Aplicatii Client-Server - BOOTP, DHCP, TELNETdorin.huzum/rc/curs07/curs8.pdf · WILL - The sender wants to set an option. ... Aplicatii Client-Server BOOTP, DHCP, TELNET noiembrie

BOOTP, DHCPTELNET: TErminaL NETwork

Aplicatii Client-ServerBOOTP, DHCP, TELNET

Gabriel Ciobanu

Facultatea de InformaticaUniversitatea A.I. Cuza, Iasi

[email protected]

Nivel Aplicatie

Dorin Huzum ([email protected])Sinica Alboaie ([email protected])

Cristian Paslaru ([email protected]

http://www.info.uaic.ro/∼dorin.huzum/rc

Aplicatii Client-Server BOOTP, DHCP, TELNET noiembrie 2007 1 / 42


Protocoale binecunoscute la nivelul aplicatie

Nivelul aplicatie pune la dispozitie o serie de serviciiInternet standard:

Configurare retea (BOOTP, DHCP)Terminal la distanta (TELNET, SSH, . . .)Posta electronica (SMTP, IMAP, POP,. . .)Transferul de fisiere (TFTP, FTP)World-Wide Web (HTTP, WWW)Conversatii instantanee (IRC, ICQ, Jabber, . . .)

Protocoale pentru rezolvarea unor sarcini de sistem -/etc/services, /etc/protocols

Sistemul de fisiere in retea (NFS)Conectivitate cu alte sisteme de fisiere (SMB)Servicii de baze de date (MySQL, PostgreSQL, . . .)



Static configuration: BOOTPDynamic configuration: DHCP

Cuprins

1 BOOTP, DHCPStatic configuration: BOOTPDynamic configuration: DHCP

2 TELNET: TErminaL NETwork




BOOTP: Bootstrap Protocol

client-server paradigm is applied to bootstrapping

A host connected to an internetwork needs to know its IPaddress before it can send or receive datagrams

Overall, a host needs to know at least the following:

its IP addressits subnet maskthe IP address of a routerthe IP address of a domain name server

BOOTP and DHCP make it possible to boot a machinethrough an IP network without using RARP

both are based on UDP, and use specific formats




BOOTP: BOOTstrap Protocol

At startup, a diskless workstation has an initializationroutine stored in ROM, which is the same for all machines(then its specific IP address cannot be stored in the ROM)

Using RARP has at least three draw-backs:

RARP is low-level network-dependent protocol, thereforeusing it requires access to the physical network interfaceThe efficiency of RARP is low, the minimum-sizeEthernet- message is 512-bit long but only contains 32bits of useful information (the IP address)Because RARP is based on the physical network address,it cannot be used in networks, where the address isassigned dynamically

BOOTP and its extension DHCP address these problems




BOOTP

BOOTP is based on UDP, and it can be implementedwithout access to the hardware levelBOOTP is more efficient than RARP (more informationthan the IP address is transferred in the message)BOOTP includes a field that makes it possible to transfervendor-dependent information using the protocolIP address 255.255.255.255 defines a limited broadcastand it can be used without knowing ones own IP address(it can use ”0.0.0.0” as its sender address)assume that work station A sends a BOOTP request toserver B, which chooses an IP address for A but cannotyet use ARP to update its own routing tableIn practice, B either broadcast its reply or derive the MACaddress of A from the BOOTREQUEST message




BOOTP packet




BOOTP option


TCP/IP Protocol Suite 8

Table 16.1 Table 16.1 Options for BOOTPOptions for BOOTP



BOOTP client-server




Two-Step Bootstrap Procedure

BOOTP does not provide clients with a memory image; itonly provides a file name. The client then uses a secondprotocol (tftp) to obtain the memory image.

client can fill BOOT FILE NAME a generic name like“uni; t. The server consults it configuration database tomap the generic name into a specific file name for client.

If client fills zeros in the BOOT FILE NAME field,BOOTP selects a memory image of the machine.

Clients use port 68 instead of an ephemeral port. This isbecause the reply from the server can be broadcast (receivedby all hosts).— Relay agent... Error control...



Figure 16.2 Client and server on two different networks



DHCP: Dynamic Host Configuration Protocol

BOOTP was designed for a static environment

BOOTP does not apply well to portable workstations,because it only provides a static mapping from theidentification of a workstation to its parameters

The parameters need to be defined and stored to theconfig. file of the BOOTP server separately for each host

If the hosts often change locations or the number of hostsexceeds the number of available IP addresses, this leadsinto difficultiesTo facilitate automatic address allocation, the IETF hasdefined DHCP, which extends BOOTP in two ways:

The host gets all configuration information in onemessageThe host receives its IP address fast and dynamically




DHCP: Dynamic Host Configuration Protocol

DHCP makes it possible to allocate an IP address in threeways, out of which the administrator can choose the onethat best suits each network or host:

Like BOOTP, DHCP allows manual configurationAutomatic configuration means, that DHCP assigns apermanent IP address for a new hostDynamic configuration means, that the host leases its IPaddress from a common address pool when it starts

Dynamic configuration is the most interesting one ofthese alternatives because (it allows any computer to beconnected to the network without prior arrangements)If the host is attached to several networks, it has toperform DHCP separately for each network interfacesBOOTP and DHCP facilitate the booting from anothernetwork by using a relay agent




DHCP packet



Table 16.2 Table 16.2 Options for DHCPOptions for DHCP



DHCP

A host becomes a DHCP client when it broadcasts theDHCPDISCOVER message to all DHCP servers of thelocal networkEach DHCP server replies with an offer, containingconfiguration information and the IP address the server isoffering to lease to the client, sent in a DHCPOFFERmessageThe host chooses one of the offers and sends aDHCPREQUEST message to the respective serverThe server confirms the lease with a DHCPACK messageA machine may terminate the lease by sending aDHCPRELEASE messageThe protocol logic of DHCP is presented as a statetransition diagram




DHCP Transition Diagram




DHCP Exchanging Messages




DHCP Exchanging Messages



Telnet

Cuprins

1 BOOTP, DHCP

2 TELNET: TErminaL NETworkTelnet



Telnet

Telnet

a general-purpose client-server program for virtualterminal

connection to a remote system such that the localterminal appears to be a terminal at the remote system



Telnet

Time-sharing environment

a large computer supports multiple users

interaction user-computer is given by a terminal

users have login names, passwords, and access rights toresources



Telnet

Time-sharing Environment (local login)



Telnet

Time-sharing Environment (remote login)



Telnet

Telnet

telnet client overrides the local interpretation the keys,allowing a user to interact with a remote machine exactlyas one would from a locally connected terminal

telnet alllows a TCP connection, and then passeskeystrokes from the local keyboard directly to the remotecomputer as if they had been typed on a keyboardattached to remote machine

pseudoterminal: impossible to build a TELNET serverunless the OS supplies such a facility

TELNET server at application level has advantages (easyto modify) and disadvantages (inefficiency).



Telnet

Network Virtual Terminal (NVT) characters



Telnet

NVT character sets: data and commands



Telnet

NVT remote control characters



Telnet

IAC (Interpret As Control)



Telnet

Option Negotiation

To negotiate an option, either side may send one of fouroption requests:

WILL - The sender wants to set an option.

WONT - The sender wants to disable the option.

DO - The sender wants the receiver to set the option.

DONT - The sender wants the receiver to disable theoption.



Telnet

Offer/request to enable



Telnet

Offer/request to disable



Telnet

Example of Echo option



Telnet

Suboption negotiation



Telnet

Controlling the server

Some of the control characters can be used to control theremote server.



Telnet

Control Functions

IP (interrupt process) function requests the server toabort the currently running user process. (e.g., IAC IP(255 244)); this function only terminates the runningprocess, not the entire remote terminal session.AO (abort output) function is used when a user processon the server has generated output the user does notwant to see; similar to IP function, except that AO willnot abort the user process.AYT (Are you there) facility enables a user to determinewhether the server is still active.EC (erase char) function is used to delete the lastpreceding undeleted character transmitted.EL (erase line) function is used to delete an entire line ofinput.



Telnet

Example of interruption



Telnet

Out-of-band signalling



Telnet

Example of interruptions



Telnet

Modes of operation

Default mode (half-duplex)

echoing is done by the clientclient does not send until whole line is complete.

Character Mode

the user enters a char that is sent to the server.server ack the receiving char, and echos the char back.client ack.

Line mode (full duplex w/o GA)



Telnet

Example 1

A client and a server negotiate the terminal type and terminalspeed and then the server checks the login and password ofthe user. We use the default mode to show the concept andits deficiencies even though it is almost obsolete today.



Telnet

Example 1



Telnet

Example 2

A client switches to the character mode. This requires a clientto ask the server to enable the SUPPRESS GO AHEAD andECHO options.



Telnet

Example 2


Documents

Aplicatii Client-Server - BOOTP, DHCP, TELNETdorin.huzum/rc/curs07/curs8.pdf · WILL - The sender wants to set an option. ... Aplicatii Client-Server BOOTP, DHCP, TELNET noiembrie