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1. Overview

MobaXterm is an SSH client for Windows that allows you to securely connect to and manage a Virtual Machine (VM) from a remote system.

This guide explains how to connect to a VM using SSH with MobaXterm.

2. Prerequisites

Before connecting to the VM, ensure you have:

• The VM’s IP address or hostname.
• A valid username (such as root or another user with SSH access).
• The SSH password.
• MobaXterm installed on your Windows computer.

3. Connect to the Virtual Machine

Step 1: Open MobaXterm

Launch MobaXterm on your Windows computer.

Step 2: Create a New Session

Click the Session button in the upper-left corner of the application.

Step 3: Select the Session Type

Choose SSH from the available session types.

Step 4: Enter the Connection Details

• In the Remote host field, enter the VM’s IP address or hostname.
• Leave the port set to 22, unless your server uses a different SSH port.
• Select Specify username and enter the appropriate username (for example, root or ubuntu).
• Click OK.

MobaXterm will prompt you to enter the SSH password.

After entering the correct password, the SSH session will be established and you will be connected to the virtual machine.

1. Overview

MTU (Maximum Transmission Unit) is the largest packet size, measured in bytes, that a network interface can send in a single frame without fragmentation.

The default MTU on most Ethernet networks is 1500 bytes.

A larger MTU, such as 9000 bytes (Jumbo Frames), is commonly used in high-performance environments, including storage networks, virtualization platforms, and cloud infrastructure.

2. Why Use MTU 9000?

Using an MTU of 9000 allows more data to be transmitted in each packet, reducing the total number of packets required.

Benefits include:

• Lower CPU usage due to fewer packets being processed.
• Higher network throughput.
• Improved performance for data-intensive workloads such as backups, virtual machine migrations, and databases.

3. Why Applications May Not Work with MTU 9000

For MTU 9000 to work correctly, every device in the network path must support Jumbo Frames.

If any device, such as a switch, router, firewall, or VPN, only supports an MTU of 1500, larger packets may be dropped or fragmented.

Example:
Server A (MTU 9000) → Switch (MTU 9000) → Router/VPN (MTU 1500) → Server B

In this case, the router cannot process 9000-byte packets, which may result in:

• Connection failures
• Packet loss
• Network timeouts
• Application issues, especially with databases, SSH, APIs, or web applications

4. Why Use MTU 1500?

• An MTU of 1500 is the standard value supported by virtually all network devices.
• If an application works with MTU 1500 but fails with MTU 9000, it usually indicates an MTU mismatch somewhere in the network path.
• Using MTU 1500 provides stable and reliable connectivity, although it may offer slightly lower performance than Jumbo Frames.

5. Check the Current Network Interface

Before changing the MTU, identify the active network interface.

Run either of the following commands:

ip link show

or

nmcli device status

Example output:

2: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
link/ether 52:54:00:ab:cd:ef brd ff:ff:ff:ff:ff:ff

In this example:

• Interface: enp1s0
• Current MTU: 9000

6. Change MTU Permanently on Ubuntu 24.x

Step 1: Back Up the Netplan Configuration

sudo cp /etc/netplan/50-cloud-init.yaml /etc/netplan/50-cloud-init.yaml.bak

Step 2: Edit the Netplan Configuration

Open the configuration file:

sudo nano /etc/netplan/50-cloud-init.yaml

Update the interface configuration to include the required MTU value:

network:
version: 2
ethernets:
enp1s0:
dhcp4: true
dhcp6: true
optional: true
mtu: 1500

Step 3: Apply the Changes

sudo netplan apply

Step 4: Verify the MTU

ip link show enp1s0 | grep mtu

7. Change MTU Permanently on AlmaLinux

Step 1: Back Up the NetworkManager Connection Profile

View the active connection:

nmcli connection show –active

Create a backup of the connection profile:

sudo cp “/etc/NetworkManager/system-connections/<profile-name>.nmconnection” “/etc/NetworkManager/system-connections/<profile-name>.nmconnection.bak”

Step 2: Configure the MTU

Set the MTU value permanently:

sudo nmcli connection modify <profile-name> 802-3-ethernet.mtu 1500

Step 3: Restart the Network Connection

Restart the connection:

sudo nmcli connection down <profile-name> && sudo nmcli connection up <profile-name>

Or restart the NetworkManager service:

sudo systemctl restart NetworkManager

Step 4: Verify the MTU

ip link show <interface-name> | grep mtu

8. Temporary MTU Change (Ubuntu and AlmaLinux)

To test a different MTU without making a permanent change, run:

sudo ip link set dev <interface-name> mtu 1500

This change is temporary and will be lost after the system is restarted.

9. Change the MTU on Windows Server Using PowerShell

Purpose

This procedure explains how to view and modify the MTU on a Windows Server using PowerShell.

Prerequisites

Before making changes:

• Ensure you have administrative privileges.
• Identify the network interface you want to modify.

Step 1: Check the Current MTU

Open PowerShell as Administrator.

Run:

Get-NetIPInterface

Review the InterfaceAlias and NlMtu values for the network interface.

Example:

ifIndex InterfaceAlias AddressFamily NlMtu(Bytes)
——- ————– ————- ————
5 Ethernet Instance 0 IPv4 9000
5 Ethernet Instance 0 IPv6 1500

Step 2: Change the MTU

To change the MTU for IPv4:

Set-NetIPInterface -InterfaceAlias “Ethernet Instance 0” -NlMtu 1500

To change the MTU for IPv6:

Set-NetIPInterface -InterfaceAlias “Ethernet Instance 0” -AddressFamily IPv6 -NlMtu 1500

Step 3: Verify the MTU

Run:

Get-NetIPInterface | Select-Object InterfaceAlias, AddressFamily, NlMtu

Example output:

InterfaceAlias AddressFamily NlMtu
————– ————- —–
Ethernet Instance 0 IPv4 1500
Ethernet Instance 0 IPv6 1500

Step 4: Restart the Network Adapter (Optional)

To apply the changes immediately:

Restart-NetAdapter -Name “Ethernet Instance 0”

Step 5: Verify the MTU is Persistent

Confirm that the MTU value has been saved to the persistent configuration:

Get-NetIPInterface -InterfaceAlias “Ethernet Instance 0” -PolicyStore PersistentStore

This verifies that the MTU configuration will remain in effect after the server is restarted.

1. Overview

RDP (Remote Desktop Protocol) is a Microsoft protocol that allows you to remotely access and control a Windows computer over a network or the internet.

By default, RDP uses port 3389.

When you connect using the Remote Desktop Connection (mstsc.exe) application, your keyboard, mouse, and screen are transmitted to the remote computer, allowing you to control it through a graphical interface.

2. Enable RDP on a New Windows Server VM

Step 1: Enable Remote Desktop

1. Open Start → Settings → System → Remote Desktop.
2. Turn Enable Remote Desktop to On.
3. Click Confirm when prompted.

Step 2: Allow RDP Through Windows Firewall

1. Open Windows Defender Firewall.
2. Click Allow an app or feature through Windows Defender Firewall.
3. Make sure Remote Desktop is enabled for the required network profiles (Private and/or Public).

Step 3: Configure User Access

1. Right-click This PC and select Properties.
2. Click Remote Settings.
3. Under Remote Desktop, click Select Users.
4. Add the user accounts that should be allowed to connect through RDP.

3. Change the Default RDP Port

Step 1: Open Registry Editor

1. Press Windows + R.
2. Type regedit and press Enter.
3. If prompted by User Account Control (UAC), click Yes.

Step 2: Locate the RDP Port Setting

Navigate to the following registry path:
HKEY_LOCAL_MACHINE
└─ System
└─ CurrentControlSet
└─ Control
└─ Terminal Server
└─ WinStations
└─ RDP-Tcp

Step 3: Change the Port Number

1. In the right pane, locate PortNumber.
2. Double-click PortNumber.
3. Select Decimal under Base.
4. Enter the new port number (for example, 3390 or 5000).
5. Click OK.

Tip: Choose a port number between 1025 and 65535 that is not already in use.

Step 4: Allow the New Port Through Windows Firewall

1. Open Windows Defender Firewall with Advanced Security.
2. Select Inbound Rules.
3. Click New Rule.
4. Select Port, then click Next.
5. Choose TCP and enter the new port number.
6. Click Next.
7. Select Allow the connection.
8. Click Next.
9. Select the required network profiles (Domain, Private, and/or Public).
10. Click Next.
11. Enter a name for the rule (for example, Custom RDP Port).
12. Click Finish.

Step 5: Apply the Changes

Restart the server.

Or restart the Remote Desktop service using Command Prompt with administrative privileges:

net stop termservice
net start termservice

Tip: Record the new RDP port number and verify that you can connect successfully before ending your current remote session. This helps prevent accidental loss of access to the server.

Overview

Rescue Mode allows you to boot a Virtual Machine (VM) into a temporary rescue environment to diagnose and repair problems without deleting or affecting the existing data.

Instead of starting from the VM’s normal operating system, the VM boots from a rescue image. The original system disk remains attached as a secondary disk so you can access files, repair the operating system, recover data, or fix configuration issues.

Common use cases include:

• Repairing boot loader problems
• Resetting or recovering SSH access
• Fixing file system errors
• Correcting network or system configuration issues
• Recovering important data from an unbootable VM

Prerequisites

Before using Rescue Mode, make sure:

• You have permission to manage the VM.
• The VM is in either Running (Active) or Stopped state.
• A compatible rescue image (Linux or Windows) is available.
• You have basic knowledge of console or SSH access.

Note: Rescue Mode cannot be started while the VM is in a transitional state (such as Starting, Stopping, or Rebooting).

How Rescue Mode Works

When Rescue Mode is enabled:

1. The VM is stopped safely (if it is running).
2. The VM boots using a rescue image.
3. The original boot disk is attached as a secondary disk (for example, /dev/vdb).
4. You can mount the original disk, inspect files, and perform repairs.
5. After troubleshooting, the VM can be returned to normal boot mode.

Entering Rescue Mode

1. Log in to the Cloud dashboard.
2. Go to Compute → Virtual Machines.
3. Select the VM that requires troubleshooting.
4. Open the Actions menu (⋯).
5. Select Rescue Mode or Enter Rescue Mode.
   
6. Choose the appropriate rescue image (Linux or Windows).
   
7. Click Start or Enter to begin.

The VM status will change to Rescue.

Note: Once the rescue environment is ready, connect using the VM console or SSH.

Using Rescue Mode

A. Access the Rescue Environment

• Open the VM console from the Cloud dashboard.
• Log in using the credentials provided for the rescue image.

B. Mount the Original Disk

After logging into the rescue environment, identify the attached disks.

Example:

fdisk -l

lsblk

Create a mount point:

mkdir /mnt/original

Mount the original system disk:

mount /dev/vdb1 /mnt/original

After mounting, you can access the original files:

cd /mnt/original

You can now inspect logs, modify configuration files, or recover data.

C. Perform Required Repairs

Common repair tasks include:

• Reset SSH keys or passwords
• Edit SSH configuration
• Correct /etc/fstab
• Repair file systems using fsck
• Restore configuration files
• Copy important data to another location

 Exiting Rescue Mode

After completing the required repairs:

1. Return to the Cloud dashboard.
2. Select the rescued VM.
3. Click Exit Rescue Mode or Unrescue VM.
4. Confirm the action.

The VM will reboot normally using its original boot disk, and its status will return to Running (Active).

Windows VM Notes

When rescuing Windows virtual machines:

• Use a rescue image that supports Windows.
• If the rescue and original disks have conflicting disk identifiers, you may need to update the disk ID using Disk Management so Windows can locate the correct boot disk.

Troubleshooting

VM Cannot Enter Rescue Mode

Possible causes:

• The VM is not in a supported state.
• The selected rescue image is missing or incompatible.
• The rescue image failed to boot.

Try using another compatible rescue image.

Cannot Access the Rescue Console

Check that:

• Console access is available.
• Firewall or security group settings allow SSH (if using SSH).
• Browser-based console access is working.

Original Disk Is Not Visible

Verify that the original disk is attached as a secondary disk (for example, /dev/vdb).
If it is missing, attach the disk again using the Cloud management interface.

VM Does Not Boot After Exiting Rescue Mode

The issue may still exist on the original operating system.

Re-enter Rescue Mode and verify:

• Boot loader configuration
• File system integrity
• System configuration files

If necessary, restore the VM from a backup or snapshot.

Best Practices

• Create a snapshot or backup before entering Rescue Mode.
• Use trusted and compatible rescue images.
• Keep the VM in Rescue Mode only for the time required.
• Record all changes made during troubleshooting for future reference.

Frequently Asked Questions (FAQ)

Which login credentials should I use?

In most cases, you can use the same username, password, or SSH keys as the original VM.

Some rescue images (such as Ubuntu-based images) may generate a temporary password during boot. This password is displayed on the VM console.

How long does Rescue Mode take?

Typical duration:

• Enter Rescue Mode: 1–2 minutes
• Troubleshooting: Depends on the issue
• Exit Rescue Mode: 1–2 minutes

If the VM remains in a transitional state for an extended period, review the console logs or contact support.

Can I access files from my original VM?

Yes.

The original system disk is attached as a secondary disk (such as /dev/vda, /dev/vdb, or /dev/vdc).

You can mount the disk and access or modify its files as needed.

Will my data be affected?

No.

Rescue Mode does not automatically modify the original disk.

• The VM only boots from a temporary rescue environment.
• Your original data remains unchanged unless you manually edit or delete files.

What should I do if Rescue Mode fails?

If the VM enters an ERROR state:

• Do not retry multiple times.
• Contact the support team.
• Provide the following information:
  • VM ID
  • Error message
  • Time the issue occurred
  • Actions performed before the error

This information will help with further troubleshooting and recovery.

Is Rescue Mode secure?

Yes.

Only authorized users with the required permissions can enable Rescue Mode.

Normal access controls and audit logging remain in effect.

• Best Practice: Exit Rescue Mode as soon as troubleshooting is complete to return the VM to its normal operating state.

Can I use a custom rescue image?

Yes, provided the rescue image includes the required properties:

hw_rescue_device=disk
hw_rescue_bus=virtio

If these properties are not present, Rescue Mode may fail with a driver-related error.