LIS508 lecture 11:TCP/IP, DNS

Thomas Krichel

2002-12-02

Structure

• Refresher on IP

• TCP

• the TCP/IP model

• DNS

IP address

• 32 bit address, usually written as a sequence of four decimal numbers between 0 and 255.

• Contains a network part and a host part– dependent on the network mask– depending on the class of the network

• All traffic that is not on the local network is sent to a gateway

finding the IP address

• A host is on the Internet when it has an IP address that others can reach. Usually that implies knowing– IP address of the machine– network mask– IP address of the gateway

• There can be written into a file on the machine. This is usually referred to as a static address.

static and dynamic addresses

• Static IP addresses have problems– waste a lot of address space on machines

that are switched off– have to be rewritten when device moves

• dynamic addressing has become more common

• the most widely used protocol is the dynamic host configuration protocol

dhcp

• When connected to a network a client software on a host machine sends out a request on the LAN it is attached to.

• A dhcp server will answer and lease an IP address to the host, sending it the network mask and the gateway address as well.

• Internet providers work like that to save address space.

• Problem: some services require IP addresses to be stable. They can not be run in such a setting.

routing

• using IP, all datagrams travel individually.• they may travel through many networks in

order to get to their destination• networks are interconnected through

routers who make the decision where to send the packet to next.

• Such decision are based on routing protocols such as OSPF or BGP etc. This is a very complicated piece of engineering.

Routing packet from home to wotantraceroute to wotan.liu.edu (148.4.2.231), 30 hops max, 38 byte packets 1 10.42.32.1 (10.42.32.1) 6.320 ms 7.262 ms 2 pos2-0-nycmnyl-ubr1.nyc.rr.com (24.29.98.157) 6.253 ms 6.686 ms 3 pos0-0-nycmnyl-rtr1.nyc.rr.com (24.29.98.153) 7.428 ms 6.599 ms 4 pos2-0-nycmnyrdc-rtr1.nyc.rr.com (24.29.98.17) 9.790 ms 17.420 ms 5 pop2-nye-P0-2.atdn.net (66.185.137.209) 22.821 ms 8.953 ms 6 level3.atdn.net (66.185.137.218) 8.021 ms 7.212 ms 7 gigabitethernet2-1.core1.NewYork1.Level3.net (64.159.17.69) 20.369ms

8.690ms 8 mny1-cr10.bbnplanet.net (209.244.160.142) 14.698 ms 11.004 ms 9 p1-0.nycmny1-nbr2.bbnplanet.net (4.24.8.169) 9.628 ms 8.604 ms 10 p1-0.nycmny1-cr2.bbnplanet.net (4.24.7.6) 9.434 ms 7.482 ms 11 s1-0.liunv.bbnplanet.net (4.24.153.106) 82.531 ms 74.760 ms 12 148.4.3.66 (148.4.3.66) 57.008 ms 46.573 ms 13 wotan (148.4.2.231) 47.840 ms 36.411 ms

frame / packet / segment

• The Ethernet frame on the LAN contains the IP packet.

• A gateway between two networks takes the packet out of the Ethernet frame of one network, and “wraps” it in a frame used on another network.

• Inside the IP packet, there is a TCP segment. The TCP segment tells the destination machine what to do with the data.

transmission control protocol

• provides a reliable service of communication– stream orientation: bytes come out from the sender,

arrive in the same order at the receiver– packet buffering: fast arrived data is stored at the

destination until it can be processed– full duplex: communication in two ways.

• if a packet has gone astray, TCP retransmit it.• uses port numbers as addresses to tell the host

what to do with arriving packets.

applications and ports

• TCP uses port numbers to detect which application protocol to send the data to.

• Some ports are well-known– 80 for http -- 20 for ssh– 23 for smtp -- 53 for dns

• firewalls can block traffic for specific ports on specific machines and therefore allow or prevent services.

summary : TCP/IP “model”

• Application layer– http, smtp, ftp, dns, ntp

• Transport layer– tcp, udp

• Network layer– IP

• Host-to-host layer– LAN and WAN protocols

Critique of tcp/ip

• Not a model but a protocol stack

• No proper distinction between – Service– Protocol– Interface

• Ad hoc set of application protocols

Application layer

• This is the top level of the network, applications that run on it.

• In fact, the Domain Name System occupies a special role because most other application layer protocols relay on it.

• Off the top of my head, only peer-to-peer communication systems don’t really use DNS

Host names

• A host name associates a human-friendly name with an IP address.

• Example: arcano.openlib.org = 148.4.16.229• Finding an IP for a name is called a name

lookup. The reverse is a reverse lookup.• Names are a sequence of labels, separated by

dot.• Names may contain letters, numbers and

hyphens. They may not start with a hyphen.• Names solve from right to left, contrary to

addresses, that resolve from left to right.

purpose

• Allows to keep constant name for– changing machines– changing the location of the machine.

• Makes it easier for humans to remember access points to services.

• Establish brand names and have an economic value

History of DNS

• In the 70s, one single file HOSTS.TXT was maintained at SRI-NIC, downloaded frequently by all hosts on the Internet.

• Problems– traffic and load– name collisions– Consistency

• 1984, Paul Mockapetris releases RFC822 and RFC883 that describe the Domain Name System DNS.

• Names are words separated by dots.

DNS and domains

• DNS is– distributed database– client server architecture

• Name servers have information about names.

– general purpose• Allows a lot of different properties to be associated

with names

– hierarchical structure• Top component of name is to the right.

– independent of physical structure

Berkeley Internet Name Domain

• BIND is an implementation of the Domain Name System (DNS) protocols and provides an openly redistributable reference implementation of the major components of the Domain name system, including– a Domain Name System server (named)– a Domain Name System resolver library– tools for verifying the proper operation of the

DNS server

Top level domains

• For the US, delimited by function: .com, .net, .org, .int, .biz, .info etc.

• For other countries, use name of the country, .to, .su, .ru.

• Below that you can register names, such as myprof.com

• And then, you can create your own names like krichel.myprof.com, daluca.myprof.com, and associate properties with them.

To register names

• There is a market of name registrars around. • You have to check that the name is not already

taken, e.g. “whois openlib.org”– Domain Name: OPENLIB.ORG– Registrar: NETWORK SOLUTIONS, INC.– Whois Server: whois.networksolutions.com– Referral URL: http://www.networksolutions.com– Name Server: UTSERV.MCC.AC.UK– Name Server: FAFNER.OPENLIB.ORG

• Many registrars will run a server for you, I run my own.

openlib.org. IN SOA wotan.liu.edu. tkrichel.wotan.liu.edu. ( 2001111300 ; Serial 10800 ; Refresh after 3 hours 3600 ; Retry after 1 hour 640800 ; Expire after 1 week 86400 ; Minimum ttl of 1 day)openlib.org. IN NS wotan.liu.edu.; primary server, the one which holds the authoritative info

(this file)openlib.org. IN NS utserv.mcc.ac.uk.; secondary servers, At least one is necssesary.openlib.org. IN A 131.227.9.154mail.openlib.org. IN CNAME wotan.liu.edu.openlib.org. IN MX 1 mail.openlib.org.trabbi.openlib.org. IN TXT "hello world"

http://openlib.org/home/krichel

Thank you for your attention!