DHCP, DNS, SNMP

1. DHCP, DNS, SNMP Debashis Saha MIS Group, IIM Calcutta ds@iimcal.ac.in

2. First Question • How does your computer know which IP to use? • How does your computer know that www.iimcal.ac.in is at IP 202.54.116.3? • How can you track your network’s health?

3. The Internet and Addressing • All machines on the Internet are accessed via their IP address • Dotted quad: xxx.xxx.xxx.xxx • Problem: IP addresses are hard to remember and hard to statically assign on large networks • Solution: Need to come up with a way to automatically assign IP addresses and a text based representation of machine addresses instead of just 4 numbers

4. Addressing Solutions • Two protocols have been developed to solve these problems • DHCP – automatic network configuration (including IP address) • DNS – translates textual based names into IP addresses and vice versa

5. DHCP – Dynamic Host Configuration Protocol

6. DHCP • Allows client machines to receive an IP address, DNS information, etc automatically • Before DHCP came into use, users had to type in all this information by hand, which is bad: • Easy to mistype something when entering by hand • Manually changing network configuration every time you move your laptop is a pain • Bootp resolved some of these issues • … and DHCP still uses the same port as bootp

7. Purpose of DHCP From RFC-2131 (the Internet standard): • The Dynamic Host Configuration Protocol (DHCP) provides a framework for passing configuration information to hosts on a TCP/IP network. • DHCP consists of two components: • a protocol for delivering host-specific configuration parameters from a DHCP server to a host and • a mechanism for allocation of network addresses to hosts.

8. DHCP functional goals • A host without a valid IP address locates and communicates with a DHCP server • A DHCP server passes configuration parameters, including an IP address, to the host • The DHCP server may dynamically allocate addresses to hosts and reuse addresses • Hosts can detect when they require a new IP address • Unavailability of DHCP server has minimal effect on operation of hosts

9. DHCP: Basics • A client leases an IP address from a DHCP server for a given amount of time • When lease expires, the client must ask DHCP server for a new address (clients attempt to renew lease after 50% of the lease time has expired) • Typical leases may last for from 30 seconds to 24 hours, or even longer.

10. What does DHCP do? • Provides protocol stack, application and other configuration parameters to hosts • Eliminates need for individual, manual configuration for hosts • Includes administrative controls for network administrators • Backward compatible packet format for BOOTP interoperation (RFC 1542) • Can coexist with hosts that have pre-assigned IP addresses and hosts that do not participate in DHCP

11. Design Goals • Eliminate manual configuration of hosts • Prevent use of any IP address by more than one host • Should not require a server on every subnet • Allow for multiple DHCP servers • Provide a mechanism, not a policy • Provide the same configuration - including IP address - to a host whenever possible

12. What can you do with DHCP • Plug-and-play • Move desktop PCs between offices • Renumber • Other restructuring - change subnet masks • Mobile IP - laptops • Moving equipment - cartable

13. What DHCP doesn’t do • Support multiple addresses per interface • Inform running host that parameters have changed • Propagate new addresses to DNS • Support inter-server communication • Provide authenticated message delivery • Configure routers and other network equipment • Design network addressing plan • Determine other configuration parameters • Locate other servers

14. DHCP: Messages Overview • Several messages are sent back and forth between a client and the DHCP server before it can successfully obtain an IP address

15. DHCP: DISCOVER • Hardcoding the addresses of DHCP servers kind of defeats the purpose of automatic configuration • Solution: A client using DHCP will broadcast a DISCOVER message to all computers on its subnet (addr 255.255.255.255) to figure out the IP address of any DHCP servers • Most routers are configured to pass this request within the campus or enterprise

16. DHCP: OFFER • (Optionally) sent from server in response to a DISCOVER • Contains an IP address, other configuration information as well (subnet mask, DNS servers, default gateway, search domains, etc) • Note that all DHCP servers that receive a DISCOVER request may send an OFFER; • since a client typically does not need more than one IP address, more messages needed to zero on one IP

17. DHCP: REQUEST • Sent by client to request a certain IP address • Usually the one sent by an OFFER, but also used to renew leases. Also can be sent to try to get same address after a reboot • This message is broadcast • Most OSs by default will send a REQUEST for the first OFFER they receive – this means that if there is a rogue DHCP server on your subnet, most clients will ignore the OFFERs from the campus DHCP servers (since the OFFER from the rogue server gets to the user’s PC first)!

18. DHCP: ACK/NACK • Sent by server in response to a REQUEST • ACK: Request accepted, client can start using the IP it REQUESTed • NACK: Something is wrong with the client’s REQUEST (for example they requested an IP address they’re not supposed to have)

19. DHCP: RELEASE • Sent by client to end a lease • Not strictly required, but is the “polite” thing to do if done with the IP (could just let the lease expire) • Some clients may not send RELEASEs in an attempt to keep the same IP address for as long as possible

20. DHCP: Big Picture

21. Summary • DHCP works today as a tool for automatic configuration of TCP/IP hosts • It is an open Internet standard and interoperable client implementations are widely available • Provides automation for routine configuration tasks, once network architect has configured network and addressing plan • Ongoing work will extend DHCP with authentication, DHCP-DNS interaction and inter-server communication

22. DHCP: Conclusion • Efficient way of assigning computers IP addresses and delivering configuration information to those computers • Does not solve other problem of addressing – that IP addresses are hard to remember

23. IETF standards • Formal process for development, review and acceptance of TCP/IP protocol suite standards • Initial specifications published as Internet Drafts (I-Ds) • Accepted specifications published as Request for Comments (RFCs)

24. Protocol status • DHCP has been accepted as a Draft Standard; the specifications are published in: • RFC 2131: Dynamic Host Configuration Protocol • RFC 2132: DHCP Options and BOOTP Vendor Extensions • Several additional options are in development

25. Implementation status • DHCP is an open standard, with freely available specifications • Can be (and has been) implemented entirely from the specification • Commercial implementations are widely available • Non-commerical implementations are also available

26. DHCP Resources • Compilation of DHCP-related WWW links and other information: http://www.dhcp.org • DHCP FAQ (maintained by John Wobus) • dhcp-v4@bucknell.edu mailing list (admin requests to listserv@bucknell.edu) • IETF information can be retrieved from: http://www.ietf.cnri.reston.va.us • I-Ds and RFCs can also be retrieved from: http://www.rfc-editor.org

27. DNS – Domain Name System

28. Second Question • How does your computer know which IP to use? • How does your computer know that www.iimcal.ac.in is at IP 202.54.116.3? • How can you track your network’s health?

29. Domain Name System (DNS) • The first IP networks distributed host files on a regular basis • This became a burden and an automated distributed solution was needed • IP Addresses are great for computers • IP address includes information used for routing. • IP addresses are tough for humans to remember. • IP addresses are impossible to guess. • ever guessed at the name of a WWW site?

30. Hostnames • DNS is born • Domain names comprise a hierarchy so that names are unique, yet easy to remember. • The domain name system is usually used to translate a host name into an IP address . • Ex: www.xxx.org <-> IP 212.87.7.181 • (which one would you rather remember?)

31. DNS: Basics • Hierarchical namespace • Distributed system – very few core servers • Stores other information than simple hostname <-> IP mappings • Request/response protocol

32. DNS Hierarchy edu com org in rpi albany ac iimcal

33. Host name structure • Each host name is made up of a sequence of labels separated by periods. • Each label can be up to 63 characters • The total name can be at most 255 characters. • Examples: • whitehouse.gov • barney.the.purple.dinosaur.com • monica.cs.rpi.edu

34. Domain Name • The domain name for a host is the sequence of labels that lead from the host (leaf node in the naming tree) to the top of the worldwide naming tree. • A domain is a subtree of the worldwide naming tree.

35. arpa com edu gov mil net org us colordao cs foobar Hierarchical Naming foobar.cs.colorado.edu

36. Top level domains • edu, gov, com, net, org, mil, … • Countries each have a top level domain (2 letter domain name). • Such as .in, .uk, .it, etc. • New top level domains include: .aero .biz .coop .info .name .pro

37. edu com gov mil org net uk fr … … … … … harvard mit cisco yahoo nasa nsf arpa navy acm ieee hbs ee physics www Domain Naming System

38. DNS: Architecture • DNS servers are responsible for one or more domains of any level • “Root servers” are maintained throughout the world (one is in Palo Alto) and are responsible for all of the top-level domains • When you register a domain, an entry for that domain is added to the appropriate root server • Owners of each regular domain or subdomain maintain (or outsource) their own DNS servers containing the correct information

39. Name Servers • Partition hierarchy into zones edu com gov mil org net uk fr … … … … … princeton mit cisco yahoo nasa nsf arpa navy acm ieee cs ee physics ux01 ux04 Root name server Each zone implements two or more name servers • Primary • secondary … Princeton Cisco name server name server … CS EE name server name server

40. arpa arpa com in gov mil net org us ac iimcal www Hierarchical Administration - “Zones” www.iimcal.ac.in

41. Administration - Zones • A zone is a subtree of the DNS tree that is independently managed • Second-level domains (“ac.in”) are usually an independent zone • Most sub-domains (“iimcal.ac.in”) are also independent. • A zone must provide multiple name servers. This server records the members in the domain. • You typically need a primary name server and one or more secondary name servers. • Secondary retrieves information from primary using a zone transfer.

42. Resolving an address • A.C.D wants to know about F.E.D D H C E Name servers A B F G • Host “A.C.D” asks “B.C.D” (the local name server) to resolve “F.E.D”

43. Resolving an address • B.C.D doesn’t know the answer. • It wants to ask the primary domain server for the “E.D” domain, so it asks the parent of the “B.C” domain (“D” in this example) to resolve “E.D”. • D asks H, the root server. • H doesn’t know the answer, but it’s the top-level domain and knows that “G.E.D” is the primary domain server for the “E.D” domain • B.C.D now knows the primary domain server for the E.D domain, and can now ask “G.E.D” about “F.E.D”

44. Domain servers • What kind of records can be requested for a given domain? • Address translation • Caching information • Mail server information • Authoritative nameserver information • How is this data requested? • Each record has a type and certain data associated with it – clients request records of a certain type from a server

45. DNS Organization • Distributed Database • The organization that owns a domain name is responsible for running a DNS server that can provide the mapping between hostnames within the domain to IP addresses. • So - some machine run by RPI is responsible for everything within the rpi.edu domain.

46. DNS Distributed Database • There is one primary server for a domain, and typically a number of secondary servers containing replicated databases. rpi.edu DNS server rpi.edu DNS DB rpi.edu DNS DB rpi.edu DNS DB rpi.edu DNS DB Authoritative Replicas

47. DNS Clients • A DNS client is called a resolver. • A call to gethostbyname()is handled by a resolver (typically part of the client). • Most Unix workstations have the file /etc/resolv.conf that contains the local domain and the addresses of DNS servers for that domain.

48. /etc/resolv.conf domain rpi.edu 128.113.1.5 128.113.1.3

49. nslookup • nslookup is an interactive resolver that allows the user to communicate directly with a DNS server. • nslookup is usually available on Unix workstations. (dig and host are also DNS clients).

50. DNS Servers • Servers handle requests for their domain directly. • Servers handle requests for other domains by contacting remote DNS server(s). • Servers cache external mappings.