by telnet E.g., the machine fraser has IP address You can login to fraser from anywhere in the world

Transcription

1 Naming and Look-Up Service Host addresses and Port numbers A client requiring a service needs to lote a server that will provide the desired service. In JavaRMI discussed in 3.3, rmiregistry provided a limited form of name service. First of all, we need a unique network address bound to each host (machine). IP addresses: Used in the Internet and are universally accepted. 32 bits long (compact). Dotted decimal notation: each of the four parts in an IP address is the decimal representation of a one-byte bit pattern. Theoretilly, 2 32 ( ) addresses are possible under this address scheme, but only a small fraction is usable. So, giving unique transport addresses to all servers is not difficult: use a two-part structure, e.g., (host address,port#), where port# indites the port on which the server is listening. On any machine attached to the Internet, host address is its IP address. 1 See Unix gethostbyname(3n). The port numbers are reserved for system-provided services, which are the same regardless of the host. Listed below are some well-known addresses (see /etc/services in a Unix machine.): A user n pick an arbitrary 16-bit port numbers ( 1024) to name a new server (accessed by RPC). In Unix, you n then register the service by lling the portmapper which listens on the well-kown port 111. (Its role is similar to that of rmiregistry, which listens on port 1099 by default.) Clients n ask the portmapper for the port number of a particular server by presenting its program number. The addresses of system-provided services on a remote host (that is known to the lol host) n be obtained by consulting the two databases, /etc/hosts for the (host name, IP address) pairs and /etc/services for the port numbers of different available services. 1 E.g., the machine fraser has IP address You n login to fraser from anywhere in the world by telnet

2 CMPT401 Chapter 3, Summer Name Port # Description rjd 5 Remote job entry echo 7 Echo users 11 List active users daytme 13 Day and time ftp-data 20 File transfer (data) ftp 21 File transfer (control) telnet 23 Virtual terminal smtp 25 Simple Mail Transfer time 37 Time service name 42 Host name server whois 43 Find nickname domain 53 DNS server bootps 67 Bootstrap protocol server bootpc 68 Bootstrap protocol client tftp 69 Trivial (or simplified) ftp finger 79 Information service about users http 80 HTTP server hostnames 101 NIC host name server ISO-tsap 102 ISO TSAP X X400 name server X400SND 104 X400.SND pop2 109 Post office protocol 2 pop3 110 Post office protocol 3 portmapper 111 Sun RPC portmapper netbios-ns 137 NETBIOS name service netbios-dg 138 NETBIOS datagram service netbios-ss 139 NETBIOS session service login 513 Unix login service shell 514 Unix shell talk 517 Unix talk service Common Interner Port Numbers Name service Requiring users to use numeric addresses of the servers creates some problems. Besides being difficult to remember, if the host IP address of a server machine is bound into the client program at compile time, replacing the server (e.g., due to repair work) by another machine necessitates the recompilation of every client program that lls it. A name server was introduced to solve this problem. Like rmiregistry, a name server accepts registration of dynamilly created service names and their addresses and helps clients in looking up servers which provide the desired service. The RMI registry requires at least one registry server in each server machine, which may not be practil. In ARPANET, the predecessor to the Internet, flat (i.e., non-hierarchil) naming was used, and a file hosts.txt was maintained at the NIC (Network Information Centre) and all hosts would

3 CMPT401 Chapter 3, Summer fetch it every night. For a network of a few hundred hosts, this was satisfactory (up to 1983), but clearly, this didn t sle well. 2 Definitions Name: Human-friendly denotation, e.g., file and service names. Address: where an object n be found, e.g., IP address. Many names (e.g., aliases) n have the same address, and the same name n have multiple addresses. 3 Name resolution: Name address mapping, e.g., telnet port 23, fraser Address resolution: Address route mapping DNS - The Internet Domain Name System In this subsection, we consider the Internet DNS, which, unlike RMI Registry, n work in large-sle distributed systems. It has the following features: 1. User friendly structured textual names (instead of numbers). 2. Distributed database (of name-mapping) under lol control. Hierarchil names prevents name collision. 3. Changes to the database require little overhead. 4. Slable, robust (replited service) and adequate performance (ching) Internet domain names (e.g., orion.csil.sfu.). Hierarchil and user-friendly. Similar to postal addresses or Unix paths. Each node in the hierarchy represents a domain, consisting of the subtree rooted at the node. 4 Component names 63 characters, the total length 255 characters. The top-level domains: several hundred domains, including com, edu, gov, org, mil, net, int (certain international organizations), as well as abbreviated country names, e.g.,, hk, jp, us. Since.com beme very crowded, recently, new top-level domains, biz, name, info, museum, etc. have been added. To obtain a domain under one of the top-level domains, an applition must be made to the NIC. However, creating subdomains under such a granted domain requires no additional permission. This is similar to IP address assignment to an organization by NIC. Within the assigned network address space, the organization is free to create subnetworks or assign IP addresses to hosts. However, subnets and subdomains are not related: Creating one does not imply creating the other. 2 In 1986, the number of hosts attached to the Internet was 3,100, and in 1995, it was about 4 million. 3 A router, for example, has an IP address in each network it belongs to. 4 A leaf domain is the name of some object, e.g., a host.

4 CMPT401 Chapter 3, Summer au edu com... sfu ubc ec uoft cs English library wormhole css fornax Figure 7: DNS hierarchy. Currently (as of 2000) there are 13 root servers, i.e., name servers for the root zone. 5 NAME Lotion Organization A.ROOT-SERVERS.NET internic.net InterNIC B.ROOT-SERVERS.NET isi.edu Information Science Institute (USC) C.ROOT-SERVERS.NET psi.net PSINet D.ROOT-SERVERS.NET umd.edu University of Maryland E.ROOT-SERVERS.NET nasa.gov NASA F.ROOT-SERVERS.NET isc.org Internet Software Consortium G.ROOT-SERVERS.NET nic.mil NIC, Dept. of Defence H.ROOT-SERVERS.NET arl.mil Army Research Laboratory I.ROOT-SERVERS.NET sunet.se Swedish University Network J.ROOT-SERVERS.NET nsi InterNIC(tmp) K.ROOT-SERVERS.NET lynx.net RIPE NCC (LYNX INTERNET & MARKETING) L.ROOT-SERVERS.NET ep.net ISI (IANA) M.ROOT-SERVERS.NET wide.ad.jp WIDE To traverse the domain hierarchy, it is convenient to keep information about its child domains at each node. If a child is a domain consisting of a single host, then this information would be (host name, IP address) pair for the host. If the child is a subdomain, then this information would be (name server, IP address) pair for the subdomain. One major advantage of hierarchil naming is that the management (e.g., cre- 5 A zone will be defined later. As seen from Figure 8, a NS may provide mapping for more than one domain. For example, the root server is shown to provide name service not only for the root of the hierarchy, but also for all top-level domains in US, e.g., edu and com, as well. The root servers on average receive several queries per second. The busiest one receives as many as 1000 queries a second.

5 CMPT401 Chapter 3, Summer root_ns au edu com... sfu ubc ec uoft _ns sfu_ns = whistler cs English library cmpt_ns = fornax wormhole css fornax IP = d Figure 8: Name servers for zones. ation/deletion of subdomains) n be delegated to lol authorities, thus avoiding bottlenecks. Distributed NS The main problem is how to provide name service (mapping between hierarchil host names and IP addresses) to remote hosts, which is robust and without performance bottlenecks and sling problem. We will talk about the database required and the searching procedure. It is based on client-server paradigm, where the client is a lol procedure lled the resolver or a name server (NS) and the server is another NS. Database It is reasonable to assume that the NS for any domain corresponding to a LAN maintains a list of lol hosts and their IP addresses, and that each host knows about the NS for the lol domain. If, for reliability reasons, there is more than one NS in a lol domain, then all their addresses must be known to each host, so that they n be contacted one by one, in se some of them have failed. A more common solution in lol area networks, however, is for a machine to broadst a query message asking for the identity of the current name server(s), on start-up and whenever it nnot communite with the name server it knows about. The first one to respond n be used, since it is probably the least loaded. In general, a NS for a node needs to maintain a database containing: 1. All lol names (of hosts and NSs for subdomains) and their IP addresses. 2. Info on looking up some remote names. A zone is a contiguous subset of a domain, consisting of a set of subdomains adjacent in the domain tree. Each zone has one primary NS and one or more secondary NSs (for reliability and load-sharing) which copy the entire database periodilly (e.g., once a day) from the primary server. The resource records (see below) for the zone maintained by both the primary and secondary NSs are authoritative. The domain administrator enters data into the primary server s database, which percolates to the secondary by periodil downloading.

6 CMPT401 Chapter 3, Summer The same NS may look after name service for more than one zone. For simplicity, we assume that a zone consists of exactly one domain, and each domain has its own NS. Resource records DNS maintains a database of resource records (RRs) of the form Domain name Time to live Class Type Value wormhole.cs.sfu IN A Time to live is specified in seconds (e.g., = 1 day), and indites how long the ched information may be valid. 6 There are about 20 different types of RRs, including: Type Value A 32-bit host IP address MX Preference (integer), host willing to accept for the domain NS Name of a NS for this domain HINFO Host info: CPU and OS in ASCII CNAME Official name for alias Domain name (Canonil name) SOA Start of data for the zone (Start of Authority) TXT Uninterpreted ASCII text An SOA-type record marks the start of data for a zone, and contains the name of the source of information about this zone, the address of the zone administrator, and parameters specifying, e.g., how often secondary NSs should refresh their copies from the primary NS. wormhole.cs.sfu IN MX fornax.cs.sfu IN CNAME adhara.cs.sfu IN NS A.ROOT-SERVERS.NET. (Note the root domain. ) A.ROOT-SERVERS.NET A for example, is an alias, which n be used without designating a specific machine; if the current WWW server fails, another n be assigned by updating the database, without invalidating the address people use to access it. The nslookup command n be used to access information in this database. wormhole% nslookup /* Command that I type. Default Server: fornax.cs.sfu. /* These two lines are response. Address: > set type=any /* To indite that I am interested in all available info at the NS. 6 If this field is empty, it defaults to the value specified in another part of the database. If it is 0, the RR should not be ched.

7 CMPT401 Chapter 3, Summer > wormhole /* I want to get info on wormhole Server: fornax.cs.sfu. Address: wormhole.cs.sfu. internet address = wormhole.cs.sfu. preference = 10, mail exchanger = cs.sfu. cs.sfu. nameserver = fornax.cs.sfu. cs.sfu. nameserver = whistler.sfu. cs.sfu. nameserver = seymour.sfu. cs.sfu. internet address = fornax.cs.sfu. internet address = whistler.sfu. internet address = seymour.sfu. internet address = > server whistler.sfu. /* I want to talk to whistler. Server: whistler.sfu. Address: > whistler.sfu. /* Tell me about yourself. Server: whistler.sfu. Address: whistler.sfu. preference = 10, mail exchanger = rm-rstar.sfu. whistler.sfu. preference = 20, mail exchanger = wedge.sfu. whistler.sfu. internet address = sfu. nameserver = whistler.sfu. sfu. nameserver = seymour.sfu. sfu. nameserver = cheddar.ucs.ubc. rm-rstar.sfu. internet address = wedge.sfu. internet address = whistler.sfu. internet address = seymour.sfu. internet address = cheddar.ucs.ubc. internet address = > root /* I want to talk to a root Default Server: a.root-servers.net Address: >. /* What do you know about the domain.? Server: a.root-servers.net Address: Non-authoritative answer:

8 CMPT401 Chapter 3, Summer nameserver = CLOUSO.RISQ.QC. nameserver = NS2.UUNET. nameserver = RS0.INTERNIC.NET nameserver = DNS2.UTCC.UTORONTO. nameserver = RELAY.CDNNET. Authoritative answers n be found from: nameserver = CLOUSO.RISQ.QC. nameserver = NS2.UUNET. nameserver = RS0.INTERNIC.NET nameserver = DNS2.UTCC.UTORONTO. nameserver = RELAY.CDNNET. CLOUSO.RISQ.QC. internet address = NS2.UUNET. internet address = RS0.INTERNIC.NET internet address = DNS2.UTCC.UTORONTO. internet address = RELAY.CDNNET. internet address = >exit Notes: 1. Each host has the IP addresses of the lol NSs of the domain it belongs to in a system config file (/etc/resolv.conf), and 2. Each NS has the addresses of the root servers (for the top-level US domains), whose IP addresses are in a system config file, and are loaded into the DNS che when it is started and is never purged (its Time to Live is ignored). 3. A domain name (e.g., may be mapped to more than one computer for load sharing. In this se, there is one RR of type A for each such computer. To assign load equally among the servers, the NS normally returns those addresses to successive queries on a round-robin basis. This effort is somewhat spoiled if server addresses are ched. Domain name resolution The resolver is a library of software procedure, that is linked into a program which requires name service. It sends a name-resolution query by the client to a lol NS. Iterative: If a NS nnot satisfy the query, it returns the IP address of another NS which may be able to resolve the name to the requesting NS. Recursive: Each NS forwards the query and passes the reply back. The lol NS is off-loaded. Not all NSs n handle queries recursively. In particular, the root servers do not provide this service.

9 CMPT401 Chapter 3, Summer Client's name resolver 1. <nl,vu,cs,ftp> 2. #<nl>, <vu,cs,ftp> 3. <vu,cs,ftp> 4. #<vu>, <cs,ftp> 5. <cs,ftp> 6. #<cs>, <ftp> 7. <ftp> 8. #<ftp> Root name server Name server nl node Name server vu node Name server cs node cs vu nl <nl,vu,cs,ftp> #<nl,vu,cs,ftp> Nodes are managed by the same server ftp Figure 9: Iterative name resolution. Fig. 9 shows an example of iterative name resolution. The resolver and NSs may che the addresses of remote NSs that they have dealt with recently. 1. address resolution for user@host.domain: The source host invokes the name resolver, which asks the NS for domain to send all MX records for the destination machine host. The source host tries to send the mail to the mail exchangers, one at a time, in the preference order, until it succeeds. This mechanism allows the administrator of the.com domain to add the following MX record to its database: tikodomain.com IN MX 1 mailserver.cs.sfu. in order to have all mail to tikodomain.com delivered to mailserver.cs.sfu.. 2. Non- access (e.g., telnet, ping): The source host invokes the name resolver, which asks the address record (type A) of host, which in this se must be a machine. 3. An important consequence of multiple types: It is entirely possible that the NS stores an A-record for host in its database but no MX record for host. In this se, you n do ping host, but if you try to send to user@host.domain, you may be informed that the destination is unkown. Failing to obtain an MX-record, some resolvers send to the IP address of host. Or conversely, there may be an MX-record but no A-record for host, in which se you n send to it but ping host will fail. The Berkeley Internet Name Domain (BIND) is the most widely used implementation of DNS (on Unix machines). The source code is available through anonymous FTP (ftp.uu.net).