No v4 address/routes on first start after boot

[klausman]

OS is Debian testing (Trixie), dhcpcd is v10.0.6-1

After a reboot, for some reason, dhcpcd does not receive a DHCP (v4) lease, while DHCPv6 works fine.

I've waited for several minutes for a lease to show up to no avail.

But if I restart dhcpcd manually, it gets a v4 lease within <1s. This only changed with a recent reinstall of the router (DUID and config remained the same), so I am very puzzled as to what is going on.

What's the best way to debug this? Mind that this is my Internet router, so reboot-cycling it for hours on end is rather disruptive. I am confident with using tshark or tcpdump and similar, but my knowledge of dhcpcd and DHCP-the-protocols is limited.

[klausman]

This was fixed (I guess) by replacing the cheapo USB Ethernet adapter with a proper PCI-E NIC. My suspicion is that on startup (as in just after link negotiation) things are a bit racy in the hardware, and the first DHCPv4 request gets lost "somehow". Might be something different, but not worth debugging any further for me.