[821] | 1 | /* vi: set sw=4 ts=4: */
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| 2 | /*
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| 3 | * Mini DNS server implementation for busybox
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| 4 | *
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| 5 | * Copyright (C) 2005 Roberto A. Foglietta (me@roberto.foglietta.name)
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| 6 | * Copyright (C) 2005 Odd Arild Olsen (oao at fibula dot no)
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| 7 | * Copyright (C) 2003 Paul Sheer
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| 8 | *
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| 9 | * Licensed under GPLv2 or later, see file LICENSE in this tarball for details.
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| 10 | *
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| 11 | * Odd Arild Olsen started out with the sheerdns [1] of Paul Sheer and rewrote
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| 12 | * it into a shape which I believe is both easier to understand and maintain.
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| 13 | * I also reused the input buffer for output and removed services he did not
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| 14 | * need. [1] http://threading.2038bug.com/sheerdns/
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| 15 | *
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| 16 | * Some bugfix and minor changes was applied by Roberto A. Foglietta who made
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| 17 | * the first porting of oao' scdns to busybox also.
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| 18 | */
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| 19 |
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[1770] | 20 | #include <syslog.h>
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| 21 | #include "libbb.h"
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[821] | 22 |
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[1770] | 23 | //#define DEBUG 1
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| 24 | #define DEBUG 0
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[821] | 25 |
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| 26 | enum {
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| 27 | MAX_HOST_LEN = 16, // longest host name allowed is 15
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| 28 | IP_STRING_LEN = 18, // .xxx.xxx.xxx.xxx\0
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| 29 |
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| 30 | //must be strlen('.in-addr.arpa') larger than IP_STRING_LEN
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| 31 | MAX_NAME_LEN = (IP_STRING_LEN + 13),
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| 32 |
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| 33 | /* Cannot get bigger packets than 512 per RFC1035
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| 34 | In practice this can be set considerably smaller:
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| 35 | Length of response packet is header (12B) + 2*type(4B) + 2*class(4B) +
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| 36 | ttl(4B) + rlen(2B) + r (MAX_NAME_LEN =21B) +
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| 37 | 2*querystring (2 MAX_NAME_LEN= 42B), all together 90 Byte
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| 38 | */
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| 39 | MAX_PACK_LEN = 512 + 1,
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| 40 |
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| 41 | DEFAULT_TTL = 30, // increase this when not testing?
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| 42 |
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| 43 | REQ_A = 1,
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| 44 | REQ_PTR = 12
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| 45 | };
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| 46 |
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| 47 | struct dns_repl { // resource record, add 0 or 1 to accepted dns_msg in resp
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| 48 | uint16_t rlen;
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| 49 | uint8_t *r; // resource
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| 50 | uint16_t flags;
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| 51 | };
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| 52 |
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| 53 | struct dns_head { // the message from client and first part of response mag
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| 54 | uint16_t id;
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| 55 | uint16_t flags;
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| 56 | uint16_t nquer; // accepts 0
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| 57 | uint16_t nansw; // 1 in response
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| 58 | uint16_t nauth; // 0
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| 59 | uint16_t nadd; // 0
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| 60 | };
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| 61 | struct dns_prop {
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| 62 | uint16_t type;
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| 63 | uint16_t class;
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| 64 | };
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| 65 | struct dns_entry { // element of known name, ip address and reversed ip address
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| 66 | struct dns_entry *next;
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| 67 | char ip[IP_STRING_LEN]; // dotted decimal IP
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| 68 | char rip[IP_STRING_LEN]; // length decimal reversed IP
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| 69 | char name[MAX_HOST_LEN];
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| 70 | };
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| 71 |
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[1770] | 72 | static struct dns_entry *dnsentry;
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[821] | 73 | static uint32_t ttl = DEFAULT_TTL;
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| 74 |
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[1770] | 75 | static const char *fileconf = "/etc/dnsd.conf";
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| 76 |
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| 77 | // Must match getopt32 call
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| 78 | #define OPT_daemon (option_mask32 & 0x10)
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| 79 | #define OPT_verbose (option_mask32 & 0x20)
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| 80 |
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| 81 |
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[821] | 82 | /*
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| 83 | * Convert host name from C-string to dns length/string.
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| 84 | */
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| 85 | static void convname(char *a, uint8_t *q)
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| 86 | {
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| 87 | int i = (q[0] == '.') ? 0 : 1;
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[1770] | 88 | for (; i < MAX_HOST_LEN-1 && *q; i++, q++)
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[821] | 89 | a[i] = tolower(*q);
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| 90 | a[0] = i - 1;
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| 91 | a[i] = 0;
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| 92 | }
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| 93 |
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| 94 | /*
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| 95 | * Insert length of substrings instead of dots
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| 96 | */
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| 97 | static void undot(uint8_t * rip)
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| 98 | {
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| 99 | int i = 0, s = 0;
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[1770] | 100 | while (rip[i])
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| 101 | i++;
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| 102 | for (--i; i >= 0; i--) {
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| 103 | if (rip[i] == '.') {
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[821] | 104 | rip[i] = s;
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| 105 | s = 0;
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| 106 | } else s++;
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| 107 | }
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| 108 | }
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| 109 |
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| 110 | /*
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| 111 | * Read one line of hostname/IP from file
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| 112 | * Returns 0 for each valid entry read, -1 at EOF
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| 113 | * Assumes all host names are lower case only
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[1770] | 114 | * Hostnames with more than one label are not handled correctly.
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[821] | 115 | * Presently the dot is copied into name without
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| 116 | * converting to a length/string substring for that label.
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| 117 | */
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[1770] | 118 | static int getfileentry(FILE * fp, struct dns_entry *s)
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[821] | 119 | {
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| 120 | unsigned int a,b,c,d;
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[1770] | 121 | char *line, *r, *name;
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[821] | 122 |
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[1770] | 123 | restart:
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| 124 | line = r = xmalloc_fgets(fp);
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| 125 | if (!r)
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[821] | 126 | return -1;
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[1770] | 127 | while (*r == ' ' || *r == '\t') {
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[821] | 128 | r++;
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[1770] | 129 | if (!*r || *r == '#' || *r == '\n') {
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| 130 | free(line);
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[821] | 131 | goto restart; /* skipping empty/blank and commented lines */
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[1770] | 132 | }
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[821] | 133 | }
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| 134 | name = r;
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[1770] | 135 | while (*r != ' ' && *r != '\t')
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[821] | 136 | r++;
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[1770] | 137 | *r++ = '\0';
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| 138 | if (sscanf(r, ".%u.%u.%u.%u"+1, &a, &b, &c, &d) != 4) {
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| 139 | free(line);
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| 140 | goto restart; /* skipping wrong lines */
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| 141 | }
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[821] | 142 |
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[1770] | 143 | sprintf(s->ip, ".%u.%u.%u.%u"+1, a, b, c, d);
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| 144 | sprintf(s->rip, ".%u.%u.%u.%u", d, c, b, a);
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[821] | 145 | undot((uint8_t*)s->rip);
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[1770] | 146 | convname(s->name, (uint8_t*)name);
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[821] | 147 |
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[1770] | 148 | if (OPT_verbose)
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| 149 | fprintf(stderr, "\tname:%s, ip:%s\n", &(s->name[1]),s->ip);
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[821] | 150 |
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[1770] | 151 | free(line);
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| 152 | return 0;
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[821] | 153 | }
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| 154 |
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| 155 | /*
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| 156 | * Read hostname/IP records from file
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| 157 | */
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[1770] | 158 | static void dnsentryinit(void)
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[821] | 159 | {
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| 160 | FILE *fp;
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| 161 | struct dns_entry *m, *prev;
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[1770] | 162 |
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[821] | 163 | prev = dnsentry = NULL;
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[1770] | 164 | fp = xfopen(fileconf, "r");
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[821] | 165 |
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| 166 | while (1) {
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[1770] | 167 | m = xzalloc(sizeof(*m));
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| 168 | /*m->next = NULL;*/
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| 169 | if (getfileentry(fp, m))
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[821] | 170 | break;
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| 171 |
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| 172 | if (prev == NULL)
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| 173 | dnsentry = m;
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| 174 | else
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| 175 | prev->next = m;
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| 176 | prev = m;
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| 177 | }
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| 178 | fclose(fp);
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| 179 | }
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| 180 |
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| 181 | /*
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| 182 | * Look query up in dns records and return answer if found
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| 183 | * qs is the query string, first byte the string length
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| 184 | */
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| 185 | static int table_lookup(uint16_t type, uint8_t * as, uint8_t * qs)
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| 186 | {
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| 187 | int i;
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[902] | 188 | struct dns_entry *d = dnsentry;
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[821] | 189 |
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[1770] | 190 | do {
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| 191 | #if DEBUG
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| 192 | char *p,*q;
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| 193 | q = (char *)&(qs[1]);
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| 194 | p = &(d->name[1]);
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| 195 | fprintf(stderr, "\n%s: %d/%d p:%s q:%s %d",
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| 196 | __FUNCTION__, (int)strlen(p), (int)(d->name[0]),
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| 197 | p, q, (int)strlen(q));
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[821] | 198 | #endif
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[1770] | 199 | if (type == REQ_A) { /* search by host name */
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| 200 | for (i = 1; i <= (int)(d->name[0]); i++)
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| 201 | if (tolower(qs[i]) != d->name[i])
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[902] | 202 | break;
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[1770] | 203 | if (i > (int)(d->name[0])) {
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[902] | 204 | strcpy((char *)as, d->ip);
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[1770] | 205 | #if DEBUG
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| 206 | fprintf(stderr, " OK as:%s\n", as);
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[821] | 207 | #endif
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[902] | 208 | return 0;
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| 209 | }
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[1770] | 210 | } else if (type == REQ_PTR) { /* search by IP-address */
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[821] | 211 | if (!strncmp((char*)&d->rip[1], (char*)&qs[1], strlen(d->rip)-1)) {
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| 212 | strcpy((char *)as, d->name);
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| 213 | return 0;
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| 214 | }
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| 215 | }
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[1770] | 216 | d = d->next;
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| 217 | } while (d);
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[821] | 218 | return -1;
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| 219 | }
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| 220 |
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| 221 |
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| 222 | /*
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| 223 | * Decode message and generate answer
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| 224 | */
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| 225 | static int process_packet(uint8_t * buf)
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| 226 | {
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| 227 | struct dns_head *head;
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| 228 | struct dns_prop *qprop;
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| 229 | struct dns_repl outr;
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| 230 | void *next, *from, *answb;
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| 231 |
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| 232 | uint8_t answstr[MAX_NAME_LEN + 1];
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| 233 | int lookup_result, type, len, packet_len;
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| 234 | uint16_t flags;
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| 235 |
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| 236 | answstr[0] = '\0';
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| 237 |
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| 238 | head = (struct dns_head *)buf;
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[1770] | 239 | if (head->nquer == 0) {
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| 240 | bb_error_msg("no queries");
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| 241 | return -1;
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| 242 | }
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[821] | 243 |
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[1770] | 244 | if (head->flags & 0x8000) {
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| 245 | bb_error_msg("ignoring response packet");
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| 246 | return -1;
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| 247 | }
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[821] | 248 |
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| 249 | from = (void *)&head[1]; // start of query string
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[902] | 250 | next = answb = from + strlen((char *)from) + 1 + sizeof(struct dns_prop); // where to append answer block
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[821] | 251 |
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| 252 | outr.rlen = 0; // may change later
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| 253 | outr.r = NULL;
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| 254 | outr.flags = 0;
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| 255 |
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| 256 | qprop = (struct dns_prop *)(answb - 4);
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| 257 | type = ntohs(qprop->type);
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| 258 |
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| 259 | // only let REQ_A and REQ_PTR pass
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| 260 | if (!(type == REQ_A || type == REQ_PTR)) {
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| 261 | goto empty_packet; /* we can't handle the query type */
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| 262 | }
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| 263 |
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| 264 | if (ntohs(qprop->class) != 1 /* class INET */ ) {
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| 265 | outr.flags = 4; /* not supported */
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| 266 | goto empty_packet;
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| 267 | }
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| 268 | /* we only support standard queries */
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| 269 |
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| 270 | if ((ntohs(head->flags) & 0x7800) != 0)
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| 271 | goto empty_packet;
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| 272 |
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| 273 | // We have a standard query
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[1770] | 274 | bb_info_msg("%s", (char *)from);
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[902] | 275 | lookup_result = table_lookup(type, answstr, (uint8_t*)from);
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[821] | 276 | if (lookup_result != 0) {
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| 277 | outr.flags = 3 | 0x0400; //name do not exist and auth
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| 278 | goto empty_packet;
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| 279 | }
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| 280 | if (type == REQ_A) { // return an address
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| 281 | struct in_addr a;
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| 282 | if (!inet_aton((char*)answstr, &a)) {//dotted dec to long conv
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| 283 | outr.flags = 1; /* Frmt err */
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| 284 | goto empty_packet;
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| 285 | }
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| 286 | memcpy(answstr, &a.s_addr, 4); // save before a disappears
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| 287 | outr.rlen = 4; // uint32_t IP
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[1770] | 288 | } else
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[821] | 289 | outr.rlen = strlen((char *)answstr) + 1; // a host name
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| 290 | outr.r = answstr; // 32 bit ip or a host name
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| 291 | outr.flags |= 0x0400; /* authority-bit */
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| 292 | // we have an answer
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| 293 | head->nansw = htons(1);
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| 294 |
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| 295 | // copy query block to answer block
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| 296 | len = answb - from;
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| 297 | memcpy(answb, from, len);
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| 298 | next += len;
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| 299 |
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| 300 | // and append answer rr
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| 301 | *(uint32_t *) next = htonl(ttl);
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| 302 | next += 4;
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| 303 | *(uint16_t *) next = htons(outr.rlen);
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| 304 | next += 2;
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| 305 | memcpy(next, (void *)answstr, outr.rlen);
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| 306 | next += outr.rlen;
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| 307 |
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[1770] | 308 | empty_packet:
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| 309 |
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[821] | 310 | flags = ntohs(head->flags);
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| 311 | // clear rcode and RA, set responsebit and our new flags
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| 312 | flags |= (outr.flags & 0xff80) | 0x8000;
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| 313 | head->flags = htons(flags);
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| 314 | head->nauth = head->nadd = htons(0);
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| 315 | head->nquer = htons(1);
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| 316 |
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[1770] | 317 | packet_len = (uint8_t *)next - buf;
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[821] | 318 | return packet_len;
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| 319 | }
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| 320 |
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| 321 | /*
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| 322 | * Exit on signal
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| 323 | */
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| 324 | static void interrupt(int x)
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| 325 | {
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[1770] | 326 | /* unlink("/var/run/dnsd.lock"); */
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| 327 | bb_error_msg("interrupt, exiting\n");
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[821] | 328 | exit(2);
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| 329 | }
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| 330 |
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[1770] | 331 | int dnsd_main(int argc, char **argv);
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[821] | 332 | int dnsd_main(int argc, char **argv)
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| 333 | {
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[1770] | 334 | const char *listen_interface = "0.0.0.0";
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| 335 | char *sttl, *sport;
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| 336 | len_and_sockaddr *lsa;
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[821] | 337 | int udps;
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| 338 | uint16_t port = 53;
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| 339 | uint8_t buf[MAX_PACK_LEN];
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| 340 |
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[1770] | 341 | getopt32(argv, "i:c:t:p:dv", &listen_interface, &fileconf, &sttl, &sport);
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| 342 | //if (option_mask32 & 0x1) // -i
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| 343 | //if (option_mask32 & 0x2) // -c
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| 344 | if (option_mask32 & 0x4) // -t
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| 345 | ttl = xatou_range(sttl, 1, 0xffffffff);
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| 346 | if (option_mask32 & 0x8) // -p
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| 347 | port = xatou_range(sport, 1, 0xffff);
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[821] | 348 |
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[1770] | 349 | if (OPT_verbose) {
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| 350 | bb_info_msg("listen_interface: %s", listen_interface);
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| 351 | bb_info_msg("ttl: %d, port: %d", ttl, port);
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| 352 | bb_info_msg("fileconf: %s", fileconf);
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[821] | 353 | }
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| 354 |
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[1770] | 355 | if (OPT_daemon) {
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| 356 | bb_daemonize_or_rexec(DAEMON_CLOSE_EXTRA_FDS, argv);
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| 357 | openlog(applet_name, LOG_PID, LOG_DAEMON);
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| 358 | logmode = LOGMODE_SYSLOG;
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| 359 | }
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[821] | 360 |
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[1770] | 361 | dnsentryinit();
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[821] | 362 |
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| 363 | signal(SIGINT, interrupt);
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[1770] | 364 | /* why? signal(SIGPIPE, SIG_IGN); */
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[821] | 365 | signal(SIGHUP, SIG_IGN);
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| 366 | #ifdef SIGTSTP
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| 367 | signal(SIGTSTP, SIG_IGN);
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| 368 | #endif
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| 369 | #ifdef SIGURG
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| 370 | signal(SIGURG, SIG_IGN);
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| 371 | #endif
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| 372 |
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[1770] | 373 | lsa = xdotted2sockaddr(listen_interface, port);
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| 374 | udps = xsocket(lsa->sa.sa_family, SOCK_DGRAM, 0);
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| 375 | xbind(udps, &lsa->sa, lsa->len);
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| 376 | /* xlisten(udps, 50); - ?!! DGRAM sockets are never listened on I think? */
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| 377 | bb_info_msg("Accepting UDP packets on %s",
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| 378 | xmalloc_sockaddr2dotted(&lsa->sa));
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[821] | 379 |
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| 380 | while (1) {
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| 381 | int r;
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[1770] | 382 | socklen_t fromlen = lsa->len;
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| 383 | // FIXME: need to get *DEST* address (to which of our addresses
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| 384 | // this query was directed), and reply from the same address.
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| 385 | // Or else we can exhibit usual UDP ugliness:
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| 386 | // [ip1.multihomed.ip2] <= query to ip1 <= peer
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| 387 | // [ip1.multihomed.ip2] => reply from ip2 => peer (confused)
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| 388 | r = recvfrom(udps, buf, sizeof(buf), 0, &lsa->sa, &fromlen);
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| 389 | if (OPT_verbose)
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| 390 | bb_info_msg("Got UDP packet");
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| 391 | if (r < 12 || r > 512) {
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| 392 | bb_error_msg("invalid packet size");
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| 393 | continue;
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| 394 | }
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| 395 | r = process_packet(buf);
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| 396 | if (r <= 0)
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| 397 | continue;
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| 398 | sendto(udps, buf, r, 0, &lsa->sa, fromlen);
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| 399 | }
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[821] | 400 | return 0;
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| 401 | }
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