Roslaunch and SSH over local and remote machines using avahi

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Some (long) hours later, here's an answer for those new to ROS who might face the same questions.

The basic idea behind avahi is... you can resolve and connect to ROS machines without worrying about the IP they're behind. This particularly when moving from one LAN to another (e.g lab to field, lab to investor, etc). For e.g. theoretically, you could just connect your robot and laptop to a hotspot on your phone and demo your kit.

Hint: a lowercase system hostname/avahi hostname will serve you well! Ssh issue avoided later.

Avahi setup There are a number of good tutorials out there. Your machines would look like something.local and somethingelse.local In my case it was remote.local and local.local Do bi-directional ping tests so both machines can ping and resolve the avahi hostnames

/etc/hosts In my opinion, this should be vanilla Ubuntu something like this:

    127.0.0.1   localhost
    # The following lines are desirable for IPv6 capable hosts
    ::1     ip6-localhost ip6-loopback
    fe00::0 ip6-localnet
    ff00::0 ip6-mcastprefix
    ff02::1 ip6-allnodes
    ff02::2 ip6-allrouters

SSH Issues: As some threads have noted, roslaunch uses an ssh lib that requires an RSA key (default is now ECDSA-SHA2).

IMPORTANT: Check your machine hostname is lowercase. ssh-server defaults to lowercase in the authorized keys file so your remote machine will keep asking you for a password when you punch in your upper case hostname.

$ ssh user@remote.local -oHostKeyAlgorithms=’ssh-rsa’

Follow the prompts and say “yes”

user@remote $ exit
$ ssh-keygen

Follow the prompt and generate a public-private key at ~/.ssh/ Provide a password

$ ssh-copy-id -i ~/.ssh/id_rsa.pub ssh user@remote.local

This copies your public key to the ~/.ssh/authorized_keys file on the other machines

$  ssh user@remote.local

Confirm a passwordless entry is possible Repeat for local.local from remote.local

~/.bashrc I’ve setup my ~/.bashrc like so

source /opt/ros/kinetic/setup.bash
export ROS_MASTER_URI=http://local.local:11311
export ROS_IP=local.local
export ROS_HOSTNAME=local.local

This allows me to launch roscore and nodes on my development machine when needed and remote connect from remote.local as well. You can replicate the script as you please on remote.local

I use env-loader from roslaunch to setup environment variables on both machines

Environment-loader script somewhere on remote.local:

export ROS_IP=remote.local
export ROS_MASTER_URI=http://remote.local:11311
export ROS_HOSTNAME=remote.local
exec "$@"
# Anything else you need….

Environment-loader script somewhere on local.local:

export ROS_IP=local.local
export ROS_MASTER_URI=http://remote.local:11311
export ROS_HOSTNAME=local.local
exec "$@"
# Anything else you need….

Again, above, my ros master needs to be on the remote robot so it’s alive even if the link dies.

BTW these scripts MUST be read-writeable for the user you’re ssh-ing into

For the launch file:

    <launch>
         <!-- Setup environment -->
        <arg name="machine_bot" default = "remote"/>
        <arg name="machine_c2i" default = "local"/>
        <arg name="ip_bot" default = "remote.local"/>
        <arg name="ip_c2i" default = "local.local"/>
        <arg name="machine_bot_user" default = "abc"/>
        <arg ...