OracleLinux9.1 : SSH , Firewalld , NTP Server

Contents

1.Setting up a remote connection via SSH
- 1.1 SSH service configuration file changes
2.Firewall (Firewalld) settings
- 2.1 How to use the "firewall-cmd" command to control "firewalld
- 2.2 Release modified SSH port 2244
3.Remote connection from Windows
4.NTP Server

1.Setting up a 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.

1.1 SSH service configuration file changes

The SSH service configuration file is "/etc/ssh/sshd_config".

# 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.

# This sshd was compiled with PATH=/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin

# 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

#PubkeyAuthentication yes

# The default is to check both .ssh/authorized_keys and .ssh/authorized_keys2
# but this is overridden so installations will only check .ssh/authorized_keys
AuthorizedKeysFile      .ssh/authorized_keys

# $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.

# This sshd was compiled with PATH=/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin

# 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

#PubkeyAuthentication yes

# The default is to check both .ssh/authorized_keys and .ssh/authorized_keys2

# but this is overridden so installations will only check .ssh/authorized_keys

AuthorizedKeysFile .ssh/authorized_keys

Line 22 "Port 22" this time change to "Port 2244" and proceed
Delete "#" from line 24 "#ListenAddress 0.0.0.0".
Line 41, "# PermitRootLogin prohibit-password" delete "#" at the beginning of the line
Since the root user already knows the user name and can log in to the server with administrator privileges once the password is known, the setting is set to deny this.

Restart SSH

# 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

In Oracle, the firewall is firewalld configured by default and 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; vendor preset: enabled)
     Active: active (running) since Tue 2022-12-27 21:18:44 JST; 50min ago
       Docs: man:firewalld(1)
   Main PID: 738 (firewalld)
      Tasks: 2 (limit: 9393)
     Memory: 42.3M
        CPU: 961ms
     CGroup: /system.slice/firewalld.service
             mq738 /usr/bin/python3 -s /usr/sbin/firewalld --nofork --nopid

Dec 27 21:18:43 Lepard systemd[1]: Starting firewalld - dynamic firewall daemon...
Dec 27 21:18:44 Lepard systemd[1]: Started firewalld - dynamic firewall daemon.
※If stopped
Active: inactive (dead) is displayed, indicating that the firewall is stopped

# systemctl status firewalld

firewalld.service - firewalld - dynamic firewall daemon

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

Active: active (running) since Tue 2022-12-27 21:18:44 JST; 50min ago

Docs: man:firewalld(1)

Main PID: 738 (firewalld)

Tasks: 2 (limit: 9393)

Memory: 42.3M

CPU: 961ms

CGroup: /system.slice/firewalld.service

mq738 /usr/bin/python3 -s /usr/sbin/firewalld --nofork --nopid

Dec 27 21:18:43 Lepard systemd[1]: Starting firewalld - dynamic firewall daemon...

Dec 27 21:18:44 Lepard systemd[1]: Started firewalld - dynamic firewall daemon.

※If stopped

Active: inactive (dead) is displayed, indicating that the firewall is stopped

➁View default zone settings

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

# firewall-cmd --list-all

public (active)

target: default

icmp-block-inversion: no

interfaces: ens160

sources:

services: cockpit dhcpv6-client ssh

ports:

protocols:

forward: no

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, you must use the "--permanent" option.
The "--permanent" option must be used to configure the settings.
If the "--permanent" option is specified, the setting is not reflected in "firewalld" as it is, so it is necessary to use "fiewall-cmd --reload" to reflect the setting.

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

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

④How to start/stop

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

Start firewalld
# systemctl start firewalld
Stopping firewalld
# systemctl stop firewalld

Start firewalld

# systemctl start firewalld

Stopping 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 ssh
ports: 2244/tcp
protocols:
forward: no
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 ssh

ports: 2244/tcp

protocols:

forward: no

masquerade: no

forward-ports:

source-ports:

icmp-blocks:

rich rules:

2244 ports have been added

3.Remote connection from Windows

Configuration in Windows

Start the setup for remote connection from Windows, using "Tera Term" as the terminal emulator.
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"" Passphrasee" and click OK.

If the information is correct, you should be able to log in normally as shown below.

4.NTP Server

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

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

# vi /etc/chrony.conf

Row 3 : comment out and add under it

#pool 2.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
===============================================================================
^- 126.99.178.217.static.us>     1   6    17    29    -10us[  -10us] +/- 8208us
^- x.ns.gin.ntt.net              2   6    17    28   +364ns[ +364ns] +/-   79ms
^- time.cloudflare.com           3   6    17    29   +943us[ +943us] +/-   66ms
^- ec2-3-114-30-212.ap-nort>     2   6    17    30  -1881us[-1874us] +/-   24ms
^* ntp-a2.nict.go.jp             1   6    17    30   -360us[ -354us] +/- 6093us
^+ ntp-a3.nict.go.jp             1   6    17    30   +123us[ +129us] +/- 5674us
^- ntp-b2.nict.go.jp             1   6    17    29    -35us[  -35us] +/- 5684us
^- ntp-b3.nict.go.jp             1   6    17    29   +140us[ +140us] +/- 5610us

# chronyc sources

MS Name/IP address Stratum Poll Reach LastRx Last sample

===============================================================================

^- 126.99.178.217.static.us> 1 6 17 29 -10us[ -10us] +/- 8208us

^- x.ns.gin.ntt.net 2 6 17 28 +364ns[ +364ns] +/- 79ms

^- time.cloudflare.com 3 6 17 29 +943us[ +943us] +/- 66ms

^- ec2-3-114-30-212.ap-nort> 2 6 17 30 -1881us[-1874us] +/- 24ms

^* ntp-a2.nict.go.jp 1 6 17 30 -360us[ -354us] +/- 6093us

^+ ntp-a3.nict.go.jp 1 6 17 30 +123us[ +129us] +/- 5674us

^- ntp-b2.nict.go.jp 1 6 17 29 -35us[ -35us] +/- 5684us

^- ntp-b3.nict.go.jp 1 6 17 29 +140us[ +140us] +/- 5610us

If it is marked with *, it has been synchronized.

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