[libvirt-users] inquiry about differences between the tap and the vnet in the virt-manager
Laine Stump
laine at laine.org
Mon Apr 20 13:36:29 UTC 2015
On 04/18/2015 10:02 PM, wh.h at foxmail.com wrote:
>
> inquiry about differences between the tap and the vnet in the virt-manager
>
>
> Greetings!
>
> I encounter a problem about the network when using virt-manager to
> create and run a VM.
>
> when I want to establish a network bridge for the guest OS, I
> generally use two kinds of ways described as fellow :
>
> *First method : create a bridge with the help of linux
> commad: brctl*, or
>
> *Second method : using virt-manager *
>
> *(1) create a bridge with the help of linux commad: brctl*
>
> I use a script to establish a bridge, there are many script examples
> on the web to establish a bridge. when I restart the network service,
> I get the bridge :
>
> *1.1 create network bridge *
>
> [root at localhost ~]# ifconfig
>
> br0 Link encap:Ethernet HWaddr C2:CD:89:61:2C:F0
>
> inet addr:192.168.1.189 Bcast:192.168.1.255 Mask:255.255.255.0
>
> inet6 addr: fe80::c0cd:89ff:fe61:2cf0/64 Scope:Link
>
> UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
>
> RX packets:0 errors:0 dropped:0 overruns:0 frame:0
>
> TX packets:16 errors:0 dropped:0 overruns:0 carrier:0
>
> collisions:0 txqueuelen:0
>
> RX bytes:0 (0.0 b) TX bytes:1080 (1.0 KiB)
>
> eth2 Link encap:Ethernet HWaddr 00:25:90:8E:C0:F0
>
> inet addr:172.16.1.189 Bcast:172.16.255.255 Mask:255.255.0.0
>
> inet6 addr: fe80::225:90ff:fe8e:c0f0/64 Scope:Link
>
> UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
>
> RX packets:918998 errors:0 dropped:0 overruns:0 frame:0
>
> TX packets:2033 errors:0 dropped:0 overruns:0 carrier:0
>
> collisions:0 txqueuelen:1000
>
> RX bytes:84486598 (80.5 MiB) TX bytes:148116 (144.6 KiB)
>
> Memory:fbe20000-fbe3ffff
>
> [root at localhost ~]# brctl show
>
> bridge name bridge id STP enabled
> interfaces
>
> br0 8000.0025908ec0f0 no
>
> virbr0 8000.525400b1de42 yes
> virbr0-nic
>
> *1.2 add bridge interface *
>
> the eth2 device is a physical ethernet device,
>
> [root at localhost ~]# brctl addif br0 eth2
>
> [root at localhost ~]# brctl show
>
> bridge name bridge id STP enabled
> interfaces
>
> br0 8000.0025908ec0f0 no
> eth2
>
> virbr0 8000.525400b1de42 yes
> virbr0-nic
>
> *1.3 create tap and add it to the bridge *
>
> *create tap:*
>
> tunctl -t tap0 -u root
>
> brctl addif br0 tap0
>
> ifconfig tap0 up
>
> chmod a+rw /dev/net/tun
>
> echo 1 > /proc/sys/net/ipv4/ip_forward
>
> iptables -t nat -A POSTROUTING -j MASQUERADE
>
The previous two lines do nothing for a bridge'd setup. Neither the
setting of ip_forward nor a POSTROUTING rule will have any effect on
packets that are bridged directly to a physical device (i.e. L2) - they
only take effect on packets that are routed via the host's IP stack
(i.e. L3).
> *add it to the bridge *
>
> [root at localhost ~]# brctl addif br0 tap0
>
> [root at localhost ~]# brctl show
>
> bridge name bridge id STP enabled
> interfaces
>
> br0 8000.0025908ec0f0 no
> eth2
>
>
> tap0
>
> virbr0 8000.525400b1de42 yes
> virbr0-nic
>
> [root at localhost kvm_img]# ifconfig
>
> br0 Link encap:Ethernet HWaddr 00:25:90:8E:C0:F0
>
> inet addr:172.16.1.189 Bcast:172.16.255.255 Mask:255.255.0.0
>
> inet6 addr: fe80::c0cd:89ff:fe61:2cf0/64 Scope:Link
>
> UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
>
> RX packets:37947 errors:0 dropped:0 overruns:0 frame:0
>
> TX packets:10485 errors:0 dropped:0 overruns:0 carrier:0
>
> collisions:0 txqueuelen:0
>
> RX bytes:2581300 (2.4 MiB) TX bytes:40996927 (39.0 MiB)
>
> eth2 Link encap:Ethernet HWaddr 00:25:90:8E:C0:F0
>
> inet6 addr: fe80::225:90ff:fe8e:c0f0/64 Scope:Link
>
> UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
>
> RX packets:957962 errors:0 dropped:34 overruns:0 frame:0
>
> TX packets:38466 errors:0 dropped:0 overruns:0 carrier:0
>
> collisions:0 txqueuelen:1000
>
> RX bytes:87721507 (83.6 MiB) TX bytes:42551007 (40.5 MiB)
>
> Memory:fbe20000-fbe3ffff
>
> tap0 Link encap:Ethernet HWaddr F2:55:FC:F8:95:DC
>
> inet6 addr: fe80::f055:fcff:fef8:95dc/64 Scope:Link
>
> UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
>
> RX packets:11 errors:0 dropped:0 overruns:0 frame:0
>
> TX packets:1328 errors:0 dropped:0 overruns:0 carrier:0
>
> collisions:0 txqueuelen:500
>
> RX bytes:846 (846.0 b) TX bytes:173159 (169.1 KiB)
>
>
>
> *1.4 boot the guest (using the tap0)*
>
> kvm -boot c -localtime -m 4G -hda /home/kvm_img/centos6p4.img -net nic
> -net tap,ifname=tap0,script=no -enable-kvm
>
I haven't seen anything in this case where you attach the host's eth2 to
the bridge. (I'm not sure why you would want to do all of this by hand
rather than using libvirt and letting it take care of the details for
you, but that's not what you asked...)
> *(2) using virt-manager*
>
> *2.1 create network bridge *
>
> [root at localhost ~]# ifconfig
>
> br0 Link encap:Ethernet HWaddr C2:CD:89:61:2C:F0
>
> inet addr:192.168.1.189 Bcast:192.168.1.255 Mask:255.255.255.0
>
> inet6 addr: fe80::c0cd:89ff:fe61:2cf0/64 Scope:Link
>
> UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
>
> RX packets:0 errors:0 dropped:0 overruns:0 frame:0
>
> TX packets:16 errors:0 dropped:0 overruns:0 carrier:0
>
> collisions:0 txqueuelen:0
>
> RX bytes:0 (0.0 b) TX bytes:1080 (1.0 KiB)
>
> eth2 Link encap:Ethernet HWaddr 00:25:90:8E:C0:F0
>
> inet addr:172.16.1.189 Bcast:172.16.255.255 Mask:255.255.0.0
>
> inet6 addr: fe80::225:90ff:fe8e:c0f0/64 Scope:Link
>
> UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
>
> RX packets:918998 errors:0 dropped:0 overruns:0 frame:0
>
> TX packets:2033 errors:0 dropped:0 overruns:0 carrier:0
>
> collisions:0 txqueuelen:1000
>
> RX bytes:84486598 (80.5 MiB) TX bytes:148116 (144.6 KiB)
>
> Memory:fbe20000-fbe3ffff
>
> [root at localhost ~]# brctl show
>
> bridge name bridge id STP enabled
> interfaces
>
> br0 8000.0025908ec0f0 no
>
> virbr0 8000.525400b1de42 yes
> virbr0-nic
>
> *2.2 add bridge interface *
>
> the eth2 device is a physical ethernet device,
>
> [root at localhost ~]# brctl addif br0 eth2
>
> [root at localhost ~]# brctl show
>
> bridge name bridge id STP enabled
> interfaces
>
> br0 8000.0025908ec0f0 no
> eth2
>
> virbr0 8000.525400b1de42 yes
> virbr0-nic
>
> *2.3 run the virt-manager and select the NIC option*
>
> I find the bridge created above , configure it as fellow :
>
> in "*Virtual Network Interface* " setting:
>
> Source device: Host device tap0 (bridge br0)
>
> Device model: e1000
>
> Mac address: 52:54:00:84:E3:62
>
> *2.4 I start the VM using virt-manager*
>
> I get a vnet0 device created by virt-manager,
>
> [root at localhost ~]# ifconfig
>
> br0 Link encap:Ethernet HWaddr 00:25:90:8E:C0:F0
>
> inet addr:172.16.1.189 Bcast:172.16.255.255 Mask:255.255.0.0
>
> inet6 addr: fe80::c0cd:89ff:fe61:2cf0/64 Scope:Link
>
> UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
>
> RX packets:62160 errors:0 dropped:1 overruns:0 frame:0
>
> TX packets:22071 errors:0 dropped:0 overruns:0 carrier:0
>
> collisions:0 txqueuelen:0
>
> RX bytes:4007196 (3.8 MiB) TX bytes:59626981 (56.8 MiB)
>
> eth2 Link encap:Ethernet HWaddr 00:25:90:8E:C0:F0
>
> inet6 addr: fe80::225:90ff:fe8e:c0f0/64 Scope:Link
>
> UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
>
> RX packets:982209 errors:0 dropped:47 overruns:0 frame:0
>
> TX packets:61101 errors:0 dropped:0 overruns:0 carrier:0
>
> collisions:0 txqueuelen:1000
>
> RX bytes:89496644 (85.3 MiB) TX bytes:61778801 (58.9 MiB)
>
> Memory:fbe20000-fbe3ffff
>
> vnet0 Link encap:Ethernet HWaddr FE:54:00:84:E3:62
>
> inet6 addr: fe80::fc54:ff:fe84:e362/64 Scope:Link
>
> UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
>
> RX packets:5 errors:0 dropped:0 overruns:0 frame:0
>
> TX packets:114 errors:0 dropped:0 overruns:0 carrier:0
>
> collisions:0 txqueuelen:500
>
> RX bytes:670 (670.0 b) TX bytes:13720 (13.3 KiB)
>
>
>
> both of the methods above seems that the network of guest OS is OK at
> most of environments, and I can get the attributes of the virtual
> network device . They are seemed to be the same.
>
> *about the tap0**：*
>
> [root at localhost ~]# ethtool -i tap0
>
> driver: tun
>
> version: 1.6
>
> firmware-version:
>
> bus-info: tap
>
> supports-statistics: no
>
> supports-test: no
>
> supports-eeprom-access: no
>
> supports-register-dump: no
>
> supports-priv-flags: no
>
> *about the vnet0 created by virt-manager :*
>
> *[root at localhost ~]# ethtool -i vnet0*
>
> driver: tun
>
> version: 1.6
>
> firmware-version:
>
> bus-info: tap
>
> supports-statistics: no
>
> supports-test: no
>
> supports-eeprom-access: no
>
> supports-register-dump: no
>
> supports-priv-flags: no
>
> *but the action between tap and vnet is not exactly the same**！I will
> describe it in the following.*
>
> *this is my project **environment*
> <http://www.iciba.com/application_environment>*:*
>
> hardware : intel xeon E5 2600 server from supermicro
>
> host OS: centos 6.4 (kernel version is changed to 3.15.10 by me)
>
> virtual software : KVM+qemu 2.1.3
>
> guest OS: centos 6.4 (kernel is not changed : 2.6.32 64bit)
>
>
>
> when I establish the network by *the first method: create a bridge
> with the help of Linux commad: brctl*, tap0 device is created in the
> host and assigned to the guest OS. tap0 device gets a name “eth0” in
> the guest OS.
>
> I use “ping ” command to test network state . I get the following result :
>
> *concept explanation : *
>
> local host: the OS where the guest OS is running
>
> remote host: another server which is connected to the local host using
> ethernet network .
>
> and this is my results:
>
> ping local host from guest OS is OK, I get reply from local host;
>
> ping remote host from local host is OK, I get reply from remote host;
>
> *ping remote host from guest OS is not OK, I don’t get any reply from
> remote host; and this is my question.*
>
>
>
> when I establish the network by *the second method: using
> virt-manager*, at this time , vnet0 device is created in the host and
> assigned to the guest OS. vnet0 device gets a name “eth0” in the guest OS.
>
> I also use “ping ” command to test network state . I get the following
> result :
>
> ping local host from guest OS is OK, I get reply from local host;
>
> ping remote host from local host is OK, I get reply from remote host;
>
> *ping remote host from guest OS is OK, I get reply from remote host;*
>
>
>
> *my question is :*
> why I cannot get reply when I ping *remote host from guest OS using
> the first method?*
>
> I use the same bridge: br0, so I think that the bridge: br0 is not
> responsible for the fault. and I have also stop the firewall .
>
> I guess the reason is that there are some differences between the tap
> device created by linux command and vnet device created by
> virt-manager. could you please kindly give my some advices to solve
> the problem ?
>
>
>
> I would be grateful if you could give me some advice . look forward to
> your reply !
>
> weihua
> ------------------------------------------------------------------------
>
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