2. Runtime Configuration Options

This section of the document by Hans, <lermen@fgan.de>. Last updated on Feb 12, 1999.

Most of DOSEMU configuration is done during runtime and per default it expects the system wide configuration file /etc/dosemu.conf optionally followed by the users  /.dosemurc and additional configurations statements on the commandline (-I option). The builtin configuration file of a DEXE file is passed using the -I technique, hence the rules of -I apply.

In fact /etc/dosemu.conf and  /.dosemurc (which have identical syntax) are included by the systemwide configuration script /var/lib/dosemu/global.conf, but as a normal user you won't ever think on editing this, only dosemu.conf and your personal  /.dosemurc. The syntax of global.conf is described in detail in README-tech.txt, so this is skipped here. However, the option -I string too uses the same syntax as global.conf, hence, if you are doing some special stuff (after you got familar with DOSEMU) you may need to have a look there.

In DOSEMU prior to 0.97.5 the private configuration file was called  /.dosrc (not to be confused with the new  /.dosemurc). This will work as expected formerly, but is subject to be nolonger supported in the near future. This (old)  /.dosrc is processed after global.conf and follows (same as -I) the syntax of global.conf (see README-tech.txt).

The first file expected (and interpreted) before any other configuration (such as global.conf, dosemu.conf and  /.dosemurc) is /etc/dosemu.users. Within /etc/dosemu.users the general permissions are set:

• which users are allowed to use DOSEMU.
• which users are allowed to use DOSEMU suid root.
• which users are allowed to have a private lib dir.
• what kind of access class the user belongs to.
• what special configuration stuff the users needs

and further more:

• wether the lib dir (/var/lib/dosemu) resides elsewhere.
• setting the loglevel.

Except for lines starting with `xxx=' (explanation below), each line in dosemu.user corresponds to exactly one valid user count, the special user `all' means any user not mentioned earlier. Format:

  [ <login> | all ] [ confvar [ confvar [ ... ] ] ]

The below example is from etc/dosemu.users.secure, which you may copy to /etc/dosemu.users.

  root c_all     # root is allowed to do all weird things
  nobody nosuidroot guest # variable 'guest' is checked in global.conf
  guest nosuidroot guest  # login guest treated as `nobody'
  myfriend c_all unrestricted private_setup
  myboss nosuidroot restricted private_setup
  all nosuidroot restricted # all other users have normal user restrictions

The use of `nosuidroot' will force a suid root dosemu binary to exit, the user may however use a non-suid root copy of the binary. For more information on this look at README-tech, chapter 11.1 `Privileges and Running as User'

Giving the keyword `private_setup' to a user means he/she can have a private DOSEMU lib under $HOME/.dosemu/lib. If this directory is existing, DOSEMU will expect all normally under /var/lib/dosemu within that directory, including `global.conf'. As this would be a security risc, it only will be allowed, if the used DOSEMU binary is non-suid-root. If you realy trust a user you may additionally give the keyword `unrestricted', which will allow this user to execute a suid-root binary even on a private lib directory (though, be aware).

In addition, dosemu.users can be used to define some global settings, which must be known before any other file is accessed, such as:

  default_lib_dir= /opt/dosemu  # replaces /var/lib/dosemu
  log_level= 2                  # highest log level

With `default_lib_dir=' you may move /var/lib/dosemu elsewere, this mostly is interesting for distributors, who want it elswere but won't patch the DOSEMU source just for this purpose. But note, the dosemu supplied scripts and helpers may need some adaption too in order to fit your new directory.

The `log_level=' can be 0 (never log) or 1 (log only errors) or 2 (log all) and controls the ammount written to the systems log facility (notice). This keyword replaces the former /etc/dosemu.loglevel file, which now is obsolete.

Nevertheless, for a first try of DOSEMU you may prefer etc/dosemu.users.easy, which just contains

  root c_all
  all c_all

After /etc/dosemu.users /etc/dosemu.conf (via global.conf) is interpreted, and only during global.conf parsing access to all configuration options is allowed. Your personal  /.dosemurc is included directly after dosemu.conf, but has less access rights (in fact the lowest level), all variables you define within  /.dosemurc transparently are prefixed with `dosemu_' such that the normal namespace cannot be polluted (and a hacker cannot overwrite security relevant enviroment variables). Within global.conf only those  /.dosemurc created variables, that are needed are taken over and may overwrite those defined in /etc/dosemu.conf.

The dosemu.conf (global.conf) may check for the configuration variables, that are set in /etc/dosemu.users and optionaly include further configuration files. But once /etc/dosemu.conf (global.conf) has finished interpretation, the access to secure relevant configurations is (class-wise) restricted while the following interpretation of (old) .dosrc and -I statements.

For more details on security setings/issues look at README-tech.txt, for now (using DOSEMU the first time) you should need only the below description of /etc/dosemu.conf ( /.dosemurc)

2.1 Format of /etc/dosemu.conf

All settings in dosemu.conf are just variables, that are interpreted in /var/lib/dosemu/global.conf and have the form of

  $_xxx = (n)
or
  $_zzz = "s"

where `n' ist a numerical or boolean value and `s' is a string. Note that the brackets are important, else the parser won't decide for a number expression. For numers you may have complete expressions ( such as (2*1024) ) and strings may be concatenated such as

  $_zzz = "This is a string containing '", '"', "' (quotes)"

Hence a comma separated list of strings is concatenated.

Controling amount of debug output

DOSEMU will help you finding problems, when you enable its debug messages. These will go into the file, that you defined via the `-o file' or `-O' commandline option (the later prints to stderr). In dosemu.conf you can preset this via

  $_debug = "-a"

Basic emulaton settings

To enable INT08 type timer interrupts set the below on or off

  $_timint = (on)

Wether a numeric processor should be shown to the DOS space

  $_mathco = (on)

Which type of CPU should be emulated (NOTE: this is not the one you are running on, but your setting may not exeed the capabilities of the running CPU). Valid values are: 80[345]86

  $_cpu = (80386)

To let DOSEMU use the Pentium cycle counter (if availabe) to do better timing use the below

  $_rdtsc = (on)   # or off

For the above `rdtsc' feature DOSEMU needs to know the exact CPU clock, it normally calibrates it itself, but is you encounter a wrong mesurement you may overide it such as

  $_cpuspeed = (166.666)  # 0 = let DOSEMU calibrate

NOTE: `$_rdtsc' and `$_cpuspeed' can not be overwritten by  /.dosemurc.

If you have a PCI board you may allow DOSEMU to access the PCI configuration space by defining the below

  $_pci = (on)    # or off

Defining the memory layout, which DOS should see:

  $_xms = (1024)          # in Kbyte
  $_ems = (1024)          # in Kbyte
  $_ems_frame = (0xe000)
  $_dpmi = (off)          # in Kbyte
  $_dosmem = (640)        # in Kbyte, < 640

There are some features in DOSEMU, that may violate system security and which you should not use on machines, which are `net open'. To have atleast a minimum of protection against intruders, use the folling:

  $_secure ="ngd"  # secure for: n (normal users), g (guest), d (dexe)

  $_secure ="d"

  $_secure ="0"

NOTE: `$_secure' can not be overwritten by  /.dosemurc.

For the similar reasons you may `backout' some host, which you don't like to have access to dosemu

  $_odd_hosts = ""    # black list such as
  $_diskless_hosts="" # black list such as "hacker1 newbee gateway1"

NOTE: `$_*_hosts' can not be overwritten by  /.dosemurc.

If you want mixed operation on the filesystem, from which you boot DOSEMU (native and via DOSEMU), it may be necessary to have two separate sets of `config.sys,autoexec.bat,system.ini'. DOSEMU can fake a different file extension, so DOS will get other files when running under DOSEMU.

  $_emusys = ""    # empty or 3 char., config.sys   -> config.XXX
  $_emubat = ""    # empty or 3 char., autoexec.bat -> autoexec.XXX
  $_emuini = ""    # empty or 3 char., system.ini   -> system.XXX

As you would realize at the first glance: DOS will not have the the CPU for its own. But how much it gets from Linux, depends on the setting of `hogthreshold'. The HogThreshold value determines how nice Dosemu will be about giving other Linux processes a chance to run.

  $_hogthreshold = (1)   # 0 == all CPU power to DOSEMU

If you have hardware, that is not supported under Linux but you have a DOS driver for, it may be necessary to enable IRQ passing to DOS.

  $_irqpassing = ""  # list of IRQ number (2-15) to pass to DOS such as

Here you tell DOSEMU what to do when DOS wants let play the speaker:

  $_speaker = ""     # or "native" or "emulated"

And with the below may gain control over real ports on you machine. But: WARNING: GIVING ACCESS TO PORTS IS BOTH A SECURITY CONCERN AND SOME PORTS ARE DANGEROUS TO USE. PLEASE SKIP THIS SECTION, AND DON'T FIDDLE WITH THIS SECTION UNLESS YOU KNOW WHAT YOU'RE DOING.

  $_ports = ""  # list of portnumbers such as "0x1ce 0x1cf 0x238"

NOTE: `$_ports' can not be overwritten by  /.dosemurc.

Code page and character set

To select the character set and code page for use with DOSEMU you have

  $_term_char_set = "XXX"

ibm

the text is taken whithout translation, it is to the user to load a proper DOS font (cp437.f16, cp850.f16 or cp852.f16 on the console).

latin

the text is processed using cp437->iso-8859-1 translation, so the font used must be iso-8859-1 (eg iso01.f16 on console); which is the default for unix in western languages countries.

latin1

like latin, but using cp850->iso-8859-1 translation (the difference between cp437 and cp850 is that cp437 uses some chars for drawing boxes while cp850 uses them for accentuated letters)

latin2

like latin1 but uses cp852->iso-8859-2 translation, so translates the default DOS charset of eastern european countries to the default unix charset for those countries.

Terminals

This section applies whenever you run DOSEMU remotely or in an xterm. Color terminal support is now built into DOSEMU. Skip this section for now to use terminal defaults, until you get DOSEMU to work.

  $_term_color = (on)   # terminal with color support
  $_term_updfreq = (4)  # time between refreshs (units: 20 == 1 second)
  $_escchar = (30)      # 30 == Ctrl-^, special-sequence prefix

`term_updfreq' is a number indicating the frequency of terminal updates of the screen. The smaller the number, the more frequent. A value of 20 gives a frequency of about one per second, which is very slow. `escchar' is a number (ascii code below 32) that specifies the control character used as a prefix character for sending alt, shift, ctrl, and function keycodes. The default value is 30 which is Ctrl-^. So, for example,

  F1 is 'Ctrl-^1', Alt-F7 is 'Ctrl-^s Ctrl-^7'.
  For online help, press 'Ctrl-^h' or 'Ctrl-^?'.

Keyboard settings

When running DOSEMU from console (also remote from console) or X you may need to define a proper keyboard layout. This is done either by choosing one on the internal keytables or by loading an external keytable from /var/lib/dosemu/keymap/* (which you may modify according to your needs). Both sets have identical names (though you may add any new one to /var/lib/dosemu/keymap/*):

  be              finnish         hu-latin2       sg-latin1
  de              finnish-latin1  it              sw
  de-latin1       fr              keyb-no         uk
  dk              fr-latin1       no-latin1       us
  dk-latin1       hr-cp852        po
  dvorak          hr-latin2       sf
  es              hu              sf-latin1
  es-latin1       hu-cwi          sg              jp106
  cz-qwerty       cz-qwertz

You define an internal keytable such as

  $_layout = "name"

  $_layout = "load de-latin1"

Note, however, that you have to set

  
  $_X_keycode = (on)

When being on console you might wish to use raw keyboard, especially together with some games, that don't use the BIOS/DOS to get their keystrokes.

  $_rawkeyboard = (1)

NOTE: `$_rawkeyboard' can not be overwritten by  /.dosemurc.

The `keybint (on)' allows more accurate of keyboard interrupts, It is a bit unstable, but makes keyboard work better when set to "on".

  $_keybint = (on)     # emulate PCish keyboard interrupt

X Support settings

If DOSEMU is running in its own X-window (not xterm), you may need to tailor it to your needs. Here a summary of the settings and a brief description what they mean. A more detailed description of values one can be found at chapter 2.2.14 (X Support settings) of README-tech.txt

$_X_updfreq = (5)       # time between refreshs (units: 20 == 1 second)
$_X_title = "DOS in a BOX" # Title in the top bar of the window
$_X_icon_name = "xdos"  # Text for icon, when minimized
$_X_keycode = (off)     # on == translate keybord via dosemu keytables
$_X_blinkrate = (8)     # blink rate for the cursor
$_X_font = ""           # basename from /usr/X11R6/lib/X11/fonts/misc/*
$_X_mitshm = (on)       # Use shared memory extensions
$_X_sharecmap = (off)   # share the colormap with other applications
$_X_fixed_aspect = (on) # Set fixed aspect for resize the graphics window
$_X_aspect_43 = (on)    # Always use an aspect ratio of 4:3 for graphics
$_X_lin_filt = (off)    # Use linear filtering for >15 bpp interpol.
$_X_bilin_filt = (off)  # Use bi-linear filtering for >15 bpp interpol.
$_X_mode13fact = (2)    # initial factor for video mode 0x13 (320x200)
$_X_winsize = ""        # "x,y" of initial windows size
$_X_gamma = (1.0)       # gamma correction
$_X_vgaemu_memsize = (1024) # size (in Kbytes) of the frame buffer
$_X_lfb = (on)  # use linear frame buffer in VESA modes
$_X_pm_interface = (on) # use protected mode interface for VESA modes
$_X_mgrab_key = ""      # KeySym name to activate mouse grab, empty == off
$_X_vesamode = ""       # "xres,yres ... xres,yres"

Video settings ( console only )

!!WARNING!!: IF YOU ENABLE GRAPHICS ON AN INCOMPATIBLE ADAPTOR, YOU COULD GET A BLANK SCREEN OR MESSY SCREEN EVEN AFTER EXITING DOSEMU. Read doc/README-tech.txt (Recovering the console after a crash).

Start with only text video using the following setup in dosemu.conf

  $_video = "vga"         # one of: plainvga, vga, ega, mda, mga, cga
  $_console = (0)         # use 'console' video
  $_graphics = (0)        # use the cards BIOS to set graphics
  $_videoportaccess = (1) # allow videoportaccess when 'graphics' enabled
  $_vbios_seg = (0xc000)  # set the address of your VBIOS (e.g. 0xe000)
  $_vbios_size = (0x10000)# set the size of your BIOS (e.g. 0x8000)
  $_vmemsize = (1024)     # size of regen buffer
  $_chipset = ""
  $_dualmon = (0)         # if you have one vga _plus_ one hgc (2 monitors)

After you get it `somehow' working and you have one of the DOSEMU supported graphic cards you may switch to graphics mode changing the below

  $_graphics = (1)        # use the cards BIOS to set graphics

If you have a 100% compatible standard VGA card that may work, however, you get better results, if your card has one of the DOSEMU supported video chips and you tell DOSEMU to use it such as

  $_chipset = "s3"        # one of: plainvga, trident, et4000, diamond, s3,

Note, `s3' is only an example, you must set the correct video chip else it most like will crash your screen.

NOTE: `video setting' can not be overwritten by  /.dosemurc.

Disks, boot directories and floppies

The parameter settings via dosemu.conf are tailored to fit the recommended usage of disk and floppy access. There are other methods too, but for these you have to look at README-tech.txt (and you may need to modify global.conf). We strongly recommend that you use the proposed techique. Here the normal setup:

  $_vbootfloppy = ""    # if you want to boot from a virtual floppy:
  $_floppy_a ="threeinch" # or "fiveinch" or empty, if not existing
  $_floppy_b = ""       # dito for B:

  $_hdimage = "hdimage.first" # list of hdimages or boot directories
  $_hdimage_r = $_hdimage # hdimages for 'restricted access (if different)

When you installed DOSEMU (`make install') you will have a bootable hdimage in the file /var/lib/dosemu/hdimage.first. It contains a very tiny installation of FreeDos, just to show you that it works. A hdimage is a file containing a virtual image of a DOS-FAT filesystem. Once you have booted it, you (or autoexec.bat) can use `lredir' to access any directory in your Linux tree as DOS drive (a -t msdos mounted too). Note, however, that the DosC kernel (FreeDos) is not capable yet for redirection, for this you need a proprietary DOS such as MSDOS, PCDOS, DRDOS. Look at chapter 6 (Using Lredir) and for more details on creating your own (better) hdimage look at chapter 4.3 of this README (Making a bootable hdimage for general purpose). Chapter 4.4 also describes how to import/export files from/to a hdimage.

Starting with dosemu-0.99.8, there is a more convenient method available: you just can have a Linux directory containing all what you want to have under your DOS C:. Copy your IO.SYS, MSDOS.SYS or what ever to that directory (e.g. /var/lib/dosemu/bootdir), put

   $_hdimage = "bootdir"

  config.sys
  autoexec.bat
  command.com -> /dos/command.com
  io.sys -> /dos/io.sys
  msdos.sys -> /dos/msdos.sys
  dos -> /dos/dos
  bc -> /dos/bc
  windows -> /dos/windows

There is, however, one drawback, you can't use the DosC kernel (FreeDos) for it, because it hasn't yet a working redirector (will hopefully be available some time in the future).

Now, what does the above `vbootfloppy' mean? Alternatively of booting from a virtual `disk' you may have an image of a virtual `floppy' which you just created such as `dd if=/dev/fd0 of=floppy_image'. If this floppy contains a bootable DOS, then

  $_vbootfloppy = "floppy_image"

will boot that floppy. Once running in DOS you can make the floppy available by (virtually) removing the `media' via `bootoff.com'. If want the disk access specified via `$_hdimage' anyway, you may add the keyword `+hd' such as

  $_vbootfloppy = "floppy_image +hd"

In some rare cases you may have problems accessing Lredir'ed drives (especially when your DOS application refuses to run on a 'network drive'), though I personally never happened to fall into one of this. For this to overcome you may need to use socalled `partition access'. The odd with this kind of access is, that you never should have those partition mounted in the Linux file system at the same time as you use it in DOSEMU (which is quite uncomfortable and dangerous on a multitasking OS such as Linux ). Though global.conf checks for mounted partitions, there may be races that are not caught. In addition, when your DOSEMU crashes, it may leave some FAT sectors unflushed to the disk, hence destroying the partition. Anyway, if you think you need it, here is how you `assign' real DOS partitions to DOSEMU:

  $_hdimage = "hdimage.first /dev/hda1 /dev/sdc4:ro"

The above will have `hdimage.first' as booted drive C:, /dev/hda1 as D: (read/write) and /dev/sdc4 as E: (readonly). You may have any kind of order within `$_hdimage', hence

  $_hdimage = "/dev/hda1 hdimage.first /dev/sdc4:ro"

would have /dev/hda1 as booted drive C:. Note that the access to the /dev/* devices must be exclusive (no other process should use it) except for `:ro'.

COM ports and mices

We have simplified the configuration for mices and serial ports and check for depencies between them. If all strings in the below example are empty, then no mouse and/or COM port is available. Note. that you need no mouse.com driver installed in your DOS environment, DOSEMU has the mousedriver builtin. The below example is such a setup

  $_com1 = ""           # e.g. "/dev/mouse" or "/dev/cua0"
  $_com2 = "/dev/modem" # e.g. "/dev/modem" or "/dev/cua1"

  $_mouse = "microsoft" # one of: microsoft, mousesystems, logitech,
  $_mouse_dev = "/dev/mouse" # one of: com1, com2 or /dev/mouse
  $_mouse_flags = ""    # list of none or one or more of:
  $_mouse_baud = (0)    # baudrate, 0 == don't set

The above example lets you have your modem on COM2, COM1 is spare (as you may have your mouse under native DOS there and don't want to change the configuration of your modem software between boots of native DOS and Linux)

However, you may use your favorite DOS mousedriver and directly let it drive COM1 by changing the below variables (rest of variables unchanged)

  $_com1 = "/dev/mouse"
  $_mouse_dev = "com1"

And finaly, when having a PS2 mouse running on your Linuxbox you use the builtin mousedriver (not your mouse.com) to get it work: ( again leaving the rest of variables unchanged)

  $_mouse = "ps2"
  $_mouse_dev = "/dev/mouse"

When using a PS2 mouse or when having more then 2 serial ports you may of course assign _any_ free serialdevice to COM1, COM2. The order doesn't matter:

  $_com1 = "/dev/cua2"
  $_com2 = "/dev/cua0"

Printers

Printer is emulated by piping printer data to your normal Linux printer. The belows tells DOSEMU which printers to use. The `timeout' tells DOSEMU how long to wait after the last output to LPTx before considering the print job as `done' and to to spool out the data to the printer.

$_printer = "lp"        # list of (/etc/printcap) printer names to appear as
$_printer_timeout = (20)# idle time in seconds before spooling out

Networking under DOSEMU

Turn the following option `on' if you require IPX/SPX emulation, there is no need to load IPX.COM within the DOS session. ( the option does not emulate LSL.COM, IPXODI.COM, etc. ) And NOTE: You must have IPX protocol configured into the kernel.

  $_ipxsupport = (on)

Enable Novell 8137->raw 802.3 translation hack in new packet driver.

  $_novell_hack = (on)

If you make use of the dosnet device driver, you may turn on 'multi' packet driver support via

  $_vnet = (on)

For more on this look at chapter 24 (Net code)

Sound

The sound driver is more or less likely to be broken at the moment. Anyway, here are the settings you would need to emulate a SB-sound card by passing the control to the Linux soundrivers.

$_sound = (off)           # sound support on/off
$_sb_base = (0x220)       # base IO-address (HEX)
$_sb_irq = (5)            # IRQ
$_sb_dma = (1)            # DMA channel
$_sb_dsp = "/dev/dsp"     # Path the sound device
$_sb_mixer = "/dev/mixer" # path to the mixer control
$_mpu_base = "0x330"      # base address for the MPU-401 chip (HEX)

Builtin ASPI SCSI Driver

The builtin ASPI driver (a SCSI interface protocol defined by Adaptec) can be used to run DOS based SCSI drivers that use this standard (most SCSI devices ship with such a DOS driver). This enables you to run hardware on Linux, that normally isn't supported otherwise, such as CD writers, Scanners e.t.c. The driver was successfully tested with Dat-streamers, EXABYTE tapedrives, JAZ drives (from iomega) and CD writers. To make it work under DOSEMU you need

• to configure $_aspi in /etc/dosemu.conf to define which of the /dev/sgX devices you want to show up in DOSEMU.
• to load the DOSEMU aspi.sys stub driver within config.sys (e.g. DEVICE=ASPI.SYS) before any ASPI using driver.

The $_aspi variable in dosemu.conf takes strings listing all generic SCSI devices, that you want give to DOSEMU. NOTE: You should make sure, that they are not used by Linux elsewere, else you would come into much trouble. To help you not doing the wrong thing, DOSEMU can check the devicetype of the SCSI device such as

$_aspi = "sg2:WORM"

                                                    
$_aspi = "sg2 sg3 sg4"

$_aspi = "sg1:Direct-Access"
or
$_aspi = "sg1:0"

$ cat /proc/scsi/scsi

$ scsicheck
sg0 scsi0 ch0 ID0 Lun0 ansi2 Direct-Access(0) IBM DCAS-34330 S61A
    $_aspi = "sg0:Direct-Access:0" (or "0/0/0/0:Direct-Access:0")
sg1 scsi0 ch0 ID5 Lun0 ansi2 Direct-Access(0) IOMEGA ZIP 100 D.08
    $_aspi = "sg1:Direct-Access:5" (or "0/0/5/0:Direct-Access:5")
sg2 scsi0 ch0 ID6 Lun0 ansi2 CD-ROM(5) TOSHIBA CD-ROM XM-5701TA 0167
    $_aspi = "sg2:CD-ROM:6" (or "0/0/6/0:CD-ROM:6") <== multiple IDs
sg3 scsi1 ch0 ID4 Lun0 ansi2 Sequential-Access(1) HP C1533A 9503
    $_aspi = "sg3:Sequential-Access:4" (or "1/0/4/0:Sequential-Access:4")
sg4 scsi1 ch0 ID6 Lun0 ansi1 WORM(4) IMS CDD522/10 1.07
    $_aspi = "sg4:WORM:6" (or "1/0/6/0:WORM:6") <== multiple IDs

In the above example there are two scsi hostadapters (scsi0 and scsi1) and DOSEMU will not show more than one hostadapter to DOS (mapping them all into one), hence you would get problems accessing sg2 and sg4. For this you may remap a different targetID such as

$_aspi = "sg2:CD-ROM:5 sg4:WORM"

$_aspi = "0/0/6/0:CD-ROM:5 1/0/6/0:WORM"

echo "scsi remove-single-device 0 0 5 0" >/proc/scsi/scsi

echo "scsi add-single-device 0 0 5 0" >/proc/scsi/scsi

Coming so far, one big problem remains: the (hard coded) buffersize for the sg devices in the Linux kernel (default 32k) may be to small for DOS applications and, if your distributor yet didn't it, you may need to recompile your kernel with a bigger buffer. The buffer size is defined in linux/include/scsi/sg.h and to be on the secure side you may define

#define SG_BIG_BUFF (128*1024-512)  /* 128 Kb buffer size */