Virtual host and DNS names enumeration techniques

Posted on January 24, 2009 in Research • 3 min read

Table of Contents

1. Why you need to enumerate
2. Techniques
2.1 DNS enumeration techniques
2.2 Banner grabbing
2.3 SSL/TLS Protocol enumeration techniques
2.4 HTTP Protocol enumeration techniques
2.5 Passive web enumeration techniques
2.6 Active web enumeration techniques

1. Why you need to enumerate

The host name discovery phase is an information gathering act to get a complete and detailed view of target resources and attack points.

During an attack or a penetration test, the attacker needs to known  as much information as possible about the entry points to attack. An entry point can be identified with an IP address, a service port, and some application level information, like the virtual host name in the case of a web server hosting several sites.

2. Techniques

There are several techniques that can be used to discover host names and virtual hosts associated with a IP address.

Some techniques described here are implemented (and the others will be implemented soon) in hostmap, a tool that I wrote to discover virtual hosts and DNS names of a given IP address. As of today, the tool is private (it does not depend on me) but I hope to release it to the public domain soon.

2.1 DNS enumeration techniques

The following enumeration techniques are based on the DNS protocol and are:

  • Reverse DNS lookup: Performs a PTR request to get the host name from IP address.
  • Name servers record lookup: Get the authoritative name server for the target host.
  • Mail exchange record lookup: Get the MX records for the target host domain.
  • DNS AXFR zone transfer: The name server that serve the target machine’s domain zone can be prone to a zone transfer attack. This allows an attacker to perform an AXFR DNS request to retrieve all of the DNS records served.
  • Host name brute forcing: Using a brute-forcing technique to guess a host name on the enumerated domain that resolve as the target ip address.

2.3 SSL/TLS Protocol enumeration techniques

The following enumeration techniques are based on the SSL/TLS protocol and is:

  • X.509 Certificate: Often the target machine exposes an HTTP over SSL service. A connection is tried to the common HTTP service ports and is tried to negotiate an SSL/TLS connection, if the remote server supply a X.509 certificate the host name is taken from the Common Name (CN) field.

2.4 HTTP Protocol enumeration techniques

  • Virtual host brute-forcing: The web server can be brute-forced to guess a website served by the target host.
  • Following redirects: It is possible to guess another website served by the target host following redirects (HTTP code 301 and 302).
  • With error pages: If you try to get an error page (code 500) sometimes you can get an error page showing a banner with the host name.

2.5 Passive web enumeration techniques

The following enumeration techniques are based on third party web sites and are:

  • Search engines: The following search engines can be used and queried using the target IP address:
  • GPG/PGP key databases: The following public databases can be used:
  • DNS/WHOIS databases: Public whois information databases like RIPE, or DNS snapshot database can be used to passively enumerate host name and track his history.

The following is a partial list of public databases that can be used:

2.6 Active web enumeration techniques

  • Crawling: All published websites can be crawled for links to other sites and checked (if they resolve as the target IP address) to get other sites hosted on the target. This technique is very time consuming.

UPDATE: hostmap is a free, automatic, hostnames and virtual hosts discovery tool written in Python. hostmap has been released in may and you can get it at