If you have been following my blogs over the years you should be aware that I’ve always been using Exchange Edge Transport servers in front of my Mailbox servers for message hygiene purposes. My last (well known) environment looked like this:
There are two Mailbox servers (Exchange 2013 and Exchange 2016) and two Edge Transport servers (also Exchange 2013 and Exchange 2016). MX records point to both Edge Transport servers and there are two Edge Synchronizations. And the Edge Transport servers were capable for DKIM signing (as posted in a previous blogpost), but lacked DKIM verification and DMARC validation.
The most important part in the Edge Transport server is the Real Time Blocklist, configured to use Spamhaus for connection filtering. While this works pretty well (there still is quite some spam that gets delivered into mailboxes) there is always room for improvement. I have been looking at cloud solution, but they didn’t always deliver what was expected.
A couple of my customers are using Cisco Email Security Appliance (previously known as IronPort) solutions on-premises and are happy with it, so time to start testing a Cisco Email Security Appliance (ESA) in my own environment.
Setting up the ESA
The ESA is available as a physical device, but also as a virtual device. Unfortunately (says the Microsoft consultant in me) only as a vmware VM, but since I also have a vmware server running that’s not a problem. Deploying the virtual machine is because of the OVF solution a piece of cake. Connect three NICs (management, internal and external) and you’re good. My Cisco contact told me it’s a common practice to use a one arm deployment, but I did a two arm deployment, resulting in a configuration like this:
The management network is (by default) 192.168.42.42, the internal network is configured to use my 10.38.96.0/24 network while the external network is configured with the 176.62.196.244 address. The FQDN of the device is smtphost.exchangelabs.nl, in DNS this points to the above mentioned IP address and I have PTR records configured for this IP address.
Afther the initial setup (where entering the license file via PUTTY in the CLI was the most challenging) you can continue with the System Settings in the GUI. The default domain is exchangelabs.nl, enter a reporting address and a time server as shown in the following screenshot:
After entering the system settings you can continue with the network configuration. Enter a default gateway (on the public interface), a public and internal IP address and enter a forwarding address for inbound messages (i.e. the Exchange 2013 and Exchange 2016 servers) as shown in the following screenshot:
The last step is to configure the message security options like enabling the Reputation filtering, IronPort anti-spam, Sophos anti-virus and outbreak filters as shown in the following screenshot:
After reviewing the configuration you’re good to go and once accepted the device configures itself. This took only a few minutes, and I immediately started receiving configuration messages from the ESA. Some of these made no sense, it looks like the device starts sending out messages before the device is fully configured, but at least it worked well.
The next step is to create a Send Connector in the Exchange configuration, using the ESA as a smarthost. All outbound mail is routed through the ESA, since this has a proper FQDN, IP address and PTR record other servers on the Internet will accept my email messages.
Adding additional domains
Besides my test domain I want my jaapwesselius.com and jaapwesselius.nl domains to use the ESA. First is to add the domains in the Recipients Address Table (RAT) so that the ESA will accept messages for this domain. The second (and last) step is to configure an SMTP route for these domains so the messages will be delivered on my Exchange servers. Note to self: use the commit changes button after entering the domain configuration data 🙂
When checking inbound email on my platform (using Remote Connectivity Analyzer) you can clearly see that mail is going through the ESA:
Message Hygiene
To understand message hygiene in the ESA we have to take a closer look at the mail flow in the ESA. This is called the Email pipeline and consists of three parts:
- Receipt – this is the initial step in receiving a message, i.e. accepting the connection, check for policies and validate the recipient
- Work queue – when connected the message enter the work queue where filtering, safelist/blocklist, anti-spam/anti-virus, outbreak filtering and quarantining takes place.
- Delivery – responsible for delivering the incoming message to the Exchange server.
One of the most important parts on any email security solution is connection filtering, you want to make sure that you only accept messages from trusted sources. Messages coming from sources known as spammers should not be accepted. On an Exchange Edge Transport server you can configure Realtime Block Lists (RBL) and I often use SpamHaus for this, as explained in my blogpost about configuring the Edge Transport Server.
The ESA is using Sender Reputation Filtering to achieve this. Sender Reputation Filtering is using SenderBase Reputation Service which is using the SenderBase affiliate network. The SenderBase Reputation Service assigns a SenderBase Reputation Score (SBRS) to email senders based on complaint rates, message volume statistics, and data from public blacklists and open proxy lists. The SenderBase Reputation Score helps to differentiate legitimate senders from spam sources. The SBRS can vary between -10 (most likely to be spam) to +10 (most likely to be legitimate).
After working with this for a couple of week now (in default configuration) I have to admit it is working extremely well. I haven’t received any spam messages so far, the only message I don’t want to receive are legitimate messages from primarily vendor sales departments.
TLS
By default, the ESA is configured with a self-signed certificate. This certificate is only used for accessing the ESA (for management purposes). To use TLS on this server you have to request (or import) a 3rd party SSL certificate.
After importing the SSL certificate you have to configure the listener and the mail flow policies on the Host Address Table (HAT). When configured you can use the http://checktls.com site to check if your site is TLS ready:
If you check the incoming TLS messages (the next day) you can see how many TLS encrypted messages are received. Personally I am a bit surprised that I still received 21 unencrypted messages yesterday (an average Tuesday).
Reporting
Reporting is always interesting, and the ESA sends out reports (in PDF) every day. One of these reports is about incoming messages. As can be seen in the following screenshot a lot of messages are blocked by the Sender Reputation Filtering:
In the same PDF you can see an overview of the sending (threat message) domains:
Outbound messages
The ESA can (should?) also be used for outbound messages. The mailflow in my environment is changed, the Edge Transport servers are no longer used. Instead, a Send Connector is created and the ESA is configured as a smart host on the Send Conector.
During installation, the 2nd network interface was configured to relay outbound messages, and the FQDN of outbound messages is configured to use smtphost.exchangelabs.nl so outbound messages appear to come from smtphost.exchangelabs.nl.
I have configured a reverse DNS entry, so when using http://misk.com/tools I can check if this is correct:
So, when a receiving SMTP host does a reverse lookup on my sending IP address they will see that there’s a match with my FQDN. So far I haven’t heard any complaints about messages not being delivered 🙂
Summary
The Exchange Edge Transport servers can be used for connection filtering and content filtering using keywords can be configured as well. Besides this the Edge Transport servers are not really an anti-spam solution. Most of the times I see Edge Transport servers they are used as a mail relay server sitting in the network’s DMZ.
For message hygiene there are several solutions, and the last couple of weeks I have been playing with the Cisco Email Security Appliance (running on Vmware). I must say, I am VERY IMPRESSED by the appliance. Installation was very easy to do, and using the ESA User Manual (consisting of 1216 pages 🙂 ) you can do a real deep dive into message hygiene.
At this moment I have an almost default configuration for inbound and outbound mailflow (including multiple domains and mailboxes) and I haven’t seen any spam since the installation.
In the upcoming blog posts I will go more into details about for example configuring anti-spam, DKIM signing, DMARC etc. using the Cisco Email Security Appliance.
Jaap Wesselius 