dd if=/dev/random of=data.bin bs=512 count=1

dd if=/dev/random of=data.bin bs=512 count=1

[Jon Grant]

Hello

Issuing this command, I expected to get a file 4096 bytes in size.
Could someone let me know if I have missed something..? Or is there a
problem with using /dev/random this way?

if I have a count=10 it hangs forever (several hours).

Changing to use /dev/urandom and it generates the file ok.

Please include my email address in replies,  as I'm not a member of this list.

Best regards, Jon

$ dd if=/dev/random of=data.bin bs=512 count=1
0+1 records in
0+1 records out
8 bytes (8 B) copied, 4.47366 s, 0.0 kB/s

$ dd --version
dd (coreutils) 6.10

$ uname -a
Linux ubuntu 2.6.28-11-generic #42-Ubuntu SMP Fri Apr 17 01:57:59 UTC
2009 i686 GNU/Linux

Re: dd if=/dev/random of=data.bin bs=512 count=1

[KOSAKI Motohiro]

> Hello
>
> Issuing this command, I expected to get a file 4096 bytes in size.
> Could someone let me know if I have missed something..? Or is there a
> problem with using /dev/random this way?
>
> if I have a count=10 it hangs forever (several hours).
>
> Changing to use /dev/urandom and it generates the file ok.

you already have the answer. /dev/random might blocked, /dev/urandom doesn't.


> Please include my email address in replies,  as I'm not a member of this list.
>
> Best regards, Jon
>
> $ dd if=/dev/random of=data.bin bs=512 count=1
> 0+1 records in
> 0+1 records out
> 8 bytes (8 B) copied, 4.47366 s, 0.0 kB/s
>
> $ dd --version
> dd (coreutils) 6.10
>
> $ uname -a
> Linux ubuntu 2.6.28-11-generic #42-Ubuntu SMP Fri Apr 17 01:57:59 UTC
> 2009 i686 GNU/Linux
> --
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Re: dd if=/dev/random of=data.bin bs=512 count=1

[Jon]

Hello KOSAKI,

Thank you for your reply.

KOSAKI Motohiro wrote:
>> Hello
>>
>> Issuing this command, I expected to get a file 4096 bytes in size.
>> Could someone let me know if I have missed something..? Or is there a
>> problem with using /dev/random this way?
>>
>> if I have a count=10 it hangs forever (several hours).
>>
>> Changing to use /dev/urandom and it generates the file ok.
> 
> you already have the answer. /dev/random might blocked, /dev/urandom doesn't.

Is this a "feature" of /dev/random ? ..it does not seem very useful to 
have it able to block for so long.

Please include my email address in replies,  as I'm not a member of this 
list.

Best regards, Jon

Re: dd if=/dev/random of=data.bin bs=512 count=1

[Samuel Thibault]

Jon, le Sat 23 May 2009 20:12:59 +0100, a écrit :
> KOSAKI Motohiro wrote:
> >>Issuing this command, I expected to get a file 4096 bytes in size.
> >>Could someone let me know if I have missed something..? Or is there a
> >>problem with using /dev/random this way?
> >>
> >>if I have a count=10 it hangs forever (several hours).
> >>
> >>Changing to use /dev/urandom and it generates the file ok.
> >
> >you already have the answer. /dev/random might blocked, /dev/urandom 
> >doesn't.
> 
> Is this a "feature" of /dev/random ?

It is, as opposed to urandom

> ..it does not seem very useful to have it able to block for so long.

The feature is about to wait for random data to be available, not only
pseudo-random data.

Samuel

Re: dd if=/dev/random of=data.bin bs=512 count=1

[Matt Mackall]

On Sat, 2009-05-23 at 20:12 +0100, Jon wrote:
> Hello KOSAKI,
> 
> Thank you for your reply.
> 
> KOSAKI Motohiro wrote:
> >> Hello
> >>
> >> Issuing this command, I expected to get a file 4096 bytes in size.
> >> Could someone let me know if I have missed something..? Or is there a
> >> problem with using /dev/random this way?
> >>
> >> if I have a count=10 it hangs forever (several hours).
> >>
> >> Changing to use /dev/urandom and it generates the file ok.
> > 
> > you already have the answer. /dev/random might blocked, /dev/urandom doesn't.
> 
> Is this a "feature" of /dev/random ? ..it does not seem very useful to 
> have it able to block for so long.

Yes. The theory is that it will be secure even in the event of a
catastrophic failure of the cryptographic compression function. But
don't worry about that; use /dev/urandom.

-- 
http://selenic.com : development and support for Mercurial and Linux

Re: dd if=/dev/random of=data.bin bs=512 count=1

[Bill Davidsen]

Jon wrote:
> Hello KOSAKI,
> 
> Thank you for your reply.
> 
> KOSAKI Motohiro wrote:
>>> Hello
>>>
>>> Issuing this command, I expected to get a file 4096 bytes in size.
>>> Could someone let me know if I have missed something..? Or is there a
>>> problem with using /dev/random this way?
>>>
>>> if I have a count=10 it hangs forever (several hours).
>>>
>>> Changing to use /dev/urandom and it generates the file ok.
>>
>> you already have the answer. /dev/random might blocked, /dev/urandom 
>> doesn't.
> 
> Is this a "feature" of /dev/random ? ..it does not seem very useful to 
> have it able to block for so long.
> 
Of course. /dev/random generates better random numbers (in the sense of less 
predictable), while /dev/urandom gives you some pseudo-random bits, which mat or 
may not be appropriate for your task. For timing disk speed /dev/zero is faster 
and less overhead.

> Please include my email address in replies,  as I'm not a member of this 
> list.
> 
> Best regards, Jon


-- 
Bill Davidsen <davidsen@tmr.com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot

Re: dd if=/dev/random of=data.bin bs=512 count=1

[Bernd Eckenfels]

In article <19ac3f7a0905170822q28255239nea01b56d52e3ddf8@mail.gmail.com> you wrote:
> Issuing this command, I expected to get a file 4096 bytes in size.

one time 512 is 512 not 4096.

> if I have a count=10 it hangs forever (several hours).

It depends on your harware and usage type (mouse atached?) how much entropy
is generated and filled into the pool.  If you drain it, it will hang for
/dev/random and generate PRNG data with /dev/urandom. 

You should think carefully about sucking too many random bytes from /dev/random.

> $ dd if=/dev/random of=data.bin bs=512 count=1
> 0+1 records in
> 0+1 records out
> 8 bytes (8 B) copied, 4.47366 s, 0.0 kB/s

I am not sure why you only get 8bytes here, but I would suggest to use "bs=1
count=256" anyway.

Gruss
Bernd