netdev - skb->protocol 0x300?!

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Date:   Fri, 1 Jul 2022 23:41:39 +0200 (CEST)
From:   Thorsten Glaser <t.glaser@...ent.de>
To:     netdev@...r.kernel.org
Subject: skb->protocol 0x300?!

Hi,

I’m needing to introspect packets to get the TCP/UDP port and
am writing packet parsing code (as apparently there is no easy
API to use). So far, so well, but my code starts with this…

	switch (skb->protocol) {
	case htons(ETH_P_IP): {
…
	    }
	case htons(ETH_P_IPV6): {
…
	    }
	default:
		// debug log
	}

… to figure out whether it’s an IP packet in the first place.

Now I’m getting 0x300 in skb->protocol for what tcpdump -X makes
me believe is a fragmented IP packet.

Why? And where is that documented? What does 0x300 there even mean?
And how can I figure out properly if a packet is IP, Legacy IP, or
something else?

Thanks,
//mirabilos
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