What is a DNS server and a DNS client? DNS server operating modes. What to do if the DNS server is not responding or is unavailable

DNS is a service that facilitates communication between different network segments. Its use can significantly reduce the time spent searching for information. In this article you will learn about the basic principles of operation of the service, as well as methods and forms of data transmission on the Internet.

How does it work

At the dawn of the Internet, there was a “flat” naming system: each user had a separate file, which contained lists of contacts he needed. When he connected to World Wide Web, then its data was sent to other devices.

However, due to rapid development The Internet needed to make data exchange as easy as possible. Therefore, it was divided into smaller segments-domains. In turn, they are divided into subdomains. At the top of the address, submitted in the nominal form, there is a root - the main domain.

Since the Internet is an American development, there are two types of primary domains:

generic domains that belong to US institutions:

com – business organizations;
gov – government agencies;
edu – educational institutions;
mil – military missions;
org – private organizations;
net – Internet provider.

The indigenous domains of other countries consist of two letters.

The second level consists of abbreviations for cities or regions, and third-order domains denote various organizations and enterprises.

The period acts as a separator between domains different order. There is no dot at the end of the name. Every separate domain with a dot is called a label.

Its length should not exceed 63 characters, and the total length of the address should be 255 characters. Basically, the Latin alphabet, numbers and hyphens are used, but several years ago they began to use prefixes based on other writing systems. Letter case does not matter.

Servers are computers that contain a list of other objects within one network level, which allows for faster exchange between users. They became the basis of the new system.

Each network layer must have own server, which contains information about the addresses of users in its segment.

Searching for the necessary data goes like this:

DNS Basics

A node consisting of several domains is called a zone. Its file contains the main parameters of its segment. This includes information about the FQDN or fully qualified domain name. If such an entry ends with a dot, this means that the object name is specified correctly.

There are several types of computers that serve DNS:

master– main agent of the network. He can change its configuration;
slave– second order devices. They serve clients equally with the master and can replace him in case of problems. This allows you to unload the network;
caching. Contains information about domains of foreign zones;
invisible. Missing from the zone description. Most often, this status is assigned to users with master status in order to protect them from attack.

The user can send one of two types of requests to them.

The browser sends it through the resolver program:

recursive. If the server does not contain the necessary information, in this case it will obtain the necessary data from the computers top level and sends a response to the client. This allows you to reduce the number of requests and saves time and your traffic;
iterative. The server sends a ready response, selecting information only from its own cache (memory). If it does not have suitable data, it provides a link to other computers. The browser then goes to this address.

There are two types of responses:

authoritative– if the data is sent from a device that serves the network;
non-authoritative. Sent another computer, which obtains the necessary data from its own cache or after an iterative query.

Video: DNS Service

Names and IP addresses

The DNS service provides translation of website names into IP addresses. On the Internet, each device can be tracked by 2 main parameters - domain name and IP address. They can be assigned to the user's computer, network printer or router.

However, this is very conditional, since a computer may not have a domain name, but use several addresses. In addition, each IP address must match all domain names. However, a domain can only contain information about one IP address.

Operating mode

Servers can operate in the following modes:

maintenance of your own zone. Data exchange takes place between the master and slave computers. However, requests from unauthorized users are not accepted;
performing a recursive question;
forwarding– the server sends a request to another zone.

Changing DNS settings

Typically these parameters are set by the network in automatic mode. In order to reset the data, you need to go to the “Network Connections” section.

Photo: changing parameters DNS server A

After that, you need to enter the protocol used to maintain the network.

In the "Properties" section you can set required parameters. Usually the main IP address of the server and an alternative one are indicated.

Message Format

The message that is used to exchange information between the service begins with a 12-byte header. This is followed by an identification field that allows you to determine which request was answered.

The flags field (the next 16 bits) includes the information:

message type;
operation code;
identification of authoritativeness (i.e. shows whether the serving computer belongs to the network);
TC flag. Displays whether the message arrived truncated or full.
recursion flag, i.e. requirements for the server to send requests to higher-order computers;
recursion capability flag. Shows the server's ability to redirect messages;
return code. Displays whether the response was sent with errors or not.

The last 16-bit field shows the total number of parameters taken into account.

Questions in DNS request

Part of the resource record in the response

Any response contains information about the party that sent the message. It contains the following data: response, server credentials and Additional Information about him.

In addition to them, the message contains:

domain name;
request type;
validity period of the cached version;
resource record length – an estimate of the amount of information.

Index queries

Pointer queries are aimed at searching for a page in inverse mode, i.e. searching for a resource name by IP address, given in the form text string, separated by dots.

To send it, the host address is written in the opposite order with the addition of a certain suffix (most often in the form in-addr.arpa).

The operation can be performed if the resource contains a PTR record. This allows control of the zone to be transferred to the owner of the IP addresses.

Resource records

This is a list of the main programs used by the service. Within one domain, these records are unique. On different levels There may be duplicates of these records on the network.

This data includes the following types entries:

SOA–start of powers. It allows you to compare a domain and the computers serving it. They also contain information about the validity period of the cached version, and contact person, which serves a server of a certain level;
A contain a list of IP addresses and their corresponding hosts. They allow you to identify the address of domain resources;
NS (Name Server) include a list of computers that serve the domain;
SRV (Service) display all resources that perform essential functions services;
MX ( Mail Exchanger) allow you to automatically configure the distribution of data to serving computers within the boundaries of one domain;
PTR (Pointer) used to search for a resource name if the user knows its IP address;
CNAME (Canonical Name) allow the server to be referred to under multiple aliases within the service.

Caching

To find the information you need, the browser can search for information in three segments. First, the necessary data is searched using the DNS service, i.e. on local level. They can be found if your computer contains a Hosts file.

However, if the operation fails, the client submits a request. To speed up the search for information, cached servers are used. If it does not find the required data, then it performs a recursive query. When served, it copies data from other networks.

Photo: setting up a caching DNS server

This allows you to save traffic without subsequently contacting authoritative users. But open entry remains valid for a limited period. Its validity period is set in the zone file. The default minimum is 1 hour.

UDP or TCP

The service supports both UDP and TCP protocols.

UDP is used to send messages over global networks. The size of messages sent via this protocol is limited. Incomplete answers contain the TS label. This means that the response size exceeded 512 bytes, so the rest did not reach the computer.

It is less reliable because it does not have a specific timeout for request response. However, such a system is suitable for transmitting huge amount information.

TCP is used to transmit such data because it allows you to receive any amount of data divided into segments of a certain size.

This protocol is also used by secondary servers when they request data from host computers every three hours to learn about updates to the network configuration file.

The DNS service has a complex hierarchical structure. However, the server system provides flexible and fast interaction between all users and devices of the Network.

To find out necessary information, the client sends a request. The response contains basic data about the object of interest and the computer serving the zone. To carry out this exchange, we use UDP protocols and TCP.

If clients are unable to access resources on the network by host name or fully qualified domain name, the cause of the problem is usually due to one of these errors:

The local name resolution cache stores a negative response
The DHCP server provided clients incorrect address name resolution server
Incorrect static parameters TCP/IP protocol
Other domain names are not listed as suffixes, which are added in order of DNS suffix iteration ( Extra options TCP/IP protocol)
Network hardware error (network adapters, cables, switches, routers, etc.)

If clients are unable to access a system using a fully qualified domain name, it is worth first determining whether the problem is with the name resolution service or with the network itself. The most in a simple way This is done by sending a test echo packet to the remote system indicating the IP address. If remote system successfully responds to the test echo packet, then the problem is indeed with the name resolution service (if not, check for problems in hardware network environment).

You can then try to send a test echo packet using the fully qualified domain name.

If ping command will display wrong address IP, then you need to find the source of the incorrect IP address. Most likely, the source of the incorrect IP address is the DNS server or the Hosts file on the client computer (which is located in the %systemroot%\system32\drivers\etc directory). If in Hosts file If static records are not specified, you can try to find the source of the incorrect conversion using the nslookup utility.

Now let's assume that using a fully qualified domain name shows the correct IP address. After this, you need to send a test echo packet, specifying only the name of the remote host. If sending a test echo packet fails, the source of the problem can be considered found, since DNS suffixes are not added to name resolution requests. This issue is resolved by adding the DNS suffixes to the appropriate suffix list. This list is available on the DNS tab in the TCP/IP protocol properties window for a specific network interface.

Common DNS Client Issues

	Possible reason
After changing a host record on the DNS server, clients are unable to access that host	The original IP address for this node is stored in the local name resolver cache. On the client system you need to run the command ipconfig /flushdns, which will clear the local name resolver cache. Incorrectly configured Hosts file on the local computer
The user receives the message "Network Path Not Found" Not Found") when trying to access a network resource	Incorrectly configured TCP/IP protocol parameters or unreceived lease from the DHCP server DNS server is unavailable Incorrect static entries in the Hosts file on the local system The user is trying to access resources that are in a different domain. Add DNS suffixes to the DNS tab of the window additional properties TCP/IP protocol on the local computer
The client receives an incorrect response from the DNS server	The Type A record for the requested resource contains an invalid IP address. Make changes to the IP address on the DNS server and clear the resolver cache on the client computer. Then force the client to execute the request again. The client's request is being made by a secondary DNS server whose zone data is out of date. To correct the situation, you need to reduce the Refresh Interval for the primary zone, or add the secondary server to the Notify List on primary server. To immediately update a zone on a secondary server, you can click on the secondary server zone right click mouse and select Transfer from Master
Client node names and IP addresses are not dynamically registered with the primary DNS server	Dynamic update is disabled in the TCP/IP protocol properties on the client computer. Check if the Register this connection’s addresses in DNS checkbox is selected. DNS zone is configured to prevent dynamic updates There is no zone on the primary DNS server for the client domain The DNS zone is configured to perform only secure dynamic updates, and the problematic client computers are not domain members.

Customer problems are usually small in scale and can be corrected quickly with the right technology. On the other hand, server problems sometimes require some time to isolate the source of the problem.

DNS client- a program (or module in a program) that provides determination of the node address by its full name(the dot at the end is usually either added by the DNS client or is implicitly implied, however, in the case of using domain suffixes, there may be differences in client behavior when determining a name with and without a dot at the end).

The DNS client function is built into almost any program designed to work in Internet networks. However, most DNS clients do not know how to perform recursion on their own, since otherwise their algorithms would have to be unnecessarily complicated. In addition, the use of recursion by DNS clients would not allow access restriction modes to be implemented in external network and DNS caching.

Specialized programs

DNS diagnostics are used specialized programs- nslookup and dig. The Windows version of nslookup (like many Windows DNS clients) supports WINS, which can create confusion if WINS and DNS servers respond differently. Some versions of nslookup on Unix also support yp (NIS) in addition to DNS, which also complicates diagnostics. When checking integrated with Active Directory DNS servers can use the dcdiag utility.

DNS server operating modes

The DNS server executing the client's request can operate in one of three modes:

* forwarding mode (transferring) requests to another DNS server - in this case, the request is almost no different from the DNS client request. (This scheme is used when using caching DNS servers and servers in the DMZ).
* mode of independent execution of a recursive query.
* zone maintenance mode (in this case, requests for extraneous (non-authoritative) zones are not accepted, instead the addresses of the root servers are returned).

In many versions of BIND, the request to other DNS servers originated from port 53 (the port on which DNS queries, both TCP and UDP), unlike client applications using arbitrary port sender (from an unregistered range).

Working with DNS of various programs

The mail server uses DNS client, for the request MX, SPF, A-record (for example, sendmail uses MX-records in ascending order of priority, but if there is no suitable route, uses the A-record as a last attempt), PTR-records (to determine the hostname). In addition, many versions mail servers use names obtained through a PTR record request to determine whether a given name has an MX record, this is used to automatically refuse delivery from non-mail servers (mostly botnets).

IN general case configuration is possible when for different servers and zones the DNS server behaves differently (for example, from local network accepts recursive requests, from external - only requests for authoritative zones, or forwards requests to another server only for specified zones).

Workstations and servers in Active Directory use many SRV DNS records to determine the location of the nearest domain controller in a site and to perform dynamic update DNS records about workstation.

Interface between the program and DNS client

Most programs that use DNS functions do not implement its functionality themselves (via sockets), but use either the DNS client functions provided by the operating system or use a ready-made client library. Minorities are mainly low-level utilities and server software that requires more specific functionality than is provided by the DNS client.

DNS client

DNS client- a program (or module in a program) that provides determination of a host address by its full name (the dot at the end is usually either added by the DNS client or is implicitly implied, however, in the case of using domain suffixes, differences in client behavior when determining a name with a dot are possible at the end and without).

DNS server operating modes

The DNS server executing a client request can operate in one of three modes:

mode of forwarding (transferring) requests to another DNS server - in this case, the request is almost no different from the DNS client request. (This scheme is used when using caching DNS servers and servers in the DMZ).
mode of independent execution of a recursive query.
zone maintenance mode (in this case, requests for extraneous (non-authoritative) zones are not accepted; instead, the addresses of the root servers are returned).

In general, a configuration is possible where the DNS server behaves differently for different servers and zones (for example, it accepts recursive requests from the local network, only requests for authoritative zones from the external network, or forwards requests to another server only for specified zones).

Workstations and servers in Active Directory use multiple SRV DNS records to determine the location of the nearest domain controller in a site and to dynamically update the workstation's DNS record.

Interface between the program and DNS client

Most programs that use DNS functions do not implement its functionality themselves (via sockets), but use either the DNS client functions provided by the operating system or use a ready-made client library. The minority are mostly low-level utilities and server software that require more specific functionality than is provided by the DNS client.

Notes

Literature

Paul Albitz and Cricket Lee - DNS and BIND, ISBN 5-93286-035-9, Symbol-Plus, 2002

Links

Wikimedia Foundation. 2010.

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Sometimes it becomes very annoying that you cannot use the Internet due to a completely banal error “DNS server is not responding.” In the vast majority of cases, when it is not possible to find the DNS address of the server, the problem can be solved in a matter of minutes. In this article we will look in detail at all the ways to solve this problem.

DNS server is a utility that redirects the Internet user to a website. The fact is that any Internet page is stored on a server that has its own IP address. To give a user access to a site, a DNS server connects his computer to the server. In other words, the DNS server is connecting link between the user and the site.

Errors “DNS server is not responding” or “DNS address cannot be found”

Often the browser complains that it cannot find the DNS address of the server. This message occurs most often among desktop users using a connection without cables (3G/LTE modem or Wi-Fi router). However, it can also appear in those who use wired Internet. This error means that the unit from which the user accesses the site cannot find a DNS address that will redirect him to the server with the page he is looking for.

What to do if the DNS server is not responding or is unavailable

Before you try to decide this problem, you need to first find out why it arose:

Because of incorrect settings modem or router;
Due to incorrect operating system settings (the site is blocked by a virus or firewall, or the Windows DNS client has failed);
Due to outdated driver network card.

To do this, you need to go to the network control panel located in the lower right corner of the task bar. It has a monitor icon, next to which there is an Enternet cable. Click on it with the left button of the manipulator. Next click right key the manipulator on the field where it says “Connected”, then go to “Properties”. Click on the “Network” tab and go to the “Properties” item, after clicking “Internet Protocol Version 4”. In the DNS addresses tab, try selecting the “Load DNS server automatically” option. If this does not help, then enter the address (preferred and alternative) yourself. It is written in the contractual connection certificates. You can also find out the DNS address from your ISP by calling them.

Advice: the correct DNS address can be registered not only in Windows settings, but also in the control panel of the router itself. If you are using software utilities from TP-LINK, then use the parameter quick setup(Quick Setup).

Often a virus carelessly downloaded by a user blocks access to other sites. To check your system for existing malware, you should scan it with an antivirus. In this case, it is better to perform scanning with a program that does not require installation on the desktop and is located on a Live-CD or Live-flash drive (Live media is storage space independent of the main system). For such purposes, we can recommend Dr. Web CureIt! Portable anti-malware are good because, being placed on a Live-CD or Live-flash drive, they cannot be infected with viruses.

Setting up a firewall

There is a possibility that access to the site has been blocked native Windows Farewall or firewall (another name for firewall) that comes with your antivirus. The firewall blocks access to sites that it considers malicious. If you know that the blocked page is truly safe, you can temporarily disable the firewall or reset its settings to the initial settings (then the list of blocked pages will be reset to zero). How to turn off Microsoft's firewall? Click Control Panel->Windows and Security-> Windows Firewall. In the left panel there will be an item “Enable and shutting down Windows Firewall". Click it, then turn all the toggle switches to “Turn off Windows Firewall.” Save these settings.

Advice: Windows firewall– key. By turning it off, you will disable other firewalls.

Updating network card drivers

Often the desktop refuses to go online due to outdated drivers network card. To check their status, use the utility Driver Booster. This application will help you find not only drivers for network controllers and install them, but also update the functionality of other components.

Advice: you can update the drivers network card and standard Windows utilities. Go to “Devices and Printers”, then double-click the left mouse button on the icon of your desktop. In the “Equipment” tab, find the components marked as “ Network adapters” and go to their “Properties”. There, click on “Driver” and select “Update”.

This method involves resetting the desktop and router settings. The sequence of actions is as follows: you need to disconnect the router from the 220V network and leave it unconnected for 5 minutes. Next, you need to restart your computer and plug the router back into the outlet.

Advice: Before turning off the router, you should go to its settings menu and reset the default settings.

This problem can be resolved in two ways. The first - the least painful - register the DNS address not through the Panel Windows management, but through the router menu. The second is to perform a system restore. Go to the Control Panel, then – “System and Security” – “Restore a previously saved desktop state”. After a few minutes, when the utility collects all the registered backup points, you need to select one of them. The date of its creation is written next to each point. Select the one where the DNS client was functioning normally and confirm the system reset.

How to find out the dns address of a server

The correct DNS address is specified in the agreement on connecting the desktop to the Network. It was compiled by the provider, so there is no possibility of error. If you do not have access to the certificate, you can call the provider or contact him via technical service support and ask him to provide the exact DNS address again.

Where can I configure the DNS server address in Windows?

It can be configured through Windows utilities (path: network icon in the taskbar - “Settings” - “Network” - “Internet Protocol v4” - “Properties” - tab with DNS addresses) or through the control panel of your router or modem.

Programs for setting up a DNS server

If the DNS server is unavailable, then the DNS Jumper utility will help fix this problem. Its advantage is that it is portable and does not require installation. In the “DNS Server Selection” tab, you can select a DNS address manually or let the utility itself select it. In this case, DNS Jumper will choose the most stable and fast server on this moment, and the “DNS server does not respond to windows” problem will be removed. You can also download the DOT VPN add-on to your browser. This extension allows you to select not only the address, but also the country from which the user will log in. That is, you can physically be in Germany, but access the site as a resident of the Netherlands. Very useful extension, since some pages are blocked by state governments, and DOT VPN allows you to bypass this ban. “ Setting up a VPN» in the Opera browser. It is turned on like this: Settings->Security->VPN (switch the toggle switch to “Enable” and select “Optimal location”).