path: root/dns.c

#include "dns.h"

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <unistd.h>
#include <fcntl.h>


/* Entry point is dns_get. */


/* Read about DNS in RFC 1034 & 1035. */
struct header {
    unsigned short id;

    char a;
    /*
    int is_response:          1;
    int opcode:               4;
    int is_authanswer:        1;
    int is_truncated:         1;
    int is_recursion_desired: 1;
    */

    char b;
    /*
    int is_recursion_available: 1;
    int:                        3;
    int response_code:          4;
    */

    unsigned short questions;
    unsigned short answers;
    unsigned short authorities;
    unsigned short additionals;
};

struct question_header {
    unsigned short type;
    unsigned short class;
};

struct resource_header {
    unsigned short type;
    unsigned short class;
    unsigned int   ttl;
    unsigned short datalen;
};


/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 * get-related
 */
static void fill_header(char **writer)
{
    struct header *head = (struct header *) *writer;
    memset(head, 0, sizeof(*head));
    /* PID is unique enough to use it as ID: */
    head->id        = (unsigned short) htons(getpid());
    head->a         = 0b00000001;  /* recursion desired */
    head->questions = htons(1);    /* we have one question */
    *writer        += sizeof(*head);
}

static void fill_name(char **writer, const char *host)
{  /* In DNS names are always starting with dot,
    * so 'example.com' need to become '.example.com'. */
    char *last = *writer;
    do {
        if (*host == '.' || !*host) {
            *last = *writer - last;
            last  = ++*writer;
        } else {
            *++*writer = *host;
        }
    } while (*host++);
    *last = 0;
    ++*writer;
}

static void fill_question(char **writer, int query_type)
{
    struct question_header *q = (struct question_header *) *writer;
    q->type  = htons(query_type);
    q->class = htons(1);
    *writer += sizeof(*q);
}

static int askudp(const char *s, const char *h, int qt, struct sockaddr_in *a)
{  /* Create socket, fill in the question and send it. */
    int sent;
    int plen = sizeof(struct header) + strlen(h) + 2 +
               sizeof(struct question_header);
    char *packet       = malloc(plen);
    char *writer       = packet;
    int   sock         = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    if (sock < 0) {
        return 0;
    }
    a->sin_family      = AF_INET;
    a->sin_port        = htons(53);
    a->sin_addr.s_addr = inet_addr(s);

    /* Socket created, now fill in the question. */
    fill_header(&writer);
    fill_name(&writer, h);
    fill_question(&writer, qt);

    /* Question is filled in, so send it. */
    sent = sendto(sock, packet, plen, 0, (struct sockaddr *) a, sizeof(*a));
    free(packet);
    if (sent != plen) {
        return 0;
    }
    return sock;
}

static int asktcp(const char *s, const char *h, int qt)
{  /* Create socket, fill in the question and send it. */
    struct sockaddr_in a;
    int sent;
    int plen = sizeof(struct header) + strlen(h) + 4 +
               sizeof(struct question_header);
    char *packet = malloc(plen);
    char *writer = packet;
    int   sock   = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (sock < 0) {
        return 0;
    }
    a.sin_family       = AF_INET;
    a.sin_port         = htons(53);
    a.sin_addr.s_addr  = inet_addr(s);
    if (connect(sock, (struct sockaddr *) &a, sizeof(a))) {
        return 0;
    }

    /* Socket created, now fill in the question.
       Note the difference from UDP: TCP has size in the start. */
    writer += 2;
    fill_header(&writer);
    fill_name(&writer, h);
    fill_question(&writer, qt);
    *((unsigned short *) packet) = htons(writer - packet - 2);

    /* Question is filled in, so send it. */
    sent = write(sock, packet, plen);
    free(packet);
    if (sent != plen) {
        return 0;
    }
    return sock;
}

static char *gettcp(int sock)
{
    short size;
    char *packet;
    if (!sock) {
        return 0;
    }
    if (read(sock, &size, 2) < 0) {
        close(sock);
        return 0;
    }
    size   = ntohs(size);
    packet = malloc(size);
    if (read(sock, packet, size) < 0) {
        close(sock);
        free(packet);
        return 0;
    }
    close(sock);
    return packet;
}

static char *get(const char *s, const char *h, int qt)
{ /* Get response in UDP, if it is truncated do it in TCP. */
    struct sockaddr_in a;
    char     *packet;
    socklen_t slen = sizeof(a);
    int       sock = askudp(s, h, qt, &a);
    if (!sock) {
        return 0;
    }
    packet = malloc(512);  /* Max size of UDP is 512 bytes. */
    if (recvfrom(sock, packet, 512, 0, (struct sockaddr *) &a, &slen) < 0) {
        free(packet);
        close(sock);
        return 0;
    } else if (packet[2] & 0b10) {  /* is truncated */
        free(packet);
        close(sock);
        return gettcp(asktcp(s, h, qt));
    }
    close(sock);
    return packet;
}


/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 * parse-related
 */
static void skip_questions(char **reader, int num)
{
    for (; num; --num) {
        *reader += strlen(*reader) + 1;
        *reader += sizeof(struct question_header);
    }
}

static char *read_name(char **extreader, char *packet)
{  /* Names are stored in binary format:
    * length followed by name part (string between dots),
    * length of zero means end of domain name,
    * and length >= 192 means it will be followed by absolute link,
    * name readed from link should be inserted instead of this part. */
    char *name   = malloc(256);  /* cannot be more =) */
    char *reader = *extreader;
    char *writer = name;
    int   jumped = 0;
    if (!*reader) {
        *name = 0;
        return name;
    }
    while (*reader) {
        unsigned char len = *reader++;
        if (len >= 192) {
            int jump = ((unsigned char) *reader) + (len - 192) * 256;
            if (!jumped) {
                *extreader += 2;
                jumped        = 1;
            }
            reader = packet + jump;
            continue;
        }
        memcpy(writer, reader, len);
        writer += len;
        reader += len;
        if (!jumped) {
            *extreader += len + 1;
        }
        *writer++ = '.';
    }
    if (!jumped) {
        ++*extreader;
    }
    *--writer = 0;
    return name;
}

static struct dns_answers *get_answers(char **reader, char *packet, int num)
{  /* Read all (num) answers into res and return it. */
    struct dns_answers     *res;
    struct resource_header *head;
    if (!num) {  /* no answers left to read */
        return 0;
    }
    /* Read one answer, starting with header: */
    res       = malloc(sizeof(*res));
    res->host = read_name(reader, packet);
    head      = (struct resource_header *) *reader;
    res->type = ntohs(head->type);
    res->size = ntohs(head->datalen);
    *reader  += 10;
    /* Analyse answer data based on answer type: */
    if (res->type == dns_type_cname) {
        /* For CNAME data is just domain name: */
        res->size--;
        res->data = read_name(reader, packet);
    } else if (res->type == dns_type_mx) {
        /* For MX data is record preference, followed by its name: */
        char *name;
        unsigned short pref = ntohs(*((unsigned short *) *reader));
        *reader  += 2;
        name      = read_name(reader, packet);
        res->data = malloc(2 + strlen(name) + 1);
        *((unsigned short *) res->data) = pref;
        memcpy(res->data + 2, name, strlen(name) + 1);
        free(name);
    } else if (res->type == dns_type_srv) {
        /* For SRV data is record priority, weight and port
         * followed by its name. */
        char *name;
        unsigned short priority;
        unsigned short weight;
        unsigned short port;
        priority  = ntohs(*((unsigned short *) *reader));
        *reader  += 2;
        weight    = ntohs(*((unsigned short *) *reader));
        *reader  += 2;
        port      = ntohs(*((unsigned short *) *reader));
        *reader  += 2;
        name      = read_name(reader, packet);
        res->data = malloc(7 + strlen(name));
        ((unsigned short *) res->data)[0] = priority;
        ((unsigned short *) res->data)[1] = weight;
        ((unsigned short *) res->data)[2] = port;
        memcpy(res->data + 6, name, strlen(name) + 1);
        free(name);
    } else {
        /* For other types just copy the data: */
        if (res->type == dns_type_txt) {
            /* Skipping leading length byte for TXT: */
            res->size--;
            ++*reader;
        }
        res->data = malloc(res->size);
        memcpy(res->data, *reader, res->size);
        *reader += res->size;
    }
    /* Start recursion for the rest of answers and add them as next: */
    res->next = get_answers(reader, packet, num - 1);
    return res;
}

static struct dns_answers *parse(char *packet)
{
    struct dns_answers *res;
    char          *reader = packet;
    struct header *head   = (struct header *) reader;
    if (!packet) {
        return 0;
    }
    head->questions = ntohs(head->questions);
    head->answers   = ntohs(head->answers);
    reader         += sizeof(*head);
    skip_questions(&reader, head->questions);
    res = get_answers(&reader, packet, head->answers);
    free(packet);
    return res;
}


/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 * MX utility
 */
char *dns_mx_server(void *data)
{
    return ((char *) data) + 2;
}

int dns_mx_preference(void *data)
{
    return *((unsigned short *) data);
}


/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 * SRV utility
 */
char *dns_srv_server(void *data)
{
    return ((char *) data) + 6;
}

int dns_srv_port(void *data)
{
    return ((unsigned short *) data)[2];
}

int dns_srv_priority(void *data)
{
    return *((unsigned short *) data);
}

int dns_srv_weight(void *data)
{
    return ((unsigned short *) data)[1];
}


/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 * Constructor and destructor
 */
struct dns_answers *dns_get(const char *server, const char *host, int query)
{  /* Acquire response in get and parse it. */
    return parse(get(server, host, query));
}

void dns_free(struct dns_answers *answers)
{
    if (answers) {
        dns_free(answers->next);
        free(answers->host);
        free(answers->data);
        free(answers);
    }
}