The sys admin's daily grind – Pdnsd
Short-Term Memory
Cache it, if you can! When the latencies of his Internet connection seem to take longer than Napoleon's reign, sys admin Charly comes up with a solution for name resolution.
It is always annoying when I need to use Internet via a satellite route. The latency is really bad. To counteract this, I use caching wherever I can. My choice of cache for DNS requests is Pdnsd [1]. More or less any fat distribution will have the lean and fast daemon in its collection. When launched, the daemon parses the content of /etc/hosts
and stores it in its cache. Any DNS requests that I make are added.
By default, the cache is 2MB. If you have built a very long /etc/hosts
throughout your IT landscape, you can modify the cache size in /etc/pdnsd.conf
. The matching option resides in the global
section. It goes by the name of perm_cache
and expects the size in bytes – I use 8192
. By the way, the option is named perm_cache
because the cache not only resides in RAM but also on the disk. In other words, Pdnsd does not need to build the cache from scratch after a reboot.
In the global
configuration section, you will find other central settings. One setting that is very important is:
server_ip = 127.0.0.1;
What this means is that Pdnsd only responds to DNS requests that come from localhost. If you want to allow other machines on the same network to submit IP requests to Pdnsd, you need to replace 127.0.0.1 with the interface that points to your internal network:
server_ip = eth1;
Options min_ttl
and max_ttl
let you define the minimum and maximum amounts of time the cache will keep an entry. The defaults – 15 minutes and one week – make a lot of sense in my opinion, and I tend to leave them that way. This is not true of the timeout
parameter, which is typically 10 seconds; this is not enough if you make generous use of a satellite route. I tend to double this value to timeout = 20s;
.
Say It!
After setting up a new Pdnsd, I like to make it more chatty by setting verbose = 3;
. (In difficult cases, I maximize the verbosity by setting debug = on
.) Once everything has reached a steady state, I comment out this option, and Pdnsd silently goes about its work.
Another option that I find useful is status_ctl = on
. It allows me to send commands to Pdnsd on the fly using the pdnsd-ctl
tool. Figure 1 shows the command
sudo pdnsd-ctl status
in action. Right at the top, you can see the cache utilization level, followed by an overview of the active threads and global configuration options. The sudo pdnsd-ctl empty-cache
command lets you empty the cache, which can be necessary after DNS changes if you do not want to wait until the TTL expires. You can type sudo pdnsd-ctl help
for an overview of the other commands.
Although Pdnsd is unable to cure latency in satellite connections, it can at least alleviate the pain – and that is a good thing.
Charly Kühnast
Charly Kühnast is a Unix operating system administrator at the Data Center in Moers, Germany. His tasks include firewall and DMZ security and availability. He divides his leisure time into hot, wet, and eastern sectors, where he enjoys cooking, freshwater aquariums, and learning Japanese, respectively.
Infos
Buy this article as PDF
(incl. VAT)
Buy Linux Magazine
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters
Support Our Work
Linux Magazine content is made possible with support from readers like you. Please consider contributing when you’ve found an article to be beneficial.

News
-
First Release Candidate for Linux Kernel 6.14 Now Available
Linus Torvalds has officially released the first release candidate for kernel 6.14 and it includes over 500,000 lines of modified code, making for a small release.
-
System76 Refreshes Meerkat Mini PC
If you're looking for a small form factor PC powered by Linux, System76 has exactly what you need in the Meerkat mini PC.
-
Gnome 48 Alpha Ready for Testing
The latest Gnome desktop alpha is now available with plenty of new features and improvements.
-
Wine 10 Includes Plenty to Excite Users
With its latest release, Wine has the usual crop of bug fixes and improvements, along with some exciting new features.
-
Linux Kernel 6.13 Offers Improvements for AMD/Apple Users
The latest Linux kernel is now available, and it includes plenty of improvements, especially for those who use AMD or Apple-based systems.
-
Gnome 48 Debuts New Audio Player
To date, the audio player found within the Gnome desktop has been meh at best, but with the upcoming release that all changes.
-
Plasma 6.3 Ready for Public Beta Testing
Plasma 6.3 will ship with KDE Gear 24.12.1 and KDE Frameworks 6.10, along with some new and exciting features.
-
Budgie 10.10 Scheduled for Q1 2025 with a Surprising Desktop Update
If Budgie is your desktop environment of choice, 2025 is going to be a great year for you.
-
Firefox 134 Offers Improvements for Linux Version
Fans of Linux and Firefox rejoice, as there's a new version available that includes some handy updates.
-
Serpent OS Arrives with a New Alpha Release
After months of silence, Ikey Doherty has released a new alpha for his Serpent OS.