Linux

1. Introduction

Linux is an open-source operating system (OS; other operating systems include Microsoft Windows and Mac OSX, for example). The physics and astrophysics communities often adopt the Linux OS as it is compact, reliable, and secure. However, it takes some time to learn how to use it and how to troubleshoot it.

My group is presently using the Cent OS Stream 9 and Rocky Linux 9 Linux distributions. They are very similar to Redhat but are free (i.e., Redhat is not free).

If you want to learn Linux, good Chinese documentation can be found at 鳥站. In particular, this website includes an introduction to how to maintain a Rocky Linux 9 server, see this page. If you do not read Chinese, this page may be workable but there should be others.

If you would like to install Linux to your laptop, instead of Rocky Linux or Cent OS, I usually also recommend Ubuntu.

Embedded below are some tips.

1.1 Frequently used commands

Text editing can use vim.
Connecting to other server: ssh, e.g., ssh -X USER_ID@IP or ssh -X USER_ID@domain_name.
Downloading or uploading can use scp or rsync, e.g.,scp -r USER_ID@domain_name:source_directory local_destination or rsync -avh -progress source_directory destination.
You can use the tar command to compress and package files; tar -tvf file.tar.gz can list the files in a tarball; tar -zxvf file.tar.gz can untar a tarball; tar -zcvf file.tar.gz ./file can package everythiing in the ./file directory into the tarball, file.tar.gz.
You can use the arp command to check the MAC address of a certain IP or hostname, e.g., arp 192.168.100.1.

1.2 ssh

The configuration file is ususally /etc/ssh/sshd_config but can be different. For details, see this webpage.

To allow one client compueter to login to the server without typing password, we can first generate a key from the client, and then copy the client’s key to the server. The steps are:

client> ssh-keygen. (you may do the following steps simply using vim)
client> scp ~/.ssh/id_rsa.pub user_ID@server:~.
server> cat id_rsa.pub >> .ssh/authorized_keys.
server> chmod .ssh/authorized_keys
on server, you can remove the id_rsa.pub file afterward.

2. Installing

You need to create a bootable USB drive.

First, insert your USB key. Use the command fdisk -l to identify this USB key from a list of devices. Use the following command to re-format the USB key to FAT32 file system, mkdosfs -I /dev/sdc -F 32 (replace /dev/sdc with the actual device-name of your USB key).

In Linux, you may use the dd command to create it. See the introduction in this Chinese webpage, or this English webpage.

Example:

dd if=xxx.iso of=/dev/sdc

If the package manager is yum, we can update the Linux system using the commnad yum update (we may use crontab to update the system regularly, e.g., root yum -y update && yum clean packages); if the package manager is apt-get, we can run apt-get update and then apt-get upgrade.

In most cases, the boot menu is managed by a program called grub. You can set your preferences by editing the file /etc/default/grub, using any text editor (for example, vim). An example of the content in the /etc/default/grub file is as follows (please google or ask ChatGPT of you would like to understand some of the lines).

GRUB_TIMEOUT=-1
GRUB_TIMEOUT_STYLE=menu
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
GRUB_DEFAULT=1
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/map
per/rl-swap rd.lvm.lv=rl/root rd.lvm.lv=rl/swap rhgb quiet"
GRUB_DISABLE_RECOVERY="true"
GRUB_ENABLE_BLSCFG=true

4. Installing packages

With CentOS or Rocky Linux, use yum install; with Ubuntu Linux, use apt-get install.

5. Accounts

5.1 root password

> sudo -i
(enter password for user)
New password:
Retype new password:
> passwd

6. Network environment

6.0 Basics

Check network status can use ifconfig or ip addr. With this, you can also obtain the ID of your network interface card (網路卡代號, e.g., eth0). If you are using wireless network, you can also try the command iwconfig.

Check your own hostname can type hostname. Check other sites’ IP can type host domain_name or nslookup domain_name (e.g., host www.google.com, nslookup www.google.com). You can also get to domain name of an IP address by typing nslookup IP.

Check route table can use route or route -n. The route command also allows you to (tentatively) set routes.

Restar your network service: /etc/init.d/network restart

You can check the rules of your firewall by > iptables -L -n or iptables-save (need to be root).

Some important configuration files:

/etc/sysconfig/network-scripts/ifcfg-eth0  # device_ID, IP, netmask, gateway, etc for eth0
/etc/sysconfig/network                     # networking, supporting ipv6, hostname
/etc/resolv.conf                           # IP of DNS servers
/etc/hosts                                 # 私有IP 主機名稱 別名

The simplest way of tentatively setting the IP for a network interface card is ifconfig ID XXX.XXX.XXX.XXX, for example, ifconfig eth0 192.168.1.100. For example, you can use the command ifconfig eth0 192.168.1.100; route add default gw 192.168.1.254 to tentatively change your network configuration without altering the configuration file. You can then run /etc/init.d/network restart to resume the configuration (in your configuration file).

If it takes a long time to login to some nodes within your LAN, you should include those nodes into the /etc/hosts file.

With Ubuntu Linux, more sophisticated way of editing the network configuration is:

root> vim /etc/netplan/50-cloud-init.yaml

network:
  ethernets:
    enp45s0:
      dhcp4: true
    enp46s0:
      dhcp4: false
      addresses: [xxx.xxx.xxx.xxx/cidr]
      gateway4: xxx.xxx.xxx.xxx
      nameservers:
        addresses: [8.8.8.8,101.101.101.101]
  version: 2

We run root> netplan apply to make the new setup in the 50-cloud-init.yaml file take effect. if we want mask to be 255.255.255.0, we put cidr to be 24. You can also set this using the GUI interface.

We can check active internet connection with the command > sudo netstat -tun; you can check the activated port on your computer by typing > sudo netstat -tunl.

We can also can the active port on our computer by typing > nmap -sTU localhost; we can scan the computers in your LAN, for example, by typing nmap -sP 192.168.1.0/24 (change the IP to the domain of your LAN).

We can check whether or not DNS is working by typing dig www.google.com.

With CentOS, more sophisticated way of editing the network configuration (e.g., for eth0) is:

vim /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE="eth0"              
HWADDR="08:00:27:71:85:BD"  # MAC address
NM_CONTROLLED="no"          # just use no in typical cases
ONBOOT="yes"                # active this device when booting
BOOTPROTO=none              <==取得IP的方式，其實關鍵字只有dhcp，手動可輸入none
IPADDR=192.168.1.100        # IP
NETMASK=255.255.255.0       
GATEWAY=192.168.1.254       
MTU=1500                    # maximum transmission unit

After setting the network configuration, you can let it take effect by typing > /etc/init.d/network restart in the command line. Or we can run

> ifconfig device_ID down (e.g., > ifconfig eth0 down)
> ifconfig device_ID up

注意若要用dhcp取得IP，不要設置gateway!

6.1 Enable port 22 ssh connection for Ubuntu (CentOS Steam 9 and Rocky Linux 9 does not require this.)

# install open ssh server
> sudo apt install openssh-server

# set the connection port (normally, port 22)
> sudo vim /etc/ssh/sshd_config
(if it is slow to login, try edit useDNS in this file to NO)

# restart ssh service
> sudo systemctl restart ssh
> sudo systemctl status ssh


# UFW is a simple firewall that is on Ubuntu
# check ufw status
> sudo ufw status

# enable ufw service
> sudo ufw enable

# enable port 22 connection
> sudo ufw allow ssh
# or
> sudo ufw 22/tcp

6.2 IP forwarding

> vim /etc/sysctl.conf
(uncomment) net.ipv4.ip_forward=1
> sysctl -p
> iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -j MASQUERADE

6.X Practice of setting up Local Area Network (LAN) 區域網路設置實務 (Ubuntu)

Here, I assume you have

One Ubuntu Linux desktop with two network interface card (網路卡). We call this desktop the Master node.
One router (probably with 1 WAN port and several LAN ports; 路由器，亦稱IP分享器), which can also share WIFI.
One Linux desktop with Linux OS systems (e.g., CentOS, Rocky Linux, Ubuntu, etc). We call this desktop the Computing node. If you have multiple computing nodes, the setup of each of them will be similar. Usually, each of your computing node only needs one network interface card.
One fixed IP, e.g., 140.117.123.456 (you need to obtain this from the IT guy).

I assume that what you want to do is connecting from outside (e.g., your home) to your Master node using ssh, and then connecting from the Master node to the computing node. In addition, you want the computing nodes to be able to access to to the internet (e.g., check e-mail, browsing, or download files).

In this case, on the Master node, you need to use one network interface card to connect to the internet (let’s call this card NIC-global), and use the other network interface card to connect to the local area network, likely from the router (let’s call this card NIC-local).

The steps you need to follow are:

Connecting the Wide Area Network (WAN) internet cable (i.e., from outside of your office) to NIC-global. Then you can set the network configuration of your Master node for NIC-global.
- set IP to 140.117.123.456 (for example). Typically, the netmask is 255.255.255.0, and typically, the gateway is 140.117.123.254 (i.e., change the last three digits of your IP to 254. Note that it is not necessarily this case. In case of problem, you need to consult the IT guy of your institution.)
- set the Domain Name System (DNS) server(s) to 8.8.8.8 and/or 101.101.101.101. If you are in NSYSU, you can also include our DNS server, 140.117.11.1.
- Apply your setup. Then with a command line, type > ping 8.8.8.8 and see if you are connected (in this case, to the DNS server of google).
Connecting another cable from NIC-local to your router, and then set the network configuration.
- We may set the IP and netmask to 192.168.100.254 and 255.255.255.0. We can leave gateway blank.
Active the IP forwarding service (IP轉發) on the Master node, and properly set the firewall (防火牆). I will first uncomment net.ipv4.ip_forward=1 in the file /etc/sysctl.conf (see also IP forwarding). I will then create a text file iptables.sh (e.g., using vim) with the content embedded below (if you use other IP for the LAN, you need to edit the IP correspondingly): and then execute this file by typing > sudo sh iptables.sh
Connect your computing node to a LAN port of your router, and then set the network configuration. You can set the IP, netmask, and gateway to be 192.168.100.ABC, 255.255.255.0, and the LAN IP of your Master node (e.g., 192.168.100.254). Here, ABC should be an integer in the range of 002~253 (reserving 000, 001, and 254 for other purposes).
On the computing node, trying pinging 8.8.8.8 and see if you are connected. You can also try using browser.
Test the connection from outside. Since the firewall of NSYSU does not permit ssh connection from outside, you need to pretent being in the domain (網域) of NSYSU using the VPN service (please see the instruction of the NSYSU VPN and download the VPN client to your outside desktop or laptop, which is not your Master or computing node). Activate the VPN service and connect it. Then open an terminal and type ssh 140.117.123.456 (i.e., ssh to the fixed IP of your Master node) to connect to the Master node. You should be able to ssh from the Master node to any of your computing nodes.
You can also connect your printer to a LAN port of the router (if possible).

Here is an example for the iptables.sh files (參考鳥哥的私房菜的例子):

#!/bin/bash
# part 1： 清除規則並設定預設政策
iptables -F
iptables -X
iptables -Z
iptables -P INPUT   DROP
iptables -P OUTPUT  ACCEPT
iptables -P FORWARD ACCEPT

# part 2； 基礎的三條防火牆規則
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -i lo -j ACCEPT
iptables -A INPUT -p icmp -j ACCEPT

# allow ssh from outside via port 22
iptables -A INPUT -p tcp --dport 22 -j ACCEPT

# part 3； 拒絕黑名單、開放白名單的設定
## 黑名單例子
# iptables -A INPUT -s 10.30.30.0/24 -j REJECT
# iptables -A INPUT -i enp1s0 -s 192.168.201.254 -j ACCEPT
## 中山校內網路設為白名單
iptables -A INPUT -i enp46s0 -s 140.117.30.0/24 -m state --state NEW -p tcp --dport 22 -j ACCEPT
## 實驗室內部網路設為白名單
iptables -A INPUT -i enp45s0 -s 192.168.100.0/24 -m state --state NEW -p tcp --dport 22 -j ACCEPT

# part 4： 一般通用放行的網際網路服務
iptables -A INPUT -m state --state NEW -p tcp --dport  80 -j ACCEPT
iptables -A INPUT -m state --state NEW -p tcp --dport 443 -j ACCEPT
iptables -A INPUT -m state --state NEW -p udp --dport  53 -j ACCEPT
iptables -A INPUT -m state --state NEW -p tcp --dport  53 -j ACCEPT

# part 5: 允許IP forwarding
iptables -t nat -A POSTROUTING -s 192.168.100.0/24 -j MASQUERADE

# part 6： 列出規則及儲存規則
iptables-save
/etc/init.d/iptables save

# 啟動IP forwarding
sysctl -p

To tentatively switch off your firewall, use > /etc/init.d/iptables stop

7. Scheduling your job

If you would like to execute something at a certain time, you can use the at command; if you would like to executing something every certain time period, you can use crontab.

`at`

It require the service, atd to be activated. If it is not activated, you can run > systemctl restart atd . You can also make it activated when booting by > systemctl enable atd. Some useful instructions can see here and here.

Examples of using at:

# check active at jobs
> atq

# remove at job
atrm job_ID

# executing a certiain scipt now
> at -f ./XXX.sh now

# executing a certiain scipt 5 minutes from now
> at -f ./XXX.sh now + 5 minutes

# executing a script at 12:30
> at -f ./XXX.sh 12:30

# executing a script at 12:30 of 2025-07-30
> at -f ./XXX.sh 12:30 2025-07-30