FreeBSD Handbook
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22.2 Installation

Linux binary compatibility is not turned on by default. The easiest way to enable this functionality is to load the linux KLD object (``Kernel LoaDable object''). You can load this module by simply typing linux at the command prompt.

If you would like Linux compatibility to always be enabled, then you should add the following line to /etc/rc.conf:

    linux_enable="YES"

This, in turn, triggers the following action in /etc/rc.i386:

    # Start the Linux binary compatibility if requested.
    #
    case ${linux_enable} in
    [Yy][Ee][Ss])
        echo -n ' linux';   linux > /dev/null 2>&1
        ;;
    esac

The kldstat(8) command can be used to verify that the KLD is loaded:

    % kldstat
    Id Refs Address    Size     Name
     1    2 0xc0100000 16bdb8   kernel
     7    1 0xc24db000 d000     linux.ko

If for some reason you do not want to or cannot load the KLD, then you may statically link Linux binary compatibility into the kernel by adding options LINUX to your kernel configuration file. Then install your new kernel as described in Chapter 9.

22.2.1 Installing Linux Runtime Libraries

This can be done one of two ways, either by using the linux_base port, or by installing them manually.

22.2.1.1 Installing Using the linux_base Port

This is by far the easiest method to use when installing the runtime libraries. It is just like installing any other port from the ports collection. Simply do the following:

    # cd /usr/ports/emulators/linux_base
    # make install distclean

You should now have working Linux binary compatibility. Some programs may complain about incorrect minor versions of the system libraries. In general, however, this does not seem to be a problem.

22.2.1.2 Installing Libraries Manually

If you do not have the ``ports'' collection installed, you can install the libraries by hand instead. You will need the Linux shared libraries that the program depends on and the runtime linker. Also, you will need to create a ``shadow root'' directory, /compat/linux, for Linux libraries on your FreeBSD system. Any shared libraries opened by Linux programs run under FreeBSD will look in this tree first. So, if a Linux program loads, for example, /lib/libc.so, FreeBSD will first try to open /compat/linux/lib/libc.so, and if that does not exist, it will then try /lib/libc.so. Shared libraries should be installed in the shadow tree /compat/linux/lib rather than the paths that the Linux ld.so reports.

Generally, you will need to look for the shared libraries that Linux binaries depend on only the first few times that you install a Linux program on your FreeBSD system. After a while, you will have a sufficient set of Linux shared libraries on your system to be able to run newly imported Linux binaries without any extra work.

22.2.1.3 How to Install Additional Shared Libraries

What if you install the linux_base port and your application still complains about missing shared libraries? How do you know which shared libraries Linux binaries need, and where to get them? Basically, there are 2 possibilities (when following these instructions you will need to be root on your FreeBSD system).

If you have access to a Linux system, see what shared libraries the application needs, and copy them to your FreeBSD system. Look at the following example:

Let us assume you used FTP to get the Linux binary of Doom, and put it on a Linux system you have access to. You then can check which shared libraries it needs by running ldd linuxdoom, like so:

    % ldd linuxdoom
    libXt.so.3 (DLL Jump 3.1) => /usr/X11/lib/libXt.so.3.1.0
    libX11.so.3 (DLL Jump 3.1) => /usr/X11/lib/libX11.so.3.1.0
    libc.so.4 (DLL Jump 4.5pl26) => /lib/libc.so.4.6.29

You would need to get all the files from the last column, and put them under /compat/linux, with the names in the first column as symbolic links pointing to them. This means you eventually have these files on your FreeBSD system:

    /compat/linux/usr/X11/lib/libXt.so.3.1.0
    /compat/linux/usr/X11/lib/libXt.so.3 -> libXt.so.3.1.0
    /compat/linux/usr/X11/lib/libX11.so.3.1.0
    /compat/linux/usr/X11/lib/libX11.so.3 -> libX11.so.3.1.0
    /compat/linux/lib/libc.so.4.6.29 /compat/linux/lib/libc.so.4 -> libc.so.4.6.29

Note: Note that if you already have a Linux shared library with a matching major revision number to the first column of the ldd output, you will not need to copy the file named in the last column to your system, the one you already have should work. It is advisable to copy the shared library anyway if it is a newer version, though. You can remove the old one, as long as you make the symbolic link point to the new one. So, if you have these libraries on your system:
    /compat/linux/lib/libc.so.4.6.27
    /compat/linux/lib/libc.so.4 -> libc.so.4.6.27
and you find a new binary that claims to require a later version according to the output of ldd:
    libc.so.4 (DLL Jump 4.5pl26) -> libc.so.4.6.29
If it is only one or two versions out of date in the in the trailing digit then do not worry about copying /lib/libc.so.4.6.29 too, because the program should work fine with the slightly older version. However, if you like, you can decide to replace the libc.so anyway, and that should leave you with:
    /compat/linux/lib/libc.so.4.6.29
    /compat/linux/lib/libc.so.4 -> libc.so.4.6.29

Note: The symbolic link mechanism is only needed for Linux binaries. The FreeBSD runtime linker takes care of looking for matching major revision numbers itself and you do not need to worry about it.

22.2.2 Installing Linux ELF Binaries

ELF binaries sometimes require an extra step of ``branding''. If you attempt to run an unbranded ELF binary, you will get an error message like the following:

    % ./my-linux-elf-binary
    ELF binary type not known
    Abort

To help the FreeBSD kernel distinguish between a FreeBSD ELF binary from a Linux binary, use the brandelf(1) utility.

    % brandelf -t Linux my-linux-elf-binary

The GNU toolchain now places the appropriate branding information into ELF binaries automatically, so you this step should become increasingly more rare in the future.

22.2.3 Configuring the Hostname Resolver

If DNS does not work or you get this message:

    resolv+: "bind" is an invalid keyword resolv+:
    "hosts" is an invalid keyword

You will need to configure a /compat/linux/etc/host.conf file containing:

    order hosts, bind
    multi on

The order here specifies that /etc/hosts is searched first and DNS is searched second. When /compat/linux/etc/host.conf is not installed, Linux applications find FreeBSD's /etc/host.conf and complain about the incompatible FreeBSD syntax. You should remove bind if you have not configured a name server using the /etc/resolv.conf file.

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