1. Internet Engineering
DHCP, DNS

2. Introduction Client administration:
IP address management:
They need to ease the process of joining the network and they do not want users to do any special configuration (DHCP)
They want to network boot their workstations
i.e. Diskless workstations or remote OS installation (acquiring the network setting during boot process)
Solution: Deploy a DHCP server
Machine names management:
They need to be able to name machines and access them by names instead of IP addresses
Solution: Deploy A DNS server
So we discuss on DHCP and DNS in this session

3. DHCP
Provides configuration parameters specific to the DHCP client host requesting, information required by the client host to participate on an IP network
Method of IP allocation
Manual
Only requesting clients with a MAC address listed in the table (MAC-IP pairs) get the IP address according to the table
Automatic
DHCP server permanently assigns to a requesting client a free IP-address from a range given by the administrator
Dynamic
The only method which provides dynamic re-use of IP addresses
The request-and-grant process uses a lease concept with a controllable time period.

4. DHCP cont. DHCP server can provide optional configuration
e.g. Subnet Mask, Router, Name Server, …
RFC 2132 defines DHCP options Usage
DHCP relay agent (mostly in network routers/high-end switches)
Relays DHCP Discover broadcasts from a LAN without DHCP to a network which has one
Usage US
Cable Internet providers use DHCP
DSL providers prefer PPPoE UK
Many broadband ISP networks use DHCP
XDSL providers use infinite lease  Semi-static IPs
Office networks, public internet access
Places where there are mobile nodes that want to access the net

5. DHCP Implementations
Microsoft introduced DHCP on their NT server with Windows NT 3.5 in late 1994
DHCP did not originate from Microsoft
Internet Software Consortium published DHCP for Unix variants
Version released on December 6, 1997
Version 2.0 on June, 1999 – A more RFC-compliant one
Novell included a DHCP server in NetWare OS since v. 5, 1999
It integrates with Novell eDirectory
Weird solutions introducing a variety of multiplatform DHCP implementations since 1997
Cisco since Cisco IOS 12.0 in February 1999
Sun added DHCP support in Solaris 8, July 2001

6. DHCP Anatomy Uses the same IANA assigned ports as BOOTP
67/udp for the server, 68/udp for the client
DHCP Messages
Discover
Client broadcasts on the local physical subnet to find servers
UDP packet (broadcast dest )
Also request last-known IP address (optional parameter)
Offer
Server determines the configuration based on the client’s MAC addr.
Server specifies the IP address and put optional parameters
Request
Client selects a configuration out the DHCP Offer packet and broadcasts it again
Acknowledge
Server acknowledges the request and sends the ack to the client

7. DHCP Anatomy cont. Inform Release
Client requests more information than the server sent with the DHCPACK, or to repeat data for a particular application (e.g. to obtain web proxy settings by a browser)
Release
Client requests the server to release the DHCP and the client unconfigures its IP address
Sending this message is not mandatory (unplug or …)

8. BOOTP BOOTstrap Protocol (RFC 951) UDP
Used to obtain IP address automatically
Usually in booting process of computers or OSs
Diskless workstations
Historically used for UNIX-like diskless workstations
Also obtains the locations of the boot image
Also can be used for installing a pre-configured OS
Protocol became embedded in the BIOS of some NICs
Allowing direct network booting without need for a floppy

9. BOOTP cont.
Recently used for booting a Windows OS in diskless standalone media center PCs
DHCP is a more advanced protocol base on BOOTP
Far more complex to implement than BOOTP
Most DHCP servers also offer BOOTP support
Duration based leases is the fundamental addition in DHCP
Dynamic in DHCP is for this

10. RARP ARP Address Resolution Protocol
Resolve a hardware address from a given IP address
Try arp command in both Windows and Linux
RARP
Reverse Address Resolution Protocol (RFC 903)
Complement of ARP
Resolve an IP address from a given hardware address
Needs manual configuration on a central server
Not scalable
Obsoleted by BOOTP and the more modern DHCP
Try rarp command in Linux (if supported by Kernel), and RARP daemon - RARPd

11. DNS Domain Name System (RFC 1034, 1035)
RFC 1034 and 1035 made RFC 882, 883 obsolete
A system that stores info associated with domain names in a distributed database on networks (such as Internet)
Many types of information for the domain provided by DNS
Most important, IP address associated with domain name
Mail eXchange servers accepting for each domain
Mainly UDP
TCP only when response data size exceeds 512 bytes or for things like zone transfer

12. DNS is Decentralized No single point of failure Less traffic volume
Easier maintenance
Scalable
Less distant (delay) issues
Delegation

13. Resolvers Clients that access name servers
Querying a name server
Interpreting responses
Returning the information to the programs that requested it
In BIND, the resolver is just a set of library routines that is linked into programs
Not even a separate process
Most of the burden of finding an answer to the query is placed on the name server
The DNS specs call this kind of resolver a stub resolver

14. Types of DNS Servers Primary master
Reads the data for the zone from a file on its host
Secondary master (Slave)
Gets the zone data from another ns that is authoritative for the zone (master server)
Often, master server is the zone’s primary master
Not always the case
Secondary master may get the info from another secondary server
Zone transfer
Contacting master ns and if necessary pulling the zone data
Redundancy
An authoritative ns may be master for some of its zones and be slave for some others
It’s imprecise to call an ns, master or slave!

15. DNS Applications Attach IP addresses to domain names (ease of use)
Many to many mapping
Virtual Hosting
Sender Policy Framework
Makes it possible for people to assign authoritative names, without needing to communicate with a central registrar
Load balancing between hosts

16. DNS History Idea in ARPAnet
Originally, each computer retrieved a file called HOSTS.TXT from SRI which contained the mappings
Hosts file exists today (Looked up before querying DNS)
/etc/hosts, C:\WINDOWS\system32\drivers\etc\hosts
Limitations
Not scalable
Each time a given computer’s address changed, all computers should update their Hosts file
DNS invented by Paul Mockapetris in 1983
First implementation was called JEEVES by himself

17. Parts of a Domain Name
Domain name consists of two or more parts separated by dots (here ce.sharif.edu for example)
Rightmost label: Top-level domain (edu)
Each label to the left specifies a subdomain of the domain above it.
Relative dependence, not absolute dependence
sharif is a subdomain of the edu domain
ce is a subdomain of the sharif.edu domain
Theoretical limits: 127 level, each level 63 chars, total domain name 255 chars
A domain name with one or more IP addresses is called a hostname (sharif.edu, ce.sharif.edu but not edu)

18. A Distributed Hierarchical Database
Root Servers (13 root servers worldwide)
TLD Servers (.com, .org, .net, .uk, .ir, …)
Authoritative DNS Servers (organization’s DNS server)

19. Local DNS Server Does not belong to hierarchy
Also called default name server
Acts as a proxy (forwarder), forwards query into hierarchy
Caches the results if of interest

20. DNS Queries Recursive Iterative
Contacted name server should recurs and find the mapping for the requesting host
Heavy load on the servers
Iterative
Contacted server replies with the name of the server to contact
An ns provides the name of the next ns
Bootstrapping problem (another query is required and …)
So the IP of the next ns is provided
Glue record

21. DNS Queries
Recursive query example

22. DNS Queries
DNS in the real world

23. DNS Caching and Updating Records
Once a name server learns mapping, it caches it
It’ll expire (TTL defined by the authoritative server)
TLD servers typically cached in local name server
Root name servers not often visited
Update/Notify Mechanisms
RFC 2136
TTL is specified in the Start Of Authority (SOA) record
Serial – Incremented when the zone file modified, others know when the zone has been changed and should be reloaded
Refresh – Number of seconds between update requests
Retry – Number of seconds between retries (if a request failed)
Expire – Number of seconds before considering the data stale
Minimum – Used for minimum TTL, used for negative caching

24. DNS Records Resource Records Types
Tuples which are stored in the distributed database
(name, value, type, ttl)
Types
There are many types, most famous ones (IPv4 mostly)
A: Maps a hostname to an IPv4 address
NS: Maps a domain name to a list of authoritative DNS servers
CNAME: Makes one domain name an alias of another
MX: Maps a domain name to a list of mail exchange servers
PTR: Maps an IPv4 address to canonical name for that host
SOA: Specifies the authoritative DNS server
Info like of the domain administrator, serial number, …

25. Advanced Features of DNS Servers
Address Match Lists and Address Control Lists
i.e. defining a network and referring to it with the name we defined. e.g.
acl “ce” { { /24; /24; }; };
DNS Notify
Notify the listed servers on zone change
DNS Dynamic Update
This permits authorized updaters to add and delete resource records from a zone for which the server is authoritative
Used in DNS, DHCP servers integration

26. Legal Users of Domains Registrant Administrative contact
Technical contact
Billing contact
Name servers
Try whois in Linux and see these information for different hosts

27. DNS - BIND
BIND (Berkeley Internet Name Domain) written for Berkeley’s 4.3BSD UNIX OS by Kevin Dunlap
It is not maintained by Internet Software Consortium
The most popular implementation of DNS today
Ported to many flavors of UNIX
Shipped as a standard part of most vendors’ UNIX offerings
Has even been ported to Microsoft Windows

28. References Wikipedia, the free encyclopedia
Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition, Jim Kurose, Keith Ross, Addison-Wesley, July 2004
DNS and BIND, 3rd edition, Cricket Liu, Paul Albitz, O’Reilly, September 1998
BIND9 Administrator Reference Manual