Backup-With-MSDOS mini-HOWTO
Christopher Neufeld, neufeld@physics.utoronto.ca
v, 5 August 1997
This HOWTO describes how to use a Linux-compatible tape drive
installed on an MS-DOS machine to back up the filesystem of a Linux
machine.
______________________________________________________________________
Table of Contents
1. Preface/Introduction
2. The technique
3. Notes
4. Copyright
______________________________________________________________________
1. Preface/Introduction
Earlier I posed the question on the Net, how does one back up a Linux
machine to a Colorado Jumbo 250 tape drive on an MS-DOS machine. From
the email I received, it seems that this is a frequently pondered
problem. Now that I've figured it out, I'm posting the method. If
anybody wants to massage this into a HOWTO document, let me know. I
should thank Jim Nance (jlnance@isscad.com) for pointing out that an
MS-DOS machine need not always be an MS-DOS machine. This technique
should also work for any other tape drive supported by the ftape
module, and for SCSI tape drives with suitable obvious changes (i.e.
substituting /dev/st0 for /dev/ftape).
The criteria I set were that the resulting setup should be as secure
as possible and should be fairly simple, and take up little or no
space on the MS-DOS machine's hard drive. It should also be capable of
recovering from the worst system corruptions, up to and including the
theft of the hard disk, requiring a restore to a bare Linux file
system. The technique described here uses no hard drive space on the
MS-DOS machine, though it requires that that machine be assigned an
IP#. You will need three formatted, blank 1.44MB diskettes.
2. The technique
Throughout this description I will refer to two machines as ``msdos''
and ``linux''. ``msdos'' is the name of the machine which has the tape
drive and is usually running MS-DOS. ``linux'' is the Linux machine
whose disk you are trying to back up or restore to the tape drive.
For simplicity I will refer to the first machine as ``msdos'' even
when it is booted into and running Linux. Further, all path names in
this document should be considered to be relative to the Linux machine
with the Search-And-Rescue (SAR) disks mounted somewhere on the
system. That means that the file /etc/passwd is the password file for
your Linux machine's hard drive, while, for instance,
/tape144/etc/passwd is the corresponding file on the floppy disk.
I am using Karel Kubat's backup scripts, version 1.03, available at
<ftp://sunsite.enc.edu:/pub/Linux/system/Backup/backup-1.03.tar.gz>
Throughout this document I will refer to these simply as ``the backup
scripts''. You do not have to use these scripts for your own backups
to tape. I like these scripts as they use afio to form an uncom
pressed archive of compressed files, rather than a compressed archive
of uncompressed files. The former is much safer if there is a media
read error during the restore. I understand that Karel is no longer
supporting backup, and now has produced 'tob', or tape oriented
backup. While I haven't tried the new package myself, it cannot make
a significant difference to the procedure outlined here.
First of all, obtain the ftape module. It is a part of all modern
kernels, but if you are using an older kernel you can find the module
at:
<ftp://sunsite.unc.edu/pub/Linux/kernel/tapes/ftape-2.05.tar.gz>
Next, get a Slackware boot disk (I got the net disk, but it doesn't
make much difference) and the tape144 root disk, and put the images
onto 3"1/2 floppies.
The ftape module will only work if it is installed in the kernel which
was running when you compiled it. I could not get it to work with the
ftape.o module on the tape144 root disk, I think because that module
has been stripped of symbols and won't install. So, you now have to
make a new kernel with network and ftape support, and if you're
running an old kernel, a new ftape.o. Read the directions which ship
with the ftape archive for directions at this stage. Remember that
the kernel you compile must support the Ethernet cards on both the
Linux machine and the MS-DOS machine.
Copy the newly created kernel image over top of the one on the Net
boot disk. Use /bin/cp, do not create a boot disk with the ``dd''
command as you would to create a bootable kernel image. Write protect
the boot disk, and label it: SAR#1.
Now, mount the tape144 root disk. I'll assume that the mount point is
/tape144, to avoid confusion in file names. We need to free some
space on it, so delete the following files:
/tape144/bin/dialog
/tape144/bin/elvis
/tape144/bin/vi
/tape144/boot/ftape.o
Now, create a new file:
/tape144/etc/exports
which contains the following line:
/mnt msdos(ro)
Where ``msdos'' should be replaced with the name or IP# of the MS-DOS
machine which has the tape drive installed.
Next, so that you don't have to rely on a name server, add lines to
the file /tape144/etc/hosts with the names and IP numbers of the Linux
and MS-DOS machines. For instance, mine contains the following two
lines:
128.100.75.114 caliban.physics.utoronto.ca caliban caliban.physics
128.100.75.111 ariel.physics.utoronto.ca ariel ariel.physics
Now, there's some sort of problem with the inetd configuration. We
have to put the full path name of the rsh daemon in it. Change line
19 of /tape144/etc/inetd.conf to read:
shell stream tcp nowait root /usr/etc/tcpd /usr/etc/in.rshd
Add local net routing information to /tape144/etc/rc.d/rc.inet1 to
enable the MS-DOS machine to use the network. The format of this
depends on your network configuration, you can just copy the
appropriate format out of your Linux /etc/rc.d/rc.inet1. For my
network, the lines that have to be added are:
/etc/ifconfig eth0 128.100.75.111 broadcast 128.100.75.0 netmask 255.255.255.0
/etc/route add -net 128.100.75.0 netmask 255.255.255.0
The IP# in the ifconfig entry is that of the MS-DOS machine.
Now, copy this file into /tape144/etc/rc.d/rc.inet1-l, and change the
IP# in the new file to reflect that of the Linux machine rather than
the MS-DOS machine.
Next, clip out lines 3 to 11 of /tape144/etc/rc.local. That's an if
statement which executes the rc.inet* files. We don't want this to
happen during the bootup.
Create a new file: /tape144/root/.rhosts containing the line:
linux root
where, again, ``linux'' is replaced with the full machine name
(including domain) or the IP# of the Linux machine.
Fill in the password field in /tape144/etc/passwd for the root login
to keep people from logging onto the MS-DOS machine while you're doing
the backup. You can do this by copying the corresponding field from
your Linux machine's /etc/passwd file.
Copy /usr/bin/rsh into /tape144/usr/bin.
Copy the following files from /usr/etc into /tape144/usr/etc:
in.rshd
rpc.mountd
rpc.nfsd
rpc.portmap
services
tcpd
Create a new script, /tape144/bin/tapesetup, which consists of the
following: (change ``linux'' to reflect your Linux machine name).
#! /bin/sh
/bin/sh /etc/rc.d/rc.inet1
/bin/sh /etc/rc.d/rc.inet2
/bin/mount linux:/nfs /mnt
/bin/insmod /mnt/ftape.o
Note that newer kernels will not require the insmod line.
Next, create another new script, /tape144/bin/msdosset, as follows:
(change ``linux'' to reflect your Linux machine name).
#! /bin/sh
/bin/sh /etc/rc.d/rc.inet1
/bin/sh /etc/rc.d/rc.inet2
mount linux:/mnt /mnt
/bin/insmod /mnt/ftape.o
As above, newer kernels will not require the insmod line.
Create a readable file, /tape144/root/notes, which contains this
helpful information for use in full recovery:
For a full recovery to a trashed hard disk,
boot the Linux machine with the SAR disks #1 and #2
then type the following:
/bin/sh /etc/rc.d/rc.inet1-l
/bin/sh /etc/rc.d/rc.inet2
/usr/etc/rpc.portmap
/usr/etc/rpc.mountd
/usr/etc/rpc.nfsd
Next, insert SAR disk #3 and type:
mount /dev/fd0 /mnt
Create a new mount point, with:
mkdir /mnt2
and mount your Linux hard disk partition on this point.
You may have to reformat the partition first, if so,
follow the directions in the Linux Installation HOWTO.
The SAR disks contain all the files necessary to do the reformat.
Finally, use disks SAR#1 and SAR#2 to boot up
the MS-DOS machine and run the /bin/msdosset script on that machine.
It will take about a minute to run that script because it is getting
an NSF file from a floppy drive, so be patient.
Now, recover the tape to /mnt2 on the Linux machine.
If you are using the backup scripts you will need to copy 'afio' into
the /tape144/local/bin subdirectory. It is not necessary to have the
rest of the backup script files on the recovery disks, an archive can
be recovered using only 'afio' and 'gzip'.
I was unable to use the backup scripts as they come shipped. The tape
archive appears to build cleanly, but it is unrecoverable. I found
that removing the block size and conversion statements fixed it. Here
is the patch to the ``netbackup'' script. Apply this patch to the
Linux machine's hard disk copy of 'netbackup' as well as to the copy
on the SAR disks.
*** netbackup.orig Mon Jan 9 17:22:32 1995
--- netbackup Mon Jan 9 17:23:25 1995
***************
*** 35,41 ****
"'mknod", devname, "p'");
exec ("su -", USERNAME, "-c",
"'rsh ", REMOTE_HOST,
! "\"dd", "of=" REMOTE_DEVICE, "obs=20k", "conv=sync\"",
"<", devname,
"'&"
);
--- 35,41 ----
"'mknod", devname, "p'");
exec ("su -", USERNAME, "-c",
"'rsh ", REMOTE_HOST,
! "\"dd", "of=" REMOTE_DEVICE, "\"",
"<", devname,
"'&"
);
***************
*** 50,56 ****
"'mknod", devname, "p'");
exec ("su", USERNAME, "-c",
"'rsh ", REMOTE_HOST,
! "\"dd", "if=" REMOTE_DEVICE, "ibs=20k", "conv=sync\"",
">", devname,
"'&"
);
--- 50,56 ----
"'mknod", devname, "p'");
exec ("su", USERNAME, "-c",
"'rsh ", REMOTE_HOST,
! "\"dd", "if=" REMOTE_DEVICE, "\"",
">", devname,
"'&"
);
You have now finished your SAR disk #2. Write protect it.
Next, mount a clean, formatted disk (create it with fdformat and
mkfs). Copy the ftape.o file onto it, and label it SAR#3. For some
reason things go badly if you write protect this disk, so leave it
write-enabled.
On the Linux machine, create a new directory for NFS file serving. I
made a directory:
/nfs
Put the ftape.o (unstripped, about 500+ kB) into this subdirectory.
Create an entry in your Linux's exports file /etc/exports:
/nfs msdos(ro)
Note that all files in your NFS directory and it's subdirectories are
not secure. Somebody else could boot the MS-DOS machine into Linux
with his own boot disks and mount this directory, so be certain that
you don't put anything sensitive in your NFS subdirectory.
Restart your NFS daemons, rpc.mountd and rpc.nfsd. They don't seem to
take kindly to a SIGHUP restart, so kill them and reinvoke them. If
you're not activating these daemons in your /etc/rc.d/rc.inet2 you
might want to do so now.
OK, now we're all set to back up and recover. To make a full backup
from the Linux machine, boot the MS-DOS machine with SAR#1. When
prompted for the second disk, load SAR#2. Log in as root, and execute
the script: /bin/tapesetup. Log out of the MS-DOS machine. If you're
using the backup scripts, the netbackup command will now work. You
can also use the ``-f msdos:/dev/ftape'' switch on GNU tar, cpio, or
mt, and make your backup this way. If you have a backup program,
which is only capable of writing to a local file, do the following.
Assume that the backup program is called ``localbackup'' and writes to
the file represented by its command line argument:
mknod /tmp/tapepipe p
rsh msdos dd of=/dev/ftape < /tmp/tapepipe &
localbackup /tmp/tapepipe
when it's done, delete /tmp/tapepipe.
Recovering to a live Linux machine: the netbackup script, tar, cpio,
and so on will all work without special actions on the part of the
operator. If you have a local recovery program which recovers from a
file, do this:
mknod /tmp/tapepipe p
rsh -n msdos dd if=/dev/ftape >> /tmp/tapepipe &
localrecovery /tmp/tapepipe
and delete /tmp/tapepipe when you're done.
Notice that I'm using 'rsh' to the root user on the MS-DOS machine.
This works with a correct .rhosts entry. The configuration on the
'tape144' disk allows rsh to root, but does not allow telnet or rlogin
to root, logins are restricted to the console. This is good for
security.
If you are worried about a root .rhost file, you can create a new user
on SAR#2, ``tapeuser'', with permissions to operate the tape drive but
not the disks (create a new group and put tapeuser in that group, then
chown and chmod the files /dev/rft* and /dev/nrft*). Your backup
program then has to know to rsh to that username rather than to root.
Of course, now there must be an .rhosts file in ~tapeuser on SAR#2.
For my own use, I have chosen this course, rather than a root .rhosts.
Finally, the directions for a complete recovery to a trashed hard
disk. This assumes that the Linux partition is completely
unrecoverable. If necessary, reformat that partition as described in
the Linux Installation HOWTO. Boot the Linux machine from SAR disk #1.
When prompted, insert disk #2. Now, follow the directions in the file
/root/notes (this was /tape144/root/notes when it was mounted on your
Linux machine). Once both machines have been booted up, run the
recovery routine you need. If you are running the backup scripts you
can do it as follows:
1. change directory to the mount point of the hard disk partition
which you will be recovering.
2. if any mounted volumes are on the backup, and you want to recover
them, create the mount points within the hard disk partition and
mount the volumes.
3. Enter the command:
rsh -n msdos dd if=/dev/ftape | afio -i -v -Z -c 1024 -
or
rsh -n -l tapeuser msdos dd if=/dev/ftape | afio -i -v -Z -c 1024 -
or
mknod /tmp/backpipe p
rsh -n msdos dd if=/dev/ftape >> /tmp/backpipe &
afio -i -v -Z -c 1024 /tmp/backpipe
This reads the tape on the remote machine, writing the result to std
out, where afio picks it up. The '-i' switch tells it to recover the
files relative to the current working directory (which is the root of
the hard disk partition). '-v' is verbose, listing the files as they
are recovered. '-Z' tells afio that this is an archive of individu
ally compressed files. '-c 1024' tells it to use a 5 MB streaming
buffer to avoid a lot of tape rewinding.
3. Notes
The commands listed in the /tape144/root/notes file could be run from
a script. When I tried, I got rpc setup errors. I suspect it was
just that the commands were run too quickly, and the portmapper hadn't
properly installed itself. I found that typing the sequence in
manually worked fine, so I've recommended that.
I think this setup is secure. Note that somebody can still get access
to all your files if they go to the tape drive and pull the tape out
before you get there, then then read the tape themselves. People with
very sensitive data might consider encrypting the stream from the
archiver. Archive to standard output and pipe the output to the
encrypter, and redirect the output of the encrypter to append to the
named pipe /tmp/tapepipe as described above. Note that errors in the
recovery process will result in all files after that point being
unrecoverable, as the entire archive is now a single DES-encrypted
stream. It is possible to use options on afio to send each file in
the archive first through gzip, then into an encryption program like
des, but note that this compressing first does provide a fair amount
of known plaintext for determined code breakers to work with, so a
better approach might be to skip the gzip step and simply encrypt it
with des, at the expense of significantly more tape area. Needless to
say, DES encrypted files don't compress.
The rc.inet1 directions I've included will allow only communication
with the local network, not the rest of the world through a gateway.
During a full recovery to a blank hard disk the SAR disk #3 provides
ftape.o to the MS-DOS machine through NFS. This is because some old
versions of the ftape module can't control some tape drives when there
is a disk mounted in the floppy drive. With newer kernels, the entire
NFS stuff can be omitted.
This is very important. ***TEST*** the SAR recovery procedure. I
did, but don't leave anything to chance. Make sure that you can
recover at least one file from your tape to the Linux machine using
only the SAR disks (i.e. without mounting the hard disk). If you
can't reboot the Linux machine without inconveniencing a lot of users,
change the setup information on the SAR disks to assign the ``linux''
identity to another MS-DOS machine and then boot the two MS-DOS
machines into Linux to make sure everything works. Then, change the
``linux'' identity back again so that you have usable SAR disks.
4. Copyright
Copyright Jan 10, 1995 by Christopher Neufeld
(neufeld@physics.utoronto.ca)
Modified Feb 6, 1996.
Modified Aug 5, 1997.
