With the development of the internet, DNS security threats have gradually increased, especially the "DNS cache poisoning" attack, which can severely impact website accessibility and security. DNS cache poisoning refers to attackers forging or tampering with records in the DNS server cache, causing users to access incorrect or malicious target websites. This attack not only renders the website unusable but also poses serious security risks to its users.

　　The working principle of DNS cache poisoning:

　　Before understanding DNS cache poisoning, we first need to understand how DNS caching works. When a user visits a website, the computer sends a request to the local DNS server to query the target website's IP address. The DNS server performs a series of queries to ultimately obtain the website's IP address and caches this result, typically keeping it valid for a certain period (determined by TTL, or "Time to Live"). This way, the next time the same website is accessed, the DNS server can directly return the IP address from the cache, thereby improving query speed and reducing network load.

　　DNS cache poisoning attacks typically occur within DNS servers. When a user sends a query request to a DNS server, an attacker inserts a forged DNS response (i.e., a fake IP address) into the DNS server's cache. These forged DNS records are cached, causing the user to access the wrong target website, or even a malicious one.

　　The DNS cache poisoning attack process is as follows:

　　When a user requests access to a domain name, the computer sends a query request to the DNS server. The attacker tampers with the original query result by sending a forged DNS response to the DNS server. For example, a user requests access to "example.com," but the attacker's forged response points to a malicious IP address (possibly a phishing website). The DNS server accepts the forged response and caches it. Future user requests will resolve to the incorrect IP address. Subsequently, when other users or systems make requests for the same domain name, the DNS server returns the malicious IP address, thus redirecting the user to a malicious website.

　　DNS cache poisoning attack methods:

　　There are several methods of DNS cache poisoning attacks, among which the common ones include:

　　Brute-force attack: Attackers send a large number of forged DNS responses to the DNS server, exploiting a time window in the system to tamper with its cache. Brute-force attacks force DNS servers to write incorrect responses into their caches by continuously sending forged DNS responses.

　　Blind attack methods: In some unprotected DNS servers, attackers can send forged DNS responses to directly tamper with cached content. Blind attacks typically rely on server security vulnerabilities.

　　DNS hijacking: Attackers control certain ISPs or local DNS servers, forging DNS responses to redirect user requests to malicious or advertising websites.

　　How to prevent DNS cache poisoning:

　　Enable DNSSEC (DNS Security Extensions). DNSSEC is a protocol that enhances DNS security by using digital signatures to ensure the authenticity of DNS records. When a DNS server receives a DNS response, DNSSEC verifies it to ensure the response has not been tampered with. If verification fails, the DNS server will not cache the response, thus preventing cache poisoning. Enabling DNSSEC can significantly improve the security of DNS resolution. Many mainstream DNS service providers support DNSSEC, and users can enable this feature by configuring their own DNS servers.

　　Regularly cleaning up the DNS cache helps reduce the risk of cache poisoning. Clearing the DNS cache ensures that DNS servers do not store outdated or inaccurate DNS records indefinitely, thus preventing attackers from affecting user access by tampering with cached records. On many operating systems, administrators can manually clear the DNS cache via command line. For Linux and macOS, users can also clear the cache using systemd or other commands.

　　To improve the security of DNS queries and prevent attackers from forging DNS responses through brute-force attacks, randomized ports and query IDs can be used. The source port and query ID for a DNS query should be randomly generated with each request. This increases the difficulty for attackers to guess the correct port and query ID, thereby reducing the success rate of DNS cache poisoning.

　　Choosing a highly secure DNS service provider can effectively prevent DNS cache poisoning. Some well-known public DNS providers offer robust security features, including encrypted transmission, DNSSEC support, and malicious URL filtering, helping users defend against DNS cache poisoning and other network attacks.

　　Deploying firewalls on DNS servers and networks, and setting strict access control policies, can prevent unauthorized access to DNS servers and reduce the risk of tampering. By setting source IP filtering and restricting external DNS queries, the occurrence of DNS cache poisoning attacks can be reduced.

　　Timely updates to DNS software and systems ensure that the DNS server's operating system and software versions are always up-to-date and that security patches are installed promptly. This is one of the fundamental measures to prevent DNS cache poisoning. Many attackers exploit vulnerabilities in DNS servers, therefore, regularly checking and updating server software is crucial to reducing security risks.

　　Monitoring DNS traffic can promptly detect abnormal DNS request and response behavior. For example, frequent requests for abnormal DNS resolution results for a specific domain may be a sign of DNS cache poisoning. Timely detection and handling of these abnormal requests can effectively prevent the spread of attacks.

　　DNS cache poisoning is a serious problem in current network security. It can affect normal website access and even lead to user data leaks and network security risks. By understanding how DNS cache poisoning works and taking a series of effective preventive measures (such as enabling DNSSEC, clearing the DNS cache, and choosing a high-security DNS service), network security can be significantly improved, preventing the potential harm caused by DNS cache poisoning. Strengthening DNS protection is an important step in ensuring internet security and a crucial issue that every website administrator and network security expert must pay attention to.