initial commit

dns.c

+#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;
+}