1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
|
<?xml version='1.0'?>
<!-- -*- DocBook -*- -->
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"/usr/share/sgml/docbook/dtd/xml/4.2/docbookx.dtd" [
<!ENTITY % sgml.features "IGNORE">
<!ENTITY % xml.features "INCLUDE">
<!ENTITY % dbcent PUBLIC
"-//OASIS//ENTITIES DocBook Character Entities V4.2//EN"
"/usr/share/sgml/docbook/dtd/xml/4.2/dbcentx.mod"> %dbcent;
<!ENTITY % commondata SYSTEM "../common.ent" > %commondata;
<!ENTITY % popcon SYSTEM "../popcon.ent" > %popcon;
<!ENTITY % pkgsize SYSTEM "../pkgsize.ent" > %pkgsize;
<!ENTITY % urlsdata SYSTEM "../urls.ent" > %urlsdata;
]>
<chapter id="thesysteminitialization">
<title>The system initialization</title>
<para>It is wise for you as the system administrator to know roughly how the Debian system is started and configured. Although the exact details are in the source files of the packages installed and their documentations, it is a bit overwhelming for most of us. </para>
<para>I did my best to provide a quick overview of the key points of the Debian system and their configuration for your reference, based on the current and previous knowledge of mine and others. Since the Debian system is a moving target, the situation over the system may have been changed. Before making any changes to the system, you should refer to the latest documentation for each package. </para>
<para/>
<section id="anoverviewofthebootstrapprocess">
<title>An overview of the boot strap process</title>
<para>The computer system undergoes several phases of <ulink url="&bootstrapprocesses;">boot strap processes</ulink> from the power-on event until it offers the fully functional operating system (OS) to the user. </para>
<para>For simplicity, I will limit discussion to the typical PC platform with the default installation. </para>
<para>The typical boot strap process is like a four-stage rocket. Each stage rocket hands over the system control to the next stage one. Here each stage corresponds to: </para>
<itemizedlist>
<listitem><para>Stage 1: the BIOS </para></listitem>
<listitem><para>Stage 2: the boot loader </para></listitem>
<listitem><para>Stage 3, the mini-Debian system </para></listitem>
<listitem><para>Stage 4: the normal Debian system </para></listitem>
</itemizedlist>
<para>Of course, these can be configured differently. For example, if you compiled your own kernel, you may be skipping the step with the mini-Debian system. So please do not assume this is the case for your system until you check it yourself. </para>
<note><para> For non-legacy PC platform such as the SUN or the Macintosh system, the BIOS on ROM and the partition on the disk may be quite different (<xref linkend="partitionconfiguration"/>). Please seek the platform specific documentations elsewhere for such a case. </para></note>
<para/>
</section>
<section id="stagebthebios">
<title>Stage 1: the BIOS</title>
<para>The <ulink url="&bios;">BIOS</ulink> is the 1st stage of the boot process which is started by the power-on event. The <ulink url="&bios;">BIOS</ulink> resides on the <ulink url="&readonlymemoryrom;">read only memory (ROM)</ulink> from the particular memory address to which the program counter of CPU is initialized by the power-on event. </para>
<para>This BIOS performs the basic initialization of the hardware (<ulink url="&postpoweronselftest;">POST: power on self test</ulink>) and hands the system control to the next step which you provide. The BIOS is usually provided with the hardware. </para>
<para>The BIOS startup screen usually indicates what key(s) to press to enter the BIOS setup screen to configure the BIOS behavior. Popular keys used are F1, F2, F10, Esc, Ins, and Del. If your BIOS startup screen is hidden by a nice graphics screen, you may press some keys such as Esc to disable this. These keys are highly dependent on the hardware. </para>
<para>The hardware location and the priority of the code started by the BIOS can be selected from the BIOS setup screen. Typically, the first few sectors of the first found selected device (hard disk, floppy disk, CD-ROM, ...) are loaded to the memory and this initial code is executed. This initial code can be: </para>
<itemizedlist>
<listitem><para>the boot loader code, </para></listitem>
<listitem>
<para>the kernel code of the stepping stone OS such as <ulink url="&freedos;">FreeDOS</ulink>, or </para>
</listitem>
<listitem><para>the kernel code of the target OS if it fits in this small space. </para></listitem>
</itemizedlist>
<para>Typically, the system is booted from the specified partition of the primary hard disk partition. The first sector of the hard disk contain the <ulink url="&masterbootrecordmbr;">master boot record (MBR)</ulink>. The disk partition information including the boot selection is recorded at the end of this MBR. The first boot loader code executed from the BIOS for the hard disk occupies the rest of this MBR. </para>
<para/>
</section>
<section id="stagecthebootloader">
<title>Stage 2: the boot loader</title>
<para>The <ulink url="&bootloader;">boot loader</ulink> is the 2nd stage of the boot process which is started by the BIOS. It loads the system kernel image and the <ulink url="&initrd;">initrd</ulink> image to the memory and hands control over to them. This initrd image is the root filesystem image and its support depends on the bootloader used. </para>
<para>The Debian system normally uses the Linux kernel as the default system kernel. The initrd image for the current 2.6 Linux kernel is technically the initramfs (initial RAM filesystem) image. The initramfs image is a gzipped cpio archive of files in the root filesystem. </para>
<para/>
<para/>
<para>The default install of the Debian system places first-stage GRUB boot loader code into the <ulink url="&mbr;">MBR</ulink> for the PC platform. There are many boot loaders and configuration options available. </para>
<table id="listofbootloaders">
<title> List of boot loaders. </title><tgroup x-pkgname=" 2 " x-popcon=" 3 " x-pkgsize=" 4 " cols="6">
<colspec colname="xxx1"/>
<colspec colname="xxx2"/>
<colspec colname="xxx3"/>
<colspec colname="xxx4"/>
<colspec colname="xxx5"/>
<colspec colname="xxx6"/>
<thead><row>
<entry>
<para>
<emphasis role="strong">bootloader</emphasis>
</para>
</entry>
<entry>
<para>
<emphasis role="strong">package</emphasis>
</para>
</entry>
<entry>
<para>
<emphasis role="strong">popcon</emphasis>
</para>
</entry>
<entry>
<para>
<emphasis role="strong">size</emphasis>
</para>
</entry>
<entry>
<para>
<emphasis role="strong">initrd</emphasis>
</para>
</entry>
<entry>
<para>
<emphasis role="strong">description</emphasis>
</para>
</entry>
</row></thead><tbody><row>
<entry>
<para>
<ulink url="&grublegacy;">GRUB Legacy</ulink>
</para>
</entry>
<entry>
<para> grub </para>
</entry>
<entry><para>&pop-grub;</para></entry>
<entry><para>&size-grub;</para></entry>
<entry>
<para> Supported </para>
</entry>
<entry>
<para> This is smart enough to understand disk partitions and file systems such as vfat, ext3, .... (etch default) </para>
</entry>
</row><row>
<entry>
<para>
<ulink url="&grubc;">GRUB 2</ulink>
</para>
</entry>
<entry>
<para> grub-pc </para>
</entry>
<entry><para>&pop-grubpc;</para></entry>
<entry><para>&size-grubpc;</para></entry>
<entry>
<para> Supported </para>
</entry>
<entry>
<para> This is smart enough to understand disk partitions and file systems such as vfat, ext3, .... (new for lenny) </para>
</entry>
</row><row>
<entry>
<para>
<ulink url="&grubc;">GRUB 2</ulink>
</para>
</entry>
<entry>
<para> grub-rescue-pc </para>
</entry>
<entry><para>&pop-grubrescuepc;</para></entry>
<entry><para>&size-grubrescuepc;</para></entry>
<entry>
<para> Supported </para>
</entry>
<entry>
<para> This is GRUB 2 bootable rescue images (CD and floppy) (PC/BIOS version) </para>
</entry>
</row><row>
<entry>
<para>
<ulink url="&lilo;">Lilo</ulink>
</para>
</entry>
<entry>
<para> lilo </para>
</entry>
<entry><para>&pop-lilo;</para></entry>
<entry><para>&size-lilo;</para></entry>
<entry>
<para> Supported </para>
</entry>
<entry>
<para> This relies on the sector locations of data on the hard disk. (Old) </para>
</entry>
</row><row>
<entry>
<para>
<ulink url="&isolinux;">Isolinux</ulink>
</para>
</entry>
<entry>
<para> syslinux </para>
</entry>
<entry><para>&pop-syslinux;</para></entry>
<entry><para>&size-syslinux;</para></entry>
<entry>
<para> Supported </para>
</entry>
<entry>
<para> This understands the ISO9660 file system. This is used by the boot CD. </para>
</entry>
</row><row>
<entry>
<para>
<ulink url="&syslinux;">Syslinux</ulink>
</para>
</entry>
<entry>
<para> syslinux </para>
</entry>
<entry><para>&pop-syslinux;</para></entry>
<entry><para>&size-syslinux;</para></entry>
<entry>
<para> Supported </para>
</entry>
<entry>
<para> This understands the <ulink url="&msdosfilesystemfat;">MSDOS file system (FAT)</ulink>. This is used by the boot floppy. </para>
</entry>
</row><row>
<entry>
<para>
<ulink url="&loadlin;">Loadlin</ulink>
</para>
</entry>
<entry>
<para> loadlin </para>
</entry>
<entry><para>&pop-loadlin;</para></entry>
<entry><para>&size-loadlin;</para></entry>
<entry>
<para> Supported </para>
</entry>
<entry>
<para> New system is started from the FreeDOS/MSDOS system. </para>
</entry>
</row><row>
<entry>
<para>
<ulink url="&neilturtonsmbr;">Neil Turton's MBR</ulink>
</para>
</entry>
<entry>
<para> mbr </para>
</entry>
<entry><para>&pop-mbr;</para></entry>
<entry><para>&size-mbr;</para></entry>
<entry>
<para> Not supported </para>
</entry>
<entry>
<para> This is free software which substitutes MSDOS <ulink url="&mbr;">MBR</ulink>. This only understands disk partitions. </para>
</entry>
</row></tbody>
</tgroup>
</table>
<para>For GRUB Legacy, the menu configuration file is located at <code>/boot/grub/menu.lst</code> . For example, it has entries like: </para>
<screen>title Debian GNU/Linux
root (hd0,2)
kernel /vmlinuz root=/dev/hda3 ro
initrd /initrd.img
</screen>
<para>For GRUB 2, the menu configuration file is located at <code>/boot/grub/grub.cfg</code>. It is automatically generated by <code>/usr/sbin/update-grub</code> using templates from <code>/etc/grub.d/*</code> and settings from <code>/etc/default/grub</code>. For example, it has entries like: </para>
<para/>
<screen>menuentry "Debian GNU/Linux" {
set root=(hd0,3)
linux /vmlinuz root=/dev/hda3
initrd /initrd.img
}
</screen>
<para>For these examples, these GRUB parameters mean: </para>
<table id="themeaningofgrubparameters">
<title> The meaning of GRUB parameters. </title><tgroup x-pkgname=" " x-popcon="" x-pkgsize="" cols="2">
<colspec colname="xxx1"/>
<colspec colname="xxx2"/>
<thead><row>
<entry>
<para>
<emphasis role="strong">GRUB parameter</emphasis>
</para>
</entry>
<entry>
<para>
<emphasis role="strong">meaning</emphasis>
</para>
</entry>
</row></thead><tbody><row>
<entry>
<para>
<code>root</code>
</para>
</entry>
<entry>
<para> Use 3rd partition on boot disk by setting it as <code>(hd0,2)</code> in GRUB legacy or as <code>(hd0,3)</code> in GRUB 2. </para>
</entry>
</row><row>
<entry>
<para>
<code>kernel</code>
</para>
</entry>
<entry>
<para> Use kernel located at <code>/vmlinuz</code> with kernel parameter: "<code>root=/dev/hda3 ro</code>". </para>
</entry>
</row><row>
<entry>
<para>
<code>initrd</code>
</para>
</entry>
<entry>
<para> Use <ulink url="&initrdinitramfs;">initrd/initramfs</ulink> image located at "<code>/initrd.img</code>". </para>
</entry>
</row></tbody>
</tgroup>
</table>
<note><para> The value of the partition number used by GRUB legacy program is one less than normal one used by Linux kernel and utility tools. GRUB 2 program fixes this problem. </para></note>
<tip><para><ulink url="&uuid;">UUID</ulink> (see <xref linkend="accessingpartitionusinguuid"/>) may be used to identify a block special device instead of its file name such as "<code>/dev/hda3</code>", e.g."<code>root=UUID=81b289d5-4341-4003-9602-e254a17ac232 ro</code>". </para></tip>
<tip><para> You can start a boot loader from another boot loader using techniques called <ulink url="&chainloading;">chain loading</ulink>. </para></tip>
<note><para> Having bootable rescue media (CD or floppy) created from images in the <code>grub-rescue-pc</code> package let you boot your system without functioning bootloader on the harddisk. </para></note>
<para>See "<code>info grub</code>" and <code>grub-install</code>(8). </para>
<para/>
</section>
<section id="stagedtheminidebiansystem">
<title>Stage 3: the mini-Debian system</title>
<para/>
<para/>
<para/>
<para/>
<para/>
<para>The mini-Debian system is the 3rd stage of the boot process which is started by the boot loader. It runs the system kernel with its root filesystem on the memory. It is an optional preparatory stage of the boot process. </para>
<note><para> The term "the mini-Debian system" is coined by the author to describe this 3rd stage boot process for this document. This system is commonly referred as the <ulink url="&initrd;">initrd</ulink> or initramfs system. Similar system on the memory is used by <ulink url="&thedebianinstaller;">the Debian Installer</ulink>. </para></note>
<para>The <code>/init</code> script is executed as the first program in this root filesystem on the memory. It is a shell script program which initializes the kernel in user space and hands control over to the next stage. This mini-Debian system offers flexibility to the boot process such as adding kernel modules before the main boot process or mounting the root file system as an encrypted one. </para>
<para>You can interrupt this part of the boot process to gain root shell by providing "<code>break=init</code>" etc. to the kernel boot parameter. See the <code>/init</code> script for more break conditions. This shell environment is sophisticated enough to make a good inspection of your machine's hardware. </para>
<para>Commands available in this mini-Debian system are stripped down ones and mainly provided by a GNU tool called <code>busybox</code>. </para>
<caution><para> You need to use "<code>-n</code>" option for <code>mount</code> command when you are on the readonly root file system. </para></caution>
<para/>
</section>
<section id="stageethenormaldebiansystem">
<title>Stage 4: the normal Debian system</title>
<para>The normal Debian system is the 4th stage of the boot process which is started by the mini-Debian system. The system kernel for the mini-Debian system continues to run in this environment. The root filesystem is switched from the one on the memory to the one on the real harddisk filesystem. </para>
<para>The <code>/sbin/init</code> program is executed as the first program and performs the main boot process. The Debian normally uses the traditional sysvinit scheme with the <code>sysv-rc</code> package. See <code>man 8 init</code>, <code>man 5 inittab</code>, and <code>/usr/share/doc/sysv-rc/README.runlevels.gz</code> for the exact explanation. Following is a simplified overview of this main boot process: </para>
<orderedlist numeration="arabic">
<listitem>
<para>The Debian system goes into <emphasis role="strong">runlevel N</emphasis> (none) to initialize the system by following the <code>/etc/inittab</code> description. </para>
</listitem>
<listitem>
<para>The Debian system goes into <emphasis role="strong">runlevel S</emphasis> to initialize the system under the single-user mode to complete hardware initialization etc. </para>
</listitem>
<listitem>
<para>The Debian system switches itself to one of the specified <emphasis role="strong">multi-user runlevels (2 to 5)</emphasis> to start the system services. </para>
</listitem>
</orderedlist>
<para>The initial runlevel used for multi-user mode is specified with the "<code>init=</code>" kernel boot parameter or in the "initdefault" line of this <code>/etc/inittab</code>. The Debian system as installed starts at the <emphasis role="strong">runlevel 2</emphasis>. </para>
<para>All scripts executed by the init system are located in the directory <code>/etc/init.d/</code>. </para>
<tip><para> For alternative boot mechanism to the <code>sysv-rc</code> package using a single configuration file, see the <code>file-rc</code> package. Both mechanisms are compatible through <code>/etc/init.d/rc</code>, <code>/etc/init.d/rcS</code>, <code>/usr/sbin/update-rc.d</code>, and <code>/usr/sbin/invoke-rc.d</code> scripts. </para></tip>
<para/>
<section id="themeaningoftherunlevel">
<title>The meaning of the runlevel</title>
<para>Each runlevel uses a directory for its configuration and has specific meaning: </para>
<table id="listofrunlevelsandmeanings">
<title> List of runlevels and meanings. </title><tgroup x-pkgname=" " x-popcon=" " x-pkgsize="" cols="3">
<colspec colname="xxx1"/>
<colspec colname="xxx2"/>
<colspec colname="xxx3"/>
<thead><row>
<entry>
<para>
<emphasis role="strong">runlevel</emphasis>
</para>
</entry>
<entry>
<para>
<emphasis role="strong">directory</emphasis>
</para>
</entry>
<entry>
<para>
<emphasis role="strong">meaning</emphasis>
</para>
</entry>
</row></thead><tbody><row>
<entry>
<para>
<code>N</code>
</para>
</entry>
<entry>
<para> none </para>
</entry>
<entry>
<para> System bootup (NONE). There is no <code>/etc/rcN.d/</code> directory.</para>
</entry>
</row><row>
<entry>
<para>
<code>0</code>
</para>
</entry>
<entry>
<para>
<code>/etc/rc0.d/</code>
</para>
</entry>
<entry>
<para> Halt the system. </para>
</entry>
</row><row>
<entry>
<para>
<code>S</code>
</para>
</entry>
<entry>
<para>
<code>/etc/rcS.d/</code>
</para>
</entry>
<entry>
<para> Single-user mode on boot. The lower case <code>s</code> can be used as alias. </para>
</entry>
</row><row>
<entry>
<para>
<code>1</code>
</para>
</entry>
<entry>
<para>
<code>/etc/rc1.d/</code>
</para>
</entry>
<entry>
<para> Single-user mode switched from multi-user mode. </para>
</entry>
</row><row>
<entry>
<para>
<code>2</code>
</para>
</entry>
<entry>
<para>
<code>/etc/rc2.d/</code>
</para>
</entry>
<entry>
<para> Multi-user mode. </para>
</entry>
</row><row>
<entry>
<para>
<code>3</code>
</para>
</entry>
<entry>
<para>
<code>/etc/rc3.d/</code>
</para>
</entry>
<entry>
<para> ,, </para>
</entry>
</row><row>
<entry>
<para>
<code>4</code>
</para>
</entry>
<entry>
<para>
<code>/etc/rc4.d/</code>
</para>
</entry>
<entry>
<para> ,, </para>
</entry>
</row><row>
<entry>
<para>
<code>5</code>
</para>
</entry>
<entry>
<para>
<code>/etc/rc5.d/</code>
</para>
</entry>
<entry>
<para> ,, </para>
</entry>
</row><row>
<entry>
<para>
<code>6</code>
</para>
</entry>
<entry>
<para>
<code>/etc/rc6.d/</code>
</para>
</entry>
<entry>
<para> Reboot the system. </para>
</entry>
</row><row>
<entry>
<para>
<code>7</code>
</para>
</entry>
<entry>
<para>
<code>/etc/rc7.d/</code>
</para>
</entry>
<entry>
<para> Valid multi-user mode but not normally used. </para>
</entry>
</row><row>
<entry>
<para>
<code>8</code>
</para>
</entry>
<entry>
<para>
<code>/etc/rc8.d/</code>
</para>
</entry>
<entry>
<para> ,, </para>
</entry>
</row><row>
<entry>
<para>
<code>9</code>
</para>
</entry>
<entry>
<para>
<code>/etc/rc9.d/</code>
</para>
</entry>
<entry>
<para> ,, </para>
</entry>
</row></tbody>
</tgroup>
</table>
<para>You can change the runlevel from the console to, e.g., 4 by: </para>
<screen>$ sudo telinit 4
</screen>
<caution><para><emphasis role="strong">The Debian system does not pre-assign any special meaning differences among the runlevels between 2 and 5.</emphasis> The system administrator on the Debian system may change this. (I.e., Debian is not RedHat nor SOLARIS nor HP-UX nor ...) </para></caution>
<caution><para> The Debian system does not populate directories for the runlevels between 7 and 9 when the package is installed. Traditional Unix variants don’t use these runlevels. </para></caution>
<para/>
</section>
<section id="theconfigurationoftherunlevel">
<title>The configuration of the runlevel</title>
<para>The names of the symlinks in the runlevel directories have the form <code>S<2-digit-number><original-name></code> or <code>K<2-digit-number><original-name></code>. The 2-digit-number is used to determine the order in which to run the scripts. 'S' is for 'Start' and 'K' is for 'Kill'. </para>
<para>When <code>init</code> or <code>telinit</code> commands change the runlevel to <n>: </para>
<orderedlist numeration="arabic">
<listitem>
<para>the script names starting with a <code>K</code> in <code>/etc/rc<n>.d/</code> are executed in alphabetical order with the single argument <code>stop</code> . (killing services) </para>
</listitem>
<listitem>
<para>the script names starting with an <code>S</code> in <code>/etc/rc<n>.d/</code> are executed in alphabetical order with the single argument <code>start</code> . (starting services) </para>
</listitem>
</orderedlist>
<para>For example, if you had the links <code>S10sysklogd</code> and <code>S20exim4</code> in a runlevel directory, <code>S10sysklogd</code> would run before <code>S20exim4</code>. </para>
<warning><para> It is not advisable to make any changes to symlinks in <code>/etc/rcS.d/</code> unless you know better than the maintainer. </para></warning>
<para/>
</section>
<section id="therunlevelmanagementexample">
<title>The runlevel management example</title>
<para>For example, let's set up runlevel system somewhat like Redhat system, i.e.: </para>
<itemizedlist>
<listitem><para>to start the system in runlevel=3 as the default, </para></listitem>
<listitem>
<para>not to start <code>gdm</code> in runlevel=(0,1,2,6), and </para>
</listitem>
<listitem>
<para>to start <code>gdm</code> in runlevel=(3,4,5). </para>
</listitem>
</itemizedlist>
<para>The easy way is to use editor on the <code>/etc/inittab</code> file to change starting runlevel and use user friendly runlevel management tools such as <emphasis role="strong"><code>sysv-rc-conf</code></emphasis> or <emphasis role="strong"><code>bum</code></emphasis> to edit the runlevel. If you are to use command line only, here is how you do it (after the default installation of <code>gdm</code> package and selecting it to be the choice of display manager): </para>
<screen># cd /etc/rc2.d ; mv S21gdm K21gdm
# cd /etc ; perl -i -p -e 's/^id:.:/id:3:/' inittab
</screen>
<para>Please note the <code>/etc/X11/default-display-manager</code> file is checked when starting the display manager daemons: <code>xdm</code>, <code>gdm</code>, <code>kdm</code>, and <code>wdm</code>. </para>
<note><para> You can still start X from any console shell with the <code>startx</code> command. </para></note>
<para/>
</section>
<section id="thedefaultparameoreachinitscript">
<title>The default parameter for each init script</title>
<para>The default parameter for each init script in <code>/etc/init.d/</code> is given by the corresponding file in <code>/etc/default/</code> which contains environment variable assignments <emphasis role="strong">only</emphasis>. The choice of directory name is specific to the Debian system. It is roughly the equivalent of the <code>/etc/sysconfig</code> directory found in Red Hat and other distributions. </para>
<para>For example, <code>/etc/default/hotplug</code> can be used to control how <code>/etc/init.d/hotplug</code> works. The <code>/etc/default/rcS</code> file can be used to customize boot-time defaults for <code>motd</code>, <code>sulogin</code>, etc. </para>
<para>If you cannot get the behavior you want by changing such variables then you may <ulink url="&modifytheinitscripts;">modify the init scripts</ulink> themselves: they are all configuration files. </para>
<para/>
</section>
<section id="thehostname">
<title>The hostname</title>
<para>The kernel maintains the system <emphasis role="strong">hostname</emphasis>. The initscript <code>/etc/init.d/hostname.sh</code> sets the system hostname at boot time (using the <code>hostname</code> command) to the name stored in <code>/etc/hostname</code>. This file should contain <emphasis role="strong">only</emphasis> the system hostname, not a fully qualified domain name. </para>
<para>To print out the current hostname run <code>hostname</code> without an argument. </para>
<para/>
</section>
<section id="networkinterfaceinitialization">
<title>Network interface initialization</title>
<para>Network interfaces are initialized under single-user mode on boot by the initscript <code>/etc/init.d/ifupdown-clean</code> and <code>/etc/init.d/ifupdown</code>. See <xref linkend="networksetup"/> for how to configure them. </para>
<para/>
</section>
<section id="networkserviceinitialization">
<title>Network service initialization</title>
<para>Many network services (see <xref linkend="networkapplications"/>) are started directly as daemon processes at boot time, e.g., <code>/etc/rc2.d/S20exim4</code> (for RUNLEVEL=2) which is a symlink to <code>/etc/init.d/exim4</code>. </para>
<para>Some network services can be started on demand using the <ulink url="&superserver;">super-server</ulink>, <code>inetd</code> (or its equivalents). The <code>inetd</code> is started at boot time by <code>/etc/rc2.d/S20inetd</code> (for RUNLEVEL=2) which is a symlink to <code>/etc/init.d/inetd</code>. Essentially, <code>inetd</code> allows one running daemon to invoke several others, reducing load on the system. </para>
<para>Whenever a request for service arrives, its protocol and service are identified by looking them up in the databases in <code>/etc/protocols</code> and <code>/etc/services</code>. <code>inetd</code> then looks up a normal Internet service in the <code>/etc/inetd.conf</code> database, or a Sun-RPC based service in <code>/etc/rpc.conf</code>. </para>
<para>For system security, make sure to disable unused services in <code>/etc/inetd.conf</code>. Sun-RPC services need to be active for NFS and other RPC-based programs. </para>
<para>Sometimes, <code>inetd</code> does not start the intended server directly but starts the <ulink url="&tcpwrapper;">TCP wrapper</ulink>, <code>tcpd</code>, with the intended server name as its argument in <code>/etc/inetd.conf</code>. In this case, <code>tcpd</code> runs the appropriate server program after logging the request and doing some additional checks using <code>/etc/hosts.deny</code> and <code>/etc/hosts.allow</code>. </para>
<para>If you have problems with remote access in a recent Debian system, comment out "ALL: PARANOID" in <code>/etc/hosts.deny</code> if it exists. (But you must be careful on security risks involved with this kind of action.) </para>
<para>For details, see <code>inetd</code>(8), <code>inetd.conf</code>(5), <code>protocols</code>(5), <code>services</code>(5), <code>tcpd</code>(8), <code>hosts_access</code>(5), and <code>hosts_options</code>(5). </para>
<para>For more information on Sun-RPC, see <code>rpcinfo</code>(8), <code>portmap</code>(8), and <code>/usr/share/doc/portmap/portmapper.txt.gz</code>. </para>
<para/>
</section>
<section id="thesystemmessage">
<title>The system message</title>
<para>The system message can be customized by <code>/etc/syslog.conf</code> for both the log file and on-screen display. See <code>syslogd</code>(8) and <code>syslog.conf</code>(5). See also <xref linkend="loganalyzer"/>. </para>
<para/>
</section>
<section id="thekernelmessage">
<title>The kernel message</title>
<para>The kernel message can be customized by <code>/etc/init.d/klogd</code> for both the log file and on-screen display. Set <code>KLOGD="-c 3"</code> in this script and run <code>/etc/init.d/klogd restart</code>. See <code>klogd</code>(8). </para>
<para>You may directly change the error message level by: </para>
<screen># dmesg -n3
</screen>
<para>Here: </para>
<table id="listofkernelerrorlevels">
<title> List of kernel error levels. </title><tgroup x-pkgname=" " x-popcon=" " x-pkgsize="" cols="3">
<colspec colname="xxx1"/>
<colspec colname="xxx2"/>
<colspec colname="xxx3"/>
<thead><row>
<entry>
<para>
<emphasis role="strong">error level value</emphasis>
</para>
</entry>
<entry>
<para>
<emphasis role="strong">error level name</emphasis>
</para>
</entry>
<entry>
<para>
<emphasis role="strong">meaning</emphasis>
</para>
</entry>
</row></thead><tbody><row>
<entry>
<para> 0 </para>
</entry>
<entry>
<para> KERN_EMERG </para>
</entry>
<entry>
<para> system is unusable </para>
</entry>
</row><row>
<entry>
<para> 1 </para>
</entry>
<entry>
<para> KERN_ALERT </para>
</entry>
<entry>
<para> action must be taken immediately </para>
</entry>
</row><row>
<entry>
<para> 2 </para>
</entry>
<entry>
<para> KERN_CRIT </para>
</entry>
<entry>
<para> critical conditions </para>
</entry>
</row><row>
<entry>
<para> 3 </para>
</entry>
<entry>
<para> KERN_ERR </para>
</entry>
<entry>
<para> error conditions </para>
</entry>
</row><row>
<entry>
<para> 4 </para>
</entry>
<entry>
<para> KERN_WARNING </para>
</entry>
<entry>
<para> warning conditions </para>
</entry>
</row><row>
<entry>
<para> 5 </para>
</entry>
<entry>
<para> KERN_NOTICE </para>
</entry>
<entry>
<para> normal but significant condition </para>
</entry>
</row><row>
<entry>
<para> 6 </para>
</entry>
<entry>
<para> KERN_INFO </para>
</entry>
<entry>
<para> informational </para>
</entry>
</row><row>
<entry>
<para> 7 </para>
</entry>
<entry>
<para> KERN_DEBUG </para>
</entry>
<entry>
<para> debug-level messages </para>
</entry>
</row></tbody>
</tgroup>
</table>
<para/>
</section>
<section id="theudevsystem">
<title>The udev system</title>
<para>For Linux kernel 2.6, <ulink url="&theudevsystem;">the udev system</ulink> provides mechanism for the automatic hardware discovery and initialization (see <code>udev</code>(7)). Upon discovery of each device by the kernel, the udev system starts a user process which uses information from the <ulink url="&sysfs;">sysfs</ulink> filesystem (see <xref linkend="procfsandsysfs"/>), loads required kernel modules supporting it using the <code>modprobe</code>(8) program (see <xref linkend="thekernelmoduleinitialization"/>), and creates corresponding device nodes. </para>
<para>The name of device nodes can be configured by files in <code>/etc/udev/rules.d/</code> (see <code>/usr/share/doc/udev/writing_udev_rules/index.html</code>). </para>
<para>Since the udev system is somewhat a moving target, I leave details to other documentations and describe the minimum information here. </para>
<para/>
</section>
<section id="thekernelmoduleinitialization">
<title>The kernel module initialization</title>
<para>The <code>modprobe</code>(8) program enables us to configure running Linux kernel from user process by adding and removing kernel modules. The udev system (see <xref linkend="theudevsystem"/>) automates its invocation to help the kernel module initialization. </para>
<para>Non-hardware modules and special hardware driver modules, such as: </para>
<itemizedlist>
<listitem>
<para><ulink url="&tuntap;">TUN/TAP</ulink> modules providing virtual Point-to-Point network device (TUN) and virtual Ethernet network device (TAP), </para>
</listitem>
<listitem>
<para><ulink url="&netfilter;">netfilter</ulink> modules providing netfilter firewall capabilities (<code>iptables</code>(8), <xref linkend="netfilter"/>), </para>
</listitem>
<listitem>
<para><ulink url="&watchdogtimer;">watchdog timer</ulink> driver modules </para>
</listitem>
</itemizedlist>
<para>need to be pre-loaded by listing them in the <code>/etc/modules</code> file (see <code>modules</code>(5)). </para>
<para>The configuration files for the <code>modprobe</code>(8) program are located under the <code>/etc/modprobes.d/</code> directory as explained in <code>modprobe.conf</code>(5). (If you want to avoid some kernel modules to be auto-loaded, consider to blacklist them in the <code>/etc/modprobes.d/blacklist</code> file.) </para>
<para>The <code>/lib/modules/<version>/modules.dep</code> file generated by the <code>depmod</code>(8) program describes module dependencies used by the <code>modprobe</code>(8) program. </para>
<para>The <code>modinfo</code>(8) program shows information about a Linux Kernel module. </para>
<para>The <code>lsmod</code>(8) program nicely formats the contents of the <code>/proc/modules</code>, showing what kernel modules are currently loaded. </para>
<tip><para> You can identify exact hardware on your system. See <xref linkend="thehardwareidentification"/>. </para></tip>
<tip><para> You may configure hardware at boot time to activate expected hardware features. See <xref linkend="thehardwareconfiguration"/>. </para></tip>
<tip><para> You can add support for your device by recompiling kernel. See <xref linkend="thekernel"/>. </para></tip>
</section>
</section>
</chapter>
|
