Firewall Management

Check firewall status:

systemctl status firewalld

Start firewall:

systemctl start firewalld

List all firewall rules and open ports:

firewall-cmd --list-all

Open a specific port (e.g., 8080/tcp):

firewall-cmd --add-port=8080/tcp --permanent

Reload firewall to apply changes:

firewall-cmd --reload

Verify if a port is open:

firewall-cmd --list-all
firewall-cmd --permanent --query-port=8080/tcp

Network Connectivity

Test remote port connectivity with telnet:

telnet 192.168.1.100 50020

If the port is reachable, a connection is established. To exit the telnet session (Ctrl+C does not work):

• Press Ctrl+]
• Then type quit and press Enter.

User Switching

Switch to another user (e.g., appuser):

su appuser

Switching from a regular user to root requires root's pasword. Switching from root to any user requires no password.

Searching Content in Files

Using vim/vi

1. Enter command mode (press Esc).
2. Type / to start a forward search.
3. Enter the keyword and press Enter.
4. Jump to the next occurrence with n, previous with N.

Using cat with grep

cat filename | grep "keyword"

Search recursively in a directory

grep -r "keyword" /path/to/directory

Process and Port Management

Find process ID by port:

netstat -nap | grep 8080

Find ports used by a specific process (by PID):

netstat -nap | grep 12345

Disk and Memory

Check disk usage:

df -h

Check memory usage (including physical, swap, and buffers):

free -m

Options: -b (bytes), -k (KB), -m (MB).

Monitor real-time system processes (CPU, memory, etc.):

top

DNS Configuration

Three levels of DNS resolution (priority from highest to lowest):

1. Local hosts file: vi /etc/hosts
   Example: 23.231.234.33 www.example.com
2. Network interface config: vi /etc/sysconfig/network-scripts/ifcfg-eth0
   Example: DNS1='114.114.114.114'
3. System resolver config: vi /etc/resolv.conf
   Example: nameserver 114.114.114.114

Using cat <<EOF

Display multiple lines until EOF:

cat <<EOF
text line 1
text line 2
EOF

Write multiple lines into a file (overwrites existing content):

cat <<EOF > /etc/test/a.txt
aaa
bbb
EOF

Common Vim Operations (Normal Mode)

Navigation

• gg - Go to start of file
• G - Go to end of file
• 0 - Go to beginning of line
• $ - Go to end of line

Editing

• dd - Delete current line
• yy - Copy currant line
• p - Paste after cursor
• u - Undo last action

Search and Replace

• /keyword - Search forward
• ?keyword - Search backward
• n - Next match
• N - Previous match
• :s/old/new/g - Replace all occurrences of 'old' with 'new' in current line
• :%s/old/new/g - Replace all occurrences in entire file

Saving and Exiting

• :w - Save file
• :q - Quit
• :wq - Save and quit
• :q! - Quit without saving changes