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The Arbitrum Nitro software includes a special
NodeInterface contract available at address
0xc8 that is only accessible via RPCs (it's not actually deployed on-chain, and thus can't be called by smart contracts). The way it works is that the node uses Geth's
InterceptRPCMessage hook to detect messages sent to the address
0xc8, and swaps out the message it's handling before deriving a transaction from it.
The reference page contains information about all methods available in the NodeInterface.