Loose Source Routing


Identifies a network path that the IP datagram should take, with variations allowed as long as all of the defined routes are taken at some point.

Class and Code

Class 0, Code 3


Varies as needed.

Copy to all fragments?


Defined In

RFC 791.




Loose Source Routing allows an originating system to list landmark routers that a datagram must visit on the way to its destination. In between these landmark routers, the datagram may be sent wherever the network tells it to go.

In order to facilitate this process, the Loose 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 next landmark to be visited. Once a landmark has been visited, the pointer is moved to an offset that points to the next landmark. If the pointer exceeds the option-length value, then no more landmarks can be used, and normal routing takes over.

Each router that touches the datagram will also record its own IP address in the option-data as well, as specified in Record Route in the previous section of this chapter. Due to the limited space available, this option is not very useful on the open Internet.

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 ...

Get Internet Core Protocols: The Definitive Guide now with the O’Reilly learning platform.

O’Reilly members experience live online training, plus books, videos, and digital content from nearly 200 publishers.