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20140616-4 选题
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How to Rescue a Non-booting GRUB 2 on Linux
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================================================================================
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Figure 1: GRUB 2 menu with cool Apollo 17 background.
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Once upon a time we had legacy GRUB, the Grand Unified Linux Bootloader version 0.97. Legacy GRUB had many virtues, but it became old and its developers did yearn for more functionality, and thus did GRUB 2 come into the world.
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GRUB 2 is a major rewrite with several significant differences. It boots removable media, and can be configured with an option to enter your system BIOS. It's more complicated to configure with all kinds of scripts to wade through, and instead of having a nice fairly simple `/boot/grub/menu.lst` file with all configurations in one place, the default is `/boot/grub/grub.cfg`. Which you don't edit directly, oh no, for this is not for mere humans to touch, but only other scripts. We lowly humans may edit `/etc/default/grub`, which controls mainly the appearance of the GRUB menu. We may also edit the scripts in `/etc/grub.d/`. These are the scripts that boot your operating systems, control external applications such as memtest and os_prober, and theming`./boot/grub/grub.cfg` is built from `/etc/default/grub` and `/etc/grub.d/*` when you run the update-grub command, which you must run every time you make changes.
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The good news is that the update-grub script is reliable for finding kernels, boot files, and adding all operating systems to your GRUB boot menu, so you don't have to do it manually.
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We're going to learn how to fix two of the more common failures. When you boot up your system and it stops at the grub> prompt, that is the full GRUB 2 command shell. That means GRUB 2 started normally and loaded the normal.mod module (and other modules which are located in /boot/grub/[arch]/), but it didn't find your grub.cfg file. If you see grub rescue> that means it couldn't find normal.mod, so it probably couldn't find any of your boot files.
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How does this happen? The kernel might have changed drive assignments or you moved your hard drives, you changed some partitions, or installed a new operating system and moved things around. In these scenarios your boot files are still there, but GRUB can't find them. So you can look for your boot files at the GRUB prompt, set their locations, and then boot your system and fix your GRUB configuration.
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### GRUB 2 Command Shell ###
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The GRUB 2 command shell is just as powerful as the shell in legacy GRUB. You can use it to discover boot images, kernels, and root filesystems. In fact, it gives you complete access to all filesystems on the local machine regardless of permissions or other protections. Which some might consider a security hole, but you know the old Unix dictum: whoever has physical access to the machine owns it.
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When you're at the `grub>` prompt, you have a lot of functionality similar to any command shell such as history and tab-completion. The `grub rescue>` mode is more limited, with no history and no tab-completion.
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If you are practicing on a functioning system, press C when your GRUB boot menu appears to open the GRUB command shell. You can stop the bootup countdown by scrolling up and down your menu entries with the arrow keys. It is safe to experiment at the GRUB command line because nothing you do there is permanent. If you are already staring at the `grub>` or `grub rescue>`prompt then you're ready to rock.
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The next few commands work with both `grub>` and `grub rescue>`. The first command you should run invokes the pager, for paging long command outputs:
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grub> set pager=1
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There must be no spaces on either side of the equals sign. Now let's do a little exploring. Type ls to list all partitions that GRUB sees:
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grub> ls
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(hd0) (hd0,msdos2) (hd0,msdos1)
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What's all this msdos stuff? That means this system has the old-style MS-DOS partition table, rather than the shiny new Globally Unique Identifiers partition table (GPT). (See [Using the New GUID Partition Table in Linux (Goodbye Ancient MBR)][1]. If you're running GPT it will say (hd0,gpt1). Now let's snoop. Use the ls command to see what files are on your system:
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grub> ls (hd0,1)/
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lost+found/ bin/ boot/ cdrom/ dev/ etc/ home/ lib/
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lib64/ media/ mnt/ opt/ proc/ root/ run/ sbin/
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srv/ sys/ tmp/ usr/ var/ vmlinuz vmlinuz.old
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initrd.img initrd.img.old
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Hurrah, we have found the root filesystem. You can omit the msdos and gpt labels. If you leave off the slash it will print information about the partition. You can read any file on the system with the cat command:
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grub> cat (hd0,1)/etc/issue
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Ubuntu 14.04 LTS \n \l
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Reading /etc/issue could be useful on a multi-boot system for identifying your various Linuxes.
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### Booting From grub> ###
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This is how to set the boot files and boot the system from the grub> prompt. We know from running the ls command that there is a Linux root filesystem on (hd0,1), and you can keep searching until you verify where /boot/grub is. Then run these commands, using your own root partition, kernel, and initrd image:
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grub> set root=(hd0,1)
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grub> linux /boot/vmlinuz-3.13.0-29-generic root=/dev/sda1
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grub> initrd /boot/initrd.img-3.13.0-29-generic
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grub> boot
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The first line sets the partition that the root filesystem is on. The second line tells GRUB the location of the kernel you want to use. Start typing /boot/vmli, and then use tab-completion to fill in the rest. Type root=/dev/sdX to set the location of the root filesystem. Yes, this seems redundant, but if you leave this out you'll get a kernel panic. How do you know the correct partition? hd0,1 = /dev/sda1. hd1,1 = /dev/sdb1. hd3,2 = /dev/sdd2. I think you can extrapolate the rest.
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The third line sets the initrd file, which must be the same version number as the kernel.
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The fourth line boots your system.
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On some Linux systems the current kernels and initrds are symlinked into the top level of the root filesystem:
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$ ls -l /
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vmlinuz -> boot/vmlinuz-3.13.0-29-generic
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initrd.img -> boot/initrd.img-3.13.0-29-generic
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So you could boot from grub> like this:
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grub> set root=(hd0,1)
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grub> linux /vmlinuz root=/dev/sda1
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grub> initrd /initrd.img
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grub> boot
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### Booting From grub-rescue> ###
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If you're in the GRUB rescue shell the commands are different, and you have to load the normal.mod andlinux.mod modules:
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grub rescue> set prefix=(hd0,1)/boot/grub
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grub rescue> set root=(hd0,1)
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grub rescue> insmod normal
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grub rescue> normal
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grub rescue> insmod linux
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grub rescue> linux /boot/vmlinuz-3.13.0-29-generic root=/dev/sda1
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grub rescue> initrd /boot/initrd.img-3.13.0-29-generic
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grub rescue> boot
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Tab-completion should start working after you load both modules.
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### Making Permanent Repairs ###
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When you have successfully booted your system, run these commands to fix GRUB permanently:
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# update-grub
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Generating grub configuration file ...
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Found background: /usr/share/images/grub/Apollo_17_The_Last_Moon_Shot_Edit1.tga
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Found background image: /usr/share/images/grub/Apollo_17_The_Last_Moon_Shot_Edit1.tga
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Found linux image: /boot/vmlinuz-3.13.0-29-generic
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Found initrd image: /boot/initrd.img-3.13.0-29-generic
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Found linux image: /boot/vmlinuz-3.13.0-27-generic
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Found initrd image: /boot/initrd.img-3.13.0-27-generic
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Found linux image: /boot/vmlinuz-3.13.0-24-generic
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Found initrd image: /boot/initrd.img-3.13.0-24-generic
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Found memtest86+ image: /boot/memtest86+.elf
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Found memtest86+ image: /boot/memtest86+.bin
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done
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# grub-install /dev/sda
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Installing for i386-pc platform.
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Installation finished. No error reported.
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When you run grub-install remember you're installing it to the boot sector of your hard drive and not to a partition, so do not use a partition number like /dev/sda1.
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### But It Still Doesn't Work ###
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If your system is so messed up that none of this works, try the [Super GRUB2 live rescue disk][2]. The official [GNU GRUB Manual 2.00][3] should also be helpful.
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--------------------------------------------------------------------------------
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via: http://www.linux.com/learn/tutorials/776643-how-to-rescue-a-non-booting-grub-2-on-linux
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译者：[译者ID](https://github.com/译者ID) 校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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[1]:http://www.linux.com/learn/tutorials/730440-using-the-new-guid-partition-table-in-linux-good-bye-ancient-mbr-
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[2]:http://www.supergrubdisk.org/
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[3]:https://www.gnu.org/software/grub/manual/grub.html
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150
sources/tech/20140616 How to diskless boot a Linux machine.md
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How to diskless boot a Linux machine
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================================================================================
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Diskless booting implies that a client computer does not have any disk storage when booting an operating system. In that case, the computer can load the kernel as well as the root filesystem from a remote NFS server over network. It may use several different methods to load the kernel and the root filesystem from an NFS server: RARP, BOOTP or DHCP protocols. In this tutorial, I will use BOOTP/DHCP protocol because they are supported by many network cards.
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### Advantage of Diskless Computers ###
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Imagine you have 30 computers in your office, all of which need to access the same application. If you are managing the computers as an administrator, what would you do? It will be a waste of your time if you install the application on every computer. On the other hand, a diskless system can eliminate the problem. With a diskless system, you just need to install the application on a central NFS server, and then boot all 30 clients over network.
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### Requirements ###
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Two or more Linux computers equipped with network cards that support DHCP protocol. The computer that will act as an NFS server should have a hard drive, and the other client computer(s) do not need any hard drive. The server and client computer(s) need to be connected to the same local network.
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There are five steps to setting up the diskless system.
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1. Install required packages
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1. Configure a TFTP server
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1. Configure a DHCP server
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1. Configure an NFS server
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1. Booting diskless clients
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In this tutorial, I assume that the computer which will run as a booting server is running Ubuntu. If you are using other Linux distribution, the principle is the same.
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### Step One: Install Required Packages ###
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Use apt-get to install all necessary packages as follows.
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$ sudo apt-get install dhcp3-server tftpd-hpa syslinux nfs-kernel-server initramfs-tools
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### Step Two: Configure a TFTP Server ###
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TFTP server is a small FTP server which is needed for automated transfer of boot files between a client computer and server in the local network.
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Add the following lines to /etc/default/tftpd-hpa
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RUN_DAEMON="yes"
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OPTIONS="-l -s /var/lib/tftpboot/"
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Next, create a boot directory.
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$ sudo mkdir -p /var/lib/tftpboot/pxelinux.cfg
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Copy the bootstrap ROM.
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$ sudo cp /usr/lib/syslinux/pxelinux.0 /var/lib/tftpboot
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Create a default boot configuration file as follows.
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$ sudo vi /tftpboot/pxelinux.cfg/default
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----------
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LABEL Ubuntu
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KERNEL vmlinuz
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APPEND root=/dev/nfs initrd=initrd.img nfsroot=10.10.101.1:/nfsroot ip=dhcp rw
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Note:
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- "root=/dev/nfs" means the network filesystem on the server (doesn't need to change).
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- "initrd=initrd.img" is a boot script for system startup.
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- "nfsroot=10.10.101.1/nfsroot" indicates the server's IP address and the NFS share folder name. Substitute the IP address with your server's address.
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- "ip=dhcp" means that client computers use DHCP addressing scheme.
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- "rw" means that the NFS share is read/write.
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Finally, restart the TFTPD service.
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sudo /etc/init.d/tftpd-hpa restart
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### Step Three: Configure DHCP Service ###
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You also need to configure DHCP service on the NFS server to allow booting with /var/lib/tftpboot/pxelinux.0. Your configuration might look like the following, assuming you using subnet 10.10.101.0.
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$ sudo vi /etc/dhcp3/dhcpd.conf
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----------
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allow booting;
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allow bootp;
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subnet 10.10.101.0 netmask 255.255.255.0 {
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range 10.10.101.2 10.10.101.254;
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option broadcast-address 10.10.101.255;
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option routers 10.10.101.1;
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filename "/pxelinux.0";
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}
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Then restart DHCP service.
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$ sudo service isc-dhcp-server restart
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### Step Four: Configure an NFS server ###
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Create a directory that holds the client root filesystem.
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$ sudo mkdir /nfsroot
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Next, configure the NFS server to export the client root filesystem. For that, add the following line to /etc/exports.
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/nfsroot *(rw,no_root_squash,async,insecure,no_subtree_check)
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Run the following command to reload modified /etc/exports.
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$ sudo exportfs -rv
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By default, Ubuntu does not add network boot support to the initrd image. Thus you need to create a new initrd.img file. For that, first add the following line to /etc/initramfs-tools/initramfs.conf
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BOOT=nfs
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MODULES=netboot
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Then run the following command to create a new initrd.img.
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$ sudo mkinitramfs -o /var/lib/tftpboot/initrd.img
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Copy the new kernel image to /var/lib/tftpboot.
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$ sudo cp /boot/vmlinuz-`uname -r` /var/lib/tfftpboot/vmlinuz
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Now it is time to copy the entire root filesystem to /nfsroot.
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Assuming tgat you are using a fresh Ubuntu server installation, you just need to clone the server filesystem to the NFS root.
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$ sudo cp -ax / /nfsroot
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Then open /nfsroot/etc/fstab with a text editor to add the following line.
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/dev/nfs / nfs defaults 1 1
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The directory /var/lib/tftpboot should have world read/write permissions. Otherwise the client would not be able to boot from network.
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$ sudo chmod -R 777 /var/lib/tfftpboot
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Lastly, to avoid any misconfiguration on the server, I recommend using a static IP address for the interface which DHCP service is running on. For example, if its network interface is named eth0, your configuration in /etc/network/interfaces should look like this:
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iface eth0 inet static
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address 10.10.101.1
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netmask 255.255.255.0
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broadcast 10.10.101.255
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network 10.10.101.0
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### Step Five: Booting a Diskless Client ###
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After you completed the configuration on the server. Boot your client from network. To boot from network, you typically need to change the boot order priority in your BIOS configuration.
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If a client booted successful, then your diskless environment is ready. You can add one or more client computers without changing anything.
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--------------------------------------------------------------------------------
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via: http://xmodulo.com/2014/06/diskless-boot-linux-machine.html
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译者：[译者ID](https://github.com/译者ID) 校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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