lecture on dhcp, dns & snmpdebashis saha thursday, may 17, 20071 dhcp, dns, snmp debashis saha...

Thursday, May 17, 2007

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Lecture on DHCP, DNS & SNMP Debashis Saha

DHCP, DNS, SNMP

Debashis SahaMIS Group, IIM Calcutta

[email protected]


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First Question

• How does your computer know which IP to use?

• How does your computer know that www.iimcal.ac.in is at IP 202.54.116.3?

• How can you track your network’s health?


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The Internet and Addressing

• All machines on the Internet are accessed via their IP address– Dotted quad: xxx.xxx.xxx.xxx

• Problem: IP addresses are hard to remember and hard to statically assign on large networks

• Solution: Need to come up with a way to automatically assign IP addresses and a text based representation of machine addresses instead of just 4 numbers


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Addressing Solutions

• Two protocols have been developed to solve these problems

– DHCP – automatic network configuration (including IP address)

– DNS – translates textual based names into IP addresses and vice versa


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DHCP – Dynamic Host Configuration Protocol


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DHCP

• Allows client machines to receive an IP address, DNS information, etc automatically

• Before DHCP came into use, users had to type in all this information by hand, which is bad:– Easy to mistype something when entering by hand– Manually changing network configuration every time

you move your laptop is a pain– Bootp resolved some of these issues

• … and DHCP still uses the same port as bootp


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Purpose of DHCP

From RFC-2131 (the Internet standard): • The Dynamic Host Configuration Protocol

(DHCP) provides a framework for passing configuration information to hosts on a TCP/IP network.

• DHCP consists of two components: – a protocol for delivering host-specific configuration

parameters from a DHCP server to a host and – a mechanism for allocation of network addresses to hosts.


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DHCP functional goals• A host without a valid IP address locates

and communicates with a DHCP server• A DHCP server passes configuration

parameters, including an IP address, to the host

• The DHCP server may dynamically allocate addresses to hosts and reuse addresses

• Hosts can detect when they require a new IP address

• Unavailability of DHCP server has minimal effect on operation of hosts


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DHCP: Basics

• A client leases an IP address from a DHCP server for a given amount of time

• When lease expires, the client must ask DHCP server for a new address (clients attempt to renew lease after 50% of the lease time has expired)

• Typical leases may last for from 30 seconds to 24 hours, or even longer.


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What does DHCP do?• Provides protocol stack, application and

other configuration parameters to hosts• Eliminates need for individual, manual

configuration for hosts• Includes administrative controls for

network administrators• Backward compatible packet format for

BOOTP interoperation (RFC 1542)• Can coexist with hosts that have pre-

assigned IP addresses and hosts that do not participate in DHCP


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Design Goals

• Eliminate manual configuration of hosts• Prevent use of any IP address by more

than one host• Should not require a server on every

subnet• Allow for multiple DHCP servers• Provide a mechanism, not a policy• Provide the same configuration -

including IP address - to a host whenever possible


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What can you do with DHCP

• Plug-and-play• Move desktop PCs between offices• Renumber• Other restructuring - change subnet

masks• Mobile IP - laptops• Moving equipment - cartable


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What DHCP doesn’t do• Support multiple addresses per interface• Inform running host that parameters have

changed• Propagate new addresses to DNS• Support inter-server communication• Provide authenticated message delivery• Configure routers and other network equipment• Design network addressing plan• Determine other configuration parameters• Locate other servers


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DHCP: Messages Overview

• Several messages are sent back and forth between a client and the DHCP server before it can successfully obtain an IP address


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DHCP: DISCOVER

• Hardcoding the addresses of DHCP servers kind of defeats the purpose of automatic configuration

• Solution: A client using DHCP will broadcast a DISCOVER message to all computers on its subnet (addr 255.255.255.255) to figure out the IP address of any DHCP servers

• Most routers are configured to pass this request within the campus or enterprise


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DHCP: OFFER

• (Optionally) sent from server in response to a DISCOVER

• Contains an IP address, other configuration information as well (subnet mask, DNS servers, default gateway, search domains, etc)

• Note that all DHCP servers that receive a DISCOVER request may send an OFFER; – since a client typically does not need more than one IP

address, more messages needed to zero on one IP


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DHCP: REQUEST

• Sent by client to request a certain IP address– Usually the one sent by an OFFER, but also used to renew

leases. Also can be sent to try to get same address after a reboot

• This message is broadcast• Most OSs by default will send a REQUEST for

the first OFFER they receive – this means that if there is a rogue DHCP server on your subnet, most clients will ignore the OFFERs from the campus DHCP servers (since the OFFER from the rogue server gets to the user’s PC first)!


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DHCP: ACK/NACK

• Sent by server in response to a REQUEST

• ACK: Request accepted, client can start using the IP it REQUESTed

• NACK: Something is wrong with the client’s REQUEST (for example they requested an IP address they’re not supposed to have)


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DHCP: RELEASE

• Sent by client to end a lease

• Not strictly required, but is the “polite” thing to do if done with the IP (could just let the lease expire)

• Some clients may not send RELEASEs in an attempt to keep the same IP address for as long as possible


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DHCP: Big Picture


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Summary• DHCP works today as a tool for

automatic configuration of TCP/IP hosts• It is an open Internet standard and

interoperable client implementations are widely available

• Provides automation for routine configuration tasks, once network architect has configured network and addressing plan

• Ongoing work will extend DHCP with authentication, DHCP-DNS interaction and inter-server communication


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DHCP: Conclusion

• Efficient way of assigning computers IP addresses and delivering configuration information to those computers

• Does not solve other problem of addressing – that IP addresses are hard to remember


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IETF standards

• Formal process for development, review and acceptance of TCP/IP protocol suite standards

• Initial specifications published as Internet Drafts (I-Ds)

• Accepted specifications published as Request for Comments (RFCs)


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Protocol status

• DHCP has been accepted as a Draft Standard; the specifications are published in:– RFC 2131: Dynamic Host Configuration Protocol– RFC 2132: DHCP Options and BOOTP Vendor

Extensions

• Several additional options are in development


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Implementation status

• DHCP is an open standard, with freely available specifications

• Can be (and has been) implemented entirely from the specification

• Commercial implementations are widely available

• Non-commerical implementations are also available


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DHCP Resources• Compilation of DHCP-related WWW links

and other information: http://www.dhcp.org

• DHCP FAQ (maintained by John Wobus)• [email protected] mailing list (admin

requests to [email protected])• IETF information can be retrieved from:

http://www.ietf.cnri.reston.va.us

• I-Ds and RFCs can also be retrieved from:http://www.rfc-editor.org


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DNS – Domain Name System


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Second Question





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Domain Name System (DNS)

• The first IP networks distributed host files on a regular basis– This became a burden and an automated distributed

solution was needed

• IP Addresses are great for computers– IP address includes information used for routing.

• IP addresses are tough for humans to remember.

• IP addresses are impossible to guess.– ever guessed at the name of a WWW site?


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Hostnames

• DNS is born• Domain names comprise a hierarchy so

that names are unique, yet easy to remember.

• The domain name system is usually used to translate a host name into an IP address .– Ex: www.xxx.org <-> IP 212.87.7.181

• (which one would you rather remember?)


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DNS: Basics

• Hierarchical namespace

• Distributed system – very few core servers

• Stores other information than simple hostname <-> IP mappings

• Request/response protocol


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DNS Hierarchy

edu com org in

rpi albany ac

iimcal


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Host name structure• Each host name is made up of a

sequence of labels separated by periods.– Each label can be up to 63 characters– The total name can be at most 255 characters.

• Examples:– whitehouse.gov– barney.the.purple.dinosaur.com– monica.cs.rpi.edu


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Domain Name

• The domain name for a host is the sequence of labels that lead from the host (leaf node in the naming tree) to the top of the worldwide naming tree.

• A domain is a subtree of the worldwide naming tree.


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Hierarchical Naming

com edu gov mil net org us

arpa

colordao

cs

foobar

foobar.cs.colorado.edu


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Top level domains

• edu, gov, com, net, org, mil, …• Countries each have a top level domain

(2 letter domain name).– Such as .in, .uk, .it, etc.

• New top level domains include:.aero .biz .coop .info .name .pro


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Domain Naming System

edu com

harvard … mit

hbs ee

www

physics

cisco … yahoonasa … nsf arpa … navy acm … ieee

gov mil org net uk fr


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DNS: Architecture

• DNS servers are responsible for one or more domains of any level

• “Root servers” are maintained throughout the world (one is in Palo Alto) and are responsible for all of the top-level domains– When you register a domain, an entry for that domain is

added to the appropriate root server

• Owners of each regular domain or subdomain maintain (or outsource) their own DNS servers containing the correct information


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Name Servers• Partition hierarchy into zones

edu com

princeton … mit

cs ee

ux01 ux04

physics

cisco … yahoo nasa … nsf arpa … navy acm … ieee

gov mil org net uk fr

Rootname server

Princetonname server

Cisconame server

CSname server

EEname server

…

…

Each zone implements two or more name servers

1. Primary2. secondary


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Hierarchical Administration - “Zones”

arpa com in gov mil net org us

arpa

ac

iimcal

www

www.iimcal.ac.in


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Administration - Zones

• A zone is a subtree of the DNS tree that is independently managed

– Second-level domains (“ac.in”) are usually an independent zone

– Most sub-domains (“iimcal.ac.in”) are also independent.

• A zone must provide multiple name servers. This server records the members in the domain.

– You typically need a primary name server and one or more secondary name servers.

– Secondary retrieves information from primary using a zone transfer.


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Resolving an address• A.C.D wants to know about F.E.D

A

C

D

E

GB F

H

• Host “A.C.D” asks “B.C.D” (the local name server) to resolve “F.E.D”

Name servers


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Resolving an address

• B.C.D doesn’t know the answer. • It wants to ask the primary domain server for the

“E.D” domain, so it asks the parent of the “B.C” domain (“D” in this example) to resolve “E.D”.

• D asks H, the root server. • H doesn’t know the answer, but it’s the top-level

domain and knows that “G.E.D” is the primary domain server for the “E.D” domain

• B.C.D now knows the primary domain server for the E.D domain, and can now ask “G.E.D” about “F.E.D”


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Domain servers

• What kind of records can be requested for a given domain?– Address translation– Caching information– Mail server information– Authoritative nameserver information

• How is this data requested?– Each record has a type and certain data associated with it

– clients request records of a certain type from a server


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DNS Organization• Distributed Database

– The organization that owns a domain name is responsible for running a DNS server that can provide the mapping between hostnames within the domain to IP addresses.

– So - some machine run by RPI is responsible for everything within the rpi.edu domain.


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rpi.eduDNS DB

rpi.eduDNS DB

DNS Distributed Database

• There is one primary server for a domain, and typically a number of secondary servers containing replicated databases.

rpi.eduDNS DB

Authoritative

rpi.eduDNS DB

Replicas

rpi.edu DNS server


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DNS Clients• A DNS client is called a resolver.

• A call to gethostbyname()is handled by a resolver (typically part of the client).

• Most Unix workstations have the file /etc/resolv.conf that contains the local domain and the addresses of DNS servers for that domain.


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/etc/resolv.conf

domain rpi.edu

128.113.1.5

128.113.1.3


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nslookup

• nslookup is an interactive resolver that allows the user to communicate directly with a DNS server.

• nslookup is usually available on Unix workstations. (dig and host are also DNS clients).


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DNS Servers

• Servers handle requests for their domain directly.

• Servers handle requests for other domains by contacting remote DNS server(s).

• Servers cache external mappings.


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Server - Server Communication• If a server is asked to provide the

mapping for a host outside it’s domain (and the mapping is not in the server cache):– The server finds a nameserver for the target domain.– The server asks the nameserver to provide the host

name to IP translation.

• To find the right nameserver, use DNS!


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DNS Data

• DNS databases contain more than just hostname-to-address records:– Name server records NS– Hostname aliases CNAME– Mail Exchangers MX– Host Information HINFO


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The Root DNS Server

• The root server needs to know the address of 1st (and many 2nd) level domain nameservers.

edu com org jp

albanyrpi


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Server Operation

• If a server has no clue about where to find the address for a hostname, ask the root server.

• The root server will tell you what nameserver to contact.

• A request may get forwarded a few times.


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DNS Message Format

HEADER

QUERIES

Response RESOURCE RECORDS

Response AUTHORITY RECORDS

Response ADDITIONAL INFORMATION

HEADER

QUERIES

Response RESOURCE RECORDS

Response AUTHORITY RECORDS

Response ADDITIONAL INFORMATION


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DNS Message Header

• query identifier• flags• # of questions• # of RRs• # of authority RRs• # of additional RRs

16 b

it f

ield

s

Response}


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Message Flags

• QR: Query=0, Response=1• AA: Authoritative Answer• TC: response truncated (> 512 bytes)• RD: recursion desired• RA: recursion available• rcode: return code


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Recursion

• A request can indicate that recursion is desired - this tells the server to find out the answer (possibly by contacting other servers).

• If recursion is not requested - the response may be a list of other name servers to contact.


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Question Format

• Name: domain name (or IP address)

• Query type (A, NS, MX, …)

• Query class (1 for IP)


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Response Resource Record

• Domain Name• Response type • Class (IP)• Time to live (in seconds) • Length of resource data • Resource data


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UDP & TCP

• Both UDP and TCP are used:– TCP for transfers of entire database to secondary

servers (replication).– UDP for lookups– If more than 512 bytes in response - requestor

resubmits request using TCP.


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Lots more

• This is not a complete description ! • If interested - look at:

– RFC 1034: DNS concepts and facilities.– RFC 1035: DNS implementation and protocol

specification.– play with nslookup.– Look at code for BIND (DNS server code).


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References

• RFC 1034• RFC 1035


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SNMP - Simple Network Management Protocol


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Third Question





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SNMP is …

• A network management tool– Designed to allow data acquisition from SNMP

capable network equipment– Allows control of SNMP capable equipment by the

distant controller

• Based on inquiry/response architecture with simple response parameters


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• Used for management of fault conditions – prevention of and response to network failure

• Designed to manage performance – coping with deterioration of performance due to congestion, changing traffic conditions

SNMP is …


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SNMP Message Protocol• Messages use UDP, ports 161

(requests/responses) and 162 (notifications)• Message types:

– GetRequest: request values of variables from device– GetNextRequest: request value of variable following

the one supplied– GetResponse: return values– SetRequest: instruct device to set values of variables– Trap: from device - notify monitor / manager of value

change


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To know more on SNMP

• “Essential SNMP”, Douglas R Mauro & Kevin J Schmidt, Shroff Publishers & Distributors, Calcutta, 2001

lecture on dhcp, dns & snmpdebashis saha thursday, may 17, 20071 dhcp, dns, snmp debashis saha...

Documents

purpose of dhcp

dns snmpdebashis saha

ip address dns

dhcp functional goals

ip addresses

dns information

valid ip address

configuration information