server/site_linux_system.py

# Lint as: python2, python3
# Copyright (c) 2011 The Chromium OS Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

import datetime
import collections
import logging
import os
import random
import time

from autotest_lib.client.common_lib import error
from autotest_lib.client.common_lib.cros import path_utils
from autotest_lib.client.common_lib.cros import virtual_ethernet_pair
from autotest_lib.client.common_lib.cros.network import interface
from autotest_lib.client.common_lib.cros.network import iw_runner
from autotest_lib.client.common_lib.cros.network import ping_runner
from autotest_lib.server.cros.network import packet_capturer
import six
from six.moves import range

NetDev = collections.namedtuple('NetDev',
                                ['inherited', 'phy', 'if_name', 'if_type'])

class LinuxSystem(object):
    """Superclass for test machines running Linux.

    Provides a common point for routines that use the cfg80211 userspace tools
    to manipulate the wireless stack, regardless of the role they play.
    Currently the commands shared are the init, which queries for wireless
    devices, along with start_capture and stop_capture.  More commands may
    migrate from site_linux_router as appropriate to share.

    """

    CAPABILITY_5GHZ = '5ghz'
    CAPABILITY_MULTI_AP = 'multi_ap'
    CAPABILITY_MULTI_AP_SAME_BAND = 'multi_ap_same_band'
    CAPABILITY_IBSS = 'ibss_supported'
    CAPABILITY_SEND_MANAGEMENT_FRAME = 'send_management_frame'
    CAPABILITY_TDLS = 'tdls'
    CAPABILITY_VHT = 'vht'
    CAPABILITY_SME = 'sme'
    CAPABILITY_SUPPLICANT_ROAMING = "supplicant_roaming"
    BRIDGE_INTERFACE_NAME = 'br0'
    HOSTAP_BRIDGE_INTERFACE_PREFIX = 'hostapbr'
    IFB_INTERFACE_PREFIX = 'ifb'
    MIN_SPATIAL_STREAMS = 2
    MAC_BIT_LOCAL = 0x2  # Locally administered.
    MAC_BIT_MULTICAST = 0x1
    MAC_RETRY_LIMIT = 1000

    _UMA_EVENTS = '/var/lib/metrics/uma-events'
    _LOG_PATH_PREFIX = '/tmp/autotest-'


    @property
    def capabilities(self):
        """@return iterable object of AP capabilities for this system."""
        if self._capabilities is None:
            self._capabilities = self.get_capabilities()
            logging.info('%s system capabilities: %r',
                         self.role, self._capabilities)
        return self._capabilities


    @property
    def board(self):
        """@return string self reported board of this device."""
        if self._board is None:
            # Remove 'board:' prefix.
            self._board = self.host.get_board().split(':')[1]
        return self._board


    def __init__(self, host, role, inherit_interfaces=False):
        self.host = host
        self.role = role
        self.inherit_interfaces = inherit_interfaces
        self.__setup()


    def __setup(self):
        """Set up this system.

        Can be used either to complete initialization of a LinuxSystem object,
        or to re-establish a good state after a reboot.

        """
        # hostapd, tcpdump, netperf, etc., may leave behind logs, pcap files,
        # etc., which can fill up tmpfs. Clear them out now.
        self.host.run('rm -rf %s*' % self._LOG_PATH_PREFIX)
        self._logdir = self.host.run('mktemp -d %sXXXXXX' %
                self._LOG_PATH_PREFIX).stdout.strip()

        # Command locations.
        cmd_iw = path_utils.must_be_installed('/usr/sbin/iw', host=self.host)
        self.cmd_ip = path_utils.must_be_installed('/usr/sbin/ip',
                                                   host=self.host)
        self.cmd_readlink = '%s -l' % path_utils.must_be_installed(
                '/bin/ls', host=self.host)

        self._packet_capturer = packet_capturer.get_packet_capturer(
                self.host, host_description=self.role, cmd_ip=self.cmd_ip,
                cmd_iw=cmd_iw, ignore_failures=True, logdir=self.logdir)
        self.iw_runner = iw_runner.IwRunner(remote_host=self.host,
                                            command_iw=cmd_iw)

        self._phy_list = None
        self.phys_for_frequency, self.phy_bus_type = self._get_phy_info()
        logging.debug('Current regulatory domain %r',
                      self.iw_runner.get_regulatory_domain())
        self._interfaces = []
        for interface in self.iw_runner.list_interfaces():
            if self.inherit_interfaces:
                self._interfaces.append(NetDev(inherited=True,
                                               if_name=interface.if_name,
                                               if_type=interface.if_type,
                                               phy=interface.phy))
            else:
                self.iw_runner.remove_interface(interface.if_name)

        self._wlanifs_in_use = []
        self._local_macs_in_use = set()
        self._capture_interface = None
        self._board = None
        # Some uses of LinuxSystem don't use the interface allocation facility.
        # Don't force us to remove all the existing interfaces if this facility
        # is not desired.
        self._wlanifs_initialized = False
        self._capabilities = None
        self._ping_runner = ping_runner.PingRunner(host=self.host)
        self._bridge_interface = None
        self._virtual_ethernet_pair = None

        # TODO(crbug.com/839164): some routers fill their stateful partition
        # with uncollected metrics.
        if self.host.path_exists(self._UMA_EVENTS):
            self.host.run('truncate -s 0 %s' % self._UMA_EVENTS,
                          ignore_status=True)

        # Tear down hostapbr bridge and intermediate functional block
        # interfaces. Run this even for pcaps, because pcap devices sometimes
        # are run as APs too.
        # TODO(crbug.com/1005443): drop the ifb hack when we deploy an AP OS
        # image that has fixes for crbug.com/960551.
        result = self.host.run('ls -d /sys/class/net/%s* /sys/class/net/%s*'
                               ' 2>/dev/null' %
                               (self.HOSTAP_BRIDGE_INTERFACE_PREFIX,
                                self.IFB_INTERFACE_PREFIX),
                               ignore_status=True)
        for path in result.stdout.splitlines():
            self.delete_link(path.split('/')[-1])


    @property
    def phy_list(self):
        """@return iterable object of PHY descriptions for this system."""
        if self._phy_list is None:
            self._phy_list = self.iw_runner.list_phys()
        return self._phy_list


    def _phy_by_name(self, phy_name):
        """@return IwPhy for PHY with name |phy_name|, or None."""
        for phy in self._phy_list:
            if phy.name == phy_name:
                return phy
        else:
            return None


    def _get_phy_info(self):
        """Get information about WiFi devices.

        Parse the output of 'iw list' and some of sysfs and return:

        A dict |phys_for_frequency| which maps from each frequency to a
        list of phys that support that channel.

        A dict |phy_bus_type| which maps from each phy to the bus type for
        each phy.

        @return phys_for_frequency, phy_bus_type tuple as described.

        """
        phys_for_frequency = {}
        phy_caps = {}
        phy_list = []
        for phy in self.phy_list:
            phy_list.append(phy.name)
            for band in phy.bands:
                for mhz in band.frequencies:
                    if mhz not in phys_for_frequency:
                        phys_for_frequency[mhz] = [phy.name]
                    else:
                        phys_for_frequency[mhz].append(phy.name)

        phy_bus_type = {}
        for phy in phy_list:
            phybus = 'unknown'
            command = '%s /sys/class/ieee80211/%s' % (self.cmd_readlink, phy)
            devpath = self.host.run(command).stdout
            if '/usb' in devpath:
                phybus = 'usb'
            elif '/mmc' in devpath:
                phybus = 'sdio'
            elif '/pci' in devpath:
                phybus = 'pci'
            phy_bus_type[phy] = phybus
        logging.debug('Got phys for frequency: %r', phys_for_frequency)
        return phys_for_frequency, phy_bus_type


    def _create_bridge_interface(self):
        """Create a bridge interface."""
        self.host.run('%s link add name %s type bridge' %
                      (self.cmd_ip, self.BRIDGE_INTERFACE_NAME))
        self.host.run('%s link set dev %s up' %
                      (self.cmd_ip, self.BRIDGE_INTERFACE_NAME))
        self._bridge_interface = self.BRIDGE_INTERFACE_NAME


    def _create_virtual_ethernet_pair(self):
        """Create a virtual ethernet pair."""
        self._virtual_ethernet_pair = virtual_ethernet_pair.VirtualEthernetPair(
                interface_ip=None, peer_interface_ip=None, host=self.host)
        self._virtual_ethernet_pair.setup()


    def _get_unique_mac(self):
        """Get a MAC address that is likely to be unique.

        Generates a MAC address that is a) guaranteed not to be in use
        on this host, and b) likely to be unique within the test cell.

        @return string MAC address.

        """
        # We use SystemRandom to reduce the likelyhood of coupling
        # across systems. (The default random class might, e.g., seed
        # itself based on wall-clock time.)
        sysrand = random.SystemRandom()
        for tries in range(0, self.MAC_RETRY_LIMIT):
            mac_addr = '%02x:%02x:%02x:%02x:%02x:%02x' % (
                (sysrand.getrandbits(8) & ~self.MAC_BIT_MULTICAST) |
                self.MAC_BIT_LOCAL,
                sysrand.getrandbits(8),
                sysrand.getrandbits(8),
                sysrand.getrandbits(8),
                sysrand.getrandbits(8),
                sysrand.getrandbits(8))
            if mac_addr not in self._local_macs_in_use:
                self._local_macs_in_use.add(mac_addr)
                return mac_addr
        else:
            raise error.TestError('Failed to find a new MAC address')


    def _phy_in_use(self, phy_name):
        """Determine whether or not a PHY is used by an active DEV

        @return bool True iff PHY is in use.
        """
        for net_dev in self._wlanifs_in_use:
            if net_dev.phy == phy_name:
                return True
        return False


    def remove_interface(self, interface):
        """Remove an interface from a WiFi device.

        @param interface string interface to remove (e.g. wlan0).

        """
        self.release_interface(interface)
        self.host.run('%s link set %s down' % (self.cmd_ip, interface))
        self.iw_runner.remove_interface(interface)
        for net_dev in self._interfaces:
            if net_dev.if_name == interface:
                self._interfaces.remove(net_dev)
                break


    def delete_link(self, name):
        """Delete link using the `ip` command.

        @param name string link name.

        """
        self.host.run('%s link del %s' % (self.cmd_ip, name),
                      ignore_status=True)


    def close(self):
        """Close global resources held by this system."""
        logging.debug('Cleaning up host object for %s', self.role)
        self._packet_capturer.close()
        # Release and remove any interfaces that we create.
        for net_dev in self._wlanifs_in_use:
            self.release_interface(net_dev.if_name)
        for net_dev in self._interfaces:
            if net_dev.inherited:
                continue
            self.remove_interface(net_dev.if_name)
        if self._bridge_interface is not None:
            self.remove_bridge_interface()
        if self._virtual_ethernet_pair is not None:
            self.remove_ethernet_pair_interface()
        self.host.close()
        self.host = None


    def reboot(self, timeout):
        """Reboot this system, and restore it to a known-good state.

        @param timeout Maximum seconds to wait for system to return.

        """
        self.host.reboot(timeout=timeout, wait=True)
        self.__setup()


    def get_capabilities(self):
        caps = set()
        phymap = self.phys_for_frequency
        if [freq for freq in six.iterkeys(phymap) if freq > 5000]:
            # The frequencies are expressed in megaherz
            caps.add(self.CAPABILITY_5GHZ)
        if [freq for freq in six.iterkeys(phymap) if len(phymap[freq]) > 1]:
            caps.add(self.CAPABILITY_MULTI_AP_SAME_BAND)
            caps.add(self.CAPABILITY_MULTI_AP)
        elif len(self.phy_bus_type) > 1:
            caps.add(self.CAPABILITY_MULTI_AP)
        for phy in self.phy_list:
            if ('tdls_mgmt' in phy.commands or
                'tdls_oper' in phy.commands or
                'T-DLS' in phy.features):
                caps.add(self.CAPABILITY_TDLS)
            if 'authenticate' in phy.commands:
                caps.add(self.CAPABILITY_SME)
            if phy.support_vht:
                caps.add(self.CAPABILITY_VHT)
            if 'roaming' not in phy.features:
                caps.add(self.CAPABILITY_SUPPLICANT_ROAMING)
        if any([iw_runner.DEV_MODE_IBSS in phy.modes
                for phy in self.phy_list]):
            caps.add(self.CAPABILITY_IBSS)
        return caps


    def start_capture(self, frequency,
                      width_type=None, snaplen=None, filename=None):
        """Start a packet capture.

        @param frequency int frequency of channel to capture on.
        @param width_type object width type from iw_runner.
        @param snaplen int number of bytes to retain per capture frame.
        @param filename string filename to write capture to.

        """
        if self._packet_capturer.capture_running:
            self.stop_capture()
        self._capture_interface = self.get_wlanif(frequency, 'monitor')
        full_interface = [net_dev for net_dev in self._interfaces
                          if net_dev.if_name == self._capture_interface][0]
        # If this is the only interface on this phy, we ought to configure
        # the phy with a channel and a width.  Otherwise, inherit the
        # settings of the phy as they stand.
        if len([net_dev for net_dev in self._interfaces
                if net_dev.phy == full_interface.phy]) == 1:
            self._packet_capturer.configure_raw_monitor(
                    self._capture_interface, frequency, width_type=width_type)
        else:
            self.host.run('%s link set %s up' %
                          (self.cmd_ip, self._capture_interface))

        # Start the capture.
        if filename:
            remote_path = os.path.join('/tmp', os.path.basename(filename))
        else:
            remote_path = None
        self._packet_capturer.start_capture(
            self._capture_interface, './debug/', snaplen=snaplen,
            remote_file=remote_path)


    def stop_capture(self, save_dir=None, save_filename=None):
        """Stop a packet capture.

        @param save_dir string path to directory to save pcap files in.
        @param save_filename string basename of file to save pcap in locally.

        """
        if not self._packet_capturer.capture_running:
            return
        results = self._packet_capturer.stop_capture(
                local_save_dir=save_dir, local_pcap_filename=save_filename)
        self.release_interface(self._capture_interface)
        self._capture_interface = None
        return results


    def sync_host_times(self):
        """Set time on our DUT to match local time."""
        epoch_seconds = time.time()
        busybox_format = '%Y%m%d%H%M.%S'
        busybox_date = datetime.datetime.utcnow().strftime(busybox_format)
        self.host.run('date -u --set=@%s 2>/dev/null || date -u %s' %
                      (epoch_seconds, busybox_date))


    def _get_phy_for_frequency(self, frequency, phytype, spatial_streams):
        """Get a phy appropriate for a frequency and phytype.

        Return the most appropriate phy interface for operating on the
        frequency |frequency| in the role indicated by |phytype|.  Prefer idle
        phys to busy phys if any exist.  Secondarily, show affinity for phys
        that use the bus type associated with this phy type.

        @param frequency int WiFi frequency of phy.
        @param phytype string key of phytype registered at construction time.
        @param spatial_streams int number of spatial streams required.
        @return string name of phy to use.

        """
        phy_objs = []
        for phy_name in self.phys_for_frequency[frequency]:
            phy_obj = self._phy_by_name(phy_name)
            num_antennas = min(phy_obj.avail_rx_antennas,
                               phy_obj.avail_tx_antennas)
            if num_antennas >= spatial_streams:
                phy_objs.append(phy_obj)
            elif num_antennas == 0:
                logging.warning(
                    'Allowing use of %s, which reports zero antennas', phy_name)
                phy_objs.append(phy_obj)
            else:
                logging.debug(
                    'Filtering out %s, which reports only %d antennas',
                    phy_name, num_antennas)

        busy_phys = set(net_dev.phy for net_dev in self._wlanifs_in_use)
        idle_phy_objs = [phy_obj for phy_obj in phy_objs
                         if phy_obj.name not in busy_phys]
        phy_objs = idle_phy_objs or phy_objs
        phy_objs.sort(key=lambda phy_obj: min(phy_obj.avail_rx_antennas,
                                              phy_obj.avail_tx_antennas),
                      reverse=True)
        phys = [phy_obj.name for phy_obj in phy_objs]

        preferred_bus = {'monitor': 'usb', 'managed': 'pci'}.get(phytype)
        preferred_phys = [phy for phy in phys
                          if self.phy_bus_type[phy] == preferred_bus]
        phys = preferred_phys or phys

        return phys[0]


    def _get_wlanif(self, phytype, spatial_streams, frequency, same_phy_as):
        """Get a WiFi device that supports the given frequency and phytype.

        We simply find or create a suitable DEV. It is left to the
        caller to actually configure the frequency and bring up the
        interface.

        @param phytype string type of phy (e.g. 'monitor').
        @param spatial_streams int number of spatial streams required.
        @param frequency int WiFi frequency to support.
        @param same_phy_as string create the interface on the same phy as this.
        @return NetDev WiFi device.

        """
        if frequency and same_phy_as:
            raise error.TestError(
                'Can not combine |frequency| and |same_phy_as|')

        if not (frequency or same_phy_as):
            raise error.TestError(
                'Must specify one of |frequency| or |same_phy_as|')

        if spatial_streams is None:
            spatial_streams = self.MIN_SPATIAL_STREAMS
        # We don't want to use the 3rd radio on Whirlwind. Reject it if someone
        # tries to add a test that uses it.
        elif spatial_streams < self.MIN_SPATIAL_STREAMS and \
             self.board == 'whirlwind':
            raise error.TestError('Requested spatial streams: %d; minimum %d' \
                                  % (spatial_streams, self.MIN_SPATIAL_STREAMS))

        if same_phy_as:
            for net_dev in self._interfaces:
                if net_dev.if_name == same_phy_as:
                    phy = net_dev.phy
                    break
            else:
                raise error.TestFail('Unable to find phy for interface %s' %
                                     same_phy_as)
        elif frequency in self.phys_for_frequency:
            phy = self._get_phy_for_frequency(
                frequency, phytype, spatial_streams)
        else:
            raise error.TestFail('Unable to find phy for frequency %d' %
                                 frequency)

        # If we have a suitable unused interface sitting around on this
        # phy, reuse it.
        for net_dev in set(self._interfaces) - set(self._wlanifs_in_use):
            if net_dev.phy == phy and net_dev.if_type == phytype:
                break
        else:
            # Because we can reuse interfaces, we have to iteratively find a
            # good interface name.
            name_exists = lambda name: bool([net_dev
                                             for net_dev in self._interfaces
                                             if net_dev.if_name == name])
            if_name = lambda index: '%s%d' % (phytype, index)
            if_index = len(self._interfaces)
            while name_exists(if_name(if_index)):
                if_index += 1
            net_dev = NetDev(phy=phy, if_name=if_name(if_index),
                             if_type=phytype, inherited=False)
            self._interfaces.append(net_dev)
            self.iw_runner.add_interface(phy, net_dev.if_name, phytype)

        # Link must be down to reconfigure MAC address.
        self.host.run('%s link set dev %s down' % (
            self.cmd_ip, net_dev.if_name))
        if same_phy_as:
            self.clone_mac_address(src_dev=same_phy_as,
                                   dst_dev=net_dev.if_name)
        else:
            self.ensure_unique_mac(net_dev)

        return net_dev


    def get_configured_interface(self, phytype, spatial_streams=None,
                                 frequency=None, same_phy_as=None):
        """Get a WiFi device that supports the given frequency and phytype.

        The device's link state will be UP, and (where possible) the device
        will be configured to operate on |frequency|.

        @param phytype string type of phy (e.g. 'monitor').
        @param spatial_streams int number of spatial streams required.
        @param frequency int WiFi frequency to support.
        @param same_phy_as string create the interface on the same phy as this.
        @return string WiFi device.

        """
        net_dev = self._get_wlanif(
            phytype, spatial_streams, frequency, same_phy_as)

        self.host.run('%s link set dev %s up' % (self.cmd_ip, net_dev.if_name))

        if frequency:
            if phytype == 'managed':
                logging.debug('Skipped setting frequency for DEV %s '
                              'since managed mode DEVs roam across APs.',
                              net_dev.if_name)
            elif same_phy_as or self._phy_in_use(net_dev.phy):
                logging.debug('Skipped setting frequency for DEV %s '
                              'since PHY %s is already in use',
                              net_dev.if_name, net_dev.phy)
            else:
                self.iw_runner.set_freq(net_dev.if_name, frequency)

        self._wlanifs_in_use.append(net_dev)
        return net_dev.if_name


    # TODO(quiche): Deprecate this, in favor of get_configured_interface().
    # crbug.com/512169.
    def get_wlanif(self, frequency, phytype,
                   spatial_streams=None, same_phy_as=None):
        """Get a WiFi device that supports the given frequency and phytype.

        We simply find or create a suitable DEV. It is left to the
        caller to actually configure the frequency and bring up the
        interface.

        @param frequency int WiFi frequency to support.
        @param phytype string type of phy (e.g. 'monitor').
        @param spatial_streams int number of spatial streams required.
        @param same_phy_as string create the interface on the same phy as this.
        @return string WiFi device.

        """
        net_dev = self._get_wlanif(
            phytype, spatial_streams, frequency, same_phy_as)
        self._wlanifs_in_use.append(net_dev)
        return net_dev.if_name


    def ensure_unique_mac(self, net_dev):
        """Ensure MAC address of |net_dev| meets uniqueness requirements.

        The Linux kernel does not allow multiple APs with the same
        BSSID on the same PHY (at least, with some drivers). Hence, we
        want to ensure that the DEVs for a PHY have unique MAC
        addresses.

        Note that we do not attempt to make the MACs unique across
        PHYs, because some tests deliberately create such scenarios.

        @param net_dev NetDev to uniquify.

        """
        if net_dev.if_type == 'monitor':
            return

        our_ifname = net_dev.if_name
        our_phy = net_dev.phy
        our_mac = interface.Interface(our_ifname, self.host).mac_address
        sibling_devs = [dev for dev in self._interfaces
                        if (dev.phy == our_phy and
                            dev.if_name != our_ifname and
                            dev.if_type != 'monitor')]
        sibling_macs = (
            interface.Interface(sib_dev.if_name, self.host).mac_address
            for sib_dev in sibling_devs)
        if our_mac in sibling_macs:
            self.configure_interface_mac(our_ifname,
                                         self._get_unique_mac())


    def configure_interface_mac(self, wlanif, new_mac):
        """Change the MAC address for an interface.

        @param wlanif string name of device to reconfigure.
        @param new_mac string MAC address to assign (e.g. '00:11:22:33:44:55')

        """
        self.host.run('%s link set %s address %s' %
                      (self.cmd_ip, wlanif, new_mac))


    def clone_mac_address(self, src_dev=None, dst_dev=None):
        """Copy the MAC address from one interface to another.

        @param src_dev string name of device to copy address from.
        @param dst_dev string name of device to copy address to.

        """
        self.configure_interface_mac(
            dst_dev,
            interface.Interface(src_dev, self.host).mac_address)


    def release_interface(self, wlanif):
        """Release a device allocated throuhg get_wlanif().

        @param wlanif string name of device to release.

        """
        for net_dev in self._wlanifs_in_use:
            if net_dev.if_name == wlanif:
                 self._wlanifs_in_use.remove(net_dev)


    def get_bridge_interface(self):
        """Return the bridge interface, create one if it is not created yet.

        @return string name of bridge interface.
        """
        if self._bridge_interface is None:
            self._create_bridge_interface()
        return self._bridge_interface


    def remove_bridge_interface(self):
        """Remove the bridge interface that's been created."""
        if self._bridge_interface is not None:
            self.host.run('%s link delete %s type bridge' %
                          (self.cmd_ip, self._bridge_interface))
        self._bridge_interface = None


    def add_interface_to_bridge(self, interface):
        """Add an interface to the bridge interface.

        This will create the bridge interface if it is not created yet.

        @param interface string name of the interface to add to the bridge.
        """
        if self._bridge_interface is None:
            self._create_bridge_interface()
        # TODO b:169251326 terms below are set outside of this codebase
        # and should be updated when possible. ("master" -> "main")
        self.host.run('%s link set dev %s master %s' %
                      (self.cmd_ip, interface, self._bridge_interface))


    def get_virtual_ethernet_main_interface(self):
        """Return the main interface of the virtual ethernet pair.

        @return string name of the main interface of the virtual ethernet
                pair.
        """
        if self._virtual_ethernet_pair is None:
            self._create_virtual_ethernet_pair()
        return self._virtual_ethernet_pair.interface_name


    def get_virtual_ethernet_peer_interface(self):
        """Return the peer interface of the virtual ethernet pair.

        @return string name of the peer interface of the virtual ethernet pair.
        """
        if self._virtual_ethernet_pair is None:
            self._create_virtual_ethernet_pair()
        return self._virtual_ethernet_pair.peer_interface_name


    def remove_ethernet_pair_interface(self):
        """Remove the virtual ethernet pair that's been created."""
        if self._virtual_ethernet_pair is not None:
            self._virtual_ethernet_pair.teardown()
        self._virtual_ethernet_pair = None


    def require_capabilities(self, requirements):
        """Require capabilities of this LinuxSystem.

        Check that capabilities in |requirements| exist on this system.
        Raise an exception to skip but not fail the test if said
        capabilities are not found.

        @param requirements list of CAPABILITY_* defined above.

        """
        missing = [cap for cap in requirements if not cap in self.capabilities]
        if missing:
            raise error.TestNAError('%s is missing required capabilites: %r'
                                    % (self.role, missing))


    def disable_antennas_except(self, permitted_antennas):
        """Disable unwanted antennas.

        Disable all antennas except those specified in |permitted_antennas|.
        Note that one or more of them may remain disabled if the underlying
        hardware does not support them.

        @param permitted_antennas int bitmask specifying antennas that we should
        attempt to enable.

        """
        for phy in self.phy_list:
            if not phy.supports_setting_antenna_mask:
                continue
            # Determine valid bitmap values based on available antennas.
            self.iw_runner.set_antenna_bitmap(phy.name,
                permitted_antennas & phy.avail_tx_antennas,
                permitted_antennas & phy.avail_rx_antennas)


    def enable_all_antennas(self):
        """Enable all antennas on all phys."""
        for phy in self.phy_list:
            if not phy.supports_setting_antenna_mask:
                continue
            self.iw_runner.set_antenna_bitmap(phy.name, phy.avail_tx_antennas,
                                              phy.avail_rx_antennas)


    def ping(self, ping_config):
        """Ping an IP from this system.

        @param ping_config PingConfig object describing the ping command to run.
        @return a PingResult object.

        """
        logging.info('Pinging from the %s.', self.role)
        return self._ping_runner.ping(ping_config)


    @property
    def logdir(self):
        """Return a directory for storing temporary logs.
        @return string path to temporary log directory.
        """
        return self._logdir