VPN-based instance with LCM

Summary

In this tutorial, you are shown how to instantiate a workspace that is accessed over VPN and for which LCM provides messaging.

If you think that something is missing, or if you find errors, please contact us or open a ticket.

Prerequisites

The instantiation process for this tutorial is entirely similar to that of the tutorial VPN-based instance with ROS and Brunel Hand through the end of the subsection Your first log-in in that tutorial. Note that the instructions here for passing LCM messages to and from your instance can be applied to any workspace type.

Building and installing LCM

This subsection provides instructions to install LCM in your fresh workspace instance. To get the latest official release, in the terminal that has an active SSH session, enter

curl -L -O https://github.com/lcm-proj/lcm/archive/v1.3.1.tar.gz shasum -a 256 v1.3.1.tar.gz

The last line of output should be

d9765731127e5138017938c2f990eda6d8a8df260c98fe3053189db7954b9a41 v1.3.1.tar.gz

Checking the SHA-256 hash of the downloaded file provides confidence that the file has not been maliciously modified.

Assuming the base image used in the tutorial with ROS and Brunel Hand, two more packages must be installed:

apt-get -y install autoconf libglib2.0-dev

Now, build LCM from the source release as follows

tar -xzf v1.3.1.tar.gz cd lcm-1.3.1/ ./configure && make && make install

As a superficial check of the installation, try

lcm-gen --version

which should print its version number. Now, finish configuring the instance host:

export LD_LIBRARY_PATH=/usr/local/lib:$LD_LIBRARY_PATH export LCM_DEFAULT_URL=udpm://239.255.76.67:7667?ttl=1 sudo route add -net 224.0.0.0 netmask 240.0.0.0 dev tap0

The last two lines are critical because they

  1. configure LCM to use TTL=1, which causes multicast messages to be sent from the host into the network (up to one hop); and
  2. add a route that sends LCM multicast messages to the tap0 device, which corresponds to the VPN of the instance.

Example: listener and sender

For the example Python LCM listener,

cd /root/lcm-1.3.1/examples/python ./gen-types.sh

which will generate the message types that are used in this example. Now

python listener.py

Open a new terminal on your computer, i.e., the computer from which you are following this tutorial, not the one in the instance. Build LCM locally if you do not have it already. Then, configure routing for the LCM multicast subnet:

sudo route add -net 224.0.0.0 netmask 240.0.0.0 dev tap0

Finally, start to send LCM messages

export LCM_DEFAULT_URL=udpm://239.255.76.67:7667?ttl=1 python send-message.py

and a message should be printed from the listener.py program that is running within the instance.