Docker daemon error on joining swarm node - could not find local IP address - unknown port

Question

Consider the following setup:

Three hosts in the same network:

• Host A - SLES
• Host B - Ubuntu
• Host C - Ubuntu

Then, the following has worked:

• Init swarm on A
• Join swarm at B, make B a manager

The following is however not working:

• Join swarm at C as worker with token issued from A
• Join swarm at C as worker with token issued from B

In both cases we get the same error:

Error response from daemon: could not find local IP address: 
dial udp: address udp/2377​: unknown port

Honestly, I have even no idea how to interpret this message.

Any ideas?..

UPD. nmap results for ports 2377,4789,7946 (hint from Felipe in a comment below) - no idea what are they good for but now it is clear that something is wrong with host C.

Host A (Primary manager)

PORT     STATE  SERVICE
2377/tcp open   unknown
4789/tcp closed unknown
7946/tcp open   unknown

Host B (Secondary manager)

PORT     STATE  SERVICE
2377/tcp open   unknown
4789/tcp closed unknown
7946/tcp open   unknown

Host C (should become a worker)

PORT     STATE  SERVICE
2377/tcp closed unknown
4789/tcp closed unknown
7946/tcp closed unknown

Host D (successfully added as a worker)

PORT     STATE  SERVICE
2377/tcp closed unknown
4789/tcp closed unknown
7946/tcp open   unknown

Finally, with host B there is the actual problem.

• swarm init => OK; port 2377 is open
• swarm leave => port is closed again
• swarm join => error "port not found"

Answer 1

Could you please try the commands above between all hosts ?

INSIDE HOST A:

nc -zv HOST_B_IP_ADDRESS 2377,4789,7946
nc -zv HOST_C_IP_ADDRESS 2377,4789,7946

INSIDE HOST B:

nc -zv HOST_A_IP_ADDRESS 2377,4789,7946
nc -zv HOST_C_IP_ADDRESS 2377,4789,7946

INSIDE HOST C:

nc -zv HOST_A_IP_ADDRESS 2377,4789,7946
nc -zv HOST_B_IP_ADDRESS 2377,4789,7946

As explained in https://docs.docker.com/engine/swarm/swarm-tutorial/#open-protocols-and-ports-between-the-hosts, some ports must be available between hosts in order for Docker Swarm to work correctly.

The error reported sounds like a port related problem. Maybe is there another application in one of these hosts already using one of the ports needed by Docker ? Maybe a firewall configuration ?