RFC 2644 / START HERE
Directed broadcast defaults, before the details
The title describes a router-default change; IPv4 means Internet Protocol version 4.
A directed broadcast addresses every host on one chosen network. RFC 2644 makes router receiving and forwarding of these broadcasts default to blocking.
The idea in one picture
Recognize the target network
Check the router gates
Block by default
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
It reduces the risk that an outside request makes a network amplify traffic toward someone else. It updates two specific RFC 1812 requirements, rather than replacing all IPv4 router rules.
A familiar example
Imagine one message addressed to everyone in an office: copying it to many desks can multiply replies. Routers should not provide that outside-to-everyone path by default. This does not authenticate the sender or stop every denial-of-service attack.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Find the directed broadcast addressIPv4 addresses have 32 bits. A directed broadcast keeps a particular network prefix and sets every host bit to one; the last-hop router needs the prefix to recognize it.→
- Block receiving by defaultReceiving a directed broadcast at the router is one decision. RFC 2644 changes its default, rather than banning every use of broadcast.→
- Check forwarding separatelyA last-hop router can deliver one directed broadcast onto the destination network. Receiving and forwarding have separate optional controls and both default to blocking.→
- Separate origination from source addressesOriginating a packet means the router creates it itself. That permission is distinct from receiving or forwarding someone else’s directed broadcast.→
- Understand the amplification risk and its limitsAn attacker may lie about a packet’s source address. A network that accepts and forwards directed broadcasts can then turn one request into traffic from several responders toward the claimed source.→
This introduction explains RFC 2644 (1999). Read the original RFC ↗