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4 Using FreeNX to Run Linux from Solaris, Mac OS X, or Linux

4 Using FreeNX to Run Linux from Solaris, Mac OS X, or Linux

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NSS—Name Service Switch

A part of many Unix and related systems that defines how lookups for information

relating to the environment of the machine are made. By default, most lookups

for names such as user passwords, groups, hosts, and so on are done via files

such as /etc/passwd or /etc/hosts. The Name Service Switch allows lookups using

other databases to discover the same information, and defines the order in

which those databases are accessed. It is through configuration of this switch

that a Linux system can be used on a Windows domain, with the Winbind NSS

module providing users and groups from a Windows domain.

NTP—Network Time Protocol

A protocol designed to allow computers on a network to synchronize their

clocks, taking into account the variable latency on a packet switched network.

Using NTP, it’s possible for all computers on a network (like the Internet) to

have clocks synchronized to within hundredths of a second. This is required for

some network activities, such as Kerberos authentication, which in part relies

upon accurate timestamps.

Null modem cable

A cable that allows a PC to connect directly to another PC via serial ports. Similar to a normal modem cable (except where receive/transmit lines would go

straight through to transmit/receive pins on the modem), a Null modem cable

swaps the lines inside the cable, allowing the two PCs to communicate using the

same serial connection software and serial ports used to connect to a modem.

NVRAM—Non-Volatile Random Access Memory

Unlike the normal RAM inside a PC, NVRAM doesn’t lose its contents when

power is removed. Various forms of NVRAM generally come with disadvantages compared to normal RAM—it’s often slower, requires more power to read,

and many times more to write, and may wear out with the masses of writing that

normal RAM requires. Different forms of NVRAM are most often used to store

some settings within a device, where only occasional writes are required, but it

can also serve as a silent replacement for a small hard drive. Flash memory is the

most well-known form of NVRAM.


OID—Object IDentifier

Within the context of SNMP, a unique identifier referring to an object within a

Management Information Base (MIB) used to store information and settings

related to a network device. The OID is represented as a string of numbers separated by dots, and refers to an object’s position in the tree structure of the MIB.

For example, would be a sibling of, and both are children of

1.3.4. The object and the information it contains can be anything relevant to the

device’s operation, from the name of the device to the speed of fans, memory

usage, bandwidth usage, or the number of hamster wheels in use.

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OSPF—Open Shortest Path First

A link-state routing protocol, implemented by routers to dynamically adjust

routing to changing network conditions. An OSPF router multicasts information to other routers when changes have occurred around its network, as well as

routine updates every 30 minutes. From this information, each individual OSPF

router builds a link-state database that contains a representation of the entire

topology of the network in tree form, with the router itself at the root. When a

router needs to forward a packet, it can use its copy of the link-state database to

calculate the best path from the root (itself) to the destination on the tree, using

a path cost as its routing metric (as opposed to RIP’s hop count). In a practical

sense, path cost is mainly determined by link speed over a given route, so a

packet is forwarded toward the fastest of multiple routes. As a network grows

larger, routers will spend more time and bandwidth talking to each other, which

consumes valuable bandwidth just keeping the network together. OSPF

addresses this issue by allowing the division of a network into areas. Areas must

all be connected to a common backbone, and the routers inside each area only

need to contain the topology for that area, with border routers communicating

between different areas. (See also RIP.)


Packet filtering

Filtering by the attributes of a packet entering a device or network. Attributes

may include the source or destination address for the device, the port, connection type, elements of the data payload, or any other number of detectable

attributes of the packet.

Packet switching

A packet switched network breaks information to be transmitted into discrete

packets, each of which is sent over a shared network used by multiple machines

or users. Each individual packet contains information pertaining to its source

and destination, and does not require a dedicated path to reach its destination;

indeed, packets may travel between the same source and destination using different paths. Multiple users may transmit packets over the same connection at the

same time, independently of one another. (Contrast with Circuit switching.)

PAM—Pluggable Authentication Modules

A system whereby applications that require authentication can use many kinds

of authentication, all using the same API. An application only needs to know it is

using PAM, and the relevant modules provide one of many kinds of authentication, transparently.



Appendix B: Glossary of Networking Terms

PBX—Private Branch eXchange

A PBX was originally a private telephone exchange that handled a business’ own

internal telephone requirements, so that an entire building’s internal phone calls

wouldn’t need to use the costly public phone network. Now, a PBX is any system that handles in-house telephony, from manual exchanges to VOIP systems

that route telephony over IP networks.

PCI—Peripheral Component Interconnect

The PCI Standard defines a 32- or 64-bit parallel bus for connecting devices to a

computer motherboard. Peripherals connected via a PCI bus vary widely, including graphics cards, network cards, modems, disk controllers, and other I/O

devices. The original PCI bus specification consisted of a 33 MHz 32-bit bus,

and has been revised multiple times, culminating in PCI-X running up to 533

MHz with 64-bit signalling. PCIe (also called PCI Express) is a far faster interface that is physically and electrically very different to PCI, but retains software

compatibility; i.e., an operating system written to talk to PCI devices won’t be

confused when it finds it’s running on a PCIe system.

PDC—Primary Domain Controller

A server catering to Windows NT style domains that can give a user access to

multiple resources on a network with the use of one login. NT Server domains

have one Primary Domain Controller, and optionally multiple Backup Domain

Controllers. While the Primary Domain Controller contains the database of

accounts and privileges in a read/write form, each Backup Domain Controller

gets a full backup of the database, but is read-only. If needed, a PDC can be

removed and a BDC can be promoted to PDC. Under Linux, Samba can perform as a PDC. (Contrast to Active Directory, which supersedes NT-style


PKI—Public Key Infrastructure

A system that handles the work of creating public-key certificates containing

identities tied to public keys and signed by a certificate authority (CA). The PKI

can publish the public-key certificates to those who wish to communicate with

the keys’ owners, and verify that a certificate containing some public key and

identity is genuine, so the public key can be trusted to belong to the owner


PPP—Point-to-Point Protocol

In its most common form, PPP is used to provide an OSI layer 2 (data link)

between two nodes over a serial modem connection to allow TCP/IP to function

and give a computer Internet access. Defined within PPP’s specification is Link

Control Protocol (LCP), which automatically configures the interfaces at each

end of the PPP connection. PPP is also used as part of PPP over Ethernet (PPPoE)

for some ADSL connections, and PPP over ATM (PPPoA) for some ADSL and

Cable Internet connections.




PPTP—Point-to-Point Tunneling Protocol

A protocol used to create a VPN over an IP-based network such as the Internet.

Network protocols on the original networks are sent over a regular PPP session

using a Generic Routing Encapsulation (GRE) tunnel. A PPTP VPN can be

encrypted using Microsoft Point to Point Encryption (MPPE), but the implementation isn’t particularly secure in comparison to the SSL-based OpenVPN.


QoS—Quality of Service

Any system whereby packets zipping around your network are handled in different ways according to their importance and need. Applications sending/receiving

data don’t all require the same performance from the network; VoIP may have

strict requirements for low delay, high quality video may need consistent high

throughput, an SSH session may require little bandwidth but must be highly

responsive, and network warnings to on-call admins (you really do want to

know when your most critical servers have something to complain about) absolutely must get through.


RAS/RRAS—Remote Access Service, Routing & Remote Access Service

RAS is Windows NT’s Remote Access Service, which allows the sharing of network services over a dial-up connection. A remote user would dial in to a server,

and then have the same access to the server’s network as if they were connected

to it physically.

RRAS is the equivalent to RAS in Windows 2000 Server and above, which not

only provides dial-up remote access, but also a VPN server, IP Routing, and


RDP—Remote Desktop Protocol

The protocol used by client software to connect to a remote Windows computer

running Microsoft Terminal Services, and to use that computer as if it were the

local machine. Currently, the server software only runs on Windows, but clients

are available for other operating systems, including Linux, Mac OS X, BSDs, and

Solaris. RDP not only allows the remote machine to display graphics on the local

screen, but applications on the remote can play audio and use serial ports, parallel ports, and printers on the local device.

Not all Windows computers can run an RDP service; notable exceptions are

Windows XP Home Edition and Windows Vista Home Basic or Home Premium.



Appendix B: Glossary of Networking Terms

RFC—Request For Comments

Documents containing standards, technical, and organizational information

about the Internet. An individual RFC is not necessarily a standard or even a

proposed standard, but may be published to provide information about how

other standards work in practice when applied to the Internet, to provide information on de facto adopted standards, or to convey new concepts related to the

Internet. RFCs are serialized, and referred to by number; for example, RFC 4406

is a document covering an experimental protocol for email authentication.

Anyone may publish a document to the Internet Engineering Task Force for inclusion as a possible RFC. The official source for RFCs is http://www.rfc-editor.org/.

RIP—Routing Information Protocol

A method by which routers within a network are able to adapt to changing

network conditions (such as a downed router or suddenly congested links) by

communicating to other routers. About every 30 seconds, a RIP-enabled router

multicasts its routing table to any other connected routers, and can be triggered

to do the same on certain events for quick response to sudden changes. As a

distance-vector routing protocol, RIP uses the hop count of a destination to

detect the most desirable path to route packets, but limits the number of hops to

15 to prevent routing loops. This creates a limit to the size of a network that can

be supported by RIP, as anything more than 15 hops away appears not to exist

to RIP routers. RIP benefits from simple configuration and low processing

requirements, so for a relatively small LAN, RIP may be ideal. (See also OSPF.)


IP Routing is the process of path selection for packets traveling through an IPbased network. Compared to bridging, which automatically discovers the route

that network traffic takes between multiple network segments, and does so via

OSI Layer 2 (the data link layer), routing relies upon a coordinated OSI Layer 3

(network layer) network, and uses the IP addresses of packets to decide where to

forward them. Routing is usually controlled by pre-constructed routing tables

that define where a packet should go. Each router only needs to know where a

packet should be sent on its next hop, and doesn’t know nor care what happens

afterward; the next hop plus one is the responsibility of the next router, and so

on through the network until a packet reaches its destination.


SBC—Single Board Computer

A computer where everything needed to function is on a single board (mostly). A

desktop computer can require a whole load of different boards and accessories

to make it work. There’s the motherboard, some RAM modules, a hard drive, a

graphics card, a keyboard, and a mouse—and that’s just for a basic system

without including extra storage, exotic graphics setups, extra USB ports, or

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specialized sound and media cards. On the other hand are the single board computers with much more modest hardware. A fanless basic processor, RAM, flash

RAM storage, multiple networking ports, and serial connections all on the one

board is the norm. There may be some basic expansion available, but it’s not

necessary for most operations. The idea is that many specialized repetitive tasks

like routing, firewalls, and some services can be handled by computers at about

the speed of an early Pentium, and that’s where these boards fit. Just cram it in a

box, add power and an operating system to its flash RAM, and you’re on your


Serial console

Any PC, laptop, or PDA that controls another machine via the serial port. Some

folks think that only a real hardware serial terminal, like a Wyse terminal, can be

called a console. Using an old PC for a serial console is a nice way to get a few

more years’ life out of an old machine.

SIP—Session Initiation Protocol

The SIP protocol is probably the most popular VoIP protocol in use now. Commercial VoIP providers like Vonage use SIP. SIP is not a multimedia protocol

itself, but rather carries any type of audio or video stream, and it creates, modifies, and terminates sessions between at least two endpoints.

SLA—Service Level Agreement

A formal agreement that defines the level of service to be expected from a provider of those services. For example, with an Internet connection, an SLA may

define the percentage of time a connection remains open and fully usable, the

average time before the helpdesk answers their phones, or the average time taken

for problems to be fixed. An SLA can also lay out billing reductions for the client or penalties for the provider if they fail to honor the level of service



A Smurf attack is a form of Denial of Service attack that exploits the response of

computers on a network to a broadcast ICMP echo request (a ping). The basic

element of a Smurf attack is a single ICMP echo request carrying a faked source

IP address, sent to a broadcast address. The routing device that receives the echo

request then broadcasts the single request to all IP addresses covered by that

broadcast address, and each one sends back an ICMP echo response directed to

the faked source IP address. In this way, a single ping request from somewhere

on the Internet can generate a much larger ping response to the faked source

address (the victim). Floods of such pings can multiply the response hundredsfold, and overwhelm the network connection or computer at the faked source IP.

SNMP—Simple Network Management Protocol

SNMP consists of managers (stations that oversee devices on a network) and

agents (inside a network device itself) communicating through a simple language. Using SNMP, a manager is able to read information from an agent, or



Appendix B: Glossary of Networking Terms

read and write information depending on the permissions it has to that agent.

Information within agents is stored by objects within a Management Information Base (MIB), and those objects may contain a wide range of information

about a device such as settings, usage statistics, performance data, or physical

properties (e.g., temperature or fan speed).

SOHO—Small Office/Home Office

A term applying to a small business with up to about 10 users. Computing

equipment labeled SOHO may be designed with some features typically for business use, but not necessarily capable of handling the requirements of large

organizations with hundreds of users.


A package for Red Hat-based Linux systems that contains source code and a

spec file that lets the rpm utility compile and build an RPM package. The resulting RPM package can then be installed and managed like any other RPM.

SSH—Secure SHell

A protocol that allows the opening of a secure, encrypted channel between two

computers with secure authentication. SSH is most often used to provide a

secure shell to log in to a remote machine, but also supports file transfers, TCP,

and X11 tunneling.

SSL/TLS—Secure Sockets Layer/Transport Layer Security

SSL and TLS are similar, related protocols for providing secure data transmission and authentication over networks, including the Internet. SSL was originally

developed by Netscape in 1994, and was revised to become SSL 3.0 in 1996,

which became the base of TLS. TLS 1.1 is the current version of the protocol. An

SSL/TLS connection is started by a client requesting a secure connection to a

server. The client and server decide on the strongest cipher and hash function

they both share, and the server presents a digital certificate that can be checked

by the client with the issuing certificate authority. Within the server’s certificate

is its public key, which the client uses to encrypt a random number to send to

the server. If the connection is genuine, the server is able to decrypt the message

and the server and client now have a matching secret random number that can

be used to generate keys for data transfer. Now that this handshaking is

complete, the server and client may communicate over a secure connection. The

client may also present a digital certificate as part of the handshaking process, so

that the server, too, can verify the client’s identity.

State (packet filtering)

Filtering on the known state of a packet, identified by previous network activity.

A single packet coming from a random machine on the Internet may be dropped

by a firewall, or it may be accepted, depending on the known state. For example, a machine behind a firewall may request a web page from a web server. The

web server then sends a response back, and the firewall allows the response

because it knows a machine requested information from that server. The same

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response from the web server would be denied if there had been no original

request passing through the firewall. While there was not necessarily any information within the packet that defined whether it was a valid response to be

passed through, its state was derived by the firewall through previous activity

between the two hosts.

Static address

A Static address is one meant to be matched to a particular computer, so that it

always has the same address. Necessary when you have a server on a network,

and must know a permanent IP address in order to use it. (Contrast with a

Dynamic address.)


In the context of an IP-based network, a subnet is a group of related IP addresses

all beginning with the same binary network part, and ending in a unique binary

sequence identifying the host within the subnet. An example might be the IP

address with subnet mask of The first 24 bits of

the address, shown by bits in the subnet mask, reveal which part is the network

address (, with the last 8 bits correspond to the hosts part (12 in

this case). The entire subnet thus spans the address range to Dividing a network into subnets in this hierarchical sense

keeps routing easy, as the IP addresses within a subnet can all be derived from

the network address.


At first glance, a switch may look very similar to a hub, but it will act far more

intelligently. Switches take note of the addresses of connected computers in

order to send only data to the correct machine. For example, a packet arrives in

a port on a switch, and is destined for one particular machine connected via

another port. The switch has previously paid attention to which machines are

connected to which port, and forwards the packet out only to the correct

machine. An unmanaged switch has no configuration options, and simply connects to multiple network computers. A managed switch can be configured for

various network fine tuning, such as limiting speed on certain ports, QoS, SNMP

reporting/control, link aggregation, and so on. (Contrast with Hub.)


Part of opening a new TCP connection. When a client wishes to connect to a

server on the Internet, it first sends a SYN packet to the server. The server

responds back with a SYN-ACK (an acknowledgment), and the client returns a

SYN-ACK-ACK (another acknowledgment). Both acknowledgments together

indicate that the server can talk to the client, the client can talk to the server, and

a TCP connection is now open for use between the two hosts.



Appendix B: Glossary of Networking Terms


TCAM—Ternary Content Addressable Memory

Unlike normal RAM in a computer where data is stored in many addresses and

the RAM can only be queried for the contents at a given address, Content

Addressable Memory (CAM) works in the other direction. CAM is provided

with content, then searches its memory in order to return a list of addresses

where the content was found. With RAM, a search requires software to repeatedly read from a memory address, compare the contents of memory to the

content being searched for, then move on to the next address, repeating until the

area of RAM to be searched is exhausted. With CAM, content can be provided,

and the list of addresses containing that content is returned in one operation,

which provides a phenomenal speedup for searching the contents of memory.

Ternary Content Addressable Memory takes this a step further. With normal

CAM, the stored data is only in the form of bits—a word at an address may be

10011101, but TCAM may contain a third state of “don’t care” or “X” in memory—so a word at an address could be 10011X01, which would match the

search for 10011101 and 10011001. CAM and TCAM are often used in switches

and routers to store MAC lookup tables and routing tables, respectively. A

router may have a network address in memory, and when a packet arrives to be

routed, its destination IP address can be searched for in TCAM, which will

instantly return the address of a routing table entry for its destination address,

stored with only the network part of the destination network as 1 or 0, and host

part as X. CAM and TCAM are far more complex, expensive, and power-hungry

memory-wise than normal RAM, but are necessary for applications like routing

where a search through a routing table must be done thousands or millions of

times per second.

TCP—Transmission Control Protocol

One of the central protocols essential to the function of the Internet, TCP allows

applications to create connections that, once established, the applications can

stream data across. TCP stacks in an operating system do the hard work of splitting the stream of data into segments with a sequence number, and sending

them out over an IP-based network. At the remote end, the TCP stack acknowledges packets that have been received (so that missing packets can be resent)

and reassembles received packets in the correct order to provide an in-order data

stream to the remote application.

TLS/SSL—Transport Layer Security/Secure Sockets Layer


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