背景

树莓派3B自带一个有线口，一个无线网卡，需要使用有线口上外网，无线连接另一个局域网

试错过程

设置rc.local无效

通过等待一分钟删除和添加相关路由的方式无法实现 开机后依然是无线作为出口网卡

sleep 60 && 
route del -net 0.0.0.0/0 gw 172.18.1.254 2>/dev/null && 
route del -net 0.0.0.0/0 gw 172.18.2.254 2>/dev/null && 
route del -net 0.0.0.0/0 gw 172.18.3.254 2>/dev/null && 
route add -net 0.0.0.0/0 gw 172.18.1.254 metric 0 dev eth0 2>/dev/null && 
route del -net 0.0.0.0/0 gw 172.17.1.254 2>/dev/null && 
route del -net 0.0.0.0/0 gw 172.17.2.254

对应帖子是 https://www.jianshu.com/p/5d07d46cd434

正确方式

在/etc/network/interfaces设置有线网卡静态IP和网关，无线网卡自动获取IP地址即可，重启多次树莓派都满足需求。

pi@oldpi:~ $ sudo cat /etc/network/interfaces
# interfaces(5) file used by ifup(8) and ifdown(8)

# Please note that this file is written to be used with dhcpcd
# For static IP, consult /etc/dhcpcd.conf and man dhcpcd.conf

# Include files from /etc/network/interfaces.d:
source-directory /etc/network/interfaces.d

auto eth0
allow-hotplug eth0
#iface eth0 inet dhcp
iface eth0 inet static
address 172.18.1.100
netmask 255.255.255.0
gateway 172.18.1.254

auto eth1
allow-hotplug eth1
iface eth1 inet dhcp

auto eth2
allow-hotplug eth2
iface eth2 inet dhcp

auto wlan0
allow-hotplug wlan0
iface wlan0 inet dhcp
	wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

有多个有线网卡的我还没有测试过，不过理论上可以的。 路由设置也正常，如下，

pi@oldpi:~ $ route -n 
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         172.18.1.254    0.0.0.0         UG    0      0        0 eth0
172.18.1.0      0.0.0.0         255.255.255.0   U     0      0        0 eth0
172.17.1.0      0.0.0.0         255.255.255.0   U     0      0        0 wlan0
pi@oldpi:~ $ 
pi@oldpi:~ $ 
pi@oldpi:~ $ date 
Tue, 02 Nov 2021 09:13:48 +0800