1. 问题发生
在使用
virt-manager
启动虚拟机后，使用一段时间后突然虚拟机网络断开，并不知道是什么原因导致。
2. 解决问题
可以看到虚拟机是使用
NAT
模式连接主机网络的。
这里使用的是
virt-manager
创建虚拟机时提供的
default
网络配置
xml
文件。
首先查看
virt-manager
的网卡接口，如下图中的
virbr0
就是
KVM
为虚拟机创建的网卡接口，也是作为一个
switch
和
bridge
，将包分发到
KVM
中的各个虚拟机。
通过
virsh
重新启动
default
接口，提示信息显示，该接口是正常工作的。
通过
brctl
命令也可以看到
virbr0
网络接口也是在正常开启的。
到这里就很奇怪了，不知道该从和下手，在突然尝试将
libvirtd
服务重新启动时，看到启动时报错了。
imaginemiracle:~$ service libvitd status
● libvirtd.service - Virtualization daemon
Loaded: loaded (/lib/systemd/system/libvirtd.service; enabled; vendor preset: enabled)
Active: active (running) since Thu 2021-07-01 16:28:46 CST; 3s ago
TriggeredBy: ● libvirtd-admin.socket
● libvirtd-ro.socket
● libvirtd.socket
Docs: man:libvirtd(8)
https://libvirt.org
Main PID: 41796 (libvirtd)
Tasks: 19 (limit: 32768)
Memory: 35.2M
CGroup: /system.slice/libvirtd.service
├─19069 /usr/sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/lib/libvirt/libvirt_leaseshelper
├─19070 /usr/sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/lib/libvirt/libvirt_leaseshelper
└─41796 /usr/sbin/libvirtd
7月 01 16:28:46 Linux-wxn systemd[1]: Starting Virtualization daemon...
7月 01 16:28:46 Linux-wxn systemd[1]: Started Virtualization daemon.
7月 01 16:28:47 Linux-wxn libvirtd[41796]: libvirt version: 6.6.0, package: 1ubuntu3.5 (Christian Ehrhardt <[email protected]> Wed, 07 Apr 2021 13:33:46 +0200)
7月 01 16:28:47 Linux-wxn libvirtd[41796]: hostname: Linux-wxn
7月 01 16:28:47 Linux-wxn libvirtd[41796]: internal error: firewalld is set to use the nftables backend, but the required firewalld 'libvirt' zone is missing. Either set the firewalld backend to 'iptables', or ensure that firewalld has a 'libvirt' zone by upgrading firewalld to a version supporting rule priorities (0.7.0+) and/or rebuilding libvirt with --with-firewalld-zone
7月 01 16:28:47 Linux-wxn dnsmasq[19069]: read /etc/hosts - 7 addresses
7月 01 16:28:47 Linux-wxn dnsmasq[19069]: read /var/lib/libvirt/dnsmasq/default.addnhosts - 0 addresses
7月 01 16:28:47 Linux-wxn dnsmasq-dhcp[19069]: read /var/lib/libvirt/dnsmasq/default.hostsfile
可以看到这个错误是由于Linux的防火墙引起的，在网上找了很多解决方案却还是不能解决，于是直接将Linux的防火墙关闭，再次重启
libvirtd
服务错误解决，
KVM
中的虚拟机正常连接网络。
imaginemiracle:~$ sudo service firewalld stop
imaginemiracle:~$ sudo service libvitd restart
● libvirtd.service - Virtualization daemon
Loaded: loaded (/lib/systemd/system/libvirtd.service; enabled; vendor preset: enabled)
Active: active (running) since Thu 2021-07-01 16:33:00 CST; 41min ago
TriggeredBy: ● libvirtd-admin.socket
● libvirtd-ro.socket
● libvirtd.socket
Docs: man:libvirtd(8)
https://libvirt.org
Main PID: 42261 (libvirtd)
Tasks: 21 (limit: 32768)
Memory: 40.8M
CGroup: /system.slice/libvirtd.service
├─19069 /usr/sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/lib/libvirt/libvirt_leaseshelper
├─19070 /usr/sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/lib/libvirt/libvirt_leaseshelper
└─42261 /usr/sbin/libvirtd
7月 01 16:33:00 Linux-wxn dnsmasq[19069]: read /etc/hosts - 7 addresses
7月 01 16:33:00 Linux-wxn dnsmasq[19069]: read /var/lib/libvirt/dnsmasq/default.addnhosts - 0 addresses
7月 01 16:33:00 Linux-wxn dnsmasq-dhcp[19069]: read /var/lib/libvirt/dnsmasq/default.hostsfile
7月 01 16:33:12 Linux-wxn libvirtd[42261]: libvirt version: 6.6.0, package: 1ubuntu3.5 (Christian Ehrhardt <[email protected]> Wed, 07 Apr 2021 13:33:46 >
7月 01 16:33:12 Linux-wxn libvirtd[42261]: hostname: Linux-wxn
7月 01 16:33:22 Linux-wxn dnsmasq-dhcp[19069]: DHCPREQUEST(virbr0) 192.168.122.245 52:54:00:11:c0:55
7月 01 16:33:22 Linux-wxn dnsmasq-dhcp[19069]: DHCPACK(virbr0) 192.168.122.245 52:54:00:11:c0:55 Linux-IM
7月 01 17:03:22 Linux-wxn dnsmasq-dhcp[19069]: DHCPREQUEST(virbr0) 192.168.122.245 52:54:00:11:c0:55
7月 01 17:03:22 Linux-wxn dnsmasq-dhcp[19069]: DHCPACK(virbr0) 192.168.122.245 52:54:00:11:c0:55 Linux-IM
3. Error starting domain: Requested operation is not valid: network ‘default’ is not active
imaginemiracle:~$ virsh net-start default
error: Failed to start network default
error: internal error: firewalld is set to use the nftables backend, but the required firewalld 'libvirt' zone is missing. Either set the firewalld backend to 'iptables', or ensure that firewalld has a 'libvirt' zone by upgrading firewalld to a version supporting rule priorities (0.7.0+) and/or rebuilding libvirt with --with-firewalld-zone
imaginemiracle:~$ sudo service firewalld stop
imaginemiracle:~$ sudo service libvirtd restart
imaginemiracle:~$ virsh net-start default
Network default started
总结
由于防火墙的原因导致虚拟机无法通过
NAT
连接主机网络，关闭防火墙即可，到此算是解决了该问题吧！(由于不明发生的原因，防火墙为什么导致libvirtd服务启动default网络失败，只能算作是表面上的解决)