字符界面配置虚拟机好像有点难，所以还是先用vnc连接服务器：https://blog.csdn.net/qq_41961459/article/details/112909800
然后看这个：https://my.oschina.net/kelvinxupt/blog/265108
其中软件安装的部分不全，要装qemu-kvm
sudo apt install qemu-kvm
解决root窗口无法弹出的问题
在执行
sudo qemu-system-x86_64 -m 4096 -enable-kvm centos.img -cdrom ~/Downloads/CentOS-8.2.2004-x86_64-minimal.iso
的时候，会提示
X11 connection rejected because of wrong authentication.
Unable to init server: Could not connect: Connection refused
gtk initialization failed
看这篇文章可以解决：https://blog.csdn.net/qq_41961459/article/details/112916589ctrl+alt+g可以取消捕获。
联网
虚拟机一开始是没有网络的。
桥接网卡
参考：https://blog.csdn.net/u014022631/article/details/53411557配置网络。
sudo apt install bridge-utils uml-utilities
先看自己的机器的网卡编号
ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eno1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
link/ether 30:9c:23:f0:33:40 brd ff:ff:ff:ff:ff:ff
inet 10.249.41.236/21 brd 10.249.47.255 scope global dynamic noprefixroute eno1
valid_lft 40668sec preferred_lft 40668sec
inet6 fe80::140d:db62:feb5:b904/64 scope link noprefixroute
valid_lft forever preferred_lft forever
可以看到我的编号是eno1。
由于操作网卡有可能导致连不上网，而连不上网的话服务器就失控了，所以最好是先设置一个定时重启，完成配置之后检测一下有没有网，有网则取消重启，否则就立即重启。由于操作过程有一段时间连不上网，所以需要把下面的保存为脚本运行：
set -e
if [ ! $1 ]; then
echo Usage: $0 net_dev
exit
fi
shutdown -r 1
brctl addbr br0
brctl addif br0 $1
brctl stp br0 off
brctl setfd br0 1
brctl sethello br0 1
ifconfig br0 0.0.0.0 promisc up
ifconfig $1 0.0.0.0 promisc up
dhclient br0
#brctl show br0
#brctl showstp br0
tunctl -t tap0 -u root # 创建一个tap0接口，只允许root用户访问
brctl addif br0 tap0 # 在虚拟网桥中增加一个tap0接口
ifconfig tap0 0.0.0.0 promisc up # 打开tap0接口
#brctl showstp br0 # 显示br0的各个接口
if ping -c 1 baidu.com; then
shutdown -c
echo Shutdown canceled.
else
shutdown -r now
fi
奇怪的是，在我的服务器上第一次失败了，但是重启之后第二次就成功了。
然后用下面的命令重新启动虚拟机：
sudo qemu-system-x86_64 -m 4096 -enable-kvm centos.img -net nic -net tap,ifname=tap0,script=no,downscript=no
如果虚拟机是centos的话，还需要在虚拟机中执行一下
dhclient
才能拿到ip。
这个相当于把虚拟机桥接到外面去了。如果外面的网络要登陆才能上网的话，还需要登陆一下。
NAT
如果装的虚拟机没有GUI，但是外面的网络又需要在网页上登陆才能使用，那桥接就不太可行了，这时要用NAT模式，使虚拟机直接使用主机的网络。
https://wiki.qemu.org/Documentation/Networking/NAT
把下面的保存为ifup_nat.sh
#!/bin/sh
#
# Copyright IBM, Corp. 2010
#
# Authors:
# Anthony Liguori <aliguori@us.ibm.com>
#
# This work is licensed under the terms of the GNU GPL, version 2. See
# the COPYING file in the top-level directory.
# Set to the name of your bridge
BRIDGE=br0
# Network information
NETWORK=192.168.53.0
NETMASK=255.255.255.0
GATEWAY=192.168.53.1
DHCPRANGE=192.168.53.2,192.168.53.254
# Optionally parameters to enable PXE support
TFTPROOT=
BOOTP=
do_brctl() {
brctl "$@"
}
do_ifconfig() {
ifconfig "$@"
}
do_dd() {
dd "$@"
}
do_iptables_restore() {
iptables-restore "$@"
}
do_dnsmasq() {
dnsmasq "$@"
}
check_bridge() {
if do_brctl show | grep "^$1" > /dev/null 2> /dev/null; then
return 1
else
return 0
fi
}
create_bridge() {
do_brctl addbr "$1"
do_brctl stp "$1" off
do_brctl setfd "$1" 0
do_ifconfig "$1" "$GATEWAY" netmask "$NETMASK" up
}
enable_ip_forward() {
echo 1 | do_dd of=/proc/sys/net/ipv4/ip_forward > /dev/null
}
add_filter_rules() {
do_iptables_restore <<EOF
# Generated by iptables-save v1.3.6 on Fri Aug 24 15:20:25 2007
*nat
:PREROUTING ACCEPT [61:9671]
:POSTROUTING ACCEPT [121:7499]
:OUTPUT ACCEPT [132:8691]
-A POSTROUTING -s $NETWORK/$NETMASK -j MASQUERADE
COMMIT
# Completed on Fri Aug 24 15:20:25 2007
# Generated by iptables-save v1.3.6 on Fri Aug 24 15:20:25 2007
*filter
:INPUT ACCEPT [1453:976046]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [1605:194911]
-A INPUT -i $BRIDGE -p tcp -m tcp --dport 67 -j ACCEPT
-A INPUT -i $BRIDGE -p udp -m udp --dport 67 -j ACCEPT
-A INPUT -i $BRIDGE -p tcp -m tcp --dport 53 -j ACCEPT
-A INPUT -i $BRIDGE -p udp -m udp --dport 53 -j ACCEPT
-A FORWARD -i $1 -o $1 -j ACCEPT
-A FORWARD -s $NETWORK/$NETMASK -i $BRIDGE -j ACCEPT
-A FORWARD -d $NETWORK/$NETMASK -o $BRIDGE -m state --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -o $BRIDGE -j REJECT --reject-with icmp-port-unreachable
-A FORWARD -i $BRIDGE -j REJECT --reject-with icmp-port-unreachable
COMMIT
# Completed on Fri Aug 24 15:20:25 2007
EOF
}
start_dnsmasq() {
do_dnsmasq \
--strict-order \
--except-interface=lo \
--interface=$BRIDGE \
--listen-address=$GATEWAY \
--bind-interfaces \
--dhcp-range=$DHCPRANGE \
--conf-file="" \
--pid-file=/var/run/qemu-dnsmasq-$BRIDGE.pid \
--dhcp-leasefile=/var/run/qemu-dnsmasq-$BRIDGE.leases \
--dhcp-no-override \
${TFTPROOT:+"--enable-tftp"} \
${TFTPROOT:+"--tftp-root=$TFTPROOT"} \
${BOOTP:+"--dhcp-boot=$BOOTP"}
}
setup_bridge_nat() {
if check_bridge "$1" ; then
create_bridge "$1"
enable_ip_forward
add_filter_rules "$1"
start_dnsmasq "$1"
fi
}
setup_bridge_vlan() {
if check_bridge "$1" ; then
create_bridge "$1"
start_dnsmasq "$1"
fi
}
setup_bridge_nat "$BRIDGE"
if test "$1" ; then
do_ifconfig "$1" 0.0.0.0 up
do_brctl addif "$BRIDGE" "$1"
fi
并给执行权限：
chmod +x ifup_nat.sh
如果不给执行权限会报错(exit code 255)。
然后启动虚拟机：
sudo qemu-system-x86_64 -m 4096 -enable-kvm centos.img -net nic -net tap,script=ifup_nat.sh
这个方法我在家里的电脑上实验成功，但是在学校的服务器上失败了，不知道为什么。