DNS (Domain Name Service)

Transcription

1 mywbut.cm DNS (Dmain Name Service) The internet primarily uses IP addresses fr lcating ndes. Hwever, its humanly nt pssible fr us t keep track f the many imprtant ndes as numbers. Alphabetical names as we see wuld be mre cnvenient t remember than the numbers as we are mre familiar with wrds. Hence, in the chatic rganizatin f numbers (IP addresses) we wuld be much relieved if we can use familiar sunding names fr ndes n the netwrk. There is als anther mtivatin fr DNS. All the related infrmatin abut a particular netwrk (generally maintained by an rganizatin, firm r university) shuld be available at ne place. The rganizatin shuld have cmplete cntrl ver what it includes in its netwrk and hw des it "rganize" its netwrk. Meanwhile, all this infrmatin shuld be available transparently t the utside wrld. Cnceptually, the internet is divide int several hundred tp level dmains where each dmain cvers many hsts. Each dmain is partitined in subdmains which may be further partitined int subsubdmains and s n... S the dmain space is partitined in a tree like structure as shwn belw. It shuld be nted that this tree hierarchy has nthing in cmmn with the IP address hierarchy r rganizatin. The internet uses a hierarchical tree structure f Dmain Name Servers fr IP address reslutin f a hst name. 1

2 mywbut.cm The tp level dmains are either generic r names f cuntries. eg f generic tp level dmains are.edu.mil.gv.rg.net.cm.int etc. Fr cuntries we have ne entry fr each cuntry as defined in ISO3166. eg..in (India).uk (United Kingdm). The leaf ndes f this tree are target machines. Obviusly we wuld have t ensure that the names in a rw in a subdmain are unique. The max length f any name between tw dts can be 63 characters. The abslute address shuld nt be mre than 255 characters. Dmain names are case insensitive. Als in a name nly letters, digits and hyphen are allwed. Fr eg. is a dmain name crrespnding t a machine named www under the subsubdmain iitk.ac.in. Resurce Recrds: Every dmain whether it is a single hst r a tp level dmain can have a set f resurce recrds assciated with it. Whenever a reslver (this will be explained later) gives the dmain name t DNS it gets the resurce recrd assciated with it. S DNS can be lked upn as a service which maps dmain names t resurce recrds. Each resurce recrd has five fields and lks as belw: 2

3 mywbut.cm Dmain Name Class Type Time t Live Value Dmain name: the dmain t which this recrd applies. Class: set t IN fr internet infrmatin. Fr ther infrmatin ther cdes may be specified. Type: tells what kind f recrd it is. Time t live: Upper Limit n the time t reach the destinatin Value: can be an IP address, a string r a number depending n the recrd type Resurce Recrd A Resurce Recrd (RR) has the fllwing: wner which is the dmain name where the RR is fund. type which is an encded 16 bit value that specifies the type f the resurce in this resurce recrd. It can be ne f the fllwing: A a hst address CNAME identifies the cannical name f an alias HINFO identifies the CPU and OS used by a hst MX identifies a mail exchange fr the dmain. NS the authritative name server fr the dmain PTR a pinter t anther part f the dmain name space SOA identifies the start f a zne f authrity class which is an encded 16 bit value which identifies a prtcl family r instance f a prtcl. class One f: IN the Internet system r CH the Chas system TTL which is the time t live f the RR. This field is a 32 bit integer in units f secnds, an is primarily used by reslvers when they cache RRs. The TTL describes hw lng a RR can be cached befre it shuld be discarded. RDATA Data in this field depends n the values f the type and class f the RR and a descriptin fr each is as fllws: fr A: Fr the IN class, a 32 bit IP address Fr the CH class, a dmain name fllwed by a 16 bit ctal Chas address. fr CNAME: a dmain name. fr MX: a 16 bit preference value (lwer is better) fllwed by a hst name willing t act as a mail exchange fr the wner dmain. fr NS: a hst name. fr PTR: a dmain name. fr SOA: several fields. Nte: While shrt TTLs can be used t minimize caching, and a zer TTL prhibits caching, the realities f Internet perfrmance suggest that these times shuld be n the rder f days fr the typical hst. If a change can be anticipated, the TTL can be reduced prir t the change t minimize incnsistency during the change, and then increased back t its frmer value fllwing the change. The data in the RDATA sectin f RRs is carried as a cmbinatin f binary strings and dmain names. The dmain names are frequently used as "pinters" t ther data in the DNS. 3

4 mywbut.cm Aliases and Cannnical Names Sme servers typically have multiple names fr cnvenience. Fr example & yamuna.iitk.ernet.in identify the same server. In additin multiple mailbxes might be prvided by sme rganizatins. Mst f these systems have a ntin that ne f the equivalent set f names is the cannical r primary name and all thers are aliases. When a name server fails t find a desired RR in the resurce set assciated with the dmain name, it checks t see if the resurce set cnsists f a CNAME recrd with a matching class. If s, the name server includes the CNAME recrd in the respnse and restarts the query at the dmain name specified in the data field f the CNAME recrd. Name Servers Name servers are the repsitries f infrmatin that make up the dmain database. The database is divided up int sectins called znes, which are distributed amng the name servers. Name servers can answer queries in a simple manner; the respnse can always be generated using nly lcal data, and either cntains the answer t the questin r a referral t ther name servers "clser" t the desired infrmatin. The way that the name server answers the query depends upn whether it is perating in recursive mde r iterative mde: The simplest mde fr the server is nn-recursive, since it can answer queries using nly lcal infrmatin: the respnse cntains an errr, the answer, r a referral t sme ther server "clser" t the answer. All name servers must implement nn-recursive queries. The simplest mde fr the client is recursive, since in this mde the name server acts in the rle f a reslver and returns either an errr r the answer, but never referrals. This service is ptinal in a name server, and the name server may als chse t restrict the clients which can use recursive mde. Recursive Query vs Iterative Query If the server is suppsed t answer a recursive quesry then the respnse is either the reurce recrd data r a errr cde. A server perating in this mde will never return the name f any frwarding name server but will cntact the apprpiate name server itself and try t get the infrmatin. In iterative mde, n the ther hand, if the server des nt have the infrmatin requested lcally then it return the address f sme name server wh might have the infrmatin abut the query. It is then the respnsibility f the cntacting applicatin t cntact the next name server t reslve its query and d this iteratively until gets an answer r and errr. Relative Names In place f giving full DNS names like cu2.cse.iitk.ac.in r bhaskar.cc.iitk.ac.in ne can give just cu2 r bhaskar.this can be used by the server side as well as the client side.but fr this ne has t manually specify these extensins in the database f the servers hlding the resurce recrds. 4

5 mywbut.cm BOOTP The BOOTP uses UDP/IP. It is run when the machine bts. The prtcl allws diskless machines t discver their IP address and the address f the server hst. Additinally name f the file t be laded frm memry and executed is als supplied t the machine. This prtcl is an imprvement ver RARP which has the fllwing limitatins: 1. Netwrks which d nt have a bradcast methd can't supprt RARP as it uses the bradcast methd f the MAC layer underneath the IP layer. 2. RARP is heavily dependent n the MAC prtcl. 3. RARP just supplies the IP address crrespnding t a MAC address It desn't supprt respnd with any mre data. 4. RARP uses the cmputer hardware's address t identify the machine and hence cannt be used in netwrks that dynamically assign hardware addresses. Events in BOOTP 1. The Client bradcasts its MAC address (r ther unique hardware identity number) asking fr help in bting. 2. The BOOTP Server respnds with the data that specifies hw the Client shuld be cnfigured (pre-cnfigured fr the specific client) Nte: BOOTP desn't use the MAC layer bradcast but uses UDP/IP. Cnfiguratin Infrmatin The imprtant infrmatins prvided are: IP address IP address f the default ruter fr that particular subnet Subnet mask IP addresses f the primary and secndary nameservers Additinaly it may als prvide: Time ffset frm GMT The IP address f a time server The IP address f a bt server The name f a bt file (e.g. bt image fr X terminals) The IP dmain name fr the client But the prblem with BOOTP is that it again can't be used fr the dynamic IP's as in RARP servers.fr getting dynamic IP's we use DHCP. DHCP (Dynamic Hst Cnfiguratin Prtcl) 5

6 mywbut.cm DHCP (Dynamic Hst Cnfiguratin Prtcl) is a prtcl that lets netwrk administratrs manage centrally and autmate the assignment f Internet Prtcl (IP) addresses in an rganizatin's netwrk. If a machine uses Internet's set f prtcl (TCP/IP), each machine that can cnnect t the Internet needs a unique IP address. When an rganizatin sets up its cmputer users with a cnnectin t the Internet, an IP address must be assigned t each machine. Withut DHCP, the IP address must be entered manually at each cmputer and, if cmputers mve t anther lcatin in anther part f the netwrk, a new IP address must be entered. DHCP lets a netwrk administratr supervise and distribute IP addresses frm a central pint and autmatically sends a new IP address when a cmputer is plugged int a different place in the netwrk. IP Address Allcatin Mechanism DHCP supprts three mechanisms fr IP address allcatin. Autmatic allcatin: DHCP assigns a permanent IP address t a hst. Dynamic allcatin: DHCP assigns an IP address t a hst fr a limited perid f time (r until the hst explicitly relinquishes the address). Manual allcatin: Hst's IP address is assigned by the netwrk administratr, and DHCP is used simply t cnvey the assigned address t the hst. A particular netwrk will use ne r mre f these mechanisms, depending n the plicies f the netwrk administratr. Messages Used by DHCP DHCP Discver - Client bradcast t lcate available servers. It is assumed atleast ne f the servers will have resurces t fulfill the request.( may include additinal pinters t specific services required eg. particular subnet, minimum time limit etc ). DHCP Offer - Server t client in respnse t DHCP Discver with ffer f cnfigratin parameters. DHCP Request - Client bradcast t servers requesting ffered parameters frm ne server and implicitly declining ffers frm all thers.( als imprtant in case f lease renewal if the allted time is abut t expire ). DHCP Decline - Client t server indicating cnfigratin parameters invalid. DHCP Release - Client t server relinquishing netwrk address and cancelling current lease.( in case f a graceful shut dwn DHCP server is sent a DHCP Release by the hst machine). DHCP Ack - Server t client with cnfigratin parameters, including cmmitted Netwrk address. DHCP Nack - Server t client refusing request fr cnfigratin parameters (eg. requested netwrk address already allcated). Timers Used Nte that lease time is the time specified by the server fr which the services have been prvided t the client. Lease Renewal Timer - When this timer expires machine will ask the server fr mre time sending a DHCP Request. 6

7 mywbut.cm Lease Rebinding Timer - Whenever this timer expires, we have nt been receiving any respnse frm the server and s we can assume the server is dwn. Thus send a DHCP Request t all the servers using IP Bradcast facility. This is nly pint f difference between Lease renewal and rebinding. Lease Expiry Timer - Whenever this timer expires, the system will have t start crashing as the hst des nt have a valid IP address in the netwrk. Timer Cnfiguratin Plicy The timers have this usual setting which can be cnfigured depending upn the usage pattern f the netwrk. An example setting has been discussed belw. Lease Renewal = 50 % Lease time Lease Rebinding = 87.5 % Lease time Lease Expiry = 100 % Lease time 7