The Black Hole of What?

In networking, black holes refer to places in the network where incoming traffic is silently discarded (or "dropped"), without informing the source that the data did not reach its intended recipient.

When examining the topology of the network, the black holes themselves are invisible, and can only be detected by monitoring the lost traffic; hence the name.

Contents 1 Dead addresses 2 Firewalls and "stealth" ports 3 Black hole filtering 4 PMTUD black holes 5 Black hole e-mail addresses 6 See also 7 External links

Dead addresses

The most common form of black hole is simply an IP address that specifies a host machine that is not running or an address to which no host has been assigned.
Even though TCP/IP provides means of communicating the delivery failure back to the sender via ICMP, traffic destined for such addresses is often just dropped.
Note that a dead address will be undetectable only to protocols that are both connectionless and unreliable (e.g., UDP). Connection-oriented or reliable protocols (TCP, RUDP) will either fail to connect to a dead address or will fail to receive expected acknowledgements.

Firewalls and "stealth" ports

Most firewalls can be configured to silently discard packets addressed to forbidden hosts or ports, resulting in small or large "black holes" in the network.

Black hole filtering

Black hole filtering refers specifically to dropping packets at the routing level, usually using a routing protocol to implement the filtering on several routers at once, often dynamically to respond quickly to distributed denial-of-service attacks.

PMTUD black holes

Main article: Path MTU discovery#Problems with PMTUD

Some firewalls incorrectly discard all ICMP packets, including the ones needed for Path MTU discovery to work correctly. This causes TCP connections from/to/through hosts with a lower MTU to hang.

Black hole e-mail addresses

A black hole e-mail address is an e-mail address which is valid (messages sent to it will not generate errors), but to which all messages sent are automatically deleted, and never stored or seen by humans. These addresses are often used as return addresses for automated e-mails.

See also

• Null route
• Internet background noise
• Packet drop attack
• DDoS

External links

• Remotely triggered black hole filtering (Cisco Systems)(link broken)
• University of Washington blackhole monitor/lookup system (link doesn't work)
• Tools for detecting a blackhole attack in an ad hoc wireless network
• Remote Triggered Black Hole Filtering