Linux rescue CD guide: fix network, firewall, and SSH issues

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What is a rescue CD and why use it?

• Access and repair non-booting systems
• Check and fix disk, network, and firewall issues
• Mount your main OS partitions and modify configurations
• Recover access when SSH or firewall rules block connectivity

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Mount and boot the SystemRescueCD

1. Log in to your VPS Management Portal.
2. From your VPS dashboard, click Mount ISO.
3. In the ISO list, select System-Rescue-CD-6-x86.iso.
4. Enable the force reset & boot option and click Mount ISO to start the process.
5. Once mounted, click Start VNC, then Open Console to access the console.
6. In the boot menu, select Boot SystemRescueCd using default options and press Enter.
7. If it boots into a command-line interface, start the graphical desktop manually:
   startx
   

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Configure network in rescue mode

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Using the graphical interface

1. Open Settings → Advanced Network Configuration.\
2. Select your wired interface (e.g., Wired connection 1).
3. Click the gear ⚙️ icon to open settings.\
4. IPv4 tab → Method: Manual → Add your IP, Netmask, and your actual Gateway.
5. DNS: e.g., 1.1.1.1, 1.0.0.1 → Save.\

ping -c 3 8.8.8.8
ping -c 3 1.1.1.1
ping -c 3 YOUR_GATEWAY   # replace with your real gateway
ping -c 3 google.com       # DNS + outbound test

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Using command line (if GUI not available)

ip a                            # identify the interface (eth0 / ens3)
ip addr add YOUR_IP/PREFIX_LENGTH dev eth0
ip route add default via YOUR_GATEWAY

echo "nameserver 1.1.1.1" > /etc/resolv.conf
echo "nameserver 8.8.8.8" >> /etc/resolv.conf

ping -c 4 8.8.8.8
ping -c 4 YOUR_GATEWAY
ping -c 4 google.com

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Check the firewall from rescue mode

1. Identify and mount your main partition:

lsblk
mkdir -p /mnt/system
mount /dev/vda1 /mnt/system
# Replace /dev/vda1 with your actual root partition.

2. Check if UFW was enabled:

cat /mnt/system/etc/ufw/ufw.conf
# look for: ENABLED=yes

3. Inspect rules:

cat /mnt/system/etc/ufw/user.rules
# or, for iptables-persistent:
cat /mnt/system/etc/iptables/rules.v4

4. Back up any configuration file before editing it. Inspect both IPv4 and IPv6 rules (user.rules and user6.rules for UFW, or rules.v4 and rules.v6 for iptables-persistent).

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Check SSH configuration and service

1. Mount the main system partition if it is not already mounted from the firewall checks:

lsblk
mkdir -p /mnt/system
mount /dev/vda1 /mnt/system
# Replace /dev/vda1 with your actual root partition.

mount | grep /mnt/system
ls /mnt/system/etc/

2. Inspect SSH config:

cd /mnt/system/etc/ssh
cat sshd_config
ls -la sshd_config.d/

3. Inspect installed SSH unit files without trying to start the installed OS:

systemctl --root=/mnt/system list-unit-files 'ssh*'

4. (Optional) Edit SSH settings:

nano /mnt/system/etc/ssh/sshd_config

5. Finish recovery and test the installed OS:

• Leave any shell whose working directory is inside /mnt/system, and unmount any filesystems you mounted. Do not force-unmount a busy filesystem; first close processes using it.
• Unmount the rescue ISO, then reboot from the virtual disk.
• In the VNC console, check SSH syntax with sudo /usr/sbin/sshd -t, inspect the SSH service and firewall, and confirm the assigned network settings.
• From a terminal on your own computer, test the VPS’s actual SSH port and attempt a fresh login:

nc -zv YOUR_VPS_IP 22
ssh username@YOUR_VPS_IP