11-Deploying And Configuring The Dhcp Service

Front 1

3 components of DHCP

Back 1

Server application - responds to client requests for TCP/IP config settingsClient -- Issues requests and applies received TCP/IP config settingsCommunications protocol - Defines formats and sequences of the messages exchanged by DHCP clients and servers

Front 2

IETF

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Internet Engineering Task Force

Front 3

Which IETF RFC describes DHCP

Back 3

RFC 2131

Front 4

Which IETF RFC describes DHCP options and bootp vendor extensions?

Back 4

RFC 2132

Front 5

3 types of DHCP allocation methods

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Dynamic allocationAutomatic allocationManual allocation

Front 6

DHCP dynamic allocation method

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Server assigns client IP lease from a scope of IPs for a period of time. Client must renew lease. If not renewed IP goes back into the scope for reassignment

Front 7

DHCP automatic allocation method

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Dynamic allocation with an indefinite lease. Reduces network traffic by eliminating lease renewals but does not allow for much change without reconfiguring the PC manually

Front 8

DHCP manual allocation method

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Server assigns IP permanently to client. Called a reservation.

Front 9

BOOTP

Back 9

Bootstrap Protocol. Allows a diskless client to boot from a network resource

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DHCP packetSize & description: op field

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AKA Op code1 byteSpecifies whether the message is a request or a reply

Front 11

DHCP packetSize & description: htype

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AKA hardware type1 bytesSpecifies the type of hardware address used in the chaddr field

Front 12

DHCP packetSize & description: hlen

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AKA hardware address length1 byteSpecifies the length of the hardware address in the chaddr field in bytes

Front 13

DHCP packetSize & description: hops

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1 byteSpecifies the number of network segments between client /server. CLient sets to 0. Each DHCP relay increments by 1

Front 14

DHCP packetSize & description: xid

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Transaction ID4 bytesContains transaction ID used to associate the request and response messages of a single DHCP transaction

Front 15

DHCP packetSize & description: secs

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Seconds2 bytesSpecifies the number of secs elaps since ip was assigned or lease last renewed

Front 16

DHCP packetSize & description: flags

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2 bytesContains broadcast flag as first bit. When set to 1 specifies DHCP servers and relay agents should use broadcasts not unicasts

Front 17

DHCP packetSize & description: ciaddr

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Client IP address4 bytesSpecifies the clients IP address in DHCPREQUEST messages transmitted while in the bound renewal or rebinding state

Front 18

DHCP packetSize & description: yiaddr

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Your IP address4 bytesSpecifies the IP address being offered or assigned by the server in DHCPOFFER or DHCPACK messages

Front 19

DHCP packetSize & description: siaddr

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Server IP address4 bytesSpecifies IP addr of the next server in bootstrap sequence. Only included when DHCP is configured to supply executable boot file to clients & files located on a different server

Front 20

DHCP packetSize & description: giaddr

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Gateway IP Address4 bytesSpecifies the ip addr of the DHCP replay agent the server should send its replies when a client and server are located at different subnets

Front 21

DHCP packetSize & description: chaddr

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Client Hardware Address16 bytesSpecifies the hardware address of the client system in CHCPDISCOVER and DCHPREQUEST messages which the server uses to address its unicast responses to the client

Front 22

DHCP packetSize & description: sname

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Server Host Name64 bytesSpecifies the optional host name of the DHCP server. More commonly used to hold overflow data from the options field

Front 23

DHCP packetSize & description: file

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Boot file name128 bytesSpecifies the name of an executable boot file for diskless client workstation. More commonly used to hold overflow data from the option field

Front 24

DHCP packetSize & description: options

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Variable size. Min of 312 bytesContains magic cookie that specifies how the rest of the field should be interpreted and the DHCP Message Type option that defines the function of the message.

Front 25

8 different message types

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DHCPDISCOVERDHCPOFFERDHCPREQUESTDHCPDECLINEDHCPACKDHCPNAKDHCPRELEASEDHCPINFORM

Front 26

The options field of a DHCP packet always begins with _____

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The magic cookie

Front 27

Magic cookie

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Portion of the DHCP options field that informs the server about what is contained in the rest of the field. 4 byte subfield.

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DHCPDISCOVER

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Used by clients requesting config params from a DHCP server

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DHCPOFFER

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Used by server to offer IP addresses to requesting clients

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DHCPREQUEST

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Used by clients to accept or renew an ip address assignment

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DHCPDECLINE

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Used by clients to reject an offered IP address

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DHCPACK

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Used by server to acknowledge a client's acceptance of an offered IP address

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DHCPNAK

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Used by servers to reject a client's acceptance of an offered IP address

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DHCPRELEASE

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Used by clients to terminate an IP address lease

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DHCPINFORM

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Used by clients to obtain additional TCP/IP config params

Front 36

Which transport layer protocol is used to carry DHCP packets?

Back 36

UDP

Front 37

What BOOTP vendor info extensions are included in the DHCP option field and used by most clients?

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Subnet maskRouter / default routedomain name serverhost namedomain name

Front 38

What DHCP message does a client send first when seeking an IP address?

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DHCPDISCOVER

Front 39

When a DHCP server received DHCPDISCOVER and has an ip available what does it send?

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DHCPOFFER

Front 40

If a client received a DHCPOFFER and wants the address what does it send?

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DHCPREQUEST

Front 41

If a client sends a server a DHCPREQUEST but the offered IP is no longer available what does the server send back?

Back 41

DHCPNAK

Front 42

If a client sends a server a DHCPREQUEST and the offered IP is still available what does the server send back?

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DHCPACK

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init state

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A client with no ip looking for the DHCP server (having sent DHCPDISCOVER messages)

Front 44

Why might A client may receive DHCPOFFERs from more than on DHCP server?

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Client sent a DHCPDISCOVER message as a broadcast. If there's more than one DHCP server both may respond.

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If a client receives DHCPACK for an IP what does it then do?

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Attempts an ARP test to confirm that the offered IP really isn't in use in its network segment.

Front 46

If an offered IP fails an ARP test what does the client send to the DHCP server?

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DHCPDECLINE

Front 47

If an offered IP passes the ARP test what happens?

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"Client is ""bound"". Transaction complete."

Front 48

What is the default ip lease time for a DHCP client in server 2012?

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8 days

Front 49

When is renewal time on a DHCP lease?

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At 50% of the way through the lease time.

Front 50

Renewal time AKA

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T1

Front 51

What state does a client enter at renewal time?

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Renewing State

Front 52

Besides at renewal time when do DHCP clients renew their lease?

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On reboot

Front 53

What happens if the DHCP server does not respond to a client's request to renew a lease via its DHCPREQUEST mesasgess?

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Client enters rebinding state and beings transmitting DHCPREQUEST messages as broadcasts attempting an IP assignment from any DHCP server on the network

Front 54

At what point does a client enter rebinding state?

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87.5% into lease duration (T2) after DHCP server failed to respond to T1 renewal requests.

Front 55

Distributed DHCP infrastructure

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Locating a DHCP server on each of your subnets -- one per broadcast domain

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Advantages of distributed DHCP infrastructure

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Clients have easy access to their DHCP serversDo not have to worry about DHCP traffic adding to the burden of your routers. All client/server traffic for DHCP will be local

Front 57

Centralized DHCP infrastructure

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DHCP servers are all placed in a single location. Request pass through subnets. Relay agents are required on each individual broadcast domain

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DHCP relay agent

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Receives DHCP broadcast traffic on a subnet / broadcast domain and sends it to a DHCP server on at least one other subnet. DHCP servers send messages back to the agent which processes and sends them to the client.

Front 59

What devices often have DHCP relay agents built in?

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Routers

Front 60

On what and how does centralizing a DHCP server create a burden?

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Creates a burden on the DHCP server and on the subnet the central server is located b/c of increased traffic

Front 61

PXE

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PreBoot Execution EnvironmentAllows a DHCP server to specify a boot file for a client to download and boot from.

Front 62

What does PXE use to transfer the boot files specified by the DHCP server?

Back 62

TFTP - Trivial File Transfer Protocol