Linux Administrator

Overview 📝

This documentation talks about the various commands that are used as part of Linux administration in day 2 day activities.

Pre-requisite Setup Instructions 🚧

Download Oracle VM Virtual Box from Click here.
Download RedHat Linux ISO Click here.

Important Commands ⌨️

Adding a User
- Add Operation
```
  useradd ant
```
- Verification, check if a folder ant is created in the /home directory
```
  cd /home/
  ls ant
```
Adding a Group
- Add Operation
```
  groupadd insects
```
- Verification, check if an entry is present in /etc/group file
```
  cat /etc/group | grep insects
```

Assigning User to Group

Modify User
```
  usermod -G insects ant
```

Verify if User is added to the Group

  cat /etc/group | grep insects
  #insects:x:1004:ant
  #group:encryptedpasswd:groupid:userid

Checking Different files under /etc

passwd [UserFile]

  cat /etc/passwd | grep ant
  #ant:x:1002:1003::/home/ant:/bin/bash
  #user:encryptedpasswd:userid:groupid:homedirectory:shell

group [GroupFile]

  cat /etc/group | grep ant
  #insects:x:1004:ant
  #group:encryptedpasswd:groupid:userid

shadow [PasswordFile]

  cat /etc/shadow | grep ant
  #ant:!!:19835:0:99999:7:::

One line command to create the User and Assign it to a Group

Create User and add to Group

      useradd -G insects -s /bin/bash -c "This is a bug" -m -d /home/bug bug
      # -G GROUP
      # -s SHELL
      # -c COMMENT
      # -m CREATE HOME DIRECTORY
      # -d HOME DIRECTORY

Create password for the User
```
  passwd bug
```

Verification

  cat /etc/shadow | grep bug
  #bug:$6$tipJYGx11g3arh2U$R7nEyhEIy90wDEtPf32Zr4T0EWHVtcrH6I6h1Nd8IpRbrHd1v4mc1Mbn4XY2A.If/OuCnW.BPshjbOLL1uIMv1:19835:0:99999:7:::
  #This is an encrypted password in shadow file

  cat /etc/passwd | grep bug
  #bug:x:1003:1005:This is bug:/home/bug:/bin/bash

  cat /etc/group | grep bug
  #insects:x:1004:ant,bug

Delete User, Group

User

  userdel -r ant
  #Remove home directory as well
  groupdel insects

Changing Password policy

Per User

  chage -m mindays -M maxdays -d lastday -I inactive -E expiredate -W warndays user
  #bug:$6$tipJYGx11g3arh2U$R7nEyhEIy90wDEtPf32Zr4T0EWHVtcrH6I6h1Nd8IpRbrHd1v4mc1Mbn4XY2A.If/OuCnW.BPshjbOLL1uIMv1:19835:0:99999:7:::

  #So from the above let's consider the following 19835:0:99999:7::: 
  # -d lastday: days since 1 Jan 1970 the password was change so we see 19835 days (April 22, 2024).
  # -m mindays: After how many days can a user change the password, currently no restriction hence 0, if set to 5, the User can change password after 5 days.
  # -M maxdays: After how many days User should be prompted to change password, Current is 99999 that is alot, hence we have to change it to something meaningful.
  # -W warning: Warn User when password is going to expire.
  # -I Inactive: Set User as inactive after number of days Password expires.
  # -E Expire: Days after which the User to expire from 1 Jan 1970.

  #Let's apply this to the user bug using chage
  chage bug -m 5 -M 90 -W 10 -I 3 
  #bug:$6$tipJYGx11g3arh2U$R7nEyhEIy90wDEtPf32Zr4T0EWHVtcrH6I6h1Nd8IpRbrHd1v4mc1Mbn4XY2A.If/OuCnW.BPshjbOLL1uIMv1:19835:5:90:10:3::

All Users

  cat /etc/login.defs
  #PASS_MAX_DAYS   99999
  #PASS_MIN_DAYS   0
  #PASS_MIN_LEN    5
  #PASS_WARN_AGE   7
  #UMASK 077

Sudo Commands
- Switch User to root
```
  su
  #Prompts for root password
```
- Switch User to bug
```
  su bug
```
- Add users to sudoer group using wheel Group to execute certain commands like dmidecode which are accessible only to root
```
  usermod -aG wheel smore

  #Test using the sudo command
  sudo dmidecode
  sudo fdisk -l
```
Other important commands - Monitoring and Manage processes
- Filesystem Usage
```
df -h
#h is human readable format
```
- Disk Usage
```
du -h
#Details of each file in the filesystem
```
- Check system uptime
```
uptime
#Gives basic information like since when is the system up and running etc.
```
- Checking processes running in server
```
top
```
- Check the swap and memory
```
free
```
- Check list of open files
```
lsof
```
- Networking command, incoming network
```
tcpdump -i enp0s3
```
- Network statistics using netstat
```
netstat
```
- Check processes runnning using ps -ef
```
ps -ef | grep java
```
- Force kill a process using kill command
```
kill -9 47474
```

Controlling Services and daemons using systemctl

Get all the services running
```
systemctl --all
```
Check a particular service status
```
systemctl status firewalld.service
```
Stop the service - i.e. it will change the status to inactive
```
systemctl stop firewalld.service
```
Restarting the firewall service
```
systemctl restart firewalld.service
```

Reload the cofiguration of the service

systemctl reload firewalld.service
#Reload the config without impacting the service

Configure the service to start during linux start time and vice-versa

systemctl enable firewalld.service
#This will add the service to start at restart
systemctl disable firewalld.service
#This will remove from the startup list

Prevent accidental starts of the service by masking the service

systemctl mask firewalld.service
#The service is masked now you wont be able to start it until it is unmasked.
systemctl unmask firewallId.service

Secure SSH login using keys

If Machine A wants to login to Machine B without providing username/password.

Generate the key pair in Machine A

ssh-keygen
#Enter password if you want to else you can ignore
cd /home/usera/.ssh
#You will find two new files, id_rsa.pub [public key] and id_rsa [private key], copy the public key

Copy the public key of the USER ONE from Machine A to Machine B USER TWO’s authorized_keys file

cd /home/userb/.ssh
#Login to server B, as USET TWO
vi authorized_keys
#Create a new file if not present and paste the content of public key copied in the above step

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Resources 📚

RedHat Doc