10.2. Configuring Linux® Binary Compatibility
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## 10.2. Configuring Linux® Binary Compatibility

By default, Linux® libraries are not installed and Linux® binary compatibility is not enabled. Linux® libraries can either be installed manually or from the FreeBSD Ports Collection.

Before attempting to build the port, load the Linux® kernel module, otherwise the build will fail:

    # kldload linux

For 64-bit compatibility:

    # kldload linux64

To verify that the module is loaded:

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

The emulators/linux_base-c7 package or port is the easiest way to install a base set of Linux® libraries and binaries on a FreeBSD system. To install the port:

    # pkg install emulators/linux_base-c7

For Linux® compatibility to be enabled at boot time, add this line to `/etc/rc.conf`:

    linux_enable="YES"

On 64-bit machines, `/etc/rc.d/abi` will automatically load the module for 64-bit emulation.

Since the Linux® binary compatibility layer has gained support for running both 32- and 64-bit Linux® binaries (on 64-bit x86 hosts), it is no longer possible to link the emulation functionality statically into a custom kernel.

### 10.2.1. Installing Additional Libraries Manually

If a Linux® application complains about missing shared libraries after configuring Linux® binary compatibility, determine which shared libraries the Linux® binary needs and install them manually.

From a Linux® system, `ldd` can be used to determine which shared libraries the application needs. For example, to check which shared libraries `linuxdoom` needs, run this command from a Linux® system that has Doom installed:

    % 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

Then, copy all the files in the last column of the output from the Linux® system into `/compat/linux` on the FreeBSD system. Once copied, create symbolic links to the names in the first column. This example will result in the following files on the 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

If a Linux® shared library already exists with a matching major revision number to the first column of the `ldd` output, it does not need to be copied to the file named in the last column, as the existing library should work. It is advisable to copy the shared library if it is a newer version, though. The old one can be removed, as long as the symbolic link points to the new one.

For example, these libraries already exist on the FreeBSD system:

    /compat/linux/lib/libc.so.4.6.27
    /compat/linux/lib/libc.so.4 -> libc.so.4.6.27

and `ldd` indicates that a binary requires a later version:

    libc.so.4 (DLL Jump 4.5pl26) -> libc.so.4.6.29

Since the existing library is only one or two versions out of date in the last digit, the program should still work with the slightly older version. However, it is safe to replace the existing `libc.so` with the newer version:

    /compat/linux/lib/libc.so.4.6.29
    /compat/linux/lib/libc.so.4 -> libc.so.4.6.29

Generally, one will need to look for the shared libraries that Linux® binaries depend on only the first few times that a Linux® program is installed on FreeBSD. After a while, there will be a sufficient set of Linux® shared libraries on the system to be able to run newly installed Linux® binaries without any extra work.

### 10.2.2. Installing Linux® ELF Binaries

ELF binaries sometimes require an extra step. When an unbranded ELF binary is executed, it will generate an error message:

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

To help the FreeBSD kernel distinguish between a FreeBSD ELF binary and a Linux® binary, use brandelf(1):

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

Since the GNU toolchain places the appropriate branding information into ELF binaries automatically, this step is usually not necessary.

### 10.2.3. Installing a Linux® RPM Based Application

To install a Linux® RPM-based application, first install the archivers/rpm4 package or port. Once installed, `root` can use this command to install a `.rpm`:

    # cd /compat/linux
    # rpm2cpio < /path/to/linux.archive.rpm | cpio -id

If necessary, `brandelf` the installed ELF binaries. Note that this will prevent a clean uninstall.

### 10.2.4. Configuring the Hostname Resolver

If DNS does not work or this error appears:

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

configure `/compat/linux/etc/host.conf` as follows:

    order hosts, bind
    multi on

This specifies that `/etc/hosts` is searched first and DNS is searched second. When `/compat/linux/etc/host.conf` does not exist, Linux® applications use `/etc/host.conf` and complain about the incompatible FreeBSD syntax. Remove `bind` if a name server is not configured using `/etc/resolv.conf`.

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