withhost

withhost is a utility that allows overriding DNS lookups for a given command invocation. It works on Linux and FreeBSD.

Some potential use-cases include accessing backend nodes behind a load balancer, checking whether a service is responding correctly after porting it to a new server (before switching the DNS over), etc.

Usage

usage: withhost [-h <host>] [--export] [--version] [--help] [<program> ...]

Overrides host lookups for a given command.

withhost can be used to override the response to a DNS lookup for a given
set of domains. Functionally, it is equivalent to adding /etc/hosts entries
manually, except it is local for a given command. That is, /etc/hosts
should be prioritized over DNS responses in /etc/nsswitch.conf for this
utility to work well.

For setuid executables, withhost must be invoked with root privileges,
otherwise it will not work.

Examples:
$ withhost --host example.com=10.0.0.1 getent hosts 10.0.0.1
10.0.0.1        example.com
$ sudo withhost --host example.com=10.0.0.1 ping example.com
PING example.com (10.0.0.1) 56(84) bytes of data.
^C
$ eval $(withhost --host example.com=10.0.0.1 --export)
$ getent hosts 10.0.0.1
10.0.0.1        example.com

optional arguments:
  -h <host>, --host <host>  specifies a host entry, in the format ${hostname}=${ip_address}
  --export                  write environment variables to standard output
  --version                 write version string to standard output
  --help                    show this help message and exit

Installation

You will need an autotools install.

$ git clone https://github.com/Xyene/withhost.git
$ cd withhost
$ autoreconf -i
$ ./configure
$ make
$ make install

Miscellaneous

/etc/nsswitch.conf resolution order

For withhost to work correctly, files must be prioritized over everything in the hosts entry of /etc/nsswitch.conf. On default installations, it usually is. Otherwise, a DNS response may be trusted over the entry in /etc/hosts, making withhost useful only for defining non-existent (e.g. NXDOMAIN) hosts.

Working with setuid executables

setuid executables will generally ignore LD_PRELOAD, which withhost uses to provide its functionality. Notably, ping does this to open raw sockets. withhost must be invoked as root when running these kinds of executables.