I believe you will find a fairly complete discussion of firewall issues with MPI on the OMPI mailing lists. Bottom line is that the firewall blocks both the ssh port plus the TCP communication ports required to wireup the MPI transports. If you are using the TCP transport, then those ports are also blocked.
The source of the problem has been determined, but not wholly understood, by fully disabling the firewall on the host to the internal network. Parallel jobs involving the host and nodes launched from a node were successful while those launched on the host were apparently blocked by the firewall. Would the former only involve the use of the ssh port on the host while the latter involve other ports?
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