Why does the Internet disappear after enabling IP Forwarding?

Question

linux

Hello! The problem is this - there is a machine on the local network. I connect to it (from another machine on the local network) via eth0 via ssh. The machine accesses the Internet through wlan0.
If I prescribe echo 1 > /proc/sys/net/ipv4/ip_forwardthen the Internet disappears. Ifconfig gives the following picture:

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.10.100  netmask 255.255.255.0  broadcast 192.168.10.255
        inet6 fe80::ba27:ebff:fea4:63d4  prefixlen 64  scopeid 0x20<link>
        ether b8:27:eb:a4:63:d4  txqueuelen 1000  (Ethernet)
        RX packets 1053  bytes 149557 (146.0 KiB)
        RX errors 0  dropped 1  overruns 0  frame 0
        TX packets 1351  bytes 1180716 (1.1 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1  (Local Loopback)
        RX packets 347  bytes 28129 (27.4 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 347  bytes 28129 (27.4 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

wlan0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.10.88  netmask 255.255.255.0  broadcast 192.168.10.255
        inet6 fe80::74cf:8cb0:cba9:2c6b  prefixlen 64  scopeid 0x20<link>
        ether 98:de:d0:1a:ab:b1  txqueuelen 1000  (Ethernet)
        RX packets 314  bytes 62309 (60.8 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 25  bytes 3153 (3.0 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

That is, the wlan0 interface receives IP 192.168.10.88, but refuses to access the Internet through this interface.
etc/network/interfaces configuration

auto lo
iface lo inet loopback

iface eth0 inet static
address 192.168.10.100
netmask 255.255.255.0
auto eth0

At the same time, if I want to return everything back - echo 0 > /proc/sys/net/ipv4/ip_forwardthen it doesn’t change anything, there is no Internet, after a reboot it doesn’t either.
What can be wrong?
How to return everything back, so that the Internet would work?

Answer 1

I think the reason is that both interfaces are on the same network (192.168.10.0/24).
We need to check the routes.