Application Layer Protocols ( DNS)
What is a DNS resolver? How does it interact with other name servers?
DNS resolver is a server that is not necessarily part of the DNS hierarchy (not server responsible for a zone) , but it is capable of taking recursive request from a client and performing a set of iterative queries, going to the root servers if necessary, to get a result.
Understand some of the key items that DNS servers know about: host address, canonical name (alias), name servers, mail exchanger
Address (A Record) - identifies the IP address for a given host name. canonical name (CNAME record) - identifies the real host name for an alias. For example, cs.rutgers.edu is really a CNAME (alias) to ww3.srv.lcsr.rutgers.edu name server (NS record) - identifies the authoritative name servers for the domain. mail exchanger (MX record) - identifies the mail server for a given host name.
What is a referral?
Referral is the name server for the next lower level when using the iterative resolution when doing a name resolution.
What is a resource record?
Resource record contains a name, value type of record, and a time to live value. The Resource record is stored in the DNS server.
What are the roles of the domain name registry operator and the domain name registrar?
domain name registry operator maintains the domain name registry. domain name reistrar is a company that provides domain registration services to customers, allowing them to register domain names for a fee.
What are generic top-level domains (gTLD) and country-code top-level domains (ccTLD)?
gTLD - .com, .edu, .gov, .net etc ccTLD - .us, .es, (country specific domains)
What is a zone?
a zone is a portion of the domain name space (a sub-tree) that is managed by a specific entity. i.e. rutgers.edu is a zone that manages all domains within rutgers.edu
What is at the top of the domain name hierarchy?
.com, .edu, .org. 1,239 top-level domains.
How is reverse DNS handled differently from looking up domain names?
An organization has a range, or several ranges, of IP addresses assigned to it. It sets up a local DNS server with PTR (pointer) records that map IP addresses to names. It then tells its ISP what DNS servers are responsible for reverse DNS lookups. The ISP knows what range of addresses belong to the organization. If it gets a query for an address in that range, it now knows which name servers to send on a referral reply. A reverse query that starts at the root will contact the root name servers. These servers, in addition to knowing the name servers of TLDs, also know the name servers for the five RIRs (ARIN, RIPE NCC, etc.) - the entities that hand out IP addresses. The root server may return a referral for the ARIN server (responsible for IP addresses in North America). The ARIN server knows the blocks of IP addresses that were allocated to various ISPs and will send a referral to the name server for the appropriate ISP. That ISP, when queried, will then respond with a referral to the name server for the organization that owns that address.
What is the in-addr.arpa domain?
It is a special domain created for reverse lookups (ARPA stands for Address and Routing Parameter area)
What is a Regional Internet Registry (RIR)?
Responsible for the part of the world's geography. They allocate blocks of IP addresses given from the IANA to ISPs within their region. US and Canada get address from ARIN. Europe and Mid-East get addresses from the RIPE Network Coordination Centre.
What is the value of caching previous lookups in DNS servers. When/how do they expire?
The value of caching previous lookups in DNS servers is that it avoids the overhead of repeated lookups for the same name and to avoid the overhead of iterative queries. They contain a Time-to-live value or TTL, usually an estimate of how long it is safe to keep the results. Usually between an 15 mins to an hour.
What is the Internet Assigned Numbers Authority (IANA)?
They are responsible for the entire set of IP addresses. It allocates blocks of addresses to Regional Internet Registries.
What is a designated registrar?
When you pay the registrar ( godaddy for example) for a domain name if its available, godaddy becomes the designated registrar meaning it has information on the owner of the domain name and transfer or changes of ownership has to be taken care of by godaddy.
What is iterative versus recursive name resolution?
iterative resolution - operates by having you walk the tree to the zone you are searching for. each level of the hierarchy either returns a definitive answer or a referral ( the name server for the next lower level, taking you closer to the destination) . recursive resolution - the DNS server does the entire job of looking up the answer, contacting other servers as needed to resolve the name. The query gets gets forwarded until its found and then send back up that chain.
What are root name servers?
root name servers are servers that can provide a list of authoritative name servers for all top-level domains. By contacting any one of these servers, you can find out the address of a name server responsible for a specific top-level domain (such as .edu)
What is authoritative name server?
the authoritative name server is responsible for answering queries about its zone. it stores and is responsible for specific DNS records (as opposed to storing a cached copy)
What is a domain name registry?
the list of registered domain names for a particular TLD.