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:

# 調(diào)整 systemd-resolved 的 DNS 緩存大小和超時(shí)時(shí)間
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 服務(wù)的 DNS 緩存
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:

# 重啟 nscd 服務(wù)
sudo systemctl restart nscd

# 清除 DNS 緩存文件
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.

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