Internet Core Protocols: The Definitive Guide

Name

Strict Source Routing

Synopsis

Identifies a network path that the IP datagram must take, without exception.

Class and Code

Class 0, Code 9

Size

Varies as needed.

Copy to all fragments?

Yes.

Defined In

RFC 791.

Status

Standard.

Notes

Strict Source Routing allows an originating system to list the specific routers that a datagram must visit on the way to its destination. No deviation from this list is allowed.

In order to facilitate this process, the Strict Source Route option uses an 8-bit pointer field that is placed at the beginning of the option-data field. The pointer indicates the byte position that contains the IP address of the next router to be visited. Once a router has been visited, the pointer is moved to an offset that points to the IP address of the next router. If the pointer exceeds the option-length value, then no more routes can be used, and normal routing takes over.

Each router also records its own IP address in the moving list of landmarks, as specified in Record Route earlier in this chapter. Due to the limited space available, this option is not very useful on the open Internet.

As with Loose Source Routing, there are some security concerns with this option. By specifying a route that datagrams must take, it is possible for an intruder to mark external datagrams as being internal to your network. Normally, any datagrams sent in response to these datagrams would never leave your network, although by specifying a source-route, the hacker can tell your systems to send the ...