LIS508 lecture 11: TCP/IP, DNS

1. LIS508 lecture 11:TCP/IP, DNS Thomas Krichel 2002-12-02

2. Structure • Refresher on IP • TCP • the TCP/IP model • DNS

3. 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

4. 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.

5. 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

6. 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.

7. 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.

8. Routing packet from home to wotan traceroute 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

9. 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.

10. 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.

11. 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.

12. 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

13. Critique of tcp/ip • Not a model but a protocol stack • No proper distinction between • Service • Protocol • Interface • Ad hoc set of application protocols

14. 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

15. 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.

16. 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

17. 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.

18. 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

19. 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

20. 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.

21. 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.

22. 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 131.227.9.154 mail.openlib.org. IN CNAME wotan.liu.edu. openlib.org. IN MX 1 mail.openlib.org. trabbi.openlib.org. IN TXT "hello world"

23. http://openlib.org/home/krichel Thank you for your attention!