Implement DNS protocol, fossa-style

Author: Marko Mikulicic

fossa.c

@@ -3384,3 +3384,190 @@ void ns_mqtt_disconnect(struct ns_connection *nc) {
 }
 
 #endif  /* NS_DISABLE_MQTT */
+/*
+ * Copyright (c) 2014 Cesanta Software Limited
+ * All rights reserved
+ */
+
+/*
+ * == DNS API
+ */
+
+#ifndef NS_DISABLE_DNS
+
+
+#define MAX_DNS_PACKET_LEN  2048
+
+static int ns_dns_tid = 0xa0;
+
+struct ns_dns_header {
+  uint16_t transaction_id;
+  uint16_t flags;
+  uint16_t num_questions;
+  uint16_t num_answers;
+  uint16_t num_authority_prs;
+  uint16_t num_other_prs;
+};
+
+/*
+ * Low-level: send a dns query to the remote end
+ */
+void ns_send_dns_query(struct ns_connection* nc, const char *name,
+                       int query_type) {
+  struct ns_dns_header header;
+  const char *s;
+  int n, name_len;
+  uint16_t num;
+  struct iobuf pkt;
+
+  iobuf_init(&pkt, MAX_DNS_PACKET_LEN);
+
+  memset(&header, 0, sizeof(header));
+  header.transaction_id = ++ns_dns_tid;
+  header.flags = htons(0x100);  /* recursion allowed */
+  header.num_questions = htons(1);
+
+  iobuf_append(&pkt, &header, sizeof(header));
+
+  name_len = strlen(name);
+  do {
+    if ((s = strchr(name, '.')) == NULL)
+      s = name + name_len;
+
+    n = s - name;              /* chunk length */
+    iobuf_append(&pkt, &n, 1); /* send length */
+    iobuf_append(&pkt, name, n);
+
+    if (*s == '.')
+      n++;
+
+    name += n;
+    name_len -= n;
+  } while (*s != '\0');
+  iobuf_append(&pkt, "\0", 1);  /* Mark end of host name */
+
+  num = htons(query_type);
+  iobuf_append(&pkt, &num, 2);
+  num = htons(0x0001);  /* Class: inet */
+  iobuf_append(&pkt, &num, 2);
+
+  /* TCP DNS requires messages to be prefixed with len */
+  if (!(nc->flags & NSF_UDP)) {
+    uint16_t len = htons(pkt.len);
+    iobuf_prepend(&pkt, &len, 2);
+  }
+
+  ns_send(nc, pkt.buf, pkt.len);
+  iobuf_free(&pkt);
+}
+
+static unsigned char *ns_parse_dns_resource_record(
+    unsigned char *data, struct ns_dns_resource_record *rr, int reply) {
+  unsigned char *name = data;
+  int chunk_len, data_len;
+
+  while((chunk_len = *data)) {
+    if (((unsigned char *)data)[0] & 0xc0) {
+      data += 1;
+      break;
+    }
+    data += chunk_len + 1;
+  }
+
+  rr->name.p = (char *) name;
+  rr->name.len = data-name+1;
+
+  data++;
+
+  rr->rtype = data[0] << 8 | data[1];
+  data += 2;
+
+  rr->rclass = data[0] << 8 | data[1];
+  data += 2;
+
+  if (reply) {
+    rr->ttl = data[0] << 24 | data[1] << 16 | data[2] << 8 | data[3];
+    data += 4;
+
+    data_len = *data << 8 | *(data+1);
+    data += 2;
+
+    rr->rdata.p = (char *) data;
+    rr->rdata.len = data_len;
+    data += data_len;
+  }
+  return data;
+}
+
+/* Low-level: parses a DNS response. */
+int ns_parse_dns(const char *buf, int len, struct ns_dns_message *msg) {
+  struct ns_dns_header *header = (struct ns_dns_header *) buf;
+  unsigned char *data = (unsigned char *) buf + sizeof(*header);
+  int i;
+  msg->pkt = buf;
+
+  if (len < (int)sizeof(*header)) {
+    return -1;
+  }
+
+  msg->num_questions = ntohs(header->num_questions);
+  msg->num_answers = ntohs(header->num_answers);
+
+  /* TODO(mkm): check bounds */
+
+  for (i = 0; i < msg->num_questions
+           && i < (int)ARRAY_SIZE(msg->questions); i++) {
+    data = ns_parse_dns_resource_record(data, &msg->questions[i], 0);
+  }
+
+  for (i = 0; i < msg->num_answers
+           && i < (int)ARRAY_SIZE(msg->answers); i++) {
+    data = ns_parse_dns_resource_record(data, &msg->answers[i], 1);
+  }
+
+  return 0;
+}
+
+/*
+ * Uncompress a DNS compressed name.
+ *
+ * The containing dns message is required because the compressed encoding
+ * and reference suffixes present elsewhere in the packet.
+ *
+ * If name is less than `dst_len` characters long, the remainder
+ * of `dst` is terminated with `\0' characters. Otherwise, `dst` is not terminated.
+ *
+ * If `dst_len` is 0 `dst` can be NULL.
+ * Returns the uncompressed name length.
+ */
+size_t ns_dns_uncompress_name(struct ns_dns_message *msg, struct ns_str *name,
+                              char *dst, int dst_len) {
+  int chunk_len;
+  char *old_dst = dst;
+  const unsigned char *data = (unsigned char *) name->p;
+
+  while((chunk_len = *data++)) {
+    int leeway = dst_len - (dst - old_dst);
+    if (chunk_len & 0xc0) {
+      uint16_t off = (data[-1] & (~0xc0)) << 8 | data[0];
+      data = (unsigned char *)msg->pkt + off;
+      continue;
+    }
+    if (chunk_len > leeway) {
+      chunk_len = leeway;
+    }
+
+    memcpy(dst, data, chunk_len);
+    data += chunk_len;
+    dst += chunk_len;
+    leeway -= chunk_len;
+    if (leeway == 0) {
+      return dst - old_dst;
+    }
+    *dst++ = '.';
+  }
+  *--dst = 0;
+  return dst - old_dst;
+}
+
+#endif  /* NS_DISABLE_DNS */

fossa.h

@@ -808,3 +808,53 @@ void ns_mqtt_pong(struct ns_connection *);
 #endif /* __cplusplus */
 
 #endif /* NS_MQTT_HEADER_INCLUDED */
+/*
+ * Copyright (c) 2014 Cesanta Software Limited
+ * All rights reserved
+ */
+
+#ifndef NS_DNS_HEADER_DEFINED
+#define NS_DNS_HEADER_DEFINED
+
+
+#ifdef __cplusplus
+extern "C" {
+#endif /* __cplusplus */
+
+#define NS_DNS_A_RECORD     0x01  /* Lookup IP address */
+#define NS_DNS_CNAME_RECORD 0x05  /* Lookup CNAME */
+#define NS_DNS_AAAA_RECORD  0x1c  /* Lookup IPv6 address */
+#define NS_DNS_MX_RECORD    0x0f  /* Lookup mail server for domain */
+
+void ns_dns_resolve(const char *, int, ns_event_handler_t);
+
+/* low-level */
+#define NS_MAX_DNS_QUESTIONS 32
+#define NS_MAX_DNS_ANSWERS   32
+
+struct ns_dns_resource_record {
+  struct ns_str name;  /* buffer with compressed name */
+  int rtype;
+  int rclass;
+  int ttl;
+  struct ns_str rdata;  /* protocol data (can be a compressed name) */
+};
+
+struct ns_dns_message {
+  const char *pkt;  /* packet body */
+  int num_questions;
+  int num_answers;
+  struct ns_dns_resource_record questions[NS_MAX_DNS_QUESTIONS];
+  struct ns_dns_resource_record answers[NS_MAX_DNS_ANSWERS];
+};
+
+void ns_send_dns_query(struct ns_connection*, const char *, int);
+int ns_parse_dns(const char *, int, struct ns_dns_message *);
+
+size_t ns_dns_uncompress_name(struct ns_dns_message *, struct ns_str *,
+                              char *, int);
+
+#ifdef __cplusplus
+}
+#endif /* __cplusplus */
+#endif  /* NS_HTTP_HEADER_DEFINED */