be included in as an additional entry in your Postfix's postscreen_access_list parameter (see Usage above). An example of the query_host_ovh file's output is included in the /examples/ folder as postscreen_ovh_whitelist.cidr.

To create additional customized query scripts for mailers that don't publish outbound IPs via SPF, copy the example query_host_ovh file to a new unique filename, edit the script's mailhost and numerical range values as required, set a unique output file (/etc/postfix/postscreen_*_whitelist.cidr), include the output file in Postfix's postscreen_access_list parameter, then configure cron to run the new query script periodically.

Depending on the size of the range you wish to query, this script could take a long time to complete. I recommend testing on a small fraction of the mailhost's range before pushing the script to a production environment.

Yahoo! Hosts

As mentioned in the Known Issues, Yahoo!'s SPF record doesn't support queries to expose their netblocks, and therefore a dynamic list of Yahoo mailers can't be built. However, Yahoo! does publish a list of outbound mailer IP addresses at https://help.yahoo.com/kb/SLN23997.html.

A list of Yahoo! outbound IP addresses, based on the linked knowledgebase article and formatted for Postwhite, is included as yahoo_static_hosts.txt. By default, the contents of this file are added to the final whitelist. To disable the Yahoo! IPs from being included in your whitelist, set the include_yahoo configuration option in /etc/postwhite.conf to include_yahoo="no".

The yahoo_static_hosts.txt file can be periodically updated by running the scrape_yahoo script, which requires either Wget or cURL (included on most systems). The scrape_yahoo script reads the Postwhite config file for the location to write the updated list of Yahoo! oubound IP addresses. Run the scrape_yahoo script periodically via cron (I recommend no more than weekly) to automatically update the list of Yahoo! IPs used by Postwhite.

Blacklisting

To enable blacklisting, set enable_blacklist=yes and then list blacklisted hosts in blacklist_hosts. Please refer to the blacklisting warning above. Blacklisting is not the primary purpose of Postwhite, and most users will never need to turn it on.

Simplify

By default, the option to simplify (remove) invididual IP addresses that are already included in CIDR ranges (handled by the SPT-Tools simplify.sh script) is set to no. Turning this feature on when building a whitelist for more than just a few mailers dramatically adds to the processing time required to run Postwhite. Feel free to turn it on to see how it affects the amount of time required to build your whitelist, but if you're whitelisting more than just 3 or 4 mailers, you'll probably want to turn it to "no" again. Having a handful of individual IP addresses in your whitelist that might be redundantly covered by CIDR ranges won't have any appreciable impact on Postscreen's performance.

Invalid hosts

You can also choose how to handle malformed or invalid CIDR ranges that appear in the mailers' SPF records (which happens more often than it should). The options are:

• remove - the default action, it removes the invalid CIDR range so it doesn't appear in the whitelist.
• keep - this keeps the invalid CIDR range in the whitelist. Postfix will log a warning about non-null host address bits, suggest the closest valid range with a matching prefix length, and harmlessly ignore the rule. Useful only if you want to see which mailers are less than careful about their SPF records (cough, cough, Microsoft, cough, cough).
• fix - this option will change the invalid CIDR to the closest valid range (the same one suggested by Postfix, in fact) and include the corrected CIDR range in the whitelist.

Other options in postwhite.conf include changing the filenames for your whitelist & blacklist, Postfix path, SPF-Tools path, and whether or not to automatically reload Postfix after you've generated a new list.

Credits

• Special thanks to Mike Miller for his 2013 gwhitelist script that initially got me tinkering with SPF-based Postscreen whitelists. The temp file creation and printf statement near the end of the Postwhite script are remnants of his original script.
• Thanks to Jan Sarenik (author of SPF-Tools).
• Thanks to Jose Borges Ferreira for patches and contributions to Postwhite, include internal code to validate CIDRs.
• Thanks to Ricardo Iván Vieitez Parra for contributions to Postwhite, including external config file support, normalization improvements, error handling, and additional modifications that allow Postwhite to run on additional systems.
• Thanks to partner (business... not life) Steve Cook for helping me cludge through Bash scripting, and for writing the initial version of the scrape_yahoo script.
• Thanks to all the generous contributors right here on GitHub who have helped move the project along!

More Info

My blog post discussing how Postwhite came to be is here:

http://www.stevejenkins.com/blog/2015/11/postscreen-whitelisting-smtp-outbound-ip-addresses-large-webmail-providers/

Known Issues

• I'd love to include Yahoo's IPs in the whitelist via the same methods used for all other mails, but their SPF record doesn't support queries to expose their netblocks. The included scrape_yahoo script, which creates a static list of Yahoo! IPs by scraping their web page, is an acceptable work-around, but if you have a suggestion for a more elegant solution, please create an issue and let me know, or create a pull request.

• I have no way of validating IPv6 CIDRs yet. For now, the script assumes all SPF-published IPv6 CIDRs are valid and includes them in the whitelist.

• I've improved the sorting by doing the uniq separately, after the sort. sort -u -V is still ideal, but it the -V option doesn't exist on all platforms (OSX doesn't support it, for example). For now, I can live with the two-step sort and uniq, even though the final output splits the IPv6 address into two grips: those that start with letters and numbers (2a00, 2a01, etc.) at the top, and those that start with numbers only (2001, 2004, etc.) at the bottom. All the IPv4 addresses in the middle are sorted properly. See the /testdata/ folder for examples of different sorting attempts or to play around with your own attempts at sorting. If you have any suggestions to improve the sorting without losing any data, I'm all ears!

Suggestions for Additional Mailers

If you're a Postfix admin who sees a good number of PASS OLD entries for Postscreen in your mail logs, and have a suggestion for an additional mail host that might be a good candidate to include in Postwhite, please comment on this issue: https://github.com/stevejenkins/postwhite/issues/2

Disclaimer

You are totally responsible for anything this script does to your system. Whether it launches a nice game of Tic Tac Toe or global thermonuclear war, you're on your own. :)