yum --enablerepo=el5_u4_base install ruby

Create user & group

groupadd -g 500 cloud
useradd -g 501 -G cloud -u 500 -d /var/lib/one -s /bin/bash oneadmin

Use visudo to add the following bits

Defaults:%cloud !requiretty
%cloud    ALL=(ALL) NOPASSWD: /usr/sbin/xm *
%cloud    ALL=(ALL) NOPASSWD: /usr/sbin/xentop *

Add the ONE frontend node's oneadmin ssh public key on the new node.

su - oneadmin
mkdir .ssh && chmod 700 .ssh
vi .ssh/authorized_keys 

Paste the content of the id_dsa.pub from the frontend node.

After this use

onehost create <newbox> im_xen vmm_xen tm_ssh

?

Double check that tm_ssh is uncommented in your /etc/one/oned.conf on the frontend node.