Lecture 2:Servers and ServicesNetwork Design & Administration

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Servers vs. Clients• Microsoft XP, Vista, Win 7 and Server 2008/R2 are similar

architecturally• All based on the NT kernel architecture• Concept of Kernel and HAL common to most resent OS’s from

Microsoft:• Kernel

• Acts a bridge between user applications and hardware• Manages computer resources

• E.g. process, memory & device management

• HAL (Hardware Abstraction Layer)• Abstract layer between hardware and operating system• Hardware specific code

• Linux uses similar concepts but slightly different names (kernel and board support package)

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XP/Server 2003 Architecture Diagram[1]

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Vista/7 Architecture Diagram[1]

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Gnu/Linux [2]

Board Support Package (BSP)

Plugins to support different file systems

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Different Server Roles

• File server• Centralised storage• Better control/backup/recovery

• Print server• Spool jobs• Track usage

• Application server• Web services e.g. Internet Information Services (IIS)• Databases e.g. SQL Server• Accounting packages• etc…

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Different Server Roles

• Mail server• Terminal Services server• Remote Access / VPN server• Hyper-V server• Domain controller

• Vital within an Active Directory Domain!

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Different Server Roles

• DNS Server• Resolves names

• DHCP server• Allocated IP address leases

• Streaming media server• WINS server

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Dynamic Host Configuration Protocol• How does your network know what the IP

addresses are?• You could set up static IP address

• Question: Why would this not be a good way to assign IP addresses?

• Better if something (apart from you) could handle this and allocate IP addresses on request

• This is what DHCP does (and has done since 1993)

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DHCP

The DHCP process works as follows:1. Client machine boots and broadcasts a DHCP Discovery

query, searching for the server.2. DHCP Server reserves an IP address for the client and sends

this as a DHCP Offer.3. Client takes up offer and must tell any other DHCP Servers of

this offer via a broadcast DHCP Request including the IP address of the server making the offer.

4. Other DHCP Servers cancel their offers to the same client and return the IP address to the pool, while the accepted server completes the process by sending a DHCP Ack message to the client, with addition info e.g. lease expiry time.

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DHCP• Note that the Discovery and

Request must be broadcast in order to inform any other DHCP Servers of the state of the transaction.

• The client does not know the addresses of any DHCP Server, so it has to broadcast on the physical subnet to find them.

• (Network administrators may allow DHCP traffic to be forwarded by a router onto another subnet) 11

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DHCP• In large networks it is usual to have more than one DHCP Server.

• Question: Why do you think more than one DHCP server should be present within a network?

• Therefore, need to decide how address ranges are to be configured

• Overlapping address ranges allows all address available even if one server goes down.• Question: What problem does that introduce?

• Could get a conflict

• Solutions:• DHCP Server Conflict Resolution• Use non-overlapping ranges • Client starts Address Resolution Probe (ARP) to validate address 12

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DHCP

• Address allocation has a few variants[2] –• Dynamic: the address is allocated for a lease period from

a pool and then re-used after the lease has expired.• Question:

• What lease would be appropriate• In the office?• On the Dell production line where they are checking PC’s

before shipping?

• Automatic: the address is assigned permanently to a client and the client is preferentially given the same address next time it asks.

• Static: a list of MAC/IP address pairs is used to assign to the client.

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DHCP

• Microsoft seem to have slightly different definitions of these terms[4]:• Static – certain machines (e.g. DHCP, DNS, WINS Servers,

Print Server, Firewall, Router) have defined addresses which are also excluded from a dynamic range (also called permanent lease)

• Client Reservation (as above, but for ordinary clients)• Automatic (Automatic Private IP Addressing, APIPA) – if

DHCP Server unavailable, client can configure itself in the 169.254.0.1 to 169.254.255.254 range and talk to other clients in the same range. Client polls regularly (but not frequently) for a DHCP Server to return, to get back to normal.

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Why use a Server?

• Why not use a desktop machine running Windows 7?• Hardware reasons:

Server Hardware

Engineered to higher standard

Extensible

Processors

High performance

I/O

High Availability

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Next Time & References

• More on Servers and services

[1] “Modern Operating Systems”, Andrew Tanenbaum, 2008[2] http://www.ibm.com/developerworks/linux/library/l-linux-kernel/[3] Wikipedia, man pages for dhcpd[4] "Windows Server 2008 - TCP/IP Fundamentals for Microsoft Windows", eBook available at: http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=8781

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