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 source tree.
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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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20 | #include "libbb.h"
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21 | #include <syslog.h>
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22 |
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23 | //#define DEBUG 1
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24 | #define DEBUG 0
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25 |
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26 | enum {
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27 | /* can tweak this */
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28 | DEFAULT_TTL = 120,
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29 |
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30 | /* cannot get bigger packets than 512 per RFC1035. */
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31 | MAX_PACK_LEN = 512,
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32 | IP_STRING_LEN = sizeof(".xxx.xxx.xxx.xxx"),
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33 | MAX_NAME_LEN = IP_STRING_LEN - 1 + sizeof(".in-addr.arpa"),
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34 | REQ_A = 1,
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35 | REQ_PTR = 12,
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36 | };
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37 |
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38 | /* the message from client and first part of response msg */
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39 | struct dns_head {
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40 | uint16_t id;
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41 | uint16_t flags;
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42 | uint16_t nquer;
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43 | uint16_t nansw;
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44 | uint16_t nauth;
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45 | uint16_t nadd;
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46 | };
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47 | /* Structure used to access type and class fields.
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48 | * They are totally unaligned, but gcc 4.3.4 thinks that pointer of type uint16_t*
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49 | * is 16-bit aligned and replaces 16-bit memcpy (in move_from_unaligned16 macro)
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50 | * with aligned halfword access on arm920t!
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51 | * Oh well. Slapping PACKED everywhere seems to help: */
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52 | struct type_and_class {
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53 | uint16_t type PACKED;
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54 | uint16_t class PACKED;
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55 | } PACKED;
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56 | /* element of known name, ip address and reversed ip address */
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57 | struct dns_entry {
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58 | struct dns_entry *next;
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59 | uint32_t ip;
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60 | char rip[IP_STRING_LEN]; /* length decimal reversed IP */
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61 | char name[1];
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62 | };
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63 |
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64 | #define OPT_verbose (option_mask32 & 1)
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65 | #define OPT_silent (option_mask32 & 2)
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66 |
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67 |
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68 | /*
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69 | * Insert length of substrings instead of dots
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70 | */
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71 | static void undot(char *rip)
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72 | {
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73 | int i = 0;
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74 | int s = 0;
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75 |
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76 | while (rip[i])
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77 | i++;
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78 | for (--i; i >= 0; i--) {
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79 | if (rip[i] == '.') {
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80 | rip[i] = s;
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81 | s = 0;
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82 | } else {
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83 | s++;
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84 | }
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85 | }
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86 | }
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87 |
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88 | /*
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89 | * Read hostname/IP records from file
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90 | */
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91 | static struct dns_entry *parse_conf_file(const char *fileconf)
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92 | {
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93 | char *token[2];
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94 | parser_t *parser;
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95 | struct dns_entry *m, *conf_data;
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96 | struct dns_entry **nextp;
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97 |
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98 | conf_data = NULL;
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99 | nextp = &conf_data;
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100 |
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101 | parser = config_open(fileconf);
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102 | while (config_read(parser, token, 2, 2, "# \t", PARSE_NORMAL)) {
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103 | struct in_addr ip;
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104 | uint32_t v32;
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105 |
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106 | if (inet_aton(token[1], &ip) == 0) {
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107 | bb_error_msg("error at line %u, skipping", parser->lineno);
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108 | continue;
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109 | }
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110 |
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111 | if (OPT_verbose)
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112 | bb_error_msg("name:%s, ip:%s", token[0], token[1]);
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113 |
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114 | /* sizeof(*m) includes 1 byte for m->name[0] */
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115 | m = xzalloc(sizeof(*m) + strlen(token[0]) + 1);
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116 | /*m->next = NULL;*/
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117 | *nextp = m;
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118 | nextp = &m->next;
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119 |
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120 | m->name[0] = '.';
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121 | strcpy(m->name + 1, token[0]);
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122 | undot(m->name);
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123 | m->ip = ip.s_addr; /* in network order */
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124 | v32 = ntohl(m->ip);
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125 | /* inverted order */
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126 | sprintf(m->rip, ".%u.%u.%u.%u",
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127 | (uint8_t)(v32),
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128 | (uint8_t)(v32 >> 8),
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129 | (uint8_t)(v32 >> 16),
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130 | (v32 >> 24)
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131 | );
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132 | undot(m->rip);
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133 | }
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134 | config_close(parser);
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135 | return conf_data;
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136 | }
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137 |
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138 | /*
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139 | * Look query up in dns records and return answer if found.
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140 | */
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141 | static char *table_lookup(struct dns_entry *d,
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142 | uint16_t type,
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143 | char* query_string)
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144 | {
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145 | while (d) {
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146 | unsigned len = d->name[0];
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147 | /* d->name[len] is the last (non NUL) char */
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148 | #if DEBUG
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149 | char *p, *q;
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150 | q = query_string + 1;
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151 | p = d->name + 1;
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152 | fprintf(stderr, "%d/%d p:%s q:%s %d\n",
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153 | (int)strlen(p), len,
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154 | p, q, (int)strlen(q)
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155 | );
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156 | #endif
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157 | if (type == htons(REQ_A)) {
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158 | /* search by host name */
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159 | if (len != 1 || d->name[1] != '*') {
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160 | /* we are lax, hope no name component is ever >64 so that length
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161 | * (which will be represented as 'A','B'...) matches a lowercase letter.
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162 | * Actually, I think false matches are hard to construct.
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163 | * Example.
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164 | * [31] len is represented as '1', [65] as 'A', [65+32] as 'a'.
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165 | * [65] <65 same chars>[31]<31 same chars>NUL
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166 | * [65+32]<65 same chars>1 <31 same chars>NUL
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167 | * This example seems to be the minimal case when false match occurs.
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168 | */
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169 | if (strcasecmp(d->name, query_string) != 0)
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170 | goto next;
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171 | }
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172 | return (char *)&d->ip;
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173 | #if DEBUG
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174 | fprintf(stderr, "Found IP:%x\n", (int)d->ip);
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175 | #endif
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176 | return 0;
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177 | }
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178 | /* search by IP-address */
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179 | if ((len != 1 || d->name[1] != '*')
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180 | /* we assume (do not check) that query_string
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181 | * ends in ".in-addr.arpa" */
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182 | && strncmp(d->rip, query_string, strlen(d->rip)) == 0
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183 | ) {
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184 | #if DEBUG
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185 | fprintf(stderr, "Found name:%s\n", d->name);
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186 | #endif
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187 | return d->name;
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188 | }
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189 | next:
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190 | d = d->next;
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191 | }
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192 |
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193 | return NULL;
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194 | }
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195 |
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196 | /*
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197 | * Decode message and generate answer
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198 | */
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199 | /* RFC 1035
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200 | ...
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201 | Whenever an octet represents a numeric quantity, the left most bit
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202 | in the diagram is the high order or most significant bit.
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203 | That is, the bit labeled 0 is the most significant bit.
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204 | ...
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205 |
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206 | 4.1.1. Header section format
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207 | 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
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208 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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209 | | ID |
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210 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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211 | |QR| OPCODE |AA|TC|RD|RA| 0 0 0| RCODE |
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212 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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213 | | QDCOUNT |
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214 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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215 | | ANCOUNT |
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216 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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217 | | NSCOUNT |
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218 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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219 | | ARCOUNT |
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220 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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221 | ID 16 bit random identifier assigned by querying peer.
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222 | Used to match query/response.
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223 | QR message is a query (0), or a response (1).
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224 | OPCODE 0 standard query (QUERY)
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225 | 1 inverse query (IQUERY)
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226 | 2 server status request (STATUS)
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227 | AA Authoritative Answer - this bit is valid in responses.
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228 | Responding name server is an authority for the domain name
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229 | in question section. Answer section may have multiple owner names
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230 | because of aliases. The AA bit corresponds to the name which matches
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231 | the query name, or the first owner name in the answer section.
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232 | TC TrunCation - this message was truncated.
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233 | RD Recursion Desired - this bit may be set in a query and
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234 | is copied into the response. If RD is set, it directs
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235 | the name server to pursue the query recursively.
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236 | Recursive query support is optional.
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237 | RA Recursion Available - this be is set or cleared in a
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238 | response, and denotes whether recursive query support is
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239 | available in the name server.
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240 | RCODE Response code.
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241 | 0 No error condition
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242 | 1 Format error
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243 | 2 Server failure - server was unable to process the query
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244 | due to a problem with the name server.
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245 | 3 Name Error - meaningful only for responses from
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246 | an authoritative name server. The referenced domain name
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247 | does not exist.
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248 | 4 Not Implemented.
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249 | 5 Refused.
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250 | QDCOUNT number of entries in the question section.
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251 | ANCOUNT number of records in the answer section.
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252 | NSCOUNT number of records in the authority records section.
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253 | ARCOUNT number of records in the additional records section.
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254 |
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255 | 4.1.2. Question section format
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256 |
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257 | The section contains QDCOUNT (usually 1) entries, each of this format:
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258 | 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
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259 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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260 | / QNAME /
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261 | / /
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262 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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263 | | QTYPE |
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264 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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265 | | QCLASS |
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266 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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267 | QNAME a domain name represented as a sequence of labels, where
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268 | each label consists of a length octet followed by that
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269 | number of octets. The domain name terminates with the
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270 | zero length octet for the null label of the root. Note
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271 | that this field may be an odd number of octets; no
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272 | padding is used.
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273 | QTYPE a two octet type of the query.
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274 | 1 a host address [REQ_A const]
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275 | 2 an authoritative name server
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276 | 3 a mail destination (Obsolete - use MX)
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277 | 4 a mail forwarder (Obsolete - use MX)
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278 | 5 the canonical name for an alias
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279 | 6 marks the start of a zone of authority
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280 | 7 a mailbox domain name (EXPERIMENTAL)
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281 | 8 a mail group member (EXPERIMENTAL)
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282 | 9 a mail rename domain name (EXPERIMENTAL)
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283 | 10 a null RR (EXPERIMENTAL)
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284 | 11 a well known service description
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285 | 12 a domain name pointer [REQ_PTR const]
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286 | 13 host information
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287 | 14 mailbox or mail list information
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288 | 15 mail exchange
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289 | 16 text strings
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290 | 0x1c IPv6?
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291 | 252 a request for a transfer of an entire zone
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292 | 253 a request for mailbox-related records (MB, MG or MR)
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293 | 254 a request for mail agent RRs (Obsolete - see MX)
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294 | 255 a request for all records
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295 | QCLASS a two octet code that specifies the class of the query.
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296 | 1 the Internet
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297 | (others are historic only)
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298 | 255 any class
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299 |
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300 | 4.1.3. Resource Record format
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301 |
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302 | The answer, authority, and additional sections all share the same format:
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303 | a variable number of resource records, where the number of records
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304 | is specified in the corresponding count field in the header.
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305 | Each resource record has this format:
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306 | 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
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307 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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308 | / /
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309 | / NAME /
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310 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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311 | | TYPE |
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312 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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313 | | CLASS |
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314 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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315 | | TTL |
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316 | | |
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317 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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318 | | RDLENGTH |
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319 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--|
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320 | / RDATA /
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321 | / /
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322 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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323 | NAME a domain name to which this resource record pertains.
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324 | TYPE two octets containing one of the RR type codes. This
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325 | field specifies the meaning of the data in the RDATA field.
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326 | CLASS two octets which specify the class of the data in the RDATA field.
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327 | TTL a 32 bit unsigned integer that specifies the time interval
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328 | (in seconds) that the record may be cached.
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329 | RDLENGTH a 16 bit integer, length in octets of the RDATA field.
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330 | RDATA a variable length string of octets that describes the resource.
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331 | The format of this information varies according to the TYPE
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332 | and CLASS of the resource record.
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333 | If the TYPE is A and the CLASS is IN, it's a 4 octet IP address.
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334 |
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335 | 4.1.4. Message compression
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336 |
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337 | In order to reduce the size of messages, domain names coan be compressed.
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338 | An entire domain name or a list of labels at the end of a domain name
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339 | is replaced with a pointer to a prior occurance of the same name.
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340 |
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341 | The pointer takes the form of a two octet sequence:
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342 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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343 | | 1 1| OFFSET |
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344 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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345 | The first two bits are ones. This allows a pointer to be distinguished
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346 | from a label, since the label must begin with two zero bits because
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347 | labels are restricted to 63 octets or less. The OFFSET field specifies
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348 | an offset from the start of the message (i.e., the first octet
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349 | of the ID field in the domain header).
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350 | A zero offset specifies the first byte of the ID field, etc.
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351 | Domain name in a message can be represented as either:
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352 | - a sequence of labels ending in a zero octet
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353 | - a pointer
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354 | - a sequence of labels ending with a pointer
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355 | */
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356 | static int process_packet(struct dns_entry *conf_data,
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357 | uint32_t conf_ttl,
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358 | uint8_t *buf)
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359 | {
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360 | struct dns_head *head;
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361 | struct type_and_class *unaligned_type_class;
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362 | const char *err_msg;
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363 | char *query_string;
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364 | char *answstr;
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365 | uint8_t *answb;
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366 | uint16_t outr_rlen;
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367 | uint16_t outr_flags;
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368 | uint16_t type;
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369 | uint16_t class;
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370 | int query_len;
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371 |
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372 | head = (struct dns_head *)buf;
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373 | if (head->nquer == 0) {
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374 | bb_error_msg("packet has 0 queries, ignored");
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375 | return 0; /* don't reply */
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376 | }
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377 | if (head->flags & htons(0x8000)) { /* QR bit */
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378 | bb_error_msg("response packet, ignored");
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379 | return 0; /* don't reply */
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380 | }
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381 | /* QR = 1 "response", RCODE = 4 "Not Implemented" */
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382 | outr_flags = htons(0x8000 | 4);
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383 | err_msg = NULL;
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384 |
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385 | /* start of query string */
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386 | query_string = (void *)(head + 1);
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387 | /* caller guarantees strlen is <= MAX_PACK_LEN */
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388 | query_len = strlen(query_string) + 1;
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389 | /* may be unaligned! */
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390 | unaligned_type_class = (void *)(query_string + query_len);
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391 | query_len += sizeof(*unaligned_type_class);
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392 | /* where to append answer block */
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393 | answb = (void *)(unaligned_type_class + 1);
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394 |
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395 | /* OPCODE != 0 "standard query"? */
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396 | if ((head->flags & htons(0x7800)) != 0) {
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397 | err_msg = "opcode != 0";
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398 | goto empty_packet;
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399 | }
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400 | move_from_unaligned16(class, &unaligned_type_class->class);
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401 | if (class != htons(1)) { /* not class INET? */
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402 | err_msg = "class != 1";
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403 | goto empty_packet;
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404 | }
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405 | move_from_unaligned16(type, &unaligned_type_class->type);
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406 | if (type != htons(REQ_A) && type != htons(REQ_PTR)) {
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407 | /* we can't handle this query type */
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408 | //TODO: happens all the time with REQ_AAAA (0x1c) requests - implement those?
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409 | err_msg = "type is !REQ_A and !REQ_PTR";
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410 | goto empty_packet;
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411 | }
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412 |
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413 | /* look up the name */
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414 | answstr = table_lookup(conf_data, type, query_string);
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415 | #if DEBUG
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416 | /* Shows lengths instead of dots, unusable for !DEBUG */
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417 | bb_error_msg("'%s'->'%s'", query_string, answstr);
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418 | #endif
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419 | outr_rlen = 4;
|
---|
420 | if (answstr && type == htons(REQ_PTR)) {
|
---|
421 | /* returning a host name */
|
---|
422 | outr_rlen = strlen(answstr) + 1;
|
---|
423 | }
|
---|
424 | if (!answstr
|
---|
425 | || (unsigned)(answb - buf) + query_len + 4 + 2 + outr_rlen > MAX_PACK_LEN
|
---|
426 | ) {
|
---|
427 | /* QR = 1 "response"
|
---|
428 | * AA = 1 "Authoritative Answer"
|
---|
429 | * RCODE = 3 "Name Error" */
|
---|
430 | err_msg = "name is not found";
|
---|
431 | outr_flags = htons(0x8000 | 0x0400 | 3);
|
---|
432 | goto empty_packet;
|
---|
433 | }
|
---|
434 |
|
---|
435 | /* Append answer Resource Record */
|
---|
436 | memcpy(answb, query_string, query_len); /* name, type, class */
|
---|
437 | answb += query_len;
|
---|
438 | move_to_unaligned32((uint32_t *)answb, htonl(conf_ttl));
|
---|
439 | answb += 4;
|
---|
440 | move_to_unaligned16((uint16_t *)answb, htons(outr_rlen));
|
---|
441 | answb += 2;
|
---|
442 | memcpy(answb, answstr, outr_rlen);
|
---|
443 | answb += outr_rlen;
|
---|
444 |
|
---|
445 | /* QR = 1 "response",
|
---|
446 | * AA = 1 "Authoritative Answer",
|
---|
447 | * TODO: need to set RA bit 0x80? One user says nslookup complains
|
---|
448 | * "Got recursion not available from SERVER, trying next server"
|
---|
449 | * "** server can't find HOSTNAME"
|
---|
450 | * RCODE = 0 "success"
|
---|
451 | */
|
---|
452 | if (OPT_verbose)
|
---|
453 | bb_error_msg("returning positive reply");
|
---|
454 | outr_flags = htons(0x8000 | 0x0400 | 0);
|
---|
455 | /* we have one answer */
|
---|
456 | head->nansw = htons(1);
|
---|
457 |
|
---|
458 | empty_packet:
|
---|
459 | if ((outr_flags & htons(0xf)) != 0) { /* not a positive response */
|
---|
460 | if (OPT_verbose) {
|
---|
461 | bb_error_msg("%s, %s",
|
---|
462 | err_msg,
|
---|
463 | OPT_silent ? "dropping query" : "sending error reply"
|
---|
464 | );
|
---|
465 | }
|
---|
466 | if (OPT_silent)
|
---|
467 | return 0;
|
---|
468 | }
|
---|
469 | head->flags |= outr_flags;
|
---|
470 | head->nauth = head->nadd = 0;
|
---|
471 | head->nquer = htons(1); // why???
|
---|
472 |
|
---|
473 | return answb - buf;
|
---|
474 | }
|
---|
475 |
|
---|
476 | int dnsd_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
|
---|
477 | int dnsd_main(int argc UNUSED_PARAM, char **argv)
|
---|
478 | {
|
---|
479 | const char *listen_interface = "0.0.0.0";
|
---|
480 | const char *fileconf = "/etc/dnsd.conf";
|
---|
481 | struct dns_entry *conf_data;
|
---|
482 | uint32_t conf_ttl = DEFAULT_TTL;
|
---|
483 | char *sttl, *sport;
|
---|
484 | len_and_sockaddr *lsa, *from, *to;
|
---|
485 | unsigned lsa_size;
|
---|
486 | int udps, opts;
|
---|
487 | uint16_t port = 53;
|
---|
488 | /* Ensure buf is 32bit aligned (we need 16bit, but 32bit can't hurt) */
|
---|
489 | uint8_t buf[MAX_PACK_LEN + 1] ALIGN4;
|
---|
490 |
|
---|
491 | opts = getopt32(argv, "vsi:c:t:p:d", &listen_interface, &fileconf, &sttl, &sport);
|
---|
492 | //if (opts & (1 << 0)) // -v
|
---|
493 | //if (opts & (1 << 1)) // -s
|
---|
494 | //if (opts & (1 << 2)) // -i
|
---|
495 | //if (opts & (1 << 3)) // -c
|
---|
496 | if (opts & (1 << 4)) // -t
|
---|
497 | conf_ttl = xatou_range(sttl, 1, 0xffffffff);
|
---|
498 | if (opts & (1 << 5)) // -p
|
---|
499 | port = xatou_range(sport, 1, 0xffff);
|
---|
500 | if (opts & (1 << 6)) { // -d
|
---|
501 | bb_daemonize_or_rexec(DAEMON_CLOSE_EXTRA_FDS, argv);
|
---|
502 | openlog(applet_name, LOG_PID, LOG_DAEMON);
|
---|
503 | logmode = LOGMODE_SYSLOG;
|
---|
504 | }
|
---|
505 |
|
---|
506 | conf_data = parse_conf_file(fileconf);
|
---|
507 |
|
---|
508 | lsa = xdotted2sockaddr(listen_interface, port);
|
---|
509 | udps = xsocket(lsa->u.sa.sa_family, SOCK_DGRAM, 0);
|
---|
510 | xbind(udps, &lsa->u.sa, lsa->len);
|
---|
511 | socket_want_pktinfo(udps); /* needed for recv_from_to to work */
|
---|
512 | lsa_size = LSA_LEN_SIZE + lsa->len;
|
---|
513 | from = xzalloc(lsa_size);
|
---|
514 | to = xzalloc(lsa_size);
|
---|
515 |
|
---|
516 | {
|
---|
517 | char *p = xmalloc_sockaddr2dotted(&lsa->u.sa);
|
---|
518 | bb_error_msg("accepting UDP packets on %s", p);
|
---|
519 | free(p);
|
---|
520 | }
|
---|
521 |
|
---|
522 | while (1) {
|
---|
523 | int r;
|
---|
524 | /* Try to get *DEST* address (to which of our addresses
|
---|
525 | * this query was directed), and reply from the same address.
|
---|
526 | * Or else we can exhibit usual UDP ugliness:
|
---|
527 | * [ip1.multihomed.ip2] <= query to ip1 <= peer
|
---|
528 | * [ip1.multihomed.ip2] => reply from ip2 => peer (confused) */
|
---|
529 | memcpy(to, lsa, lsa_size);
|
---|
530 | r = recv_from_to(udps, buf, MAX_PACK_LEN + 1, 0, &from->u.sa, &to->u.sa, lsa->len);
|
---|
531 | if (r < 12 || r > MAX_PACK_LEN) {
|
---|
532 | bb_error_msg("packet size %d, ignored", r);
|
---|
533 | continue;
|
---|
534 | }
|
---|
535 | if (OPT_verbose)
|
---|
536 | bb_error_msg("got UDP packet");
|
---|
537 | buf[r] = '\0'; /* paranoia */
|
---|
538 | r = process_packet(conf_data, conf_ttl, buf);
|
---|
539 | if (r <= 0)
|
---|
540 | continue;
|
---|
541 | send_to_from(udps, buf, r, 0, &from->u.sa, &to->u.sa, lsa->len);
|
---|
542 | }
|
---|
543 | return 0;
|
---|
544 | }
|
---|