In networking, black holes refer to places in the network where incoming or outgoing 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.
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 a 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 (and routers for household use) can be configured to silently discard packets addressed to forbidden hosts or ports, resulting in small or large “black holes” in the network.
Personal firewalls that do not respond to ICMP echo requests (“ping”) have been designated by some vendors as being in “stealth mode”.
Despite this, in most networks the IP addresses of hosts with firewalls configured in this way are easily distinguished from invalid or otherwise unreachable IP addresses: On encountering the latter, a router will generally respond with an ICMP network rsp. host unreachable error. NAT, as used in home and office routers, is generally a more effective way of obscuring the layout of an internal network.
Black Hole Filtering
A null route or black hole route is a network route (routing table entry) that goes nowhere. Matching packets are dropped (ignored) rather than forwarded, acting as a kind of very limited firewall. The act of using null routes is often called blackhole filtering. The rest of this article deals with null routing in the Internet Protocol (IP).
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.
Remote Triggered Black Hole Filtering (RTBH) is a technique that provides the ability to drop undesirable traffic before it enters a protected network. The Internet Exchange (IX) provider usually acquires this technology to help its members or participants to filter such attack.
Null routes are typically configured with a special route flag, but can also be implemented by forwarding packets to an illegal IP address such as 0.0.0.0, or the loopback address.
Null routing has an advantage over classic firewalls since it is available on every potential network router (including all modern operating systems), and adds virtually no performance impact. Due to the nature of high-bandwidth routers, null routing can often sustain higher throughput than conventional firewalls.
For this reason, null routes are often used on high-performance core routers to mitigate large-scale denial-of-service attacks before the packets reach a bottleneck, thus avoiding collateral damage from DDoS attacks — although the target of the attack will be inaccessible to anyone. Blackhole filtering can also be abused by malicious attackers on compromised routers to filter out traffic destined to a certain address.
Routing typically only works on the Internet Protocol layer and is very limited in packet classification. It is bound to be stateless due to the nature of IP routers. Typically, classification is limited to the destination IP address prefix, source IP address and incoming network interface.