SLDR: Super Light DNS Resolver

SLDR is a tiny asynchronous DNS resolver written in C. It is capable of doing A, AAAA and MX requests. SLDR is supposed to be used on embedded systems and in applications that performs large amount of DNS requests. SLDR keeps query cache, therefore subsequent calls to resolve the same host hit the cache, saving time and traffic.

SLDR usage pattern is as follows:

Application clreates SLDR context by means of sldr_create()
Application schedules queries on SLDR context with sldr_queue(), which sends DNS UDP request and remembers which callback to call on success or failure, with optional unique application-defined parameter (application context).
Application does sldr_poll() which reads DNS replies and calls respective callbacks. sldr_poll() Is supposed to be called in a loop, with sldr_queue() calls in between to schedule new queries.
When done, application calls sldr_destroy()
Queued queries could be canceled with sldr_cancel()

API

struct sldr *sldr_create(void);
void sldr_destroy(struct sldr **);

Create and destroy SLDR instance, which is handled by an opaque struct sldr.

typedef void (*sldr_callback_t)(struct sldr_cb_data *);
void sldr_queue(struct sldr *, void *context, const char *host,
                enum dns_query_type type, sldr_callback_t callback);

Queue DNS request. context is an optional application-defined parameter, host is the hostname to resolve, type is DNS query type (either DNS_A_RECORD, or DNS_MX_RECORD, or DNS_AAAA_RECORD), and callback is a function to call on success or error. Callback function will be called with struct sldr_cb_data as a parameter, which is defined as:

struct sldr_cb_data {
  void *context;
  enum sldr_error error;
  enum dns_query_type query_type;
  const char *name;               // Requested host name
  const unsigned char *addr;      // Resolved address
  size_t addr_len;                // Resolved address len
};

From struct sldr_cb_data, an application can fetch resolved address, or error code.

int sldr_poll(struct sldr *, int milliseconds);

Read DNS replies and call user-defined callbacks. Wait not more than milliseconds for DNS replies to arrive.

int sldr_get_fd(struct sldr *);

Return UDP socket used to send and receive DNS queries. Could be used by an application to multiplex IO, or to set specific socket option, for example to increase kernel socket buffer.

void sldr_cancel(struct sldr *, const void *context);

Cancel queued DNS query.

Example

#include <stdio.h>
#include "sldr.h"

static const struct dns_callback(struct sldr_cb_data *cbd) {
  if (cbd->error == SLDR_OK) {
    printf("%s: %u.%u.%u.%u\n", cbd->name,
           cbd->addr[0], cbd->addr[1], cbd->addr[2], cbd->addr[3]);
  } else {
    printf("Error: %d\n", cbd->error);
  }
}

int main(void) {
  struct sldr *sldr = sldr_create();
  sldr_queue(sldr, NULL, "google.com", DNS_A_RECORD, dns_callback);
  sldr_poll(sldr, 5 * 1000); // Resolve, wait no more then 5 sec
  sldr_destroy(&sldr);
  return 0;
}

Using SLDR as a command line tool

SLDR could be built as a command line tool:

cc -o sldr sldr.c -DSLDR_CLI      # On Unix
cl sldr.c /DSLDR_CLI              # On Windows

Then, one can send queries like:

$ ./sldr google.com
google.com: 74.125.24.113
$ ./sldr google.com aaaa
google.com: 2a00:1450:400b:0c02:0000:0000:0000:0066
$ ./sldr google.com mx
alt3.aspmx.l.google.com

Licensing

SLDR is released under GNU GPL v.2. Businesses have an option to get non-restrictive, royalty-free commercial license and professional support from Cesanta Software.

Super Light Regular Expression library, Mongoose web server are other projects by Cesanta Software, developed with the same philosophy of functionality and simplicity.

tovenja/sldr

SLDR: Super Light DNS Resolver

API

Example

Using SLDR as a command line tool

Licensing