xfl-brfw 0.1


INTRODUCTION

    This guide produces a small Linux distribution which 
    allows an x86 system to act as an ethernet bridging
    firewall.

    The distribution consists of xfbl, Linux 5.18.5, with
    BusyBox 1.35.0 and ebtables linked against musl 1.2.3.

    The reader is expected to identify and install any
    dependencies missing from their build system.


PREPARE ENVIRONMENT

    export XFL=$(readlink -f .)
    export HOST=i686-linux-gnu
    mkdir src dist


DOWNLOAD & EXTRACT LINUX

    cd "$XFL/src"
    wget https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/snapshot/linux-5.18.5.tar.gz 
    tar xzf linux-5.18.5.tar.gz
    cd linux-5.18.5


PREPARE LINUX BUILD

    mkdir build
    cd build


SELECT BASE LINUX CONFIGURATION

    cp "$XFL/conf/linux/5.18.5/brfw.config" .config
    make -f ../Makefile oldconfig


ADJUST LINUX CONFIGURATION AS NEEDED

    make menuconfig


BUILD & DISTRIBUTE LINUX KERNEL IMAGE 

    make bzImage
    cp arch/x86/boot/bzImage "$XFL/dist/bzImage"


INSTALL LINUX HEADERS

    make headers_install INSTALL_HDR_PATH="$XFL/tools"


DOWNLOAD & EXTRACT MUSL

    cd "$XFL/src"
    wget https://musl.libc.org/releases/musl-1.2.3.tar.gz
    tar xzf musl-1.2.3.tar.gz
    cd musl-1.2.3


CONFIGURE MUSL

    mkdir build
    cd build
    ../configure --host=$HOST --prefix="$XFL/tools"


BUILD & INSTALL MUSL

    make
    make install


PREPARE TOOLS FOR BUSYBOX

    export PATH="$XFL/tools/bin":$PATH
    ln -s "$(command -v $HOST-ar)" "$XFL/tools/bin/musl-ar"
    ln -s "$(command -v $HOST-strip)" "$XFL/tools/bin/musl-strip"


DOWNLOAD & EXTRACT BUSYBOX

    cd "$XFL/src"
    wget https://busybox.net/downloads/busybox-1.35.0.tar.bz2
    tar xjf busybox-1.35.0.tar.bz2
    cd busybox-1.35.0


PREPARE BUSYBOX BUILD

    mkdir build
    cd build


SELECT A BUSYBOX CONFIGURATION

    cp "$XFL/conf/busybox/1.35.0/brfw.config" .config
    make -f ../Makefile KBUILD_SRC=.. oldconfig


ADJUST BUSYBOX CONFIGURATION AS NEEDED

    make menuconfig


BUILD BUSYBOX

    make
    make install


CLONE EBTABLES

    cd "$XFL/src"
    git clone git://git.netfilter.org/ebtables
    cd ebtables


PREPARE EBTABLES FOR CONFIGURATION

    ./autogen.sh


PATCH EBTABLES

    sed -i -e 's/^static_LDFLAGS = -static/static_LDFLAGS = -all-static/g' Makefile.am


CONFIGURE EBTABLES

    mkdir build
    cd build
    ../configure --host=i686-linux-gnu --prefix="$XFL/root" \
                 --disable-shared --enable-static \
                 CC="$XFL/tools/bin/musl-gcc" CFLAGS="-I$XFL/tools/include" \
                 LD="$XFL/tools/bin/musl-gcc"


BUILD EBTABLES

    make static
    musl-strip static


INSTALL EBTABLES

    cp static "$XFL/root/sbin/ebtables"


PREPARE ROOT FILESYSTEM DIRECTORIES

    cd "$XFL/root"
    mkdir dev lib lib/firmware proc run sys tmp


CREATE TTY DEVICES

    mknod -m 600 dev/tty1 c 4 1
    mknod -m 600 dev/tty2 c 4 2
    mknod -m 600 dev/tty3 c 4 3
    mknod -m 600 dev/tty4 c 4 4
    mknod -m 600 dev/tty5 c 4 5


EDIT EBTABLES RULES AS NEEDED

    vi etc/rc.d/filter


GENERATE INITRAMFS

    find . | cpio -H newc -o | xz --check=crc32 > "$XFL/dist/initramfs"


CLONE XFBL
 
    cd "$XFL/src"
    git clone https://github.com/jhswartz/xfbl.git 
    cd xfbl


CREATE LINKS TO KERNEL & INITRAMFS

    ln -s "$XFL/dist/bzImage" ext/kernel
    ln -s "$XFL/dist/initramfs" ext/initramfs


AMMEND KERNEL COMMAND LINE AS NEEDED

    echo -n "auto" > ext/cmdline


GENERATE 1.44MB FLOPPY IMAGE

    make dist
    ls -al dist/1440k.img
    sha256sum dist/1440k.img


GENERATE BOOTABLE ISO9660 IMAGE

    make iso
    ls -al iso/1440k.iso
    sha256sum dist/1440k.iso