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/* dnslogger.c
* By Ron Bowes
* Created January, 2010
* (See LICENSE.txt)
* [[dnslogger]] has two primary functions:
* # Print all received DNS requests
* # Reply to them with an error or a static ip address (IPv4 or IPv6)
* This is obviously very simple, but is also powerful.
* -h --help
* Help (this page)
* --test <domain>
* Test to see if we are the authoritative nameserver for the given domain.
* -s --source <address>
* The local address to bind to. Default: any (
* -p --port <port>
* The local port to listen on. I don't recommend changing this.
* default: 53
* -A <address>
* The A record to return when a record is requested. Default: NXDOMAIN.
* --AAAA <address>
* The AAAA record to return when a record is requested. Default: NXDOMAIN.
* --TTL <time>
* The time-to-live value to send back, in seconds. Default: 1 second.
* -u --username
* Drop privileges to this user after opening socket (default: 'nobody')
* -V --version
* Print the version and exit
*==Printing requests==
* Printing DNS requests has a lot of uses. Essentially, it'll tell you if
* a program tried to connect to your site, without the program ever
* attempting the connection. There are a great number of possible uses for
* that:
* * Finding open proxies without making an actual connection through it
* * Finding open mail relays without sending an email through it
* * Finding errors in mail-handling code on a site
* * Finding shell injection on a Web application without outbound traffic or delays
* * Checking if a user visited a certain page
* In every one of those cases, the server will try to look up the domain name
* to perform some action, and fails. For example, to find an open proxy you
* can connect to the potential proxy and send it "CONNECT <yourdomain>". If
* the proxy server is indeed open, it'll do a lookup on <yourdomain> and
* you'll see the request. Then, by default, an error is returned, so the proxy
* server gives up on attempting the connection and it's never logged. That's
* really the key -- the connection attempt never gets logged.
* Likewise, shell injection. If you're testing an application for shell
* injection, you can send it the payload 'ping <yourdomain>' to run. a
* vulnerable server will attempt to ping the domain and perform a DNS lookup.
* By default, the DNS lookup will fail, and the server won't perform the ping.
* It'll look like this:
* $ ping
* ping: unknown host
* dnslogger, however, will have seen the request and we therefore know that
* the application is vulnerable. This is far more reliable than the classic
* 'ping localhost 4 times and see if it takes 3 seconds' approach to finding
* shell injection.
* One final note is discovering Web applications that handle email incorrectly.
* A classic vulnerability when sending email, besides shell injection, is
* letting the user terminate the email with a "." on its own line, then start
* a new email. Something like this:
* This is my email, hello!
* .
* mail from:
* rcpt to: test@<yourdomain>
* data
* This email won't get sent!
* So the first email was terminated on the second line, with a period. A new
* email is composed to test@<yourdomain>. If the application is vulnerable
* to this type of attack, it will attempt to look up <yourdomain> so it can
* send an email there. We'll see the request, respond with an error, and the
* request will never be sent.
*==Controlling the response==
* In addition to logging requests, dnslogger can also respond with arbitrary
* A or AAAA records to any incoming request. A long time ago, at work, I used
* a Visual Basic program I found somewhere called "FakeDNS" that accomplished
* a similar task, but I've since lost it and decided to implement it myself.
* Some potential uses of this program are:
* * Investigating malware that connects to a remote host
* * Redirecting users if you control their DNS server
* * Redirecting a legitimate program to your own server
* The first use is actually the one I created this for -- investigating
* malware. One of the most common types of malware I'm asked to investigate
* at work is a classic downloader, which reaches out to the Internet and
* downloads its payload. Almost always, it uses a DNS server to find the
* malware. By setting the system's dns to the dnslogger DNS server, all DNS
* lookups will be seen (for later investigation), and you can control which
* server it tries to connect to to download the files.
* Another potential use, and somewhat malicious, is, if you control the DHCP
* server on a victim's computer, you can point their DNS to a malicious host,
* perhaps one running a password-stealer or Metasploit payload, and do what
* you want.
* One final use, which takes me back to the old days of programming,
* is redirecting a legitimate program with a hardcoded domain. For example,
* used to default to,, etc.
* Although you could change these servers in the registry, another option is
* to point your system DNS to dnslogger and let it redirect the requests for
* you.
*==Authoritative DNS server==
* Many functions of this tool require you to be the authoritative nameserver
* for a domain. This typically costs money, but is fairly cheap and has a lot
* of benefits. If you aren't sure whether or not you're the authority, you
* can use the --test argument to this program, or you can directly run the
* [[dnstest]] program, also included.
#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <time.h>
#ifdef WIN32
#include <winsock2.h>
#include <unistd.h>
#include "buffer.h"
#include "dns.h"
#include "memory.h"
#include "my_getopt.h"
#include "select_group.h"
#include "types.h"
#include "udp.h"
#define NAME "dnslogger"
typedef struct
int server_socket;
select_group_t *select_group;
char *user;
char *source;
int port;
char *A;
#ifndef WIN32
char *AAAA;
int TTL;
} settings_t;
/* We need this for catching signals. */
settings_t *global_settings = NULL;
static SELECT_RESPONSE_t dns_callback(void *group, int socket, uint8_t *packet, size_t packet_length, char *addr, uint16_t port, void *s)
settings_t *settings = (settings_t*) s;
dns_t *response;
uint8_t *response_packet;
uint32_t response_packet_length;
/* Parse the DNS packet. */
dns_t *request = dns_create_from_packet(packet, packet_length);
/* Create the response packet. */
response = dns_create();
response->trn_id = request->trn_id;
response->flags = 0x8000;
if(request->question_count > 0)
int i;
/* Display the questions. */
for(i = 0; i < request->question_count; i++)
/* Grab the question and display it. */
question_t this_question = request->questions[i];
fprintf(stderr, "Question %d: %s (0x%04x 0x%04x)\n", i,, this_question.type, this_question.class);
/* Add an answer, if appropriate. */
dns_add_question(response,, this_question.type, this_question.class);
if(settings->A && (this_question.type == DNS_TYPE_A || this_question.type == DNS_TYPE_ANY))
fprintf(stderr, "(Responding with %s)\n", settings->A);
dns_add_answer_A(response,, 0x0001, settings->TTL, settings->A);
#ifndef WIN32
else if(settings->AAAA && this_question.type == DNS_TYPE_AAAA)
fprintf(stderr, "(Responding with %s)\n", settings->AAAA);
dns_add_answer_AAAA(response,, 0x0001, settings->TTL, settings->AAAA);
/* If we have any answers, send back our packet. */
if(response->answer_count > 0)
/* Send the packet. */
response_packet = dns_to_packet(response, &response_packet_length);
udp_send(socket, addr, port, response_packet, response_packet_length);
/* Send back an error. */
response_packet = dns_create_error_string(request->trn_id, request->questions[0], &response_packet_length);
udp_send(socket, addr, port, response_packet, response_packet_length);
/* Delete the response. */
/* Delete the request. */
return SELECT_OK;
static void dns_poll(settings_t *s)
/* Create the select group in 'settings' -- this is so we can free it on a signal. */
s->select_group = select_group_create();
/* Add the server socket. */
select_group_add_socket(s->select_group, s->server_socket, SOCKET_TYPE_DATAGRAM, s);
select_set_recv(s->select_group, s->server_socket, dns_callback);
select_group_do_select(s->select_group, -1);
select_group_destroy(s->select_group); /* Note: we don't get here. */
void cleanup(void)
/* Free memory. */
/* Print allocated memory. This will only run if -DTESTMEMORY is given. */
void interrupt(int signal)
/* Note: exiting like this will call the atexit() function, cleanup(). */
fprintf(stderr, "punt!\n");
static void usage(char *program)
fprintf(stderr, NAME", by Ron Bowes <>\n");
fprintf(stderr, "\n");
fprintf(stderr, "%s [options]\n", program);
fprintf(stderr, "\n");
fprintf(stderr, " -h --help\n");
fprintf(stderr, " Help (this page)\n");
fprintf(stderr, " --test <domain>\n");
fprintf(stderr, " Test to see if we are the authoritative nameserver for the given domain.\n");
fprintf(stderr, " -s --source <address>\n");
fprintf(stderr, " The local address to bind to. Default: any (\n");
fprintf(stderr, " -p --port <port>\n");
fprintf(stderr, " The local port to listen on. I don't recommend changing this.\n");
fprintf(stderr, " default: 53\n");
fprintf(stderr, " -A <address>\n");
fprintf(stderr, " The A record to return when a record is requested. Default: NXDOMAIN.\n");
#ifndef WIN32
fprintf(stderr, " --AAAA <address>\n");
fprintf(stderr, " The AAAA record to return when a record is requested. Default: NXDOMAIN.\n");
fprintf(stderr, " --TTL <time>\n");
fprintf(stderr, " The time-to-live value to send back, in seconds. Default: 1 second.\n");
fprintf(stderr, " -u --username\n");
fprintf(stderr, " Drop privileges to this user after opening socket (default: 'nobody')\n");
fprintf(stderr, " -V --version\n");
fprintf(stderr, " Print the version and exit\n");
fprintf(stderr, "\n");
static void version()
fprintf(stderr, "%s is part of %s\n", NAME, NBTOOL_NAME_VERSION);
int main(int argc, char *argv[])
settings_t *s = safe_malloc(sizeof(settings_t));
char c;
int option_index;
const char *option_name;
/* Build the long-options array for parsing the options. */
struct option long_options[] =
/* General options. */
{"A", required_argument, 0, 0}, /* A record. */
#ifndef WIN32
{"AAAA", required_argument, 0, 0}, /* A record. */
{"help", no_argument, 0, 0}, /* Help. */
{"h", no_argument, 0, 0},
{"H", no_argument, 0, 0},
{"port", required_argument, 0, 0}, /* Local port. */
{"p", required_argument, 0, 0},
{"source", required_argument, 0, 0}, /* Source. */
{"s", required_argument, 0, 0},
{"test", required_argument, 0, 0}, /* Test the DNS authority. */
{"TTL", required_argument, 0, 0}, /* Time to live */
{"username", required_argument, 0, 0}, /* Username (for dropping privileges). */
{"u", required_argument, 0, 0},
{"version", no_argument, 0, 0}, /* Version. */
{"V", no_argument, 0, 0},
{0, 0, 0, 0}
/* Initialize Winsock. */
/* Get ready to randomize. */
srand((unsigned int)time(NULL));
/* Clear the settings. */
memset(s, sizeof(s), 0);
/* Set some defaults. */
s->user = "nobody";
s->port = 53;
s->source = "";
s->TTL = 1;
/* Catch SIGINT. */
signal(SIGINT, interrupt);
/* Catch all exit events. */
/* Parse the commandline options. */
opterr = 0;
while((c = getopt_long_only(argc, argv, "", long_options, &option_index)) != EOF)
case 0:
option_name = long_options[option_index].name;
/* General options. */
if(!strcmp(option_name, "A"))
s->A = optarg;
#ifndef WIN32
else if(!strcmp(option_name, "AAAA"))
s->AAAA = optarg;
else if(!strcmp(option_name, "help") || !strcmp(option_name, "h") || !strcmp(option_name, "H"))
else if(!strcmp(option_name, "port") || !strcmp(option_name, "p"))
s->port = atoi(optarg);
else if(!strcmp(option_name, "source") || !strcmp(option_name, "s"))
s->source = optarg;
else if(!strcmp(option_name, "test"))
dns_do_test(optarg); /* Doesn't return. */
else if(!strcmp(option_name, "TTL"))
s->TTL = atoi(optarg);
else if(!strcmp(option_name, "username") || !strcmp(option_name, "u"))
s->user = optarg;
else if(!strcmp(option_name, "version") || !strcmp(option_name, "V"))
case '?':
fprintf(stderr, "Couldn't parse arguments\n\n");
#ifndef WIN32
/* Check for the root user. */
if(getuid() != 0)
fprintf(stderr, "WARNING: If the bind() fails, please re-run as root (privileges will be dropped as soon as the socket is created).\n");
/* Create a socket for the server. */
s->server_socket = udp_create_socket(s->port, s->source);
/* Display what we're doing. */
fprintf(stderr, "Listening for requests on %s:%d\n", s->source, s->port);
fprintf(stderr, "Will respond to A requests with %s\n", s->A);
#ifndef WIN32
fprintf(stderr, "Will respond to AAAA requests with %s\n", s->AAAA);
/* Drop privileges. */
/* Set the global settings -- this lets us clean up when a signal is caught. */
global_settings = s;
/* Poll for data. */
return 0;