Lecture on DHCP DNS SNMP Debashis Saha

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

Lecture on DHCP, DNS & SNMP Debashis Saha 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? Thursday, May 17, 2007 2

Lecture on DHCP, DNS & SNMP Debashis Saha The Internet and Addressing • All machines on the Internet are accessed via their IP address – Dotted quad: 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 Thursday, May 17, 2007 3

Lecture on DHCP, DNS & SNMP Debashis Saha 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 Thursday, May 17, 2007 4

Lecture on DHCP, DNS & SNMP Debashis Saha DHCP – Dynamic Host Configuration Protocol Thursday, May 17, 2007 5

Lecture on DHCP, DNS & SNMP Debashis Saha 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 Thursday, May 17, 2007 6

Lecture on DHCP, DNS & SNMP Debashis Saha 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. Thursday, May 17, 2007 7

Lecture on DHCP, DNS & SNMP Debashis Saha 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 Thursday, May 17, 2007 8

Lecture on DHCP, DNS & SNMP Debashis Saha 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. Thursday, May 17, 2007 9

Lecture on DHCP, DNS & SNMP Debashis Saha 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 preassigned IP addresses and hosts that do not participate in DHCP Thursday, May 17, 2007 10

Lecture on DHCP, DNS & SNMP Debashis Saha 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 Thursday, May 17, 2007 11

Lecture on DHCP, DNS & SNMP Debashis Saha 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 Thursday, May 17, 2007 12

Lecture on DHCP, DNS & SNMP Debashis Saha 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 Thursday, May 17, 2007 13

Lecture on DHCP, DNS & SNMP Debashis Saha 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 Thursday, May 17, 2007 14

Lecture on DHCP, DNS & SNMP Debashis Saha 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) to figure out the IP address of any DHCP servers • Most routers are configured to pass this request within the campus or enterprise Thursday, May 17, 2007 15

Lecture on DHCP, DNS & SNMP Debashis Saha 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 Thursday, May 17, 2007 16

Lecture on DHCP, DNS & SNMP Debashis Saha 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)! Thursday, May 17, 2007 17

Lecture on DHCP, DNS & SNMP Debashis Saha 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) Thursday, May 17, 2007 18

Lecture on DHCP, DNS & SNMP Debashis Saha 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 Thursday, May 17, 2007 19

Lecture on DHCP, DNS & SNMP Debashis Saha DHCP: Big Picture Thursday, May 17, 2007 20

Lecture on DHCP, DNS & SNMP Debashis Saha 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 Thursday, May 17, 2007 21

Lecture on DHCP, DNS & SNMP Debashis Saha 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 Thursday, May 17, 2007 22

Lecture on DHCP, DNS & SNMP Debashis Saha 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) Thursday, May 17, 2007 23

Lecture on DHCP, DNS & SNMP Debashis Saha 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 Thursday, May 17, 2007 24

Lecture on DHCP, DNS & SNMP Debashis Saha 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 Thursday, May 17, 2007 25

Lecture on DHCP, DNS & SNMP Debashis Saha DHCP Resources • Compilation of DHCP-related WWW links and other information: http: //www. dhcp. org • DHCP FAQ (maintained by John Wobus) • dhcp-v 4@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 Thursday, May 17, 2007 26

Lecture on DHCP, DNS & SNMP Debashis Saha DNS – Domain Name System Thursday, May 17, 2007 27

Lecture on DHCP, DNS & SNMP Debashis Saha 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? Thursday, May 17, 2007 28

Lecture on DHCP, DNS & SNMP Debashis Saha 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? Thursday, May 17, 2007 29

Lecture on DHCP, DNS & SNMP Debashis Saha 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? ) Thursday, May 17, 2007 30

Lecture on DHCP, DNS & SNMP Debashis Saha DNS: Basics • Hierarchical namespace • Distributed system – very few core servers • Stores other information than simple hostname <-> IP mappings • Request/response protocol Thursday, May 17, 2007 31

Lecture on DHCP, DNS & SNMP Debashis Saha DNS Hierarchy edu rpi albany com org in ac iimcal Thursday, May 17, 2007 32

Lecture on DHCP, DNS & SNMP Debashis Saha 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 Thursday, May 17, 2007 33

Lecture on DHCP, DNS & SNMP Debashis Saha 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. Thursday, May 17, 2007 34

Lecture on DHCP, DNS & SNMP Debashis Saha Hierarchical Naming arpa com edu gov mil net org us colordao cs foobar. cs. colorado. edu foobar Thursday, May 17, 2007 35

Lecture on DHCP, DNS & SNMP Debashis Saha 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 Thursday, May 17, 2007 36

Lecture on DHCP, DNS & SNMP Debashis Saha Domain Naming System edu harvard …mit hbs com gov mil org net uk fr cisco…yahoonasa…nsf arpa…navy acm…ieee ee physics www Thursday, May 17, 2007 37

Lecture on DHCP, DNS & SNMP Debashis Saha 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 subdomaintain (or outsource) their own DNS servers containing the correct information Thursday, May 17, 2007 38

Lecture on DHCP, DNS & SNMP Debashis Saha Name Servers • Partition hierarchy into zones edu princeton … mit cs ee ux 01 ux 04 com gov cisco … yahoo nasa … nsf mil org arpa … navy acm … ieee uk fr physics Root name server Each zone implements two or more name servers … Princeton name server 1. Primary … CS EE 2. secondary name server Thursday, May 17, 2007 net Cisco name server 39

Lecture on DHCP, DNS & SNMP Debashis Saha Hierarchical Administration - “Zones” arpa com in gov mil net org us ac iimcal www. iimcal. ac. in www Thursday, May 17, 2007 40

Lecture on DHCP, DNS & SNMP Debashis Saha 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. Thursday, May 17, 2007 41

Lecture on DHCP, DNS & SNMP Debashis Saha 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” Thursday, May 17, 2007 42

Lecture on DHCP, DNS & SNMP Debashis Saha 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” Thursday, May 17, 2007 43

Lecture on DHCP, DNS & SNMP Debashis Saha 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 Thursday, May 17, 2007 44

Lecture on DHCP, DNS & SNMP Debashis Saha 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. Thursday, May 17, 2007 45

Lecture on DHCP, DNS & SNMP Debashis Saha 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 Authoritative Thursday, May 17, 2007 rpi. edu DNS DB Replicas 46

Lecture on DHCP, DNS & SNMP Debashis Saha 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. Thursday, May 17, 2007 47

Lecture on DHCP, DNS & SNMP Debashis Saha /etc/resolv. conf domain rpi. edu 128. 113. 1. 5 128. 113. 1. 3 Thursday, May 17, 2007 48

Lecture on DHCP, DNS & SNMP Debashis Saha 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). Thursday, May 17, 2007 49

Lecture on DHCP, DNS & SNMP Debashis Saha 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. Thursday, May 17, 2007 50

Lecture on DHCP, DNS & SNMP Debashis Saha Server - Server Communication • If a server is asked to provide the mapping for a host outside it’s domain (and the mapping is not in the server cache): – The server finds a nameserver for the target domain. – The server asks the nameserver to provide the host name to IP translation. • To find the right nameserver, use DNS! Thursday, May 17, 2007 51

Lecture on DHCP, DNS & SNMP Debashis Saha DNS Data • DNS databases contain more than just hostname-to-address records: – – Name server records Hostname aliases Mail Exchangers Host Information Thursday, May 17, 2007 NS CNAME MX HINFO 52

Lecture on DHCP, DNS & SNMP Debashis Saha The Root DNS Server • The root server needs to know the address of 1 st (and many 2 nd) level domain nameservers. edu rpi Thursday, May 17, 2007 com org jp albany 53

Lecture on DHCP, DNS & SNMP Debashis Saha Server Operation • If a server has no clue about where to find the address for a hostname, ask the root server. • The root server will tell you what nameserver to contact. • A request may get forwarded a few times. Thursday, May 17, 2007 54

Lecture on DHCP, DNS & SNMP Debashis Saha DNS Message Format Thursday, May 17, 2007 55

Lecture on DHCP, DNS & SNMP Debashis Saha 16 bit fields DNS Message Header • • • Thursday, May 17, 2007 query identifier flags # of questions # of RRs # of authority RRs # of additional RRs } Response 56

Lecture on DHCP, DNS & SNMP Debashis Saha Message Flags • • • QR: Query=0, Response=1 AA: Authoritative Answer TC: response truncated (> 512 bytes) RD: recursion desired RA: recursion available rcode: return code Thursday, May 17, 2007 57

Lecture on DHCP, DNS & SNMP Debashis Saha Recursion • A request can indicate that recursion is desired - this tells the server to find out the answer (possibly by contacting other servers). • If recursion is not requested - the response may be a list of other name servers to contact. Thursday, May 17, 2007 58

Lecture on DHCP, DNS & SNMP Debashis Saha Question Format • Name: domain name (or IP address) • Query type (A, NS, MX, …) • Query class (1 for IP) Thursday, May 17, 2007 59

Lecture on DHCP, DNS & SNMP Debashis Saha Response Resource Record • • • Domain Name Response type Class (IP) Time to live (in seconds) Length of resource data Resource data Thursday, May 17, 2007 60

Lecture on DHCP, DNS & SNMP Debashis Saha UDP & TCP • Both UDP and TCP are used: – TCP for transfers of entire database to secondary servers (replication). – UDP for lookups – If more than 512 bytes in response - requestor resubmits request using TCP. Thursday, May 17, 2007 61

Lecture on DHCP, DNS & SNMP Debashis Saha Lots more • This is not a complete description ! • If interested - look at: – – RFC 1034: DNS concepts and facilities. RFC 1035: DNS implementation and protocol specification. play with nslookup. Look at code for BIND (DNS server code). Thursday, May 17, 2007 62

Lecture on DHCP, DNS & SNMP Debashis Saha References • RFC 1034 • RFC 1035 Thursday, May 17, 2007 75

Lecture on DHCP, DNS & SNMP Debashis Saha SNMP - Simple Network Management Protocol Thursday, May 17, 2007 76

Lecture on DHCP, DNS & SNMP Debashis Saha Third 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? Thursday, May 17, 2007 77

Lecture on DHCP, DNS & SNMP Debashis Saha SNMP is … • A network management tool – Designed to allow data acquisition from SNMP capable network equipment – Allows control of SNMP capable equipment by the distant controller • Based on inquiry/response architecture with simple response parameters Thursday, May 17, 2007 78

Lecture on DHCP, DNS & SNMP Debashis Saha SNMP is … • Used for management of fault conditions – prevention of and response to network failure • Designed to manage performance – coping with deterioration of performance due to congestion, changing traffic conditions Thursday, May 17, 2007 79

Lecture on DHCP, DNS & SNMP Debashis Saha SNMP Message Protocol • Messages use UDP, ports 161 (requests/responses) and 162 (notifications) • Message types: – Get. Request: request values of variables from device – Get. Next. Request: request value of variable following the one supplied – Get. Response: return values – Set. Request: instruct device to set values of variables – Trap: from device - notify monitor / manager of value change Thursday, May 17, 2007 80

Lecture on DHCP, DNS & SNMP Debashis Saha To know more on SNMP • “Essential SNMP”, Douglas R Mauro & Kevin J Schmidt, Shroff Publishers & Distributors, Calcutta, 2001 Thursday, May 17, 2007 81