Mongoose connection error: MongoError: failed to connect to server - mongodb

Question

I was running mongo 3.4 on centOS. It was using authorization. I needed to upgrade it to mongo 3.6. I upgraded it and now I'm not able to connect it through any means remotely. Neither with the shell nor with the node server itself.
Here is the mongoose connection.
const uri = 'mongodb://admin:12345#host:27017/db?authSource=admin';
mongoose.connect(uri);
Here is mongod.conf
# where to write logging data.
systemLog:
destination: file
logAppend: true
path: /var/log/mongodb/mongod.log
# Where and how to store data.
storage:
dbPath: /var/lib/mongo
journal:
enabled: true
# engine:
# mmapv1:
# wiredTiger:
# how the process runs
processManagement:
fork: true # fork and run in background
pidFilePath: /var/run/mongodb/mongod.pid # location of pidfile
# network interfaces
net:
port: 27017
# bindIp: 127.0.0.1 # Listen to local interface only, comment to listen on all interfaces.
security:
authorization: enabled
#operationProfiling:
#replication:
#sharding:
## Enterprise-Only Options
#auditLog:
#snmp:

Answer 1

Probably the upgrade was not successful, and the restart of the mongod service failed.
View the logs in /var/log/mongodb/mongod.log and check for any inconsistency in the mongod.conf.
Check if the service is up and if it is listening on port 27017.
service mongod status
netstat -tl | grep 27017 # or using the ss command
ss -tl | grep 27017
From the official documentation:
Starting in MongoDB 3.6, mongod and mongos instances bind to localhost by default. Remote clients cannot connect to an instance bound only to localhost. To override and bind to other ip addresses, use the net.bindIp configuration file setting or the --bind_ip command-line option to specify a list of ip addresses.
Try the following setting to enable the service to listen on all the interfaces
net:
port: 27017
bindIp: 0.0.0.0