Foreword: The Domain Name System (DNS) is the Internet's phone book. Map IP addresses that are difficult for humans to remember to be relatively easy to remember in English, provide network services, and access information online through domain names such as nytimes.com or espn.com Web browsers interact through Internet Protocol (IP) addresses. DNS converts domain names to IP addresses so that browsers can load Internet resources. Each device connected to the Internet has a unique IP address that other computers can use to find the device. The DNS server does not require human memory IP addresses, such as 192.168.1.1 (in IPv4), or more complex new alphanumeric IP addresses, such as 2400: cb00: 2048: 1 :: c629: d7a2 (in IPv6). DNS domain name structure Each IP address can have a host name. The host name is composed of one or more character strings, and the strings are separated by a decimal point through the host name. The process of finally obtaining the IP address corresponding to the host name is called domain name resolution. Generally, the domain name structure of an Internet host is: host name. Third-level domain name. Second-level domain name. Top-level domain name. The Internet's top-level domain name is registered and managed by the Internet Network Association's domain name registration query committee responsible for network address allocation. It also assigns a unique IP address to each host on the Internet. Top-level domain: Cn --- is China Us ---is the United States Jp ---is Japan secondary domain: .com---Generally used for commercial institutions or companies .net---Generally used for organizations or companies engaged in Internet-related network services .top---generally used for enterprises and personal organizations .org---generally used for non-profit organizations and groups .gov---for government departments How does DNS work? Enter the www.baidu.com domain name in the browser. The operating system will first check whether its local hosts file has this URL mapping relationship. If so, it will first call this IP address mapping to complete the domain name resolution. If there is no mapping of this domain name in the hosts, then look up the local DNS resolver cache, if there is this URL mapping relationship, if there is, return directly to complete the domain name resolution. If there is no corresponding URL mapping relationship between the hosts and the local DNS resolver cache, we will first find the preferred DNS server set in the TCP / IP parameters, here we call it the local DNS server, When this server receives the query, if the domain name to be queried is included in the local configuration area resource, it will return the resolution result to the client to complete the domain name resolution. This resolution is authoritative. If the domain name to be queried is not resolved by the local DNS server area, but the server has cached this URL mapping relationship, then this IP address mapping is called to complete the domain name resolution, which is not authoritative. If both the local zone file and the cache resolution of the local DNS server are invalid, query according to the settings of the local DNS server (whether or not to set a forwarder), If the forwarding mode is not used, the local DNS will send the request to the "root DNS server". After receiving the request, the "root DNS server" will determine who the domain name (.com) is to authorize management and return a responsible domain name. An IP of the server. After the local DNS server receives the IP information, it will contact the server responsible for the .com domain. After the server responsible for the .com domain receives the request, if it cannot resolve it, It will find a lower DNS server address (baidu.com) that manages the .com domain to the local DNS server. When the local DNS server receives this address, it will find the baidu.com domain server, repeat the above actions, and query until it finds the www.baidu.com host. If the forwarding mode is used, the DNS server will forward the request to the upper-level DNS server for resolution by the upper-level server. , Cycle through this. Regardless of whether the local DNS server is used for forwarding or root hints, the result is finally returned to the local DNS server, and then the DNS server is returned to the client. Inquiry mode The query from the host to the local domain name server is generally recursive. The so-called recursive query is: if the local domain name server inquired by the host does not know the IP address of the domain name being queried, the local domain name server acts as a DNS client, Instead of sending the host to perform the next query, it will continue to send query request messages to other root domain name servers (that is, continue to query for the host). Therefore, the query result returned by the recursive query is either the IP address to be queried, or an error is reported, indicating that the required IP address cannot be queried. A Iterative query of the local domain name server to the root domain name server. Features of iterative query: When the root domain name server receives the iterative query request message from the local domain name server, it either gives the IP address to be queried or tells the local server: "Which domain name server should you query next" . Then let the local server perform subsequent queries. The root domain name server usually tells the local domain name server the IP address of the top-level domain name server that it knows, and then the local domain name server queries the top-level domain name server. After receiving the query request from the local domain name server, the top-level domain name server either gives the IP address to be queried, or tells the local server which authority domain name server to query next. Finally, know the IP address to be resolved or report an error, and then return this result to the host that initiated the query Basic configuration example SERVER (config) #ip dns server        //Enable its own ability to resolve domain names SERVER (config) #ip host r1 192.168.1.1  //On the DNS server, create a 'parse entry' SERVER (config) #ip host r2 192.168.1.2   //On the DNS server, create a 'parse entry' CLIENT (config) #ip name-server 192.168.1.1   //Set the DNS server, that is, point to the DNS server IP, when there is no resolution entry locally, iteratively query the next server CLIENT # telnet r1 (Execute the telnet command to check) Translating "r1"… domain server (192.168.1.1) [OK]

To fully understand BGP, we must first answer the following seemingly simple questions: why BGP is needed, that is, how BGP is generated, and what problems does it solve. With the above questions, let us briefly review the development trajectory of a routing protocol. First of all, the essence of routing is to describe the expression of a network structure. The routing table is actually a collection of results. In the early ARPANet network era, the network scale was limited and the number of routes was not large. Therefore, all routers can maintain the entire network topology. The routing protocol used at that time was called GGP (Gateway-to-Gateway Protocol). GGP naturally became the first internal gateway protocol (IGP). At that time, network managers encountered a similar problem to today: the number of routes caused by the expansion of the network scale continues to increase. In order to solve this problem of network size growth, an autonomous system concept (AS) is proposed, which can also be called a routing management domain. Use one routing protocol inside the AS, and then use another routing protocol between the AS. The benefits of this are obvious. Different networks can choose the IGP protocol and then interconnect through a unified inter-AS protocol.         In the development field of IGP, first RIP became the mainstream of IP routing, and then more advanced IGP protocols including OSP and ISIS appeared. These protocols are more automated, smarter and more reliable. There is a mutual trust relationship between routers in the same AS, and these routers are often maintained by the same management personnel. Therefore, IGP's automatic discovery and routing calculation information flooding are completely open, and there is relatively little manual intervention. The need for interconnection of different ASs has promoted the generation of external gateway protocol (EGP). The main purpose of EGP is to transfer routing protocols between different ASs. And different ASs are often directly connected, most AS interconnection behavior only involves a small number of border routers (ASBR), so the design of EGP is also very simple. EGP's RFC827 was released in 1982, and it seems to be earlier than RIP's first standard FRC1058, but in fact RIP has been widely used before RFC1058. At the time, RIP + EGP became a standard routing combination. EGP was designed so simple that it quickly failed to meet the requirements of network management. EGP simply publishes network reachability information without making any optimization or considering loop avoidance. Some people even think that EGP is not a routing protocol. Many of EGP's shortcomings are eventually replaced by BGP. BGP's first FRC1105 was released in 1989. Compared with EGP, BGP is more like a routing protocol, with many routing protocol features, such as solving loop problems, convergence problems, triggering updates, and so on. It's like different companies have their own corporate culture and standards, but the interaction between companies must follow a unified code of conduct and standards. There must also be a unified standard for routing interaction between ASs. The many advantages of BGP over EGP make BGP the only external gateway protocol and widely used on the Internet. In summary, BGP is an external gateway protocol that appears to replace EGP. It must be able to perform route selection, avoid routing loops, be able to deliver routes more efficiently, and maintain a large number of routes. Because BGP is deployed between ASs that do not have a complete trust relationship, BGP needs to have rich routing control capabilities, and BGP can be extended through some simple and uniform methods. BGP development BGPv1 (RFC1105) defines some of the most basic protocol features of BGP. BGP passes routes between ASs, so it is very important. In order to ensure the reliable transmission of BGP, TCP is used as the transport layer protocol. The advantages of using TCP are obvious. BGP can use TCP's existing reliable transmission mechanism, retransmission, sequencing and other mechanisms to ensure the reliability of protocol message interaction. The benefits of TCP extension can also be inherited, for example, MD5 authentication of TCP can be used by BGP.     BGP is established between two different AS and there is a trust problem. Therefore, BGP cannot be discovered automatically. Instead, it needs to manually configure neighbors and establish TCP relationships using specified addresses. The BGP relationship established with AS external nodes is called EBGP relationship, and the BGP relationship established with AS internal nodes is IBGP relationship. One of the most important concepts of BGP is to use the AS number to solve the loop problem between AS. If a certain routing information is received with its own AS number, it means that this route is a known route and it will not be processed anymore. If the AS number is duplicated, it means that there is a routing loop. There is no concept of AS-path in BGPv1, and this concept is made clear in BGPv2. BGP is constantly improving from v1, v2, v3, and now v4. BGP4 + is mainly an extension of multi-protocol BGP, also known as MP-BGP. The concept of MP-BGP will not be discussed in this article. Within the AS, because there is no change in the AS number, other methods are needed to prevent loops. BGP stipulates that the routes learned from IBGP neighbors will not be passed to another IBGP neighbor. Simply put, the route between IBGPs will only be transmitted by one hop, and the route will only be transmitted once. Of course, there is no problem of looping. At the same time, all routers within the AS are required to establish IBGP relationships in pairs. This is the BGP full connection in BGP technology. Full connectivity is unthinkable in a large network, so two technologies (RFC1966 and RFC1965) were later derived from route reflector and BGP alliance.   The route reflector designates a node as a reflector in the AS, all other nodes establish an IBGP relationship with the reflector, and the reflector acts as an intermediate node to pass routes between any other two IBGPs. Therefore, in theory, the reflector should not change the path attribute information when routing, otherwise it will destroy the principle of BGP avoiding loops inside the AS. However, from the perspective of practical application, different vendors have made many features on the function of the reflector, which requires careful use by BGP deployers. The BGP alliance is re-planned within the AS, and a flat AS is divided into multiple private ASs. The benefits of doing this can be a layered management of a large AS on the one hand, and on the other hand through the layer , Naturally reducing the need for full connectivity.   BGP messages use the TLV structure, which is very conducive to expansion and backward compatibility. Therefore, with the development of the network, a large number of RFCs on BGP extensions have been generated, which makes BGP an external gateway protocol that keeps youth forever.