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Customizing the FreeBSD Kernel (eng) (freebsd kernel howto)

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From: Michael Lucas <> Subject: Customizing the FreeBSD Kernel (eng) Customizing the FreeBSD Kernel FreeBSD for the Linux administrator Summary As Linux and FreeBSD often run side by side, Linux administrators would do well to learn how to configure FreeBSD to meet their needs. Michael Lucas presents a guide to customizing the FreeBSD kernel, written for the Linux oriented. (3,400 words) By Michael Lucas C ustomizing a Unix kernel is a rite of passage for the system administrator. Linux and many System V Unixes have a friendly, menu-driven kernel configuration system. A system administrator needs to have a good grounding in a variety of Unixes, however. You would configure FreeBSD's kernel in a manner completely different than the one in which you would System V or Linux. Linux and FreeBSD are frequently found side by side. They're both free Unix-like systems. Each has particular strengths; FreeBSD is known for its network services, and Linux has strong desktop facilities. A multiple-boot system can have both FreeBSD and Linux partitions. With FreeBSD's Linux compatibility and the right kernel options, you can even use both simultaneously. This step-by-step guide includes a discussion of some of the core differences between the FreeBSD kernel and the Linux kernel; descriptions of the kernel configuration, build processes, and common kernel options; ways you can gather more information; and steps to take if you have trouble. The FreeBSD kernel building system is very powerful, and it only requires a bit of familiarity to use it quickly and efficiently. We'll only discuss the i386 kernel here. The Alpha kernel is handled in a very similar manner, but most of us aren't lucky enough to have an Alpha lying around to test new operating systems. Alphas also have slightly different kernel requirements, and not all functions are fully implemented. Check LINT (more on this later) for details. You can also apply most of the information discussed here to NetBSD and OpenBSD, with some minor changes. Your hardware Before building your kernel, you need to know what hardware you have. If you're not sure, a good place to start is the file /var/run/dmesg.boot. This records the results of the dmesg command immediately after boot. The dmesg command displays the contents of the system message buffer; if your system has been up for a while, the buffer probably contains system information other than boot probes. /var/run/dmesg.boot remains, however. If a device shows up as not found, you may in fact not have the device; your hardware or kernel could also be misconfigured. The kernel probe routines are fairly intelligent, but some hardware doesn't lend itself to easy detection. For example, if you have an old-fashioned, dumb-as-a-brick NE2000 network card sitting on IRQ 11 and port 0x320, the default kernel setting of IRQ 10 and port 0x280 isn't going to find it. You will need to change one or the other. Basic kernel information The Linux kernel appears in frequent, incremental releases. Every so often Linus Torvalds double-checks what he has, tars it up, assigns it a version number, and posts it. The FreeBSD kernel, on the other hand, is in a state of continuous release. The kernel and user programs are available via CVS (Cyclic Software's Concurrent Version System version control software). Users are expected to be able to compile the current kernel at any time. Similarly, Linux kernels are expected to have certain minimal levels of glibc and other system utilities. The FreeBSD kernel is expected to be in sync with the user programs. This makes sense, given the different development methods: the FreeBSD project is responsible for both the kernel and user programs, while Linux is strictly speaking just the kernel, with distribution designers being responsible for user programs. We'll assume that you have a FreeBSD system on which the kernel and user programs are already in sync, because it installs and stays that way unless you make a CVS error. The kernel code is stored under /sys. Don't bother backing up this directory; if there's an error, you could use CVS to back it out. To configure your kernel, look under /sys/i386/conf. You will find a variety of files. The important ones for our purposes are: * GENERIC: A default kernel configuration * LINT: A kernel configuration that contains all possible options Kernel configurations are simple text files stating which options, device drivers, and functions the kernel will support. The FreeBSD kernel has thousands and thousands of possible combinations of options; a menu system would run several screens deep and take far too long to use. Building your own kernel config file In Linux, to build a kernel config file you would unpack your new kernel source or apply the patch from the previous version. You'd then either make config, make menuconfig, or make xconfig. Each of these tools brings up a configuration interface, which would take the form of a simple string of questions, a cursor-based menu, or an X Window point-and-click configuration tool. All of these options are simple enough that almost any computer-literate person can use them. FreeBSD, on the other hand, has no menu configuration tool. The configuration is stored in a simple text file. Functions are enabled and disabled via flags on each device. A graphic interface to this system would run several levels deep, and each menu would be different. The best place to start building your custom kernel is the GENERIC file. Do not edit this file directly; the next system upgrade will overwrite your changes. Copy GENERIC to another file, which we'll call NEWSERVER. It's fairly traditional to name your kernel configuration after the machine itself, and to use all capital letters. The apparent problem with the GENERIC file is that it isn't self documenting. GENERIC isn't supposed to include extensive documentation. That's what the LINT file is for. First, you will want to go through your config file and change any occurrence of the word GENERIC to NEWSERVER. Then you might want to comment out unnecessary entries. Removing drivers for unnecessary devices will shrink your kernel. This also means that you have to rebuild your kernel to add hardware. If your hardware changes a lot, leave more drivers in; if it's constant, trim them. The equipment that makes up your system is also important in deciding whether to leave a feature-rich or stripped-down kernel. My desktop machine is an AMD K6 350 with 192 MB of RAM, a 19-inch monitor, and a 2 GB hard drive; the kernel is lugging around drivers for hardware I hope to be able to afford one day. Meanwhile, the office firewall is a 486/25 with 8 MB of RAM. Its kernel is so stripped it could be arrested for indecent exposure. GENERIC is divided into sections for various types of cards, controllers, and functions. Some are easy to find; if you have an ATAPI-only system, you can delete all the SCSI devices. Some are less obvious, and you need to check the LINT file for the entry's description. A few of the obvious ones to check are: * I386_CPU, I486_CPU, I586_CPU, and I686_CPU: These are for CPU types. The I586 is a Pentium; Pentium II and above are I686. If you're not sure what your CPU probes as, check /var/run/dmesg.boot. Some chips don't probe as you might expect; for example, some older AMD chips probe as a lower-level CPU than their clock speed seems to indicate. If you don't include the proper CPU in your finished kernel, the machine will no longer boot. You might as well remove the drivers for all the CPUs you don't have; changing CPU class will probably mean so many other hardware changes that you'll need a new kernel anyway. * MFS, MFS_ROOT, NFS, NFS_ROOT, MSDOSFS: If you're not using Memory File System, Network File System, or FAT filesystem, you can remove these. The filesystem features are all available as KLMs (kernel loadable modules) if you later discover that you do need them. * wdc*, wd*, acd*, wfd*: These are standard ATAPI controllers, disks, CD-ROMs, and LS-120 floppies. You probably want to leave the wdc controllers; most motherboards have them, even if there are no ATAPI devices attached. * SCSI: If you don't have any SCSI systems, you can remove all the SCSI controllers and peripherals. If you do this, be certain you have eliminated all the SCSI devices. They're all arranged in a nice little block. If you pull all the controllers, but leave a peripheral or two, the kernel will not compile. * Network card: Check /var/run/dmesg.boot to see if your NIC was detected. Any modern NIC probably will be. If not, check the NIC list in LINT to see what driver your card should use, and how to set its port and IRQ, if necessary. You can remove drivers for all the network devices you don't have. * Pseudodevices: The ppp device is used for kernel PPP, sl is used for slip, and tun is used for user programs' PPP. You can remove whichever devices you don't need. The BPF pseudodevice allows you to sniff network traffic. If you use DHCP, you need it. If you don't want the machine to have Ethernet packet sniffing capabilities, you want to lose it. Now that you've trimmed the kernel to the reasonable minimum needed to support your system, see what you want to add. Scroll through LINT looking for features you might like. Adding extra features won't hurt; the kernel will grow, but on most modern systems that's not a serious problem. If you're not running an original first-edition Pentium, for example, you should use the NO_F00F_HACK option. (Foof was a bug in the original Pentium design that allowed users to lock up the system. If your CPU doesn't have the bug, you don't need kernel protection against it.) For Linux compatibility, you need the POSIX options and the USER_LDT option. If you have a Linux partition, you probably want the EXT2FS option as well. Don't be surprised if you run into a fundamental design difference between the Linux and FreeBSD kernels. Linux tends to install everything that might be needed; FreeBSD has a more spartan core. Everything you need is there, but you must add the appropriate options. This philosophy holds true both in the kernel and across the user programs. Kernel tweaking Once you have all the functions you want in your kernel, you might find that it doesn't work exactly as you'd like. Check LINT for more information on each feature. Also, every device driver has a man page; check there for details on how to tweak the configuration. Many Linux command-line options are kernel options in FreeBSD; similarly, some Linux kernel options are controlled on the FreeBSD command line. For example, the wd (aka IDE) driver in GENERIC isn't configured to do DMA (direct memory access) by default. Check the wd man page for the proper flags to use. In this particular case, you probably want to add flags 0xa0ffa0ff to the IDE controller. Do you want your console to default out a particular serial port? Go to the man page for sio. Do you want to force an Intel EtherExpress network card to run at 10BaseT instead of 100BaseTX? Check the man page for fxp. You might find that some functions should be configured in /etc/rc.conf instead of in your kernel, or that you need to drop a two-line shell script into /usr/local/etc/rc.d instead of building it into the kernel. Kernel building Now that you have your kernel configuration file in shape, do the following as root: %config NEWSERVER Don't forget to do a make depend Kernel build directory is ../../compile/NEWSERVER % The config program parses your config file and makes sure that you have no egregious errors. It also makes a compile directory if one doesn't exist already, and installs all the header and source files needed to start the build process therein. If config returns a compile directory, your config file parsed properly. This doesn't guarantee that it will compile; it just means that the file passed config's simple checks. You might get errors and a compile directory. Fix the errors before trying to build a kernel. In the best case, the kernel won't compile. Alternately, it might compile but not boot -- and at worst, it could boot and break something. If the latter occurs, go to the compile directory and type: make depend all install This is fairly similar to the Linux kernel build procedure. You don't need to make clean. The kernel will build and install as /kernel. Your old kernel will be moved to /kernel.old. On your next reboot, you will be running your customized kernel. There is no need to muck about with the loader (FreeBSD's LILO equivalent) to install a kernel, as you would in Linux. Updating your kernel When you update your system source code, either through CVSup and make world, or off a CD-ROM, you will want to rebuild your kernel to take advantage of the new code. Glance through /usr/src/UPDATING to see if any of your preferred kernel options have changed. If you're feeling paranoid, check GENERIC and LINT as well. Then simply do the following: * %cd /sys/i386/conf * %config NEWSERVER * %cd ../../compile/NEWSERVER * %make depend && make all install This will rebuild only the parts of the kernel that have changed and assemble them into the new kernel. If a kernel subsystem hasn't changed, it won't be recompiled. Building new kernels can be extremely rapid if the configuration or source code hasn't changed much. If you don't want to keep your prior kernel around as /kernel.old, you can use make depend && make all reinstall. This will simply overwrite your old kernel. Again, you shouldn't have to make clean as you would in Linux. The old object files are used to build the new kernel more quickly; only changed objects are built. If the build fails, however, you might try make clean before make depend. If you are doing a large upgrade (for example, FreeBSD 3-STABLE to 4-STABLE), you are probably better off starting with the new GENERIC config file and rebuilding your configuration from scratch. If you want to use your own config, I highly recommend diff -uw GENERIC MYSERVER to generate a unified diff. Go through the diff carefully and be sure you understand every option you need to add or remove. For those of us who upgrade our entire systems from source every week, however, the quick configure and compile is a major timesaver. Troubleshooting If your kernel build fails, look at the last lines of the compile output. At times the problem is obvious; perhaps a particular kernel component cannot coexist with another, or an option requires another option. Many of these errors are fairly self-explanatory. Others aren't. Consider the example below: ===> sys/modules/xl cc -O -pipe -D_KERNEL -Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes -Wmissing-prototypes -Wpointer-arith -Winline -Wcast-qual -fformat-extensions -ansi -DKLD_MODULE -nostdinc -I- -I. -I@ -I@/../include -mpreferred-stack-boundary=2 -c /usr/src/sys/modules/xl/../../pci/if_xl.c /usr/src/sys/modules/xl/../../pci/if_xl.c:155: syntax error before `<' cpp: output pipe has been closed *** Error code 1 Stop in /usr/src/sys/modules/xl. *** Error code 1 Stop in /usr/src/sys/modules. *** Error code 1 Stop in /usr/src/sys. *** Error code 1 Stop in /usr/src. *** Error code 1 Stop in /usr/src. *** Error code 1 Everything after the first *** is noise.The error is probably immediately before this. If you get an error you don't understand, or aren't up to reading kernel code to debug it at the moment, take the last bit of the compile failure (ignoring the error code 1 lines and other associated crud) and go to the FreeBSD Webpage. (See Resources for a link.) Search the freebsd-questions mailing list archive for your particular error. Unless you've discovered a new error, someone will have already had the problem, and someone else will have provided a solution. If your answer isn't there, email the list yourself at Include the following information: * Your FreeBSD version number * The contents of /var/run/dmesg/boot * The output of uname -a * The kernel config file * The end of the output of the failed compile Yes, it's going to be a long message; but if you don't include all this information, someone will probably write back and ask for the missing piece. Disaster recovery You might install a kernel only to discover that you've blown away some vital function. If you're one of the brave souls running FreeBSD-current, you might find that your new kernel is phoning the police every hour and telling them you're smuggling gerbils. Avoid this. Keep a good kernel around. The kernel install procedure copies the old kernel to /kernel.old. You can boot this kernel instead of /kernel, the default. When the machine first boots, you will see a countdown: booting kernel in ... seconds. Hit the space bar to interrupt the autoboot, and type: disk1s1a>unload disk1s1a>load kernel.old disk1s1a>boot (Your prompt may differ, depending on exactly when you interrupt the autoboot.) I keep a known good kernel, /kernel.good, on all my systems. This way, if I have two goofball kernels in a row I can still boot the machine. The system installs a /kernel.GENERIC for this purpose, but GENERIC might lack functions you need or have some incorrect settings. You can also use RCS (Revision Control System) on your kernel config file to allow you to back up to a prior version; this is a good idea when you're just learning about all the options. If you find that you goofed in building your new kernel and assigned a dumb device the wrong memory address or IRQ, you don't have to crack your case or rebuild your kernel. Interrupt the boot process as above and type the following: disk1s1a>boot -c This will bring you up into the kernel configuration tool. You can alter IRQs, memory ports, and other system information here. The next time you have to rebuild your kernel, you can fix it properly. Summary FreeBSD's kernel build process is configured just like Samba, named, Apache, or any other common network utility: it is done with a text configuration file. The average Linux administrator is generally comfortable with this sort of configuration, and shouldn't be put off just because it's the kernel.

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