Tuesday, June 12, 2007


Probably the single best thing I like about Sguil is the development philosophy. Most of the features are suggested by working intrusion analysts. Just the other day, we were chatting on IRC (freenode's #snort-gui) about DNS lookups. Specifically, I asked about having the Sguil client modified to give the analyst more control of DNS lookups. I am always concerned that when I ask the GUI to resolve an IP for me, I might be tipping off an attacker who is monitoring the DNS server for their zone. I mentioned that it'd be cool to have the ability to look up internal IPs in my own DNS servers but forward all external IPs to some third party (e.g., OpenDNS) in order to mask the true source of the queries.

Based on some of the discussion, I decided it'd be very handy to have a lightweight DNS proxy server that could use a few simple rules to determine which servers to query. I had some time this afternoon, so I hacked one up in Perl. It turns out that the same feature has now made it into the CVS version of Sguil, but I thought that my proxy would still be of use for other purposes. So without further ado, I hereby post it for your downloading pleasure.

You'll need to make a few edits to the script to tailor it to your own environment. First, set up the @LOCAL_NETS list to contain a list of regexps for your local network. All incoming queries will be compared against these expressions, and any that match will be considered "local" addresses. Queries that don't match anything in the list will be considered "public" queries. Note that reverse lookups require the "in-addr.arpa" address format. Here's a sample:

@LOCAL_NETS = ( "\.168\.192\.in-addr\.arpa",
"\.mydomain\.com" );

You'll also need to set the $LOCAL_DNS and $PUBLIC_DNS strings to space delimited lists of DNS servers to use for each type of query, like so:

$LOCAL_DNS = qw(""); # My DNS Servers
$PUBLIC_DNS = qw(""); # OpenDNS Servers

Once you've done this, just run the script (probably in the background) and it will start listening for DNS requests on port 53 UDP & TCP. Then configure your system's /etc/resolve.conf to use the local proxy and you're all set.

If you want to open it up a little and provide DNS proxy service for other systems on your network, you can do this by changing the LocalAddr parameter in the call to Nameserver->new(). Set the IP to your system's actual address and anyone who can connect to your port 53 will be able to resolve through you. I haven't really done any security testing on that, so it probably has some nasty remotely exploitable bugs. You have been warned!

Anyway, I hope you find it useful. It only does straight forward and reverse lookups (A and PTR records), so it will fail if you ask for any other types of requests. If these things are really necessary, though, let me know and I'll consider adding them. If you decide to use this at your site, please drop me a comment to let me know (if you can).

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