parse a DNS header!

CMakeLists.txt

@@ -1,6 +1,7 @@
 cmake_minimum_required(VERSION 4.0)
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_BUILD_TYPE Debug)
 
 project(pupdns VERSION 0.1)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)

include/dns_packet_buffer.h

@@ -5,7 +5,7 @@
 #define PB_OUT_OF_BOUNDS 9999;
 
 struct DNSPacketBuffer {
-    char buf[PB_SIZE];
+    unsigned char buf[PB_SIZE];
     size_t pos;
 };
 
@@ -13,4 +13,4 @@ struct DNSPacketBuffer *new_dns_packet_buffer();
 
 struct DNSPacketHeader *dns_header(struct DNSPacketBuffer);
 
-char pb_read_byte(struct DNSPacketBuffer *);
+unsigned char pb_read_byte(struct DNSPacketBuffer *);

include/dns_packets.h

@@ -1,6 +1,7 @@
 #pragma once
 #include <stddef.h>
 
+#define DNS_HEADER_BYTES 12
 
 struct DNSPacketHeader {
     /*
@@ -8,14 +9,34 @@ struct DNSPacketHeader {
      * a random ID is assigned to query packets.
      * Response packets must contain the same ID as the query.
      */
-    size_t packet_id : 16;
+    unsigned int packet_id : 16;
     
     /*
-     * QR (Query/Response)
+     * RD (Recursion Desired)
-     * 0 if the packet is a query
+     * if set by the client, the server should
-     * 1 if the packet is a response
+     * attempt to resolve the query recursively
+     * 
+     * the server may not allow recursive resolution.
+     * (see RA / Recursion Available)
      */
-    size_t is_response : 1;
+    unsigned int recursion_desired : 1;
+
+    /*
+     * TC (Truncated)
+     * if true, this message was truncated.
+     * usually happens when the response payload
+     * is larger than the max UDP packet size
+     *
+     * if set, the query may be re-issued over TCP for the full payload.
+     */
+    unsigned int is_truncated : 1;
+
+    /*
+     * AA (Authoritative Answer)
+     * 
+     * 1 if the responding server is authoritative for the queried domain
+     */
+    unsigned int is_authoritative : 1;
 
     /*
      * OPCODE (Operation Code)
@@ -27,51 +48,14 @@ struct DNSPacketHeader {
      *
      * almost always will be 0
      */
-    size_t opcode : 4;
+    unsigned int opcode : 4;
 
     /*
-     * AA (Authoritative Answer)
+     * QR (Query/Response)
-     * 
+     * 0 if the packet is a query
-     * 1 if the responding server is authoritative for the queried domain
+     * 1 if the packet is a response
      */
-    size_t is_authoritative : 1;
+    unsigned int is_response : 1;
-
-    /*
-     * TC (Truncated)
-     * if true, this message was truncated.
-     * usually happens when the response payload
-     * is larger than the max UDP packet size
-     *
-     * if set, the query may be re-issued over TCP for the full payload.
-     */
-    size_t is_truncated : 1;
-
-    /*
-     * RD (Recursion Desired)
-     * if set by the client, the server should
-     * attempt to resolve the query recursively
-     * 
-     * the server may not allow recursive resolution.
-     * (see RA / Recursion Available)
-     */
-    size_t recursion_desired : 1;
-
-
-    /*
-     * RA (Recursion Available)
-     * if set by the server, incicates that the server allows
-     * recursive queries.
-     */
-    size_t recursion_available : 1;   
-
-    /*
-     * Z (Reserved)
-     * in the original spec, this is reserved for future use.
-     *
-     * in later RFCs, it is used for DNSSEC queries.
-     * (to be implemented)
-     */
-    size_t z : 3;
 
     /*
      * RCODE (Response Code)
@@ -86,31 +70,47 @@ struct DNSPacketHeader {
      * 5    - Refused - the nameserver refuses to perform the requested operation. for example, it may refuse a zone transfer.
      * 6-15 - Reserved for future use
      */
-    size_t response_code : 4;
+    unsigned int response_code : 4;
+
+    /*
+     * Z (Reserved)
+     * in the original spec, this is reserved for future use.
+     *
+     * in later RFCs, it is used for DNSSEC queries.
+     * (to be implemented)
+     */
+    unsigned int z : 3;
+
+    /*
+     * RA (Recursion Available)
+     * if set by the server, incicates that the server allows
+     * recursive queries.
+     */
+    unsigned int recursion_available : 1;   
 
     /*
      * QDCOUNT (Question Count)
      * the number of entries in the question section
      */
-    size_t question_count : 16;
+    unsigned int question_count : 16;
 
     /*
      * ANCOUNT (Answer Count)
      * the number of entries in the answer section
      */
-    size_t answer_count : 16;
+    unsigned int answer_count : 16;
 
     /*
      * NSCOUNT (Nameserver Count)
      * the number of entries in the authority records section
      */
-    size_t ns_count : 16;
+    unsigned int ns_count : 16;
 
     /*
      * ARCOUNT (Additional Records Count)
      * the number of entries in the additional records section
      */
-    size_t ar_count : 16;
+    unsigned int ar_count : 16;
 };
 
 
@@ -125,26 +125,26 @@ struct RecordPreamble {
      * TYPE
      * the record type
      */
-    size_t type : 16;
+    unsigned int type : 16;
 
     /*
      * CLASS
      * specifies the class of the data in the RDATA field.
      * in practice, always set to 1
      */
-    size_t class : 16;
+    unsigned int class : 16;
 
     /*
      * TTL (Time-To-Live)
      * how long a record can be cached before it should be requeried.
      */
-    size_t ttl : 32;
+    unsigned int ttl : 32;
 
     /*
      * RDLENGTH (Record Data Length)
      * length of the record-type-specific data.
      */
-    size_t rd_data_length: 16;
+    unsigned int rd_data_length: 16;
 };
 
 /*
@@ -153,5 +153,5 @@ struct RecordPreamble {
  */
 struct ARecord {
     struct RecordPreamble preamble;
-    size_t ip : 32;
+    unsigned int ip : 32;
 };

include/dns_request.h

@@ -7,4 +7,4 @@ struct DNSRequest {
     struct DNSPacketHeader *header;
 };
 
-struct DNSRequest *new_dns_request(struct DNSPacketBuffer *pb);
+struct DNSRequest *parse_dns_query(struct DNSPacketBuffer *pb);

src/dns_packet_buffer.c

@@ -16,9 +16,9 @@ struct DNSPacketHeader *dns_header(struct DNSPacketBuffer)
 
 
 // read a single byte and move the cursor one byte forward
-char pb_read_byte(struct DNSPacketBuffer *pb)
+unsigned char pb_read_byte(struct DNSPacketBuffer *pb)
 {
-    char b = pb->buf[pb->pos];
+    unsigned char b = pb->buf[pb->pos];
     pb->pos++;
 
     if (pb->pos >= PB_SIZE) {
@@ -32,7 +32,7 @@ char pb_read_byte(struct DNSPacketBuffer *pb)
     return b;
 }
 
-char pb_get_byte(struct DNSPacketBuffer *pb)
+unsigned char pb_get_byte(struct DNSPacketBuffer *pb)
 {
     if (pb->pos >= PB_SIZE) {
        printf(

src/dns_request.c

@@ -1,14 +1,66 @@
 #include <stdlib.h>
+#include <arpa/inet.h>
 
 #include "dns_request.h"
 
+/*
+ *  get k'th bit from byte c
+ *  because of how the DNS spec works, this starts counting from the LEFTMOST bit.
+ */
+// unsigned int get_bit(unsigned char c, unsigned int k)
+// {
+//     unsigned int flag = 1U; // flag = 0000.....00001
+//
+//     flag = flag << (8U-k); // flag = 0000...010...000   (shifted k positions)
+//
+//     return c & flag; 
+// }
 
+/*
+ *  get n bits from byte c starting at position k
+ *  because of how the DNS spec works, this starts counting from the LEFTMOST bit.
+ */
+// unsigned int get_bits(unsigned char c, unsigned int n, unsigned int k)
+// {
+//     unsigned int ret = 0;
+//     unsigned int flag = 1U; // flag = 0000.....00001
+//
+//     for (int j = 0; j < n; j++) {
+//         unsigned int bit_pos = 8U - k + j;
+//         ret = ret | get_bit(c, k+j) << bit_pos;
+//     }
+//
+//     return ret;
+// }
 
-struct DNSRequest *new_dns_request(struct DNSPacketBuffer *pb)
+struct DNSRequest *parse_dns_query(struct DNSPacketBuffer *pb)
 {
     struct DNSRequest *req;
     req = malloc(sizeof(struct DNSRequest));
-
     req->header = (struct DNSPacketHeader *) pb->buf;
+
+    /*
+     * casting the buffer directly into the packet header struct like this
+     * works okay, but because of the nature of the DNS protocol, it needs
+     * a lot of manual tweaking.
+     *
+     * the struct fields are defined in a weird order (compared to the spec)
+     * because C wants to read each byte from right to left,
+     * but DNS fields go left to right.
+     *
+     * so for each set of fields adding up to 8 bits of data, i reversed the
+     * order of the fields in the struct definition so that the fields end up
+     * with the correct bits off the wire.
+     *
+     * DNS also has multi-bit (but less than a byte) fields such as 
+     * opcode and response code. fortunately we can skip these for now,
+     * since opcode is practically always zero for our purposes,
+     * and response_code only makes sense in the server response.
+     *
+     * but we do need to take care of all the network-order 16-bit fields.
+     */
+    req->header->packet_id = ntohs(req->header->packet_id);
+    req->header->question_count = ntohs(req->header->question_count);
+
     return req;
 }

src/udp_server.c

@@ -2,10 +2,10 @@
 #include "dns_packet_buffer.h"
 #include "dns_request.h"
 
+#include <stdlib.h>
 #include <stdio.h>
 #include <unistd.h>
 #include <fcntl.h>
-#include <string.h>
 #include <arpa/inet.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
@@ -37,20 +37,37 @@ int start_server()
         return -1;
     }
 
-    socklen_t len = 0;
+    while (true) {
+        socklen_t len = 0;
 
-    int n = recvfrom(sockfd, (char *)pb->buf, sizeof(pb->buf), MSG_WAITALL, 0, &len);
+        int n = recvfrom(sockfd, (char *)pb->buf, sizeof(pb->buf), MSG_WAITALL, 0, &len);
 
-    printf("received %d bytes\n", n);
+        printf("===== NEW REQUEST =====\n");
+        printf("received %d bytes\n", n);
 
-    struct DNSRequest *req = new_dns_request(pb);
+        struct DNSRequest *req = parse_dns_query(pb);
 
-    printf("packet_id: %x\n", req->header->packet_id);
+        printf("packet_id: %x\n", req->header->packet_id);
-    printf("is_response: %d\n", req->header->is_response);
+        printf("is_response: %d\n", req->header->is_response);
-    printf("opcode: %d\n", req->header->opcode);
+        printf("opcode: %d\n", req->header->opcode);
-    printf("is_authoritative: %d\n", req->header->is_authoritative);
+        printf("is_authoritative: %d\n", req->header->is_authoritative);
-    printf("is_truncated: %d\n", req->header->is_truncated);
+        printf("is_truncated: %d\n", req->header->is_truncated);
-    printf("recursion_desired: %d\n", req->header->recursion_desired);
+        printf("recursion_desired: %d\n", req->header->recursion_desired);
+        printf("recursion_available: %d\n", req->header->recursion_available);
+        printf("reserved: %d\n", req->header->z);
+        printf("response_code: %d\n", req->header->response_code);
+        printf("question_count: %d\n", req->header->question_count);
+        printf("answer_count: %d\n", req->header->answer_count);
+        printf("ns_count: %d\n", req->header->ns_count);
+        printf("ar_count: %d\n", req->header->ar_count);
+
+        // for (int i=0; i<n; i++) {
+        //     printf("%d = %02X\n", i, (unsigned char)pb->buf[i]);
+        // }
+
+        free(pb);
+        free(req);
+    }
 
     close(sockfd);
     return 0;