2007.05_Ask Klaus.pdf

KNOW-HOW

Ask Klaus!

ASK

Klaus Knopper is the creator of

On a system in which the computer

is using the onboard audio incorporated

on the motherboard, the audio chipsets

have limitations. Usually the Linux sys-

tem sound server is running, and thus

the ports are locked.

The only access to sound is through

the Linux Sound Server (because it’s

running). Xine-based players default to

Alsa or OSS, which can’t access the

cheap soundcard because the port is

locked and not open.

Xmms will work using the arts plugin.

It also depends on how the Linux system

sound is accessing the card. The Linspire

sound server system uses Jack, so pro-

grams using Jack will work with these

limited soundcards (or arts). Anyway,

onboard motherboard audio or a cheap

audio card isn’t the best hardware setup

for a standard Linux distro.

is being stored into memory, and the

address is sent to the soundchip DMA

controller, so it plays a certain amount

of sound at a time, from the given start

to the given end address.

Some cards have an internal buffer for

sound, so you can send a limited

amount of sound data to that buffer first,

which is then played by the card.

This method is usually used for stor-

ing instrument patches or fast effect

sounds. More expensive soundcards may

support multiple buffers or DMA chan-

nels that are first filled and then mixed,

at once, using the soundcard’s own me-

chanics. The usual sound hardware has

one stream that is supported at a time,

so naturally most sound drivers are de-

signed to lock the software-side port, the

sound device file, once the card is play-

ing a sound.

If you would really be able to write

concurrent sound data to the same ad-

dress, the result would not sound similar

to any of these two sound streams. In

the best case, you would get some ran-

dom noise. In the worst case, it will be

really loud noise.

So, if only one input stream is really

supported by the hardware, what could

be done from the software side is mixing

different sound resources in memory

first, like:

Knoppix and co-founder of the

LinuxTag expo. He currently

works as a teacher, program-

mer, and consultant. If you have

a configuration problem, or if

you just want to learn more

about how Linux works, send

your questions to:

klaus@linux-magazine.com

Soundcards

I always read your articles, and

I love Knoppix, I was curious

about your answer in Febuary

2007 to a user having problems playing

back audio from his DVDs. A common

problem for any Linux distro is a com-

puter in which the soundcard(s) cannot

share the audio hardware ports. Mainly

the onboard audio chipsets on most MBs

cause this problem; they cannot share

the ports due to hardware limitations.

I’ve found that soundcards for Linux

installations need to have chipsets that

can leave their ports open. Most sound-

cards that work on this level are Creative

Labs, M-Audio, Turtle Beach, etc. Linux

works great with these soundcards.

The problem that you describe

(the fact that a soundcard can

just be “opened for write” by

one program at a time) is more subtle. A

“failed open” on /dev/dsp or /dev/snd/

pcm* indeed means that another process

has that device open.

The most “apparent” solution

would be just allowing the sound

driver concurrent “open” accesses.

However, there is a reason for

locking the sound device exclu-

sively for one program, and that

has to do with the way sound

output works.

You cannot just write a wave

or MP3 file to the sound hard-

ware directly, like you would do

on a tape or printer device. The

way it works is that sound data

ISSUE 78 MAY 2007

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KLAUS!

Ask Klaus!

KNOW-HOW

sound_out = U

( sound_1 + sound_2 ) / 2

Every time Linux boots up it

asks for a password. This is obvi-

ously most annoying.

Can it be set up not to ask for

this password?

And what happens if you lose

the password? How is a user able

to get into the system without

have to reformat the HDD and

then reloading Ubuntu?

and sending the result to the card. Or a

different plan would be to create a

queue, like you have for sending print

jobs to the printer, and play each sound

after the other.

This can be important if you don’t

want to lose any information that can be

given with sound, like in text-to-speech.

There is even a “speech-dispatcher”

available with a scheme that allows you

to interrupt spoken text with more im-

portant spoken text and continue with

the old text later.

One or both of these approaches are

what sound servers like artsd and esd

are supposed to do. But once they are

running, the sound device is completely

blocked for other programs until the

server releases the device; it is not a

good solution when there are programs

that are not aware of sound servers.

To find the process that is guilty of

blocking your soundcard, you can use:

I do get a lot of email

asking me to explain all

of modern computer

technology in just a few sen-

tences that will be easy for be-

ginners to understand, but this is

unfortunately not possible. I’m

just a computer user, too, and I

have to look up information

every now and then myself. The

Internet is a good source of infor-

mation on Linux.

Ubuntu provides online help in several

forms. See page 19 of the magazine in

which you got the DVD for a summary

of support sites.

The first thing to understand when

you are learning to find your way around

Linux is that “Linux” is a computer oper-

ating system , which is something most

users don’t “use” directly.

More often, you will probably be using

an application, like OpenOffice for writ-

ing documents, GIMP for professional

photo postprocessing, or Firefox and

Thunderbird for accessing services on

the Internet.

Be as specific as you can about what

application you actually need help with

before going to books and online help

sources. Keep your focus on knowing

what you want to do with the computer

as a tool, rather than learning the techni-

cal details of how a computer works.

The specific details are not really that

important unless you happen to be inter-

ested in them.

When you are watching TV, for in-

stance, you probably don’t want to know

what kind of electrical components are

inside and what they are doing. (Of

course, some people do find that kind

of thing extremely interesting.)

You have to decide first what you want

to do, then choose from the many possi-

ble tools for accomplishing that goal. I

would need to know more about your

modem to help you configure it.

Figure 1: Enabling password-free login in Ubuntu.

A good starting point is the Linux

Modem-HOWTO, which is available

through the Linux Documentation

Project ( http://tldp.org/HOWTO/

Modem-HOWTO.html ). Check Ubuntu

sites for Ubuntu-specific information.

Yes, it is possible to configure your

Linux system so you can log in without

a password. Every distribution has a

way to do this.

You don’t mention which version of

Ubuntu you are using.

In Ubuntu 6.06 “Dapper Drake” you

can select System in the main window

and choose Login Window . You’ll then

have to provide a password to access ad-

ministrative functions.

In the Login Window dialog, you will

select the Security tab and check Enable

Automatic Login (see Figure 1). Make

sure your account is selected in the User

box. Next, click Close to close the Login

Window dialog.

You can still make your system usable

again if you lost or forgot all passwords,

provided you have physical access to the

storage media and you did not encrypt

the data.

This type of troubleshooting is an

important reason for live CDs such as

Knoppix. (The April 2007 issue of Linux

Magazine includes a Knoppix DVD.)

It is even possible to use a Linux sys-

tem for rescuing data from and repairing

Windows installations, but again, the

details will depend upon your situation.

Users who are considering a change

fuser -v /dev/dsp

( /dev/dsp is, in the ALSA driver case,

the OSS-compatible device when the

snd-oss-pcm module is active).

Some of the ALSA drivers are capable

of (almost) realtime mixing of different

sound resources. This allows seemingly

concurrent and parallel access to the

sound device and “blended” sound out-

put.

There is a (built-in) plugin, dmix, for

ALSA that does this. The dmix setup is a

little more complex and also depends on

the type of card used.

More information on how to set up

dmix is on the ALSA unofficial wiki at:

http://alsa.opensrc.org/index.php?title=

DmixPlugin .

Desperation

Can you help me please? I am

desperate. I want to escape the

clutches of Microsoft and re-

cently loaded Linux Ubuntu from the CD

that came with your magazine. However,

how does one use Linux? There is no

help or manual that I can find.

I want to access the Internet. How

does one do this? How does one set up

the modem, etc.? I cannot find any way

to do this in the menus.

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KNOW-HOW

Ask Klaus!

from Windows to Linux may be inter-

ested in a document titled “Linux Is Not

Windows,” by Dominic Humphries:

http://linux.oneandoneis2.org/LNW.htm .

nodns

nonisdomain

Malware

Attacks

Why is it

so hard to

find arti-

cles about security

in Linux? Is Linux

free from spyware,

viruses, and mal-

ware attacks? If

not, how do I pro-

tect my system? Is

there a scan pack-

age available, at

least for spyware?

DHCP Woes

I have broadband with Optus in

Australia. Optus provided me

with a Siemens modem with

pppoe built in. I originally set it up with

Windows. Most live CD and DVD distros

wouldn’t connect, however, Gentoo

2006.1 did. The difference seems to be

that it uses dhcpcd rather tham pump

or dhclient.

I have since tried several 2.6 live CDs,

and if I mount my Gentoo partition and

invoke its dhcpcd ./dhcpcd -h `host-

name` , they will connect. pump or dhcli-

ent must be stopped first, of course.

Is there a way to get pump or dhclient

to work since dhcpcd is working? The

modem has upnp enabled.

Figure 2: The Common Vulnerabilities and Exposures Project (CVE)

provides updates on recently discovered exploits.

It’s not too hard to find informa-

tion about Linux security, or

rather, Free and Open Source

software security. There are many secu-

rity portals and “Bug of the month”

mailing lists, for virtually every part of

Linux. GNU/Linux, like any complex

system, is not free of errors or bugs, se-

curity flaws, and possibilities for circum-

venting malformed security restrictions.

But, because of its open source nature,

bugs are found and eliminated quickly,

and you don’t have to wait for a “patch-

day” to make your system secure again.

Viruses are very rare (if not even non-

existent) in Linux because of the very

strict privilege separation in Unix sys-

tems. Trojans and Worms usually need

some kind of interaction with the system

administrator in order to get installed, or

at least they require very weak permis-

sion settings (like global write access to

device files).

Some Worms try to exploit flaws like

buffer overflows in software running

with root permissions. But systems with

a higher security level, like a restrictive

SElinux setup, are even

immune to that. (I

would not say that it’s

impossible to break in,

but it’s at least very

hard, even if you have a

lot of knowledge about

system internals and

possible exploits.)

All in all, no operat-

ing system I know of is

absolutely immune, but

a well-known fact is

that “Security by Obscurity” has never

worked. The closed-source practice of

keeping errors and information about

weaknesses secret does not make an op-

erating system secure. Secretiveness just

puts crackers at an unfair advantage be-

cause they get to know break-in possibil-

ities first, weeks prior to the system ad-

ministrators who could have fixed the

problem had it been published properly.

Check the security page for your Linux

distro for periodic updates on security

problems. (See the “Insecurity News” on

page 16 for more on obtaining security

updates for popular Linux variants.).

You’ll also find information on exploits

at the Common Vulnerabilities and Ex-

posures project at http://cve.mitre.org/ .

A number of tools help system admin-

istrators check for potential problems

and identify break-in attempts (even

successful ones). I recommend Nessus

( http://www.nessus.org/ ) for checking

for open ports and vulnerable network

services running on your computers,

Tiger ( http://www.net.tamu.edu/

network/tools/tiger.html ) or Tripwire

( http://www.tripwire.org ) for local secu-

rity and file system integrity checks, and

Snort ( http://www.snort.org ) as an intru-

sion watchdog.

Several virus-checking tools are avail-

able for Linux. One popular solution is

the open-source ClamAV anti-virus tool-

kit. For more information, visit: http://

www.clamav.net/ . ■

You could try some of the set-

tings mentioned in the pump

manpage. Unfortunately, there

does not seem to be a specific program

that always works best. In some cases,

pump does fetch IP addresses when dh-

client or dhcpcd fail. pump may require

some cards to be activated with the com-

mand ifconfig eth0 0.0.0.0 up (replace

eth0 is the actual device name).

You may also want to experiment with

some settings in /etc/pump.conf :

# Number of retries

retries 16

# Giving up after this time

timeout 60

# Skip nameserver (if running U

a local cache with a fixed U

/etc/resolv.conf entry)

Send your Linux questions to

klaus@linux-magazine.com.

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