update code

Makefile

@@ -1,12 +1,13 @@
 GCC=gcc
 CL=clang-11
 CFLAGS=-Wall
-LIBS=-lbpf
+LIBS=-L../libbpf/src -l:libbpf.a -lelf -lz
+#LIBS=-lbpf
 
 all: dns-trace.ebpf.o dns-trace
 
 dns-trace.ebpf.o: src/dns-trace.ebpf.c
-	$(CL) -g -O2 -target bpf -c src/dns-trace.ebpf.c -o src/dns-trace.ebpf.o
+	$(CL) -g -O2 -target bpf -D __TARGET_ARCH_x86_64 -D __BPF_TRACING__ -L../libbpf/src -l:libbpf.a -c src/dns-trace.ebpf.c -o src/dns-trace.ebpf.o
 
 dns-trace: src/dns-trace.c
 	$(GCC) $(CFLAGS) src/dns-trace.c -o dns-trace $(LIBS)

README.md

@@ -0,0 +1,18 @@
+# Introduction
+
+## Requirements
+First, you need to install these packages:
+
+```
+apt-get install bpftool clang libbpf-dev gcc-multilib 
+```
+
+Clone the project and compile it:
+```
+$ git clone https://github.com/libbpf/libbpf/
+cd libbpf/src && make
+```
+
+After that, you can execute the program:
+
+

src/common.h

@@ -3,6 +3,13 @@
 
 #define QNAME_SIZE 128
 
+#define REQ_QUERY   0x00
+#define REQ_ANSWER  0x01
+
+/* See section 2.3.4 RFC 1035 */
+#define MAX_UDP_PAYLOAD 512
+#define MAx_NAME_LEN    255 
+
 struct dnshdr {
     uint16_t transactionID;
     uint16_t flags;
@@ -12,19 +19,31 @@ struct dnshdr {
     uint16_t nbAdditionalRRs;
 };
 
-/*struct dns_query {
-    char *name;
-    uint16_t type;
-    uint16_t class;
-};*/
-
 struct event {
-    uint32_t saddr;
+    uint32_t client;
     int dport;
     int sport;
+    uint16_t tid;
+    int req_type;
     char qname[QNAME_SIZE];
-    int class;
-    int type;
+    uint16_t class;
+    uint16_t type;
+    uint16_t numAns;
+    unsigned char buf[MAX_UDP_PAYLOAD]; // On stocke la data au format size + data
+};
+
+struct query_section{
+    char qname[QNAME_SIZE];
+    size_t qname_len;
+    uint16_t class;
+    uint16_t type;
+};
+
+struct dns_answer {
+    char data[512];
+    uint16_t class;
+    uint16_t type;
+    uint32_t ttl;
 };
 
 #endif

src/dns-trace.c

@@ -101,93 +101,211 @@ static int open_raw_sock(const char *name)
 	return sock;
 }
 
-static void mapClass(const int class){
+static char *mapReqType(const int req){
+    char *tmp = malloc(8);
+    if (tmp == NULL)
+        return NULL;
+
+    switch(req){
+        case 0x00:
+            strncpy(tmp, "Query", 6);
+            break;
+        case 0x01:
+            strncpy(tmp, "Answer", 7);
+            break;
+        default:
+            strncpy(tmp, "Unknown", 8);
+    };
+    return tmp;
+}
+static char *mapClass(const int class){
+    char *tmp = malloc(8);
+    if (tmp == NULL)
+        return NULL;
+
+    memset(tmp, 0, 8);
     switch(class){
         case 1:
-           printf("IN\n");
+           strncpy(tmp, "IN", 3);
            break;
         case 2:
-           printf("CS\n");
+           strncpy(tmp, "CS", 3);
            break;
         case 3:
-           printf("CH\n");
+           strncpy(tmp, "CH", 3);
            break;
         case 4:
-           printf("HS\n");
+           strncpy(tmp, "HS", 3);
            break;
         default:
-           printf("Unknown\n");
+           strncpy(tmp, "Unknown", 8);
            break;
     }
+    return tmp;
 }
-static void mapType(const int type){
+static char *mapType(const int type){
+    char *tmp = malloc(8);
+    if (tmp == NULL)
+        return NULL;
+
+    /*
+     * type DNS defined in RFC:
+     * https://datatracker.ietf.org/doc/html/rfc1035#section-3.2.2
+     * https://datatracker.ietf.org/doc/html/rfc3596
+     */
     switch(type){
         case 1:
-           printf("A");
+           strncpy(tmp, "A", 2);
            break;
         case 2:
-           printf("NS");
+           strncpy(tmp, "NS", 3);
            break;
         case 3:
-           printf("MD");
+           strncpy(tmp, "MD", 3);
            break;
         case 4:
-           printf("MF");
+           strncpy(tmp, "MF", 3);
            break;
         case 5:
-           printf("CNAME");
+           strncpy(tmp, "CNAME", 6);
            break;
         case 6:
-           printf("SOA");
+           strncpy(tmp, "SOA", 4);
            break;
         case 7:
-           printf("MB");
+           strncpy(tmp, "MB", 3);
            break;
         case 8:
-           printf("MG");
+           strncpy(tmp, "MG", 3);
            break;
         case 9:
-           printf("MR");
+           strncpy(tmp, "MR", 3);
            break;
         case 10:
-           printf("NULL");
+           strncpy(tmp, "NULL", 5);
            break;
         case 11:
-           printf("WKS");
+           strncpy(tmp, "WKS", 4);
            break;
         case 12:
-           printf("PTR");
+           strncpy(tmp, "PTR", 4);
            break;
         case 13:
-           printf("HINFO");
+           strncpy(tmp, "HINFO", 6);
            break;
         case 14:
-           printf("MINFO");
+           strncpy(tmp, "MINFO", 6);
            break;
         case 15:
-           printf("MX");
+           strncpy(tmp, "MX", 3);
            break;
         case 16:
-           printf("TXT");
+           strncpy(tmp, "TXT", 4);
+           break;
+        case 28:
+            strncpy(tmp, "AAAA", 5);
             break;
         default:
-           printf("Unknown\n");
+           strncpy(tmp, "Unknown", 8);
            break;
     }
-    printf("\n");
+    return tmp;
 }
 
+static void print_query(struct event *s_event){
+    char *req_type, *class, *type;
+    printf("%s:%-10d", inet_ntoa(*(struct in_addr*)&s_event->client), s_event->dport);
+    printf("%-5x", s_event->tid);
+
+    req_type = mapReqType(s_event->req_type);
+    printf("%-10s", req_type);
+    free(req_type);
+
+    printf("%-30s", s_event->qname);
+
+    class = mapClass(s_event->class);
+    printf("%-5s", class);
+    free(class);
+
+    type = mapType(s_event->type);
+    printf("%-5s", type);
+    free(type);
+
+}
+static void get_labels(unsigned char *buf, char *qname){
+    int pos = 0;
+    while (*buf++ != '\0') {
+        if((*buf >= 'a' && *buf <= 'z') || (*buf >= 'A' && *buf <= 'Z'))
+            *(qname + pos) = *buf;
+        else if (*buf >= '0' && *buf <= '9')
+            *(qname + pos) = *buf;
+        else
+            *(qname + pos) = '.';
+        pos++;
+    }
+    qname[pos - 1] = '\0';
+}
 int handle_event(void *ctx, void *data, size_t data_sz){
     struct event *s_event = (struct event*)data;
-    printf("IP: %s\n", inet_ntoa(*(struct in_addr*)&s_event->saddr));
-    printf("dport: %d\n", s_event->dport);
-    printf("sport: %d\n", s_event->sport);
-    printf("qname: %s\n", s_event->qname);
-    printf("Class: ");
-    mapClass(s_event->class);
-    printf("Type: ");
-    mapType(s_event->type);
+    if (s_event->req_type == REQ_QUERY){
+        print_query(s_event);
+    }
+    if (s_event->req_type == REQ_ANSWER){
+        int pos = 0;
+        /*for (int i = 0; i < 50; i++)
+            printf("%d ", s_event->buf[i]);
+        printf("\n");*/
+        for (int i = 0; i < s_event->numAns; i++){
+            print_query(s_event);
+
+            uint16_t msg = s_event->buf[pos++];
+            msg |= s_event->buf[pos++] << 8;
+
+            uint16_t type = s_event->buf[pos++];
+            type |= s_event->buf[pos++] << 8;
+            uint16_t class = s_event->buf[pos++];
+            class |= s_event->buf[pos++] << 8;
+
+            uint32_t ttl = s_event->buf[pos++];
+            ttl |= s_event->buf[pos++] << 8;
+            ttl |= s_event->buf[pos++] << 16;
+            ttl |= s_event->buf[pos++] << 24;
+            uint16_t size = s_event->buf[pos++];
+            size |= s_event->buf[pos++] << 8;
+
+            type = ntohs(type);
+            class = ntohs(class);
+            ttl = ntohl(ttl);
+            size = ntohs(size);
+            if (type == 1) { // -> A
+                uint32_t ip = s_event->buf[pos] + (s_event->buf[pos+1] << 8) + (s_event->buf[pos+2] << 16) + (s_event->buf[pos+3] << 24);
+                printf("%s (%d)%5d", inet_ntoa(*(struct in_addr*)&ip), type, ttl);
+            }
+            if (type == 5) { // -> CNAME
+                char cname[size];
+                get_labels(s_event->buf + pos, cname);
+                printf("%s ", cname);
+            }
+            if (type == 28){ // -> AAAA
+                int p = 0;
+                for (int i = 0; i < size; i++){
+                    if (i % 2 == 0)
+                        printf("%x", s_event->buf[pos + p++]);
+                    else{
+                        if (i<size - 1)
+                            printf("%x:", s_event->buf[pos + p++]);
+                        else
+                            printf("%x", s_event->buf[pos + p++]);
+                    }
+                }
+            }
+            pos += size;
             printf("\n");
+        }
+    }
+
+    printf("\n");
+    
     return 0;
 }
 int main(int argc, char *argv[]){
@@ -234,7 +352,8 @@ int main(int argc, char *argv[]){
 
     bpf_program__attach(programSkb);
     // int sock = open_raw_sock("wlp0s20f3");
-    int sock = open_raw_sock("enx98e743c667fc");
+    //int sock = open_raw_sock("enx98e743c667fc");
+    int sock = open_raw_sock("lo");
     printf("Socket: %d\n", sock);
     int prog_fd = bpf_program__fd(programSkb);
     printf("Program fd: %d\n", prog_fd);

src/dns-trace.ebpf.c

@@ -2,6 +2,7 @@
 #define __TARGET_ARCH_x86
 #include <linux/bpf.h>
 #include <bpf/bpf_helpers.h>
+#include <bpf/bpf_endian.h>
 #include <bpf/bpf_tracing.h>
 #include <bpf/bpf_core_read.h>
 #include <linux/if_ether.h>
@@ -28,21 +29,12 @@ struct {
     __uint(max_entries, 256 * 1024 /* 256kb */);
 } m_data SEC(".maps");
 
-/*
- * This function get the query field and the return the length of it
- */
-static size_t get_query(struct __sk_buff *skb, struct event *s_event, uint16_t *class, uint16_t *type, size_t tlen){
-    size_t len;
-    char buf[QNAME_SIZE] = {0};
-    int index = 0;
-    int qname_len = 0; // Full length of the qname field
-    char qname[QNAME_SIZE] = {0};
-    char *c;
-
-    
-    bpf_skb_load_bytes(skb, tlen, &buf, 41);
-    c = buf;
+static size_t get_labels(struct __sk_buff *skb, size_t offset, struct event *s_event){
+    char c;
+    int qname_len = 0;
 
+    bpf_skb_load_bytes(skb, offset, &c, 1); // Get the first byte, which is the length
+    int pos = 1;
     /*
      * The qname is composed by a the number of bytes then follow by the label
      * For instance, for the qname www.bucchino.org,
@@ -52,127 +44,312 @@ static size_t get_query(struct __sk_buff *skb, struct event *s_event, uint16_t *
      * For instance, the result is:
      * 03 77 77 77 08 62 75 63 63 68 69 6e 6f 03 6f 72 67 00
      */
-    while (*(c++) != '\0') {
-        if(*c >= 'a' && *c <= 'z')
-            s_event->qname[index] = *c;
-        else if(*c >= 'A' && *c <= 'Z')
-            s_event->qname[index] = *c;
+    while (c != '\0') {
+        bpf_skb_load_bytes(skb, offset + pos++, &c, 1);
+
+        if(c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z')
+            s_event->qname[qname_len] = c;
+        else if(c >= '0' && c <= '9')
+            s_event->qname[qname_len] = c;
         else
-            s_event->qname[index] = '.';
-        index++;
+            s_event->qname[qname_len] = '.';
         qname_len++;
+        if (pos == 128 || c == '\0')
+            break;
     }
-    s_event->qname[--index] = '\0';
-    qname_len++; // For the null character
-    bpf_printk("%s (%d) %d", s_event->qname, index, qname_len);
+    s_event->qname[qname_len - 1] = '\0';
+    qname_len++;
+    // dquery->qname_len = qname_len;
+    // bpf_printk("qname: %s", s_event->qname);
+    // bpf_printk("qname len: %d", qname_len);
+    return qname_len;
+}
+
+/*
+ * This function get the query field and the return the length of it
+ */
+static size_t get_query_section(struct __sk_buff *skb, struct event *s_event, uint8_t offset){
+    size_t len;
+    size_t qname_len = 0; // Full length of the qname field
+    uint16_t class, type;
+
+    offset += sizeof(struct dnshdr);
+    qname_len = get_labels(skb, offset, s_event);
 
     // Get class and type
     len = qname_len;
-    bpf_skb_load_bytes(skb, tlen + qname_len, type, sizeof(uint16_t));
+    bpf_skb_load_bytes(skb, offset + qname_len, &type, sizeof(uint16_t));
+    s_event->type = ntohs(type);
     len += 2;
-    bpf_skb_load_bytes(skb, tlen + qname_len + 2, class, sizeof(uint16_t));
+    bpf_skb_load_bytes(skb, offset + qname_len + 2, &class, sizeof(uint16_t));
+    s_event->class = ntohs(class);
     len += 2;
 
     return len;
 }
+static unsigned int get_answer(struct __sk_buff *skb, struct event *s_event, size_t tlen, unsigned int index){
+    unsigned char buf[2] = {0}; // Need to be unsigned, otherwise, the result is fffff
+    unsigned int offset = index;
 
+    // Get the 2 first bytes to identify if it's a message compression or not
+    if(bpf_skb_load_bytes(skb, tlen, &buf, 2) < 0)
+        return 0;
+    tlen += 2; // For the message compression
+
+    /*
+     * According to the RFC 1035 (https://datatracker.ietf.org/doc/html/rfc1035#section-4.1.4)
+     * In the section 4.1.4, message compression, the first two bits are set at 11 (0xc),
+     * that's means, it's a pointer.
+     * For instance, the two bytes 0xc00c, 0xc (11) it's the pointer and 0x00c is the position in the DNS header
+     */
+    if (buf[0] == 0xc0){
+        /*
+         * I cannot read the labels, because the eBPF verifier considerate as a infinity loop
+         */
+
+        /* 
+         * According to the RFC 1035, the structure of answer is like that:
+         * https://datatracker.ietf.org/doc/html/rfc1035#section-4.1.3
+         */
+
+        // Get the class and type
+        if ((void*)(offset) >= MAX_UDP_PAYLOAD - sizeof(uint16_t))
+            return 0;
+
+        bpf_skb_load_bytes(skb, tlen, s_event->buf + offset, sizeof(uint16_t));
+        uint16_t type = s_event->buf[0] + (s_event->buf[1] << 8);
+        tlen += 2;
+        if ((void*)(offset += 2) >= MAX_UDP_PAYLOAD - sizeof(uint16_t))
+            return 0;
+        //offset += 2;
+
+        // For class
+        if(bpf_skb_load_bytes(skb, tlen, s_event->buf + offset, sizeof(uint16_t)) < 0)
+            return 0;
+        tlen += 2;
+        if ((void*)(offset += 2) >= MAX_UDP_PAYLOAD - sizeof(uint16_t))
+            return 0;
+
+        // Get ttl
+        if(bpf_skb_load_bytes(skb, tlen, s_event->buf + offset, sizeof(uint32_t)) < 0)
+            return 0;
+        if ((void*)(offset += 4) >= MAX_UDP_PAYLOAD - sizeof(uint32_t))
+            return 0;
+        tlen += 4;
+
+        // Get data size
+        uint16_t size;
+        bpf_skb_load_bytes(skb, tlen, &size, sizeof(uint16_t));
+        bpf_skb_load_bytes(skb, tlen, s_event->buf + offset, sizeof(uint16_t));
+        if ((void*)(offset += 2) >= MAX_UDP_PAYLOAD - sizeof(uint16_t))
+            return 0;
+        tlen += 2;
+
+        uint32_t data;
+        if (s_event->type == 1) { // -> A
+            bpf_skb_load_bytes(skb, tlen, s_event->buf + offset, sizeof(uint32_t)); 
+        }
+        if ((void*)(offset += ntohs(size)) >= MAX_UDP_PAYLOAD - sizeof(uint16_t))
+            return 0;
+        tlen += ntohs(size);
+    }
+    else {
+        // get_labels(skb, sizeof(struct ethhdr) + sizeof(struct iphdr) + sizeof(struct udphdr) + sizeof(struct dnshdr), s_event);
+    }
+    bpf_printk("End offset: %d", offset);
+    return offset;
+}
 /*
  * https://datatracker.ietf.org/doc/html/rfc1035
  */
 static int dnsquery(struct __sk_buff *skb, struct ethhdr eth, struct iphdr ip, struct udphdr udp, int dport, int sport){
     struct event *s_event;
     struct dnshdr dns = {0};
-    char saddr[32];
-    uint16_t class, type;
-    // bpf_printk("udp len: %d", ntohs(udp.len));
+    struct query_section dquery = {0};
+
+    /* Get DNS header */
+    bpf_skb_load_bytes(skb, sizeof(struct ethhdr) + sizeof(struct iphdr) + sizeof(struct udphdr), &dns, sizeof(struct dnshdr));
+
+    // Check OpCode
+    uint16_t qr = ntohs(dns.flags) & 0xF000; // Get the QR code: 0 -> query, 1 -> response
+    /* If it's not a query, we do not continue */
+    if(qr != 0x0)
+        return 0;
+
+    if (ntohs(dns.nbQuestions) == 0)
+        return 0;
 
     s_event = bpf_ringbuf_reserve(&m_data, sizeof(*s_event), 0);
     if (!s_event)
        return 0;
 
+    s_event->req_type = REQ_QUERY;
+
     /* Get IP header */
-    s_event->saddr = ip.saddr;
+    s_event->client = ip.saddr;
 
-    /* Get DNS header */
-    bpf_skb_load_bytes(skb, sizeof(struct ethhdr) + sizeof(struct iphdr) + sizeof(struct udphdr), &dns, sizeof(struct dnshdr));
+    s_event->tid = ntohs(dns.transactionID);
 
-    if (ntohs(dns.nbQuestions) == 0){
-        bpf_ringbuf_discard(s_event, 0);
-        return 0;
-    }
-
-
-    bpf_printk("tid: %x", ntohs(dns.transactionID)); // Use as key map
-    bpf_printk("nb question: %d", ntohs(dns.nbQuestions));
-
-    //struct dns_query dquery;
-    //bpf_skb_load_bytes(skb, sizeof(struct ethhdr) + sizeof(struct iphdr) + sizeof(struct udphdr) + sizeof(struct dnshdr), &dquery, sizeof(struct dns_query));
-    // bpf_printk("size: %d %d %d", tlen, skb->len, (skb->len - tlen));
-    //dlen = (skb->len - tlen);
-    //bpf_printk("DNS packet len: %d", dlen);
-    //qlen = dlen - sizeof(struct dnshdr);
-    //bpf_printk("size: %d %d", sizeof(struct dnshdr), qlen);
-
-
-    /* Get the query structure */
-    size_t tlen = sizeof(struct ethhdr) + sizeof(struct iphdr) + sizeof(struct udphdr) + sizeof(struct dnshdr);
-    size_t query_len = get_query(skb, s_event, &class, &type, tlen);
+    /* Get the query section */
+    uint8_t tlen = sizeof(struct ethhdr) + sizeof(struct iphdr) + sizeof(struct udphdr);
+    size_t query_len = get_query_section(skb, s_event, tlen);
 
     // https://docs.cilium.io/en/stable/reference-guides/bpf/progtypes/
     s_event->dport = dport;
     s_event->sport = sport;
-    s_event->class = ntohs(class);
-    s_event->type = ntohs(type);
-    //if(bpf_probe_read_user_str(&s_event->qname, sizeof(s_event->qname), qname) < 0)
-    //    bpf_printk("Failed to copy qname");
-
-    // Add to map
 
     bpf_ringbuf_submit(s_event, 0);
 
     return 0;
 }
 
-static int dnsanswer(struct __sk_buff *skb, struct ethhdr eth, struct iphdr ip, struct udphdr udp, int dport, int sport){
-    return 0;
+static void dnsanswer_old(struct __sk_buff *skb, struct iphdr ip, struct udphdr udp, int dport, int sport){
+     char buf[256] = {0}; // Max dns domain name length
+    //__u16 udplen = 0U;
+
+    // Check with ip.len
+    //const __u32 tot_len = ntohs(ip.tot_len);
+    // __u32 payload_len = (ntohs(ip.tot_len) - sizeof(struct iphdr) - sizeof(struct udphdr));
+    __u32 payload_len = (ntohs(ip.tot_len) - sizeof(struct iphdr) - sizeof(struct udphdr)) & 0xff;
+
+    if (payload_len > skb->len)
+        return;
+
+    bpf_printk("payload len %d", payload_len);
+    if (payload_len <= -1) {
+        bpf_printk("payload len %d", payload_len);
     }
+    
+    // Get udp len
+    //udplen = ntohs(udp.len);
+
+    //bpf_printk("udp len: %d", udplen);
+
+    //if (udplen <= 0)
+    //    return 0;
+
+    /*if (offset + udplen > skb->len) {
+        bpf_printk("outbound");
+        plen = sizeof(struct dnshdr);
+    }*/
+    /*long err = bpf_skb_load_bytes(skb, offset, &buf, payload_len);
+    if(err < 0){
+        bpf_printk("failed");
+        // bpf_ringbuf_discard(s_event, 0);
+        return;
+    }*/
+    // Cast to dnshdr
+    // dns = (struct dnshdr*)buf;
+
+}
+
+
+static void dnsanswer(struct __sk_buff *skb, struct iphdr ip, struct udphdr udp, int dport, int sport){
+    struct event *s_event;
+    struct dnshdr dns;
+    uint16_t tid = 0U;
+    uint32_t offset = sizeof(struct ethhdr) + sizeof(struct iphdr) + sizeof(struct udphdr);
+    size_t tlen = ntohs(udp.len);
+    int index = 0;
+
+    if (tlen < 0 || tlen >= 256)
+        return;
+    
+    bpf_printk("udp len: %d", tlen);
+
+    // Load dns header
+    if (bpf_skb_load_bytes(skb, offset, &dns, sizeof(struct dnshdr)) < 0)
+        return;
+
+    // Check OpCode
+    uint16_t qr = ntohs(dns.flags) & 0xF000; // Get the QR code: 0 -> query, 1 -> response
+    if(qr != 0x8000) // Not a response, we do not continue
+        return;
+
+    if (ntohs(dns.nbQuestions) == 0)
+        return;
+
+    s_event = bpf_ringbuf_reserve(&m_data, sizeof(*s_event), 0);
+    if (!s_event)
+       return;
+
+    s_event->req_type = REQ_ANSWER;
+
+    /* Get IP header */
+    s_event->client = ip.daddr;
+
+    s_event->dport = dport;
+    s_event->sport = sport;
+
+    /* Get the Transaction ID */
+    s_event->tid = ntohs(dns.transactionID);
+
+    /* Get the query section */
+    size_t query_len = get_query_section(skb, s_event, offset);
+
+    /* Get answer section */
+    uint16_t ans = ntohs(dns.nbAnswerRRs);
+    if (ans <= 0){
+        bpf_ringbuf_discard(s_event, 0);
+        return;
+    }
+
+    /*if (ans > 0){
+        s_event->numAns = ans;
+    }*/
+    s_event->numAns = ans;
+
+
+    // Load query and answer
+    /*
+     * Load query and answers
+     * It's a little dirty to do that, to load byte by byte,
+     * otherwise, I have an issue with the eBPF verifier
+     */
+    offset += sizeof(struct dnshdr) + query_len;
+    //offset += 2; // We bypass message compression
+    while (index < tlen){
+        bpf_skb_load_bytes(skb, offset + index, s_event->buf + index, 1);
+        index++;
+    }
+    bpf_ringbuf_submit(s_event, 0);
+    //if(bpf_skb_load_bytes(skb, offset, &buf, tlen) < 0)
+    //    bpf_printk("Failed");
+}
+
+/*
+ * skb -> http://oldvger.kernel.org/~davem/skb_data.html
+ */
 SEC("socket")
 int detect_dns(struct __sk_buff *skb) {
-    //void *data = (void *)(long)skb->data;
-    //void *data_end = (void *)(long)skb->data_end;
-    //struct ethhdr *eth = data;
     struct ethhdr eth = {0};
     struct iphdr ip = {0};
     struct udphdr udp = {0};
-    unsigned long long h_proto, p;
-    unsigned long long dport;
-    unsigned long long sport;
+    __u32 h_proto, p;
+    __u32 dport;
+    __u32 sport;
 
-    //if (data + sizeof(struct ethhdr) + sizeof(struct iphdr) + sizeof(struct udphdr) > data_end)
-    //    return 0;
 
-    //bpf_skb_load_bytes(skb, 12, &p, 2);
+    if (skb->len < sizeof(struct ethhdr) + sizeof(struct iphdr) + sizeof(struct udphdr))
+        return 0;
+
     bpf_skb_load_bytes(skb, 0, &eth, sizeof(struct ethhdr));
     p = eth.h_proto;
     if (ntohs(p) != ETH_P_IP)
         return 0;
 
-    // bpf_printk("ip: %d",ntohs(p));
-
-    //ip = (struct iphdr*)(data + sizeof(struct ethhdr));
     bpf_skb_load_bytes(skb, sizeof(struct ethhdr), &ip, sizeof(struct iphdr));
 
-    //bpf_skb_load_bytes(data, sizeof(struct ethhdr), ip, sizeof(struct ip));
     h_proto = ip.protocol;
     // If not UDP packet
     if (h_proto != 17) 
         return 0;
 
-    // bpf_printk("proto: %d", h_proto);
-    //udp = (struct udphdr*)(data + sizeof(struct ethhdr) + sizeof(struct iphdr));
-
     bpf_skb_load_bytes(skb, sizeof(struct ethhdr) + sizeof(struct iphdr), &udp, sizeof(struct udphdr));
 
+    if (udp.len == 0)
+        return 0;
+
     // Check if DNS port
     dport = ntohs(udp.dest);
     sport = ntohs(udp.source);
@@ -180,35 +357,8 @@ int detect_dns(struct __sk_buff *skb) {
     if (dport == 53)
         dnsquery(skb, eth, ip, udp, dport, sport);
     else if(sport == 53)
-        bpf_printk("Response");
+        dnsanswer(skb, ip, udp, dport, sport);
 
     return 0;
 }
-/*SEC("xdp")
-int detect_dns(struct xdp_md *ctx){
-    void *data_end = (void *)(long)ctx->data_end;
-    void *data = (void *)(long)ctx->data;
-    struct ethhdr *eth = data;
-    struct iphdr *ip;
-    struct udphdr *udp;
-    __u16 h_proto;
-    __u16 dport;
-
-    if (data + sizeof(struct ethhdr) + sizeof(struct iphdr) + sizeof(struct udphdr) > data_end)
-        return XDP_DROP;
-
-    ip = (struct iphdr*)(data + sizeof(struct ethhdr));
-    udp = (struct udphdr*)(data + sizeof(struct ethhdr) + sizeof(struct iphdr));
-
-    h_proto = ip->protocol;
-    // If not UDP packet
-    if (h_proto != 17) 
-        return XDP_PASS;
-
-    // Check if DNS port
-    dport = udp->dest;
-    bpf_printk("Dport: %d", (dport));
-
-    return XDP_PASS;
-}*/
 char LICENSE[] SEC("license") = "GPL";

test.c

@@ -0,0 +1,14 @@
+#include <stdio.h>
+
+int main(void){
+    char buf[128] = "Hello world!";
+    char t = 'a';
+    //char *c = buf;
+    char *c = &t;
+
+    printf("%c\n", *c);
+    *c = buf[0];
+    printf("%c\n", *c);
+
+    return 0;
+}