Interop: Exchange Addresses to Postfix Server

(Note this was originally published 2010-04-05 and has not been updated)

Pulling your Exchange valid addresses to a Postfix server

WARNING: Make sure you understand the security consequences of creating a passwordless/passphraseless account on your system. Appropriate security measures should be taken to prevent systems from being compromised.

You’ll need to decide where to set up this function. You can do it directly on the receiving Postfix server, but since it’s probably in a DMZ, that would mean allowing access from your DMZ back into your protected LAN with AD. It’s generally a bad idea to open access from the DMZ to the LAN (obviously, there are times where this is unavoidable). Instead, what I recommend is you have a system on your LAN that will push the information up to the DMZ host. This is more secure, since the connection is initiated from your LAN.

The general method is as follows:

  1. Management box polls AD servers for addresses and builds a file
  2. Management box then sends the file to Postfix server
  3. Management box initiates remote update of the Postfix server mapping

What you need on the Management box: You’ll need a working install of Perl, with the Net::LDAP modules installed. You’ll also need this script, which polls the LDAP information and builds the file for sending to Postfix.

First off, grab that script. You’ll need to edit the settings at the beginning to fit your AD domain. You’ll need an AD account that can read AD information. DO NOT USE A DOMAIN ADMIN ACCOUNT! You should use an account with privileges to only access the domain. The password for the account will be in the file as plain-text, so choose accordingly. You can also change the name and location of the resultant file the script creates; I’ve left in my default of /scripts/exchange_recipients

Once the script is appropriately modified, upload it to the management box and place it wherever you like to keep scripts. I generally make a /scripts directory and put it there. I’ll assume for the remainder of the tutorial you’ve done the same. You’ll need to make sure it’s executable as well, so run ‘chmod 664 /scripts/‘ on it.

Now, let’s create our automation account. First on the management server, add a new user, with a blank password. For the guide, lets refer to this account as autoscript. Once the account is created, you’ll need to change to it ‘su autoscript‘ and create an SSH key for it. First change to the home directory ‘cd ~‘ and create a new directory, ‘mkdir .ssh‘. Set permissions: ‘chmod 700 .ssh‘. Change to: ‘cd .ssh‘. Create an RSA SSH keypair by running ‘ssh-keygen -t rsa‘. Answer the questions, leaving the passphrase blank. You should have two files in your .ssh directory now, one is the private key (id_rsa by default) and the other is the public key ( by default).

Now, on the Postfix server, you’ll need to create the same account and .ssh directory. Once you’ve done this, copy the file from the Management server, to the Postfix server. Then, from wherever you have stored the public key, you’ll want to run the following commands:

  1. cat >> /home/autoscript/.ssh/authorized_keys
  2. cat >> /home/autoscript/.ssh/authorized_keys2
  3. chmod 664 /home/autoscript/.ssh/authorized_keys
  4. chmod 664 /home/autoscript/.ssh/authorized_keys2

While we are on the Postfix server, we need to do some other setup. First off, lets make it so that the autoscript account can run certain commands as root without providing a password. To do this, we’ll need to edit the sudo control file: ‘visudo‘. Add the following line at the bottom of the file: ‘%autoscript ALL=NOPASSWD: /usr/sbin/postmap‘. This allows autoscript to run postmapwithout providing a password.

We also need to set up some files in the postfix directory to receive our Exchange address file. Do the following (assuming postfix is in /etc/postfix):

  1. cd /etc/postfix
  2. touch exchange_recipients (assuming default name of file)
  3. touch exchange_recipients.db
  4. chown root:autoscript exchange_recipients
  5. chown root:autoscript exchange_recipients.db
  6. chmod 664 exchange_recipients
  7. chmod 664 exchange_recipients.db

This precreates the files that will be updated by our script with the proper permissions. If you try and write these files remotely, it will fail because autoscript doesn’t have privileges to create files in the /etc/postfix directory.

Almost done!

TEST THE SCRIPT!– Make sure the script is creating a valid file and placing it where you want it. Once you’ve verified this works, continue on.

On the management server, you’ll need to create a script to run and send the results to the Postfix server. Lets do that now:

vi /scripts/

Copy in the following code, updating $REMOTESERVER for the IP or hostname of the Postfix server (I recommend IP for this):


#Transfer and Update Script for Postfix
#Requires and exchange_recipients file to be created and accessible by autoscript user
#Remote server needs to be prepared before update script will work: remote exchange_recipients and .db file need
#to be writeable by autoscript user

scp /scripts/exchange_recipients autoscript@$REMOTESERVER:/etc/postfix/exchange_recipients
ssh autoscript@$REMOTESERVER sudo postmap/etc/postfix/exchange_recipients


Now you’ll need to set permissions and ownership of the script files so they can be executed.

  1. chown autoscript:autoscript /scripts/
  2. chmod 755 /scripts/‘.
  3. chown root:autoscript /scripts/
  4. chown autoscript:autoscript /scripts/exchange_recipients
  5. chmod 774 /scripts/exchange_recipients

That should set ownership and permissions correctly. Switch to the autoscript user and run ‘/scripts/‘. Look for any errors. If it asks you for a password, make sure you didn’t misspell either of the authorized_keys files on either server. Otherwise, you should just see some quick text about copying the file.

Two things left: Set the Postfix server to use this file, and add this job to the cron to run automatically.

On the Postfix server, edit and add another or edit your relay_recipient_maps to use the new file like this: relay_recipient_maps = /etc/postfix/exchange_recipients. Make sure to reload Postfix to use the new configuration.

On the management server, you’ll need to edit the cron for the autoscript user (NOT root’s cron!). Do this by changing the the autoscript user and running ‘crontab -e‘. Then add in the following line to the crontab: ‘0 * * * * /scripts/‘ This runs the script at the start of every hour, every day. You can adjust the timing to fit your needs.

Congratulations, you’re done!

(System commands based on Ubuntu 8.04. Thanks to Chris Covington for writing the original script.)

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