- Blackhole server
Blackhole DNS servers are DNS servers that return a "nonexistent address" answer to
reverse DNS lookup s for addresses reserved for private use.Background
RFC 1918 reserves several ranges of network addresses for use on private intranets. This is required to avoid address collisions between nodes on the Internet and nodes on the intranet. The addresses reserved for this use by RFC 1918 are:
* 10.0.0.0 - 10.255.255.255
* 172.16.0.0 - 172.31.255.255
* 192.168.0.0 - 192.168.255.255Even though traffic to or from these addresses should never appear on the public Internet, it is not uncommon for such traffic to appear anyway. Some servers are configured (usually for logging reasons) to perform areverse DNS lookup on clients' IP address. If the server encounters a packet originating from an RFC1918 address, it may try performing such a lookup on that address. This causes unnecessary network traffic and may also impair the functionality of the server (because the query would go unanswered and the server would have to wait for the query to time out).Role
To deal with this problem,
IANA has set-up three special DNS servers called "blackhole servers". Currently the blackhole servers are:
* blackhole-1.iana.org
* blackhole-2.iana.org
* prisoner.iana.orgThese servers are registered in the DNS directory as the authoritative servers for the reverse lookup zone of the RFC1918 addresses. These servers are configured to answer any query with a "nonexistent address" answer. This helps reducing 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 receive thousands of queries every second.External links
* [http://www.iana.org/abuse/faq.html The IANA abuse faq] which contains information about the blackhole servers.
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