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OpenSUSE15.4 ; Firewall , SSH、NTP Server

1.Setting up remote connection by 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

Modify the configuration file to change the SSH service settings.
The SSH service configuration file is "/etc/ssh/sshd_config".

①Find "Port 22" and change it to any port number other than the Wernon port, this time to "Port 2244" and proceed (changing this port number alone will reduce unauthorized access).
②Find "#ListenAddress 0.0.0.0" and delete the preceding "#".
③Look for "#PermitRootLogin yes" and change it to "PermitRootLogin no".
 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

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

2.How to set up a firewall (firewalld)

In openSUSE, the firewall is set to firewalld by default and is enabled during OS installation.

To briefly explain firewalld, when setting up a communication control policy, communication permission/blocking rules are applied to predefined zones, and the zones are assigned to each NIC (network adapter).

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

1)About Zones
Nine zones are provided by default

zone Communication Permission Service Description.
block None All external connections are blocked Setting cannot be changed Return "CIMP Pohibited" when blocked, return communication is also allowed when communicating from inside
dmz ssh Zones defined for dmz
drop None All external communications are blocked Cannot change settings Internal communication can be sent, but return communication from the outside is blocked, resulting in all communication not being possible.
external ssh Zone defined for use in connection with external networks, such as routers with IP masquerade enabled.
home dhcpv6-client  ipp-client  mdns  samba-client  ssh Zones defined for use at home, etc.
internal dhcpv6-client  ipp-client  mdns  samba-client  Zone defined for use in the internal network.
public dhcpv6-client ssh Zones defined for use in public places.
trusted Allow all communication Setting cannot be changed
work dhcpv6-client ipp-client ssh Zones defined for use in work areas such as the workplace.

2)Command to check the status and settings of firewalld

①Check firewalld operation status

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

OR

※If stopped
Active: inactive (dead) is displayed, indicating that firewalld is stopped

Show default zone settings

In the above example, the "public" zone is the "default" setting and is assigned to the "eth0" NIC, and the services "dhcpv6-client" and "ssh" are allowed, etc.

➂Show the settings for the specified zone.
The following example shows how to display the settings for the "dmz" zone

About the "--permanent" option

In order to prevent the settings from being lost when the server is restarted or the "firewalld" service is restarted, the "--permanent" option must be used.
To prevent the settings from being lost when the server is restarted or the "firewalld" service is restarted, the "--permanent" option must be used to configure the settings. In this case, if the "--permanent" option is specified, the settings will not be reflected in "firewalld" as they are, so it is necessary to reflect the settings using "fiewall-cmd --reload".
As an example, to use the HTTP service permanently without being initialized even if the system is rebooted

Adding and removing services to and from a zone

To add an already defined service to a zone, use "-add-service" to specify the service
# firewall-cmd [--permanent] --zone=zone name --add-service=service name
If you want to permanently add services to a zone, you must configure it with the "--permanent" option.

Example configuration for temporarily adding a service

Configuration example for adding a temporary http service

⑥service deletion
Use "--remove-service" to remove a service configured for a zone

Remove the "http" service from the "public" zone as an example

Add or remove ports to a zone
To add a communication that has not been added as a service to the zone, add it by specifying the port number and protocol

Adding a port
Use "--add-port" to add a port to the zone

Configuration example
Added rules for port number 10022 and protocol TCP in the "public" zone.

Deleting a port
Use "--remove-port" to remove a port from a zone

Configuration example
Delete the "10022/tcp" rule in the "public" zone that we just added

⑧How to start and stop

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

Stop firewalld

2.2 Allow modified SSH port 2244

3.Remote connection from Windows

Configuration in Windows

Start setting up a 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 the following information on the next screen.
Host : Server IP Address
TCP port : SSH port number changed above



Enter the following information on the next screen.
User name : General Login Username
Passphrase : Password for the above user

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

4. NTP Server Settings

Install Chrony and build an NTP server for time synchronization. Note that NTP uses 123/UDP.

4.1 Chrony Install

4.2 Chrony Settings

4.3 Open NTP port

If Firewalld is enabled, permission for NTP service is required. Note that NTP uses 123/UDP.

4.4 operation check

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