Topics: AIX, System Admin

Configuring dsh

The dsh (distributed shell) is a very useful (and powerful) utility that can be used to run commands on multiple servers at the same time. By default it is not installed on AIX, but you can install it yourself:

First, install the dsm file sets. DSM is short for Distributed Systems Management, and these filesets include the dsh command. These file sets can be found on the AIX installation media. Install the following 2 filesets:

# lslpp -l | grep -i dsm
  dsm.core       7.1.4.0  COMMITTED  Distributed Systems Management
  dsm.dsh        7.1.4.0  COMMITTED  Distributed Systems Management
Next, we'll need to set up some environment variables that are being used by dsh. The best way to do it, is by putting them in the .profile of the root user (in ~root/.profile), so you won't have to bother setting these environment variables manually every time you log in:
# cat .profile
alias bdf='df -k'
alias cls="tput clear"
stty erase ^?
export TERM=vt100

# For DSH
export DSH_NODE_RSH=/usr/bin/ssh
export DSH_NODE_LIST=/root/hostlist
export DSH_NODE_OPTS="-q"
export DSH_REMOTE_CMD=/usr/bin/ssh
export DCP_NODE_RCP=/usr/bin/scp
export DSH_CONTEXT=DEFAULT
In the output from .profile above, you'll notice that variable DSH_NODE_LIST is set to /root/hostlist. You can update this to any file name you like. The DSH_NODE_LIST variable points to a text file with server names in them (1 per line), that you will use for the dsh command. Basically, every host name of a server that you put in the list that DSH_NODE_LIST refers to, will be used to run a command on using the dsh command. So, if you put 3 host names in the file, and then run a dsh command, that command will be executed on these 3 hosts in parallel.

Note: You may also use the environment variable WCOLL instead of DSH_NODE_LIST.

So, create file /root/hostlist (or any file that you've configured for environment variable DSH_NODE_LIST), and add host names in it. For example:
# cat /root/hostlist
host1
host2
host3
Next, you'll have to set up the ssh keys for every host in the hostlist file. The dsh command uses ssh to run commands, so you'll have to enable password-less ssh communication from the host where you've installed dsh on (let's call that the source host), to all the hosts where you want to run commands using dsh (and we'll call those the target hosts).

To set this up, follow these steps:
  • Run "ssh-keygen -t rsa" as user root on the source and all target hosts.
  • Next, copy the contenst of ~root/.ssh/id_rsa.pub from the source host into file ~root/.ssh/authorized_keys on all the target hosts.
  • Test if you can ssh from the source hosts, to all the target hosts, by running: "ssh host1 date", for each target host. If you're using DNS, and have fully qualified domain names configured for your hosts, you will want to test by performing a ssh to the fully qualified domain name instead, for example: "ssh host1.domain.com". This is because dsh will also resolve host names through DNS, and thus use these instead of the short host names. You will be asked a question when you run ssh for the first time from the source host to the target host. Answer "yes" to add an entry to the known_host file.
Now, ensure you log out from the source hosts, and log back in again as root. Considering that you've set some environment variables in .profile for user root, it is necessary that file .profile gets read, which is during login of user root.

At this point, you should be able to issue a command on all the target hosts, at the same time. For example, to run the "date" command on all the servers:
# dsh date
Also, you can now copy files using dcp (notice the similarity between ssh and dsh, and scp and dcp), for example to copy a file /etc/exclude.rootvg from the source host to all the target hosts:
# dcp /etc/exclude.rootvg /etc/exclude.rootvg
Note: dsh and dcp are very powerful to run commands on multiple servers, or to copy files to multiple servers. However, keep in mind that they can be very destructive as well. A command, such as "dsh halt -q", will ensure you halt all the servers at the same time. So, you probably may want to triple-check any dsh or dcp commands that you want to run, before actually running them. That is, if you value your job, of course.