0.0.0.0 has the specific meaning "unspecified". So you really should enter the specific ip address for the slave in the master pg_hba.conf.
I tried this, but I am getting this error when I do that:
FATAL: no pg_hba.conf entry for replication connection from host "104.131.66.183", user "rep", SSL off
DETAIL: Could not resolve client IP address to a host name: Name or service not known.
Is ${REP_USER} defined on BOTH master & slave? IS ${REP_USER} defined as a valid user that can login in the database?
Yes, the variables are defined in the environment for both master and slave. These variables are passed through a preprocessor and replaced with their actual values. But the REP_USER only exists in the master database.
