python中的BPF来嗅探多个TCP端口的数据包

Question

我从http://allanrbo.blogspot.in/2011/12/raw-sockets-with-bpf-in-python.html得到了代码。它工作正常，但我想嗅探多个TCP端口像端口9000，80，22交通......

所以我已经修改了filter_list像打击

filters_list = [ 
    # Must have dst port 67. Load (BPF_LD) a half word value (BPF_H) in 
    # ethernet frame at absolute byte offset 36 (BPF_ABS). If value is equal to 
    # 67 then do not jump, else jump 5 statements. 
    bpf_stmt(BPF_LD | BPF_H | BPF_ABS, 36), 
    bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 9000, 0, 5), <===== Here I added another port 
    bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 80, 0, 5), 


    # Must be UDP (check protocol field at byte offset 23) 
    bpf_stmt(BPF_LD | BPF_B | BPF_ABS, 23), 
    bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 0x06, 0, 3), #<==Changed for TCP "0x06" 

    # Must be IPv4 (check ethertype field at byte offset 12) 
    bpf_stmt(BPF_LD | BPF_H | BPF_ABS, 12), 
    bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 0x0800, 0, 1), 

    bpf_stmt(BPF_RET | BPF_K, 0x0fffffff), # pass 
    bpf_stmt(BPF_RET | BPF_K, 0), # reject ] 

的事情是，有时有时候它不是工作，就像只有9000而不是80的流量，有时80的流量。我没有完全理解代码。任何帮助？

Answer 1

据我所知，问题似乎来自您的前两个条件跳转的逻辑。具体做法是：

bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 9000, 0, 5), # if false, skip 5 instructions 
bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 80, 0, 5), 

的指令bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, <val>, <jtrue>, <jfalse>)意味着

if value currently in register K is equal to <val> 
    then add <jtrue> to instruction pointer 
     (i.e. skip the next <jtrue> instructions), 
    else add <jfalse> instead` 

因此，两行表示：

if port is 9000 
    then if port is 80 
     then go on with checks… 
    else skip 5 instructions (i.e. reject) 
else 
    skip 5 instructions (i.e. pass, as jump offset was not updated from 5 to 6) 

虽然你可能想要的东西，看起来更像是：

if port is 9000 
    then go on with checks… 
else 
    if port is 80 
     then go on with checks… 
    else reject 

我h AVE没有测试过，但得到这个逻辑，我会说，你需要适应跳偏移如下：

bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 9000, 1, 0), # if true skip 1 insn 
               # (i.e. port 80 check) else 0 
               # and check for port 80 
bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 80, 0, 5), # if true skip 0 else skip 5 
               # (and land on “reject”) 

编辑1：然后用于过滤三个端口，这将成为：

bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 8084, 2, 0), # skip the next 2 checks if true 
bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 9000, 1, 0), # skip the next check if true 
bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 22, 0, 5), # if true go on else reject 

编辑2：要同时过滤源端口（除目的端口），你可以尝试这样的事情（仍然没有在我的身边测试）：

# Load TCP src port into register K, and check port value 
# For packets with IP header len == 20 bytes, TCP src port should be at offset 34 
# We adapt the jump offsets to go to next check if no match (or to “reject” after 
# the last check), or to skip all remaining checks on ports if a match is found. 
bpf_stmt(BPF_LD | BPF_H | BPF_ABS, 34),   # 34 == offset of src port 
bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 8084, 6, 0), 
bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 9000, 5, 0), 
bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 22, 4, 0), 

# As before: if no match on src port, check on dst port 
bpf_stmt(BPF_LD | BPF_H | BPF_ABS, 36), 
bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 8084, 2, 0), 
bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 9000, 1, 0), 
bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 22, 0, 5), 

…