source: MondoRescue/branches/stable/mindi-busybox/networking/dnsd.c@ 1346

/* 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 <unistd.h>
#include <string.h>
#include <signal.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <ctype.h>
#include "busybox.h"

static char *fileconf = "/etc/dnsd.conf";
#define LOCK_FILE       "/var/run/dnsd.lock"
#define LOG_FILE        "/var/log/dnsd.log"

#define is_daemon()  (flags&16)
#define is_verbose() (flags&32)
//#define DEBUG 


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 = NULL;
static int daemonmode = 0;
static uint32_t ttl = DEFAULT_TTL;

/*
 * 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++;
    }
}

/*
 * Append message to log file
 */
static void log_message(char *filename, char *message)
{
    FILE *logfile;
    if (!daemonmode)
        return;
    logfile = fopen(filename, "a");
    if (!logfile)
        return;
    fprintf(logfile, "%s\n", message);
    fclose(logfile);
}

/*
 * 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 is 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, int verb)
{
    unsigned int a,b,c,d;
    char *r, *name;

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

    sprintf(s->ip,"%u.%u.%u.%u",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(verb)
        fprintf(stderr,"\tname:%s, ip:%s\n",&(s->name[1]),s->ip);

    return 0; /* warningkiller */
}

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

    fp = bb_xfopen(fileconf, "r");

    while (1) {
        m = xmalloc(sizeof(struct dns_entry));

        m->next = NULL;
        if (getfileentry(fp, m, verb))
            break;

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


/*
 * Set up UDP socket
 */
static int listen_socket(char *iface_addr, int listen_port)
{
    struct sockaddr_in a;
    char msg[100];
    int s;
    int yes = 1;
    s = bb_xsocket(PF_INET, SOCK_DGRAM, 0);
#ifdef SO_REUSEADDR
    if (setsockopt(s, SOL_SOCKET, SO_REUSEADDR, (char *)&yes, sizeof(yes)) < 0)
        bb_perror_msg_and_die("setsockopt() failed");
#endif
    memset(&a, 0, sizeof(a));
    a.sin_port = htons(listen_port);
    a.sin_family = AF_INET;
    if (!inet_aton(iface_addr, &a.sin_addr))
        bb_perror_msg_and_die("bad iface address");
    bb_xbind(s, (struct sockaddr *)&a, sizeof(a));
    listen(s, 50); /* bb_xlisten? */
    sprintf(msg, "accepting UDP packets on addr:port %s:%d\n",
        iface_addr, (int)listen_port);
    log_message(LOG_FILE, msg);
    return s;
}

/*
 * 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;

    if(d) do {
#ifdef DEBUG
        if(qs && d) {
            char *p,*q;
            q = (char *)&(qs[1]);
            p = &(d->name[1]);
            fprintf(stderr, "\n%s: %d/%d p:%s q:%s %d", 
                __FUNCTION__, strlen(p), (int)(d->name[0]), 
                p, q, 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])) {
#ifdef DEBUG
                fprintf(stderr, " OK");
#endif
                strcpy((char *)as, d->ip);
#ifdef DEBUG
                fprintf(stderr, " 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;
            }
        }
    } while ((d = d->next) != NULL);
    return -1;
}


/*
 * Decode message and generate answer
 */
#define eret(s) do { fprintf (stderr, "%s\n", s); return -1; } while (0)
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)
        eret("no queries");

    if ((head->flags & 0x8000))
        eret("ignoring response packet");

    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
    log_message(LOG_FILE, (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 = next - (void *)buf;
    return packet_len;
}

/*
 * Exit on signal
 */
static void interrupt(int x)
{
    unlink(LOCK_FILE);
    write(2, "interrupt exiting\n", 18);
    exit(2);
}


int dnsd_main(int argc, char **argv)
{
    int udps;
    uint16_t port = 53;
    uint8_t buf[MAX_PACK_LEN];
    unsigned long flags = 0;
    char *listen_interface = "0.0.0.0";
    char *sttl=NULL, *sport=NULL;

    if(argc > 1)
        flags = bb_getopt_ulflags(argc, argv, "i:c:t:p:dv", &listen_interface, &fileconf, &sttl, &sport);
    if(sttl)
        if(!(ttl = atol(sttl)))
            bb_show_usage();
    if(sport)
        if(!(port = atol(sport)))
            bb_show_usage();

    if(is_verbose()) {
        fprintf(stderr,"listen_interface: %s\n", listen_interface);
        fprintf(stderr,"ttl: %d, port: %d\n", ttl, port);
        fprintf(stderr,"fileconf: %s\n", fileconf);
    }

    if(is_daemon())
#ifdef BB_NOMMU
        /* reexec for vfork() do continue parent */
        vfork_daemon_rexec(1, 0, argc, argv, "-d");
#else
        bb_xdaemon(1, 0);
#endif

    dnsentryinit(is_verbose());

    signal(SIGINT, interrupt);
    signal(SIGPIPE, SIG_IGN);
    signal(SIGHUP, SIG_IGN);
#ifdef SIGTSTP
    signal(SIGTSTP, SIG_IGN);
#endif
#ifdef SIGURG
    signal(SIGURG, SIG_IGN);
#endif

    udps = listen_socket(listen_interface, port);
    if (udps < 0)
        exit(1);

    while (1) {
        fd_set fdset;
        int r;

        FD_ZERO(&fdset);
        FD_SET(udps, &fdset);
        // Block until a message arrives
        if((r = select(udps + 1, &fdset, NULL, NULL, NULL)) < 0)
            bb_perror_msg_and_die("select error");
        else
        if(r == 0)
            bb_perror_msg_and_die("select spurious return");

        /* Can this test ever be false? */
        if (FD_ISSET(udps, &fdset)) {
            struct sockaddr_in from;
            int fromlen = sizeof(from);
            r = recvfrom(udps, buf, sizeof(buf), 0,
                     (struct sockaddr *)&from,
                     (void *)&fromlen);
            if(is_verbose())
                fprintf(stderr, "\n--- Got UDP size=%d ", r);
            log_message(LOG_FILE, "\n--- Got UDP ");

            if (r < 12 || r > 512) {
                bb_error_msg("invalid packet size");
                continue;
            }
            if (r > 0) {
                r = process_packet(buf);
                if (r > 0)
                    sendto(udps, buf,
                           r, 0, (struct sockaddr *)&from,
                           fromlen);
            }
        }       // end if
    }           // end while
    return 0;
}