sdns: An HTML repository from maroofi

sdns - low-level DNS library in C

A small DNS library written in C

(if you are interested in the Lua binding of the library, check the lua directory.)

How to compile

sdns does not have any external dependency. However, all the sdns_json_* functions are based on libjansson library. So if you don't need JSON output, you can compile it just by making it.

# compile the library
make

If you also need sdns_json_* functions, you need to have libjansson installed.

# make sure jaonson library is installed
sudo apt-get install libjansson-dev

# now you can build the library
make with-json

If you want to build the library with Lua binding, you must have Lua installed. I only show the 5.4 version but you can build it for 5.3 and 5.2 as well:

# make sure you have Lua dev installed
sudo apt-get install liblua5.4-dev lua5.4

# now you can build the library
make with-lua

To have both lua binding and sdns_json_* functions:

# make sure you have lua and libjansson installed
sudo apt-get install liblua5.4-dev lua5.4 libjansson-dev

# and then
make all

The make file is quite easy and you can change it to whatever you want. You can also manually build the library as it has only 5 .C files.

make commmand will create a bin directory and inside you will have libsdns.so file.

Now you can use it in your project by passing -lsdns switch. For example, here is the command to build the test1.c file in test directory:

gcc  -o test/test -Iinclude test/test1.c  -Lbin -lsdns -ljansson

NOTE: Another way of using this library in you project is to simply copy all the src/*.c and include/*.h files into your project.

Running tests

Tests are using Python3 and bash. You can optionally use valgrind to check memory leaks as well. You need to have 'jsoncomparison' package installed.

# make sure you make the library with-json
make with-json

cd test
pip install jsoncomparison

# Running the tests without valgrind
./sdns_test.sh

# Or running the tests with valgrind
./sdns_test.sh with-valgrind

Documentation

Here is the online documentation of the library: https://maroofi.github.io/sdns

Or you can compile it yourself:

doxygen Doxygen

Example sourece code

(this source code creates a DNS packet the same as using dig command: dig google.com IN A +noedns)

#include <assert.h>
#include <stdio.h>
#include <string.h>
#include "sdns.h"

int main(){
    sdns_context * dnsctx = sdns_init_context();
    if (NULL == dnsctx){    // we are done as we can create the context
        fprintf(stderr, "Can not create a DNS context\n");
        return 1;
    }
    char * qname = strdup("google.com");
    int res = sdns_make_query(dnsctx, sdns_rr_type_A, sdns_q_class_IN, qname, 0);
    if (res != 0){  
        // sdns_make_query() failed. The return value can tell us why.
        // we have to call sdns_error_string() with 'res' to know the reason.
        fprintf(stderr, "Can not create the query packet\n");
        sdns_free_context(dnsctx);
        return 1;
    }
    // we are done. Now if we want to get the binary data to send it over the socket
    // we have to call sdns_to_wire() function.
    res = sdns_to_wire(dnsctx);
    if (res != 0){
        fprintf(stderr, "Can not covert to binary data\n");
        sdns_free_context(dnsctx);
        return 1;
    }
    // let's print the hex presentation of the data
    fprintf(stdout, "Raw data ready for the socket:\n");
    for (int i=0; i < dnsctx->raw_len; ++i){
        fprintf(stdout, "%02x ", (unsigned char)dnsctx->raw[i]);
    }
    fprintf(stdout, "\n");
    // let's free the context and we are done
    sdns_free_context(dnsctx);
    return 0;
}

paractical tutorials of using sdns library

Checkout the first tutorial: In this tutorial, we are making a naive dig using sdns library.

The second tutorial: Creating a DNS packet sniffer (naive-wireshark for DNS)