busybox/networking/dnsd.c

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/* 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 "libbb.h"
#include <syslog.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 {
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#if DEBUG
char *p,*q;
q = (char *)&(qs[1]);
p = &(d->name[1]);
fprintf(stderr, "\n%s: %d/%d p:%s q:%s %d",
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__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);
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#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");
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return -1;
}
if (head->flags & 0x8000) {
bb_error_msg("ignoring response packet");
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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) MAIN_EXTERNALLY_VISIBLE;
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->u.sa.sa_family, SOCK_DGRAM, 0);
xbind(udps, &lsa->u.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->u.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->u.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->u.sa, fromlen);
}
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return 0;
}