X server may be configured to allow """any""" connection from specified host. xhost is the utility allowing to control what hosts are allowed to connect.

Basic synopsis is

xhost -	enables host authentication protocol: only authenticated hosts may connect.
xhost +	disables any security protocols: any host may connect
xhost +host	adds specified host to the allowed hosts list
xhost -host	removes host from the allowed hosts list

host may contain inet:hostname to allow clients from hostname to connect, local to allow connection from local clients and other, less used now, forms.

For the full information about xhost read xhost(1x) manual page and Xsecurity(7x) manual pages.

It is highly insecure to use host-based authentication protocols in the non-controlled networks, so you should probably consider using other authentication protocol, such as MIT-MAGIC-COOKIE-1, or even SshTunneling. But in some cases, such as virtual machines inside one hardware machine, host-based authentication is ok.

Example configuration using host-based authentication is the following: host1 runs x2x and wants to control display on host2.

On the host2:

host2% echo $DISPLAY
:0.0
host2% xhost +inet:host1 (or just 'xhost +host1')
host2%

On the host1:

host1% x2x -to host2:0