Introduction to DNS
Domain Name System DNS (Domain Name System) is a distributed database for TCP/IP applications, providing translation services between domain names and IP addresses.
Each host in the network is identified by an IP address, and the user can only successfully implement the access operation if he obtains the IP address of the host to be accessed. For users, it is quite difficult to remember the IP address of the host, so a host naming mechanism in the form of strings is designed, and these host names correspond to IP addresses one by one, which is the domain name system. The domain name system solves the problem that IP address information is not easy to remember. When the user accesses the network host, he can directly use the easy-to-remember and meaningful domain name, and the domain name resolution server in the network will resolve the domain name to the correct IP address.
How DNS and DDNS work
DNS domain name resolution is divided into static domain name resolution and dynamic domain name resolution, which can be used together. When resolving domain names, first use static domain name resolution (find the static domain name resolution table), and then use dynamic domain name resolution if the static domain name resolution is unsuccessful. Since dynamic domain name resolution may take a certain amount of time and requires the cooperation of the domain name server, some commonly used domain names can be put into the static domain name resolution table, which can greatly improve the efficiency of domain name resolution.
- Static DNS:
Static domain name resolution is performed through the static domain name resolution table. The user manually establishes a correspondence table between domain names and IP addresses, and puts some commonly used domain names into the table. When the client needs the IP address corresponding to the domain name, it first looks up the specified domain name in the static domain name resolution table to obtain the corresponding IP address and improve the efficiency of domain name resolution.
- Dynamic DNS:
User programs (such as Ping, Tracert) access the domain name server (DNS Server) through an address resolver (Resolver) of the DNS client (DNS Client). The relationship between the user program, the resolver and the domain name server, and the cache area on the resolver is shown in the following figure.
The resolver and the cache area are integrated together to form the DNS Client, and its function is to receive the DNS request of the user program and respond to it. In general, user programs (eg Ping, Tracert), caches and resolvers are on the same host, and domain name servers are on different hosts.
The working process of DNS
When a user uses a domain name to access some application services, the user program first sends a request to the resolver in the DNS client.
After the resolver receives the DNS request sent by the user program, it first queries the local domain name cache.
( 1 ) There is a mapping table entry corresponding to the domain name in the local domain name cache, and the resolver returns the IP address corresponding to the domain name to the user program.
( 2 ) If the local domain name cache does not find the mapping item to be searched, the resolver sends a query request to the domain name server (DNS Server).
After the domain name server receives the query message, it first judges whether the requested domain name is in the subdomain that it is authorized to manage, and then sends a corresponding response message to the DNS Client according to different judgment results.
( 1 ) If it is within the scope of the subdomain that it is authorized to manage, the server first looks up the IP address corresponding to the domain name from its own database.
( 2 ) If it is judged that the domain name does not belong to the scope of this domain, the request will be handed over to the upper-level domain name resolution server for processing until the resolution is completed, and the result of the resolution will be returned to the DNS client.
After receiving the response message from the domain name server, the DNS client returns the resolution result to the application program.
Dynamic domain name resolution supports cache function. The mapping between the domain name and IP address that is successfully dynamically resolved each time is stored in the dynamic domain name cache area. When the same domain name is queried next time, it can be read directly from the cache area without making a request to the domain name server.
The mapping in the cache area will be aged and deleted after a period of time to ensure that the latest content can be obtained from the domain name server in time. The aging time is set by the domain name server, and the DNS client obtains the aging time from the protocol packet.
Domain name suffix list function:
DDNS supports the domain name suffix list function. Users can preset some domain name suffixes. During domain name resolution, users only need to input some fields of the domain name, and the system will automatically add different suffixes to the input domain name for resolution.
The working mode of DDNS
- DDNS overview:
DNS can be used to resolve domain names into IP addresses, so as to use domain names to access nodes in the network. However, DNS only provides a static correspondence between domain names and IP addresses. When the IP addresses of nodes change, the DNS server cannot dynamically update the mapping between domain names and IP addresses.
At this time, if you still use the domain name to access the node, the IP address obtained through domain name resolution is wrong, resulting in access failure. The Dynamic Domain Name System (DDNS) is used to dynamically update the mapping relationship between domain names and IP addresses on the DNS server to ensure that the correct IP address is resolved through the domain name.
- DDNS working mode:
DDNS adopts the client/server working mode and provides two update methods:
(1) DDNS update method defined by RFC2136: The device acts as a DDNS client to dynamically update the mapping relationship between domain names and IP addresses in the DNS server.
(2) The update method implemented by the DDNS server: as a DDNS client, the device sends the mapping relationship between the domain name and the IP address to the DDNS server at the specified URL address, and then the DDNS server notifies the DNS server to dynamically update the mapping between the domain name and the IP address relation. The following figure is a typical network diagram of DDNS that implements updates through a DDNS server.
(3) DDNS client: When the IP address changes, it needs to dynamically update the mapping relationship between its domain name and IP address on the DNS server. Internet users usually use domain names to access servers that provide application layer services, such as HTTP and FTP servers. In order to ensure that other users can still access these servers through the domain name when the IP address of the server providing application layer services changes, the server will act as a DDNS client and send a DDNS update request to the DDNS server to update the mapping relationship between the domain name and IP address.
(4) DDNS server: responsible for notifying the DNS server to dynamically update the mapping relationship between domain names and IP addresses. After receiving the update request from the DDNS client, the DDNS server notifies the DNS server to re-establish the mapping relationship between the domain name and the IP address of the DDNS client, so that even if the IP address of the DDNS client changes, Internet users can still access the DDNS client through the same domain name end.
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