Linux 中的 DNS 缓存

我对 DNS 缓存感到困惑。我正在编写一个小型的转发代理服务器，并希望在 Linux 系统上使用 OS DNS 缓存。

如果我理解正确，在浏览器级别有 DNS 缓存。然后有 DNS 缓存在操作系统级别(Windows 有它。我不确定 Linux 发行版是否默认使用它)。

浏览器或代理服务器如何使用 OS DNS 缓存？我试图找出我是否可以依靠 Linux 进行 DNS 缓存，而不是在我的代理中独自进行。

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最佳答案

On Linux (and probably most Unixes), there is no OS-level DNS caching unless nscd is installed and running. Even then, the DNS caching feature of nscd is disabled by default at least in Debian because it's broken. The practical upshot is that your Linux system very probably does not do any OS-level DNS caching.

You could implement your own cache in your application (like they did for Squid, according to diegows's comment), but I would recommend against it. It's a lot of work, it's easy to get it wrong (nscd got it wrong!!!), it likely won't be as easily tunable as a dedicated DNS cache, and it duplicates functionality that already exists outside your application.

If an end user using your software needs to have DNS caching because the DNS query load is large enough to be a problem or the round-trip time to the external DNS server is long enough to be a problem, they can install a caching DNS server such as Unbound on the same machine as your application, configured to cache responses and forward misses to the regular DNS resolvers.

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Here are two other software packages which can be used for DNS caching on Linux:

  • dnsmasq
  • bind

After configuring the software for DNS forwarding and caching, you then set the system's DNS resolver to 127.0.0.1 in /etc/resolv.conf.

If your system is using NetworkManager you can either try using the dns=dnsmasq option in /etc/NetworkManager/NetworkManager.conf or you can change your connection settings to Automatic (Address Only) and then use a script in the /etc/NetworkManager/dispatcher.d directory to get the DHCP nameserver, set it as the DNS forwarding server in your DNS cache software and then trigger a configuration reload.

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Firefox contains a dns cache. To disable the DNS cache:

  1. Open your browser
  2. Type in about:config in the address bar
  3. Right click on the list of Properties and select New > Integer in the Context menu
  4. Enter 'network.dnsCacheExpiration' as the preference name and 0 as the integer value

When disabled, Firefox will use the DNS cache provided by the OS.

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Here you have an example of DNS caching in Debian using dnsmasq: Local DNS caching, article on ManageaCloud.

Configuration summary:

/etc/default/dnsmasq

# Ensure you add this line
DNSMASQ_OPTS="-r /etc/resolv.dnsmasq"

/etc/resolv.dnsmasq

# Your preferred servers
nameserver 1.1.1.1
nameserver 8.8.8.8
nameserver 2001:4860:4860::8888

/etc/resolv.conf

nameserver 127.0.0.1

Then just restart dnsmasq.

Benchmark test using DNS 1.1.1.1:

for i in {1..100}; do time dig slashdot.org @1.1.1.1; done 2>&1 | grep ^real | sed -e s/.*m// | awk '{sum += $1} END {print sum / NR}'

Benchmark test using your locally caching DNS forwarder (dnsmasq):

for i in {1..100}; do time dig slashdot.org; done 2>&1 | grep ^real | sed -e s/.*m// | awk '{sum += $1} END {print sum / NR}'
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DNS caching is implemented nowadays by systemd-resolved at the OS level :

https://fedoraproject.org/wiki/Changes/systemd-resolved#Caching


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