Plesk Migrator cannot connect to source server: migration can not be started

Symptoms

Getting the following error while connecting to the source server using Plesk Migrator:

Looks like migration can not be started. The most common cause: wrong IP address, administrator login or password specified

The following error can be found in /opt/panel-migrator/logs/panel-migrator.debug.log :

    stderr:ssh_exchange_identification: read: Connection timed out
    rsync: connection unexpectedly closed (0 bytes received so far) [sender]
    rsync error: unexplained error (code 255) at io.c(226) [sender=3.1.1]

Source and destination servers are on different networks.

Cause

An intermediary intrusion prevention system bans rsync connections.

Multiple rsync connections required for the migration are banned by an intermediary host. New connections are possible in few minutes.To reproduce the issue, execute the following command several times one after one in short period of time (id_dsa key should be replaced with actual one):

# rsync -rltgo -e "ssh -i /usr/local/psa/admin/.ssh/id_dsa.9p3YUB6W -p 22 -o StrictHostKeyChecking=no -o GSSAPIAuthentication=no" /usr/local/psa/admin/plib/modules/panel-migrator/backend/lib/python/parallels/plesk/source/plesk/extras/pmm_12_5/ <source_server_IP>:/tmp/panel_migrator/migration_agent/tmp.mu3Q73SebA

Resolution

Contact network administrator to allow multiple simultaneous connections for rsync on an intermediary IPS host.