7. Solutions to Common Miscellaneous Problems
7.1. 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.:
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.
7.2. 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.
7.3. 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.
7.4. 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.
7.5. 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.
7.6. 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..''
7.7. 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.
7.8. 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.
7.9. 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.
7.12. 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:
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'').
7.13. 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.
7.14. 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.
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
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:
-
Save the output of ``stty -g'' to a file. In this example, the file is named ``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 |
-
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 |
-
Add executable permissions to ``termset'' and use as a shell script:
$ chmod +x termset
$ ./termset |
[Floyd L. Davidson, Bernhard Gabler]
7.15. 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).
7.16. 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.
Change that to:
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]
7.17. 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.
7.18. 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:
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? ".
7.20. 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.
7.21. 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]
7.22. 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.
7.24. 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.
