Aso'K's questions

I installed postgres and tried to start postgres service but failed with errors

[root@c1152-node3 ~]# psql
psql: could not connect to server: No such file or directory
    Is the server running locally and accepting
    connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?

[root@c1152-node3 log]# service postgres start
    Redirecting to /bin/systemctl start postgres.service
    Failed to start postgres.service: Unit not found.

I spent two days trying to figure out what went wrong. Finally, I installed postgresql-server. I am still to start the postgres service. But at least now, I am getting meaningful response from service xxxx start command. Question:- What is the difference between below two packages?

Jul 31 17:39:56 Installed: postgresql-9.2.23-3.el7_4.x86_64
Aug 02 16:45:25 Installed: postgresql-server-9.2.23-3.el7_4.x86_64

Now:-

[root@c1152-node3 log]# systemctl status postgresql
● postgresql.service - PostgreSQL database server
   Loaded: loaded (/usr/lib/systemd/system/postgresql.service; disabled; vendor preset: disabled)
   Active: failed (Result: exit-code) since Thu 2018-08-02 16:46:35 UTC; 34min ago
  Process: 7817 ExecStartPre=/usr/bin/postgresql-check-db-dir ${PGDATA} (code=exited, status=1/FAILURE)
Failed to get boot id: No such file or directory

We have multiple users accounts with /bin/bash shell allocated to them. An another application "su" to these accounts for controlling their service statuses and monitoring purposes. Passwords for these users are not set though. What shall I do to secure these users.

Refer doc https://unix.stackexchange.com/questions/113754/allow-user1-to-su-user2-without-password

would it not be a potential exploit window if user account can be logged into using pam modules. i.e. without sudo and without passwords. What can be done about this?

COMMAND   PID   USER   FD   TYPE DEVICE SIZE/OFF      NODE NAME
httpd    1744 apache   16w   REG  202,1        0 167906281 phpapi_access_log
httpd    2334 apache   16w   REG  202,1        0 167906281 phpapi_access_log

Above is the output of lsof for my phpapi_access_log file. Logging frequency is very low so I cannot determine if logs are being written to the file or not. I enabled logrotate for the file, till now the file is empty. How can I verify logs will be written to the file when required? What is SIZE/OFF and why is it "0" ?

I want to set up security group for an ec2 instance(Linux). There are services running on it like mem cached, python, dhclient, nodejs etc. How do i know which IP/CIDR-block should my server be listening to on a particular port? Eg. if mysqld is running on port 3306, what should be the source CIDR/IP/SG and how to find it out? (inbound rules)

p.s. It is a vpc with various webservers, database-servers, CMS, Solr etc. I need to setup security groups for each of these. Specifically, I need to know what instance is talking to what other machines in the VPC. Presently security group setup is open for all. I need to secure it