Understanding Linux Network Internals

Transmitting ICMP Messages

The two classes of ICMP messages introduced in the section "ICMP Header," errors and queries, are transmitted using two different routines:

icmp_send

Used by the kernel to transmit ICMP error messages when specific conditions are detected.

icmp_reply

Used by the ICMP protocol to reply to ingress ICMP request messages that require a response.

Both routines receive an skb buffer in input. However, the one used as input to icmp_send represents the ingress IP packet that triggered the transmission of the ICMP message, whereas the one in input to icmp_reply represents an ingress ICMP request message that requires a response.

The code in net/core/icmp.c processes incoming ICMP messages, and therefore always uses icmp_reply to transmit an ICMP message in response to another one received in input. Other kernel network subsystems (i.e., routing, IP, etc.) use icmp_send when they need to generate ICMP messages, as shown in Figure 25-8.

Figure 25-8. Subsystems using icmp_send/icmp_reply

In both cases:

• ip_route_output_key is used to find the route to the destination (see Chapter 33).

• The two routines ip_append_data and ip_push_pending_frames are used to request a transmission to the IP layer. These routines are described in Chapter 21.

• ICMP messages generated in kernel space are rate limited (if the kernel has been configured to do it via /proc) with icmpv4_xrlim_allow (see ...