Understanding Linux Network Internals

L4 Protocol Registration

The L4 protocols that rest on IPv4 are defined by net_protocol data structures, defined in include/net/protocol.h, which consist of the following three fields:

int (*handler)(struct sk_buff *skb)

Function registered by the protocol as the handler for incoming packets. This is discussed further in the section "L3 to L4 Delivery: ip_local_deliver_finish." It is possible to have protocols that share the same handler for both IPv4 and IPv6 (e.g., SCTP).

void (*err_handler)(struct sk_buff *skb, u32 info)

Function used by the ICMP protocol handler to inform the L4 protocol about the reception of an ICMP UNREACHABLE message. We will see in Chapter 35 when a Linux system generates ICMP UNREACHABLE messages, and we will see in Chapter 25 how the ICMP protocol uses err_handler.

int no_policy

This field is consulted at certain key points in the network stack and is used to exempt protocols from IPsec policy checks: 1 means that there is no need to check the IPsec policies for the protocol. Do not confuse the no_policy field of the net_protocol structure with the field bearing the same name in the ipv4_devconf structure: the former applies to a protocol; the latter applies to a device. See the sections "L3 to L4 Delivery: ip_local_deliver_finish" and "IPsec" for how no_policy is used.

The include/linux/in.h file contains a list of L4 protocols defined as IPPROTO_ XXX symbols. (For a more complete list, see the /etc/protocols file, or RFC 1700 and its successor RFCs.) ...