Processing Passive DNS With the nmsg C API

Introduction

At Farsight, we work with a large amount of Passive DNS data, and we’re always looking for more. While this data powers our flagship DNSDB, we also broadcast the raw Passive DNS data on the Security Information Exchange (SIE) (as channel 202) for those interested in an unfiltered real-time view of worldwide DNS activity.

In this article, we present a simple example program showing how to filtersome potentially interesting information from our raw Passive DNS feed, usingthe Farsight Security-maintained open source C APIs

nmsg

and

wdns

. If youhaven’t already familiarized yourself with the NMSG C API, please readthis articlefor an introduction to the basics.

A Motivating Example

The DNSSEC suite of standards allows domain owners to sign their DNSinformation, authoritative DNS servers to serve up this signature information,and DNS clients to verify the signatures to prevent DNS forgery. Thesestandards require DNSSEC transactions to use a standardized DNS extensionmechanism (

EDNS0

). Because Farsight’sraw passive DNS information includes complete queries and responses, we candirectly observe

EDNS0

use.

We can also directly observe

EDNS0

non-compliance, and thus find some nameservers which are not yet ready for DNSSEC, and the hostnames for which theyare authoritative. We present an example program to extract this information:

Code Part 1, Basic Plumbing and Filtering

The full code for the program, dnsqr_filter, can be found onFarsight Security’s blog-code GitHub page.

main.c

is a basic skeleton for filtering the raw passive DNS NMSG payloadsavailable on SIE. It provides a command-line interface to a basic

nmsg

I/O loop supporting a subset of the

nmsg

sources and destinations supportedby nmsgtool, and evaluates

nmsg

payloads with a filter function

dnsqr_filter()

to select which packets arewritten to the output file.

The filter for this sample program resides in

edns_filter.c

.It looks for Passive DNS messages containing non-

EDNS0

responses to

ENDS0

queries, and tells the main loop to output these messages.

Code Part 2, Digging In

The filter code starts by declaring variables to hold return codes, parsed DNSmessages, pointers to

nmsg

data, and the result of our filtering.

int
dnsqr_filter(nmsg_message_t msg) {
nmsg_res nres;
wdns_res wres;

wdns_message_t response = {0}, query = {0};
uint8_t *data;
size_t len;
int result = 0;


It then fetches the DNS response from the

base

:

dnsqr

encoded NMSG,returning if no response is present.

nres = nmsg_message_get_field(msg, "response", 0, (void **)&data, &len);
if (nres != nmsg_res_success) return result;


Next, we parse the DNS response using the low-level C library

wdns

:

wres = wdns_parse_message(&response, data, len);
if (wres != wdns_res_success) return result;


If the response did not use

EDNS0

,

response.edns.present

will be false. Ifthis is the case, we check if the request used

EDNS0

, and tell the main loopto save this message if it did:

if (!dns.edns.present) {
nres = nmsg_message_get_field(msg, "query", 0,
(void **)&data, &len);
if (nres == nmsg_res_success) {
wres = wdns_parse_message(&query, data, len);
if (wres == wdns_res_success)
result = query.edns.present;
}
}


Finally, we clean up our parsed

wdns

messages and return our result.

wdns_clear_message(&query);
wdns_clear_message(&response);
return result;
}


Building and Running

Once you’ve downloaded the code, and made sure you’ve installed the

nmsg

and

wdns

libraries, building and running should be as simple as:

$ make
cc -c main.c
cc -c edns_filter.c
cc -o dnsqr_edns_filter main.o edns_filter.o -lnmsg -lwdns
$ dnsqr_edns_filter -C ch202 -w edns_failures.nmsg
... wait a few seconds ...
^C


And you should have a file full of non-

EDNS0

DNS activity, which you caninspect withnmsgtool,manipulate using

python

scripts and the python NMSG API, or crunch with more elaborate C programs if you’re so inclined.

Running Outside of SIE

If you are not on SIE, you can run this code on your own recursive DNS serversby using

nmsgtool

to gather passive DNS information. You first need totell

nmsgtool

the query source IP addresses or subnets your recursive DNSserver will be using:

$ export DNSQR_RES_ADDRS="192.168.0.1, 172.16.1.0/27"


Then run

dnsqr_edns_filter

listening on a socket, andfeed it with

nmsgtool

:

$ dnsqr_edns_filter -l 127.0.0.1/5353 -w edns_failures.nmsg &
$ nmsgtool -i (interface) -V base -T dnsqr -s 127.0.0.1/5353


or you can use

nmsgtool

to save a batch of

base

:

dnsqr

messages for later filtering:

$ nmsgtool -i (interface) -V base -T dnsqr -c (count) -w (file)
$ dnsqr_edns_filter -r (file) -w (somewhat-more-interesting-file)


Of course, if you have this sort of interesting data to work with,you should consider e-mailing [email protected] to get your data in front of even more security-mindedpeople!

Chris Mikkelson is a Senior Distributed Systems Engineer for Farsight Security, Inc.