source: MondoRescue/branches/2.2.5/mindi-busybox/networking/dnsd.c@ 1765

/* vi: set sw=4 ts=4: */
/*
 * Mini DNS server implementation for busybox
 *
 * Copyright (C) 2005 Roberto A. Foglietta (me@roberto.foglietta.name)
 * Copyright (C) 2005 Odd Arild Olsen (oao at fibula dot no)
 * Copyright (C) 2003 Paul Sheer
 *
 * Licensed under GPLv2 or later, see file LICENSE in this tarball for details.
 *
 * Odd Arild Olsen started out with the sheerdns [1] of Paul Sheer and rewrote
 * it into a shape which I believe is both easier to understand and maintain.
 * I also reused the input buffer for output and removed services he did not
 * need.  [1] http://threading.2038bug.com/sheerdns/
 *
 * Some bugfix and minor changes was applied by Roberto A. Foglietta who made
 * the first porting of oao' scdns to busybox also.
 */

#include <syslog.h>
#include "libbb.h"

//#define DEBUG 1
#define DEBUG 0

enum {
    MAX_HOST_LEN = 16,      // longest host name allowed is 15
    IP_STRING_LEN = 18,     // .xxx.xxx.xxx.xxx\0

//must be strlen('.in-addr.arpa') larger than IP_STRING_LEN
    MAX_NAME_LEN = (IP_STRING_LEN + 13),

/* Cannot get bigger packets than 512 per RFC1035
   In practice this can be set considerably smaller:
   Length of response packet is  header (12B) + 2*type(4B) + 2*class(4B) +
   ttl(4B) + rlen(2B) + r (MAX_NAME_LEN =21B) +
   2*querystring (2 MAX_NAME_LEN= 42B), all together 90 Byte
*/
    MAX_PACK_LEN = 512 + 1,

    DEFAULT_TTL = 30,       // increase this when not testing?

    REQ_A = 1,
    REQ_PTR = 12
};

struct dns_repl {       // resource record, add 0 or 1 to accepted dns_msg in resp
    uint16_t rlen;
    uint8_t *r;     // resource
    uint16_t flags;
};

struct dns_head {       // the message from client and first part of response mag
    uint16_t id;
    uint16_t flags;
    uint16_t nquer;     // accepts 0
    uint16_t nansw;     // 1 in response
    uint16_t nauth;     // 0
    uint16_t nadd;      // 0
};
struct dns_prop {
    uint16_t type;
    uint16_t class;
};
struct dns_entry {      // element of known name, ip address and reversed ip address
    struct dns_entry *next;
    char ip[IP_STRING_LEN];     // dotted decimal IP
    char rip[IP_STRING_LEN];    // length decimal reversed IP
    char name[MAX_HOST_LEN];
};

static struct dns_entry *dnsentry;
static uint32_t ttl = DEFAULT_TTL;

static const char *fileconf = "/etc/dnsd.conf";

// Must match getopt32 call
#define OPT_daemon  (option_mask32 & 0x10)
#define OPT_verbose (option_mask32 & 0x20)


/*
 * Convert host name from C-string to dns length/string.
 */
static void convname(char *a, uint8_t *q)
{
    int i = (q[0] == '.') ? 0 : 1;
    for (; i < MAX_HOST_LEN-1 && *q; i++, q++)
        a[i] = tolower(*q);
    a[0] = i - 1;
    a[i] = 0;
}

/*
 * Insert length of substrings instead of dots
 */
static void undot(uint8_t * rip)
{
    int i = 0, s = 0;
    while (rip[i])
        i++;
    for (--i; i >= 0; i--) {
        if (rip[i] == '.') {
            rip[i] = s;
            s = 0;
        } else s++;
    }
}

/*
 * Read one line of hostname/IP from file
 * Returns 0 for each valid entry read, -1 at EOF
 * Assumes all host names are lower case only
 * Hostnames with more than one label are not handled correctly.
 * Presently the dot is copied into name without
 * converting to a length/string substring for that label.
 */
static int getfileentry(FILE * fp, struct dns_entry *s)
{
    unsigned int a,b,c,d;
    char *line, *r, *name;

 restart:
    line = r = xmalloc_fgets(fp);
    if (!r)
        return -1;
    while (*r == ' ' || *r == '\t') {
        r++;
        if (!*r || *r == '#' || *r == '\n') {
            free(line);
            goto restart; /* skipping empty/blank and commented lines  */
        }
    }
    name = r;
    while (*r != ' ' && *r != '\t')
        r++;
    *r++ = '\0';
    if (sscanf(r, ".%u.%u.%u.%u"+1, &a, &b, &c, &d) != 4) {
        free(line);
        goto restart; /* skipping wrong lines */
    }

    sprintf(s->ip, ".%u.%u.%u.%u"+1, a, b, c, d);
    sprintf(s->rip, ".%u.%u.%u.%u", d, c, b, a);
    undot((uint8_t*)s->rip);
    convname(s->name, (uint8_t*)name);

    if (OPT_verbose)
        fprintf(stderr, "\tname:%s, ip:%s\n", &(s->name[1]),s->ip);

    free(line);
    return 0;
}

/*
 * Read hostname/IP records from file
 */
static void dnsentryinit(void)
{
    FILE *fp;
    struct dns_entry *m, *prev;

    prev = dnsentry = NULL;
    fp = xfopen(fileconf, "r");

    while (1) {
        m = xzalloc(sizeof(*m));
        /*m->next = NULL;*/
        if (getfileentry(fp, m))
            break;

        if (prev == NULL)
            dnsentry = m;
        else
            prev->next = m;
        prev = m;
    }
    fclose(fp);
}

/*
 * Look query up in dns records and return answer if found
 * qs is the query string, first byte the string length
 */
static int table_lookup(uint16_t type, uint8_t * as, uint8_t * qs)
{
    int i;
    struct dns_entry *d = dnsentry;

    do {
#if DEBUG
        char *p,*q;
        q = (char *)&(qs[1]);
        p = &(d->name[1]);
        fprintf(stderr, "\n%s: %d/%d p:%s q:%s %d",
            __FUNCTION__, (int)strlen(p), (int)(d->name[0]),
            p, q, (int)strlen(q));
#endif
        if (type == REQ_A) { /* search by host name */
            for (i = 1; i <= (int)(d->name[0]); i++)
                if (tolower(qs[i]) != d->name[i])
                    break;
            if (i > (int)(d->name[0])) {
                strcpy((char *)as, d->ip);
#if DEBUG
                fprintf(stderr, " OK as:%s\n", as);
#endif
                return 0;
            }
        } else if (type == REQ_PTR) { /* search by IP-address */
            if (!strncmp((char*)&d->rip[1], (char*)&qs[1], strlen(d->rip)-1)) {
                strcpy((char *)as, d->name);
                return 0;
            }
        }
        d = d->next;
    } while (d);
    return -1;
}


/*
 * Decode message and generate answer
 */
static int process_packet(uint8_t * buf)
{
    struct dns_head *head;
    struct dns_prop *qprop;
    struct dns_repl outr;
    void *next, *from, *answb;

    uint8_t answstr[MAX_NAME_LEN + 1];
    int lookup_result, type, len, packet_len;
    uint16_t flags;

    answstr[0] = '\0';

    head = (struct dns_head *)buf;
    if (head->nquer == 0) {
        bb_error_msg("no queries");
        return -1;
    }

    if (head->flags & 0x8000) {
        bb_error_msg("ignoring response packet");
        return -1;
    }

    from = (void *)&head[1];    //  start of query string
    next = answb = from + strlen((char *)from) + 1 + sizeof(struct dns_prop);   // where to append answer block

    outr.rlen = 0;          // may change later
    outr.r = NULL;
    outr.flags = 0;

    qprop = (struct dns_prop *)(answb - 4);
    type = ntohs(qprop->type);

    // only let REQ_A and REQ_PTR pass
    if (!(type == REQ_A || type == REQ_PTR)) {
        goto empty_packet;  /* we can't handle the query type */
    }

    if (ntohs(qprop->class) != 1 /* class INET */ ) {
        outr.flags = 4; /* not supported */
        goto empty_packet;
    }
    /* we only support standard queries */

    if ((ntohs(head->flags) & 0x7800) != 0)
        goto empty_packet;

    // We have a standard query
    bb_info_msg("%s", (char *)from);
    lookup_result = table_lookup(type, answstr, (uint8_t*)from);
    if (lookup_result != 0) {
        outr.flags = 3 | 0x0400;    //name do not exist and auth
        goto empty_packet;
    }
    if (type == REQ_A) {    // return an address
        struct in_addr a;
        if (!inet_aton((char*)answstr, &a)) {//dotted dec to long conv
            outr.flags = 1; /* Frmt err */
            goto empty_packet;
        }
        memcpy(answstr, &a.s_addr, 4);  // save before a disappears
        outr.rlen = 4;          // uint32_t IP
    } else
        outr.rlen = strlen((char *)answstr) + 1;    // a host name
    outr.r = answstr;           // 32 bit ip or a host name
    outr.flags |= 0x0400;           /* authority-bit */
    // we have an answer
    head->nansw = htons(1);

    // copy query block to answer block
    len = answb - from;
    memcpy(answb, from, len);
    next += len;

    // and append answer rr
    *(uint32_t *) next = htonl(ttl);
    next += 4;
    *(uint16_t *) next = htons(outr.rlen);
    next += 2;
    memcpy(next, (void *)answstr, outr.rlen);
    next += outr.rlen;

 empty_packet:

    flags = ntohs(head->flags);
    // clear rcode and RA, set responsebit and our new flags
    flags |= (outr.flags & 0xff80) | 0x8000;
    head->flags = htons(flags);
    head->nauth = head->nadd = htons(0);
    head->nquer = htons(1);

    packet_len = (uint8_t *)next - buf;
    return packet_len;
}

/*
 * Exit on signal
 */
static void interrupt(int x)
{
    /* unlink("/var/run/dnsd.lock"); */
    bb_error_msg("interrupt, exiting\n");
    exit(2);
}

int dnsd_main(int argc, char **argv);
int dnsd_main(int argc, char **argv)
{
    const char *listen_interface = "0.0.0.0";
    char *sttl, *sport;
    len_and_sockaddr *lsa;
    int udps;
    uint16_t port = 53;
    uint8_t buf[MAX_PACK_LEN];

    getopt32(argv, "i:c:t:p:dv", &listen_interface, &fileconf, &sttl, &sport);
    //if (option_mask32 & 0x1) // -i
    //if (option_mask32 & 0x2) // -c
    if (option_mask32 & 0x4) // -t
        ttl = xatou_range(sttl, 1, 0xffffffff);
    if (option_mask32 & 0x8) // -p
        port = xatou_range(sport, 1, 0xffff);

    if (OPT_verbose) {
        bb_info_msg("listen_interface: %s", listen_interface);
        bb_info_msg("ttl: %d, port: %d", ttl, port);
        bb_info_msg("fileconf: %s", fileconf);
    }

    if (OPT_daemon) {
        bb_daemonize_or_rexec(DAEMON_CLOSE_EXTRA_FDS, argv);
        openlog(applet_name, LOG_PID, LOG_DAEMON);
        logmode = LOGMODE_SYSLOG;
    }

    dnsentryinit();

    signal(SIGINT, interrupt);
    /* why? signal(SIGPIPE, SIG_IGN); */
    signal(SIGHUP, SIG_IGN);
#ifdef SIGTSTP
    signal(SIGTSTP, SIG_IGN);
#endif
#ifdef SIGURG
    signal(SIGURG, SIG_IGN);
#endif

    lsa = xdotted2sockaddr(listen_interface, port);
    udps = xsocket(lsa->sa.sa_family, SOCK_DGRAM, 0);
    xbind(udps, &lsa->sa, lsa->len);
    /* xlisten(udps, 50); - ?!! DGRAM sockets are never listened on I think? */
    bb_info_msg("Accepting UDP packets on %s",
            xmalloc_sockaddr2dotted(&lsa->sa));

    while (1) {
        int r;
        socklen_t fromlen = lsa->len;
// FIXME: need to get *DEST* address (to which of our addresses
// this query was directed), and reply from the same address.
// Or else we can exhibit usual UDP ugliness:
// [ip1.multihomed.ip2] <=  query to ip1  <= peer
// [ip1.multihomed.ip2] => reply from ip2 => peer (confused)
        r = recvfrom(udps, buf, sizeof(buf), 0, &lsa->sa, &fromlen);
        if (OPT_verbose)
            bb_info_msg("Got UDP packet");
        if (r < 12 || r > 512) {
            bb_error_msg("invalid packet size");
            continue;
        }
        r = process_packet(buf);
        if (r <= 0)
            continue;
        sendto(udps, buf, r, 0, &lsa->sa, fromlen);
    }
    return 0;
}