46 lines
1.5 KiB
Python
46 lines
1.5 KiB
Python
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"""
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ARP responder — answers chip ARP requests for the synthetic gateway
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and any IP the chip wants to talk to.
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The cyw43 lwIP stack will do an ARP lookup for the destination MAC of
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its first packet to a remote host. We don't run a real network — there's
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no real MAC behind the gateway IP. So we synthesize the gateway MAC
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(``GATEWAY_MAC``) for everything, exactly the way QEMU's slirp does.
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"""
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from __future__ import annotations
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from .consts import ARP_REPLY, ARP_REQUEST, GATEWAY_MAC, STA_IP, ip_to_bytes
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from .protocols import Arp, Ethernet, make_frame_arp
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class ArpResponder:
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"""Stateless. Decides whether to answer an ARP request, and how."""
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def handle(self, frame: Ethernet) -> bytes | None:
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try:
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arp = Arp.parse(frame.payload)
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except ValueError:
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return None
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if arp.opcode != ARP_REQUEST:
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# We don't track replies; they shouldn't happen anyway since
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# we never send ARP requests to the chip.
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return None
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# Only answer requests that come from our own STA.
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if bytes(arp.spa) != ip_to_bytes(STA_IP):
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return None
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target_ip = bytes(arp.tpa)
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# We answer for *any* IP — the only network the chip can reach
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# is via the gateway, so all IPs map to GATEWAY_MAC.
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reply = Arp(
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opcode=ARP_REPLY,
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sha=GATEWAY_MAC,
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spa=target_ip,
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tha=arp.sha,
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tpa=arp.spa,
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)
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return make_frame_arp(arp.sha, GATEWAY_MAC, reply)
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