From 01a89d9ace95304db5ec5612ad3e72481599193c Mon Sep 17 00:00:00 2001
From: erwan <erwan@deuxfleurs.fr>
Date: Sun, 25 Apr 2021 01:05:40 +0200
Subject: [PATCH] Init add

---
 XDP/xdp_udp.c | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)
 create mode 100644 XDP/xdp_udp.c

diff --git a/XDP/xdp_udp.c b/XDP/xdp_udp.c
new file mode 100644
index 0000000..de1b221
--- /dev/null
+++ b/XDP/xdp_udp.c
@@ -0,0 +1,68 @@
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_endian.h>
+#include <linux/if_ether.h>
+#include <linux/ip.h>
+#include <linux/in.h>
+
+	//Partie offset header en commentaire pour économiser les bytes sur la mémoire réservée pour le Kernel
+	/*
+	const unsigned short macLen = 6;	//2bytes
+	const unsigned short macSrc = macLen;	//2bytes
+	const unsigned short macDest = macLen;	//2bytes
+	const unsigned short pktType =2;	//2bytes
+	const unsigned short ethHead = macSrc+macDest+pktType; //=14
+
+	//Partie offet IP
+	const unsigned short iPVersionAndHeaderLen = 1;
+	const unsigned short DSF = 1;
+	const unsigned short totalLen = 2;
+	const unsigned short id = 2;
+	const unsigned short offsetIP = iPVersionAndHeaderLen + DSF + totalLen + id; //=6
+
+	const unsigned short DF_offset = offsetIP + ethHead; //=20
+	*/
+
+
+
+SEC("xdp_udp")
+int xdp_udp_func(struct xdp_md *ctx)
+{
+	void *data_end = (void *)(long)ctx->data_end;
+	void *data = (void *)(long)ctx->data;
+
+	unsigned short mask = 0x4000;//0b 0100 0000 0000 0000
+	int DF_offset = 21;
+
+	struct ethhdr *eth = data; //structure ethernet from if_ether.h
+	int rc = XDP_PASS; // return code
+	int ipsize;
+	__u16 header_proto; __u64 length_header_eth;
+
+	length_header_eth = sizeof(*eth);//length in bits of header eth
+
+	if(data + length_header_eth > data_end)//monitor the bound data
+		return rc;
+	header_proto = eth->h_proto;
+
+	//Enter only if it's IPV4
+	if(header_proto == bpf_htons(ETH_P_IP))
+	{
+		//We monitor protocol IPV4, we want only UDP packet
+		struct iphdr *ip = data + length_header_eth;
+		ipsize = sizeof(struct iphdr);
+		if(ip + ipsize > data_end) {
+			return rc;
+		}
+		if(ip->protocol == IPPROTO_UDP) {
+			//On souhaite modifier le 21eme byte
+			// Unsigned short fait 2 bytes
+			unsigned short *p = data + DF_offset +1 ;
+			if(p > data_end) return rc;
+			p[0] = p[0] || mask;
+		}
+	}
+
+
+	return XDP_PASS;
+}