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
Putting it all together
- 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
Domain Name System
- 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
Domain Name Service
- 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
zone file
- 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