netdev - Re: why does DCCP SO_REUSEADDR have to be SOL

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Date:	Sat, 2 Feb 2008 00:52:59 -0200
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	Rick Jones <rick.jones2@...com>
Cc:	Linux Network Development list <netdev@...r.kernel.org>
Subject: Re: why does DCCP SO_REUSEADDR have to be SOL_DCCP?

Em Fri, Feb 01, 2008 at 05:42:23PM -0800, Rick Jones escreveu:
> Hi -
>
> I'm tweaking the netperf omni tests to be able to run over DCCP.  I've run 
> across a not-unorecedented problem with getaddrinfo() not groking either 
> SOCK_DCCP or IPPROTO_DCCP in the hints, and that I can more or less live 
> with - I had to do a kludge for getaddrinfo() for IPPROTO_SCTP under Linux 
> at one point and I can see how the two are not necessarily going to be in 
> sync.

See the ttcp patch where we do a xgetaddrinfo crude hack to handle dccp:

http://vger.kernel.org/~acme/dccp/ttcp.c

> And I've worked-around no user-level include files (ie without setting 
> __KERNEL__) define the DCCP stuff, and that is OK too, albeit somewhat 
> inconvenient.

Humm, for what? Again, see the ttcp code above:

> My question though is why on earth does an SO_REUSEADDR setsockopt() 
> against a DCCP socket have to be SOL_DCCP?  SCTP and TCP are quite happy 
> with SOL_SOCKET, and it might be foolish consistency, but since the option 
> _does_ begin with SO_ I'd have expected it to work for SOL_SOCKET, but 
> (again RHEL5.1, yes, I do plan on getting upstream but have to satisfy 
> several masters) it doesn't seem to be the case - a subsequent listen() or 
> connect() call after an SOL_SOCKET SO_REUSEADDR against a DCCP socket 
> leaves one SOL as it were...

Strange, lemme check...

 1. sys_socketcall ->
 2.  sys_setsockopt ->
 3.    if (level == SOL_SOCKET) {
 4.      sock_setsockopt:
 5.        case SO_REUSEADDR:
 6.          sk->sk_reuse = valbool;
 7.    } else
 8.      sock->ops->setsockopt = inet_dccp_ops->setsockopt =
 9.        inet_dccp_ops->setsockopt = sock_common_setsockopt ->
10.          sk->sk_prot->setsockopt = dccp_v4_prot->setsockopt =
11.	    dccp_setsockopt
12.              if (level != SOL_DCCP)
13.                return inet_csk(sk)->icsk_af_ops->setsockopt() =
14.		  ip_setsockopt
15.              return do_dccp_setsockopt()

SO_REUSEADDR is handled in 4, if you pass SOL_SOCKET.

If instead you pass SOL_DCCP we'll go down the rabbit hole till
do_dccp_setsockopt() and SO_REUSEADDR, that is equal to 2, will be
interpreted as DCCP_SOCKOPT_SERVICE, that is also equal to 2, so you'll
be setting the service, not changing the SO_REUSEADDR setting.

The problem here is that you need to use:

setsockopt(fd, SOL_DCCP, DCCP_SOCKOPT_PACKET_SIZE, service,
           sizeof(service));

Again, take a look at the ttcp patch, the other patches for iperf,
netcat, etc handles this.

> Of course the setsockopt(SO_REUSEADDR) against the DCCP socket using 
> SOL_SOCKET itself doesn't fail, only the later listen() or connect() 
> call...
>
> happy benchmarking,

Look forward for a happy DCCP netperf bencharking session!

Thanks a lot,

- Arnaldo
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