Understanding Linux Network Internals

Data Structures Featured in This Chapter

The three main data structures used by the ICMP code are:

icmphdr

ICMP header .

icmp_control

ICMP message type descriptor. Among its fields is the routine used to process ingress messages.

icmp_bxm

Input structure given as a parameter to the two transmit routines described in the section "Transmitting ICMP Messages." It includes all the information necessary to transmit an ICMP message.

icmphdr Structure

We saw in Figure 25-1 the structure of an ICMP message. The following, from include/linux/icmp.h, is the data structure used to define an ICMP header:

    struct icmphdr {
      _ _u8    type;
      _ _u8    code;
      _ _u16    checksum;
      union {
            struct {
                _ _u16    id;
                _ _u16    sequence;
            } echo;
            _ _u32    gateway;
            struct {
                _ _u16    _ _unused;
                _ _u16    mtu;
            } frag;
      } un;
    };

First come the three fields common to all ICMP types, and then a union that provides different fields based on the message type. For example, un.frag is used by ICMP_FRAG_NEEDED messages, and un.echo by the query messages (i.e., ICMP_ECHO, ICMP_ECHOREPLY, etc.).

icmp_control Structure

For each ICMP type there is an instance of an icmp_control data structure (defined in net/ipv4/icmp.c). Among other fields, it includes a pointer to the routine that is to be called to process ingress ICMP messages. Here are its fields:

int output_entry

int input_entry

Indexes used by the receive routine icmp_rcv and the transmission routines in the section "Transmitting ICMP Messages" to update the right SNMP counter in an array. See ...