LPI Linux Certification in a Nutshell Part 36

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128 191 129 190 62

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224 255 225 254 30 240

On the ExamBe prepared to define network and host addresses when provided an IP address and a subnet mask. Practice with a few subnet sizes within at least one cla.s.sification (A, B, or C). Also, because the use of decimal notation can cloud human interpretation of IP addresses and masks, be ready to do binary-to-decimal conversion on address numbers.

As you can see, as the number of subnets increases, the total number of hosts that can be deployed within the original cla.s.s C address range reduces. This is due to the loss of both broadcast addresses and network addresses to the additional subnets.

Protocols TCP/IP is a suite of Internet protocols, including the Transmission Control Protocol (TCP), Internet Protocol (IP), User Datagram Protocol (UDP), and Internet Control Message Protocol (ICMP), among others. Some protocols use handshaking handshaking (the exchange of control information among communicating systems) to establish and maintain a connection. Such a protocol is said to be connection-oriented and reliable, because the protocol itself is responsible for handling transmission errors, lost packets, and packet arrival order. A protocol that does not exchange control information is said to be connectionless and unreliable. In this context, "unreliable" simply means that the protocol doesn't handle transmission problems itself; they must be corrected in the application or system libraries. Connectionless protocols are simpler and have fewer overheads than connection-oriented protocols. TCP/IP is a (the exchange of control information among communicating systems) to establish and maintain a connection. Such a protocol is said to be connection-oriented and reliable, because the protocol itself is responsible for handling transmission errors, lost packets, and packet arrival order. A protocol that does not exchange control information is said to be connectionless and unreliable. In this context, "unreliable" simply means that the protocol doesn't handle transmission problems itself; they must be corrected in the application or system libraries. Connectionless protocols are simpler and have fewer overheads than connection-oriented protocols. TCP/IP is a stack stack of protocols because protocols are built in a hierarchy of of protocols because protocols are built in a hierarchy of layers layers. Low-level protocols are used by higher-level protocols on adjacent layers of the protocol stack: TCP TCP is a connection-oriented transport agent used by applications to establish a network connection. TCP transports information across networks by handshaking and retransmitting information as needed in response to errors on the network. TCP guarantees packet arrival and provides for the correct ordering of received packets. TCP is used by many network services, including FTP, Telnet, and SMTP. By using TCP, these applications don't need to establish their own error-checking mechanisms, thus making their design simpler and easier to manage.

IP IP can be thought of as the fundamental building block of the Internet. IP, which is connectionless, defines datagrams (the basic unit of transmission), establishes the addressing scheme (the IP address), and provides for the routing of datagrams between networks. IP is said to provide a datagram delivery service datagram delivery service. Other higher-level protocols use IP as an underlying carrier.

UDP UDP is a connectionless transport agent. It provides application programs direct access to IP, allowing them to exchange information with a minimum of protocol overhead. On the other hand, because UDP offers no a.s.surance that packets arrive at destinations as intended, software must manage transmission errors and other problems such as missing and incorrectly ordered packets. UDP is used by applications such as DNS and NFS.

ICMP ICMP is a connectionless transport agent that is used to exchange control information among networked systems. It uses IP datagrams for the following control, error-reporting, and informational functions:Flow controlSometimes inbound traffic becomes too heavy for a receiving system to process. In such cases, the receiving system can send a message via ICMP to the source instructing it to temporarily stop sending datagrams.Detecting unreachable destinationsVarious parts of network infrastructure are capable of detecting that a network destination is unreachable. In this case, ICMP messages are sent to the requesting system.Redirecting routesICMP is used among network components to instruct a sender to use a different gateway.Checking remote hostsHosts can transmit echo messages via ICMP to verify that a remote system's Internet Protocol is functioning. If so, the original message is returned. This is implemented in the ping ping command. command.

PPP Point-to-Point Protocol (PPP) is used for TCP/IP dial-up network access via modem.On the ExamYou will need a general understanding of the control messages sent via ICMP. In particular, note that ICMP does not transmit data and that it is used by ping ping.

Services When an inbound network request is made, such as that from a web browser or FTP client, it is sent to the IP address of the server. In addition, the request carries inside it a port number port number (or just (or just port port), which is a 16-bit value placed near the beginning of a network packet. The port number defines the type of server software that should respond to the request. For example, by default, web browsers send requests encoded for port 80. Web servers "listen" to port 80 and respond to incoming requests. The encoded port can be considered part of the address of a request. While the IP address specifies a particular interface (or host), the port specifies a specific service available on that host. Many port numbers are predefined, and the list is expanded as needed to accommodate new technologies. The official list of port number a.s.signments is managed by the Internet a.s.signed Numbers Authority (IANA). The ports known by your system are listed in /etc/services /etc/services.

Port numbers 1 through 1023 are often referred to as privileged ports privileged ports because the services that use them often run with superuser authority. Many of these, such as ports used for FTP (21), Telnet (23), and HTTP (80), are often referred to as because the services that use them often run with superuser authority. Many of these, such as ports used for FTP (21), Telnet (23), and HTTP (80), are often referred to as well-known ports well-known ports because they are standards. Port numbers from 1024 through 65535 (the maximum) are because they are standards. Port numbers from 1024 through 65535 (the maximum) are unprivileged ports unprivileged ports and can be used by applications run by ordinary system users. and can be used by applications run by ordinary system users.

During the initial contact, the client includes a local (randomly selected) unprivileged port on the client machine for the server to use when responding to the request. Client-to-server communications use the well-known port, and the server-to-client communications use the randomly selected port. This Objective requires you to be familiar with the privileged port numbers detailed in Table19-4 Table19-4.

Table19-4.Common privileged port numbers

Port number a.s.signed use Description 20 and 21 FTP data FTP control When an FTP session is opened, the binary or ASCII data flows to the server using port 20, while control information flows on port 21. During use, both ports are managed by an FTP daemon, such as vftpd.

23 Telnet server Inbound Telnet requests are sent to server port 23 and processed by telnetd.

25 SMTP server This port is used by mail transfer agents (MTAs), such as Sendmail.

53 DNS server Used by the Domain Name System (DNS) server, named.

67 BOOTP/DHCP server Hands out IP addresses to workstations dynamically.

68 BOOTP/DHCP client The client side for BOOTP/DHCP.

80 HTTP server Web servers, such as Apache (httpd), usually listen in on this port.

110 POP3 The Post Office Protocol (POP) is used by mail client programs to transfer mail from a server.

119 NNTP server This port is used by news servers for Usenet news.

139 NetBIOS Reserved for Microsoft's LAN Manager.

143 IMAP An alternate to POP3, Internet Message Access Protocol (IMAP) is another type of mail protocol.

161 SNMP Agents running on monitored systems use this port for access to the Simple Network Management Protocol (SNMP).

This list is a tiny fraction of the many well-known ports, but it may be necessary for you to know those in the list both by name and by number.

On the ExamYou should commit the list of ports in Table19-4 Table19-4 to memory so you can recognize a type of network connection solely by its port number. Your exam is likely to have at least one question on how a specific port is used. to memory so you can recognize a type of network connection solely by its port number. Your exam is likely to have at least one question on how a specific port is used.

Utilities The following popular applications, although not strictly a part of TCP/IP, are usually provided along with a TCP/IP implementation.

Name dig Syntax dighostname Description dig obtains information from DNS servers. Note that additional command-line arguments and options are available for obtains information from DNS servers. Note that additional command-line arguments and options are available for dig dig but are beyond the scope of Exam 102. but are beyond the scope of Exam 102.

Example $digredhat.com ;<>>DiG9.3.2<>>redhat.com ;;globaloptions:printcmd ;;Gotanswer: ;;->>HEADER<-opcode:query,status:noerror,id:41163 ;;flags:qrrdra;query:1,answer:1,authority:0,additional:0="">

;;QUESTIONSECTION: ;redhat.com.INA

;;ANSWERSECTION: redhat.com.60INA209.132.177.50

;;Querytime:43msec ;;SERVER:68.87.68.166#53(68.87.68.166) ;;WHEN:FriSep1806:28:082009 ;;MSGSIZErcvd:44

Name ftp Syntax ftp[options]host ...interactivecommands...

Description Establish an interactive FTP connection with host host to transfer binary or text files. FTP creates an interactive dialog and allows for two-way file transfer. The dialog includes username/pa.s.sword authentication, user commands, and server responses. to transfer binary or text files. FTP creates an interactive dialog and allows for two-way file transfer. The dialog includes username/pa.s.sword authentication, user commands, and server responses.

Frequently used options -i Turn off interactive prompting during multiple file transfers (also see the prompt prompt command in the next list). command in the next list).

-v Set verbose mode; display server responses and transfer statistics.

Frequently used commands ascii, binary binary Establish the transfer mode for files. ASCII mode is provided to correctly transfer text among computer architectures where character encoding differs.

get file file Receive a single file file from the server. from the server.

mget files files Receive multiple files files from the server. from the server. files files can be specified using normal file glob patterns. can be specified using normal file glob patterns.

ls [ [files]

Obtain a directory listing from the server, optionally listing files files.

put file file Send a single file file to the server. to the server.

mput files files Send multiple files files to the server. to the server.

prompt Toggle on and off interactive prompting during mget mget and and mput mput (also see the (also see the -i -i option in the previous list). option in the previous list).

pwd Print the working remote directory.

quit, exit Cleanly terminate the FTP session.

Example 1 Get a file from machine smp smp: $ftp-vsmp Connectedtosmp.

220smpFTPserver(Versionwu-2.4.2-VR17(1) MonApr1909:21:53EDT1999)ready.

Name(smp:root):jdean 331Pa.s.swordrequiredforjdean.

Pa.s.sword:

230Userjdeanloggedin.

RemotesystemtypeisUNIX.

Usingbinarymodetotransferfiles.