# Kernel arguments

The default bootc model uses "type 1" bootloader config files stored in `/boot/loader/entries`, which define arguments provided to the Linux kernel.

The set of kernel arguments can be machine-specific state, but can also be managed via container updates.

The bootloader entries are currently written by the OSTree backend.

More on Linux kernel arguments: [Kernel Parameters](https://docs.kernel.org/admin-guide/kernel-parameters.html)

## /usr/lib/bootc/kargs.d

Many bootc use cases will use generic "OS/distribution" kernels. In order to support injecting kernel arguments, bootc supports a small custom config file format in `/usr/lib/bootc/kargs.d` in TOML format, that have the following structure:

```toml
[kargs]
append = ["console=ttyS0", "quiet"]
prepend = ["rd.luks.uuid=12345678-1234-1234-1234-123456789abc"]
```

The `append` and `prepend` arrays contain kernel arguments that will be added to the kernel command line. Arguments in `prepend` are added at the beginning, while `append` arguments are added at the end.

## Local kernel argument management

It is currently undefined behavior to remove kernel arguments locally that are included in the base image via `/usr/lib/bootc/kargs.d`.

## Injecting default arguments into custom kernels

The Linux kernel supports building in arguments into the kernel binary, at the time of this writing via the `config CMDLINE` build option. If you are building a custom kernel, then it often makes sense to use this instead of `/usr/lib/bootc/kargs.d` for example.

## Debian-Specific Considerations

### Debian Kernel Management

When working with Debian bootc images:

- **Kernel packages**: Debian provides multiple kernel packages (linux-image-generic, linux-image-cloud, etc.)
- **Kernel headers**: Install `linux-headers-*` packages for development
- **Kernel modules**: Located in `/lib/modules/$(uname -r)/`

### Example Debian Kernel Configuration

```dockerfile
FROM debian:bookworm-slim

# Install kernel and bootc dependencies
RUN apt update && \
    apt install -y linux-image-generic linux-headers-generic bootc ostree && \
    apt clean && \
    rm -rf /var/lib/apt/lists/*

# Configure kernel arguments for Debian
COPY kargs.d/99-debian.conf /usr/lib/bootc/kargs.d/
```

### Debian Bootloader Integration

Debian uses GRUB as the default bootloader:

- **GRUB configuration**: `/etc/default/grub`
- **GRUB scripts**: `/etc/grub.d/`
- **Update GRUB**: `update-grub` command

### Example kernel arguments configuration

Create `/usr/lib/bootc/kargs.d/99-debian.conf`:

```toml
[kargs]
append = [
    "console=ttyS0",
    "quiet",
    "splash",
    "systemd.show_status=false"
]
prepend = [
    "rd.luks.uuid=12345678-1234-1234-1234-123456789abc"
]
```

### Debian Security Considerations

For Debian bootc images, consider these security-related kernel arguments:

- **AppArmor**: `apparmor=1 security=apparmor`
- **SELinux**: `selinux=1 security=selinux` (if using SELinux)
- **KASLR**: `kaslr` (Kernel Address Space Layout Randomization)
- **SMEP/SMAP**: `nosmep nosmap` (if needed for compatibility)

### Hardware-Specific Arguments

Debian bootc images may need hardware-specific kernel arguments:

- **Virtualization**: `console=ttyS0` for cloud instances
- **Storage**: `root=UUID=...` for specific root device
- **Network**: `net.ifnames=0` for predictable network interface names

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