Unexpected behavior with MWI_APP_HOST environment variable

[dnmiller]

Summary

Hello,

I'm running Matlab 2025b on an Ubuntu 20.20 system, connecting to it from Safari on a Mac.

The URL that's printed to the terminal looks something like
http://localhost:8000/matlab?mwi-auth-token=SdLCi56uNeRxO07hReW8S_Jl9-gkhqwV_D3RWeEcHAM
This could be more useful, since I have to copy this URL into Safari, replace "localhost" with my actual host name, and then use Matlab via the proxy.

I've tried to add the following to a Python script I'm using to launch Matlab:
os.environ["MWI_APP_HOST"] = socket.gethostname()
This prints what looks like a correct URL to the terminal, something like
http://[my_ubuntu_host_name]:8000/matlab?mwi-auth-token=SdLCi56uNeRxO07hReW8S_Jl9-gkhqwV_D3RWeEcHAM
But the link doesn't work. Instead Safari gives me an error saying it can't connect to the server.

Any idea what's going on here? The manual editing of the URL always works, but the environment variable doesn't.

Browser(s) used

Safari

Script output

No response

[krisctl]

Hello,

When we set the environment variable to the hostname (say fully qualified domain name of your linux server), what ends up happening behind the scenes is that Python would need to find the ip address for this host so that it can bind the aiohttp web application to a respective network interface. There is a complex setup to achieve that and has a set order of precedence to start searching for the hostname information in /etc/hosts followed by checking with the dns server.

In this case, the hostname information is coming from the /etc/hosts (in most likeliness) and gets mapped to a address of the kind 127.0.X.X or something similar. One can verify that by executing netstat -lnp | grep <port on which matlab-proxy is running> and verifying that something similar as below is printed:

tcp        0      0 127.0.X.X:12345         0.0.0.0:*               LISTEN      2713582/python3.13 

This would mean that the web application is bound to that 127.0.X.X ip address only and would only respond when that ip address is present in the URL being accesssed.

The issue happens when this ip address differs from what the DNS resolver resolves to for this hostname.
From the remote machine, one can run dig to find the ip address on which the browser will attempt to connect. Let's say, it returns something like 204.121.23.12, so the actual URL tried is: http://204.121.23.12:<port>?mwi-auth-token=xxxxx, which doesn't work because the web application is configured to only respond at 127.0.X.X.

When we don't supply the MWI_APP_HOST information, one can access on all ip addresses because the socket binding is happening with 0.0.0.0 (all interfaces).

While I work with the development team some more to figure out the use case for MWI_APP_HOST, as a workaround, you could run the below command to replace localhost in the URL with the hostname information at run time.

matlab-proxy-app 2>&1 | sed "s|http://localhost:\([0-9]*\)|http://$(hostname -A | awk '{print $1}'):\1|g"

Hope that was helpful!

[dnmiller]

Thanks. The sed workaround addresses the main issue I had. It might make sense to just have a flag that uses the hostname instead of localhost when printing that specific message.