Networking in RHEL/LINUX

In Red Hat Enterprise Linux (RHEL), managing networking involves several commands for configuring network interfaces, checking connectivity, and troubleshooting network-related issues. Here are some essential networking commands frequently used in RHEL:

1. Network Configuration

1.1. NetworkManager Commands

NetworkManager is the default service for managing network interfaces in RHEL. Here are some commands to manage NetworkManager:

View NetworkManager Status:
```
  systemctl status NetworkManager
```

Start/Stop/Restart NetworkManager:

  systemctl start NetworkManager
  systemctl stop NetworkManager
  systemctl restart NetworkManager

Enable/Disable NetworkManager (for startup):

  systemctl enable NetworkManager
  systemctl disable NetworkManager

List Network Interfaces Managed by NetworkManager:
```
  nmcli device status
```
Configure Network Interface with nmcli:
```
  nmcli connection add type ethernet ifname eth0 con-name "Wired Connection 1" autoconnect yes
```
Replace eth0 with your interface name and adjust connection parameters as needed.

1.2. Legacy Network Configuration (ifcfg files)

For systems using traditional network configuration files (ifcfg files under /etc/sysconfig/network-scripts/), commands typically include:

List Network Interfaces:
```
  ip addr show
```
Edit Network Interface Configuration:
```
  sudo vi /etc/sysconfig/network-scripts/ifcfg-eth0
```
Adjust eth0 based on your interface name.
Restart Network Service:
```
  sudo systemctl restart network
```

2. Network Connectivity

2.1. Checking IP Address and Routes

View IP Address of Interfaces:
```
  ip addr show
```
Display Routing Table:
```
  ip route show
```

2.2. Testing Connectivity

Ping a Remote Host:
```
  ping -c 4 google.com
```
Replace google.com with the host you want to ping.
Check Open Ports (for troubleshooting):
```
  sudo netstat -tuln
```

3. DNS Configuration

3.1. Checking DNS Settings

Check Current DNS Servers (via systemd-resolved):
```
  systemd-resolve --status
```
Edit DNS Configuration: Edit /etc/resolv.conf or use NetworkManager's nmcli for dynamic management.

4. Firewall and Security

Firewalld Commands:

  systemctl status firewalld
  sudo firewall-cmd --list-all
  sudo firewall-cmd --zone=public --add-service=http --permanent
  sudo firewall-cmd --reload

5. Network Diagnostics and Troubleshooting

Network Troubleshooting with ip and nmcli:

  ip link
  ip addr
  ip route
  nmcli connection show
  nmcli device show

Packet Capture (using tcpdump):
```
  sudo tcpdump -i eth0
```
Replace eth0 with your interface.