Le Blog Utux

In Azure, the Application Gateway is basically a reverse proxy, passing traffic to some backends. While testing the SSL certificate of a listener with ssllabs, I have been surprised by the "HTTP server signature" field: Nexus/2.14.10-01. Indeed, the backend was a Nexus service, but I was expecting to see the server signature of the Application Gateway (which is Nginx), or nothing, but not the backend.

Turns out the HTTP response contains the following header:

< HTTP/1.1 200 OK
< Server: Nexus/2.14.10-01
[...]

Many sysadmins and organizations consider this as a security issue, giving attackers information about the server. While I do not totally agree with this statement (Obfuscation), I often disable this information. This is simple with Nginx and Apache, but what about Azure Application Gateway?

Application Gateway -> Your application gateway -> Rewrites -> + Rewrite set

• Name: give a name to your rewrite set. Ie: DisableServerTokens.
• Associated routing rules: Select the rule associated to your backend(s)

• Rewrite rule name: give a name to your rewrite rule name. Ie: DisableServerHeader.
• Action type: Delete
• Header type: Response
• Header name: Common header
• Common header: Server

Then Save your modifications. This should remove the "Server" HTTP header.

It took me some time to figure out how to use Azure tags as a filter for Ansible. Well it's not that hard, azure_rm.py will automatically generate groups based on tags with this pattern:

• Tag: role:webserver
• Becomes group: role_webserver

Yes, that's the trick, ":" becomes "_". Let's say you want to run a playbook only on machines tagged with role:webserver, just use the following command:

$ ansible-playbook -i azure_rm.py playbook.yml --limit role_webserver