Question

Hello! I want to know if it is possible to export ROS master to a server that is sitting in another building. Both the server and my local computer are sitting over the same network at my University and it is possible to ping each over the network. I'm trying to export the ROS master to the server so it can do processing on the information posted to the topics and sending commands back to the local computer. If it is possible to export the ROS master, how would I go about this?

I've tried setting it up as follows:

Server (172.27.15.xx):

export ROS_MASTER_URI=http://172.27.18.xxx:11311
export ROS_HOSTNAME=172.27.15.xx
export ROS_IP = 172.27.15.xx

Local PC (172.27.18.xxx):

export ROS_MASTER_URI=http://172.27.18.xxx:11311
export ROS_HOSTNAME=172.27.18.xxx
export ROS_IP = 172.27.18.xxx

I simply get 'host is not set to this machine' and the ip of the rosmaster is set to localhost.

Thanks in Advance!

Asked by MartinJensen37 on 2020-11-03 06:32:00 UTC

Answer 1

I run into this issue often enough to where I created a gist for it: https://gist.github.com/chfritz/8c2adab45a94e091be77c55b0432ad2e

Note that one pitfall is that the ROS_HOSTNAME used by either machine needs to be resolvable by the other machine. This is because ROS uses that hostname to register with the master, and when a direct connection between the two machines is being established (e.g., for rostopic echo), that hostname is given to the machines to start talking with each other. This is where I have seen people fail most often. If rostopic list works, but rostopic echo doesn't, then this latter problem is what you are dealing with most likely.

Asked by chfritz on 2020-11-03 10:35:36 UTC

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Please update your gist to not use both ROS_IP and ROS_HOSTNAME. Choose one.

If you have working DNS, use ROS_HOSTNAME. If not: use ROS_IP.

Asked by gvdhoorn on 2020-11-03 11:47:54 UTC

Thanks for the suggestion. Can you elaborate on why it is bad to set both?

Asked by chfritz on 2020-11-03 11:50:24 UTC

It's not bad, but it's not a good idea imo.

ROS_HOSTNAME will take precedence over ROS_IP, so if you set both, ROS_IP will be ignored (code). So it's redundant in the best case, and can lead to strange errors in the worst (users with ROS_HOTSNAME and then ROS_IP with an IP which also has a typo, or an "old" IP (from a previous lease)). For this reason alone I recommend to set one, not both.

And personally I feel ROS_HOSTNAME should really only be used for hostnames (as that's what the name implies). I typically use ROS_IP on multi-homed systems where ROS_HOSTNAME can resolve to an IP which is not reachable on all interfaces. You could set ROS_HOSTNAME to an IP in that case as well, but then we get back to variable name.

Asked by gvdhoorn on 2020-11-03 12:08:25 UTC

This is where I have seen people fail most often. If rostopic list works, but rostopic echo doesn't, then this latter problem is what you are dealing with most likely.

I just had this exact thing happen, weirdly. I have an onboard computer and a remote computer. The onboard has nodes launched by robot upstart that the remote could list but not echo. Notably, I could pub from the onboard command line and echo on the remote just fine. Both computers were both setting the ROS_IP and ROS_MASTER_URI correctly. What eventually fixed the problem was noticing that the onboard computer's name was mistyped in the remote computer's /etc/hosts file (so it had the correct IP and the corresponding name was not the actual computer name). Correcting it to use the right computer name in the /etc/hosts did the trick and fixed the entire problem.

Is this expected behavior, given the ROS_HOSTNAME of both were not set? Mostly just looking for clarification here, as the problem is resolved

Asked by APettinger on 2020-11-03 12:27:57 UTC

Exporting ROS master to a server at another location

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