树莓派4插入eth0网线后USB无线网卡wlan1热点客户端无法ping通接口问题求助

我在树莓派4（Raspbian系统）上成功用hostapd和isc-dhcpd把USB无线网卡wlan1配置成了AP（dhcpd只监听wlan1接口）。不管板载WiFi wlan0有没有连接到外网热点，wlan1的客户端都能ping通wlan1接口。IP转发和伪装都正常，当wlan0连上网时，wlan1的客户端也能正常访问外网，一切都很完美！

但是一插上eth0网线，情况就不对了：wlan1的客户端虽然显示已连接，但完全ping不通wlan1接口了，外网访问也跟着失效。

这太奇怪了！DHCP服务器给客户端分配的还是之前的有效IP，wlan1的静态IP也没变（eth0和wlan1都是静态IP）。树莓派本地能ping通wlan1，但外部客户端就是不行。拔掉eth0网线后，一切又恢复正常了。

到底哪里出问题了？为什么连接口都ping不通了？已经开启了sysctl IPv4转发。

网络接口信息 #

$ ip link
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
3: wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
4: wlan1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000

IP地址配置 #

$ ip addr
...
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
link/ether e4:5f:01:84:db:34 brd ff:ff:ff:ff:ff:ff
inet 10.8.0.2/8 brd 10.255.255.255 scope global noprefixroute eth0
valid_lft forever preferred_lft forever
...
3: wlan0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN group default qlen 1000
link/ether e4:5f:01:84:db:37 brd ff:ff:ff:ff:ff:ff
4: wlan1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
link/ether a0:f3:c1:23:2d:62 brd ff:ff:ff:ff:ff:ff
inet 10.8.5.1/8 brd 10.255.255.255 scope global noprefixroute wlan1
valid_lft forever preferred_lft forever
...

nftables规则 #

$nft list ruleset
...
table ip nat {
chain postrouting {
type nat hook postrouting priority srcnat; policy accept;
masquerade
}
}
...

hostapd配置文件内容 #

$ cat /etc/hostapd/hostapd.conf
interface=wlan1
#If this fails, try rt1871xdrv a
#driver=nl80211
# Name of the new network: best use the hostname
ssid=xxx
# Pick a channel not already in use
channel=1
# Change to b for older devices?
hw_mode=g
macaddr_acl=0
auth_algs=3
# Disable this to insure the AP is visible:
ignore_broadcast_ssid=0
wpa=2
wpa_passphrase=xxx
wpa_key_mgmt=WPA-PSK
wpa_pairwise=TKIP
rsn_pairwise=CCMP
#bridge=br0 # no bridge! We're using routing
country_code=JP

dhcpcd配置文件内容 #

$ cat /etc/dhcpcd.conf
...
interface eth0
static ip_address=10.8.0.2/8
static routers=10.8.0.1
static domain_name_servers=127.0.0.1

interface wlan1
static ip_address=10.8.5.1/8
nohook wpa_supplicant
static domain_name_servers=127.0.0.1

interface wlan0
static domain_name_servers=127.0.0.1

dhcpd配置文件内容 #

$ cat /etc/dhcp/dhcpd.conf
...
authoritative;

subnet 10.8.5.0 netmask 255.255.255.0 {
range 10.8.5.201 10.8.5.250;
option broadcast-address 10.8.5.255;
option routers 10.8.5.1;
default-lease-time 600;
max-lease-time 7200;
option domain-name "local";
option domain-name-servers 10.8.5.1;
}

host ares {
hardware ethernet 04:ea:56:ae:58:03;
fixed-address 10.8.5.20;
}

Windows 11客户端IP配置信息 #

$ ipconfig # on Windows 11 Client
...
Wireless LAN adapter Wi-Fi:
Connection-specific DNS Suffix  . : local
Description . . . . . . . . . . . : Intel(R) Dual Band Wireless-AC 8265
Physical Address. . . . . . . . . : 04-EA-56-AE-58-03
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
Link-local IPv6 Address . . . . . : fe80::9300:7ccf:3e03:eaf9%17(Preferred)
IPv4 Address. . . . . . . . . . . : 10.8.5.20(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Lease Obtained. . . . . . . . . . : Friday, 16 December 2022 22:56:38
Lease Expires . . . . . . . . . . : Friday, 16 December 2022 23:06:37
Default Gateway . . . . . . . . . : 10.8.5.1
DHCP Server . . . . . . . . . . . : 10.8.5.1
DHCPv6 IAID . . . . . . . . . . . : 168094294
DHCPv6 Client DUID. . . . . . . . : 00-01-00-01-26-1E-05-25-48-2A-E3-3A-C9-F0
DNS Servers . . . . . . . . . . . : 10.8.5.1
NetBIOS over Tcpip. . . . . . . . : Enabled
...

备注：内容来源于stack exchange，提问作者Arakis