WARNING - OLD ARCHIVES

This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-users

Re: [Xen-users] Re: [Xen-devel] dom0 and domU /dev/urandom generating to

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: Re: [Xen-users] Re: [Xen-devel] dom0 and domU /dev/urandom generating too less entropy
From: Robbie Dinn <robbie@xxxxxxxxxxxx>
Date: Thu, 11 Oct 2007 16:28:29 +0100
Cc: Stephan Seitz <s.seitz@xxxxxxxxxxxx>, XEN Devel - listmembers <xen-devel@xxxxxxxxxxxxxxxxxxx>, XEN User - listmembers <xen-users@xxxxxxxxxxxxxxxxxxx>
Delivery-date: Thu, 11 Oct 2007 08:29:16 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxx
In-reply-to: <470E0CA4.4000909@xxxxxxxxxxxx>
List-help: <mailto:xen-users-request@lists.xensource.com?subject=help>
List-id: Xen user discussion <xen-users.lists.xensource.com>
List-post: <mailto:xen-users@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-users>, <mailto:xen-users-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-users>, <mailto:xen-users-request@lists.xensource.com?subject=unsubscribe>
References: <C3338EE1.EB45%Keir.Fraser@xxxxxxxxxxxx> <470E0CA4.4000909@xxxxxxxxxxxx>
Sender: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: Thunderbird 1.5.0.12 (X11/20060911)
Robbie Dinn wrote:
> Keir Fraser wrote:
>> On 10/10/07 21:00, "Stephan Seitz" <s.seitz@xxxxxxxxxxxx> wrote:
>>
>>> Do you know about a workaround, or maybe the possibility for another
>>> (xen-specific) RNG
>>> besides of /dev/urandom ?
>> I'm surprised you see failures. By my understanding, /dev/urandom is always
>> supposed to return the request number of bytes, but their randomness depends
>> on the amount of entropy currently in the pool. Perhaps sshd explicitly
>> interrogates urandom to find out how much entropy it has gathered?
> I haven't checked (I am too laxy to strace it) but I believe that sshd
> is using /dev/random not /dev/urandom. You can see how much entropy is
> available by cat'ing /proc/sys/kernel/random/entropy_avail .

No I am wrong. I did an strace of sshd and it does read /dev/urandom
not /dev/random as I claimed.

read(5, "\0\0\0\1\212\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n"..., 752) = 752
open("/dev/urandom", O_RDONLY|O_NONBLOCK|O_NOCTTY) = 3
fstat64(3, {st_mode=S_IFCHR|0644, st_rdev=makedev(1, 9), ...}) = 0
poll([{fd=3, events=POLLIN, revents=POLLIN}], 1, 10) = 1
read(3, "p:\326\232y\326f\364<#L\204(<\244\"\275,r\263\r%Z\f\304"..., 32) = 32
close(3)                                = 0

Sorry for the noise.

>> Anyway, the domU kernel gathers entropy from the interrupt delivery times of
>> the netfront and blkfront drivers. This is similar to what a native kernel
>> does. It's not clear how we can easily improve on that without e.g.,
>> plumbing through a hardware RNG to domUs.
> 
> I had a similar problem on a mail server providing a pop3 service. Every
> time a client machine connected to the pop3 daemon (cyrus imap actually),
> it consumed entropy. More entropy was consumed for each connection
> than was provided by the packets arriving. The machine ran of entropy
s/machine ran of entropy/machine ran out of entropy/

> and stopped providing bytes via /dev/random. The pop3 daemon ground
> to a halt because it was waiting to read bytes from /dev/random.
> 
> The work around was to feed entropy into the random number generator.
> There is a user space tool to do this called 'rngd'.
> 
> The correct way to do this would be, as you say, to get the the entropy
> from outside the domU. I used a dirty hack instead, I ran
> 
> /sbin/rngd --rng-device=/dev/urandom
> 
> Yes is wrong and evil but it got me up and running again.
s/Yes is wrong/Yes, it is wrong/

_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users