Rocky Linux8.6 : SSH、firewall、NTP

1. Setting up SSH remote connection

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

# $OpenBSD: sshd_config,v 1.103 2018/04/09 20:41:22 tj 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.

# 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 ::

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

# This system is following system-wide crypto policy. The changes to
# crypto properties (Ciphers, MACs, ...) will not have any effect here.
# They will be overridden by command-line options passed to the server
# on command line.
# Please, check manual pages for update-crypto-policies(8) and sshd_config(5).

# Logging
#SyslogFacility AUTH
SyslogFacility AUTHPRIV
#LogLevel INFO

# Authentication:

#LoginGraceTime 2m
PermitRootLogin no
#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 17 "Port 22" This time change to "Port 2244" and proceed.
Delete the "#" in line 20 "#ListenAddress"
Change "PermitRootLogin yes" to "PermitRootLogin no" on line 44.
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
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. How to set up a firewall (firewalld)

In Rocky, 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"

① Check firewalld operation status

# firewall-cmd --state
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 2022-06-07 22:09:30 JST; 4min 27s ago
Docs: man:firewalld(1)
Main PID: 993 (firewalld)
Tasks: 2 (limit: 11170)
Memory: 34.1M
CGroup: /system.slice/firewalld.service
mq993 /usr/libexec/platform-python -s /usr/sbin/firewalld --nofork --nopid

*If stopped
Active: inactive (dead) 

➁ View default zone settings

# firewall-cmd --list-all
public (active)
target: default
icmp-block-inversion: no
interfaces: ens160
services: cockpit dhcpv6-client ssh
forward: no
masquerade: no
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
The "--permanent" option must be used to configure the settings.
If the "--permanent" option is specified, it is necessary to use "fiewall-cmd --reload" to reflect the settings, as they are not reflected in "firewalld" as they are.

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
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
services: cockpit dhcpv6-client ssh
ports: 2244/tcp
forward: no
masquerade: no
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" Passphrasee and click "OK".

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

4. NTP Server Settings

Build an NTP server to synchronize the server time with Japan Standard Time

① Chrony Installation and Configuration

# dnf -y install chrony
# vi /etc/chrony.conf
Line 3 : Make it commented out and add under it
#pool iburst
pool iburst
②Restart chrony and enable chrony after system reboot
# systemctl enable chronyd.service
# systemctl restart chronyd.service
③NTP port release in firewall
# firewall-cmd --add-service=ntp --permanent
# firewall-cmd --reload
④Checking chronyd status
# chronyc sources
MS Name/IP address Stratum Poll Reach LastRx Last sample
^+ 1 6 77 27 -50us[ -342us] +/- 5432us
^+ 1 6 77 26 +103us[ +103us] +/- 5681us
^* 1 6 77 26 -250us[ -544us] +/- 3445us
^+ 1 6 77 27 -211us[ -503us] +/- 5513us

If it is marked with *, it has been synchronized. (It takes about 10 minutes to synchronize after startup.)