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7 <title>The rEFInd Boot Manager: Methods of Booting Linux</title>
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12 <h1>The rEFInd Boot Manager:<br />Methods of Booting Linux</h1>
13
14 <p class="subhead">by Roderick W. Smith, <a
15 href="mailto:rodsmith@rodsbooks.com">rodsmith@rodsbooks.com</a></p>
16
17 <p>Originally written: 3/19/2012; last Web page update:
18 10/6/2012, referencing rEFInd 0.4.6</p>
19
20
21 <p>I'm a technical writer and consultant specializing in Linux technologies. This Web page is provided free of charge and with no annoying outside ads; however, I did take time to prepare it, and Web hosting does cost money. If you find this Web page useful, please consider making a small donation to help keep this site up and running. Thanks!</p>
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86
87 <hr />
88
89 <p>This page is part of the documentation for the rEFInd boot manager. If a Web search has brought you here, you may want to start at the <a href="index.html">main page.</a></p>
90
91 <hr />
92
93 <p>Windows and Mac OS X both provide relatively simple EFI boot loader programs. Launch them, and if they're launched from the correct locations and have the correct files in place, they'll boot their respective OSes. This makes rEFInd's job easy; it just locates the boot loader program files and runs them.</p>
94
95 <p>Under Linux, by contrast, things can get complicated. As detailed on my <a href="http://www.rodsbooks.com/efi-bootloaders/index.html">Managing EFI Boot Loaders for Linux</a> page, several different EFI boot loaders for Linux exist, and all of them require configuration. If you're lucky, your distribution will have set up a Linux boot loader in a sensible way, in which case rEFInd should detect it and it will work as easily as a Windows or Mac OS X boot loader. If you're not lucky, though, you may need to configure it further. rEFInd offers options to help out with this task. Specifically, you can use a traditional Linux boot loader or configure an EFI stub loader.</p>
96
97 <h2>Using a Traditional Linux Boot Loader</h2>
98
99 <p>I consider <a href="http://www.rodsbooks.com/efi-bootloaders/elilo.html">ELILO,</a> <a href="http://www.rodsbooks.com/efi-bootloaders/grub_legacy.html">GRUB Legacy,</a> and <a href="http://www.rodsbooks.com/efi-bootloaders/grub2.html">GRUB 2</a> to be traditional Linux boot loaders. These programs all exist as EFI programs that are independent of the Linux kernel, but that can load a kernel and hand off control to it. All three programs have their own configuration files that reside in the same directory as the boot loader itself (or optionally elsewhere, in the case of GRUB 2).</p>
100
101 <p>Ordinarily, rEFInd will detect these traditional boot loaders and provide main menu entries for them. If the boot loader exists in a directory with a name that matches a Linux distribution's icon filename, you'll automatically get a distribution-specific icon to refer to the boot loader.</p>
102
103 <p>If you prefer, you can disable automatic scanning and create an entry in <tt>refind.conf</tt> for your distribution, as described on the <a href="configfile.html">Configuring the Boot Manager</a> page. This method is harder to set up but can be preferable if you want to customize your options.</p>
104
105 <h2>Using the EFI Stub Loader</h2>
106
107 <p>The Linux <a href="http://www.rodsbooks.com/efi-bootloaders/efistub.html">EFI stub loader</a> is a way to turn a Linux kernel into an EFI application. In a sense, the kernel becomes its own boot loader. This approach to booting Linux is very elegant in some ways, but as described on the page to which I just linked, it has its disadvantages, too. One challenge to booting in this way is that modern Linux installations typically require that the kernel be passed a number of options at boot time. These tell the kernel where the Linux root (<tt>/</tt>) filesystem is, where the initial RAM disk is, and so on. Without these options, Linux won't boot. These options are impossible for a generic boot loader to guess without a little help. It's possible to build a kernel with a default set of options, but this is rather limiting. Thus, rEFInd provides configuration options to help.</p>
108
109 <p>With all versions of rEFInd, you can create manual boot loader stanzas
110 in the <tt>refind.conf</tt> file to identify a Linux kernel and to pass it
111 all the options it needs. This approach is effective and flexible, but it
112 requires editing a single configuration file for all the OSes you want to
113 define in this way. If a computer boots two different Linux distributions,
114 and if both were to support rEFInd, problems might arise as each one tries
115 to modify its own rEFInd configuration; or the one that controls rEFInd
116 might set inappropriate options for another distribution. This is a problem
117 that's been a minor annoyance for years under BIOS, since the same
118 potential for poor configuration applies to LILO, GRUB Legacy, and GRUB 2
119 on BIOS. The most reliable solution there is to chainload one boot loader
120 to another. The same solution is possible under EFI, but rEFInd offers
121 another possibility.</p>
122
123 <p>rEFInd 0.2.1 and later supports semi-automatic Linux EFI stub loader detection. This feature works as part of the standard boot loader scan operation, but it extends it as follows:</p>
124
125 <ol>
126
127 <li>rEFInd looks for boot loaders whose names include the strings
128 <tt>bzImage</tt> or <tt>vmlinuz</tt> and that end in <tt>.efi</tt>. For
129 instance, <tt>bzImage-3.3.0.efi</tt> or <tt>vmlinuz-3.3.0-fc17.efi</tt>
130 would match, and trigger subsequent steps in this procedure. Beginning
131 with version 0.3.0, if you uncomment the
132 <tt>scan_all_linux_kernels</tt> option in <tt>refind.conf</tt>, rEFInd
133 will also scan for kernels <i>without</i> a <tt>.efi</tt> filename
134 extension. This option is not the default, though, because it can pick
135 up old kernels that lack EFI stub loader support and even non-kernel
136 files.</li>
137
138 <p class="sidebar">A kernel whose filename lacks a version string matches an initial RAM disk that also lacks a version string in its filename. Note that you can reliably use only <i>one</i> kernel and initial RAM disk per directory that lack version numbers in their filenames.</p>
139
140 <li>rEFInd looks for an initial RAM disk in the same directory as the
141 kernel file. A matching initial RAM disk has a name that begins with
142 <tt>init</tt> and that includes the same version string as the kernel.
143 The version string is defined as the part of the filename from the
144 first digit to the last digit, inclusive. Note that the version string
145 can include non-digits. For instance, the version string for
146 <tt>bzImage-3.3.0.efi</tt> is <tt>3.3.0</tt>, which matches
147 <tt>initramfs-3.3.0.bz</tt>; and <tt>vmlinuz-3.3.0-fc17.efi</tt>'s
148 version string is <tt>3.3.0-fc17</tt>, which matches
149 <tt>initrd-3.3.0-fc17.img</tt>. Many other matches are possible. If an
150 initial RAM disk is identified, rEFInd passes a suitable
151 <tt>initrd=</tt> option to the kernel when it boots.</li>
152
153 <p class="sidebar">rEFInd 0.2.1 and 0.2.2 used a filename of <tt>linux.conf</tt> to hold Linux kernel options; however, the Linux kernel developers plan to use this name themselves, so I've switched to <tt>refind_linux.conf</tt> as of rEFInd 0.2.3. Through version 0.4.2, rEFInd still supported the <tt>linux.conf</tt> filename as a backup to <tt>refind_linux.conf</tt>, but as of version 0.4.3, <tt>linux.conf</tt> no longer works, so you should rename rEFInd's <tt>linux.conf</tt> file to <tt>refind_linux.conf</tt> if you're upgrading.</p>
154
155 <li>rEFInd looks for a file called <tt>refind_linux.conf</tt> in the same
156 directory as the kernel file. This file is a practical requirement for
157 booting from an auto-detected kernel. It consists of a series of lines,
158 each of which consists of a label followed by a series of kernel
159 options. The first line sets default options, and subsequent lines set
160 options that are accessible from the main menu tag's submenu
161 screen.</li>
162
163 </ol>
164
165 <p>The intent of this system is that distribution maintainers can place their kernels, initial RAM disks, and a <tt>refind_linux.conf</tt> file in their own subdirectory on the ESP. rEFInd will detect their kernels and create one main menu entry for each kernel. Each entry will implement as many options as there are lines in the <tt>refind_linux.conf</tt> file. In this way, two or more distributions can each maintain their boot loader entries, without being too concerned about who maintains rEFInd as a whole.</p>
166
167 <p>The <tt>scan_all_linux_kernels</tt> option is intended to help users and distribution maintainers when rEFInd is used in conjunction with a Linux filesystem driver for EFI or when the ESP is mounted as the Linux <tt>/boot</tt> partition. In these cases, if all the kernels in Linux's <tt>/boot</tt> directory include EFI stub loader support, rEFInd will automatically detect and use kernels installed in the usual way, such as via an automatic system update. You won't even need to move or rename your kernels. You will need to set up a <tt>refind_linux.conf</tt> file and you may need to install a driver or set the <tt>also_scan_dirs</tt> option in <tt>refind.conf</tt>; but these are one-time requirements. Set up in this way, ongoing maintenance to handle kernel updates drops to zero!</p>
168
169 <p>As an example, consider the following file configuration:</p>
170
171 <pre class="listing">
172 $ <b>ls -l /boot/efi/EFI/ubuntu/</b>
173 total 17943
174 -rwxr-xr-x 1 root root 4781632 2012-03-18 12:01 bzImage-3.3.0.efi
175 -rwxr-xr-x 1 root root 131072 2011-10-14 04:10 grubx64.EFI
176 -rwxr-xr-x 1 root root 13459936 2012-03-18 12:02 initrd.img-3.3.0
177 -rwxr-xr-x 1 root root 266 2012-03-26 19:39 refind_linux.conf
178 </pre>
179
180 <p>When rEFInd scans this directory, it will find two EFI boot loaders in <tt>EFI/ubuntu</tt>: <tt>grubx64.EFI</tt> and <tt>bzImage-3.3.0.efi</tt>. rEFInd will create two main-menu tags for these two loaders, one of which will launch Ubuntu's standard GRUB and the other of which will launch the 3.3.0 kernel file directly. The <tt>refind_linux.conf</tt> file contains a list of labels and options:</p>
181
182 <pre class="listing">
183 "Boot with defaults" "root=/dev/sda3 ro quiet splash vt.handoff=7"
184 "Boot into single-user mode" "root=UUID=1cd95082-bce0-494c-a290-d2e642dd82b7 ro single"
185 "Boot without graphics" "root=UUID=1cd95082-bce0-494c-a290-d2e642dd82b7 ro"
186 # "Boot alternate install" "root=/dev/sdb9 ro quiet splash vt.handoff=7"
187 </pre>
188
189 <p>Ordinarily, both fields in this file must be enclosed in quotes. You can include as much white space as you like between options. You can also place comments in the file, or remove an option by commenting it out with a leading hash mark (<tt>#</tt>), as in the fourth line in this example.</p>
190
191 <p>In the preceding example, the first line sets the options that rEFInd passes to the kernel by default (along with the name of the <tt>initrd.img-3.3.0</tt> file, since its version string matches that of the kernel). The next two lines set options that you can obtain by pressing Insert, F2, or + on the main menu, as shown here:</p>
192
193 <br /><center><img src="automatic-submenu.png" align="center"
194 width="376" height="279" alt="rEFInd can load Linux boot options from
195 a refind_linux.conf file in the Linux kernel's directory."
196 border=2></center><br />
197
198 <p>Note that the first entry shown here takes a name that's set in rEFInd rather than the one specified in the <tt>refind_linux.conf</tt> file. The remaining names match those specified in the file, though.</p>
199
200 <p>From a user's perspective, the submenus defined in this way work just like submenus defined via the <tt>submenuentry</tt> options in <tt>refind.conf</tt>, or like the submenus that rEFInd creates automatically for Mac OS X or ELILO. There are, however, limitations in what you can accomplish with this method:</p>
201
202 <ul>
203
204 <li>Your kernels must be compiled with EFI stub loader support.</li>
205
206 <li>You can't set a submenu option to boot via a different boot loader,
207 such as ELILO or GRUB; all the submenu options apply to a single boot
208 loader&mdash;that is, a single kernel. (rEFInd will still detect other
209 boot loaders and provide separate main-menu tags for them,
210 though.)</li>
211
212 <li>If an installation includes two or more kernel files, each one receives
213 its own main-menu entry; you can't combine them together in one menu
214 item. This is essentially a corollary of the preceding limitation. The
215 result can be an overburdened main menu if your system has many
216 kernels.</li>
217
218 <li>All the kernels in a given directory use the same
219 <tt>refind_linux.conf</tt> file. If you need to set different options
220 for different kernels, you'll need to place those kernels in different
221 directories.</li>
222
223 <li>You must place your kernels in a directory other than the one that
224 holds the main rEFInd <tt>.efi</tt> file. This is because rEFInd does
225 not scan its own directory for boot loaders.</li>
226
227 </ul>
228
229 <p>Ordinarily, a kernel booted in this way must reside on the ESP, or at least on another FAT partition. On a Macintosh, though, you can use HFS+ to house your kernel files. In fact, that may be necessary; my Mac Mini hangs when I try to boot a Linux kernel via an EFI stub loader from the computer's ESP, but it works fine when booting from an HFS+ partition. If you use <a href="drivers.html">EFI drivers,</a> though, you can place your kernel on any filesystem for which an EFI driver exists. This list is currently rather limited (ext2fs/ext3fs, ReiserFS, ISO-9660, and HFS+), but even just one or two options might help a lot if you've got an undersized ESP or if copying your kernel file to the ESP is a hassle you'd rather avoid.</p>
230
231 <p>Beginning with version 0.3.1, rEFInd sorts boot loader entries <i>within each directory</i> by time stamp, so that the most recent entry comes first. Thus, if you specify a directory name (or a volume label, for loaders stored in a volume's root directory) as the <tt>default_selection</tt>, rEFInd will make the most recent loader in the directory the default. This can obviate the need to adjust this configuration parameter when you add a new kernel; chances are you want the most recently-added kernel to be the default, and rEFInd makes it so when you set the <tt>default_selection</tt> in this way. If you <i>don't</i> want the latest kernel to become the default, you can use <tt>touch</tt> to give the desired kernel (or other boot loader) in the directory a more recent time stamp, or you can set <tt>default_selection</tt> to a value that uniquely identifies your desired default loader. One caveat you should keep in mind is that the EFI and Windows interpret the hardware clock as local time, whereas Mac OS X uses <a href="http://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated Universal Time (UTC)</a>. Linux can work either way. Thus, time stamps for boot loaders can be skewed by several hours depending on the environment in which they were created or last modified.</p>
232
233 <p class="sidebar"><b>Tip for distribution maintainers:</b> If you maintain an <tt>EFI/<tt class="variable">distname</tt></tt> directory for your kernels, you can place your version of rEFInd in a directory called <tt>EFI/<tt class="variable">distname</tt>/refind</tt>. This will avoid collisions with duplicate rEFInd installations from other distributions.</p>
234
235 <p>On the whole, this method of configuration has a lot going for it. For distribution maintainers, if you place your Linux kernel files (with EFI stub support) on the ESP, with suitable filenames, matching initial RAM disk files, and a <tt>refind_linux.conf</tt> file, then any rEFInd 0.2.3 or later installation should detect your files, even if the user installs another distribution with another rEFInd that takes over from yours. (If the user, or this other rEFInd installation, disables auto-detection, this won't work.)</p>
236
237 <p>For end users, this method is simpler than maintaining manual configurations in <tt>refind.conf</tt> (or equivalents for ELILO or GRUB). To install a new kernel, you need only copy it and its initial RAM disk, under suitable names, to a scanned directory on the ESP. There's no need to touch any configuration file, provided you've already set up <tt>refind_linux.conf</tt> in your kernel's directory. You will, however, have to adjust <tt>refind_linux.conf</tt> if you make certain changes, such as if your root directory identifier changes.</p>
238
239 <hr/>
240
241 <p>copyright &copy; 2012 by Roderick W. Smith</p>
242
243 <p>This document is licensed under the terms of the <a href="FDL-1.3.txt">GNU Free Documentation License (FDL), version 1.3.</a></p>
244
245 <p>If you have problems with or comments about this Web page, please e-mail me at <a href="mailto:rodsmith@rodsbooks.com">rodsmith@rodsbooks.com.</a> Thanks.</p>
246
247 <p><a href="index.html">Go to the main rEFInd page</a></p>
248
249 <p><a href="revisions.html">Learn about rEFInd's history</a></p>
250
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