DHCP Server Interview Questions & Answers

1. What is DHCP?

DHCP stands for "Dynamic Host Configuration Protocol".

2.What is DHCP's purpose?

DHCP's purpose is to enable individual computers on an IP network to extract

their configurations from a server (the 'DHCP server') or servers, in particular,

servers that have no exact information about the individual computers until they

request the information. The overall purpose of this is to reduce the work

necessary to administer a large IP network. The most significant piece of

information distributed in this manner is the IP address.

3. Can DHCP work with AppleTalk or IPX?

No, it is too tied to IP. Furthermore, they don't need it since they have always

had automated mechanisms for assigning their own network addresses.

4. Who Created It? How Was It Created?

DHCP was created by the Dynamic Host Configuration Working Group of the

Internet Engineering Task Force (IETF; a volunteer organization which defines

protocols for use on the Internet). As such, it's definition is recorded in an

Internet RFC and the Internet Activities Board (IAB) is asserting its status as to

Internet Standardization. As of this writing (June 1998), DHCP is an Internet

Draft Standard Protocol and is Elective. BOOTP is an Internet Draft Standard

Protocol and is recommended. For more information on Internet standardization,

see RFC2300 (May 1998)

5. How is it different than BOOTP or RARP?

DHCP is based on BOOTP and maintains some backward compatibility. The main

difference is that BOOTP was designed for manual pre-configuration of the host

information in a server database, while DHCP allows for dynamic allocation of

network addresses and configurations to newly attached hosts. Additionally,

DHCP allows for recovery and reallocation of network addresses through a

leasing mechanism.

RARP is a protocol used by Sun and other vendors that allows a computer to find

out its own IP number, which is one of the protocol parameters typically passed

to the client system by DHCP or BOOTP. RARP doesn't support other parameters

and using it, a server can only serve a single LAN. DHCP and BOOTP are

designed so they can be routed.

6.How is it different than VLANs?

DHCP and VLANs, which are very different in concept, are sometimes cited as

different solutions to the same problem. While they have a goal in common

(easing moves of networked computers), VLANs represent a more revolutionary

change to a LAN than DHCP. A DHCP server and forwarding agents can allow you

to set things up so that you can unplug a client computer from one network or

subnet and plug it into another and have it come alive immediately, it having

been reconfigured automatically. In conjunction to Dynamic DNS, it could

automatically be given its same name in its new place. VLAN-capable LAN

equipment with dynamic VLAN assignment allows you to configure things so a

client computer can be plugged into any port and have the same IP number (as

well as name) and be on the same subnet. The VLAN-capable network either has

its own configuration that lists which MAC addresses are to belong to each VLAN,

or it makes the determination from the source IP address of the IP packets that

the client computer sends. Some differences in the two approaches:

• DHCP handles changes by reconfiguring the client while a VLAN-capable

network handles it by reconfiguring the network port the client is moved to.

• DHCP dynamic reconfiguration requires a DHCP server, forwarding agent

in each router, and DHCP capability in each client's TCP/IP support. The

analogous capability in VLANs requires that all hubs throughout the

network be VLAN-capable, supporting the same VLAN scheme. To this

point VLAN support is proprietary with no vendor interoperability, but

standards are being developed.

• DHCP can configure a new client computer for you while a VLAN-capable

network can't.

• DHCP is generally aimed at giving "easy moves" capability to networks

that are divided into subnets on a geographical basis, or on separate

networks. VLANs are generally aimed at allowing you to set up subnets

on some basis other than geographical, e.g. instead of putting everyone

in one office on the same subnet, putting each person on a subnet that

has access to the servers that that person requires.

There is an issue with trying to use DHCP (or BOOTP) and VLANs at the same

time, in particular, with the scheme by which the VLAN-capable network

determines the client's VLAN based upon the client computer's source IP

address. Doing so assumes the client computer is already configured, which

precludes the use of network to get the configuration information from a DHCP

or BOOTP server.

7. What protocol and port does DHCP use?

DHCP, like BOOTP runs over UDP, utilizing ports 67 and 68.

8. What is an IP address?

An IP address (also called an IP number) is a number (typically written as four

numbers separated by periods, i.e. 107.4.1.3 or 84.2.1.111) which uniquely

identifies a computer that is making use of the Internet. It is analogous to your

telephone number in that the telephone number is used by the telephone

network to direct calls to you. The IP address is used by the Internet to direct

data to your computer, e.g. the data your web browser retrieves and displays

when you surf the net. One task of DHCP is to assist in the problem of getting a

functional and unique IP number into the hands of the computers that make use

of the Internet.

9. What is a MAC address?

A MAC address (also called an Ethernet address or an IEEE MAC address) is a

number (typically written as twelve hexadecimal digits, 0 through 9 and A

through F, or as six hexadecimal numbers separated by periods or colons, i.e.

0080002012ef, 0:80:0:2:20:ef) which uniquely identifes a computer that has an

Ethernet interface. Unlike the IP number, it includes no indication of where your

computer is located. In DHCP's typical use, the server uses a requesting

computer's MAC address to uniquely identify it.

10. What is a DHCP lease?

A DHCP lease is the amount of time that the DHCP server grants to the DHCP

client permission to use a particular IP address. A typical server allows its

administrator to set the lease time.

11. What is a Client ID?

What is termed the Client ID for the purposes of the DHCP protocol is whatever

is used by the protocol to identify the client computer. By default, DHCP

implementations typically employ the client's MAC address for this purpose, but

the DHCP protocol allows other options. Some DHCP implementations have a

setup option to specify the client ID you want. One alternative to the MAC

address is simply a character string of your choice. In any case, in order for

DHCP to function, you must be certain that no other client is using the client ID

you choose, and you must be sure the DHCP server will accept it.

12.Can DHCP support statically defined addresses?

Yes. At least there is nothing in the protocol to preclude this and one expects it

to be a feature of any DHCP server. This is really a server matter and the client

should work either way. The RFC refers to this as manual allocation.

13. How does DHCP and BOOTP handle multiple subnets?

For the situations where there is more than one LAN, each with its own subnet

number, there are two ways. First of all, you can set up a seperate server on

each subnet. Secondly, a feature of some routers known as "BOOTP forwarding"

to forward DHCP or BOOTP requests to a server on another subnet and to

forward the replies back to the client. The part of such a router (or server acting

as a router) that does this is called a "BOOTP forwarding agent". Typically you

have to enable it on the interface to the subnet to be served and have to

configure it with the IP address of the DHCP or BOOTP server. On a Cisco router,

the address is known as the "UDP Helper Address".

14. Can a BOOTP client boot from a DHCP server?

Only if the DHCP server is specifically written to also handle BOOTP queries.

15. Can a DHCP client boot from a BOOTP server?

Only if the DHCP client were specifically written to make use of the answer from

a BOOTP server. It would presumably treat a BOOTP reply as an unending lease

on the IP address.

In particular, the TCP/IP stack included with Windows 95 does not have this

capability.

16. Is a DHCP server "supposed to" be able to support a BOOTP client?

The RFC on such interoperability (1534) is clear: "In summary, a DHCP server:

... MAY support BOOTP clients," (section 2). The word "MAY" indicates such

support, however useful, is left as an option.

A source of confusion on this point is the following statement in section 1.5 of

RFC 1541: "DHCP must provide service to existing BOOTP clients." However, this

statement is one in a list of "general design goals for DHCP", i.e. what the

designers of the DHCP protocol set as their own goals. It is not in a list of

requirements for DHCP servers.

17. Is a DHCP client "supposed to" be able to use a BOOTP server?

The RFC on such interoperability (1534) is clear: "A DHCP client MAY use a reply

from a BOOTP server if the configuration returned from the BOOTP server is

acceptable to the DHCP client." (section 3). The word "MAY" indicates such

support, however useful, is left as an option.

18. Can a DHCP client or server make a DNS server update the client's DNS entry to match the client's dynamically assigned address?

RFCs 2136 and 2137 indicate a way in which DNS entries can be updated

dynamically. Using this requires a DNS server that supports this feature and a

DHCP server that makes use of it. The RFCs are very recent (as of 5/97) and

implementations are few. In the mean time, there are DNS and DHCP servers

that accomplish this through proprietary means.

19. Can a DHCP server back up another DHCP server?

You can have two or more servers handing out leases for different addresses. If

each has a dynamic pool accessible to the same clients, then even if one server

is down, one of those clients can lease an address from the other server.

However, without communication between the two servers to share their

information on current leases, when one server is down, any client with a lease

from it will not be able to renew their lease with the other server. Such

communication is the purpose of the "server to server protocol" (see next

question). It is possible that some server vendors have addressed this issue with

their own proprietary server-to-server communication.

20. When will the server to server protocol be defined?

The DHC WG of the IETF is actively investigating the issues in inter-server

communication. The protocol should be defined "soon".

21.Where is DHCP defined?

In Internet RFCs.

22. Can DHCP support remote access?

PPP has its own non-DHCP way in which communications servers can hand

clients an IP address called IPCP (IP Control Protocol) but doesn't have the same

flexibility as DHCP or BOOTP in handing out other parameters. Such a

communications server may support the use of DHCP to acquire the IP addresses

it gives out. This is sometimes called doing DHCP by proxy for the client. I know

that Windows NT's remote access support does this.

A feature of DHCP under development (DHCPinform) is a method by which a

DHCP server can supply parameters to a client that already has an IP number.

With this, a PPP client could get its IP number using IPCP, then get the rest of its

parameters using this feature of DHCP.

SLIP has no standard way in which a server can hand a client an IP address, but

many communications servers support non-standard ways of doing this that can

be utilized by scripts, etc. Thus, like communications servers supporting PPP,

such communications servers could also support the use of DHCP to acquire the

IP addressees to give out.

The DHCP protocol is capable of allocating an IP address to a device without an

IEEE-style MAC address, such as a computer attached through SLIP or PPP, but

to do so, it makes use of a feature which may or may not be supported by the

DHCP server: the ability of the server to use something other than the MAC

address to identify the client. Communications servers that acquire IP numbers

for their clients via DHCP run into the same roadblock in that they have just one

MAC address, but need to acquire more than one IP address. One way such a

communications server can get around this problem is through the use of a set

of unique pseudo-MAC addresses for the purposes of its communications with

the DHCP server. Another way (used by Shiva) is to use a different "client ID

type" for your hardware address. Client ID type 1 means you're using MAC

addresses. However, client ID type 0 means an ASCII string.

23.How can I relay DHCP if my router does not support it?

A server on a net(subnet) can relay DHCP or BOOTP for that net. Microsoft has

software to make Windows NT do this.

24.What is DHCP Spoofing?

Ascend Pipeline ISDN routers (which attach Ethernets to ISDN lines) incorporate

a feature that Ascend calls "DHCP spoofing" which is essentially a tiny server

implementation that hands an IP address to a connecting Windows 95 computer,

with the intention of giving it an IP number during its connection process.

25. How long should a lease be?

A very relevant factor is that the client starts trying to renew the lease when it is

halfway through: thus, for example, with a 4 day lease, the client which has lost

access to its DHCP server has 2 days from when it first tries to renew the lease

until the lease expires and the client must stop using the network. During a 2-

day outage, new users cannot get new leases, but no lease will expire for any

computer turned on at the time that the outage commences.

Another factor is that the longer the lease the longer time it takes for client

configuration changes controlled by DHCP to propogate.

25. How can I control which clients get leases from my server?

There is no ideal answer: you have to give something up or do some extra work.

• You can put all your clients on a subnet of your own along with your own

DHCP server.

• You can use manual allocation.
• Perhaps you can find DHCP server software that allows you to list which

MAC addresses the server will accept. DHCP servers that support roaming

machines may be adapted to such use.

• You can use the user class option assuming your clients and server

support it: it will require you to configure each of your clients with a user

class name. You still depend upon the other clients to respect your

wishes.

26. How can I prevent unauthorized laptops from using a network that uses DHCP for dynamic addressing?

This would have to be done using a mechanism other than DHCP. DHCP does not

prevent other clients from using the addresses it is set to hand out nor can it

distinguish between a computer's permanent MAC address and one set by the

computer's user. DHCP can impose no restrictions on what IP address can use a

particular port nor control the IP address used by any client.

27. What features or restrictions can a DHCP server have?

While the DHCP server protocol is designed to support dynamic management of

IP addresses, there is nothing to stop someone from implementing a server that

uses the DHCP protocol, but does not provide that kind of support. In particular,

the maintainer of a BOOTP server-implementation might find it helpful to

enhance their BOOTP server to allow DHCP clients that cannot speak "BOOTP" to

retrieve statically defined addresses via DHCP. The following terminology has

become common to describe three kinds of IP address allocation/management.

These are independent "features": a particular server can offer or not offer any

of them:

• Manual allocation: the server's administrator creates a configuration for

the server that includes the MAC address and IP address of each DHCP

client that will be able to get an address: functionally equivalent to

BOOTP though the protocol is incompatible.

• Automatic allocation: the server's administrator creates a configuration

for the server that includes only IP addresses, which it gives out to

clients. An IP address, once associated with a MAC address, is

permanently associated with it until the server's administrator intervenes.

• Dynamic allocation: like automatic allocation except that the server will

track leases and give IP addresses whose lease has expired to other

DHCP clients.

Other features which a DHCP server may or may not have:

• Support for BOOTP clients.
• Support for the broadcast bit.
• Administrator-settable lease times.
• Administrator-settable lease times on manually allocated addresses.
• Ability to limit what MAC addresses will be served with dynamic

addresses.

• Allows administrator to configure additional DHCP option-types.
• Interaction with a DNS server. Note that there are a number of

interactions that one might support and that a standard set & method is

in the works.

• Interaction with some other type of name server, e.g. NIS.
• Allows manual allocation of two or more alternative IP numbers to a

single MAC address, whose use depends upon the gateway address

through which the request is relayed.

• Ability to define the pool/pools of addresses that can be allocated

dynamically. This is pretty obvious, though someone might have a server

that forces the pool to be a whole subnet or network. Ideally, the server

does not force such a pool to consist of contiguous IP addresses.

• Ability to associate two or more dynamic address pools on separate IP

networks (or subnets) with a single gateway address. This is the basic

support for "secondary nets", e.g. a router that is acting as a BOOTP

relay for an interface which has addresses for more than one IP network

or subnet.

• Ability to configure groups of clients based upon client-supplied user

and/or vendor class. Note: this is a feature that might be used to assign

different client-groups on the same physical LAN to different logical

subnets.

• Administrator-settable T1/T2 lengths.
• Interaction with another DHCP server. Note that there are a number of

interactions that one might support and that a standard set & method is

in the works.

• Use of PING (ICMP Echo Request) to check an address prior to

dynamically allocating it.

• Server grace period on lease times.
• Ability to force client(s) to get a new address rather than renew.

DHCP Discovery:

The client broadcasts on the local physical subnet to find available servers. Network

administrators can configure a local router to forward DHCP packets to a DHCP server on a

different subnet. This client-implementation creates a UDP packet with the broadcast

destination of 255.255.255.255 or subnet broadcast address and also requests its last-known

IP address (in the example below, 192.168.1.100) although the server may ignore this optional

parameter....

DHCP Offers:

When a DHCP server receives an IP lease request from a client, it extends an IP lease offer.

This is done by reserving an IP address for the client and broadcasting a DHCPOFFER message

across the network. This message contains the client's MAC address, followed by the IP

address that the server is offering, the subnet mask, the lease duration, and the IP address of

the DHCP server making the offer.

The server determines the configuration, based on the client's hardware address as specified in

the CHADDR field. Here the server, 192.168.1.1, specifies the IP address in the YIADDR field.

DHCP Requests:

Whenever a computer comes on line, it checks to see if it currently has an IP address leased. If

it does not, it requests a lease from a DHCP server. Because the client computer does not know

the address of a DHCP server, it uses 0.0.0.0 as its own IP address and 255.255.255.255 as

the destination address. Doing so allows the client to broadcast a DHCPDISCOVER message

across the network. Such a message consists of the client computer's Media Access Control

(MAC) address (the hardware address built into the network card) and its NetBIOS name.

The client selects a configuration out of the DHCP "Offer" packets it has received and

broadcasts it on the local subnet. Again, this client requests the 192.168.1.100 address that

the server specified. In case the client has received multiple offers it specifies the server from

which it has accepted the offer.

DHCP Acknowledgement:

When the DHCP server receives the DHCPREQUEST message from the client, it initiates the

final phase of the configuration process. This acknowledgement phase involves sending a

DHCPACK packet to the client. This packet includes the lease duration and any other

configuration information that the client might have requested. At this point, the TCP/IP

configuration process is complete.

The server acknowledges the request and sends the acknowledgement to the client. The

system as a whole expects the client to configure its network interface with the supplied

options.