Blackhole DNS servers are Domain Name System (DNS) servers that return a "nonexistent address" answer to reverse DNS lookups for addresses reserved for private use.

Background

There are several ranges of network addresses reserved for use on private networks in IPv4:[1]

Reserved private IPv4 network ranges[2]
NameCIDR blockAddress rangeNumber of addressesClassful description
24-bit block10.0.0.0/810.0.0.0 – 10.255.255.25516777216Single Class A
20-bit block172.16.0.0/12172.16.0.0 – 172.31.255.2551048576Contiguous range of 16 Class B blocks
16-bit block192.168.0.0/16192.168.0.0 – 192.168.255.25565536Contiguous range of 256 Class C blocks

Even though traffic to or from these addresses should never appear on the public Internet, it is not uncommon for such traffic to appear anyway.

Role

To deal with this problem, the Internet Assigned Numbers Authority (IANA) has set up three special DNS servers called "blackhole servers". Currently the blackhole servers are:[3]

  • blackhole-1.iana.org (192.175.48.6)
  • blackhole-2.iana.org (192.175.48.42)
  • prisoner.iana.org (192.175.48.1)

These servers are registered in the DNS directory as the authoritative servers for the reverse lookup zone of the 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 addresses. These servers are configured to answer any query with a "nonexistent address" answer. This helps to reduce wait times because the (negative) answer is given immediately and thus no wait for a timeout is necessary. Additionally, the answer returned is also allowed to be cached by recursive DNS servers. This is especially helpful because a second lookup for the same address performed by the same node would probably be answered from the local cache instead of querying the authoritative servers again. This helps reduce the network load significantly. According to IANA, "the blackhole servers generally answer thousands of queries per second".[4] Because the load on the IANA blackhole servers became very high, an alternative service, AS112, has been created, mostly run by volunteer operators.

AS112

The AS112 project is a group of volunteer name server operators joined in an autonomous system. They run anycasted instances of the name servers that answer reverse DNS lookups for private network and link-local addresses sent to the public Internet. These queries are ambiguous by their nature, and cannot be answered correctly. Providing negative answers reduces the load on the public DNS infrastructure.

History

Before 2001, the in-addr.arpa zones for the private networks[1] were delegated to a single instance of name servers, blackhole-1.iana.org and blackhole-2.iana.org, called the blackhole servers. The IANA-run servers were under increasing load from improperly-configured NAT networks, leaking out reverse DNS queries, also causing unnecessary load on the root servers. The decision was made by a small subset of root server operators to run the reverse delegations; each announcing the network using the autonomous system number of 112.[5] Later, the group of volunteers has grown to include many other organizations.

An alternative approach, using DNAME redirection, was adopted by the IETF in May 2015.[6][7]

Answered zones

The name servers participating in the AS112 project are each configured to answer authoritatively for the following zones:

  • For the 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 private networks:[1]
    • 10.in-addr.arpa
    • 16.172.in-addr.arpa
    • 17.172.in-addr.arpa
    • 18.172.in-addr.arpa
    • 19.172.in-addr.arpa
    • 20.172.in-addr.arpa
    • 21.172.in-addr.arpa
    • 22.172.in-addr.arpa
    • 23.172.in-addr.arpa
    • 24.172.in-addr.arpa
    • 25.172.in-addr.arpa
    • 26.172.in-addr.arpa
    • 27.172.in-addr.arpa
    • 28.172.in-addr.arpa
    • 29.172.in-addr.arpa
    • 30.172.in-addr.arpa
    • 31.172.in-addr.arpa
    • 168.192.in-addr.arpa
  • For the 169.254.0.0/16 link-local addresses:[8]
    • 254.169.in-addr.arpa
  • For unique identification purposes:
    • hostname.as112.net

References

  1. 1 2 3 Y. Rekhter; B. Moskowitz; D. Karrenberg; G. J. de Groot; E. Lear (February 1996). Address Allocation for Private Internets. Network Working Group. doi:10.17487/RFC1918. BCP 5. RFC 1918. Updated by RFC 6761.
  2. ↑ Y. Rekhter; B. Moskowitz; D. Karrenberg; G. J. de Groot; E. Lear (February 1996). Address Allocation for Private Internets. Network Working Group. doi:10.17487/RFC1918. BCP 5. RFC 1918. Best Common Practice. Obsoletes RFC 1627 and 1597. Updated by RFC 6761.
  3. ↑ J. Abley; W. Maton (July 2011). I'm Being Attacked by PRISONER.IANA.ORG!. IETF. doi:10.17487/RFC6305. ISSN 2070-1721. RFC 6305.
  4. ↑ "Common questions regarding abuse issues". IANA.
  5. ↑ T. Hardie (April 2002). Distributing Authoritative Name Servers via Shared Unicast Addresses. Network Working Group IETF. doi:10.17487/RFC3258. RFC 3258.
  6. ↑ J. Abley; W. Sotomayor (May 2015). AS112 Nameserver Operations. IETF. doi:10.17487/RFC7534. RFC 7534. Obsoletes RFC 6304.
  7. ↑ J. Abley; B. Dickson; W. Kumari; G. Michaelson (May 2015). AS112 Redirection Using DNAME. IETF. doi:10.17487/RFC7535. RFC 7535.
  8. ↑ S. Cheshire; B. Aboba; E. Guttman (May 2005). Dynamic Configuration of IPv4 Link-Local Addresses. Network Working Group IETF. doi:10.17487/RFC3927. RFC 3927.
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