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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20 | #include <unistd.h>
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21 | #include <string.h>
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22 | #include <signal.h>
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23 | #include <arpa/inet.h>
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24 | #include <sys/socket.h>
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25 | #include <ctype.h>
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26 | #include "busybox.h"
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27 |
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28 | static char *fileconf = "/etc/dnsd.conf";
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29 | #define LOCK_FILE "/var/run/dnsd.lock"
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30 | #define LOG_FILE "/var/log/dnsd.log"
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31 |
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32 | enum {
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33 | MAX_HOST_LEN = 16, // longest host name allowed is 15
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34 | IP_STRING_LEN = 18, // .xxx.xxx.xxx.xxx\0
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35 |
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36 | //must be strlen('.in-addr.arpa') larger than IP_STRING_LEN
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37 | MAX_NAME_LEN = (IP_STRING_LEN + 13),
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38 |
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39 | /* Cannot get bigger packets than 512 per RFC1035
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40 | In practice this can be set considerably smaller:
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41 | Length of response packet is header (12B) + 2*type(4B) + 2*class(4B) +
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42 | ttl(4B) + rlen(2B) + r (MAX_NAME_LEN =21B) +
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43 | 2*querystring (2 MAX_NAME_LEN= 42B), all together 90 Byte
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44 | */
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45 | MAX_PACK_LEN = 512 + 1,
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46 |
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47 | DEFAULT_TTL = 30, // increase this when not testing?
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48 |
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49 | REQ_A = 1,
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50 | REQ_PTR = 12
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51 | };
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52 |
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53 | struct dns_repl { // resource record, add 0 or 1 to accepted dns_msg in resp
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54 | uint16_t rlen;
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55 | uint8_t *r; // resource
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56 | uint16_t flags;
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57 | };
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58 |
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59 | struct dns_head { // the message from client and first part of response mag
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60 | uint16_t id;
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61 | uint16_t flags;
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62 | uint16_t nquer; // accepts 0
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63 | uint16_t nansw; // 1 in response
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64 | uint16_t nauth; // 0
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65 | uint16_t nadd; // 0
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66 | };
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67 | struct dns_prop {
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68 | uint16_t type;
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69 | uint16_t class;
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70 | };
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71 | struct dns_entry { // element of known name, ip address and reversed ip address
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72 | struct dns_entry *next;
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73 | char ip[IP_STRING_LEN]; // dotted decimal IP
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74 | char rip[IP_STRING_LEN]; // length decimal reversed IP
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75 | char name[MAX_HOST_LEN];
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76 | };
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77 |
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78 | static struct dns_entry *dnsentry = NULL;
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79 | static int daemonmode = 0;
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80 | static uint32_t ttl = DEFAULT_TTL;
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81 |
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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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88 | for(; i < MAX_HOST_LEN-1 && *q; i++, q++)
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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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100 | while(rip[i]) i++;
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101 | for(--i; i >= 0; i--) {
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102 | if(rip[i] == '.') {
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103 | rip[i] = s;
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104 | s = 0;
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105 | } else s++;
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106 | }
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107 | }
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108 |
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109 | /*
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110 | * Append message to log file
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111 | */
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112 | static void log_message(char *filename, char *message)
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113 | {
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114 | FILE *logfile;
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115 | if (!daemonmode)
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116 | return;
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117 | logfile = fopen(filename, "a");
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118 | if (!logfile)
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119 | return;
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120 | fprintf(logfile, "%s\n", message);
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121 | fclose(logfile);
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122 | }
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123 |
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124 | /*
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125 | * Read one line of hostname/IP from file
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126 | * Returns 0 for each valid entry read, -1 at EOF
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127 | * Assumes all host names are lower case only
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128 | * Hostnames with more than one label is not handled correctly.
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129 | * Presently the dot is copied into name without
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130 | * converting to a length/string substring for that label.
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131 | */
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132 |
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133 | static int getfileentry(FILE * fp, struct dns_entry *s, int verb)
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134 | {
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135 | unsigned int a,b,c,d;
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136 | char *r, *name;
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137 |
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138 | restart:
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139 | if(!(r = bb_get_line_from_file(fp)))
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140 | return -1;
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141 | while(*r == ' ' || *r == '\t') {
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142 | r++;
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143 | if(!*r || *r == '#' || *r == '\n')
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144 | goto restart; /* skipping empty/blank and commented lines */
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145 | }
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146 | name = r;
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147 | while(*r != ' ' && *r != '\t')
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148 | r++;
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149 | *r++ = 0;
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150 | if(sscanf(r,"%u.%u.%u.%u",&a,&b,&c,&d) != 4)
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151 | goto restart; /* skipping wrong lines */
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152 |
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153 | sprintf(s->ip,"%u.%u.%u.%u",a,b,c,d);
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154 | sprintf(s->rip,".%u.%u.%u.%u",d,c,b,a);
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155 | undot((uint8_t*)s->rip);
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156 | convname(s->name,(uint8_t*)name);
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157 |
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158 | if(verb)
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159 | fprintf(stderr,"\tname:%s, ip:%s\n",&(s->name[1]),s->ip);
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160 |
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161 | return 0; /* warningkiller */
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162 | }
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163 |
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164 | /*
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165 | * Read hostname/IP records from file
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166 | */
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167 | static void dnsentryinit(int verb)
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168 | {
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169 | FILE *fp;
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170 | struct dns_entry *m, *prev;
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171 | prev = dnsentry = NULL;
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172 |
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173 | fp = bb_xfopen(fileconf, "r");
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174 |
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175 | while (1) {
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176 | m = xmalloc(sizeof(struct dns_entry));
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177 |
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178 | m->next = NULL;
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179 | if (getfileentry(fp, m, verb))
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180 | break;
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181 |
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182 | if (prev == NULL)
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183 | dnsentry = m;
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184 | else
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185 | prev->next = m;
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186 | prev = m;
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187 | }
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188 | fclose(fp);
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189 | }
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190 |
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191 |
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192 | /*
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193 | * Set up UDP socket
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194 | */
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195 | static int listen_socket(char *iface_addr, int listen_port)
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196 | {
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197 | struct sockaddr_in a;
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198 | char msg[100];
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199 | int s;
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200 | int yes = 1;
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201 | s = bb_xsocket(PF_INET, SOCK_DGRAM, 0);
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202 | #ifdef SO_REUSEADDR
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203 | if (setsockopt(s, SOL_SOCKET, SO_REUSEADDR, (char *)&yes, sizeof(yes)) < 0)
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204 | bb_perror_msg_and_die("setsockopt() failed");
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205 | #endif
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206 | memset(&a, 0, sizeof(a));
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207 | a.sin_port = htons(listen_port);
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208 | a.sin_family = AF_INET;
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209 | if (!inet_aton(iface_addr, &a.sin_addr))
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210 | bb_perror_msg_and_die("bad iface address");
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211 | bb_xbind(s, (struct sockaddr *)&a, sizeof(a));
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212 | listen(s, 50); /* bb_xlisten? */
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213 | sprintf(msg, "accepting UDP packets on addr:port %s:%d\n",
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214 | iface_addr, (int)listen_port);
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215 | log_message(LOG_FILE, msg);
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216 | return s;
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217 | }
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218 |
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219 | /*
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220 | * Look query up in dns records and return answer if found
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221 | * qs is the query string, first byte the string length
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222 | */
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223 | static int table_lookup(uint16_t type, uint8_t * as, uint8_t * qs)
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224 | {
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225 | int i;
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226 | struct dns_entry *d=dnsentry;
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227 |
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228 | do {
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229 | #ifdef DEBUG
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230 | char *p,*q;
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231 | q = (char *)&(qs[1]);
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232 | p = &(d->name[1]);
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233 | fprintf(stderr, "\ntest: %d <%s> <%s> %d", strlen(p), p, q, strlen(q));
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234 | #endif
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235 | if (type == REQ_A) { /* search by host name */
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236 | for(i = 1; i <= (int)(d->name[0]); i++)
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237 | if(tolower(qs[i]) != d->name[i])
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238 | continue;
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239 | #ifdef DEBUG
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240 | fprintf(stderr, " OK");
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241 | #endif
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242 | strcpy((char *)as, d->ip);
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243 | #ifdef DEBUG
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244 | fprintf(stderr, " %s ", as);
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245 | #endif
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246 | return 0;
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247 | }
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248 | else if (type == REQ_PTR) { /* search by IP-address */
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249 | if (!strncmp((char*)&d->rip[1], (char*)&qs[1], strlen(d->rip)-1)) {
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250 | strcpy((char *)as, d->name);
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251 | return 0;
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252 | }
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253 | }
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254 | } while ((d = d->next) != NULL);
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255 | return -1;
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256 | }
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257 |
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258 |
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259 | /*
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260 | * Decode message and generate answer
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261 | */
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262 | #define eret(s) do { fprintf (stderr, "%s\n", s); return -1; } while (0)
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263 | static int process_packet(uint8_t * buf)
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264 | {
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265 | struct dns_head *head;
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266 | struct dns_prop *qprop;
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267 | struct dns_repl outr;
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268 | void *next, *from, *answb;
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269 |
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270 | uint8_t answstr[MAX_NAME_LEN + 1];
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271 | int lookup_result, type, len, packet_len;
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272 | uint16_t flags;
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273 |
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274 | answstr[0] = '\0';
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275 |
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276 | head = (struct dns_head *)buf;
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277 | if (head->nquer == 0)
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278 | eret("no queries");
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279 |
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280 | if ((head->flags & 0x8000))
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281 | eret("ignoring response packet");
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282 |
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283 | from = (void *)&head[1]; // start of query string
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284 | next = answb = from + strlen((char *)&head[1]) + 1 + sizeof(struct dns_prop); // where to append answer block
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285 |
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286 | outr.rlen = 0; // may change later
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287 | outr.r = NULL;
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288 | outr.flags = 0;
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289 |
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290 | qprop = (struct dns_prop *)(answb - 4);
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291 | type = ntohs(qprop->type);
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292 |
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293 | // only let REQ_A and REQ_PTR pass
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294 | if (!(type == REQ_A || type == REQ_PTR)) {
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295 | goto empty_packet; /* we can't handle the query type */
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296 | }
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297 |
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298 | if (ntohs(qprop->class) != 1 /* class INET */ ) {
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299 | outr.flags = 4; /* not supported */
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300 | goto empty_packet;
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301 | }
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302 | /* we only support standard queries */
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303 |
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304 | if ((ntohs(head->flags) & 0x7800) != 0)
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305 | goto empty_packet;
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306 |
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307 | // We have a standard query
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308 |
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309 | log_message(LOG_FILE, (char *)head);
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310 | lookup_result = table_lookup(type, answstr, (uint8_t*)(&head[1]));
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311 | if (lookup_result != 0) {
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312 | outr.flags = 3 | 0x0400; //name do not exist and auth
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313 | goto empty_packet;
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314 | }
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315 | if (type == REQ_A) { // return an address
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316 | struct in_addr a;
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317 | if (!inet_aton((char*)answstr, &a)) {//dotted dec to long conv
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318 | outr.flags = 1; /* Frmt err */
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319 | goto empty_packet;
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320 | }
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321 | memcpy(answstr, &a.s_addr, 4); // save before a disappears
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322 | outr.rlen = 4; // uint32_t IP
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323 | }
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324 | else
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325 | outr.rlen = strlen((char *)answstr) + 1; // a host name
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326 | outr.r = answstr; // 32 bit ip or a host name
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327 | outr.flags |= 0x0400; /* authority-bit */
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328 | // we have an answer
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329 | head->nansw = htons(1);
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330 |
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331 | // copy query block to answer block
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332 | len = answb - from;
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333 | memcpy(answb, from, len);
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334 | next += len;
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335 |
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336 | // and append answer rr
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337 | *(uint32_t *) next = htonl(ttl);
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338 | next += 4;
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339 | *(uint16_t *) next = htons(outr.rlen);
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340 | next += 2;
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341 | memcpy(next, (void *)answstr, outr.rlen);
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342 | next += outr.rlen;
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343 |
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344 | empty_packet:
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345 |
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346 | flags = ntohs(head->flags);
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347 | // clear rcode and RA, set responsebit and our new flags
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348 | flags |= (outr.flags & 0xff80) | 0x8000;
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349 | head->flags = htons(flags);
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350 | head->nauth = head->nadd = htons(0);
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351 | head->nquer = htons(1);
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352 |
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353 | packet_len = next - (void *)buf;
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354 | return packet_len;
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355 | }
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356 |
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357 | /*
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358 | * Exit on signal
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359 | */
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360 | static void interrupt(int x)
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361 | {
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362 | unlink(LOCK_FILE);
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363 | write(2, "interrupt exiting\n", 18);
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364 | exit(2);
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365 | }
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366 |
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367 | #define is_daemon() (flags&16)
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368 | #define is_verbose() (flags&32)
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369 | //#define DEBUG 1
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370 |
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371 | int dnsd_main(int argc, char **argv)
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372 | {
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373 | int udps;
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374 | uint16_t port = 53;
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375 | uint8_t buf[MAX_PACK_LEN];
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376 | unsigned long flags = 0;
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377 | char *listen_interface = "0.0.0.0";
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378 | char *sttl=NULL, *sport=NULL;
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379 |
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380 | if(argc > 1)
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381 | flags = bb_getopt_ulflags(argc, argv, "i:c:t:p:dv", &listen_interface, &fileconf, &sttl, &sport);
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382 | if(sttl)
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383 | if(!(ttl = atol(sttl)))
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384 | bb_show_usage();
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385 | if(sport)
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386 | if(!(port = atol(sport)))
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387 | bb_show_usage();
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388 |
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389 | if(is_verbose()) {
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390 | fprintf(stderr,"listen_interface: %s\n", listen_interface);
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391 | fprintf(stderr,"ttl: %d, port: %d\n", ttl, port);
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392 | fprintf(stderr,"fileconf: %s\n", fileconf);
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393 | }
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394 |
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395 | if(is_daemon())
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396 | #ifdef BB_NOMMU
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397 | /* reexec for vfork() do continue parent */
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398 | vfork_daemon_rexec(1, 0, argc, argv, "-d");
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399 | #else
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400 | bb_xdaemon(1, 0);
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401 | #endif
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402 |
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403 | dnsentryinit(is_verbose());
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404 |
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405 | signal(SIGINT, interrupt);
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406 | signal(SIGPIPE, SIG_IGN);
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407 | signal(SIGHUP, SIG_IGN);
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408 | #ifdef SIGTSTP
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409 | signal(SIGTSTP, SIG_IGN);
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410 | #endif
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411 | #ifdef SIGURG
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412 | signal(SIGURG, SIG_IGN);
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413 | #endif
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414 |
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415 | udps = listen_socket(listen_interface, port);
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416 | if (udps < 0)
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417 | exit(1);
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418 |
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419 | while (1) {
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420 | fd_set fdset;
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421 | int r;
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422 |
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423 | FD_ZERO(&fdset);
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424 | FD_SET(udps, &fdset);
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425 | // Block until a message arrives
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426 | if((r = select(udps + 1, &fdset, NULL, NULL, NULL)) < 0)
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427 | bb_perror_msg_and_die("select error");
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428 | else
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429 | if(r == 0)
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430 | bb_perror_msg_and_die("select spurious return");
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431 |
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432 | /* Can this test ever be false? */
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433 | if (FD_ISSET(udps, &fdset)) {
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434 | struct sockaddr_in from;
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435 | int fromlen = sizeof(from);
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436 | r = recvfrom(udps, buf, sizeof(buf), 0,
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437 | (struct sockaddr *)&from,
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438 | (void *)&fromlen);
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439 | if(is_verbose())
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440 | fprintf(stderr, "\n--- Got UDP ");
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441 | log_message(LOG_FILE, "\n--- Got UDP ");
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442 |
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443 | if (r < 12 || r > 512) {
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444 | bb_error_msg("invalid packet size");
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445 | continue;
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446 | }
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447 | if (r > 0) {
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448 | r = process_packet(buf);
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449 | if (r > 0)
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450 | sendto(udps, buf,
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451 | r, 0, (struct sockaddr *)&from,
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452 | fromlen);
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453 | }
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454 | } // end if
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455 | } // end while
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456 | return 0;
|
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457 | }
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458 |
|
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459 |
|
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