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1Spring Semester 2009, Dept. of Computer Science, Technion

Internet NetworkingInternet Networking

recitation #2recitation #2

DNS and DHCPDNS and DHCP

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Domain Name SystemDomain Name System -- DNSDNS

Provides mapping from ASCII domain names to IP

addresses

Hierarchical naming system.

The set of names is divided into mutually exclusive parts.

Supplies mechanism forglobal data storage and

information retrieval.

Most important principles:

Distributed system set of servers sharing information. Efficiency - most of the requests resolved by local servers.

Use of Caching.

Reliability works even if some of the servers fail.

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DNS treeDNS tree

root

orgcom gov net il

cnnibm govcoac

tautechnion

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Reliability overUDPReliability overUDP

DNS is an application whichruns overUDP

Advantage: not as complex as TCP.

Disadvantage: requires reliability implementation at

application level .

Reliability scheme:

Time out.

Retransmissions - resends timed out query to a different

server.

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DNS TermsDNS Terms

Domain name: any name represented in the DNSformat

mail.yahoo.com

\.name.example

DNS label: each string between two ".

ac.il 2 labels

DNS zone: a set of names that are under the sameauthority

cs.technion.ac.il,ee.technion.ac.il andwww.technion.ac.il

Delegation: transferof authority fora domain

example.org is a delegation from org.

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DNS ElementsDNS Elements

Resolver:

stub: simple, only asks questions;

recursive: takes simple query and makes all necessarysteps to get the full answer.

Server (some perform bothroles at the same time): authoritative: the servers that contain the zone file fora

zone, one Primary, one ormore Secondaries;

caching: a recursive resolver that stores priorresults andreuses them.

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How Does DNS Work?How Does DNS Work?

The resolversends a DNS request message overUDP to a local

domain name server.

DNS caches store data fora short time defined by TTL on the

Record.

When a name serverdoes not have the requested information,

it starts at longest match on query name it has when looking for

data.

The root serverknows the IP address of each second-level

domain name server.

Every serverknows how to reach servers that are authorities fornames furtherdown the hierarchy.

The resolver follows delegations until it receives an answer.

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DNS query example (from IETF slides)DNS query example (from IETF slides)

Root Server

Ietf.org Server

Org

Server

Ask org NS

Ask ietf.org NS

www.ietf.org A

65.256.255.51

www.ietf.org A

65.256.255.51

Recursive

Resolver

www.ietf.org

Stub resolver

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Dynamic Host Configuration ProtocolDynamic Host Configuration Protocol -- DHCPDHCP

RFC 2131

Runs overUDP as well

Provides automaticconfiguration of the host

connected to network orbooted

Provides hosts with initial configuration information

upon bootup:

IP address with subnet mask,

default gateway,

IP address of the DNS server .

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DHCP (cont.)DHCP (cont.)

server Aclient

server B

determine

configuration

determine

configuration

DHCP discoverDHCP discover

DHCP offerDHCP offer

select configuration

DHCP requestDHCP request

DHCP ack

Initialization completes

graceful shutdownDHCP release

discard lease

using the allocated configuration

(selected)(not selected)

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DHCP MessagesDHCP Messages

DHCPDISCOVER The client broadcasts message in search of available DHCP

servers.

DHCPOFFER

The serverresponse to the client DHCPDISCOVER with offerof

configuration parameters . DHCPREQUEST

The client broadcasts to the server,requesting offered parameters

from one serverspecifically.

Confirms correctness of previously allocated address after, e.g.,

system reboot.

Extends the lease on a particularnetwork address.

DHCPRELEASE

The client-to-servercommunication,relinquishing network address

and canceling remaining lease.

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DHCPACK

The server-to-client communication withconfiguration parameters,

including committed network address.

DHCPNAK

Server to client indicating client's notion of network address is

incorrect (e.g.,client has moved to new subnet) orclient's lease asexpired

DHCPDECLINE

The client-to-servercommunication, indicating that the network

address is already in use.

DHCPINFORM The client-to-servercommunication, asking foronly local

configuration parameters that the client already has externally

configured as an address.

DHCP Messages (Cont.)DHCP Messages (Cont.)

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DHCP message formatDHCP message format

xid

Transaction ID.

ciaddr

Client IP address

yiaddr

Your IP address.

siaddr

Server IP address. chaddr

Client MAC address

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DHCP message format (cont.)DHCP message format (cont.)

Transaction ID - a random number that is used by the clientto match incoming DHCP messages with pending requests.

Client IP address - used when client knows its IP address.

Your IP address - the DHCP serverwill place the offeredclient IP address in this field, if the client IP address is 0.0.0.0.

Server IP address - the DHCP serveraddress, if it is known.Otherwise, it is used in DHCPOFFER and DHCPACK.

options - DHCP defines a 'client identifier' option that is used to

pass an explicit client identifier to a server.

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DHCP message filteringDHCP message filtering

DHCPOFFER Only the client waiting for this message will accept it in

UDP layer.

But what if two clients are waiting for the offer

simultaneously?

The filtering is possible only at the application layer. The transaction ID field is responsible for this filtering.

DHCPREQUEST

The message is broadcasted to all the servers.

The serverknows if its offerwas accepted only uponreading the server identifier field in the DHCP message.

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DHCP as UDP applicationDHCP as UDP application

DHCP server - port 67,client - port 68

Reliability is not provided by UDP.

Client is responsible forreliability

Th

eclient implements time

rto measu

re timeout fo

rth

emessages that were not responded.

The client adopts a retransmission strategy that incorporates

a randomized exponential backoff algorithm to determine the

delay between retransmissions.

Every next message acts as an acknowledgment for the

previous step.

Forexample, DHCPREQUEST is an ACK forDHCPOFFER.