ICMP - Redirect Message

Introduction

The ICMP - Redirect message is always sent from a gateway to the host and the example below will illustrate when this is used.

Putting it simply (before we have a look at the example) the ICMP - Redirect message occurs when a host sends a datagram (or packet) to its gateway (destination of this datagram is a different network), which in turn forwards the same datagram to the next gateway (next hop) and this second gateway is on the same network as the host. The second gateway will generate this ICMP message and send it to the host from which the datagram originated.

There are 4 different ICMP - Redirect message types and these are:

The format of this ICMP message is as follows: ICMP - Redirect (0, 1, 2, 3 or 4) message.

Our example:

The gateway (Win2k Server) sends a redirect message (arrow No. 3) to the host in the following situation:

Gateway 1 (the linux server), receives an Internet datagram (arrow No. 1) from a host on the same network. The gateway checks its routing table and obtains the address of the next gateway (hop) on the route to the datagram's Internet destination network and sends the datagram to it (arrow No. 2).

Now, gateway 2 receives the datagram and, if the host identified by the Internet source address of the datagram (in other words, it checks the source IP of the datagram, which will still be 192.168.0.100), is on the same network, a redirect message (arrow No. 3) is sent to the host. The redirect message advises the host to send its traffic for the Internet network directly to gateway 2 as this is a shorter path to the destination. The gateway then forwards the original datagram's data (arrow No. 1) to its Internet destination (arrow No.4).

For datagrams (or packets) with the IP source options and the gateway address in the destination address field, a redirect message is not sent even if there is a better route to the ultimate destination than the next address in the source route.

Analysis

Let's have a look at the structure of an ICMP - Redirect message:

That's all about ICMP - Redirect messages !