ICS 351: Today's plan

Dynamic Host Configuration Protocol

in our lab, computers have statically-configured IP addresses
we also manually reconfigure IP addresses when needed
when there are lots of computers or non-technical users, this becomes very cumbersome
instead, a network administrator could decide the assignment of IP addresses centrally, and let a central computer distribute IP addresses
however, the computer is not really on the Internet until it has an IP address
so DHCP (like ARP) cannot use IP packets
each address is leased for a given period of time, then must be renewed
in most DHCP servers, renewal is automatic unless the network administrator decides otherwise
so the lease expiration simply makes it easier to recycle addresses
the system administrator decides the lengths of leases, which IP address ranges are available for DHCP, etc

many small office / home office (SOHO) networks are connected to the Internet by a single "router"
this "router" runs no routing protocols, and may use DHCP to obtain its address from the Internet Service Provider
or, if there are significant servers on the SOHO network, the IP address may be static and manually configured
this "router" acts as a default router for the SOHO network, forwarding to its own default router (configured, or obtained by DHCP) all packets from the inside to the outside
this "router" always performs NAT, to allow the sharing of the IP address
this "router" usually performs some sort of firewalling
for example, the firewall might by default allow all outgoing connections/streams, initiated by a computer inside, but not connections or streams initiated by outside computers
but data for valid connections (streams) must still flow in both directions

Hierarchical system of names
top-level domain names include .edu, .org, .com, .net, and many country top-level domains
root is just "."
so the fully qualified domain name is, e.g., www.hawaii.edu.
administration of domains is delegated according to the hierarchy

the domain names are administratively divided into zones
each zone must be contiguous in the hierarchy tree:
- .edu. is one zone
- hawaii.edu. and most names below it are one zone,
- ics.hawaii.edu. and all names below it are one zone,
in the Internet, each zone should have at least two authoritative servers
the root has 13 servers, A.root-servers.net through M.root-servers.net
clients (resolvers) send requests to servers to resolve a domain name to an IP address or vice-versa
DNS is designed to run over either TCP or UDP, but commonly runs over UDP for name resolution
large transfers such as zone exchanges usually use TCP

the network administrator must configure each of the servers with a description of name to IP address mappings
this is loosely known as a zone file, even though several files might be needed to accomplish the overall configuration
if there are multiple servers for one zone (should be at least two) their zone files should describe the same zone
A is not the only record that can be served:
- MX identifies the mail server for a given name
- NS identifies the name server for a given name
- CNAME identifies the "main" (canonical) name for a given name
- the SOA (start of authority) record can be used by a slave server to download the records from the master server
DNS names in zone files end in '.' if they are absolute, otherwise they are relative to the zone