The skadnsd program

skadnsd is the skadns daemon. It reads a series of queries from the client its stdin socket, resolves them asynchronously, and sends the answers back to the client as soon as it gets them. It exits 0 when its stdin closes. It exits 111 on any serious error, writing the error message to stderr.

skadnsd is a stub resolver. It reads /etc/resolv.conf at start looking for a "nameserver" line containing the address of a DNS cache (aka full resolver). It will exit 111 if it cannot find any valid cache address in /etc/resolv.conf. If the DNSCACHEIP environment variable is set, its value overrides what /etc/resolv.conf says.


skadnsd does not fork, does not background itself automatically, and does not use syslog. It is not meant to be run directly by the user: it will be invoked and spawned by the skadns library calls.

There are 2 ways to use skadnsd:

  1. (preferred) Use the skadns_startf() library call. A skadnsd child will then be spawned from your calling process, and automatically reaped when you call skadns_end(). It requires care with applications that trap SIGCHLD.
  2. Use the skadns_start() library call, together with a skadnsd service.

Running skadnsd as a child process

This is the simplest and safest way of using skadns. Forget about skadnsd: just start your library calls with skadns_startf() and end them with skadns_end(). Be careful though: if you're using SIGCHLD handlers, make sure they do not interfere with the child processes your application has without knowing. This is a general Unix programming rule.

Running a skadnsd as a daemon: the skadnsd service

In this mode, you set up a daemon listening on a Unix domain socket, and clients connect to this socket to access the service. The advantage of this setup is that it works even with badly written clients that have trouble handling a child process; the drawback is that it requires support from the system administrator.

skadnsd has no "standalone" mode: it is designed to work with a Unix domain superserver, like s6-ipcserver. skadnsd follows the UCSPI interface, it can be directly executed from the superserver.

You should run skadnsd (and its Unix superserver) under a specific user and group, for elementary security reasons; and you should run its dedicated logger as another specific user. Do NOT run skadnsd as root; check your super-server documentation to find how to run it under a specific account.