Recompile A FreeBSD Kernel With A Custom Configuration


FreeBSD is a free Unix-like operating system descended from Research Unix via the Berkeley Software Distribution (BSD). Although for legal reasons FreeBSD cannot use the Unix trademark, it is a direct descendant of BSD, which was historically also called “BSD Unix” or “Berkeley Unix”. The first version of FreeBSD was released in 1993, and today FreeBSD is the most widely used open-source BSD distribution, accounting for more than three-quarters of all installed systems running open-source BSD derivatives.

FreeBSD has similarities with Linux, with two major differences in scope and licensing: FreeBSD maintains a complete operating system, i.e. the project delivers kernel, device drivers, userland utilities and documentation, as opposed to Linux delivering a kernel and drivers only and relying on third-parties for system software and FreeBSD source code is generally released under a permissive BSD license as opposed to the copyleft GPL.

It uses the GENERIC kernel by default. FreeBSD’s kernel provides support for some essential tasks such as managing processes, communication, booting and filesystems. In this article, we will show you how you can recompile a FreeBSD kernel with a custom configuration.

Some Features of FreeBSD and kernel

From the different features we can list the following ones:

  • FreeBSD 10.0 now supports a truly tickless kernel, enhancing battery performance on laptops and general resource effectiveness in virtual machines.
  • AMD GPUs kernel mode setting supports the use of newer xf86-video-ati drivers and AMD GPUs
  • FreeBSD 10.0 brings with it support for ZFS TRIM and it also supports LZ4 compression support which compresses much better (up to 50%) than the default LZJB compression
  • BSD-kernel are not stand-alone kernels but are developed as being part of a whole. Of course, this is merely a philosophical point of view and not a technical one, but this give system coherence


As prerequisites for this article you need to need just to have a one FreeBSD 10.1 Droplet to be able to use the following commands; we assume that you are a FreeBSD user.

Recompile A FreeBSD Kernel

We will start by getting the operating system source code. As a characteristic of FreeBSD its ability to provide the source code to its operating system to be able to modify and download the source code easily. And to be able to recompile the kernel you have to use the following command in order to pull this source code from FreeBSD’s control system:

sudo pkg install subversion

Then you have to use the following command to rearrange the directory tables since the FreeBSD’s shell utilizes an internel hash table for commands in $PATH:


Then use the following command to check out the latest stable branch to the 



sudo svn co /usr/src

You will be asked to accept the server certificate type the “p” command to accept it. Now we will create our custom configuration for kernel. We will call our configuration CONF_anis you have the choice to choose another name of course. The kernel configuration files are in the 

/usr/src/sys/<span class="highlight">architecture</span>/conf


So move to the configuration directory using the following command:

cd /usr/src/sys/amd64/conf

Then use the following command to create and open the CONF_anis file in order to edit it:

sudo nano CONF_anis

Then copy the following text into your created file:

ident		CONF_anis

options 	SCHED_ULE		
options 	PREEMPTION		
options 	INET			
options 	INET6			
options 	TCP_OFFLOAD		
options 	SCTP			
options 	FFS			
options 	SOFTUPDATES		
options 	UFS_ACL			
options 	UFS_DIRHASH		
options 	UFS_GJOURNAL		
options 	QUOTA			
options 	MD_ROOT			
options 	NFSCL			
options 	NFSD			
options 	NFSLOCKD		
options 	NFS_ROOT	
options 	CD9660			
options 	PROCFS			
options 	PSEUDOFS		
options 	GEOM_PART_GPT		
options 	GEOM_RAID		
options 	GEOM_LABEL		
options 	SCSI_DELAY=5000		
options 	KTRACE			
options 	STACK			
options 	SYSVSHM			
options 	SYSVMSG			
options 	SYSVSEM			
options 	PRINTF_BUFR_SIZE=128	
options 	HWPMC_HOOKS		
options 	AUDIT			
options 	PROCDESC		
options 	MAC			
options 	KDTRACE_FRAME		
options 	KDTRACE_HOOKS		
options 	DDB_CTF			

options 	KDB			
options 	KDB_TRACE		

options 	SMP			

device		cpufreq

device		acpi
options 	ACPI_DMAR
device		pci

device		ata			
options 	ATA_STATIC_ID		

device		scbus			
device		da			
device		cd			
device		pass			

device		atkbdc			
device		atkbd			

device		vga			

device		splash			

device		sc
options 	SC_PIXEL_MODE		

device		uart			

device		loop			
device		random			
device		padlock_rng		
device		rdrand_rng		
device		ether			
device		vlan			
device		tun			
device		md			
device		gif			
device		faith			
device		firmware		

device		bpf			

options 	USB_DEBUG		
device		uhci			
device		usb			

device		virtio			
device		virtio_pci		
device		vtnet			
device		virtio_blk		
device		virtio_scsi		
device		virtio_balloon		

device		pf			
device		pflog			

options		ALTQ
options		ALTQ_CBQ        
options		ALTQ_RED       
options		ALTQ_RIO        
options		ALTQ_HFSC       
options		ALTQ_PRIQ       
device		crypto		
device		aesni		
options 	GEOM_ELI	
options 	IPSEC		
options		IPSEC_DEBUG

Then save and close your file.

 Now we will start the installation of our new Kernel. So we will move to the 


directory and use the following commands to use our new configuration file:

cd /usr/src
sudo make buildkernel KERNCONF=CONF_anis

It will require some time to finish that. As example of average equation for a 0.5 GB Droplet you need 45 minutes. After finishing your kernel recompilation, use the following command to start the installation:

sudo make installkernel KERNCONF=CONF_anis

After making the installation of your new system to reboot it, use the following command:

sudo shutdown -r now

And to check the functionality of your new kernel configuration use the following command:

sysctl kern.conftxt | grep ident

To be sure that everything works good you have to receive something like this:

ident    CONF_anis


In this article we explained for you how you can get a compiled and configured kernel. Now you get it and everything works good if you received the final message while checking the functionality of your system