Good evening, I'm Lao Yang.
As we know, TCP/IP provides the ability to connect to devices by IP address.
However, it is quite difficult for users to remember the IP address of a certain device, so someone has specially designed a host naming mechanism in the form of strings, and these host names correspond to IP addresses.
There needs to be a translation and query mechanism between IP addresses and host names, and the system that provides this mechanism is the Domain Name System (DNS).
There may be Xiaobai's concept of DNS is a little unclear, Lao Yang today's article will start from several aspects, all of which will be told to you.
Today's article reads: DNS and BSECOND (5th Edition)
If you need it, you can send a private message to Lao Yang, remark "DNS", and the top 20 will send the resource for free.
01 What exactly is DNS?
DNS is equivalent to a phone book, which can find the corresponding phone number according to the name of the person, and DNS can find the corresponding IP address according to the domain name.
Some whites may ask: Why do you have to have the entire IP address, just the domain name? This starts with the knowledge of Internet communication protocols.
Simply put, the IP address is the unique identification of each computer, just like each person's social security number is unique, only through the IP address to uniquely identify a computer.
But because of the IP address a series of numbers, memory trouble, so the domain name, domain name is equivalent to the nickname of IP address, but it and IP address is a one-to-many relationship, that is, a domain name may correspond to multiple IP addresses, when the user accesses, only one of them will be directed.
Therefore, it is necessary to look up the IP address through DNS. So how exactly does DNS work?
To illustrate the workflow of DNS as an example, let's use http://www.baidu.com as an example:
com is called a "top-level domain";
baidu and so-called "second-level domains";
www is called hostname, also known as "tertiary domain name".
Common domain names tend to show only three levels, such as the http://www.baidu.com mentioned above. This is because the last root domain name of all domain names is the same .root, so it is usually omitted.
It should actually be: www.baidu.com.root
So, a domain name is actually a hierarchical structure with four levels, namely:
Host name. Sub-domain. TLD. The root domain name, which is host.sld.tld.root
When DNS resolves domain names, it is according to the hierarchical structure, starting from the root domain name and leveling down the hierarchy, such as let's check the http://www.baidu.com.
First visit the root name server and query which top-level name server the .com (top-level domain) is maintained;
According to the returned results, visit the top-level name servers and query which secondary name server the .baidu (sub-domain name) is maintained;
According to the returned result, access the secondary name server and query the IP address of www (hostname);
The above query process is done by the local DNS server, not the local host.
When localhost accesses a domain name through a browser:
First, it will query the /etc/hosts file of the local host to see if the domain name and IP mapping are customized (for example, if the http://www.baidu.com is mapped to 127.0.0.1 in the /etc/hosts file, then the browser will access the localhost when visiting the http://www.baidu.com)
If there are no corresponding customizations, the local DNS server is accessed.
The IP address of the local DNS server can be viewed in the /etc/resolv.conf file
The local DNS server will first check the local cache, whether there is a mapping of the corresponding domain name and IP, and if so, it will directly return to the local host, if not, the above query process will be carried out.
02 What is a DNS root server?
DNS root servers, which are the name servers responsible for DNS and the entire Internet function, are the first step in determining the name of any domain name, DNS can translate domain names into IP addresses.
The root zone is a global list of top-level domains that contains generic top-level domains (.com, . net、. org), country code top-level domains (.no, . se、. uk) and internationalized top-level domains, which are ccTLDs written in national local characters, with roots from the Internet Corporation for Assigned Numbers (IANA), which is part of the Internet Corporation for Assigned Names and Numbers (ICANN).
The mapping of domain names to IP addresses is done in hierarchical order using DNS zones, which are served by root servers, which are at the top of the hierarchy and publish the root zone file, which in turn contains the resource records for all TLD authority servers.
Therefore, they can work in two ways:
- Respond directly to requests for this resource record in the root zone
- Forward the request to the appropriate name server for the requested TLD
Although in the latter case they do not directly participate in name resolution, root servers are located at the root of the entire Internet infrastructure, and without them most of the networks you know and use today would be unusable.
There are currently a total of 13 DNS servers in the world, one of which is the main root server in the United States, operated by the American Internet Agency. The remaining 12 root servers are secondary root servers, 9 are also in the United States, 2 are in Europe, and 1 is in Asia.
Due to the limitations of the local DNS infrastructure, it uses 32-byte IPv4, and the IP address must correspond to a packet, which is limited to 512 bytes at that time, so each IPv4 address is 32 bits, of which 13 bits are 416 bytes, and the remaining 96 bytes are protocol information.
Initially, each of the 13 IP addresses had a server, each IP address was given a cluster of servers, using broadcast routing to create a network of hundreds of servers around the world.
This helps balance decentralization and ensures reliability, guaranteeing the availability of the network even if one of the root servers is due to DDoS attacks or other types of DNS attacks.
After the launch of the "Yeti Project" in 2016, 16 countries around the world will complete the construction of 25 IPv6 root servers, which means that there will be more root servers in the future.
Why are there so many root servers, none of which are Chinese?
Because the protocol contained in the original DNS server did not have such a powerful ability to distribute a separate computer to China.
That is to say, due to limited technical capabilities, there is no way to distribute the 13 root servers in the world to China, which means that the mainland was indeed embattled in the field of DNS root servers.
Of course, in China, there are also 26 root nameserver nodes:
- Beijing I, L, J, K, F
- Hong Kong Special Administrative Region A, I, H, F, F, E, J
- Taipei City I, E, F, F, K, L
- Macao SAR E and F
- Shanghai L
- Hangzhou F
- Zhengzhou City L
- Wuhan City L
- Xining City L
- Guiyang City K
However, this does not mean that there is no room for development of China's root servers in the future, after all, the mainland has already occupied a place in IPv6 root servers.
With the country's support for IPv6, China has made amazing speed in IPv6 network construction. Among the ensuing ipv6 root DNS servers, China has 4 servers, of which one is the primary root and the other three are secondary roots.
Therefore, in the future, there will definitely be a position in China in the field of root servers.
03 6 good DNS servers, which is your thing?
01 Ali DNS (Alidns)
This set of DNS is provided by Alibaba, domestic connectivity is still good, overseas parts of the connectivity is not particularly good, you can test it.
Alibaba DNS provides both IPv4/IPv6 DNS and DoT/DoH services.
IPv4:
223.5.5.5
223.6.6.6
IPv6:
2400:3200::1
2400:3200:father::1
DoH Address:
https://dns.alidns.com/dns-query
DoT Address:
http://dns.alidns.com
02 Tencent DNS (DNSPod)
The public free DNS provided by DNSPod, later acquired by Tencent, is now owned by Tencent, and the stability and connectivity are also good, and can be used overseas after testing.
DNSPod now supports both IPv6 DNS and DoT/DoH services in addition to IPv4.
IPv4:
119.29.29.29
IPv6:
2402:4e00::
DoH Address:
https://doh.pub/dns-query
DoH (State Secret SM2, based on Tencent Cloud Government and Enterprise State Secret Solution) Address:
https://sm2.doh.pub/dns-query
DoT Address:
dot.pub
03 Google Public DNS
Google Public DNS is one of the most popular free DNS servers, its servers are hosted in data centers around the world, once connected to their IP address, it will automatically assign the nearest server, able to provide people in different regions with a stable and fast access experience
Google DNS has a very memorable IP address 8.8.8.8, by changing your default DNS to Google's public DNS server, you can have faster browsing speed and higher security, and it provides two DNS server addresses that you can set as your preferred and alternate DNS servers respectively
Address:
8.8.8.8
8.8.4.4
04 360 Secure DNS
360 provides DNS services, only IPv4 and DoT/DoH services.
IPv4 for China Telecom/Tietong/Mobile:
101.226.4.6
218.30.118.6
IPv4 for China Unicom:
123.125.81.6
140.207.198.6
DoH Address:
https://doh.360.cn
DoT Address:
http://dot.360.cn
China Telecom IPv6 DNS
If you use China Telecom's broadband, you should automatically allocate these two IPs
240e:4c:4008::1
240e:4c:4808::1
China Unicom IPv6 DNS
If you use China Unicom's broadband, you should automatically assign these two IPs
2408:8899::8
2408:8888::8
China Mobile IPv6 DNS
China Mobile Broadband automatically assigns two IPs
2409:8088::a
2409:8088::b
114DNS
114.114.114.114
114.114.115.115
05 Baidu DNS
The public DNS provided by Baidu, China's largest search engine, has been tested for overseas connectivity is also good, and for the time being, Baidu only provides a set of IPv4 and IPv6 DNS addresses.
IPv4:
180.76.76.76
IPv6:
2400:da00::6666
06 CNNIC DNS
It is a free public DNS provided by CNNIC, the China Internet Information Center, and supports both IPv4 and IPv6.
IPv4:
1.2.4.8
210.2.4.8
IPv6:
2001:dc7:1000::1
Finishing: Lao Yang丨10 years of senior network engineer, more network workers to improve dry goods, please pay attention to the public number: network engineer club