SO_REUSEPORT and multicasting

SO_REUSEPORT and multicasting

[Jason]

I wrote a program that uses multicasting to send data.
 It works great on HP-UX but does not work on Fedora
Core 5.  I emailed the fedora list but they were of
little to no help.  

Does the kernel support SO_REUSEPORT?  If so can
anyone give me some suggestions why my program does
not work on Linux?  I did a route add -net 224.0.0.0/4
dev eth0 but that did not do anything.

Thanks,
Jesse

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Re: SO_REUSEPORT and multicasting

[Jan Engelhardt]

>
>Does the kernel support SO_REUSEPORT?

IIRC no, *BSD being the only one(s) I know of.

> If so can
>anyone give me some suggestions why my program does
>not work on Linux?  I did a route add -net 224.0.0.0/4
>dev eth0 but that did not do anything.
>
Have you bound to a multicast group in your program?


Jan Engelhardt
-- 

Re: SO_REUSEPORT and multicasting

[Jason]

--- Jan Engelhardt <jengelh@linux01.gwdg.de> wrote:
> >Does the kernel support SO_REUSEPORT?
> 
> IIRC no, *BSD being the only one(s) I know of.
> 
> > If so can
> >anyone give me some suggestions why my program does
> >not work on Linux?  I did a route add -net
> 224.0.0.0/4
> >dev eth0 but that did not do anything.
> >
> Have you bound to a multicast group in your program?

If BSD supports it why can't Linux?  

Yes.  My code works great on HP-UX but does nothing on
Linux.  It will not compile because it can't find
SO_REUSEPORT.  I don't have the exact error message at
hand, but I could send it if it would help.  If I
switch to SO_REUSEADDR the code compiles but the
server does not receive anything.

Thanks,




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Re: SO_REUSEPORT and multicasting

[Alan Cox]

Ar Mer, 2006-06-14 am 07:26 -0700, ysgrifennodd Jason:
> > Have you bound to a multicast group in your program?
> 
> If BSD supports it why can't Linux?  

Linux follows the standards draft.  SO_REUSEPORT is fairly obscure
BSDism.

> Yes.  My code works great on HP-UX but does nothing on
> Linux.  

That doesn't actually prove very much. Its very easy to write incorrect
code that only works on one system, especially when endian-ness gets
involved with network code.

In particular if writing portable code you must remember to join the
group. You must also remember that the various htons/htonl macros need
to be correctly used or your code will break on little-endian systems.

The default IP_MULTICAST_LOOP value is probably also worth overriding if
you are being fairly paranoid.

Re: SO_REUSEPORT and multicasting

[Jason]

--- Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
> Linux follows the standards draft.  SO_REUSEPORT is
> a fairly obscure BSDism.

Standards are good.  I like standards.  I was not
aware that SO_REUSEPORT was that obscure.  My
programming instructor made it sound like every OS
supports it.  I guess he was wrong.  Is there a Linux
equivalent to SO_REUSEPORT?

> > Yes.  My code works great on HP-UX but does
> > nothing on Linux.  
> 
> That doesn't actually prove very much. Its very easy
> to write incorrect
> code that only works on one system, especially when
> endian-ness gets
> involved with network code.

I have written other unicast programs on Linux and
sftp'd them to HP-UX, compiled, and they ran great. 
The only change I have to make between Linux and HP-UX
is in the Makefile.  I have change the following line:
INCS= -I. -include /usr/include/errno.h #to use fedora

I comment out the -include on the HP-UX and everything
is great.

This multicast application was based on its TCP
brother.  The only difference is removing TCP and
installing multicast support.

> In particular if writing portable code you must
> remember to join the
> group. You must also remember that the various
> htons/htonl macros need
> to be correctly used or your code will break on
> little-endian systems.

All of those macros are used in the multicast.c file I
was given.

> The default IP_MULTICAST_LOOP value is probably also
> worth overriding if
> you are being fairly paranoid.
 
The wrapper I used has a function to set the TTL and
the loop values.  I set the TTL to 3 and the loop to
0.

Thanks,



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Re: SO_REUSEPORT and multicasting

[David Miller]

Instead of degenerating this mailing list into a BSD socket
programming class, you may find this informative:

	http://www.unixguide.net/network/socketfaq/4.11.shtml

and it's covered extensively in W. Richard Steven's book, TCP/IP
Illustrated, Volume 2.  It is considered the bible on BSD socket
programming.

Particularly telling is the final paragraph from that web page which
reads:

	Basically SO_REUSEPORT is a BSD'ism that arose when
	multicast was added, evne though it was not used in the
	original Steve Deering code.  I believe some BSD-derived
	systems may also include it.  SO_REUSEPORT lets you bind
	the same address *and* port, but only if all the binders
	have specified it.  But when binding to multicast address
	(its main use), SO_REUSEADDR is considered identical to
	SO_REUSEPORT (p. 731, "TCP/IP Illustrated, Volume 2").
	So for portability of multicast applications, I always
	use SO_REUSEADDR.

I STRONGLY suggest you go read that reference to page 731 in
the Steven book.

Re: SO_REUSEPORT and multicasting

[Jason]

--- David Miller <davem@davemloft.net> wrote:
> Instead of degenerating this mailing list into a BSD
> socket
> programming class, you may find this informative:
> 
> 
>
http://www.unixguide.net/network/socketfaq/4.11.shtml
> 
> and it's covered extensively in W. Richard Steven's
> book, TCP/IP
> Illustrated, Volume 2.  It is considered the bible
> on BSD socket
> programming.
> 
> Particularly telling is the final paragraph from
> that web page which
> reads:
> 
> 	Basically SO_REUSEPORT is a BSD'ism that arose when
> 	multicast was added, evne though it was not used in
> the
> 	original Steve Deering code.  I believe some
> BSD-derived
> 	systems may also include it.  SO_REUSEPORT lets you
> bind
> 	the same address *and* port, but only if all the
> binders
> 	have specified it.  But when binding to multicast
> address
> 	(its main use), SO_REUSEADDR is considered
> identical to
> 	SO_REUSEPORT (p. 731, "TCP/IP Illustrated, Volume
> 2").
> 	So for portability of multicast applications, I
> always
> 	use SO_REUSEADDR.
> 
> I STRONGLY suggest you go read that reference to
> page 731 in
> the Steven book.


I agree that I don't want to start a flamewar or get
off topic.  However, I did state earlier that I did
switch to use SO_REUSEADDR and while the program
compiled on Linux (without error) the server did not
receive any packets.  Hence my questions regarding
Linux supporting multicasting.  

Thanks,

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Re: SO_REUSEPORT and multicasting

[David Miller]

From: Jason <bofh1234567@yahoo.com>
Date: Wed, 14 Jun 2006 15:57:01 -0700 (PDT)

> I agree that I don't want to start a flamewar or get
> off topic.  However, I did state earlier that I did
> switch to use SO_REUSEADDR and while the program
> compiled on Linux (without error) the server did not
> receive any packets.  Hence my questions regarding
> Linux supporting multicasting.  

We do have a mailing list where the networking developers
actually hang out, netdev@vger.kernel.org

It seems you're not configuring the multicast route correctly
or something like that, the socket stuff should all work just
fine because David Stevens at IBM is testing this stuff all
the time.

Why don't you move your multicast route setup questions over
to the netdev list?

Thanks.

Re: SO_REUSEPORT and multicasting

[Jason]

--- David Miller <davem@davemloft.net> wrote:

> We do have a mailing list where the networking
> developers
> actually hang out, netdev@vger.kernel.org
> 
> Why don't you move your multicast route setup
> questions over
> to the netdev list?
> 
> Thanks.

Thank you for the information.  I will send my request
there.
 


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Re: SO_REUSEPORT and multicasting

[James Courtier-Dutton]

Jason wrote:
> I wrote a program that uses multicasting to send data.
>  It works great on HP-UX but does not work on Fedora
> Core 5.  I emailed the fedora list but they were of
> little to no help.  
>
> Does the kernel support SO_REUSEPORT?  If so can
> anyone give me some suggestions why my program does
> not work on Linux?  I did a route add -net 224.0.0.0/4
> dev eth0 but that did not do anything.
>
> Thanks,
> Jesse
>
>   
That address is a multicast address, and therefore needs to go in the 
multicast routing table, and not the unicast one.
You are using a command that only modifies the unicast table.

James

Re: SO_REUSEPORT and multicasting

[Jason]

--- James Courtier-Dutton <James@superbug.co.uk>
wrote:
> That address is a multicast address, and therefore
> needs to go in the 
> multicast routing table, and not the unicast one.
> You are using a command that only modifies the
> unicast table.
> 
> James

The broadcasts don't have to leave my PC so I
shouldn't need a multicast router, do I?  The client
is running in one terminal window and the server in a
second terminal window.

I checked the router man page and found the following:
route add -net 224.0.0.0 netmask 240.0.0.0 dev eth0
This is an obscure one documented so people know how
to do it. This sets all of the class D (multicast) IP
routes to go via "eth0".  This is the correct normal
configuration line with a multicasting kernel.

If there is another command, will you please tell me
what it is?

Thanks,

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