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h_dns_server.c revision 1.2 
      1 /*	$NetBSD: h_dns_server.c,v 1.2 2014/01/06 16:42:57 gson Exp $	*/
      2 
      3 /*-
      4  * Copyright (c) 2013 The NetBSD Foundation, Inc.
      5  * All rights reserved.
      6  *
      7  * This code is derived from software contributed to The NetBSD Foundation
      8  * by Andreas Gustafsson.
      9  *
     10  * Redistribution and use in source and binary forms, with or without
     11  * modification, are permitted provided that the following conditions
     12  * are met:
     13  * 1. Redistributions of source code must retain the above copyright
     14  *    notice, this list of conditions and the following disclaimer.
     15  * 2. Redistributions in binary form must reproduce the above copyright
     16  *    notice, this list of conditions and the following disclaimer in the
     17  *    documentation and/or other materials provided with the distribution.
     18  *
     19  * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
     20  * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
     21  * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
     22  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
     23  * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
     24  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
     25  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
     26  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
     27  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
     28  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
     29  * POSSIBILITY OF SUCH DAMAGE.
     30  */
     31 
     32 /*
     33  * A minimal DNS server capable of providing canned answers to the
     34  * specific queries issued by t_hostent.sh and nothing more.
     35  */
     36 
     37 #include <sys/cdefs.h>
     38 __RCSID("$NetBSD: h_dns_server.c,v 1.2 2014/01/06 16:42:57 gson Exp $");
     39 
     40 #include <ctype.h>
     41 #include <err.h>
     42 #include <errno.h>
     43 #include <fcntl.h>
     44 #include <memory.h>
     45 #include <paths.h>
     46 #include <stdio.h>
     47 #include <stdlib.h>
     48 #include <unistd.h>
     49 
     50 #include <sys/socket.h>
     51 
     52 #include <netinet/in.h>
     53 #include <netinet6/in6.h>
     54 
     55 union sockaddr_either {
     56 	struct sockaddr s;
     57 	struct sockaddr_in sin;
     58 	struct sockaddr_in6 sin6;
     59 };
     60 
     61 /* A DNS question and its corresponding answer */
     62 
     63 struct dns_data {
     64 	size_t qname_size;
     65 	const char *qname; /* Wire-encode question name */
     66 	int qtype;
     67 	size_t answer_size;
     68 	const char *answer; /* One wire-encoded answer RDATA */
     69 };
     70 
     71 /* Convert C string constant to length + data pair */
     72 #define STR_DATA(s) sizeof(s) - 1, s
     73 
     74 /* Canned DNS queestion-answer pairs */
     75 struct dns_data data[] = {
     76 	/* Forward mappings */
     77 	/* localhost IN A -> 127.0.0.1 */
     78 	{ STR_DATA("\011localhost\000"), 1,
     79 	  STR_DATA("\177\000\000\001") },
     80 	/* localhost IN AAAA -> ::1 */
     81 	{ STR_DATA("\011localhost\000"), 28,
     82 	  STR_DATA("\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\001") },
     83 	/* sixthavenue.astron.com IN A -> 38.117.134.16 */
     84 	{ STR_DATA("\013sixthavenue\006astron\003com\000"), 1,
     85 	  STR_DATA("\046\165\206\020") },
     86 	/* sixthavenue.astron.com IN AAAA -> 2620:106:3003:1f00:3e4a:92ff:fef4:e180 */
     87 	{ STR_DATA("\013sixthavenue\006astron\003com\000"), 28,
     88 	  STR_DATA("\x26\x20\x01\x06\x30\x03\x1f\x00\x3e\x4a\x92\xff\xfe\xf4\xe1\x80") },
     89 	/* Reverse mappings */
     90 	{ STR_DATA("\0011\0010\0010\003127\007in-addr\004arpa\000"), 12,
     91 	  STR_DATA("\011localhost\000") },
     92 	{ STR_DATA("\0011\0010\0010\0010\0010\0010\0010\0010"
     93 		   "\0010\0010\0010\0010\0010\0010\0010\0010"
     94 		   "\0010\0010\0010\0010\0010\0010\0010\0010"
     95 		   "\0010\0010\0010\0010\0010\0010\0010\0010"
     96 		   "\003ip6\004arpa\000"), 12,
     97 	  STR_DATA("\011localhost\000") },
     98 	{ STR_DATA("\00216\003134\003117\00238"
     99 		   "\007in-addr\004arpa\000"), 12,
    100 	  STR_DATA("\013sixthavenue\006astron\003com\000") },
    101 	{ STR_DATA("\0010\0018\0011\001e\0014\001f\001e\001f"
    102 		   "\001f\001f\0012\0019\001a\0014\001e\0013"
    103 		   "\0010\0010\001f\0011\0013\0010\0010\0013"
    104 		   "\0016\0010\0011\0010\0010\0012\0016\0012"
    105 		   "\003ip6\004arpa\000"), 12,
    106 	  STR_DATA("\013sixthavenue\006astron\003com\000") },
    107 	/* End marker */
    108 	{ STR_DATA(""), 0, STR_DATA("") }
    109 };
    110 
    111 /*
    112  * Compare two DNS names for equality.	If equal, return their
    113  * length, and if not, return zero.  Does not handle compression.
    114  */
    115 static int
    116 name_eq(const unsigned char *a, const unsigned char *b) {
    117 	const unsigned char *a_save = a;
    118 	for (;;) {
    119 		int i;
    120 		int lena = *a++;
    121 		int lenb = *b++;
    122 		if (lena != lenb)
    123 			return 0;
    124 		if (lena == 0)
    125 			return a - a_save;
    126 		for (i = 0; i < lena; i++)
    127 			if (tolower(a[i]) != tolower(b[i]))
    128 				return 0;
    129 		a += lena;
    130 		b += lena;
    131 	}
    132 }
    133 
    134 /* XXX the daemon2_* functions should be in a library */
    135 
    136 int __daemon2_detach_pipe[2];
    137 
    138 static int
    139 daemon2_fork(void)
    140 {
    141 	int r;
    142 	int fd;
    143 	int i;
    144 
    145 	/*
    146 	 * Set up the pipe, making sure the write end does not
    147 	 * get allocated one of the file descriptors that will
    148 	 * be closed in daemon2_detach().
    149 	 */
    150 	for (i = 0; i < 3; i++) {
    151 	    r = pipe(__daemon2_detach_pipe);
    152 	    if (r < 0)
    153 		    return -1;
    154 	    if (__daemon2_detach_pipe[1] <= STDERR_FILENO &&
    155 		(fd = open(_PATH_DEVNULL, O_RDWR, 0)) != -1) {
    156 		    (void)dup2(fd, __daemon2_detach_pipe[0]);
    157 		    (void)dup2(fd, __daemon2_detach_pipe[1]);
    158 		    if (fd > STDERR_FILENO)
    159 			    (void)close(fd);
    160 		    continue;
    161 	    }
    162 	    break;
    163 	}
    164 
    165 	r = fork();
    166 	if (r < 0) {
    167 		return -1;
    168 	} else if (r == 0) {
    169 		/* child */
    170 		close(__daemon2_detach_pipe[0]);
    171 		return 0;
    172        }
    173        /* Parent */
    174 
    175        (void) close(__daemon2_detach_pipe[1]);
    176 
    177        for (;;) {
    178 	       char dummy;
    179 	       r = read(__daemon2_detach_pipe[0], &dummy, 1);
    180 	       if (r < 0) {
    181 		       if (errno == EINTR)
    182 			       continue;
    183 		       _exit(1);
    184 	       } else if (r == 0) {
    185 		       _exit(1);
    186 	       } else { /* r > 0 */
    187 		       _exit(0);
    188 	       }
    189        }
    190 }
    191 
    192 static int
    193 daemon2_detach(int nochdir, int noclose)
    194 {
    195 	int r;
    196 	int fd;
    197 
    198 	if (setsid() == -1)
    199 		return -1;
    200 
    201 	if (!nochdir)
    202 		(void)chdir("/");
    203 
    204 	if (!noclose && (fd = open(_PATH_DEVNULL, O_RDWR, 0)) != -1) {
    205 		(void)dup2(fd, STDIN_FILENO);
    206 		(void)dup2(fd, STDOUT_FILENO);
    207 		(void)dup2(fd, STDERR_FILENO);
    208 		if (fd > STDERR_FILENO)
    209 			(void)close(fd);
    210 	}
    211 
    212 	while (1) {
    213 		r = write(__daemon2_detach_pipe[1], "", 1);
    214 		if (r < 0) {
    215 			if (errno == EINTR)
    216 				continue;
    217 			/* May get "broken pipe" here if parent is killed */
    218 			return -1;
    219 		} else if (r == 0) {
    220 			/* Should not happen */
    221 			return -1;
    222 		} else {
    223 			break;
    224 		}
    225 	}
    226 
    227 	(void) close(__daemon2_detach_pipe[1]);
    228 
    229 	return 0;
    230 }
    231 
    232 int main(int argc, char **argv) {
    233 	int s, r, protocol;
    234 	union sockaddr_either saddr;
    235 	struct dns_data *dp;
    236 	unsigned char *p;
    237 	char pidfile_name[40];
    238 	FILE *f;
    239 	int one = 1;
    240 
    241 	daemon2_fork();
    242 
    243 	if (argc < 2 || ((protocol = argv[1][0]) != '4' && protocol != '6'))
    244 		errx(1, "usage: dns_server 4 | 6");
    245 	s = socket(protocol == '4' ? PF_INET : PF_INET6, SOCK_DGRAM, IPPROTO_UDP);
    246 	if (s < 0)
    247 		err(1, "socket");
    248 	if (protocol == '4') {
    249 		memset(&saddr.sin, 0, sizeof(saddr.sin));
    250 		saddr.sin.sin_family = AF_INET;
    251 		saddr.sin.sin_len = sizeof(saddr.sin);
    252 		saddr.sin.sin_port = htons(53);
    253 		saddr.sin.sin_addr.s_addr = INADDR_ANY;
    254 	} else {
    255 		static struct in6_addr loopback = IN6ADDR_LOOPBACK_INIT;
    256 		memset(&saddr.sin6, 0, sizeof(saddr.sin6));
    257 		saddr.sin6.sin6_family = AF_INET6;
    258 		saddr.sin6.sin6_len = sizeof(saddr.sin6);
    259 		saddr.sin6.sin6_port = htons(53);
    260 		saddr.sin6.sin6_addr = loopback;
    261 	}
    262 
    263 	r = setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &one, sizeof one);
    264 	if (r < 0)
    265 		err(1, "setsockopt");
    266 
    267 	r = bind(s,
    268 		 (struct sockaddr *) &saddr,
    269 		 protocol == '4' ? sizeof(struct sockaddr_in) :
    270 				   sizeof(struct sockaddr_in6));
    271 	if (r < 0)
    272 		err(1, "bind");
    273 
    274 	snprintf(pidfile_name, sizeof pidfile_name,
    275 		 "dns_server_%c.pid", protocol);
    276 	f = fopen(pidfile_name, "w");
    277 	fprintf(f, "%d", getpid());
    278 	fclose(f);
    279 	daemon2_detach(0, 0);
    280 
    281 	for (;;) {
    282 		unsigned char buf[512];
    283 		union sockaddr_either from;
    284 		ssize_t nrecv, nsent;
    285 		socklen_t fromlen =
    286 			protocol == '4' ? sizeof(struct sockaddr_in) :
    287 					  sizeof(struct sockaddr_in6);
    288 		memset(buf, 0, sizeof buf);
    289 		nrecv = recvfrom(s, buf, sizeof buf, 0, &from.s, &fromlen);
    290 		if (nrecv < 0)
    291 			err(1, "recvfrom");
    292 		if (nrecv < 12)
    293 			continue; /* Too short */
    294 		if ((buf[2] & 0x80) != 0)
    295 			continue; /* Not a query */
    296 		if (!(buf[4] == 0 && buf[5] == 1))
    297 		    continue; /* QDCOUNT is not 1 */
    298 
    299 		for (dp = data; dp->qname_size != 0; dp++) {
    300 			int qtype, qclass;
    301 			p = buf + 12; /* Point to QNAME */
    302 			int n = name_eq(p, (const unsigned char *) dp->qname);
    303 			if (n == 0)
    304 				continue; /* Name does not match */
    305 			p += n; /* Skip QNAME */
    306 			qtype = *p++ << 8;
    307 			qtype |= *p++;
    308 			if (qtype != dp->qtype)
    309 				continue;
    310 			qclass = *p++ << 8;
    311 			qclass |= *p++;
    312 			if (qclass != 1) /* IN */
    313 				continue;
    314 			goto found;
    315 		}
    316 		continue;
    317 	found:
    318 		buf[2] |= 0x80; /* QR */
    319 		buf[3] |= 0x80; /* RA */
    320 		memset(buf + 6, 0, 6); /* Clear ANCOUNT, NSCOUNT, ARCOUNT */
    321 		buf[7] = 1; /* ANCOUNT */
    322 		memcpy(p, dp->qname, dp->qname_size);
    323 		p += dp->qname_size;
    324 		*p++ = dp->qtype >> 8;
    325 		*p++ = dp->qtype & 0xFF;
    326 		*p++ = 0;
    327 		*p++ = 1; /* IN */
    328 		memset(p, 0, 4); /* TTL = 0 */
    329 		p += 4;
    330 		*p++ = 0;		/* RDLENGTH MSB */
    331 		*p++ = dp->answer_size;	/* RDLENGTH LSB */
    332 		memcpy(p, dp->answer, dp->answer_size);
    333 		p += dp->answer_size;
    334 		nsent = sendto(s, buf, p - buf, 0, &from.s, fromlen);
    335 		if (nsent != p - buf)
    336 			warn("sendto");
    337 	}
    338 }
    339