9:20 a.m.
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@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@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@localhost ~]# brctl addif br0 eth2
[root@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).
///////////////////////////
weihua:
thanks for you suggestion !
I want to use "ping" command to test network state , so I make these setting .
///////////////////////////
add it to the bridge
[root@localhost ~]# brctl addif br0 tap0
[root@localhost ~]# brctl show
bridge name bridge id STP enabled interfaces
br0 8000.0025908ec0f0 no eth2
tap0
virbr0 8000.525400b1de42 yes virbr0-nic
[root@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...)
///////////////////////////
weihua:
I use this command to add it to the bridge :
brctl addif br0 eth2
[root@localhost ~]# brctl show
bridge name bridge id STP enabled interfaces
br0 8000.0025908ec0f0 no eth2
tap0
virbr0 8000.525400b1de42 yes virbr0-nic
what I want to do this by hand is that I want to manage the VM by script commands . But
the virt-manager is a graphic tool.
Would you please tell me something about the differences between the tap created by linux
command and the vnet device created by libvirt? so I could write some scripts to make the
tap the same as the vnet device .
thanks a lot!
///////////////////////////
(2) using virt-manager
2.1 create network bridge
[root@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@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@localhost ~]# brctl addif br0 eth2
[root@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@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@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@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:
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