IP addresses (even 4-byte IPv4 ones), are generally not easy to remember, which is why humans prefer to type domain names into their browser address window, even if they are longer. It is job of the Domain Name System (DNS) to translate the domain name into the correct IP address, which is placed in the IP header and enables proper forwarding.
The DNS works recursively in the following way. When my application (for instance, my browser) needs the IP address for some domain name, it queries the operating system’s DNS resolver. If the resolver already knows the IP address (for example, it is preconfigured, or that domain name has been recently looked up and is cached) it returns it to the application. If not, the resolver will query a DNS server, that has been configured or found using DHCP. If this server knows the IP address (i.e., it is cached there), it returns it in an “A record” (or an “AAAA record” for IPv6 addresses); otherwise it recursively queries until it finds a server that has the required “A record”. It may eventually get to the authoritative DNS server for the domain in question; that is, the name server that didn’t learn the IP address from another name server, but was configured with it.
This system is hierarchical and distributed and thus very scalable, but is not very secure. The archetypical attack is DNS cache poisoning, which is carried out by impersonating a name server that knows the desired IP address, and causing a name server closer to the resolver to cache the incorrect result. When queried the attacker’s IP address is returned to the user who then browses to a malicious site where it is tricked into accepting fallacious content or infected with viruses to be exploited later.
DNSSEC (Domain Name System Security Extensions) adds source authentication and integrity checking to the DSN system in a backwards compatible way. In DNSSEC the DNS responses are cryptographically signed with public key signatures, and thus can’t be forged. This thwarts cache poisoning exploits. In addition, DNSSEC can also be used to protect non-DNS data, such as “CERT records” that can be used to authenticate emails.
DNSSEC is described in RFCs 4033, 4034, and 4035 from 2005, but the root zone of the Internet was only signed in July, 2010. This major milestone was celebrated last week at the Wednesday IETF-78 plenary with glasses of champagne and the handing out of stickers declaring IETF – DNSSEC – SIGNED.