Subdomain Enumeration

As with most things related to pen-testing, there are many different ways to enumerate the subdomains of your target. One promising tool I’ve been playing with recently is recon-ng. I won’t be at all surprised if recon-ng becomes as popular for the reconnaissance phase of a pentest as metasploit has become for the exploit phase. Today, though, I want to talk about a fun method I used a few weeks ago to find out more about the subdomains of my target. But first, here are some methods you can use completely passively

Subdomains - Passively

There are a few great resources for enumerating the subdomains of sites that can be crawled by search engines. Here’s how I would look a domain up using Google:

site:example.com

On a recent assessment, I was kinda surprised to discover that Wolfram Alpha had a very useful subdomain list in their site results. In the “web statistics” box, there’s a button labeled “subdomains.” This returned a very accurate list of subdomains for my target, and I’ve since taken to using wolfram to aid in site enumeration.

Recon-Ng

You can view Tim Tome’s DerbyCon 2013 talk on using it here - this is a fantastic talk highly worth a watch. Hell, even if you don’t care about pen-testing, it’s a fantastic talk to see just how much useful information is out there, (almost) freely available to those who seek it.

I start with recon-ng by setting my workspace to whatever my current domain target is, just to keep things sane.

recon-ng > set workspace example.com

Next, I use the five major search engine site modules to look up subdomains for the domain I am interested in.

recon-ng > use api/google_site
recon-ng [google_site] > set domain example.com
DOMAIN > example.com
recon-ng [google_site] > run
[*] Searching Google API for: site:example.com
...

Repeat the usage of different site modules (google_site, baidu_site, bing_site, google_site, yahoo_site) and all of the results will be funneled into your hosts table. You can see all of the hosts discovered by typing

recon-ng > show hosts

The Fun Way

This method was used to verify and supplement the results obtained via the other methods I’ve outlined above. Remember, being thorough pays, and it never hurts to try and have the most information about your target as possible. The basic outline of this method is:

  1. Use wget on the main page of the target’s domain (usually www.example.com)
  2. Parse the results for all subdomains linked to

Wget

Grab the site’s main page:

wget example.com

This will return index.html.

Parse index.html

We’ll use a combination of grep, cut, and sort to pull out subdomains returned from index.html.

grep "href=" index.html | cut -d"/" -f3 | grep "\." | cut -d'"' -f1 | sort -u

This series of commands will look for all links in index.html by looking for all instances of “href=”, trims up the link so we can easily use it in another command or save it to a text file containing many links. Here’s a visual aid:

  1. grep "href=" index.html returns <a href="http://blog.example.com" >
  2. <a href="http://blog.example.com" /> passed to cut -d"/" -f3 returns blog.example.com"
  3. blog.example.com" passed to cut -d'"' returns blog.example.com

Now, if this command was run on an index.html file containing many links, it would put them all in order, one per line, for easy scriptability. If saved to list.txt, we can easily pull out the subdomains like so to get IP addresses.:

for url in $(cat list.txt); do host $url; done | grep "has address" | cut -d" " -f4 | sort -u

These results can be saved to a file, and you have a list of subdomains with IPs for each.

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