Second stage PXE ignore DHCP offer in Pi 3B

Start by comparing the Pi 3B and Pi 3B+ second-stage PXE behavior using the supplied UART output, DHCP server log, and tcpdump observations. Check the bootcode.bin version and commit identified in the report, then trace why the second-stage DHCP query ignores the offer. Done means the Pi 3B accepts the second-stage DHCP offer and continues PXE booting.

I am PXE booting a Pi 3B without any SD card or disk. Sometimes the ROM-program successfully downloads a bootcode.bin, but then when ROM hands over, the second stage DHCP query fails. I'm working around the network flush bug with dummy broadcasts so that's likely not the issue(?). Plus, the problem is in a recent bootcode.bin where I guess the flush bug has been fixed? Only my Pi 3B seems affected though. A parallel Pi 3B+ works.

Here is the second stage serial UART console:

Raspberry Pi Bootcode
USB ethernet boot
Sending DHCP request
Waiting for dhcp_reply
Sending DHCP request
Waiting for dhcp_reply
Sending DHCP request
Waiting for dhcp_reply
Sending DHCP request
Waiting for dhcp_reply
Sending DHCP request
Waiting for dhcp_reply
Sending DHCP request
Waiting for dhcp_reply

According to the console log bootcode.bin query DHCP endlessly (it never stops). But this is not true. Network querys are only sent for about one minute and then ends, as can be seen below in the DHCP server log. (Verified with tcpdump as well).

Mar 25 04:28:54 dhcpd: DHCPDISCOVER from b8:27:eb:5d:5d:74 via eth1
Mar 25 04:28:59 dhcpd: DHCPOFFER on 192.168.1.253 to b8:27:eb:5d:5d:74 via eth1

Mar 25 04:28:59 xinetd[2024]: START: tftp from=192.168.1.253
Mar 25 04:28:59 in.tftpd[31426]: RRQ from 192.168.1.253 filename bootcode.bin
Mar 25 04:28:59 in.tftpd[31427]: RRQ from 192.168.1.253 filename bootsig.bin

Mar 25 04:29:04 dhcpd: DHCPDISCOVER from b8:27:eb:5d:5d:74 via eth1
Mar 25 04:29:04 dhcpd: DHCPOFFER on 192.168.1.253 to b8:27:eb:5d:5d:74 via eth1

Mar 25 04:29:09 dhcpd: DHCPDISCOVER from b8:27:eb:5d:5d:74 via eth1
Mar 25 04:29:09 dhcpd: DHCPOFFER on 192.168.1.253 to b8:27:eb:5d:5d:74 via eth1

Mar 25 04:29:14 dhcpd: DHCPDISCOVER from b8:27:eb:5d:5d:74 via eth1
Mar 25 04:29:14 dhcpd: DHCPOFFER on 192.168.1.253 to b8:27:eb:5d:5d:74 via eth1

Mar 25 04:29:19 dhcpd: DHCPDISCOVER from b8:27:eb:5d:5d:74 via eth1
Mar 25 04:29:19 dhcpd: DHCPOFFER on 192.168.1.253 to b8:27:eb:5d:5d:74 via eth1

Mar 25 04:29:24 dhcpd: DHCPDISCOVER from b8:27:eb:5d:5d:74 via eth1
Mar 25 04:29:24 dhcpd: DHCPOFFER on 192.168.1.253 to b8:27:eb:5d:5d:74 via eth1

Mar 25 04:29:29 dhcpd: DHCPDISCOVER from b8:27:eb:5d:5d:74 via eth1
Mar 25 04:29:29 dhcpd: DHCPOFFER on 192.168.1.253 to b8:27:eb:5d:5d:74 via eth1

Mar 25 04:29:34 dhcpd: DHCPDISCOVER from b8:27:eb:5d:5d:74 via eth1
Mar 25 04:29:34 dhcpd: DHCPOFFER on 192.168.1.253 to b8:27:eb:5d:5d:74 via eth1

Mar 25 04:29:39 dhcpd: DHCPDISCOVER from b8:27:eb:5d:5d:74 via eth1
Mar 25 04:29:39 dhcpd: DHCPOFFER on 192.168.1.253 to b8:27:eb:5d:5d:74 via eth1

Mar 25 04:29:44 dhcpd: DHCPDISCOVER from b8:27:eb:5d:5d:74 via eth1
Mar 25 04:29:44 dhcpd: DHCPOFFER on 192.168.1.253 to b8:27:eb:5d:5d:74 via eth1

Mar 25 04:29:49 dhcpd: DHCPDISCOVER from b8:27:eb:5d:5d:74 via eth1
Mar 25 04:29:49 dhcpd: DHCPOFFER on 192.168.1.253 to b8:27:eb:5d:5d:74 via eth1

Mar 25 04:29:54 dhcpd: DHCPDISCOVER from b8:27:eb:5d:5d:74 via eth1
Mar 25 04:29:54 dhcpd: DHCPOFFER on 192.168.1.253 to b8:27:eb:5d:5d:74 via eth1

Versions used:

• bootcode.bin: 1.20190215 (98997f363e3683ead4f50c37902169248628303a)
• ISC DHCP server: v3
• Pi 3B, board revision a02082