Black hole (networking)

Part of a network quietly losing traffic

In networking, a black hole refers to a place in the network where incoming or outgoing traffic is discarded[1] (or "dropped"[2] or sinkholed[3][4]) without informing the source that the data did not reach its intended recipient.[1]

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

The name[3] is presumed to have originated from a concept of an astronomical location, a black hole.

## Addresses

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A black hole IP address specifies a host machine that is not running or an address to which no host has been assigned.[1]

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.

Blackholed addresses are 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 blackholed address or will fail to receive expected acknowledgements.

For IPv6, the black hole prefix is 100::/64.[4]

For IPv4, no black hole address is explicitly defined, however the reserved IP addresses can help achieve a similar effect. For example, 198.51.100.0/24 is reserved for use in documentation and examples[5]; while the RFC advises that the addresses in this range are not routed, this is not a requirement.

With the invention of cryptocurrencies addresses as locations where monies exist, there is the possibility of blackholing a quantity of crypto.[6]

## Firewalls and "stealth" ports

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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[7] 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.[8] Network address translation (NAT), as used in home and office routers, is generally a more effective way of obscuring the layout of an internal network.[9] [10] [11]

### PMTUD black holes

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Main article: Path MTU discovery § Problems

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 filtering

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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.[12]

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 (DDoS).

Remote Triggered Black Hole Filtering (RTBH) is a technique that provides the ability to drop undesirable traffic before it enters a protected network.[2] The Internet Exchange (IX) provider usually acquires this technology to help its members or participants to filter such attacks.[13]

Null routes are typically configured with a special route flag; for example, the standard iproute2 command `ip route` allows to set route types `unreachable, blackhole, prohibit` which discard packets. Alternatively, a null route can 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.

## DNS-based Blackhole List

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Main article: Domain Name System-based Blackhole List

A DNS-based Blackhole List (DNSBL) or Real-time Blackhole List (RBL) is a list of IP addresses published through the Internet Domain Name System (DNS) either as a zone file that can be used by DNS server software, or as a live DNS zone that can be queried in real-time. DNSBLs are most often used to publish the addresses of computers or networks linked to spamming; most mail server software can be configured to reject or flag messages which have been sent from a site listed on one or more such lists. The term "Blackhole List" is sometimes interchanged with the term "blacklist" and "blocklist".

A DNSBL is a software mechanism, rather than a specific list or policy. There are dozens of DNSBLs in existence,[14] which use a wide array of criteria for listing and delisting of addresses. These may include listing the addresses of zombie computers or other machines being used to send spam, listing the addresses of ISPs who willingly host spammers, or listing addresses which have sent spam to a honeypot system.

Since the creation of the first DNSBL in 1997, the operation and policies of these lists have been frequently controversial,[15][16] both in Internet advocacy and occasionally in lawsuits. Many email systems operators and users[17] consider DNSBLs a valuable tool to share information about sources of spam, but others including some prominent Internet activists have objected to them as a form of censorship.[18][19][20][21] In addition, a small number of DNSBL operators have been the target of lawsuits filed by spammers seeking to have the lists shut down altogether.[22]

## Black hole e-mail addresses

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A black hole[23] e-mail address is an e-mail address which is valid (messages sent to it will not generate errors), but all the received messages are automatically deleted, and never stored or seen by humans. These addresses are often used as return addresses for automated e-mails.

## See also

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* Bit bucket
* Internet background noise
* IP blocking
* Null device
* Packet drop attack

## References

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1. ^ a b c "GuardDuty EC2 finding types: Trojan:EC2/BlackholeTraffic". amazon.com. Amazon. 2024.
2. ^ a b "Blackhole" (PDF). cisco.com. CISCO. Retrieved 25 June 2023.
3. ^ a b "DNS Blackhole (AS112) Service". www.iana.org. Internet Corporation for Assigned Names and Numbers. 15 June 2010. Retrieved 24 February 2025.
4. ^ a b N. Hilliard; D. Freedman (August 2012). A Discard Prefix for IPv6. Internet Engineering Task Force. doi:10.17487/RFC6666. ISSN 2070-1721. RFC 6666. Informational.
5. ^ J. Arkko; M. Cotton; L. Vegoda (January 2010). IPv4 Address Blocks Reserved for Documentation. Internet Engineering Task Force. doi:10.17487/RFC5737. ISSN 2070-1721. RFC 5737. Informational. Updates RFC 1166.
6. ^ "XRP LEDGER LEARNING PORTAL". xrpl.org. XRP LEDGER FOUNDATION. Retrieved 24 February 2025.
7. ^ Apple Inc., botXhacker" "About the Application Firewall"
8. ^ Kurose, J. F.; Ross, K. W. (2021). Computer Networking: A Top-Down Approach (8th ed.). Pearson. ISBN 978-0136681557.
9. ^ Strebe, Matthew (2019). Network Security Foundations: Technology Fundamentals for IT Success. Sybex. ISBN 978-0782143270.
10. ^ "RFC 3022: Traditional IP Network Address Translator (Traditional NAT)". IETF. January 2001.
11. ^ Wool, Avishai (2004). "A quantitative study of firewall configuration errors". IEEE Computer. 37 (6): 62–67\. doi:10.1109/MC.2004.2.
12. ^ "Palo Alto Networks Support for Null Routes". knowledgebase.paloaltonetworks.com. PALO ALTO NETWORKS. 25 September 2018.
13. ^ "HKIX".
14. ^ "DNS & RHS blackhole lists". Archived from the original on 21 March 2013. Retrieved 26 March 2013.
15. ^ C. Lewis; M. Sergeant (January 2012). Overview of Best Email DNS-Based List (DNSBL) Operational Practices. Internet Research Task Force (IRTF). doi:10.17487/RFC6471. ISSN 2070-1721. RFC 6471. Informational.
16. ^ "RBLMon.com: What are RBLs and How do they Work?". Archived from the original on 4 September 2017. Retrieved 26 March 2013.
17. ^ "Revealing Botnet Membership Using DNSBL Counter-Intelligence" (PDF). Retrieved 26 March 2013.
18. ^ "RBL Criticism". 11 February 2008. Retrieved 26 March 2013.
19. ^ "Electronic Frontier Foundation, EFFector, Vol. 14, No. 31, Oct. 16, 2001". 12 January 2012. Retrieved 26 March 2013.
20. ^ "Verio gags EFF founder over spam". The Register. Retrieved 26 March 2013.
21. ^ "Choosing Spam over Censorship". Archived from the original on 21 April 2003. Retrieved 26 March 2013.
22. ^ "EMarketersAmerica.org sues anti-spam groups". Retrieved 26 March 2013.
23. ^ Exim internet mailer specification document,the Redirect router University of Cambridge

## External links

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* Remotely triggered black hole filtering (Cisco Systems)
* University of Washington blackhole monitor/lookup system
* Tools for detecting a blackhole attack in an ad hoc wireless network
* Remote Triggered Black Hole Filtering