Quentin
70aee3087b
- We can use the structure not only to read the packet but also to modify it - Binary OR/AND are done with single chars, eg. & or | while logical OR/AND are done with 2 chars, eg. && or ||
57 lines
1.6 KiB
C
57 lines
1.6 KiB
C
#include <linux/bpf.h>
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#include <bpf/bpf_helpers.h>
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#include <bpf/bpf_endian.h>
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#include <linux/if_ether.h>
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#include <linux/ip.h>
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#include <linux/in.h>
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//Partie offset header en commentaire pour économiser les bytes sur la mémoire réservée pour le Kernel
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/*
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const unsigned short macLen = 6; //2bytes
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const unsigned short macSrc = macLen; //2bytes
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const unsigned short macDest = macLen; //2bytes
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const unsigned short pktType =2; //2bytes
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const unsigned short ethHead = macSrc+macDest+pktType; //=14
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//Partie offet IP
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const unsigned short iPVersionAndHeaderLen = 1;
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const unsigned short DSF = 1;
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const unsigned short totalLen = 2;
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const unsigned short id = 2;
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const unsigned short offsetIP = iPVersionAndHeaderLen + DSF + totalLen + id; //=6
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const unsigned short DF_offset = offsetIP + ethHead; //=20
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*/
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SEC("xdp_udp")
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int xdp_udp_func(struct xdp_md *ctx)
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{
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void *data_end = (void *)(long)ctx->data_end;
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void *data = (void *)(long)ctx->data;
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struct ethhdr *eth = data; //structure ethernet from if_ether.h
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// Check that our packet ethernet headers is big enough, ie. at least the size of Ethernet's fixed headers
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if((void*)(eth + 1) > data_end) {
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return XDP_PASS;
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}
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// Check that the ethernet header declares an IPv4 packet
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if(eth->h_proto == bpf_htons(ETH_P_IP)) {
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struct iphdr *ip = (struct iphdr*)(eth + 1);
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// Check that the IP packet is big enough, ie. at least the size of IP's fixed headers
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if((void*)(ip + 1) > data_end) {
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return XDP_PASS;
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}
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if(ip->protocol == IPPROTO_UDP) {
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ip->frag_off |= 0x4000; //Set the DF flag to 1 -> 0b 0100 0000 0000 0000
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}
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}
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return XDP_PASS;
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}
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