6. Solutions to Common Miscellaneous Problems
6.1. FTP Transfers Seem to Hang.
FTP transfers that die suddenly are due, apparently, to some form of
overrunning buffer. It occurs both with Linux and Microsoft servers.
On Linux systems, The problem seems to occur most commonly with the
distribution's server software.
If you receive ftp: connection refused errors, then the problem is
likely due to a lack of authentication. Refer to "FTP or Telnet Server
Won't Allow Logins.."
One remedy is to be replacing the distribution FTP server with the
Linux port of the OpenBSD FTP server. The home page is:
http://www.eleves.ens.fr:8080/home/madore/programs/
To install the BSD server, follow the installation instructions, and
refer to the manual pages for inetd and inetd.conf. (If you have the
newer xinetd, see below.) Be sure to tell inetd to run the BSD daemon
alone, not as a subprocess of, for example, tcpd. Comment out the line
that begins "ftp" in the /etc/inetd.conf file and replace it with a
line similar to (if you install the new ftpd in /usr/local/sbin/):
# Original entry, commented out.
#ftp stream tcp nowait root /usr/sbin/tcpd /usr/sbin/in.ftpd
# Replacement entry:
ftp stream tcp nowait root /usr/local/sbin/ftpd -l
The replacement daemon will become effective after rebooting or
sending (as root) a SIGHUP to inetd, e.g.:
# kill -HUP inetd
To configure xinetd, create an entry in /etc/xinetd.d per the
instructions in the xinetd.conf manual page. Make sure, again, that
the command-line arguments for ftpd are correct, and that you have
installed the /etc/ftpusers and /etc/pam.d/ftp files. Then restart
xinetd with the command: /etc/rc.d/init.d/xinetd restart. The command
should report "OK," and the restart will be noted in the system
message log.
6.2. 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.
6.3. Netscape Crashes Frequently.
Netscape shouldn't crash, if it and the network are properly
configured. Some things to check:
* Make sure that the MOZILLA_HOME environment variable is correctly
set. If you installed Netscape under /usr/local/netscape/, for
example, that should be the value of MOZILLA_HOME. Set it from the
command line (e.g, "export MOZILLA_HOME="/usr/local/netscape""
under bash or add it to one your personal or system initialization
files. Refer to the manual page for your shell for details.
* If you have a brand-new version of Netscape, try a previous
version, in case the run-time libraries are slightly incompatible.
For example, if Netscape version 4.75 is installed (type "netscape
--version" at the shell prompt), try installing version 4.7. All
versions are archived at ftp://ftp.netscape.com/.
* Netscape uses its own Motif and Java Runtime Environment
libraries. If a separate version of either is installed on your
system, ensure that they aren't interfering with Netscape's
libraries; e.g., by un-installing them.
* Make sure that Netscape can connect to its default name servers.
The program will appear to freeze and time out after several
minutes if it can't. This indicates a problem with the system's
Internet connection; likely, the system can't connect to other
sites, either.
6.4. FTP or Telnet Server Won't Allow Logins.
This applies to server daemons that respond to clients, but don't
allow logins. On new systems that have Pluggable Authentication
Modules installed, look for a file named, "ftp," or "telnet," in the
directory /etc/pam/ or /etc/pam.d/. If the corresponding
authentication file doesn't exist, the instructions for configuring
FTP and Telnet authentication and other PAM configuration, should be
in /usr/doc/pam-<version>. Refer also to the answer for "FTP server
says: "421 service not available, remote server has closed
connection."."
If it's an FTP server on an older system, make sure that the account
exists in /etc/passwd, especially "anonymous."
This type of problem may also be caused a failure to resolve the host
addresses properly, especially if using Reverse Address Resolution
Protocol (RARP). The simple answer to this is to list all relevant
host names and IP addresses in the /etc/hosts files on each machine. (
Refer to the example /etc/hosts and /etc/resolv.conf files in:
"Sendmail Pauses for Up to a Minute at Each Command..") If the network
has an internal DNS, make sure that each host can resolve network
addresses using it.
If the host machine doesn't respond to FTP or Telnet clients at all,
then the server daemon is not installed correctly, or at all. Refer to
the manual pages: inetd and inetd.conf on older systems, or xinetd and
xinetd.conf, as well as ftpd, and telnetd.
6.5. How To Keep Track of Bookmarks in Netscape?
This probably applies to most other browsers, too. In the
Preferences/Navigator menu, set your home page to Netscape's
bookmarks.html file, which is located in the .netscape (with a leading
period) subdirectory. For example, if your login name is "smith," set
the home page to:
file://home/smith/.netscape/bookmarks.html
Setting up your personal home page like this will present you with a
nicely formatted (albeit possibly long) page of bookmarks when
Netscape starts. And the file is automatically updated whenever you
add, delete, or visit a bookmarked site.
6.6. The Computer Has the Wrong Time.
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 (now called hwclock in many distributions).
Refer to: man 8 clock or man 8 hwclock.
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), Network Time Protocol clients like netdate,
getdate, and xntp, or NTP client-server suite like chrony. Refer to:
"How to Find a Particular Application.."
6.7. 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 than 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.
6.8. 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.
6.9. When Adding 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.
6.10. Some Programs (E.g. xdm) Won't Allow Logins.
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
ftp://tsx-11.mit.edu:/pub/linux/sources/usr.bin/shadow/. This is the
source code. The binaries are probably in linux/binaries/usr.bin/.
6.11. Some Programs Allow Logins with No Password.
You probably have the same problem as in ("Some Programs (E.g. xdm)
Won't Allow Logins."), 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.
6.12. The Machine Runs Very Slowly with 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 To Upgrade/Recompile a 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").
6.13. System Only Allows Root Logins.
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.
6.14. The 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,
then 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.
Another possible command is an alias, "sane," that can work with
generic terminals:
$ alias sane='echo -e "\\033c";tput is2; \
> stty sane line 1 rows $LINES columns $COLUMNS'
The alias is enclosed with open quotes (backticks), not single quotes.
The line break is included here for clarity, and is not required.
Make sure that $LINES and $COLUMNS are defined in the environment with
a command similar to this in ~/.cshrc or ~/.bashrc,
$ LINES=25; export $LINES; $COLUMNS=80; export $COLUMNS
using the correct numbers of $LINES and $COLUMNS for the terminal.
Finally, the output of "stty -g" can be used to create a shell script
that will reset the terminal:
1. Save the output of "stty -g" to a file. In this example, the file
is named "termset.":
$ stty -g >termset
The output of "stty -g" (the contents of "termset") will look
something like:
500:5:bd:8a3b:3:1c:7f:15:4:0:1:0:11:13:1a:0:12:f:17:16:0:0:73
2. Edit "termset" to become a shell script; adding an interpreter and
"stty" command:
#!/bin/bash
stty 500:5:bd:8a3b:3:1c:7f:15:4:0:1:0:11:13:1a:0:12:f:17:16:0:0:73
3. Add executable permissions to "termset" and use as a shell script:
$ chmod +x termset
$ ./termset
[Floyd L. Davidson, Bernhard Gabler]
6.15. I Screwed Up the 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
ftp://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).
6.16. I Forgot the root Password.
Note: Incorrectly editing any of the files in the /etc/ directory can
severely screw up a system. Please keep a spare copy of any files in
case you make a mistake.
If your Linux distribution permits, try booting into single-user mode
by typing "single" at the BOOT lilo: prompt. With more recent
distributions, you can boot into single-user mode when prompted by
typing "linux 1," "linux single," or "init=/bin/bash."
If the above doesn't work for you, boot from the installation or
rescue floppy, and switch to another virtual console with Alt-F1 --
Alt-F8, and then mount the root file system on /mnt. Then proceed with
the steps below to determine if your system has standard or shadow
passwords, and how to remove the password.
Using your favorite text editor, edit the root entry of the
/etc/passwd file to remove the password, which is located between the
first and second colons. Do this only if the password field does not
contain an "x," in which case see below.
root:Yhgew13xs:0:0: ...
Change that to:
root::0:0: ...
If the password field contains an "x," then you must remove the
password from the /etc/shadow file, which is in a similar format.
Refer to the manual pages: "man passwd," and "man 5 shadow."
[Paul Colquhuon, Robert Kiesling, Tom Plunket]
6.17. There's a Huge Security Hole in rm!
No there isn'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.
6.18. lpr and/or lpd 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? ".
6.19. 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.
6.20. How To 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.
6.21. How To Make Sure the System Boots after Re-Installing the Operating
System.
This should work whether you're re-installing Linux or some other,
commercial, operating system:
* Insert a blank, formatted floppy in drive A:
* Save a copy of the boot hard drive's Master Boot Record to the
floppy, by executing the command:
#dd if=/dev/hda of=/dev/fd0 count=1
dd is a standard program on Linux systems. A MS-Windows compatible
version is available from ftp://ftp.gnu.org/, as well as many MS
software archives.
* Test that the floppy boots the system by rebooting with the floppy
in the A: drive.
* Then you should be able to install the other operating system (on
a different hard drive and/or partition, if you don't want to
uninstall Linux).
* After installation, boot Linux again from the floppy, and
re-install the MBR with the command: /sbin/lilo.
[Jacques Guy]
6.22. The PCMCIA Card Doesn't Work after Upgrading the Kernel.
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 that 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 kludgey, 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 insmod, 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 the lsmod output might look like this:
Module Pages Used by
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
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 installing the pcmcia
routines at boot time with insmod. 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 the lsmod 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 the lsmod 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.
6.23. How To Remove (or Change) the Colors in the ls Display.
The shell command, "unalias ls," should completely unset the
configuration that some distributions provide as standard. To change
the colors, refer to the ls man page ("man ls").
6.24. Why Won't a Program Work in the Current Directory?
Because the current directory (i.e., ".") is not in the search path,
for security reasons, as well as to insure that the correct program
versions are used. If an intruder is able to write a file to a
world-writable directory, like /tmp, presumably he or she would be
able to execute it if the directory were in the search path. The
solution to this is to include the directory in the command; e.g.,
"./myprog," instead of "myprog." Or add the current directory to your
PATH environment variable; e.g., "export PATH=".:"$PATH" using bash,
although this is discouraged for the reasons mentioned above.
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