DOWNLOAD PDF RESOLVING HIGH-LEVEL MACHINE NAMES

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1 Chapter 1 : resolving host names VMware Communities DNS works fine for all of the machines in our network. Except for a handful. These machines are on a subnet. Only half of the machines on this subnet are able to fully resolve in DNS. Each name server only has information about machines in its own domain, as well as information about other name servers. The root name servers only have information on the IP addresses of the name servers of. Your own machine will defined in its configuration files a name server that is geographically close to it. The responsibilities of this name server will be to directly answer any queries about its own domain that it has information about and to answer any other queries by querying as many other name servers on the Internet as is necessary. The following sequence of lookups takes place to resolve this name into an IP address. This procedure is called host name resolution and the algorithm that performs this operation is called the resolver. The application checks certain special databases on the local machine. If it can get an answer directly from them, it proceeds no further. If it has, there is no need to ask further, since the result would be stored in a local cache. That is, whether it is a computer about which it has direct information. In this case, this would only be true if the ns were cranzgot. It queries a root name server, asking what name server is responsible for. The answer will be ucthpx. The answer will be secdns1. The answer will be pizza. The answer will be These are actually the files: These are the three and only files that specify how all applications are going to look up IP numbers; and have nothing to do with the configuration files of the name server daemon itself, even though a name server daemon might be running on the local machine. When an application needs to look up a host name, it goes through the following procedure. However, this is a detail you need not be concerned about. The configuration files that follow are taken from an actual installation. An example is given below. There can actually be three of these lines so that if one name server fails, the application can try the next in turn. The application sends to the name server a query with the host name. If the host name is unqualified, then the application, before trying the query, appends to the host name a local domain name. This just saves you having to type in the full host name for computers within your own organization. The name server proceeds with the hierarchical queries described from step 4 onward. Page 1

2 Chapter 2 : macos - Why can't I resolve machine names on my LAN to their IP address? - Ask Different I have a Win2k domain that has recently started having problems with clients connecting to network shares. I attempted to disjoin/rejoin a client and have not been able to get it reconnected. The row of switches at the bottom can be used to toggle in a machine language program. Machine code is the only language a computer can process directly without a previous transformation. Currently, programmers almost never write programs directly in machine code, because it requires attention to numerous details that a high-level language handles automatically. Furthermore it requires memorizing or looking up numerical codes for every instruction, and is extremely difficult to modify. True machine code is a stream of raw, usually binary, data. Although few programs are written in machine language, programmers often become adept at reading it through working with core dumps or debugging from the front panel. A function in hexadecimal representation of bit x86 machine code to calculate the nth Fibonacci number: Typically, one machine instruction is represented as one line of assembly code. Assemblers produce object files that can link with other object files or be loaded on their own. Most assemblers provide macros to generate common sequences of instructions. Note that in this assembly language, there is no concept of returning a value. The result having been stored in the EAX register, the RET command simply moves code processing to the code location stored on the stack usually the instruction immediately after the one that called this function and it is up to the author of the calling code to know that this function stores its result in EAX and to retrieve it from there. Compare this with the same function in C: While the input parameter n is loaded from the stack, its precise position on the stack is not specified. The C compiler calculates this based on the calling conventions of the target architecture. The assembly language version loads the input parameter from the stack into a register and in each iteration of the loop decrements the value in the register, never altering the value in the memory location on the stack. The C compiler could do the same or could update the value in the stack. Which one it chooses is an implementation decision completely hidden from the code author and one with no side effects, thanks to C language standards. The local variables a, b and c are abstractions that do not specify any specific storage location on the hardware. The C compiler decides how to actually store them for the target architecture. The return function specifies the value to return, but does not dictate how it is returned. The C compiler for any specific architecture implements a standard mechanism for returning the value. Compilers for the x86 architecture typically but not always use the EAX register to return a value, as in the assembly language example the author of the assembly language example has chosen to copy the C convention but assembly language does not require this. These abstractions make the C code compilable without modification on any architecture for which a C compiler has been written. The x86 assembly language code is specific to the x86 architecture. One method for this is Inline assembly, in which assembly code is embedded in a high-level language that supports this feature. Some of these languages also allow architecture-dependent compiler optimization directives to adjust the way a compiler uses the target processor architecture. Page 2

3 Chapter 3 : Low-level programming language - Wikipedia Any time you try to resolve a domain name, it goes to a DNS to resolve into an IP, whether it's to load a website or ping or ssh or any kind of connection by name. On my DNS, I have blog.quintoapp.com as a zone. Resolving Domain Names 13 September When we type www. To illustrate - if we use curl to send a GET request to www. But how does this resolution process actually work? The NSS is a generic facility used by services to determine which sources, and in what order, they should use to obtain name-service information. Likewise, the same NSS configuration file can be used to determine which sources to use to resolve hostnames. The left-most value is the IP it should resolve to, the next value is the hostname that should resolve to that IP address, and the rest are aliases of that hostname. For example, your host file may have the following entry: However, this is not feasible because there are hundreds of millions of IP addresses, so that file would be gigantic. There are consolidated hosts files available that are used to block dubious sites, such as fake news sites, gambling sites, and pornographic sites. You can find one such file at github. What is a domain? When we talk about a "domain", such as google. Using our previous analogy, an FQDN is similar to a full postal address. There are different parts to a postal address, each giving increasingly more granular details about the location of the destination. For example, when we read the address Main Street, Washington, Connecticut, USA from right to left, we first get information that this address is in the USA, then it becomes more specific and tells us that it is in the state of Connecticut, then the city of Washington, and finally the actual street address. Likewise, an FQDN also has this property of increasing specificity. A valid FQDN contains at least two parts â a top-level domain and a secondary domain. For example, the domain google. The specificity increases from right to left - starting with. However, since this is a fixed requirement for all domains, your browser allows you to leave out the trailing period so as to improve user experience. There are many types of nameservers - resolving nameserves, TLD nameservers and root nameservers. Resolving nameservers Normally, the only type of nameserver your machine will interact with is a resolving nameserver, which is usually provided by your Internet Service Provider ISP or another business such as Google which provides the 8. The resolving nameserver acts as an intermediary between your machine and the more "central" nameservers, and would send the requests to those nameservers on your behalf. It may have to query multiple nameservers before it gets the actual IP address. The benefits of using a resolving nameserver is that the results can be cached. For example, if there were requests for the website yahoo. All your internet traffic passes through your ISP, who provides the resolving nameserver service. However, even if you do end-to-end encryption on all your messages, your ISPs will still get information about which sites you are communicating with. For example, if you visit e-commerce shops often, your ISP can pick up on this, because they are the ones who are resolving the domain for you. One way you can prevent this data from being recorded is to use a virtual private network VPN, where you send all requests through the VPN, which then relays your messages onto the intended hosts. In this setup, they act as a proxy server. However, you must do your due diligence to ensure that the VPN can be trusted, and that they do not hold on to traffic information for too long. Root and TLD nameservers When your resolving nameserver cannot find a cached record, or it wants to refresh its cache, it will send a request to the broader DNS network, which is made up of many root servers and top-level domain TLD servers. The DNS system is hierarchical. At the top of this system are the root servers, and one layer below them are the TLD servers. For example, the TLD server for. Requests can be made to any nameservers, and if that server does not have a record of the domain, it will give you the IP of a TLD server that does. If it does not know which TLD server can resolve this domain, it will return you the IP of one of the root servers, which will be able to tell you which TLD is able to resolve it. There are currently 13 root servers, operated by 12 different organizations, and each one is mirrored. The interesting thing about these root servers is that all the mirrors have the same IP address, and so act as if they are a single machine. You can see all root servers at root-servers. TLD server to domain-level nameservers When we register a domain, we do so through a domain registrar, which is a business entity that has been accepted by ICANN to register domain names. A registrar would check with a registry to see if the domain is available and, if so, registers it for you. Page 3

4 Once your registration is complete, the registrar would submit a record to the relevant TLD server, which is run by the registry for that TLD. The record consists of one or more pointers that point the domain name to the IP address of one or more domain-level nameserver s a. The domain nameserver s has on record a zone file that maps a host to an IP address. A domain nameserver will use the zone files it holds to resolve a hostname to the actual IP address of the corresponding resource. If it cannot it, it will check with one of the root or TLD nameservers, which will return the IP address of a domain nameserver that holds the zone file for that domain. The domain nameserver will then return the actual IP address of the resource that was originally requested. Page 4

5 Chapter 4 : Domain Name System - Wikipedia Try blog.quintoapp.com to the ends of the names of the machines you're trying to reach. This uses Bonjour to look up the names instead of your router's DNS, which should work if the computer you're trying to reach is a Mac. History[ edit ] Originally, the top-level domain space was organized into three main groups: Countries, Categories, and Multiorganizations. This group consists of one domain, the Address and Routing Parameter Area. Top-level domains with three or more characters restricted generic top-level domains grtld: These domains are proposed and sponsored by private agencies or organizations that establish and enforce rules restricting the eligibility to use the TLD. Use is based on community theme concepts; these domains are managed under official ICANN accredited registrars. Two-letter domains established for countries or territories. With some historical exceptions, the code for any territory is the same as its two-letter ISO code. These domains were installed under. Countries are designated in the Domain Name System by their two-letter ISO country code ; [15] there are exceptions, however e. This group of domains is therefore commonly known as country-code top-level domains cctld. Since, countries with nonâ Latin-based scripts may apply for internationalized country code top-level domain names, which are displayed in end-user applications in their language-native script or alphabet, but use a Punycode -translated ASCII domain name in the Domain Name System. Generic top-level domains formerly Categories initially consisted of gov, edu, com, mil, org, and net. More generic TLDs have been added, such as info. Internationalized country code TLDs[ edit ] An internationalized country code top-level domain IDN cctld is a top-level domain with a specially encoded domain name that is displayed in an end user application, such as a web browser, in its language-native script or alphabet, such as the Arabic alphabet, or a non-alphabetic writing system, such as Chinese characters. However, after it had been used for reverse DNS lookup, it was found impractical to retire it, and is used today exclusively for Internet infrastructure purposes such as in-addr. For historical reasons, arpa is sometimes considered to be a generic top-level domain. Reserved domains[ edit ] RFC reserves the following four top-level domain names to avoid confusion and conflict. These names can only be resolved by a Tor client because of the use of onion routing to protect the anonymity of users. NATO considered none of the then existing TLDs as adequately reflecting their status as an international organization. The nato TLD, no longer used, was finally removed in July Page 5

6 Chapter 5 : Types of Computer Languages with Their Advantages and Disadvantages Owlcation Now I can ping the Windows machine by NAME and IP ADDRESS. But I cannot Ping/Resolve ANY other machine on the remote local intranet by NAME (x, Y or Z), ONLY by IP ADDRESS. Why can't I resolve a machine name on my remote network via the VPN connection? Although the name resolution that Azure provides does not require any configuration, it is not the appropriate choice for all deployment scenarios, as seen on the preceding table. Features and considerations Features: No configuration is required to use name resolution that Azure provides. The name resolution service that Azure provides is highly available. The name resolution service that Azure provides can be used along with your own DNS servers to resolve both on-premises and Azure hostnames. Name resolution is provided between virtual machines in virtual networks without need for the FQDN. You can use hostnames that best describe your deployments rather than working with auto-generated names. The DNS suffix that Azure creates cannot be modified. You cannot manually register your own records. Hostnames must be DNS-compatible. See RFC Section 2. DNS query traffic is throttled for each virtual machine. If request throttling is observed, ensure that client-side caching is enabled. For more information, see Getting the most from name resolution that Azure provides. Getting the most from name resolution that Azure provides Client-side caching: Some DNS queries are not sent across the network. Client-side caching helps reduce latency and improve resilience to network inconsistencies by resolving recurring DNS queries from a local cache. As a result, client-side caching is suitable for most situations. Some Linux distributions do not include caching by default. Several different DNS caching packages, such as dnsmasq, are available. Here are the steps to install dnsmasq on the most common distributions: Before you use it, check its suitability for your needs and that no other cache is installed. Windows operating systems retry after one second and then again after another two, four, and another four seconds. The default Linux setup retries after five seconds. You should change this to retry five times at one-second intervals. Name resolution using your own DNS server Your name resolution needs may go beyond the features that Azure provides. For example, you might require DNS resolution between virtual networks. To cover this scenario, you can use your own DNS servers. DNS servers within a virtual network can forward DNS queries to recursive resolvers of Azure to resolve hostnames that are in the same virtual network. This functionality enables virtual machines to see both your entries in your zone files and hostnames that Azure provides via the forwarder. Access to the recursive resolvers of Azure is provided via the virtual IP DNS forwarding also enables DNS resolution between virtual networks and enables your on-premises machines to resolve hostnames that Azure provides. Because the DNS suffix is different in each virtual network, you can use conditional forwarding rules to send DNS queries to the correct virtual network for resolution. The following image shows two virtual networks and an on-premises network doing DNS resolution between virtual networks by using this method: When you use your own name resolution solution, this suffix is not supplied to virtual machines because the suffix interferes with other DNS architectures. For virtual networks that are managed by Azure Resource Manager, the suffix is available via the network interface card resource. Your DNS solution needs to: Provide appropriate hostname resolution, for example via DDNS. Provide appropriate recursive resolution to allow resolution of external domain names. Be secured against access from the Internet to mitigate threats posed by external agents. Page 6

7 Chapter 6 : DNS and Name Resolution By it's very nature, everything in /etc/hosts only works on that exact machine. If you need the entry to work on more than one machine, you need to put into in every one or use DNS anyway. If you need the entry to work on more than one machine, you need to put into in every one or use DNS anyway. Microsoft Windows clients will follow a sequence of methods in attempting to resolve a name to an address, stopping the search when it successfully matches a name to an IP address. There are two main sequences used in almost all cases: For additional information, please see the following article in the Microsoft Knowledge Base: The most common example of this is a Web browser such as Microsoft Internet Explorer. Examples of these types of applications are Outlook and Exchange. When troubleshooting name resolution issues, it is important to narrow down whether the application is resolving a NetBIOS name or a host name. In the context of this article, the term "client" does not necessarily refer to a workstation. A Windows NT server will take the role of client when it requires access to resources that require host name resolution. Host name resolution generally uses the following sequence: The client checks to see if the name queried is its own. The client then searches a local Hosts file, a list of IP address and names stored on the local computer. The Hosts file location depends on the operating system: A sample hosts file, Hosts. The Windows client will try each of these methods until it either successfully resolves the name or exhausts these methods. For additional information, please see the following articles in the Microsoft Knowledge Base: For example, if there is no hosts file on the system, then it will skip step 2 above and try a query to a DNS server. The method for changing host name resolution order differs among operating systems and versions. These are documented in the Resource Kits for the specific operating systems, as well as in the Microsoft Knowledge Base. Client is unable to resolve a host name. If a client cannot resolve a host name, then it is best to verify the Host name resolution sequence listed above that the client should be using. If the name does not exist in any of the resources that the client uses, then you must decide to which resource to add it. If the name exists in one of the resources, such as a DNS server or a Windows Internet Name Service WINS server and the client is not resolving the name correctly, focus your attention on troubleshooting that specific resource. Many applications have multiple methods that they can utilize to resolve names, this is especially true of mail and database applications. Depending on the client configuration the client may bypass host name resolution. Client resolves a name very slowly, or fails to resolve a name and takes a long time to report a failure. The client will then attempt to query a second DNS server if one is configured and take the same time to fail. There are three ways to approach this issue. If the host name is correctly entered in a host file, it will be resolved before the client attempts to query DNS. This solution works well if DNS servers are temporarily unreachable and there is a small number of host names that need to be resolved. Manually configuring Hosts files for numerous clients may be prohibitive. Even if a DNS server reports that it cannot resolve a name, this will happen much faster than if the client cannot reach a DNS server at all. The client will then bypass the DNS lookup without delay. This will usually cause a delay of just a few seconds. Page 7

8 Chapter 7 : How To Access Your Machines Using DNS Names with DD-WRT Re: resolving host names WoodyZ Apr 18, PM (in response to philswenson) The problem with taking it different places is it get different IP Address so even if you add an entry to the "hosts" file you will have to constantly edit it to update the IP Address Change. In order to access machine B from machine A using the name of machine B, machine A has to have some way to map the name of B to its IP address. There are three ways to declare machine names on A: This is a simple text file that maps names to addresses. This is the method used on the global Internet. For example, when you load this page in a browser, the first thing your computer does is to make a DNS request to know the address of unix. If your network uses one of these, you have a professional network administrator and should ask him what to do. It can be cumbersome if you have several machines, because you have to update every machine when the name of one machine changes. A hosts file is a simple list of lines mapping names to IP addresses. It looks like this: Just about every operating system that you can connect to the Internet has a similar file; Wikipedia has a list. To add an entry for B in the hosts file of A: Determine the IP address of B. The output will contain lines like this: If there are several inet addr: Edit the hosts file on A. This method assumes your computers get their IP addresses over DHCP, which is a method for computers to automatically retrieve an IP address when they connect to the network. First, you need to figure out the MAC address of B. Each network device has a unique MAC address. The name and location of the setting is completely dependent on the router model, but most have a similar set of basic settings. Most home routers are preconfigured for an address range of the form Now you need to do several things. Tell Dnsmasq to serve your host names in addition to the ones it gets from the Internet. Lines that begin with a are commented out. Tell Dnsmasq how to obtain IP addresses for names of machines on the Internet. In most common cases, everything will work out of the box. If you see a line like nameserver 8. Tell S to use the Dnsmasq service for all host name requests. Tell the other machines to use S as the DNS server. You can also use Dnsmasq as a DHCP server, so that machines can obtain the address corresponding to their name automatically. Note that there can only be a single DHCP server on a given local network the exact definition of local network is beyond the scope of this answer. You can still use any of the techniques above, except that the parts involving DHCP are only applicable within a local network. Alternatively, if your machines have public IP addresses, you can register your own public name for them. Getting your own domain name You can get your own domain name and assign IP addresses to host names inside this domain. Not all domain name providers support dynamic DNS, so shop before you buy. Page 8

9 Chapter 8 : Resolving IP Addresses - Vantage Documentation Windows 7 can't resolve host names Anyone know why Windows 7 should suddenly stop resolving host names? I've had the machine working fine for ages, but all of a sudden - without anything changing as far as I know - it now won't even ping a host (immediate response of "Ping request could not find host ", even for host names in the hosts file). This helps make the information that is presented in Reports more useful. Instead of changing the IP addresses to domain names in your storages, Vantage Ultimate uses Aliases to display domain names in your Reports. In order to resolve IP addresses to domain names, you need to specify an alias to resolve them into. You can resolve all IP addresses in your storage, or only the IP addresses for a specific summary. Resolving all IP addresses can take some time, depending on the amount of data in your storage. To resolve all IP addresses: Select the Aliases tab. Select a Storage and schema, click Next. The Alias you selected in Step 1 will be automatically selected, but you can change it on this page if necessary. Filters can be added to the Resolve IP function if needed. Please see Filtering for more information. Click OK to complete the wizard. To resolve IP addresses at a specific summary level: Select the Summaries tab from the top of the screen. Your computer must be connected to the Internet for this to work. Resolving IP addresses may take some time. To stop resolving IP addresses at any time, click the Stop button in the top right corner. Vantage Ultimate will create alias groups using the domain name and add the IP addresses as item to the groups. Any IP addresses that cannot be resolved will remain listed as an IP address, while IP addresses that have been resolved will appear as fully qualified domain names. For example, if you are reporting on data three months ago, the IPs in that data may not resolve to the same machine names today. Page 9

10 Chapter 9 : What Is Resolving Name Server? Each name server only has information about machines in its own domain, as well as information about other name servers. The root name servers only have information on the IP addresses of the name servers blog.quintoapp.com,.edu,.za, etc. Function[ edit ] An often-used analogy to explain the Domain Name System is that it serves as the phone book for the Internet by translating human-friendly computer hostnames into IP addresses. For example, the domain name www. Users take advantage of this when they use meaningful Uniform Resource Locators URLs, and addresses without having to know how the computer actually locates the services. An important and ubiquitous function of DNS is its central role in distributed Internet services such as cloud services and content delivery networks. The key functionality of DNS exploited here is that different users can simultaneously receive different translations for the same domain name, a key point of divergence from a traditional phone-book view of the DNS. This process of using the DNS to assign proximal servers to users is key to providing faster and more reliable responses on the Internet and is widely used by most major Internet services. That data can be used to gain insight on, and track responsibility for, a given host on the Internet. Postel directed the task of forging a compromise between five competing proposals of solutions to Paul Mockapetris. Mockapetris instead created the Domain Name System. It was widely distributed, especially on Unix systems, and is still the most widely used DNS software on the Internet. Each node or leaf in the tree has a label and zero or more resource records RR, which hold information associated with the domain name. The domain name itself consists of the label, possibly concatenated with the name of its parent node on the right, separated by a dot. A DNS zone may consist of only one domain, or may consist of many domains and sub-domains, depending on the administrative choices of the zone manager. DNS can also be partitioned according to class where the separate classes can be thought of as an array of parallel namespace trees. Authority over the new zone is said to be delegated to a designated name server. The parent zone ceases to be authoritative for the new zone. A domain name consists of one or more parts, technically called labels, that are conventionally concatenated, and delimited by dots, such as example. The right-most label conveys the top-level domain ; for example, the domain name www. The hierarchy of domains descends from right to left; each label to the left specifies a subdivision, or subdomain of the domain to the right. For example, the label example specifies a subdomain of the com domain, and www is a subdomain of example. This tree of subdivisions may have up to levels. The null label, of length zero, is reserved for the root zone. The full domain name may not exceed the length of characters in its textual representation. The characters allowed in labels are a subset of the ASCII character set, consisting of characters a through z, A through Z, digits 0 through 9, and hyphen. This rule is known as the LDH rule letters, digits, hyphen. Domain names are interpreted in case-independent manner. Name servers[ edit ] The Domain Name System is maintained by a distributed database system, which uses the clientâ server model. The nodes of this database are the name servers. Each domain has at least one authoritative DNS server that publishes information about that domain and the name servers of any domains subordinate to it. The top of the hierarchy is served by the root name servers, the servers to query when looking up resolving a TLD. Authoritative name server[ edit ] An authoritative name server is a name server that only gives answers to DNS queries from data that has been configured by an original source, for example, the domain administrator or by dynamic DNS methods, in contrast to answers obtained via a query to another name server that only maintains a cache of data. An authoritative name server can either be a master server or a slave server. A master server is a server that stores the original master copies of all zone records. A slave server uses a special automatic updating mechanism in the DNS protocol in communication with its master to maintain an identical copy of the master records. Every DNS zone must be assigned a set of authoritative name servers. This set of servers is stored in the parent domain zone with name server NS records. An authoritative server indicates its status of supplying definitive answers, deemed authoritative, by setting a protocol flag, called the "Authoritative Answer" AA bit in its responses. Please help improve this article by adding citations to reliable sources. Unsourced material may be Page 10

11 challenged and removed. September Learn how and when to remove this template message Address resolution mechanism[ edit ] Domain name resolvers determine the domain name servers responsible for the domain name in question by a sequence of queries starting with the right-most top-level domain label. A DNS resolver that implements the iterative approach mandated by RFC ; in this case, the resolver consults three name servers to resolve the fully qualified domain name "www. For proper operation of its domain name resolver, a network host is configured with an initial cache hints of the known addresses of the root name servers. The hints are updated periodically by an administrator by retrieving a dataset from a reliable source. Assuming the resolver has no cached records to accelerate the process, the resolution process starts with a query to one of the root servers. In typical operation, the root servers do not answer directly, but respond with a referral to more authoritative servers, e. The resolver now queries the servers referred to, and iteratively repeats this process until it receives an authoritative answer. The diagram illustrates this process for the host that is named by the fully qualified domain name "www. This mechanism would place a large traffic burden on the root servers, if every resolution on the Internet required starting at the root. In practice caching is used in DNS servers to off-load the root servers, and as a result, root name servers actually are involved in only a relatively small fraction of all requests. Recursive and caching name server[ edit ] In theory, authoritative name servers are sufficient for the operation of the Internet. However, with only authoritative name servers operating, every DNS query must start with recursive queries at the root zone of the Domain Name System and each user system would have to implement resolver software capable of recursive operation. To improve efficiency, reduce DNS traffic across the Internet, and increase performance in end-user applications, the Domain Name System supports DNS cache servers which store DNS query results for a period of time determined in the configuration time-to-live of the domain name record in question. Typically, such caching DNS servers also implement the recursive algorithm necessary to resolve a given name starting with the DNS root through to the authoritative name servers of the queried domain. With this function implemented in the name server, user applications gain efficiency in design and operation. The combination of DNS caching and recursive functions in a name server is not mandatory; the functions can be implemented independently in servers for special purposes. Internet service providers typically provide recursive and caching name servers for their customers. In addition, many home networking routers implement DNS caches and recursors to improve efficiency in the local network. A resolver is responsible for initiating and sequencing the queries that ultimately lead to a full resolution translation of the resource sought, e. DNS resolvers are classified by a variety of query methods, such as recursive, non-recursive, and iterative. A resolution process may use a combination of these methods. A recursive query is one for which the DNS server answers the query completely by querying other name servers as needed. In typical operation, a client issues a recursive query to a caching recursive DNS server, which subsequently issues non-recursive queries to determine the answer and send a single answer back to the client. The resolver, or another DNS server acting recursively on behalf of the resolver, negotiates use of recursive service using bits in the query headers. DNS servers are not required to support recursive queries. Each server refers the client to the next server in the chain, until the current server can fully resolve the request. For example, a possible resolution of www. Circular dependencies and glue records[ edit ] Name servers in delegations are identified by name, rather than by IP address. This means that a resolving name server must issue another DNS request to find out the IP address of the server to which it has been referred. If the name given in the delegation is a subdomain of the domain for which the delegation is being provided, there is a circular dependency. In this case, the name server providing the delegation must also provide one or more IP addresses for the authoritative name server mentioned in the delegation. This information is called glue. The delegating name server provides this glue in the form of records in the additional section of the DNS response, and provides the delegation in the authority section of the response. A glue record is a combination of the name server and IP address. For example, if the authoritative name server for example. As ns1 is contained in example. To break the dependency, the name server for the top level domain org includes glue along with the delegation for example. The glue records are address records that provide IP addresses for ns1. Record caching[ edit ] A standard practice in implementing name resolution in applications is to reduce the load on the Domain Name System servers by caching results locally, or in intermediate resolver hosts. Results Page 11

12 obtained from a DNS request are always associated with the time to live TTL, an expiration time after which the results must be discarded or refreshed. The period of validity may vary from a few seconds to days or even weeks. As a result of this distributed caching architecture, changes to DNS records do not propagate throughout the network immediately, but require all caches to expire and to be refreshed after the TTL. Some resolvers may override TTL values, as the protocol supports caching for up to sixty-eight years or no caching at all. Negative caching, i. Multiple domain names may be associated with an IP address. The DNS stores IP addresses in the form of domain names as specially formatted names in pointer PTR records within the infrastructure top-level domain arpa. For IPv4, the domain is in-addr. For IPv6, the reverse lookup domain is ip6. The IP address is represented as a name in reverse-ordered octet representation for IPv4, and reverse-ordered nibble representation for IPv6. When performing a reverse lookup, the DNS client converts the address into these formats before querying the name for a PTR record following the delegation chain as for any DNS query. For example, assuming the IPv4 address Instead DNS resolution takes place transparently in applications such as web browsers, clients, and other Internet applications. When an application makes a request that requires a domain name lookup, such programs send a resolution request to the DNS resolver in the local operating system, which in turn handles the communications required. The DNS resolver will almost invariably have a cache see above containing recent lookups. If the cache can provide the answer to the request, the resolver will return the value in the cache to the program that made the request. If the cache does not contain the answer, the resolver will send the request to one or more designated DNS servers. In the case of most home users, the Internet service provider to which the machine connects will usually supply this DNS server: In any event, the name server thus queried will follow the process outlined above, until it either successfully finds a result or does not. It then returns its results to the DNS resolver; assuming it has found a result, the resolver duly caches that result for future use, and hands the result back to the software which initiated the request. Broken resolvers[ edit ] Some large ISPs have configured their DNS servers to violate rules, such as by disobeying TTLs, or by indicating that a domain name does not exist just because one of its name servers does not respond. Page 12