For example, if you want to redirect all traffic to lan device which ip address is 192.168.8.123, edit /etc/config/firewall and add those lines:

config redirect               
        option target 'DNAT'   
        option src 'wan'
        option dest 'lan'
        option proto 'tcp udp'
        option dest_port '80'
        option name 'test'      
        option dest_ip '192.168.8.123'
        option src_dport '80'

For more details Firewall Redirect Docs.

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