How to view and refresh dns cache in Linux
DNS (Domain Name System) is a system used in the Internet to convert domain names into corresponding IP addresses.
In Linux systems, DNS caching is a mechanism that stores the mapping relationship between domain names and IP addresses locally, which can increase the speed of domain name resolution and reduce the burden on the DNS server. DNS caching allows the system to quickly retrieve the IP address when subsequently accessing the same domain name without having to issue a query request to the DNS server each time, thereby improving network performance and efficiency.
This article will discuss with you how to view and refresh the DNS cache on Linux, as well as related details and sample code.
The Importance of DNS Caching
In Linux systems, DNS caching plays a key role. Its existence can not only effectively reduce the time of DNS resolution and speed up network access, but also effectively reduce the load on the DNS server.
When the system accesses a specific domain name, if the resolution result of the domain name is already stored in the local cache, the system can directly obtain the result from the cache without sending a query request to the DNS server again. This mechanism effectively saves time and network bandwidth. By utilizing local caching, the system can improve access efficiency, especially when accessing the same domain name frequently. This method not only reduces the burden on the DNS server, but also reduces the consumption of network traffic, optimizing the overall network performance. Therefore, the caching mechanism plays an important role in improving system response speed and reducing resource consumption
How to check DNS cache
In the Linux operating system, there are many ways to view the contents of the DNS cache.
a. Use dig command
dig The command is a powerful DNS tool that can be used to query DNS information, including domain name resolution results and DNS cache content.
dig example.com
Output result:
; <<>> DiG 9.16.1-Ubuntu <<>> example.com ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 62233 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1 ;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 65494 ;; QUESTION SECTION: ;example.com. INA ;; ANSWER SECTION: example.com.604800INA 93.184.216.34 ;; Query time: 43 msec ;; SERVER: 127.0.0.53#53(127.0.0.53) ;; WHEN: Sat Feb 19 12:00:00 UTC 2024 ;; MSG SIZErcvd: 57
b. View nscd cache
nscd (Name Service Cache Daemon) is a daemon process responsible for managing the system name service cache.
You can use the nscd command to view the contents of the DNS cache.
nscd -g
Output result:
hosts cache: yescache is enabled yescache is persistent yescache is shared 211suggested size 216320total data pool size 1408used data pool size 7200seconds time to live for positive entries 20seconds time to live for negative entries 20cache hits on positive entries 0cache hits on negative entries 3cache misses on positive entries 0cache misses on negative entries 100 % cache hit rate 8current number of cached values 9maximum number of cached values 0maximum chain length searched 0number of delays on rdlock 0number of delays on wrlock 0memory allocations failed
c. View systemd-resolved cache
systemd-resolved is a system service responsible for resolving DNS queries. It also maintains a DNS cache.
You can use the systemd-resolve command to view the cache content.
systemd-resolve --statistics
Output result:
DNSSEC supported by current servers: no Transactions Current Transactions: 0 Total Transactions: 2422 Cache Current Cache Size: 15 Cache Hits: 2312 Cache Misses: 110
How to refresh DNS cache
Sometimes, you may need to manually refresh the DNS cache to ensure that the system uses the latest DNS resolution results.
a. Use systemd-resolved
You can use the systemd-resolved command to refresh the DNS cache of the systemd-resolved service.
sudo systemd-resolve --flush-caches
b. Restart nscd service
If the nscd service is running on the system, you can try to refresh the DNS cache by restarting the service.
sudo systemctl restart nscd
c. Clear DNS cache files
You can also manually delete the DNS cache file to clear the DNS cache.
sudo rm -rf /var/cache/bind/named_dump.db
Advanced Usage and Precautions
a. Use DNS caching to optimize performance
DNS caching can significantly increase the speed of domain name resolution and reduce the request pressure on the DNS server.
By properly configuring the DNS cache, system performance and network connection speed can be further optimized.
The size and timeout of the DNS cache can be adjusted to meet needs.
Sample code:
# 调整 systemd-resolved 的 DNS 缓存大小和超时时间 sudo systemctl edit systemd-resolved.service # 查看 systemd-resolved 的配置 cat /etc/systemd/resolved.conf
b. Pay attention to DNS cache consistency
When refreshing the DNS cache, you need to pay attention to ensure that all related DNS caches are refreshed to avoid DNS cache inconsistency.
This may cause the application to be unable to access a specific domain name or connect to the wrong IP address.
Sample code:
# 刷新 systemd-resolved 服务的 DNS 缓存 sudo systemd-resolve --flush-caches
c. Avoid frequently refreshing DNS cache
Although refreshing the DNS cache can solve some DNS-related problems, refreshing the DNS cache too frequently may affect system performance and network connection speed.
It is recommended to flush the DNS cache only when necessary, and make sure to understand the impact of the flush operation before flushing.
Sample code:
# 重启 nscd 服务 sudo systemctl restart nscd # 清除 DNS 缓存文件 sudo rm -rf /var/cache/bind/named_dump.db
Summarize
In Linux systems, viewing and refreshing the DNS cache is an important step in managing network connections and optimizing system performance.
By using command line tools such as dig, nscd and systemd-resolved, you can easily view the current system’s DNS cache information and take necessary actions Measures to flush the cache to ensure availability of the latest data.
The above is the detailed content of How to view and refresh dns cache in Linux. For more information, please follow other related articles on the PHP Chinese website!
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