Linux-Misc Digest #501, Volume #21 Sun, 22 Aug 99 04:13:09 EDT
Contents:
Linux Frequently Asked Questions with Answers (Part 4 of 6) ([EMAIL PROTECTED])
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From: [EMAIL PROTECTED]
Crossposted-To: news.answers,comp.answers
Subject: Linux Frequently Asked Questions with Answers (Part 4 of 6)
Date: 22 Aug 1999 07:20:35 GMT
Archive-Name: linux/faq/part4
URL: http://www.mainmatter.com/
Posting-Frequency: weekly
Last-modified: 08/15/99
Kermit is distributed under a non-GPL copyright that makes its terms
of distribution somewhat different. The sources and some binaries are
available on kermit.columbia.edu.
The WWW Home Page of the Columbia University Kermit project is
http://www.columbia.edu/kermit/.
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5.16 I want to use Linux with my cable modem.
The www.CablemodemInfo.com and xDSL Web page at
http://www.cablemodeminfo.com/ has a section devoted to Linux.
======================================================================
6. Solutions to common miscellaneous problems.
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6.1 Free dumps core.
In Linux 1.3.57 and later, the format of /proc/meminfo was changed in
a way that the implementation of free doesn't understand.
Get the latest version, from metalab.unc.edu, in
/pub/Linux/system/Status/ps/procps-0.99.tgz.
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6.2 My clock is very wrong.
There are two clocks in your computer. The hardware (CMOS) clock runs
even when the computer is turned off, and is used when the system
starts up and by DOS (if you use DOS). The ordinary system time, shown
and set by "date," is maintained by the kernel while Linux is
running.
You can display the CMOS clock time, or set either clock from the
other, with /sbin/clock program--see "man 8 clock."
There are various other programs that can correct either or both
clocks for system drift or transfer time across the network. Some of
them may already be installed on your system. Try looking for adjtimex
(corrects for drift), netdate, and getdate (get the time from the
network), or xntp (accurate, full-featured network time daemon).
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6.3 Setuid scripts don't seem to work.
That's right. This feature has been disabled in the Linux kernel on
purpose, because setuid scripts are almost always a security hole.
Sudo and SuidPerl can provide more security that setuid scripts or
binaries, especially if execute permissions are limited to a certain
user ID or group ID.
If you want to know why setuid scripts are a security hole, read the
FAQ for comp.unix.questions.
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6.4 Free memory as reported by free keeps shrinking.
The "free" figure printed by free doesn't include memory used as a
disk buffer cache--shown in the "buffers" column. If you want to
know how much memory is really free add the "buffers" amount to
"free"--newer versions of free print an extra line with this info.
The disk buffer cache tends to grow soon after starting Linux up. As
you load more programs and use more files, the contents get cached. It
will stabilize after a while.
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6.5 When I add more memory, the system slows to a crawl.
This is a common symptom of a failure to cache the additional memory.
The exact problem depends on your motherboard.
Sometimes you have to enable caching of certain regions in your BIOS
setup. Look in the CMOS setup and see if there is an option to cache
the new memory area which is currently switched off. This is
apparently most common on a '486.
Sometimes the RAM has to be in certain sockets to be cached.
Sometimes you have to set jumpers to enable caching.
Some motherboards don't cache all of the RAM if you have more RAM per
amount of cache than the hardware expects. Usually a full 256K cache
will solve this problem.
If in doubt, check the manual. If you still can't fix it because the
documentation is inadequate, you might like to post a message to
comp.os.linux.hardware giving all of the details--make, model number,
date code, etc., so other Linux users can avoid it.
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6.6 Some programs (e.g. xdm) won't let me log in.
You are probably using non-shadow password programs and are using
shadow passwords.
If so, you have to get or compile a shadow password version of the
programs in question. The shadow password suite can be found at
tsx-11.mit.edu:/pub/linux/sources/usr.bin/shadow/. This is the source
code. The binaries are probably in linux/binaries/usr.bin/.
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6.7 Some programs let me log in with no password.
You probably have the same problem as in "Some programs (e.g. xdm)
won't let me log in.", with an added wrinkle.
If you are using shadow passwords, you should put a letter `x' or an
asterisk in the password field of /etc/passwd for each account, so
that if a program doesn't know about the shadow passwords it won't
think it's a passwordless account and let anyone in.
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6.8 My machine runs very slowly when I run GCC / X / ...
You may have too little real memory. If you have less RAM than all the
programs you're running at once, Linux will swap to your hard disk
instead and thrash horribly. The solution in this case is to not run
so many things at once or buy more memory. You can also reclaim some
memory by compiling and using a kernel with less options configured.
See "How do I upgrade/recompile my kernel?".
You can tell how much memory and swap you're using with the free
command, or by typing:
cat /proc/meminfo
If your kernel is configured with a RAM disk, this is probably wasted
space and will cause things to go slowly. Use LILO or rdev to tell the
kernel not to allocate a RAM disk (see the LILO documentation or type
man rdev).
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6.9 I can only log in as root.
You probably have some permission problems, or you have a file
/etc/nologin.
In the latter case, put rm -f /etc/nologin in your /etc/rc.local or
/etc/rc.d/* scripts.
Otherwise, check the permissions on your shell, and any file names
that appear in error messages, and also the directories that contain
these files, up to and including the root directory.
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6.10 My screen is all full of weird characters instead of letters.
You probably sent some binary data to your screen by mistake. Type
echo '\033c' to fix it. Many Linux distributions have a command,
"reset," that does this.
If that doesn't help, try a direct screen escape command.
echo <Ctrl-V><Ctrl-O>
This resets the default font of a Linux console. Remember to hold down
the Control key and type the letter, instead of, for example,
`Ctrl-V'. The sequence
echo <Ctrl-V><Esc>c
causes a full screen reset. If there's data left on the shell command
line after typing a binary file, press Ctrl-C a few times to restore
the shell command line.
[Bernhard Gabler]
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6.11 I have screwed up my system and can't log in to fix it.
Reboot from an emergency floppy or floppy pair. For example, the
Slackware boot and root disk pair in the install subdirectory of the
Slackware distribution.
There are also two, do-it-yourself rescue disk creation packages in
metalab.unc.edu/pub/Linux/system/Recovery. These are better because
they have your own kernel on them, so you don't run the risk of
missing devices and file systems.
Get to a shell prompt and mount your hard disk with something like
mount -t ext2 /dev/hda1 /mnt
Then your file system is available under the directory /mnt and you
can fix the problem. Remember to unmount your hard disk before
rebooting (cd somewhere else first, or it will say it's busy).
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6.12 I've discovered a huge security hole in rm!
No you haven't. You are obviously new to unices and need to read a
good book to find out how things work. Clue: the ability to delete
files depends on permission to write in that directory.
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6.13 lpr(1) and/or lpd(8) don't work.
First make sure that your /dev/lp* port is correctly configured. Its
IRQ (if any) and port address need to match the settings on the
printer card. You should be able to dump a file directly to the
printer.
cat the_file >/dev/lp1
If lpr gives you a message like "myname@host: host not found," it
may mean that the TCP/IP loopback interface, lo, isn't working
properly. Loopback support is compiled into most distribution kernels.
Check that the interface is configured with the ifconfig command. By
Internet convention, the network number is 127.0.0.0, and the local
host address is 127.0.0.1. If everything is configured correctly, you
should be able to telnet to your own machine and get a login prompt.
Make sure that /etc/hosts.lpd contains the machine's host name.
If your machine has a network-aware lpd, like the one that comes with
LPRng, make sure that /etc/lpd.perms is configured correctly.
Also look at the Printing-HOWTO "Where can I get the HOWTO's and
other documentation?".
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6.14 Timestamps on files on MS-DOS partitions are set incorrectly.
There is a bug in the program "clock" (often found in /sbin). It
miscounts a time zone offset, confusing seconds with minutes or
something like that. Get a recent version.
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6.15 How do I get LILO to boot the kernel image?
From kernel versions 1.1.80 on, the compressed kernel image, which is
what LILO needs to find, is in arch/i386/boot/zImage, or
arch/i386/boot/bzImage when it is built, and is normally stored in the
/boot/ directory. The /etc/lilo.conf file should refer to the vmlinuz
symbolic link, not the actual kernel image.
This was changed to make it easier to build kernel versions for
several different processors from one source tree.
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6.16 I upgraded the kernel and now my PCMCIA card doesn't work.
The PCMCIA Card Services modules, which are located in
/lib/modules/<version>/pcmcia, where <version> is the version number
of the kernel, use configuration information that is specific to that
kernel image only. The PCMCIA modules on your system will not work
with a different kernel image. You need to upgrade the PCMCIA card
modules when you upgrade the kernel.
When upgrading from older kernels, make sure you have the most recent
version of the run-time libraries, the modutils package, and so on.
Refer to the file Documentation/Changes in the kernel source tree for
details.
Important: If you use the PCMCIA Card Services, do not enable the
"Network device support/Pocket and portable adapters" option of the
kernel configuration menu, as this conflicts with the modules in Card
Services.
Knowing the PCMCIA module dependencies of the old kernel is useful.
You need to keep track of them. For example, if your PCMCIA card
depends on the serial port character device being installed as a
module for the old kernel, then you need to ensure that the serial
module is available for the new kernel and PCMCIA modules as well.
The procedure described here is somewhat kludgy, but it is much easier
than re-calculating module dependencies from scratch, and making sure
the upgrade modules get loaded so that both the non-PCMCIA and PCMCIA
are happy. Recent kernel releases contain a myriad of module options,
too many to keep track of easily. These steps use the existing module
dependencies as much as possible, instead of requiring you to
calculate new ones.
However, this procedure does not take into account instances where
module dependencies are incompatible from one kernel version to
another. In these cases, you'll need to load the modules yourself with
insmod, or adjust the module dependencies in the /etc/conf.modules
file. The Documentation/modules.txt file in the kernel source tree
contains a good description of how to use the kernel loadable modules
and the module utilities like insmode, modprobe, and depmod.
Modules.txt also contains a recommended procedure for determining
which features to include in a resident kernel, and which to build as
modules.
Essentially, you need to follow these steps when you install a new
kernel.
* Before building the new kernel, make a record with the lsmod
command of the module dependencies that your system currently
uses. For example, part of lsmod's output might look like this:
memory_cs 2 0
ds 2 [memory_cs] 3
i82365 4 2
pcmcia_core 8 [memory_cs ds i82365] 3
sg 1 0
bsd_comp 1 0
ppp 5 [bsd_comp] 0
slhc 2 [ppp] 0
serial 8 0
psaux 1 0
lp 2 0
Module Pages Used by
This tells you for example that the memory_cs module needs the ds
and pcmcia_core modules loaded first. What it doesn't say is that,
in order to avoid recalculating the module dependencies, you may
also need to have the serial, lp, psaux, and other standard
modules available to prevent errors when insmod'ing the pcmcia
routines at boot time. A glance at the /etc/modules file will tell
you what modules the system currently loads, and in what order.
Save a copy of this file for future reference, until you have
successfully installed the new kernel's modules. Also save lsmod's
output to a file, for example, with the command:
lsmod >lsmod.old-kernel.output
* Build the new kernel, and install the boot image, either zImage or
bzImage, to a floppy diskette. To do this, change to the
arch/i386/boot directory (substitute the correct architecture
directory if you don't have an Intel machine), and, with a floppy
in the diskette drive, execute the command:
dd if=bzImage of=/dev/fd0 bs=512
If you built the kernel with the "make bzImage" command, and if
your floppy drive is /dev/fd0. This results in a bootable kernel
image being written to the floppy, and allows you to try out the
new kernel without replacing the existing one that LILO boots on
the hard drive.
* Boot the new kernel from the floppy to make sure that it works.
* With the system running the new kernel, compile and install a
current version of the PCMCIA Card Services package, available
from metalab.unc.edu as well as other Linux archives. Before
installing the Card Services utilities, change the names of
/sbin/cardmgr and /sbin/cardctl to /sbin/cardmgr.old and
/sbin/cardctl.old. The old versions of these utilities are not
compatible with the replacement utilities that Card Services
installs. In case something goes awry with the installation, the
old utilities won't be overwritten, and you can revert to the
older versions if necessary. When configuring Card Services with
the "make config" command, make sure that the build scripts know
where to locate the kernel configuration, either by using
information from the running kernel, or telling the build process
where the source tree of the new kernel is. The "make config" step
should complete without errors. Installing the modules from the
Card Services package places them in the directory
/lib/modules/<version>/pcmcia, where <version> is the version
number of the new kernel.
* Reboot the system, and note which, if any, of the PCMCIA devices
work. Also make sure that the non-PCMCIA hardware devices are
working. It's likely that some or all of them won't work. Use
lsmod to determine which modules the kernel loaded at boot time,
and compare it with the module listing that the old kernel loaded,
which you saved from the first step of the procedure. (If you
didn't save a listing of lsmod's output, go back and reboot the
old kernel, and make the listing now.)
* When all modules are properly loaded, you can replace the old
kernel image on the hard drive. This will most likely be the file
pointed to by the /vmlinuz symlink. Remember to update the boot
sector by running the lilo command after installing the new kernel
image on the hard drive.
Also look at the questions, How do I upgrade/recompile my kernel? and
Modprobe can't locate module, "XXX," and similar messages.
======================================================================
7. How do I do this or find out that ... ?
----------------------------------------------------------------------
7.1 How can I get scrollback in text mode?
With the default US keymap, you can use Shift with the PageUp and
PageDown keys. (The gray ones, not the ones on the numeric keypad.)
With other keymaps, look in /usr/lib/keytables. You can remap the
ScrollUp and ScrollDown keys to be whatever you like. For example, to
remap them to the keys on an 84-key, AT keyboard.
The "screen" program,
http://vector.co.jp/vpack/browse/person/an010455.html provides a
searchable scrollback buffer and the ability to take "snapshots" of
text-mode screens.
You can't increase the amount of scrollback, because it is implemented
using the video memory to store the scrollback text. You may be able
to get more scrollback in each virtual console by reducing the total
number of VC's. See linux/tty.h.
----------------------------------------------------------------------
7.2 How do I switch virtual consoles? How do I enable them?
In text mode, press Left Alt-F1 to Alt-F12 to select the consoles tty1
to tty12; Right Alt-F1 gives tty13 and so on. To switch out of X you
must press Ctrl-Alt-F1, etc; Alt-F5 or whatever will switch back.
If you want to use a VC for ordinary login, it must be listed in
/etc/inittab, which controls which terminals and virtual consoles have
login prompts. The X Window System needs at least one free VC in order
to start.
----------------------------------------------------------------------
7.3 How do I set the time zone?
Change directory to /usr/lib/zoneinfo/. Get the time zone package if
you don't have this directory. The source is available in
metalab.unc.edu/pub/Linux/system/admin/time/.
Then make a symbolic link named localtime pointing to one of the files
in this directory (or a subdirectory), and one called posixrules
pointing to localtime. For example:
ln -sf US/Mountain localtime
ln -sf localtime posixrules
This change will take effect immediately--try date(1).
The manual page for tzset describes setting the time zone. Some
programs recognize the TZ environment variable, but this is not
POSIX-correct.
You should also make sure that your Linux kernel clock is set to the
correct GMT time--type date -u and check that the correct UTC time is
displayed. ("My clock is very wrong.")
----------------------------------------------------------------------
7.4 What version of Linux and what machine name am I using?
Type:
uname -a
----------------------------------------------------------------------
7.5 How can I enable or disable core dumps?
By using the ulimit command in bash, the limit command in tcsh, or the
rlimit command in ksh. See the appropriate manual page for details.
This setting affects all programs run from the shell (directly or
indirectly), not the whole system.
If you wish to enable or disable core dumping for all processes by
default, you can change the default setting in linux/sched.h--see the
definition of INIT_TASK, and look also in linux/resource.h.
----------------------------------------------------------------------
7.6 How do I upgrade/recompile my kernel?
See the Kernel HOWTO or the README files which come with the kernel
release on ftp.cs.helsinki.fi, in /pub/Software/Linux/Kernel/ and
mirrors. (See "Where can I get Linux material by FTP?") You may
already have a version of the kernel source code installed on your
system, but if it is part of a standard distribution it is likely to
be somewhat out of date (this is not a problem if you only want a
custom configured kernel, but it probably is if you need to upgrade.)
With newer kernels you can (and should) make all of the following
targets. Don't forget that you can specify multiple targets with one
command.
make clean dep install modules modules_install
Also remember to update the module dependencies.
depmod -a
This command can be run automatically at boot time. On Debian/GNU
Linux systems, the command is part of the /etc/init.d/modutils script,
and can be linked appropriately in the /etc/rc(x).d/ directories. For
more information on depmod, see the manual page.
Make sure you are using the most recent version of the modutils
utilities, as well as all other supporting packages. Refer to the file
Documentation/Changes in the kernel source tree for specifics, and be
sure to consult the README file in the modutils package.
Remember that to make the new kernel boot you must run the "lilo"
command after copying the kernel into your root partition--the
Makefile in some kernels have a special zlilo target for this; try:
make zlilo
On current systems, however, you can simply copy the zImage or bzImage
file (in arch/i386/boot/ to the /boot/ directory on the root file
system, or to a floppy using the dd command. Refer also to the
question, How do I get LILO to boot the kernel image?
Kernel version numbers with an odd minor version (ie, 1.1.x, 1.3.x)
are the testing releases; stable production kernels have even minor
versions (1.0.x, 1.2.x). If you want to try the testing kernels you
should probably subscribe to the linux-kernel mailing list. (See
"What mailing lists are there?")
Also refer to the questions, I upgraded the kernel and now my PCMCIA
card doesn't work. , and How do I get LILO to boot the kernel image?
----------------------------------------------------------------------
7.7 Can I have more than 3 serial ports by sharing interrupts?
Yes, but you won't be able to use simultaneously two ordinary ports
which share an interrupt (without some trickery). This is a limitation
of the ISA Bus architecture.
See the Serial HOWTO for information about possible solutions and
workarounds for this problem.
----------------------------------------------------------------------
http://peloncho.fis.ucm.es/~inaky/uusbd-www/.
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7.9 How do I make a bootable floppy?
Make a file system on it with bin, etc, lib and dev
directories--everything you need. Install a kernel on it and arrange
to have LILO boot it from the floppy (see the LILO documentation, in
lilo.u.*.ps).
If you build the kernel (or tell LILO to tell the kernel) to have a
RAM disk the same size as the floppy the RAM disk will be loaded at
boot time and mounted as root in place of the floppy.
See the Bootdisk HOWTO.
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7.10 How do I remap my keyboard to UK, French, etc.?
For recent kernels, get /pub/Linux/system/Keyboards/kbd-0.90.tar.gz
from metalab.unc.edu. Make sure you get the appropriate version; you
have to use the right keyboard mapping package for your kernel
version.
For older kernels you have to edit the top-level kernel Makefile, in
/usr/src/linux.
You may find more helpful information in The Linux Keyboard and
Console HOWTO, by Andries Brouwer, at
metalab.unc.edu/pub/Linux/docs/HOWTO.
----------------------------------------------------------------------
7.11 How do I get NUM LOCK to default to on?
Use the setleds program, for example (in /etc/rc.local or one of the
/etc/rc.d/* files):
for t in 1 2 3 4 5 6 7 8
do
setleds +num < /dev/tty$t > /dev/null
done
Setleds is part of the kbd package ("How do I remap my keyboard to
UK, French, etc.?").
Alternatively, patch your kernel. You need to arrange for KBD_DEFLEDS
to be defined to (1 << VC_NUMLOCK) when compiling
drivers/char/keyboard.c.
----------------------------------------------------------------------
7.12 How do I set (or reset) my initial terminal colors?
The following shell script should work for VGA consoles:
for n in 1 2 4 5 6 7 8; do
setterm -fore yellow -bold on -back blue -store > /dev/tty$n
done
Substitute your favorite colors, and use /dev/ttyS$n for serial
terminals.
To make sure they are reset when people log out (if they've been
changed):
Replace the references to "getty" (or "mingetty" or "uugetty" or
whatever) in /etc/inittab with references to "/sbin/mygetty."
#!/bin/sh
setterm -fore yellow -bold on -back blue -store > $1
exec /sbin/mingetty $@
[Jim Dennis]
----------------------------------------------------------------------
7.13 How can I have more than 128Mb of swap?
Use several swap partitions or swap files--Linux supports up to 16
swap areas, each of up to 128Mb.
Very old kernels only supported swap partition sizes up to 16Mb.
Linux on machines with 8KB paging, like Alpha and Sparc64, support a
swap partition up to 512MB. The 128MB limitation comes from
PAGE_SIZE*BITSPERBYTE on machines with 4KB paging, but is 512KB on
machines with 8KB paging. The limit is due to the use of a single page
allocation map.
The file mm/swapfile.c has all of the gory details.
[Peter Moulder, Gordon Weast]
======================================================================
8. Miscellaneous information and questions answered.
----------------------------------------------------------------------
8.1 How do I program XYZ under Linux?
Read the manuals, or a good book on Unix. Manual pages (type "man
man") are usually a good source of reference information on exactly
how to use a particular command or function.
There is also a lot of GNU Info documentation, which is often more
useful as a tutorial. Run Emacs and type C-h i, or type info info if
you don't have or don't like Emacs. Note that the Emacs libc node may
not exactly describe the latest Linux libc, or GNU glibc2. But the GNU
project and LDP are always looking for volunteers to upgrade their
library documentation.
Anyway, between the existing Texinfo documentation, and the manual
pages in sections 2 and 3, should provide enough information to get
started.
As with all free software, the best tutorial is the source code
itself.
The latest release of the Linux manual pages, a collection of useful
GNU Info documentation, and various other information related to
programming Linux, can be found on metalab.unc.edu in
/pub/Linux/docs/man-pages.
----------------------------------------------------------------------
8.2 What's all this about ELF?
See the ELF HOWTO by Daniel Barlow--note, this is not the file
move-to-elf, which is a blow-by-blow account of how to upgrade to ELF
manually.
Linux has two different formats for executables, object files, and
object code libraries, known as, "ELF." (The old format is called
`a.out'.) They have advantages, including better support for shared
libraries and dynamic linking.
Both a.out and ELF binaries can coexist on a system. However, they use
different shared C libraries, both of which have to be installed.
If you want to find out whether your system can run ELF binaries, look
in /lib for a file named, "libc.so.5." If it's there, you probably
have ELF libraries. If you want to know whether your installation
actually is ELF you can pick a representative program, like ls, and
run file on it:
-chiark:~> file /bin/ls
/bin/ls: Linux/i386 impure executable (OMAGIC) - stripped
valour:~> file /bin/ls
------------------------------
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End of Linux-Misc Digest
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