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Microsoft Outlook through Citrix Access Gateway SSL IP VPN

One of our customers wants you use their locally installed Microsoft Outlook through a Citrix Access Gateway (CAG). Sales people from that customer travel through the country and use the Outlook offline to read or prepare e-mail to send later. These people use UMTS technology to connect their laptops to the Internet. The customers wants these sales people to have the ability to use their Outlook offline and actually send/receive mail when connected to a network with Internet access.

The customer is using CAG’s to publish multiple services to the Internet, so together with my colleague Edwin Houben from DigiPulse, we started to look at a suitable solution. The CAG is located behind a CheckPoint firewall and traffic to the internal network needs to go through an ISA server firewall.

First we started to look at the ports Microsoft Outlook uses to connect to the Exchange server. Looking at the settings from a laptop, the connection is made by FQDN of the Exchange server. While performing a netstat -na we noticed that Outlook uses two ports to connect to the Exchange server.

PORT DESCRIPTION
TCP/135 EPMAP
TCP/1536 AMPR-INTER

The Outlook clients connects to the Exchange server on FQDN. So the laptop needs to have an IP connection to the Exchange server. So we decided to use the Citrix Secure Access Client to give the user the ability to establish an secure IP connection to the network.

Looking at the customers network, we had to configure access-lists on two locations to make the solution more secure. The first location is a Network Resource in the CAG. The Network Resource enables only the above ports to the Exchange server IP address. The second location is allowing the IP address of the CAG to connect to the Exchange server on the above port numbers through the ISA server.

After configuring both access-list, we did some testing and the solution works perfectly. You can now use the laptop on the internal network and externally with the Citrix Secure Access Client without making any changes in the Outlook configuration.

Later, the customer noticed that he couldn’t use Microsoft Outlook anymore in conjunction with the Citrix Secure Access Client. After digging a bit deeper in the traffic flow between Microsoft Outlook and the Exchange server, I noticed that, beside TCP/135, random ports above 1024 are used. So I changed the Network Resource  and the ISA servers to allow TCP/135 and the range TCP/1024-2000. I haven’t used the complete range of registered port numbers, so I hope Exchange doesn’t use a port above TCP/2000.

FUNNY ADD-ON

I didn’t some Googleing (or Googling or whatever) on TCP port 135 and I found some “funny” things:

Some well known Root kits also use this port to transmit data back to home base and download more malware. I also suspect may be an entry point for some root kit /malware for un patched systems or systems that did not patch correctly. Source 

Currently inbound scans are likely the Nachi or MSBlast worms. Source

The problem with port TCP 135 is that it is used for multiple services, which are listed below. So blocking port TCP 135 could affect communication between devices or the usage of services.

 

Client/Server Communication DCOM DHCP Manager
Exchange Administrator Microsoft Message Queue Server RPC User Manager
RPC Service Manager RPC Port Mapper SCM used by DCOM
SQL Session Mapper WINS Manager