How to access IP_DF and IP_MF from iph->frag_offset when using netfilter

问题

I am trying to write a netfilter module and want to access the fields of the IPHeader that denotes the DF and MF fields. I can access most other fields as desired but I think I have to extract the DF and MF fields from the ipheader in the struct and manipulate them say for example I want to set or unset the DF bit depending on the type of packet I receive.

The below structure has 'frag_off' how do I access/rewrite IP_DF and IP_MF from this?

struct iphdr {
    #if defined(__LITTLE_ENDIAN_BITFIELD)
        __u8    ihl:4,
                version:4;
    #elif defined (__BIG_ENDIAN_BITFIELD)
        __u8    version:4,
                ihl:4;
    #else
        #error  "Please fix <asm/byteorder.h>"
    #endif
         __u8   tos;
         __u16  tot_len;
         __u16  id;
         __u16  frag_off;
         __u8   ttl;
         __u8   protocol;
         __u16  check;
         __u32  saddr;
         __u32  daddr;
         /*The options start here. */
};

#define IP_MF 0x2000 /* Flag: "More Fragments" */
#define IP_OFFSET 0x1FFF /* "Fragment Offset" part */
#define IP_DF 0x4000 /* dont fragment flag */
      printk(KERN_INFO "IP_FRAG_OFF : %d", (iph->frag_off & IP_OFFSET));
      printk(KERN_INFO "MF: %d", (iph->frag_off & IP_MF));

回答1:

The field frag_off is 16 bits. The first 3 are the flags, the rest 13 bits is the offset.

The flags bits are: first is reserved and must be 0. second is DF and third is MF. So, to access the DF you should isolate the second bit by "frag_off & 0x4000", and IP_DF declared as 0x4000, so you can do "iph->frag_off & IP_DF".

Same for MF, 0x2000, and for the offset IP_OFFSET, 0x1FFF

来源：https://stackoverflow.com/questions/51699201/how-to-access-ip-df-and-ip-mf-from-iph-frag-offset-when-using-netfilter

标签

linux-kernel

kernel

netfilter