DNS control device and DNS control method

Abstract

In order to suppress delays in data updating, increases in load, delays in access using a host name, and the like in a DNS system, this DNS control device controls a DNS cache server, and includes: a DNS control unit that manages change content of a DNS record; a setting unit that sets the change content of the DNS record in a DNS content server; and a cache server control unit that instructs the DNS cache server to discard a cache corresponding to the DNS record.

Description

DNS Control Device and DNS Control Method

[0001] The present invention relates to a DNS control device and a DNS control method.

[0002] A DNS (Domain Name System) cache refers to the DNS cache server retaining the result of name resolution for a certain period of time and then responding with the cached data to the client thereafter. By responding with the cached data, it becomes unnecessary to query the upper DNS content server every time.

[0003] TTL (Time To Live) refers to the time for which a resource record may be held in a DNS cache server and is specified, for example, in seconds. After the data is held in the cache, the TTL decreases, and when it reaches zero, the data held in the cache is deleted.

[0004] RFC1034, "Domain Names - Concepts and Facilities", [Searched on October 4, 2024], Internet, <https: / / jprs.jp / tech / material / rfc / RFC1034-ja.txt>.

[0005] As a countermeasure against large-scale disasters such as building disasters, buildings in remote locations may be configured as redundant systems so that disaster recovery can be achieved. In the case of switching between buildings, for example, network restrictions such as a change in the IP address range may occur. In this case as well, by changing the IP address resolved by DNS to the changed address, the client can access using the domain name (FQDN: Fully Qualified Domain Name) in the same way as before the switch.

[0006] However, for example, when the TTL is long, at the time of switching between buildings, even if the IP address of the communication terminal is changed, there is a problem that the client cannot use the service until the TTL of the DNS cache expires and the data is updated in the DNS cache server.

[0007] On the other hand, if the TTL is short, the frequency of queries from the DNS cache server increases, which increases the load on the DNS content server, and the cache does not persist, resulting in problems such as slower access using hostnames.

[0008] Embodiments of the present invention have been made in view of the above-mentioned problems, and suppress delays in data updates, increased load, and delays in access using hostnames in a DNS system.

[0009] To solve the above problems, an embodiment of the present invention provides a DNS control device for controlling a DNS cache server, comprising: a DNS control unit for managing changes to DNS records; a setting unit for setting the changes to DNS records on a DNS content server; and a cache server control unit for instructing the DNS cache server to discard the cache corresponding to the DNS records.

[0010] According to embodiments of the present invention, in a DNS system, delays in data updates, increased load, and delays in access using hostnames can be suppressed.

[0011] This figure shows an example of the system configuration of the DNS system according to this embodiment. This figure shows an example of the state management table according to this embodiment. This figure is for explaining the overview of the processing according to this embodiment. This flowchart shows an example of the processing of the DNS control device according to Embodiment 1. This flowchart shows an example of the processing of the DNS control device according to Embodiment 2. This flowchart shows an example of the processing of the DNS control device according to Embodiment 3. This figure shows an example of the hardware configuration of the computer. This is a figure (1) for explaining the background. This is a figure (2) for explaining the background. This is a figure for explaining the problem.

[0012] Hereinafter, embodiments of the present invention (this embodiment) will be described with reference to the drawings. The embodiments described below are merely examples, and the embodiments to which the present invention is applied are not limited to the embodiments described below.

[0013] <Background and Challenges> As a countermeasure against large-scale disasters such as building damage, for example, a building in a remote location may be configured as a redundant system to enable disaster recovery. When switching between buildings, network limitations may arise, such as a change in the IP address range. Even in this case, for example, as shown in Figure 8, by changing the IP address resolved by DNS (Domain Name System) to the new address, the client 12 can access the building using the domain name (FQDN: Fully Qualified Domain Name) as before the switch.

[0014] In the example shown in Figure 8, for instance, if building 10a is damaged and server 11a becomes unavailable, the DNS switches the IP address "IP1" of server 11a to the IP address "IP2" of server 11b. As a result, client 12 can access server 11b located in building 10b using the domain name (FQDN) from before the disaster.

[0015] Similarly, as shown in Figure 9, for example, a domain name (FQDN) can be assigned to the communication terminal 21, and the building 20b can be configured as a redundant system for disaster recovery even if building 20a is damaged.

[0016] In the example shown in Figure 9, for instance, if building 20a is damaged and the communication terminal 21 in building 20a becomes unusable, the DNS switches the IP address "IP1" of the communication terminal 21 to the IP address "IP2" of server 11b. As a result, client 22 can access the communication terminal 21 in building 20b using the domain name (FQDN) from before the disaster.

[0017] (Challenge) DNS caching refers to a DNS cache server holding the results of name resolution for a certain period of time, and thereafter responding to the client with cached data. By responding with cached data, it becomes unnecessary to query the upstream DNS content server every time.

[0018] For example, in Figure 10, client 31 queries DNS cache server 32 for the IP address of "xxx.example.com" (step S1).

[0019] In response, the DNS cache server 32 queries the DNS content server (root) 33a for the IP address of "xxx.example.com" (step S2). The DNS content server 33a has zone information under its management and information about the delegated DNS content server (.com) 33b. If it does not have information about the IP address being queried, it returns information about the delegated DNS content server 33b.

[0020] As a result, the DNS cache server 32 queries the delegated DNS content server 33b for the IP address of "xxx.example.com" (step S3). The DNS content server 33b also has zone information under its control and information about the delegated DNS content server 33b. If it does not have information about the IP address being queried, it returns information about the delegated DNS content server (example.com) 33c.

[0021] As a result, the DNS cache server 32 queries the delegated DNS content server 33c for the IP address of "xxx.example.com" (step S4). The DNS content server 33c also has zone information under its control and information about the delegated DNS content server 33b. Here, the DNS content server 33c has the IP address information "xxx.example.com" that was queried, and returns the IP address "192.168.0.1" corresponding to "xxx.example.com" to the DNS cache server 32.

[0022] As a result, the DNS cache server 32 responds to the client 31 that the IP address of "xxx.example.com" that was queried is "192.168.0.1" (step S5).

[0023] Furthermore, the DNS cache server 32 stores the query result (that the IP address of "xxx.example.com" is "192.168.0.1") in the cache 34. Also, for subsequent queries, the DNS cache server 32 responds with the information stored in the cache 34, so the processing in steps S2 to S4 can be omitted.

[0024] The information stored in the cache 34 is deleted after the TTL (Time To Live) has elapsed. Here, TTL refers to the time during which resource records can be retained in the DNS cache server 32, and is specified, for example, in seconds. The TTL decreases after data is stored in the cache, and when the TTL reaches zero, the data is deleted from the cache 34.

[0025] However, if the TTL is long, for example, when switching between buildings, even if the IP address of the communication terminal changes, there is a problem that the client cannot use the service until the DNS cache TTL expires and the data is updated on the DNS cache server.

[0026] On the other hand, if the TTL is short, the frequency of queries from the DNS cache server increases, leading to increased load on the DNS content server. Furthermore, the cache may not persist, resulting in slower access using hostnames. Additionally, this increases the vulnerability to cache poisoning attacks.

[0027] Therefore, if you want to comply with the Service Level Agreement (SLA) for site switching, the Time To Live (TTL) of the DNS cache server must be shorter than the switching time defined in the SLA. On the other hand, if the TTL is too short, the load on the DNS cache server and DNS content server will increase, leading to security risks. Here, an SLA is the level of quality assurance agreed upon between the service provider and the client for the services provided.

[0028] Therefore, in order to suppress delays in data updates, increased load, and delays in access using hostnames, the DNS system according to this embodiment has a system configuration such as that shown in Figure 1.

[0029] <System Configuration> Figure 1 shows an example of the system configuration of a DNS system according to this embodiment. In the example in Figure 1, the DNS system 100 includes a DNS control device 110, a DNS content server 101, and a DNS cache server 102, etc. Note that the DNS content server 101 and the DNS cache server 102 may be devices or services outside the DNS system 100.

[0030] The DNS control device 110 is an information processing device equipped with a computer configuration, or a system including multiple computers. For example, the DNS control device 110 realizes the various functional configurations shown in Figure 1 by executing a predetermined program on the computer equipped with the DNS control device 110. In the example in Figure 1, the DNS control device 110 has various functional configurations such as a change reception unit 111, a DNS control unit 112, a setting unit 113, a cache server control unit 114, and a storage unit 115. At least some of the above functional configurations may be realized by hardware.

[0031] The change reception unit 111 executes a change reception process to receive the changes to the DNS record. For example, the change reception unit 111 provides an API (Application Programming Interface) to an external device to receive the changes to the DNS record.

[0032] The DNS control unit 112 executes DNS control processing to manage the changes to the DNS record received by the change reception unit 111. For example, the DNS control unit 112 stores and manages the changes received by the change reception unit 111 in a status management table 116 or the like.

[0033] Figure 2 shows an example of a state management table according to this embodiment. In the example in Figure 2, the state management table 116 includes information such as hostname, IP address, and update status as items. The hostname is, for example, the hostname of the communication terminal 21 described in Figure 9. The IP address is the IP address corresponding to the hostname.

[0034] The update status is information indicating the update status corresponding to the hostname. The update status includes, for example, three states: "Before Update," "Updating," and "Updated."

[0035] Note that the status management table 116 shown in Figure 2 is just one example. The status management table 116 may also contain other information, such as MSISDN (Mobile Station International Subscriber Directory Number) or domains.

[0036] Now, let's return to Figure 1 and continue explaining the functional configuration of the DNS control device 110. The setting unit 113 performs a setting process to set the changes to the DNS record to the DNS content server 101. For example, the setting unit 113 instructs the DNS content server 101 to change the DNS record according to the control from the DNS control unit 112.

[0037] The cache server control unit 114 executes cache server control processing, which instructs the DNS cache server 102 to discard the cache corresponding to the DNS record whose changes have been set by the setting unit 113.

[0038] The memory unit 115 stores various data, information, and / or programs, such as a state management table 116.

[0039] The DNS content server 101 is a DNS server that holds zone information where domain names and IP addresses are associated, and can return responses without querying other DNS servers. The DNS content server 101 is also called an authoritative DNS server.

[0040] The DNS cache server 102 is a DNS server that receives inquiries from clients and instead makes inquiries to the Internet. For example, the DNS cache server 102 receives a name resolution inquiry for a domain name from a client and makes an inquiry to the DNS content server 101 that manages the corresponding domain name. Also, the DNS cache server 102 responds to the client with the IP address corresponding to the domain name and caches the result of the inquiry.

[0041] Note that the functional configuration of the DNS control device 110 shown in FIG. 1 is an example. For example, at least a part of the change reception unit 111, the setting unit 113, and the cache server control unit 114 may be provided inside the DNS control unit 112. Also, the functions of the DNS control unit 112 may be distributed and provided to the change reception unit 111, the setting unit 113, the cache server control unit 114, and the like.

[0042] Also, each functional configuration of the DNS control device 110 shown in FIG. 1 may be distributed and provided to a plurality of devices. In this case, each functional configuration of the DNS control device 110 shown in FIG. 1 only needs to be possessed by the DNS system 100.

[0043] <Overview of Processing> FIG. 3 is a diagram for explaining the overview of the processing according to the present embodiment. The change reception unit 111 of the DNS control device 110 receives the change content of the DNS record from, for example, the reception device 301 or the NW (Network) switching unit 304.

[0044] The reception device 301 receives, for example, change content such as registration, change, or deletion of a DNS record by a user, and notifies the received change content to the DNS control device 110.

[0045] The NW switching unit 304 executes an NW switching process to switch the network to another non-faulty site (e.g., building 303b) for communication restoration in a situation where a certain site (e.g., building 303a) is affected by a disaster and a failure occurs in the communication equipment during a disaster. When the network is switched, the NW switching unit 304 issues a new IP address to the communication terminal 307 connected to the mobile network because the sites are separated. At this time, since the IP address of the communication terminal 307 is changed, the NW switching unit 304 notifies the DNS control device 110 of the change details of the IP address of the communication terminal 307.

[0046] The server (DNS client) 302 makes a recursive inquiry to the DNS cache server 102 (stub resolver) about the FQDN of the communication terminal 307 as the requester of name resolution.

[0047] (Premise) It is a system configured within a closed network for mobile devices. Assume that the DNS control device 110 has information about the DNS cache server 102 existing for each user environment. Also, assume that the IP address issued to the communication terminal 307 is determined in advance for each building, and when network switching occurs, the IP address of the communication terminal 307 is switched all at once. Assume that the query processing performance of the DNS content server 101 is 5 api / sec.

[0048] (Assumed scenario) A case where building 303a is affected by a disaster and the IP address issued to the communication terminal 307 changes along with the switching of the network to the redundant line of building 303b. In this case, since rapid communication restoration is required, it is necessary to change the DNS record all at once for the changed IP address.

[0049] (Summary of the process) When building 303a is affected by a disaster, the NW switching unit 304 notifies the DNS control device 110 of a network switching event (step S11). The DNS control device 110 sets the DNS record change to the DNS content server 101 (step S12). The DNS control device 110 also instructs the DNS cache server 102 to discard the cache (step S13). The DNS cache server 102 discards the cache according to the instructions from the DNS control device 110 (step S14).

[0050] In this state, when the DNS cache server 102 receives a name resolution request from the server 302, it finds that no cache exists and queries the DNS content server 101 for name resolution. The DNS cache server 102 also stores the name resolution result in its cache and returns the result to the server 302.

[0051] Furthermore, if we consider an internet environment rather than a closed network, there are countless DNS cache servers 102, so the cache updates of DNS cache servers 102 will be specified by TTL (Time To Live) to have an expiration time.

[0052] On the other hand, in this embodiment, since it is based on a network service that constitutes a closed network, the DNS control device 110 can identify the DNS cache server 102. Therefore, the DNS control device 110 can proactively delete the cache of the DNS cache server 102 in conjunction with the update of the DNS content server 101. As a result, according to this embodiment, network switching that is independent of TTL can be performed quickly.

[0053] <Processing Flow> Next, the specific processing flow of the DNS control method according to this embodiment will be explained.

[0054] [Example 1] Figure 4 is a flowchart showing an example of processing performed by the DNS control device according to Example 1. This processing shows, for example, an example of processing performed by the DNS control device 110 in the processing described in Figure 3.

[0055] In step S401, when the change reception unit 111 receives the changes to the DNS record, the DNS control device 110 executes the processing from step S402 onward.

[0056] In step S402, the DNS control unit 112 stores the changes received by the change reception unit 111 in a status management table 116, for example, as shown in Figure 2, and sets the update status of the stored DNS record to "before update".

[0057] In step S403, the setting unit 113 sets the changes to the DNS records whose update status is "before update" in the status management table 116 to the DNS content server 101. The DNS control unit 112 also changes the update status of the DNS records set by the setting unit 113 to "updating".

[0058] In step S404, when the setting unit 113 receives confirmation from the DNS content server 101 that the update is complete, in step S405, the DNS control unit 112 changes the update status of the DNS record set by the setting unit 113 to "updated".

[0059] In step S406, the cache server control unit 114 instructs the DNS cache server 102 to delete all caches.

[0060] In step S407, when the cache server control unit 114 receives a message from the DNS cache server 102 indicating that the cache has been deleted, in step S408, the DNS control unit 112 deletes the "updated" DNS record from the status management table 116.

[0061] As shown in Figure 4, the DNS control device 110 can suppress delays in data updates, increased load, and delays in access using hostnames in the DNS system 100.

[0062] [Example 2] Figure 5 is a flowchart showing an example of the processing performed by the DNS control device according to Example 2. This processing shows another example of the processing performed by the DNS control device 110 in the processing described in Figure 3. In Example 2, although immediate communication restoration is required, a case is described in which DNS registration changes can only be performed on a domain basis due to the product specifications of the DNS content server 101.

[0063] In step S501, when the change reception unit 111 receives the changes to the DNS record, the DNS control device 110 executes the processing from step S502 onward.

[0064] In step S502, the DNS control unit 112 stores the changes received by the change reception unit 111 in a status management table 116, for example, as shown in Figure 2, and sets the update status of the updated DNS record to "before update".

[0065] In step S503, the DNS control unit 112 declares the variable DOMAIN and assigns NULL as its initial value.

[0066] In step S504, the DNS control unit 112 obtains a list of domains, deletes duplicate rows, selects one domain whose update status is "before update", and assigns it to the variable DOMAIN.

[0067] In step S505, the DNS control unit 112 determines whether DOMAIN == NULL. If DOMAIN is not NULL, the DNS control unit 112 proceeds to step S506. On the other hand, if DOMAIN is NULL, the DNS control unit 112 terminates the process shown in Figure 5.

[0068] When the process moves to step S506, the configuration unit 113 sets the DNS content server 101 with the changes corresponding to DOMAIN among the DNS records whose update status is "before update" in the status management table 116. The DNS control unit 112 also changes the update status of the DNS records set by the configuration unit 113 to "updating".

[0069] In step S507, when the setting unit 113 receives confirmation from the DNS content server 101 that the update is complete, in step S508, the DNS control unit 112 changes the update status of the DNS record set by the setting unit 113 to "updated".

[0070] In step S509, the cache server control unit 114 instructs the DNS cache server 102 to delete the cache corresponding to DOMAIN.

[0071] In step S510, when the cache server control unit 114 receives a message from the DNS cache server 102 indicating that the cache has been deleted, the DNS control unit 112 executes the process in step S511.

[0072] In step S511, the DNS control unit 112 deletes the "updated" DNS record from the status management table 116 and returns the process to step S503.

[0073] The process shown in Figure 5 makes it possible to suppress delays in data updates, increased load, and delays in access using hostnames, even when the DNS content server 101 can only change DNS registrations on a domain-by-domain basis.

[0074] The restriction that DNS registration changes can only be made on a domain basis is just one example of a limitation of the DNS content server 101. The limitations of the DNS content server 101 may also be such that registration changes can only be made on a zone basis, a subdomain basis, or the like. In this case, the DNS control device 110 can perform the processing shown in Figure 5 on a zone basis or a subdomain basis.

[0075] [Example 3] Figure 6 is a flowchart showing an example of the processing of the DNS control device according to Example 3. This process describes, for example, the case in Figure 3 where the registration information in the DNS content server 101 is updated in response to a resource record change request from the user. In this case, unlike when switching locations, the DNS record is changed in response to a change order from the user, so the change is made one record at a time.

[0076] In step S601, when the change reception unit 111 receives the change content of the DNS record, the DNS control device 110 executes the processes after step S602. In addition, in the third embodiment, it is assumed that the change reception unit 111 receives a DNS record change request from the user from the reception device 301 described in FIG. 3.

[0077] In step S602, the DNS control unit 112 stores the change content received by the change reception unit 111 in, for example, the state management table 116 as shown in FIG. 2, and sets the update state of the updated DNS record to "before update".

[0078] In step S603, the DNS control unit 112 sets the counter i to 0 and sets size to the number of records to be changed.

[0079] In step S604, the DNS control unit 112 determines whether i < size. If i < size, the DNS control unit 112 transfers the process to step S605. On the other hand, if i < size is not satisfied, the DNS control unit 112 ends the process of FIG. 6.

[0080] When transferring to step S605, the DNS control unit 112 selects one row with the update state of "before update" from the state management table 116 and assigns it to the variable ROW.

[0081] In step S606, the setting unit 113 sets the change content corresponding to ROW to the DNS content server 101. In addition, the DNS control unit 112 changes the update state of the DNS record set by the setting unit 113 to "updating".

[0082] In step S607, when the setting unit 113 receives the update completion from the DNS content server 101, in step S608, the DNS control unit 112 changes the update state of the DNS record set by the setting unit 113 to "updated".

[0083] In step S609, the cache server control unit 114 instructs the DNS cache server 102 to delete the cache corresponding to ROW.

[0084] In step S610, when the cache server control unit 114 receives a message from the DNS cache server 102 indicating that the cache has been deleted, in step S611, the DNS control unit 112 deletes the "updated" DNS record from the status management table 116.

[0085] In step S612, the DNS control unit 112 adds 1 to counter i and returns the process to step S604.

[0086] As shown in Figure 6, the DNS control device 110 can suppress delays in data updates, increases in load, and delays in access using hostnames, even when it receives a resource record change request from a user.

[0087] Furthermore, Examples 1 to 3 can be implemented in combination. For example, the setting unit 113 of the DNS control device 110 may select simultaneous switching, switching per domain, or switching per DNS record, depending on the changes to the DNS records or the capabilities of the DNS content server 101, and set the changes to the DNS records in the DNS content server 101.

[0088] <Hardware Configuration> The DNS control device 110 according to this embodiment has a hardware configuration of a computer 700 as shown in Figure 7. The DNS control device 110 may be implemented by multiple computers 700. Furthermore, the computer is not limited to a physical machine, but may be a virtual machine on the cloud, for example.

[0089] Figure 7 shows an example of a computer hardware configuration. In the example in Figure 7, the computer 700 includes a drive device 701, an auxiliary storage device 702, a memory device 703, a CPU 704, an interface device 705, a display device 706, an input device 707, and an output device 708, all of which are interconnected by bus B. The computer 700 may also include other processors such as a GPU (Graphics Processing Unit).

[0090] The program that enables processing on the computer 700 is provided on a recording medium (storage medium) 711, such as a CD-ROM or memory card. When the recording medium 711 containing the program is set in the drive device 701, the program is installed from the recording medium 711 to the auxiliary storage device 702 via the drive device 701. However, the program does not necessarily have to be installed from the recording medium 711; it may also be downloaded from another computer via a network. The auxiliary storage device 702 stores the installed program as well as necessary files and data.

[0091] The memory device 703 reads and stores a program from the auxiliary storage device 702 when a program startup command is received. The CPU 704 implements functions related to the DNS control device 110, for example, according to the program stored in the memory device 703. The interface device 705 is used as an interface for connecting to a communication network, etc. The display device 706 displays a GUI (Graphical User Interface) etc., based on a program. The input device 707 consists of a keyboard, mouse, buttons, and / or touch panel etc., and is used to input various operation commands. The output device 708 outputs the calculation results.

[0092] The CPU 704 may also be other processors, such as a DSP (Digital Signal Processor), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array).

[0093] <Effects of the Embodiment> According to this embodiment, delays in data updates, increased load, and delays in access using hostnames can be suppressed in the DNS system 100. Furthermore, according to this embodiment, it becomes possible to change DNS records within the time defined in the SLA while avoiding increased load on the DNS servers (DNS content server 101, DNS cache server 102) and security risks.

[0094] <Summary of Embodiments> This specification discloses at least the following DNS control devices, DNS control methods, and DNS systems: (Section 1) A DNS control device for controlling a DNS cache server, comprising: a DNS control unit for managing changes to DNS records; a setting unit for setting the changes to the DNS records on a DNS content server; and a cache server control unit for instructing the DNS cache server to discard the cache corresponding to the DNS records. (Section 2) The DNS control device according to Section 1, further comprising a change reception unit for receiving changes to the DNS records, wherein the changes to the DNS records include changes to the IP addresses of the plurality of communication terminals due to switching of a network to which the plurality of communication terminals are connected. (Clause 3) The DNS control device according to paragraph 1 or 2, wherein the setting unit selects a simultaneous switch, a domain-by-domain switch, or a DNS record-by-DNS switch according to the changes in the DNS record or the capabilities of the DNS content server, and sets the changes in the DNS record to the DNS content server. (Clause 4) A DNS control method in which a computer controlling a DNS cache server performs the following: a process for managing changes in the DNS record; a process for notifying the DNS content server of the changes in the DNS record; and a process for instructing the DNS cache server to discard the cache corresponding to the DNS record. (Clause 5) A DNS system for controlling a DNS cache server, comprising: a DNS control unit for managing changes in the DNS record; a setting unit for setting changes in the DNS record to the DNS content server; and a cache server control unit for instructing the DNS cache server to discard the cache corresponding to the DNS record.

[0095] Although this embodiment has been described above, the present invention is not limited to this specific embodiment, and various modifications and changes are possible within the scope of the gist of the invention as described in the claims.

[0096] 100 DNS system 101 DNS content server 102 DNS cache server 110 DNS control device 111 Change reception unit 112 DNS control unit 113 Setting unit 114 Cache server control unit 115 Storage unit 116 Status management table 301 Reception device 302 Server (DNS client) 304 Network switching unit 307 Communication terminal

Claims

1. A DNS control device for controlling a DNS cache server, comprising: a DNS control unit for managing changes to DNS records; a setting unit for setting the changes to DNS records on a DNS content server; and a cache server control unit for instructing the DNS cache server to discard the cache corresponding to the DNS records.

2. The DNS control device according to claim 1, further comprising a change reception unit that receives changes to the DNS record, wherein the changes to the DNS record include changes to the IP addresses of the multiple communication terminals due to switching of the network to which the multiple communication terminals are connected.

3. The DNS control device according to claim 1 or 2, wherein the setting unit selects a simultaneous switch, a domain-by-domain switch, or a DNS record-by-DNS switch according to the changes in the DNS record or the capabilities of the DNS content server, and sets the changes in the DNS record to the DNS content server.

4. A DNS control method in which a computer controlling a DNS cache server performs the following: a process for managing changes to DNS records; a process for notifying a DNS content server of the changes to the DNS records; and a process for instructing the DNS cache server to discard the cache corresponding to the DNS records.

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