Domain Name System - Wikipedia
Unknown command or computer name, or unable to find computer address For cases where name lookup is needed you can stop the router from That's a long time and that's why its takes so long for the connections to fail. when trying to configure mafiathegame.info as an NTP server on a Cisco box!. Bind, the most popular DNS server, is used as the backbone of much of the This avoids the peril of having a single point of failure for your DNS servers. We need to configure this so that it correctly identifies each server's hostname and FQDN. We specify relationship of this DNS server to the zone. Symptom: The "ntp server" and "ntp peer" commands allow a DNS error on its console: % NTP: IP name lookup for this hostname failed.
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. For example, a simple stub resolver running on a home router typically makes a recursive query to the DNS server run by the user's ISP.
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. The resolver uses one or more of these IP addresses to query one of the domain's authoritative servers, which allows it to complete the DNS query.
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 obtained from a DNS request are always associated with the time to live TTLan 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 cachingi.
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 ARIN's servers delegate Instead DNS resolution takes place transparently in applications such as web browserse-mail clientsand 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 aboveuntil 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.
This practice can add extra difficulty when debugging DNS issues, as it obscures the history of such data. These caches typically use very short caching times — in the order of one minute. Other applications[ edit ] The Domain Name System includes several other functions and features.
Hostnames and IP addresses are not required to match in a one-to-one relationship. Multiple hostnames may correspond to a single IP address, which is useful in virtual hostingin which many web sites are served from a single host.
Alternatively, a single hostname may resolve to many IP addresses to facilitate fault tolerance and load distribution to multiple server instances across an enterprise or the global Internet. DNS serves other purposes in addition to translating names to IP addresses. For instance, mail transfer agents use DNS to find the best mail server to deliver e-mail: An MX record provides a mapping between a domain and a mail exchanger; this can provide an additional layer of fault tolerance and load distribution.
A common method is to place the IP address of the subject host into the sub-domain of a higher level domain name, and to resolve that name to a record that indicates a positive or a negative indication. It points to 5. This hostname is either not configured, or resolves to E-mail servers can query blacklist. Many of such blacklists, either subscription-based or free of cost, are available for use by email administrators and anti-spam software.
To provide resilience in the event of computer or network failure, multiple DNS servers are usually provided for coverage of each domain. At the top level of global DNS, thirteen groups of root name servers exist, with additional "copies" of them distributed worldwide via anycast addressing. Each message consists of a header and four sections: A header field flags controls the content of these four sections. The identification field can be used to match responses with queries.
The flag field consists of several sub-fields. The first is a single bit which indicates if the message is a query 0 or a reply 1. This is "master" in this case since we are configuring this machine as the master name server for all of our zones. We also point Bind to the file that holds the actual resource records that define the zone. We are going to keep our master zone files in a subdirectory called zones within the Bind configuration directory.
We will call our file db. Our block will look like this now: A Bit About Reverse Zones If the organization that gave you your IP addresses did not give you a network range and delegate responsibility for that range to you, then your reverse zone file will not be referenced and will be handled by the organization itself.
With hosting providers, the reverse mapping is usually taken care of by the company itself. For instance, with DigitalOcean, reverse mappings for your servers will be automatically created if use the machine's FQDN as the server name in the control panel. For instance, the reverse mappings for this tutorial could be created by naming the servers like this: In instances like these, since you have not been allocated a chunk of addresses to administer, you should use this strategy.
The strategy outlined below is covered for completeness and to make it applicable if you have been delegated control over larger groups of contiguous addresses. Reverse zones are used to connect an IP address back to a domain name. However, the domain name system was designed for the forward mappings originally, so some thought is needed to adapt this to allow for reverse mappings.
The pieces of information that you need to keep in mind to understand reverse mappings are: In a domain, the most specific portion is of the address is on the left.
For an IP address, the most specific portion is on the right. The most specific part of a domain specification is either a subdomain or a host name. This is defined in the zone file for the domain. Each subdomain can, in turn, define more subdomains or hosts. All reverse zone mappings are defined under the special domain in-addr.
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Under this domain, a tree exists that uses subdomains to map out each of the octets in an IP address. To make sure that the specificity of the IP addresses mirrors that of normal domains, the octets of the IP addresses are actually reversed.How to Turn a Computer into an NTP Time Server
When we add this host specification as a hierarchy existing under the in-addr. Since we define individual hosts like the leading "1" here within the zone file itself when using DNS, the zone we would be configuring would be 2. Now that you know how to specify the reverse zone name, the actual definition is exactly the same as the forward zone. Again, this is probably only necessary if you were delegated control over a block of addresses: This is specific about what the zone configures and is more readable than the reverse notation.
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Create the Forward Zone File We have told Bind about our forward and reverse zones now, but we have not yet created the files that will define these zones. If you recall, we specified the file locations as being within a subdirectory called zones. We need to create this directory: For the forward zone, the db. Copy that file into the zones subdirectory with the name used in the named. We will use the db. We need to replace the localhost.
Domain Name System
This portion of the record is used to define any name server that will respond authoritatively for the zone being defined. This will be the machine we are configuring now, ns1. This is important for our entry to register correctly! We also want to change the next piece, which is actually a specially formatted email address with the replaced by a dot. We want our emails to go to an administer of the domain, so the traditional email is admin example. We would translate this so it looks like admin.
The next piece we need to edit is the serial number. The value of the serial number is how Bind tells if it needs to send updated information to the slave server. Failing to increment the serial number is one of the most common mistakes that leads to issues with zone updates. Each time you make an edit, you must bump the serial number. One common practice is to use a convention for incrementing the number. So the first revision made on June 05, could have a serial number of and an update made later that day could have a serial number of The value can be a 10 digit number.
It is worth adopting a convention for ease of use, but to keep things simple for our demonstration, we will just set ours to 5 for now: The first thing we want to establish after the SOA record are the name servers for our zone. We specify the domain and then our two name servers that are authoritative for the zone, by name. Since these name servers will be hosts within the domain itself, it will look a bit self-referential. For our guide, it will look like this.
Again, pay attention to the ending dots!: Since the purpose of a zone file is mainly to map host names and services to specific addresses, we are not done yet.
Any software reading this zone file is going to want to know where the ns1 and ns2 servers are in order to access the authoritative zones. So next, we need to create the A records that will associate these name server names to the actual IP addresses of our name servers: Remember, we have a web server on one of our hosts that we want to use to serve our site.
We will point requests for the general domain example. It will look like this: Reference our DNS basics guide to get familiar with some of your options with creating additional records.
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When you are finished, your file should look something like this: Create the Reverse Zone File Now, we have the forward zone configured, but we need to set up the reverse zone file that we specified in our configuration file. We already created the file at the beginning of the last section. Open the file in your text editor with sudo privileges: We will go through much of the same procedure as we did with the forward zone.
First, adjust the domain name, the admin email, and the serial number to match exactly what you had in the last file The serial number can be different, but should be incremented: Each IP address should only have a single PTR record to avoid problems in some software, so you must choose the host name you wish to reverse map to. For instance, if you have a mail server set up, you probably want to set up the reverse mapping to the mail name, since many systems use the reverse mapping to validate addresses.
First, we need to set our name servers again: Next, you will use the last octet of the IP address you are referring to and point that back to the fully qualified domain name you want to return with. For our example, we will use this: When you are finished, the file should look something like this: Testing the Files and Restarting the Service The configuration for the master server is now complete, but we still need to implement our changes.
Before we restart our service, we should test all of our configuration files to make sure that they're configured correctly. We have some tools that can check the syntax of each of our files. First, we can check the named.