Commit 7721f9c1 authored by Pieter Wuille's avatar Pieter Wuille
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initial commit

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#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdint.h>
#include <sys/types.h>
#include <arpa/inet.h>
#define BUFLEN 512
typedef enum {
CLASS_IN = 1,
QCLASS_ANY = 255
} dns_class;
typedef enum {
TYPE_A = 1,
TYPE_NS = 2,
TYPE_CNAME = 5,
TYPE_SOA = 6,
TYPE_MX = 15,
TYPE_AAAA = 28,
TYPE_SRV = 33,
QTYPE_ANY = 255
} dns_type;
int port = 53;
char *host = "seedbeta.bitcoin.sipa.be";
// 0: ok
// -1: premature end of input
// -2: unsufficient space in output
int parse_name(const unsigned char **inpos, const unsigned char *inend, char *buf, size_t bufsize) {
size_t bufused = 0;
do {
if (*inpos == inend)
return -1;
int octet = *(inpos++);
if (octet == 0) {
buf[bufused] = 0;
return 0;
}
while (octet) {
if (*inpos == inend)
return -1;
if (bufused == bufsize-1)
return -2;
octet--;
buf[bufused++] = *(inpos++);
}
if (bufused == bufsize-1)
return -2;
buf[bufused++] = '.';
} while(1);
}
// 0: k
// -1: component > 63 characters
// -2: insufficent space in output
// -3: two subsequent dots
int write_name(unsigned char** outpos, unsigned char *outend, char *name) {
while (*name != 0) {
char *dot = strchr(name, '.');
char *fin = dot;
if (!dot) fin = name + strlen(name);
if (fin - name > 63) return -1;
if (fin == name) return -3;
if (outend - *outpos < fin - name + 2) return -2;
*(outpos++) = fin - name;
memcpy(*outpos, name, fin - name);
outpos += fin - name;
if (!dot) break;
name = dot + 1;
}
if (outend == *outpos) return -2;
*(outpos++) = 0;
return 0;
}
int write_record_a(unsigned char** outpos, unsigned char *outend, char *name, int cls, int ttl, uint32_t ip) {
unsigned char *oldpos = *outpos;
// name
if (write_name(outpos, outend, name)) goto error;
if (outend - *outpos < 14) goto error;
// type
*(outpos++) = TYPE_A >> 8; *(outpos++) = TYPE_A & 0xFF;
// class
*(outpos++) = cls >> 8; *(outpos++) = cls & 0xFF;
// ttl
*(outpos++) = (ttl >> 24) & 0xFF; *(outpos++) = (ttl >> 16) & 0xFF; *(outpos++) = (ttl >> 8) & 0xFF; *(outpos++) = ttl & 0xFF;
// rdlength
*(outpos++) = 0; *(outpos++) = 4;
// rdata
*(outpos++) = (ip >> 24) & 0xFF; *(outpos++) = (ip >> 16) & 0xFF; *(outpos++) = (ip >> 8) & 0xFF; *(outpos++) = ttl & 0xFF;
return 0;
error:
*outpos = oldpos;
return -1;
}
ssize_t dnshandle(const unsigned char *inbuf, size_t insize, unsigned char* outbuf) {
int error = 0;
if (insize < 12) // DNS header
return -1;
// copy id
outbuf[0] = inbuf[0];
outbuf[1] = inbuf[1];
// copy flags;
outbuf[2] = inbuf[2];
outbuf[3] = inbuf[3];
// clear error
outbuf[3] &= ~240;
// check qr
if (inbuf[2] & 1) { error = 1; goto error; }
// check opcode
if (((inbuf[2] & 30) >> 1) != 0) { error = 4; goto error; }
// check Z
if (((inbuf[3] & 14) >> 1) != 0) { error = 1; goto error; }
// unset TC
outbuf[2] &= ~64;
// unset RA
outbuf[3] &= ~1;
// check questions
int nquestion = inbuf[4] << 8 + inbuf[5];
if (nquestion == 0) { error = 0; goto error; }
if (nquestion > 0) { error = 4; goto error; }
const unsigned char *inpos = inbuf + 12;
const unsigned char *inend = inbuf + insize;
char name[256];
int ret = parse_name(&inpos, inend, name, 256))
if (ret == -1) { error = 1; goto error; }
if (ret == -2) { error = 5; goto error; }
if (strcmp(name, host)) { error = 0; goto error; }
if (inend - inpos < 4) { error = 1; goto error; }
// copy question to output
memcpy(outbuf+12, inbuf+12, inpos+4 - (inbuf+12));
// set counts
outbuf[4] = 0; outbuf[5] = 1;
outbuf[6] = 0; outbuf[7] = 0;
outbuf[8] = 0; outbuf[9] = 0;
outbuf[10] = 0; outbuf[11] = 0;
int typ = inpos[0] << 8 + inpos[1];
int cls = inpos[2] << 8 + inpos[3];
inpos += 4;
unsigned char *outpos = outbuf+(inpos-inbuf);
unsigned char *outend = outbuf + BUFLEN;
uint32_t ip = 0x01101102;
while (!write_record_a(&outpos, outend, host, CLASS_IN, 1, ip)) {
ip += 0x01101102;
outbuf[7] ++;
}
// set AA
outbuf[2] |= 32;
return outpos - outbuf;
error:
// set error
outbuf[3] |= error << 4;
// set counts
outbuf[4] = 0; outbuf[5] = 0;
outbuf[6] = 0; outbuf[7] = 0;
outbuf[8] = 0; outbuf[9] = 0;
outbuf[10] = 0; outbuf[11] = 0;
return 12;
}
int dnsserver(void) {
struct sockaddr_in si_me, si_other;
int s, i, slen=sizeof(si_other);
unsigned char inbuf[BUFLEN], outbuf[BUFLEN];
if ((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))==-1)
return -1;
memset((char *) &si_me, 0, sizeof(si_me));
si_me.sin_family = AF_INET;
si_me.sin_port = htons(port);
si_me.sin_addr.s_addr = INADDR_ANY;
if (bind(s, &si_me, sizeof(si_me))==-1)
return -2;
do {
ssize_t insize = recvfrom(s, inbuf, BUFLEN, 0, &si_other, &slen);
if (insize > 0) {
ssize_t ret = dnshandle(inbuf, insize, outbuf);
if (ret > 0)
sendto(s, outbuf, ret, &si_other, &slen);
}
} while(1);
return 0;
}
int main(void) {
dnsserver();
return 0;
}
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