netdev - Re: vnet problem (bug? feature?)

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Message-ID: <20150214101547.GA9335@faui40p.informatik.uni-erlangen.de>
Date:	Sat, 14 Feb 2015 11:15:48 +0100
From:	Toerless Eckert <tte@...fau.de>
To:	Cong Wang <cwang@...pensource.com>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: vnet problem (bug? feature?)

Thanks for replying, Cong.

On Fri, Feb 13, 2015 at 03:48:14PM -0800, Cong Wang wrote:
> > - Created vnet pair
> > - NOT putting them into different namespaces.
> > - Unicast across them works fine.
> > - When sending IP multicsast into one end, i can not receive it on the other side
> >   (with normal socket API applications).
> 
> Hmm, what does your routing table look like?
>
> They are in the same namespace, so in the same stack, so their IP addresses
> belong to the same stack.

Sure, but it must be possible to send/receive multicast packets to/from a specific
interface. For example link-local-scope multicast. Which works.

Just repeated with a mint 17, 3.13 kernel, same result:

ip link add name veth1 type veth peer name veth2
ip addr add 10.0.0.1/24 dev veth1
ip addr add 10.0.0.2/24 dev veth2
ip link set dev veth1 up
ip link set dev veth2 up

Receiver socket, eg: on veth2:
   socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)
   setsockopt(SO_REUSEADDR, 1)
   bind(0.0.0.0/<port>)
   setsockopt(IP_ADD_MEMBERSHIP, 224.0.0.33/10.0.0.2)

   check wih "netstat -gn" that there is IGMP membership on veth2:
   veth2           1      224.0.0.33

Sender socket, eg: on veth1:
   socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)
   setsockopt(SO_REUSEADDR, 1)
   bind(10.0.0.1/7000)
   connect(224.0.0.33/<port>)

Sending packet, check how they're transmitted:
   - TX countes on veth1 go up (ifconfig output)
   - RX counters on veth2 go up (ifconfig output)
   - tcpdump -i veth2 -P in shows packets being received
   - tcpdump -i veth1 -P out shows packets being sent

Played around with lots of parameters:
   - same behavior for non-link-local-scope multicast, TTL > 1 doesn't elp.
   - same behavior if setting "multicast, "allmulticast", "promiscuous" on the veth
   - same behavior when setting IP_MULTICAST_LOOP on sender.

Routing table:
netstat -r -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
0.0.0.0         192.168.1.254   0.0.0.0         UG        0 0          0 eth1
10.0.0.0        0.0.0.0         255.255.255.0   U         0 0          0 veth1
10.0.0.0        0.0.0.0         255.255.255.0   U         0 0          0 veth2
192.168.1.0     0.0.0.0         255.255.255.0   U         0 0          0 eth1

And of course it works if one side is put into a separate namespace,
but that doesn't help me.

But: it really seems to be a problem with the kernel/sockets, not with veth.
Just replaced the veth pair with a pair of ethernets with a loopback cable and
pretty much exactly the same result (except that receiver side does not see
packets in RX unless it's promiscuous or has a real receiver socket, but that's
perfect). But not being a veth problem but other kernel network stack "feature"
doesn't make it right IMHO. I can't see by which "logic" the receiver socket
seemingly does not care about these packets even though it's explicitly bound
to the interface and the multicast group. "Gimme the darn packets, socket,
they are received on the interface"! ;-))

I can play around with the receiver side socket API call details, but i really
don't see why those should be different if the packets happen to be looped
than if they're not.

Cheers
    Toerless
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