Android cannot forward packets Target:
Perform IP forwarding for devices within the same local network. (The real target is transparent proxy but the IP forwarding is a prerequisite)
Network setup:
All devices are in the same local network 192.168.0.0/16.

main router 192.168.1.1
Android phone 192.168.2.1 (network interface wlan0)
computer 192.168.2.10 gateway set as 192.168.2.1
others like Raspberry Pi at 192.168.1.11

Android setup:

Data disabled, only WiFi connects.
rooted Android 10 (Pixel 2 XL)
enabled ip forwarding by sysctl -w net.ipv4.ip_forward=1
netfilter: on all tables of nat, mangle, filter and raw, run iptables -F -t &lt;table_name&gt; and iptables -X -t &lt;table_name&gt;
rp_filter: Disable by sysctl -w net.ipv4.conf.all.rp_filter=0 and sysctl -w net.ipv4.conf.wlan0.rp_filter=0

Result:

the computer cannot access to the Internet, ping, dig, browser, nothing works.
the computer connects to local devices without any problem (like ping router 192.168.1.1, SSH to the Raspberry Pi, or use FireFox through the SOCK5 proxy hosted on the Pi).
Android phone connects to the Internet perfectly.

Debug:

using iptables -t &lt;table_name&gt; -L -v to show packet count, all chains have some packets, *except the FORWARD chain of both raw, mangle tables shows Chain FORWARD (policy ACCEPT 0 packets, 0 bytes). After some googling, rp_filter is the suspect but I'm sure it's turned off.
Thanks to @Irfan Latif, I found the solution myself.
It's right that packets are sent to the last unreachable rule of ip rule, because they fail to match any previous rules:
# result of `ip rule`
... (no rules match)
22000:  from all fwmark 0x0/0xffff iif lo lookup wlan0 
32000:  from all unreachable

So I added a rule to route incoming traffic from local network to route table wlan0 by:
ip rule add from 192.168.0.0/16 iif wlan0 table wlan0