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I am confused about DNS caching. I am writing a small forward proxy server and want to use OS DNS cache on a Linux system.
If I understand correctly, then there is DNS caching at the browser level. Then there is DNS caching at OS level (Windows has it. I am not sure if Linux distros have it by default).
So, how does a browser/proxy_server use OS DNS caching? I am trying to find if I can rely on Linux for DNS caching instead of doing it on my own inside my proxy.
Thanks
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In Debian/Ubuntu, that file is /var/cache/nscd/hosts for the hosts/DNS cache, so you can run strings /var/cache/nscd/hosts to see the hosts in cache.
On Linux, there is no OS-level DNS caching unless a caching service such as Systemd-Resolved, DNSMasq, or Nscd is installed and running. The process of clearing the DNS cache is different depending on the Linux distribution and the caching service you're using.
Disable DNS Cache Log in to your system with the user has sudo privileges and Edit NetworkManager configuration file in your favorite text editor. Now comment the following entry by added # symbol to start of line like below. Save the configuration file. Vi users use ESC + :wq to save file and quit.
If you have a proxy, the DNS cache is in the proxy. Squid for example, has its own DNS cache and resolver. On Linux (and probably most Unix), 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.
It is the preferred caching service for most Redhat-based systems. If your Linux system is using nscd caching service, you can clear or flush the local DNS caching service by simply restarting the nscd service like below: 3. Clear Bind/Named DNS caching service
In other words, the DNS cache contains recent DNS lookups. When you visit the same website multiple times, the OS retrieves the DNS records of that website from the local DNS cache database instead of the actual public DNS server. Hence DNS caching improves the website loading time and reduces the origin DNS server's bandwidth/CPU consumption.
Maybe you need to flush the DNS cache. Jack Wallen shows you how. The DNS cache is a temporary database on an operating system that stores DNS lookups (information about visited websites and internet domains).
On Linux (and probably most Unix), 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 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 RTT 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:
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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