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I now realise that rls was deliberately using the same sequence of
random numbers to satisfy RFC4662 §5.5<br>
<blockquote>
<p>5.5. Instance Attributes</p>
<p> Each resource element contains zero or more instance elements.
These<br>
instance elements are used to represent a single notifier for
the<br>
resource. For event packages that allow forking, multiple
virtual<br>
subscriptions may exist for a given resource. Multiple virtual<br>
subscriptions are represented as multiple instance elements in
the<br>
corresponding resource element. For subscriptions in which
forking<br>
does not occur, at most one instance will be present for a given<br>
resource.</p>
<p> The "id" attribute contains an opaque string used to uniquely<br>
identify the instance of the resource. The "id" attribute is
unique<br>
only within the context of a resource. Construction of this
string<br>
is an implementation decision. Any mechanism for generating this<br>
string is valid, as long as uniqueness within the resource is<br>
assured.</p>
</blockquote>
Seeding srand() with the values 0, 1, 2... always gives the same
sequence of ID strings. Maybe someone has an opinion on whether this
should be implemented differently, or whether a value should be
stored for situations where multiple resource instances are
removed/reordered.<br>
<br>
Maybe kamailio should use a unique ID (from srutils) in the Via
header for uac requests (in modules/tm/h_table.h) anyway, to prevent
a module from affecting Via branch params by using srand().<br>
<br>
Regards,<br>
Hugh<br>
<br>
On 13/04/2012 15:26, Hugh Waite wrote:
<blockquote cite="mid:4F883782.6060706@crocodile-rcs.com"
type="cite">Hi,
<br>
One of the places that uses rand() is
modules/tm/h_table.c:init_synonym_id . Requests generated by
kamailio (as a uac) use this to generate the Via branch tag. If
any module calls srand(), it affects the Via branch for subsequent
requests.
<br>
<br>
The actual bug we found is in rls/notify.c:generate_string(). The
add_resource_instance() function will re-seed srand() with 0
(zero), leading to nearly every NOTIFY sent by rls having the same
"random" number in the Via branch. I am sure this was the cause of
lost replies, timeouts and dropped subscriptions that we were
seeing (and appears to have gone away after removing it).
<br>
<br>
Although, I could just remove the srand from rls notify.c, I
wondered if it should be using a different random function, and
also whether init_synonym_id should use something more unique for
the Via branch parameter.
<br>
<br>
A quick check has shown a few places that call srand() within the
code, although they probably have less drastic consequences.
<br>
<br>
Regards,
<br>
Hugh
<br>
<br>
On 13/04/2012 14:01, Daniel-Constantin Mierla wrote:
<br>
<blockquote type="cite">Hello,
<br>
<br>
I haven't looked at the issue reported by Hugh, so by now just
comments on srutils/sruid.
<br>
<br>
The idea was to have an unique id generator without linking to
an external library -- my first purpose was to use it for
temporary GRUU ids (RFC5627), I am just about to push to master
the gruu support in registrar/usrloc.
<br>
<br>
Then I thought it might be useful in other places, such as
dialog unique id.
<br>
<br>
I added it as part of lib, since its target usage was for
modules so far, but if needed for some core processing, the two
files (rather small by now) can be moved in the core.
<br>
<br>
Cheers,
<br>
Daniel
<br>
<br>
On 4/13/12 2:50 PM, Henning Westerholt wrote:
<br>
<blockquote type="cite">On Friday 13 April 2012, Hugh Waite
wrote:
<br>
<blockquote type="cite">I have a question about random number
generation within kamailio.
<br>
<br>
A number of modules use rand() to get a random value and in
some places
<br>
is re-seeding with srand(). I believe this is dangerous
because rand()
<br>
is used in the Via branch tag generator.
<br>
We have detected some real bugs (where srand is reseeding
with 0 for
<br>
every message, causing transaction mis-matching) but I'm not
sure of the
<br>
correct way to fix this (other than remove srand()).
<br>
<br>
Should all modules be using a 'core' random function (e.g.
in srutils?)
<br>
? And if so, is this library documented?
<br>
<br>
Regards,
<br>
Hugh
<br>
</blockquote>
Hi Hugh,
<br>
<br>
for the purpose getting a pseudo-random number (i.e. not for
cryptographic
<br>
functionality) we should consolidate on a single random
function. There is the
<br>
recent introduced srutils/sruid code, then there exists a
(IMHO stronger)
<br>
pseudo-random number generator in rand/fastrand and then there
is of course
<br>
rand().
<br>
<br>
Maybe Daniel can comment about the purpose of the srutils
function, IMHO
<br>
consolidating on fastrand or one of the stronger function
(d_rand etc..) from
<br>
stdlib.h would be fine.
<br>
<br>
The re-seeding the internal state of rand() with srand during
runtime sounds
<br>
wrong toe me and should be removed/ fixed.
<br>
<br>
Viele Grüße/ best regards,
<br>
<br>
Henning Westerholt
<br>
<br>
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</blockquote>
<br>
</blockquote>
<br>
<br>
</blockquote>
<br>
<br>
<pre class="moz-signature" cols="72">--
Hugh Waite
Senior Design Engineer
Crocodile RCS Ltd.</pre>
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