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
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
(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 ?
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