Ansible and ControlMaster file naming

This is a short story of how Ansible and SSH (using the default `ControlMaster` path format) bite me.

Lets take this network layout for example, where we have the same IP behind different machines.

                                                     +              +           +            +
                                                     |  dc1+jumper  +-----^-----+  dc1+db    |
                                              +------>  8.8.8.8     |           |  10.0.0.1  |
                                              |      |              |           |            |
+----------+          +-------------+         |      +--------------+           +------------+
|          |          |             |         |
|  Ansible +---------->  mgmt+host  +---------+
|  Client  |          |             |         |      +--------------+           +------------+
|          |          +-------------+         |      |              +-----^-----+            |
+----------+                                  |      |  dc2+jumper  |           |  dc2+db    |
                                              +------>  8.8.4.4     |           |  10.0.0.1  |
                                                     |              |           |            |
                                                     +--------------+           +------------+

And we have this lines in our ssh config file,

Host dc1-jumper
  HostName dc1-jumper
  User root
  Port 22
  UserKnownHostsFile /dev/null
  StrictHostKeyChecking no

Host dc2-jumper
  HostName dc2-jumper
  User root
  Port 22
  UserKnownHostsFile /dev/null
  StrictHostKeyChecking no

Host dc1-db
  HostName 10.0.0.2
  User root
  Port 22
  UserKnownHostsFile /dev/null
  StrictHostKeyChecking no
  ProxyCommand ssh dc1-jumper -W %h:%p

Host dc2-db
  HostName 10.0.0.2
  User root
  Port 22
  UserKnownHostsFile /dev/null
  StrictHostKeyChecking no
  ProxyCommand ssh dc1-jumper -W %h:%p

Host *
   ControlMaster auto
   ControlPath ~/.ssh/master-socket/%r@%h:%p
   ControlPersist 6s

Now the problem is that dc1-db and dc2-db share the same IP, and the master socket file which is created (based on the above placeholders `%r@%h:%p`) will use the same file name, so if you try to connect to dc1-db right after you have connected to dc2-db, guess where you well end up ?

$ ll ~/.ssh/master-socket
0 srw-------. 1  Jan 31 17:42 [email protected]:22
0 srw-------. 1  Jan 31 17:41 root@dc1-jumper:22
0 srw-------. 1  Jan 31 17:41 root@dc2-jumper:22

I only notice this after I saw my Ansible playbook is changing the same file over and over, the file was a simple yum repo definition, which should not change after the first setup, but this repo template had a ansible fact as place holder which was the distro version (rhel6/rhel7), and based on which connection was establish first (`dc1-db` vs `dc2-db`) this fact was set based on the first machine which was connected. So each run one of the machine will register a change.

After looking into ssh documentation I found out there are other place holders I can use. There is `%C` which didn’t help in my case, as it seem to generate the same string, but using `%n` did the trick as it used the connection name and not the Hostname name/ip for the socket file.

ControlPath ~/.ssh/master-socket/%r-%n-%p

$ ll ~/.ssh/master-socket
0 srw-------. 1  Jan 31 17:42 root-dc1-db-22
0 srw-------. 1  Jan 31 17:42 root-dc2-db-22
0 srw-------. 1  Jan 31 17:41 root-dc1-jumper-22
0 srw-------. 1  Jan 31 17:41 root-dc2-jumper-22