Reverse ssh tunnel: connexion refused
By default tunnel endpoints only listen on 127.0.0.1, so they can't be accessed from other machines. To change this you first need to add this to /etc/ssh/sshd_config on the server:
GatewayPorts clientspecified
and then when establishing the tunnel specify the listen address as 0.0.0.0:
ssh -p 443 -NR 0.0.0.0:19001:localhost:21 userOnB@B