Do not resolve the interface name globally

[guedou]

This PR fixes #3191. I do not know how to test it =/

[polybassa]

If you need to create multiple interfaces to create a unit test, you could use virtual CAN interfaces. At least on Linux machines.
You can create as many devices as you want:

sudo modprobe vcan
sudo ip link add dev vcan0 type vcan
sudo ip link set up vcan0
sudo ip link add dev anotherinterface type vcan
sudo ip link set up anotherinterface
sudo ip link add dev onemoreinterface type vcan
sudo ip link set up onemoreinterface

sniff(["vcan0", "anotherinterface", "onemoreinterface"])

If you think that could work for a testcase, I could provide one.

[gpotter2]

Don't we need to put this in the isinstance()s to make sure it always work?

if isinstance(iface, list):
    L2socket = resolve_iface(iface[0] or conf.iface).l2listen()
...
elif isinstance(iface, dict):
    L2socket = resolve_iface(next(iter(iface.values()) or conf.iface).l2listen()
....
.....

[gpotter2]

We can also just use TUN/TAPS polybassa, but good call.

[gpotter2]

I'd go for

diff --git a/scapy/sendrecv.py b/scapy/sendrecv.py
index 7a847a03..130f8700 100644
--- a/scapy/sendrecv.py
+++ b/scapy/sendrecv.py
@@ -1108,22 +1108,22 @@ class AsyncSniffer(object):
                             quiet=quiet)
                 )] = offline
         if not sniff_sockets or iface is not None:
-            if L2socket is None:
-                L2socket = resolve_iface(iface or conf.iface).l2listen()
+            _RL2 = lambda i: L2socket or resolve_iface(i).l2listen()
             if isinstance(iface, list):
                 sniff_sockets.update(
-                    (L2socket(type=ETH_P_ALL, iface=ifname, **karg),
+                    (_RL2(ifname)(type=ETH_P_ALL, iface=ifname, **karg),
                      ifname)
                     for ifname in iface
                 )
             elif isinstance(iface, dict):
                 sniff_sockets.update(
-                    (L2socket(type=ETH_P_ALL, iface=ifname, **karg),
+                    (_RL2(ifname)(type=ETH_P_ALL, iface=ifname, **karg),
                      iflabel)
                     for ifname, iflabel in six.iteritems(iface)
                 )
             else:
-                sniff_sockets[L2socket(type=ETH_P_ALL, iface=iface,
+                sniff_sockets[_RL2(iface)(type=ETH_P_ALL, iface=iface,
                                        **karg)] = iface

         # Get select information from the sockets

[p-l-]

I totally missed your PR and took a different approach (#3234). Feel free to use this idea if you want.