因为觉得openwrt的18的配置检查功能很费时，特别是遇到ar93xx慢的真可以，所以决定从18.0.1降回到17.0.6上
先把18.0.1的配置backup出来，然后刷17.0.6，再把backup导入回去，结果路由器能起来，但就是LAN无法上网，各种检查后发现原来18导出来的文件中/etc/sysctl.conf里面是空的，需要手工在17.0.6上补回去：
kernel.panic=3
kernel.core_pattern=/tmp/%e.%t.%p.%s.core
net.ipv4.conf.default.arp_ignore=1
net.ipv4.conf.all.arp_ignore=1
net.ipv4.ip_forward=1
net.ipv4.icmp_echo_ignore_broadcasts=1
net.ipv4.icmp_ignore_bogus_error_responses=1
net.ipv4.igmp_max_memberships=100
net.ipv4.tcp_fin_timeout=30
net.ipv4.tcp_keepalive_time=120
net.ipv4.tcp_syncookies=1
net.ipv4.tcp_timestamps=1
net.ipv4.tcp_sack=1
net.ipv4.tcp_dsack=1
net.ipv6.conf.default.forwarding=1
net.ipv6.conf.all.forwarding=1
net.netfilter.nf_conntrack_acct=1
net.netfilter.nf_conntrack_checksum=0
net.netfilter.nf_conntrack_max=16384
net.netfilter.nf_conntrack_tcp_timeout_established=7440
net.netfilter.nf_conntrack_udp_timeout=60
net.netfilter.nf_conntrack_udp_timeout_stream=180
# disable bridge firewalling by default
net.bridge.bridge-nf-call-arptables=0
net.bridge.bridge-nf-call-ip6tables=0
net.bridge.bridge-nf-call-iptables=0
补回去后重启openwrt，马上回复青春了