UML Kernel + SLIRP + Linux cloud images

This guide shows how to boot an User-Mode Linux (UML) kernel with a Linux cloudimg using SLIRP for network connectivity.

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1. Prerequisites

Make sure you have in the same directory:

• linux → UML kernel binary
• slirp → static SLIRP binary
• disk.img → Any Linux cloudimg

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2. Boot UML

Run the following command from the directory containing the files:

./linux \
    mode=skas0 \
    mem=2048M \
    ubd0=disk.img \
    root=/dev/ubda1 \
    rw \
    init=/lib/systemd/systemd \
    eth0=slirp,,./slirp \
    con0=fd:0,fd:1 \
    con=null

Explanation of key options:

• mode=skas0 → required for SKAS0 mode for UML
• mem=2048M → allocate 2GB memory to the guest
• ubd0=... → the root filesystem image
• root=/dev/ubda1 rw → use first partition (partition 1) of UML disk as root, read-write mode
• init=/lib/systemd/systemd → start systemd as init
• eth0=slirp,,./slirp → connect guest eth0 to SLIRP for network
• con0=fd:0,fd:1 → console attached to current terminal
• con=null → disable other consoles

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3. Setup Networking inside UML

Once inside the UML guest, configure the network to access the internet:

# Configure eth0 IP and bring it up
ip a add dev eth0 10.0.2.1
ip l set eth0 up

# Set default route
ip r add default dev eth0

# Configure DNS
echo "nameserver 10.0.2.3" > /etc/resolv.conf

After this, the guest should be able to access the internet via SLIRP.

For port forwarding, create a file named slirp_config in same folder and add forwarded port in format [guest_port]:[host_port] like this:

22:2222
80:8080
...

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4. Notes

• You can use any Linux Cloud Image compatible with UML.
• Adjust mem parameter according to your host's available RAM.
• SLIRP allows network access without needing root privileges.