CentOS - May 2020 - looking for ideas about how to create a constant data stream

Hi,

I'm looking for a good way to create a constant data stream that will occupy
a
bandwidth of about 2--5Mbit/sec between two remote hosts over the internet.  I 
have full access to the hosts involved.

My first attempt to use scp to copy data from /dev/null on host A to /dev/null 
on host B, but scp says '/dev/null: not a regular file'.  If something
like
that would work, I would be able to limit the bandwidth of this transfer in 
the router(s) involved so that it won't occupy all the bandwidth.

Of course, it would be better if I could limit the bandwidth on the sending 
side rather than dropping packages. I could probably write some program to do 
that, but since I have never programmed such a kind of network application, it 
would be rather time consuming.  Maybe there's already a kind of tool around
that can do this.

I need this to work around whatever settings my ISP has made 3 days ago that 
block my VPN connection so that I effectively can't reasonably work anymore.
I do know what the problem with the connection is and that occupying some 
bandwidth would unblock the VPN; only there doesn't seem to be anything else
I
could about it.

On 30/05/2020 12:32, hw at gc-24.de wrote:

Hi hw,
> I'm looking for a good way to create a constant data stream that will
occupy a
> bandwidth of about 2--5Mbit/sec between two remote hosts over the internet.
I
> have full access to the hosts involved.
> 
> My first attempt to use scp to copy data from /dev/null on host A to
/dev/null
> on host B, but scp says '/dev/null: not a regular file'.  If
something like
> that would work, I would be able to limit the bandwidth of this transfer in
> the router(s) involved so that it won't occupy all the bandwidth.
You can't read from /dev/null. You get nothing from it. You're better 
off using /dev/random, for example. That will give you a continuous 
stream of random bytes.

However, that's not the focus of this. You want a sustain a stream of 
packets between two hosts. You're better off using UDP for this. And a 
good tool for generating such packets would be "iperf". It can measure
bandwidth between two nodes more accurately.

Regards,
Anand

On Sat, 30 May 2020 12:32, hw at ... wrote:
> Hi,
>
> I'm looking for a good way to create a constant data stream that will
occupy a
> bandwidth of about 2--5Mbit/sec between two remote hosts over the internet.
I
> have full access to the hosts involved.
>
> My first attempt to use scp to copy data from /dev/null on host A to
/dev/null
> on host B, but scp says '/dev/null: not a regular file'.  If
something like
> that would work, I would be able to limit the bandwidth of this transfer in
> the router(s) involved so that it won't occupy all the bandwidth.
>
> Of course, it would be better if I could limit the bandwidth on the sending
> side rather than dropping packages. I could probably write some program to
do
> that, but since I have never programmed such a kind of network application,
it
> would be rather time consuming.  Maybe there's already a kind of tool
around
> that can do this.
>
> I need this to work around whatever settings my ISP has made 3 days ago
that
> block my VPN connection so that I effectively can't reasonably work
anymore.
> I do know what the problem with the connection is and that occupying some
> bandwidth would unblock the VPN; only there doesn't seem to be anything
else I
> could about it.
Hmm, last time I had such issues (~10 years ago), I had a ssh-server on
one side running, and used scp from the other side:
  scp -l [banwidth in Kbit/sec] /dev/zero [user at remote host]:/dev/null

For me at the time 150 kbit/sec was enough to keep my channel open.

Others used netcat (nc) in a script to get the similar results
  (feeding it "lines" at a certain rate to limit the traffic)


Have a nice weekend.
  - Yamaban

On Saturday, May 30, 2020 12:46:02 PM CEST you wrote:
> On 30/05/2020 12:32, hw at gc-24.de wrote:
> 
> Hi hw,
> 
> > I'm looking for a good way to create a constant data stream that
will
> > occupy a bandwidth of about 2--5Mbit/sec between two remote hosts over
> > the internet.  I have full access to the hosts involved.
> > 
> > My first attempt to use scp to copy data from /dev/null on host A to
> > /dev/null on host B, but scp says '/dev/null: not a regular
file'.  If
> > something like that would work, I would be able to limit the bandwidth
of
> > this transfer in the router(s) involved so that it won't occupy
all the
> > bandwidth.
> 
> You can't read from /dev/null. You get nothing from it. You're
better
> off using /dev/random, for example. That will give you a continuous
> stream of random bytes.
Oh, ok, yes, of course, that makes sense :)
> However, that's not the focus of this. You want a sustain a stream of
> packets between two hosts. You're better off using UDP for this. And a
> good tool for generating such packets would be "iperf". It can
measure
> bandwidth between two nodes more accurately.
Hm, iperf came to mind, and I looked at the manpage again.  It doesn't seem
to
have a way to transmit/receive indefinitely, though it seems it has basically 
everything I'm looking for except for unlimited transfers.  I'll try it
out; I
can always look at the source code and try to do about something about the 
limit if I need to.

On 5/30/20 5:46 AM, Anand Buddhdev wrote:
> On 30/05/2020 12:32, hw at gc-24.de wrote:
> 
> Hi hw,
> 
>> I'm looking for a good way to create a constant data stream that
will
>> occupy a
>> bandwidth of about 2--5Mbit/sec between two remote hosts over the 
>> internet.? I
>> have full access to the hosts involved.
>>
>> My first attempt to use scp to copy data from /dev/null on host A to 
>> /dev/null
>> on host B, but scp says '/dev/null: not a regular file'.? If
something
>> like
>> that would work, I would be able to limit the bandwidth of this 
>> transfer in
>> the router(s) involved so that it won't occupy all the bandwidth.
> 
> You can't read from /dev/null. You get nothing from it. You're
better
> off using /dev/random, for example. That will give you a continuous 
> stream of random bytes.
I would recommend to read from /dev/zero instead, it will give you a 
stream of zeroes. Using /dev/random is OK, but has one disadvantage in 
the OP case: you exhaust machine's accumulated entropy which may be more 
needed for other tasks (like ssh or ssl connections)...

Just my 2 cents.

Valeri
> 
> However, that's not the focus of this. You want a sustain a stream of 
> packets between two hosts. You're better off using UDP for this. And a 
> good tool for generating such packets would be "iperf". It can
measure
> bandwidth between two nodes more accurately.
> 
> Regards,
> Anand
> _______________________________________________
> CentOS mailing list
> CentOS at centos.org
> https://lists.centos.org/mailman/listinfo/centos
-- 
++++++++++++++++++++++++++++++++++++++++
Valeri Galtsev
Sr System Administrator
Department of Astronomy and Astrophysics
Kavli Institute for Cosmological Physics
University of Chicago
Phone: 773-702-4247
++++++++++++++++++++++++++++++++++++++++

On May 30, 2020, at 06:46, Anand Buddhdev <anandb at ripe.net>
wrote:
> 
> You can't read from /dev/null. You get nothing from it. You're
better off using /dev/random, for example. That will give you a continuous
stream of random bytes.
/dev/random will block when you run out of entropy, so you won?t get a
consistent flow of data after some time.  /dev/zero should always return data,
though.   It I agree it makes more sense to use iperf.

--
Jonathan Billings <billings at negate.org>