path: root/networking/dnsd.c

/*
 * 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 <ctype.h>
#include "libbb.h"

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

#define  MAX_HOST_LEN 16        // longest host name allowed is 15
#define IP_STRING_LEN 18        // .xxx.xxx.xxx.xxx\0

//must be strlen('.in-addr.arpa') larger than IP_STRING_LEN
static const int 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
*/
static const int MAX_PACK_LEN = 512 + 1;

#define  DEFAULT_TTL 30;        // increase this when not testing?

static const int REQ_A = 1;
static const int 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 insetad 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;

        if(!(fp = fopen(fileconf, "r")))
		bb_perror_msg_and_die("open %s",fileconf);

        while (1) {
                if(!(m = (struct dns_entry *)malloc(sizeof(struct dns_entry))))
			bb_perror_msg_and_die("malloc 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;
        if ((s = socket(PF_INET, SOCK_DGRAM, 0)) < 0) 
                bb_perror_msg_and_die("socket() failed");
#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");
        if (bind(s, (struct sockaddr *)&a, sizeof(a)) < 0)
                bb_perror_msg_and_die("bind() failed");
        listen(s, 50);
        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;

        do {
#ifdef DEBUG
        	char *p,*q;
		q = (char *)&(qs[1]);
		p = &(d->name[1]);
                fprintf(stderr, "\ntest: %d <%s> <%s> %d", strlen(p), 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])
					continue;
#ifdef DEBUG
			fprintf(stderr, " OK");
#endif
			strcpy((char *)as, d->ip);
#ifdef DEBUG
			fprintf(stderr, " %s ", 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 *)&head[1]) + 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 *)head);
        lookup_result = table_lookup(type, answstr, (uint8_t*)(&head[1]));
        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);
}

#define is_daemon()  (flags&16)
#define is_verbose() (flags&32)
//#define DEBUG 1

int dnsd_main(int argc, char **argv)
{
        int i, 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()) 
#if defined(__uClinux__)
		/* reexec for vfork() do continue parent */
		vfork_daemon_rexec(1, 0, argc, argv, "-d");
#else							/* uClinux */
		if (daemon(1, 0) < 0) {
			bb_perror_msg_and_die("daemon");
		}
#endif							/* uClinuvx */
			
        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  ");
                        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;
}