From 01ed0bd7ddfda6b3eecaeca5804e4e6862b603b0 Mon Sep 17 00:00:00 2001
From: Ako <emi.boucly38@gmail.com>
Date: Sat, 23 Aug 2025 18:51:10 +0200
Subject: [PATCH] feat: dns header decoding

---
 .gitignore            |  1 +
 .vscode/settings.json |  6 +++
 dns.c                 | 89 +++++++++++++++++++++++++++++++++++++++++++
 dnshdr.h              | 15 ++++++++
 4 files changed, 111 insertions(+)
 create mode 100644 .gitignore
 create mode 100644 .vscode/settings.json
 create mode 100644 dns.c
 create mode 100644 dnshdr.h

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..01d7a5d
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+dns
\ No newline at end of file
diff --git a/.vscode/settings.json b/.vscode/settings.json
new file mode 100644
index 0000000..cd32f93
--- /dev/null
+++ b/.vscode/settings.json
@@ -0,0 +1,6 @@
+{
+    "files.associations": {
+        "stdio.h": "c",
+        "netdb.h": "c"
+    }
+}
\ No newline at end of file
diff --git a/dns.c b/dns.c
new file mode 100644
index 0000000..6f3d4eb
--- /dev/null
+++ b/dns.c
@@ -0,0 +1,89 @@
+#include <netdb.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <arpa/inet.h> 
+#include <errno.h>
+#include <sys/socket.h>
+#include <unistd.h>
+#include "dnshdr.h"
+
+
+int main() 
+{
+    int sockfd;
+    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
+
+    if (sockfd < 0) {
+        perror("Couldn't open socket. Is port available ?");
+    }
+
+    struct sockaddr_in server_addr;
+    memset(&server_addr, 0, sizeof(server_addr));
+
+    server_addr.sin_family = AF_INET;
+    server_addr.sin_addr.s_addr = INADDR_ANY;
+    server_addr.sin_port = htons(5353);
+
+    if (bind(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr)) < 0) {
+        perror("Couldn't bind socket to ip/port.");
+        close(sockfd);
+        return 1;
+    }
+
+
+    unsigned char* buffer = calloc(512, sizeof(char*));
+    struct sockaddr_in client_addr;
+    socklen_t client_len = sizeof(client_addr); 
+    
+    while (1) {
+        int buflen;
+        buflen = recvfrom(sockfd, buffer, 512, 0, (struct sockaddr*)&client_addr, &client_len);
+
+        if (buflen < 0) {
+            perror("Unable to receive data");
+            continue;
+        }
+
+        printf("===== New request! %d bytes. =====\n", buflen);
+
+        printf("Hexdump: \n");
+        for(int i = 0; i<buflen; i++) 
+        {
+            printf("%02x ", buffer[i]);
+            if ((i + 1) % 8 == 0) printf("  ");
+            if ((i + 1) % 32 == 0) printf("\n");
+        }
+        printf("\n");
+
+        struct dnshdr *hdr = (struct dnshdr*) buffer;
+        uint16_t flags = ntohs(hdr->flags);
+
+        printf("Id       : %d\n", ntohs(hdr->id));
+        printf("Flags    : QR=%d OPCODE=%d AA=%d TC=%d RD=%d RA=%d Z=%d AD=%d CD=%d RCODE=%d\n",
+            (flags & 0x8000) >> 15, 
+            (flags & 0x7800) >> 11,
+            (flags & 0x0400) >> 10,
+            (flags & 0x0200) >> 9,
+            (flags & 0x0100) >> 8,
+            (flags & 0x0080) >> 7,
+            (flags & 0x0040) >> 6,
+            (flags & 0x0020) >> 5,
+            (flags & 0x0010) >> 4,
+            (flags & 0X000F)
+        );
+        printf("Qdcount  : %d\n", ntohs(hdr->qdcount));
+        printf("Ancount  : %d\n", ntohs(hdr->ancount));
+        printf("Nscount  : %d\n", ntohs(hdr->nscount));
+        printf("Arcount  : %d\n", ntohs(hdr->arcount));
+
+        
+        
+
+    }
+
+    free(buffer);
+    close(sockfd);
+    return 1;
+
+}
\ No newline at end of file
diff --git a/dnshdr.h b/dnshdr.h
new file mode 100644
index 0000000..6ad8422
--- /dev/null
+++ b/dnshdr.h
@@ -0,0 +1,15 @@
+#ifndef DNSHDR_H
+#define DNSHDR_H
+
+#include <stdint.h> 
+
+struct dnshdr {
+    uint16_t id; /* Identifiant sur 16 bit à faire correspondre pour la réponse */
+    uint16_t flags; /* Liste des flags => QR, Opcode, AA, TC, RD, RA, Z, RCODE */
+    uint16_t qdcount; /* Quantité d'entrées dans la section de questions */
+    uint16_t ancount; /* Quantité d'entrées dans la section de réponses */
+    uint16_t nscount; /* Quantité d'entrées de serveur de noms dans la section d'autorité */
+    uint16_t arcount; /* Quantité d'entrées dans la section de ressources additionnelles */
+} __attribute__((packed));
+
+#endif