CentOS Stream9 ; SSH , Firewalld , NTP

Contents

1.Setting up remote connection via SSH
2.Firewall (firewalld) settings
4.NTP Server Settings

1.Setting up remote connection via SSH

SSH is a service for connecting remotely to a server and is basically running immediately after the OS is installed, but the default settings are somewhat insecure.
Here we will configure the default settings to increase the security of ssh connections.

# vi /etc/ssh/sshd_config

1	# vi /etc/ssh/sshd_config

#       $OpenBSD: sshd_config,v 1.104 2021/07/02 05:11:21 dtucker Exp $

# This is the sshd server system-wide configuration file.  See
# sshd_config(5) for more information.


# The strategy used for options in the default sshd_config shipped with
# OpenSSH is to specify options with their default value where
# possible, but leave them commented.  Uncommented options override the
# default value.

# To modify the system-wide sshd configuration, create a  *.conf  file under
#  /etc/ssh/sshd_config.d/  which will be automatically included below
Include /etc/ssh/sshd_config.d/*.conf

# If you want to change the port on a SELinux system, you have to tell
# SELinux about this change.
# semanage port -a -t ssh_port_t -p tcp #PORTNUMBER
#
#Port 22
Port 2244
#AddressFamily any
ListenAddress 0.0.0.0
#ListenAddress ::

#HostKey /etc/ssh/ssh_host_rsa_key
#HostKey /etc/ssh/ssh_host_ecdsa_key
#HostKey /etc/ssh/ssh_host_ed25519_key

# Ciphers and keying
#RekeyLimit default none

# Logging
#SyslogFacility AUTH
#LogLevel INFO

# Authentication:

#LoginGraceTime 2m
PermitRootLogin prohibit-password
#StrictModes yes
#MaxAuthTries 6
#MaxSessions 10

# $OpenBSD: sshd_config,v 1.104 2021/07/02 05:11:21 dtucker Exp $

# This is the sshd server system-wide configuration file. See

# sshd_config(5) for more information.

# The strategy used for options in the default sshd_config shipped with

# OpenSSH is to specify options with their default value where

# possible, but leave them commented. Uncommented options override the

# default value.

# To modify the system-wide sshd configuration, create a *.conf file under

# /etc/ssh/sshd_config.d/ which will be automatically included below

Include /etc/ssh/sshd_config.d/*.conf

# If you want to change the port on a SELinux system, you have to tell

# SELinux about this change.

# semanage port -a -t ssh_port_t -p tcp #PORTNUMBER

#Port 22

Port 2244

#AddressFamily any

ListenAddress 0.0.0.0

#ListenAddress ::

#HostKey /etc/ssh/ssh_host_rsa_key

#HostKey /etc/ssh/ssh_host_ecdsa_key

#HostKey /etc/ssh/ssh_host_ed25519_key

# Ciphers and keying

#RekeyLimit default none

# Logging

#SyslogFacility AUTH

#LogLevel INFO

# Authentication:

#LoginGraceTime 2m

PermitRootLogin prohibit-password

#StrictModes yes

#MaxAuthTries 6

#MaxSessions 10

Line 21 "Port 22" changed to "Port 2244" this time
Delete the "#" in line 24 "#ListenAddress 0.0.0.0
Line 41, "#PermitRootLogin prohibit-password", delete "#".
The root user can log in to the server with administrator privileges if the user name is already known and the password is known, so the settings are set to deny this.

SSH restart

# systemctl restart sshd.service

1	# systemctl restart sshd.service

If this is not done, the next time you reboot, you will not be able to connect remotely via SSH. Please free SSH port 2244 in the following firewall settings.

2.Firewall (firewalld) settings

On CentOS9 ST, the firewall is set to firewalld by default and is enabled during OS installation.

To briefly explain "firewalld," when setting communication control policies, communication permission/blocking rules are applied to predefined zones, and these zones are then assigned to each NIC (network adapter).

2.1 How to use the "firewall-cmd" command to control "firewalld"

1)Command to check the status and settings of "firewalld

①Check firewalld operation status

# firewall-cmd --state 
running 
If "firewalld" is running, the message "running" will be displayed; if it is not running, the message "not running" will be displayed.

# firewall-cmd --state

running

If "firewalld" is running, the message "running" will be displayed; if it is not running, the message "not running" will be displayed.

# systemctl status firewalld
● firewalld.service - firewalld - dynamic firewall daemon
     Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; preset>
     Active: active (running) since Tue 2023-02-21 14:24:05 JST; 2h 38min ago
       Docs: man:firewalld(1)
   Main PID: 869 (firewalld)
      Tasks: 3 (limit: 10936)
     Memory: 41.7M
        CPU: 1.060s
     CGroup: /system.slice/firewalld.service
             mq869 /usr/bin/python3 -s /usr/sbin/firewalld --nofork --nopid

Feb 21 14:24:05 localhost systemd[1]: Starting firewalld - dynamic firewall dae>
Feb 21 14:24:05 localhost systemd[1]: Started firewalld - dynamic firewall daem>

# systemctl status firewalld

● firewalld.service - firewalld - dynamic firewall daemon

Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; preset>

Active: active (running) since Tue 2023-02-21 14:24:05 JST; 2h 38min ago

Docs: man:firewalld(1)

Main PID: 869 (firewalld)

Tasks: 3 (limit: 10936)

Memory: 41.7M

CPU: 1.060s

CGroup: /system.slice/firewalld.service

mq869 /usr/bin/python3 -s /usr/sbin/firewalld --nofork --nopid

Feb 21 14:24:05 localhost systemd[1]: Starting firewalld - dynamic firewall dae>

Feb 21 14:24:05 localhost systemd[1]: Started firewalld - dynamic firewall daem>

Active: active (running)と表示されています

➁View default zone settings

# firewall-cmd --list-all

1	# firewall-cmd --list-all

public (active)
target: default
icmp-block-inversion: no
interfaces: ens160
sources:
services: cockpit dhcpv6-client ssh
ports:
protocols:
forward: yes
masquerade: no
forward-ports:
source-ports:
icmp-blocks:
rich rules:

public (active)

target: default

icmp-block-inversion: no

interfaces: ens160

sources:

services: cockpit dhcpv6-client ssh

ports:

protocols:

forward: yes

masquerade: no

forward-ports:

source-ports:

icmp-blocks:

rich rules:

In the above example, we can see that the services "cockpit", "dhcpv6-client", "ssh" are allowed, etc.

③About the "--permanent" option
To prevent the settings from disappearing when the server is restarted or the "firewalld" service is restarted, the "--permanent" option must be used to configure the settings. If the "--permanent" option is specified, the settings will not be reflected in "firewalld" as is, so it is necessary to reflect the settings using "fiewall-cmd --reload".
As an example, to use the HTTP service permanently without initialization even if the system is rebooted

# firewall-cmd --add-service=http --permanent
# firewall-cmd --reload

1 2	# firewall-cmd --add-service=http --permanent # firewall-cmd --reload

④How to start/stop
Since "firewalld" is controlled by "systemd", use the "systemctl" command to start and stop it.

Start firewalld
# systemctl start firewalld
Stop firewalld
# systemctl stop firewalld

Start firewalld

# systemctl start firewalld

Stop firewalld

# systemctl stop firewalld

2.2 Release modified SSH port 2244

# firewall-cmd --add-port=2244/tcp --permanent
# firewall-cmd --reload
# firewall-cmd --list-all

public (active)
  target: default
  icmp-block-inversion: no
  interfaces: ens160
  sources:
  services: cockpit dhcpv6-client http ssh
  ports: 2244/tcp
  protocols:
  forward: yes
  masquerade: no
  forward-ports:
  source-ports:
  icmp-blocks:
  rich rules:

# firewall-cmd --add-port=2244/tcp --permanent

# firewall-cmd --reload

# firewall-cmd --list-all

public (active)

target: default

icmp-block-inversion: no

interfaces: ens160

sources:

services: cockpit dhcpv6-client http ssh

ports: 2244/tcp

protocols:

forward: yes

masquerade: no

forward-ports:

source-ports:

icmp-blocks:

rich rules:

2244 ports have been added

3.Remote connection from Windows

Configuration in Windows
The terminal emulator is "Tera Term".
Start Tera Term, cancel the startup screen, and then select "New Connection" from "File" in the Tera Term menu.

Enter your own settings in the "Server IP Address" and "TCP Port Number" fields. Finally, click "OK.

After clicking "OK," click "Continue" on the security confirmation screen.

Enter "User name" and "Passphrase" and click "OK

If the information is correct, you should be able to log in successfully.

4.NTP Server Settings

Build an NTP server to synchronize the server time with Japan Standard Time
①Chrony Installation and Configuration

# dnf -y install chrony

1	# dnf -y install chrony

Change time synchronization server

# vi /etc/chrony.conf
Line 3: Comment out and add under it
#pool 2.centos.pool.ntp.org iburst
pool ntp.nict.jp iburst

# vi /etc/chrony.conf

Line 3: Comment out and add under it

#pool 2.centos.pool.ntp.org iburst

pool ntp.nict.jp iburst

②Restart chrony and enable chrony after system reboot

# systemctl enable chronyd.service
# systemctl restart chronyd.service

1 2	# systemctl enable chronyd.service # systemctl restart chronyd.service

③NTP port release in firewall

# firewall-cmd --add-service=ntp --permanent
# firewall-cmd --reload

1 2	# firewall-cmd --add-service=ntp --permanent # firewall-cmd --reload

④Check chronyd status (behavior)

# chronyc sources

MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
^+ ntp-b2.nict.go.jp             1   6   377     3   -797us[ -938us] +/- 7344us
^* ntp-k1.nict.jp                1   6   377     1   -123us[ -264us] +/- 3424us
^+ ntp-b3.nict.go.jp             1   6   377    63   +187us[  +69us] +/- 6099us
^+ ntp-a2.nict.go.jp             1   6   377     1   -817us[ -958us] +/- 7079us

# chronyc sources

MS Name/IP address Stratum Poll Reach LastRx Last sample