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Anycast is a type of addressing in computer networks in which a whole group of computers is assigned a common address, but which can only be used to reach the computer that can be reached via the shortest route . This technique is used in accordance with the OSI model in the network layer to the application.


Anycast is implemented by distributing several servers of the same type on spatially separate IP networks. In practice, at least one server is often installed on every continent or in every country in a region. Each of these computers receives the same IP address and propagates a corresponding route using a routing protocol ( BGP on the Internet ). In the event of failure or inaccessibility, the route disappears and all subsequent packets are forwarded to another server. The desired service can thus be provided even if one or more servers fail. This increases availability . A server must also be able to be addressed directly for administration. Anycast servers therefore have an additional unicast address in almost all cases .

From a client's point of view, there is no difference between unicast and anycast. In contrast to multicast environments, which generate packets for all group members, the sender here only generates a single packet. Anycast is a feature that is particularly supported in IPv6 . A packet with an anycast address is only delivered to one - generally the nearest - interface in the group. The entry in the routing table of the router closest to the sender determines which one it is.

This concept enables companies to use an isolated anycast address to deliver a packet to multiple routers of the provider that are members of an anycast group - i.e. that is, they have the same anycast address. If the company has several access points to the Internet, the failure of a connection to the provider or a router at the provider can be compensated immediately by another router of the same anycast group, which is then reached via one of the alternative connections. However, the sender has no option to choose the physical receiver (the receiving interface); the destination is determined exclusively by the routing protocol.

Similarly, load distribution can be achieved by always delivering packets from senders to the closest server. If several servers of an anycast group exist in different networks, this can ensure that the load is distributed over many servers without the knowledge or involvement of the sender. This will u. a. for the DNS - root servers used (see below).


The most important advantages of anycast are load balancing and shorter access times . In addition, there is almost complete transparency (the user does not even notice that he is communicating with an anycast server) and excellent scalability (servers can be added or removed at any time). A disadvantage is the complex internal synchronization, since all servers involved always have to provide identical data. In addition, troubleshooting is made more difficult in the event of a malfunction, as it is often not possible to determine which server is responsible without further analysis.

In some of the example Anycast root name servers and top-level domain server as well as the IPv6 -in- IPv4 tunneling mechanism 6to4 used. Anycast is also used to defend against distributed denial-of-service attacks (DDoS attacks) by companies such as Cloudflare and offered as a service, for example for websites. Such protected websites are accessed using a reverse proxy . For example, several DoS attacks on root name servers in 2006 and 2007 were largely unsuccessful due to the anycast load distribution. One of the largest DDoS attacks against Cloudflare with a peak load of more than 300 Gbps as of 2013 could also be stopped by the anycast used.

Norms and standards

  • RFC 1546 , Host Anycasting Service (1993).

Individual evidence

  1. ICANN Factsheet: Root server attack on February 6, 2007
  2. Black Hat USA 2013 - Lessons from Surviving a 300Gbps Denial of Service Attack