dataplane: extract VLAN from AF_PACKET auxiliary data
The original reason we switched to libpcap was that it handled VLAN offload for
us. Now that libpcap isn't reliable on Linux (TPACKET_V3 is broken) we have to
reconstruct the VLAN tag ourselves.
diff --git a/src/python/oftest/afpacket.py b/src/python/oftest/afpacket.py
new file mode 100644
index 0000000..d02e8b0
--- /dev/null
+++ b/src/python/oftest/afpacket.py
@@ -0,0 +1,112 @@
+"""
+AF_PACKET receive support
+
+When VLAN offload is enabled on the NIC Linux will not deliver the VLAN tag
+in the data returned by recv. Instead, it delivers the VLAN TCI in a control
+message. Python 2.x doesn't have built-in support for recvmsg, so we have to
+use ctypes to call it. The recv function exported by this module reconstructs
+the VLAN tag if it was offloaded.
+"""
+
+import socket
+import struct
+from ctypes import *
+
+ETH_P_8021Q = 0x8100
+SOL_PACKET = 263
+PACKET_AUXDATA = 8
+TP_STATUS_VLAN_VALID = 1 << 4
+
+class struct_iovec(Structure):
+ _fields_ = [
+ ("iov_base", c_void_p),
+ ("iov_len", c_size_t),
+ ]
+
+class struct_msghdr(Structure):
+ _fields_ = [
+ ("msg_name", c_void_p),
+ ("msg_namelen", c_uint32),
+ ("msg_iov", POINTER(struct_iovec)),
+ ("msg_iovlen", c_size_t),
+ ("msg_control", c_void_p),
+ ("msg_controllen", c_size_t),
+ ("msg_flags", c_int),
+ ]
+
+class struct_cmsghdr(Structure):
+ _fields_ = [
+ ("cmsg_len", c_size_t),
+ ("cmsg_level", c_int),
+ ("cmsg_type", c_int),
+ ]
+
+class struct_tpacket_auxdata(Structure):
+ _fields_ = [
+ ("tp_status", c_uint),
+ ("tp_len", c_uint),
+ ("tp_snaplen", c_uint),
+ ("tp_mac", c_ushort),
+ ("tp_net", c_ushort),
+ ("tp_vlan_tci", c_ushort),
+ ("tp_padding", c_ushort),
+ ]
+
+libc = CDLL("libc.so.6")
+recvmsg = libc.recvmsg
+recvmsg.argtypes = [c_int, POINTER(struct_msghdr), c_int]
+recvmsg.retype = c_int
+
+def enable_auxdata(sk):
+ """
+ Ask the kernel to return the VLAN tag in a control message
+
+ Must be called on the socket before afpacket.recv.
+ """
+ sk.setsockopt(SOL_PACKET, PACKET_AUXDATA, 1)
+
+def recv(sk, bufsize):
+ """
+ Receive a packet from an AF_PACKET socket
+ @sk Socket
+ @bufsize Maximum packet size
+ """
+ buf = create_string_buffer(bufsize)
+
+ ctrl_bufsize = sizeof(struct_cmsghdr) + sizeof(struct_tpacket_auxdata) + sizeof(c_size_t)
+ ctrl_buf = create_string_buffer(ctrl_bufsize)
+
+ iov = struct_iovec()
+ iov.iov_base = cast(buf, c_void_p)
+ iov.iov_len = bufsize
+
+ msghdr = struct_msghdr()
+ msghdr.msg_name = None
+ msghdr.msg_namelen = 0
+ msghdr.msg_iov = pointer(iov)
+ msghdr.msg_iovlen = 1
+ msghdr.msg_control = cast(ctrl_buf, c_void_p)
+ msghdr.msg_controllen = ctrl_bufsize
+ msghdr.msg_flags = 0
+
+ rv = recvmsg(sk.fileno(), byref(msghdr), 0)
+ if rv < 0:
+ raise RuntimeError("recvmsg failed: rv=%d", rv)
+
+ # The kernel only delivers control messages we ask for. We
+ # only enabled PACKET_AUXDATA, so we can assume it's the
+ # only control message.
+ assert msghdr.msg_controllen >= sizeof(struct_cmsghdr)
+
+ cmsghdr = struct_cmsghdr.from_buffer(ctrl_buf)
+ assert cmsghdr.cmsg_level == SOL_PACKET
+ assert cmsghdr.cmsg_type == PACKET_AUXDATA
+
+ auxdata = struct_tpacket_auxdata.from_buffer(ctrl_buf, sizeof(struct_cmsghdr))
+
+ if auxdata.tp_vlan_tci != 0 or auxdata.tp_status & TP_STATUS_VLAN_VALID:
+ # Insert VLAN tag
+ tag = struct.pack("!HH", ETH_P_8021Q, auxdata.tp_vlan_tci)
+ return buf.raw[:12] + tag + buf.raw[12:rv]
+ else:
+ return buf.raw[:rv]