DHCPINFORM spam if "Automatically configure empty options" is enabled in Buster

Bug #1863713 reported by demux
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This bug affects 1 person
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Raspbian
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Bug Description

If you enable "Automatically configure empty options" in Network Preferences GUI on Raspbian Buster, and have _any_ of the fields filled out, the client will start spamming the DHCP server with DHCPINFORM.

* Example Raspbian Buster config with only IPv4 Address filled in (to provoke this problem): https://i.imgur.com/JV4Rme0.png

* Example syslog server showing DHCP client and DHCP server communications: https://i.imgur.com/6dRW6z6.png (Deckard = pfSense DHCP server, Holden = Raspbian Buster client)

Note that if _all_ fields are empty, the DHCP client behaves normally, and will shut up after it gets its config.

After some research, it seems like you stop this misbehavior by letting the DHCP server reply with the string "\n" (incl. the double quotes) on DHCP option 252 . Which I have set up in my pfSense box: https://i.imgur.com/YM8lSHf.png

But Raspbian Buster is ignoring this, and keeps on spamming.

I see this behaviour on all my Raspbian Buster devices (both RPi devices, and x86 VBoxVM). Raspbian Stretch (both RPi devices, and x86 VBoxVM) does not display this misbehaviour.

Let me know if you need more info. This problem should be easy to reproduce, as I am confident this is not restricted to my own setups here.

---Buster--------------

pi@raspberry-buster-vm:~ $ uname -a
Linux raspberry-buster-vm 4.19.0-8-686 #1 SMP Debian 4.19.98-1 (2020-01-26) i686 GNU/Linux

pi@raspberry-buster-vm:~ $ dpkg -s libc6 | grep ^Version
Version: 2.28-10

journalctl | grep -Ei 'dhcp' shows this: https://pastebin.com/9XWQB1hi (this is from a different RPi3 system, start of log is being cut off at the VM system as I'm unable to capture the start, but the behaviour is identical)

/etc/dhcpcd.conf: https://pastebin.com/XCTvLSvJ

---Stretch--------------

pi@raspberry-vm:~ $ uname -a
Linux raspberry-vm 4.9.0-11-686 #1 SMP Debian 4.9.189-3+deb9u2 (2019-11-11) i686 GNU/Linux

pi@raspberry-vm:~ $ dpkg -s libc6 | grep ^Version
Version: 2.24-11+deb9u4

journalctl | grep -Ei 'dhcp' shows this: https://pastebin.com/LKNqJPw0

/etc/dhcpcd.conf: https://pastebin.com/HirYtzRW

demux (demux4555)
description: updated
tags: added: dhcpcd
description: updated
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