Blocking IM

[Ref: Skype Access Controls]

Table of Contents:

  • Domain Name
  • Browser Identification
  • CONNECT and numeric IP Addresses

My primary purpose, is to block general access to Skype and Instant Messaging, because they are out of context for our organisation provisioned services. Came across the WWW howtos, and did something slightly differently (aggregating what knowledge is out there) so I’m recording it here so I can readily get at the information for other squid installs I deploy.

Skype, and other Instant Mesaging (IM) clients, make it their business to circumvent Proxies so they can reach as many clients as possible (even if you don’t want those clients connecting through the Internet.) Some of the methods used by IMs you can block using Squid, providing you have broad controls of how your internal users gain access to the Internet.

If all Internet access is proxied by one service or another, then you are in a good position for successfully blocking IMs.

There are generally 3 different ways that you can identify Instant Message clients.

  • The Domain Name’s they are trying to contact
  • The Browser Id they use
  • The method of connection

Domain Name

Most clients will initially attempt to connect to their parent/central host by the hosts’ domain name. Such as:


With the above case, we can begin the 1st layer of blocking IMs by blocking access to the above domain names:

File Snippet: /etc/squid/squid.conf

acl block_im dstdom -i "/etc/squid/block_im.txt"

http_access deny block_im

File snippet: /etc/squid/block_im.txt


Now, as we’re using regular expressions, we are going to be explicit with our domain names so as not to inadvertently block legitimate sites that may have the same ending domain name.

Use “^” to specify DOM begins at this point, and explicitly specify a “.” by using “.” because “.” by itself generally means “any character”.

Wrapping the search criteria in brackets “(” regexp “|” regexp “)” simplifies the line.

Browser Identification

Not as useful, because remember as mentioned above the IM providers don’t want to be blocked, and have no obligation to make life easier for you by telling us who they are.

acl Skype_UA browser ^skype

http_access deny Skype_UA

CONNECT and Numeric IPs

[Ref: Regular expression that matches valid IPv6 addresses]

An avoidance method used by IM clients, and malware sites, is to:

  • use HTTPS to prevent casual viewing of their data
  • use the HTTPS CONNECT method to provide a real-time communications mechanism.
  • use Numeric IP addresses, to bypass standardfilters

In practise, port 443 is the “accepted” standard for HTTPS traffic, so our first inclination is to not allow HTTPS traffic on any other port.

The next step, as per above, is to identify connections to Numeric IP Addresses, and explicitly deny use of the HTTPS CONNECT METHOD for these addresses.

acl numeric_IPs dstdom_regex ^((25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9])\.){3,3}(25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9])|){1,4}:((25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9]).){3,3}(25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9]))$


File extract: /etc/squid/squid.conf

acl block_im dstdom -i "/etc/squid/block_im.txt"
acl allow_skypeusers src "/etc/squid/allow_skypeusers.txt"
acl numeric_IPs dstdom_regex ^((25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9])\.){3,3}(25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9])|){1,4}:((25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9]).){3,3}(25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9]))$
acl Skype_UA browser ^skype

http_access deny CONNECT !Safe_ports
http_access deny CONNECT !SSL_ports
http_access deny CONNECT numeric_IPs !allow_skypeusers
http_access deny Skype_UA !allow_skypeusers
http_access deny CONNECT block_im
http_access deny block_im