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insomniacslk / dhcp

DHCPv6 and DHCPv4 packet library, client and server written in Go
BSD 3-Clause "New" or "Revised" License
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What does the NumSeconds option stand for? #530

Open sebastiaopamplona opened 2 months ago

sebastiaopamplona commented 2 months ago

https://github.com/insomniacslk/dhcp/blob/f1cffa2c0c49cf0a0dddde82d51ff4e529a49f73/dhcpv4/dhcpv4.go#L64

By reading the DHCP RFC, I can't figure out what the field NumSeconds stands for.

I'm super interested in knowing this because I'm facing some flaky issues where with high load, it seems some UDP packets are dropped (my theory), by looking at the DHCP logs (I wrote a wrapper around this lib):

2024-04-20 11:27:28.136 2024-04-20T10:27:28.003604751Z stdout F DEB: [eno1] (mgmt+controlplane) -->> DHCPACK 0.0.0.0 01:01:01:01:01:01 10.175.137.22 renewal DHCPv4 Message | opcode: BootReply | hwtype: Ethernet | hopcount: 0 | transaction ID: 0xfebe4920 | num seconds: 0 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 10.175.137.22 | server IP: 10.175.136.253 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Subnet Mask: fffff800
2024-04-20 11:27:28.135 2024-04-20T10:27:28.000826672Z stdout F DEB: [eno1] (mgmt+controlplane) <<-- DHCPREQUEST 10.175.137.22 01:01:01:01:01:01 <nil>  DHCPv4 Message | opcode: BootRequest | hwtype: Ethernet | hopcount: 0 | transaction ID: 0xfebe4920 | num seconds: 0 | flags: Unicast (0x00) | client IP: 10.175.137.22 | your IP: 0.0.0.0 | server IP: 0.0.0.0 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    DHCP Message Type: REQUEST
2024-04-20 11:17:51.585 2024-04-20T10:17:51.421998303Z stdout F DEB: [eno1] (mgmt+controlplane) -->> DHCPACK 0.0.0.0 01:01:01:01:01:01 10.175.137.22 renewal DHCPv4 Message | opcode: BootReply | hwtype: Ethernet | hopcount: 0 | transaction ID: 0xfebe4920 | num seconds: 0 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 10.175.137.22 | server IP: 10.175.136.253 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Subnet Mask: fffff800
2024-04-20 11:17:51.585 2024-04-20T10:17:51.419534977Z stdout F DEB: [eno1] (mgmt+controlplane) <<-- DHCPREQUEST 0.0.0.0 01:01:01:01:01:01 10.175.137.22  DHCPv4 Message | opcode: BootRequest | hwtype: Ethernet | hopcount: 0 | transaction ID: 0xfebe4920 | num seconds: 0 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 0.0.0.0 | server IP: 0.0.0.0 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Requested IP Address: 10.175.137.22
2024-04-20 11:17:51.585 2024-04-20T10:17:51.419208119Z stdout F DEB: [eno1] (mgmt+controlplane) -->> DHCPOFFER 0.0.0.0 01:01:01:01:01:01 10.175.137.22  DHCPv4 Message | opcode: BootReply | hwtype: Ethernet | hopcount: 0 | transaction ID: 0xfebe4920 | num seconds: 0 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 10.175.137.22 | server IP: 10.175.136.253 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Subnet Mask: fffff800
2024-04-20 11:17:51.585 2024-04-20T10:17:51.418071975Z stdout F DEB: [eno1] (mgmt+controlplane) <<-- DHCPDISCOVER 0.0.0.0 01:01:01:01:01:01  DHCPv4 Message | opcode: BootRequest | hwtype: Ethernet | hopcount: 0 | transaction ID: 0xfebe4920 | num seconds: 0 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 0.0.0.0 | server IP: 0.0.0.0 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    DHCP Message Type: DISCOVER
2024-04-20 11:12:54.801 2024-04-20T10:12:54.79915605Z stdout  F DEB: [eno1] (mgmt+controlplane) -->> DHCPACK 0.0.0.0 01:01:01:01:01:01 10.175.137.22 renewal DHCPv4 Message | opcode: BootReply | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x485a3625 | num seconds: 0 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 10.175.137.22 | server IP: 10.175.136.253 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Subnet Mask: fffff800
2024-04-20 11:12:54.801 2024-04-20T10:12:54.796111834Z stdout F DEB: [eno1] (mgmt+controlplane) <<-- DHCPREQUEST 0.0.0.0 01:01:01:01:01:01 10.175.137.22  DHCPv4 Message | opcode: BootRequest | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x485a3625 | num seconds: 18 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 0.0.0.0 | server IP: 0.0.0.0 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Requested IP Address: 10.175.137.22
2024-04-20 11:12:51.293 2024-04-20T10:12:51.288099565Z stdout F DEB: [eno1] (mgmt+controlplane) -->> DHCPOFFER 0.0.0.0 01:01:01:01:01:01 10.175.137.22  DHCPv4 Message | opcode: BootReply | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x485a3625 | num seconds: 0 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 10.175.137.22 | server IP: 10.175.136.253 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Subnet Mask: fffff800

(!) unexpected DISCOVER after the OFFER, and it has num seconds: 10
2024-04-20 11:12:51.292 2024-04-20T10:12:51.284731773Z stdout F DEB: [eno1] (mgmt+controlplane) <<-- DHCPDISCOVER 0.0.0.0 01:01:01:01:01:01  DHCPv4 Message | opcode: BootRequest | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x485a3625 | num seconds: 10 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 0.0.0.0 | server IP: 0.0.0.0 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    DHCP Message Type: DISCOVER

2024-04-20 11:12:49.787 2024-04-20T10:12:49.553584078Z stdout F DEB: [eno1] (mgmt+controlplane) -->> DHCPOFFER 0.0.0.0 01:01:01:01:01:01 10.175.137.22  DHCPv4 Message | opcode: BootReply | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x485a3625 | num seconds: 0 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 10.175.137.22 | server IP: 10.175.136.253 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Subnet Mask: fffff800

(!) unexpected DISCOVER after the ACK, and it has num seconds: 4
2024-04-20 11:12:49.787 2024-04-20T10:12:49.552500635Z stdout F DEB: [eno1] (mgmt+controlplane) <<-- DHCPDISCOVER 0.0.0.0 01:01:01:01:01:01  DHCPv4 Message | opcode: BootRequest | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x485a3625 | num seconds: 4 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 0.0.0.0 | server IP: 0.0.0.0 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    DHCP Message Type: DISCOVER

2024-04-20 11:12:45.777 2024-04-20T10:12:45.735970726Z stdout F DEB: [eno1] (mgmt+controlplane) -->> DHCPACK 0.0.0.0 01:01:01:01:01:01 10.175.137.22 renewal DHCPv4 Message | opcode: BootReply | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x54d9f7a5 | num seconds: 0 | flags: Broadcast (0x8000) | client IP: 0.0.0.0 | your IP: 10.175.137.22 | server IP: 10.175.136.253 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Subnet Mask: fffff800
2024-04-20 11:12:45.776 2024-04-20T10:12:45.733277856Z stdout F DEB: [eno1] (mgmt+controlplane) <<-- DHCPREQUEST 0.0.0.0 01:01:01:01:01:01 10.175.137.22  DHCPv4 Message | opcode: BootRequest | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x54d9f7a5 | num seconds: 0 | flags: Broadcast (0x8000) | client IP: 0.0.0.0 | your IP: 0.0.0.0 | server IP: 0.0.0.0 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Requested IP Address: 10.175.137.22
2024-04-20 11:12:42.265 2024-04-20T10:12:42.15815988Z stdout  F DEB: [eno1] (mgmt+controlplane) -->> DHCPOFFER 0.0.0.0 01:01:01:01:01:01 10.175.137.22  DHCPv4 Message | opcode: BootReply | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x54d9f7a5 | num seconds: 0 | flags: Broadcast (0x8000) | client IP: 0.0.0.0 | your IP: 10.175.137.22 | server IP: 10.175.136.253 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Subnet Mask: fffff800
2024-04-20 11:12:42.265 2024-04-20T10:12:42.156941314Z stdout F DEB: [eno1] (mgmt+controlplane) <<-- DHCPDISCOVER 0.0.0.0 01:01:01:01:01:01  DHCPv4 Message | opcode: BootRequest | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x54d9f7a5 | num seconds: 0 | flags: Broadcast (0x8000) | client IP: 0.0.0.0 | your IP: 0.0.0.0 | server IP: 0.0.0.0 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    DHCP Message Type: DISCOVER
insomniacslk commented 2 months ago

According to RFC2131 section 2it is defined as follows:

Filled in by client, seconds elapsed since client began address acquisition or renewal process.

It's not used anywhere in this library, as it's always set to 0. I don't think it's related to the issue you're seeing. Do you see lost packets with tcpdump?

sebastiaopamplona commented 2 months ago

Thanks for the swift reply :)

So that makes a lot of sense since the client is sending another DHCPDISCOVER 4 seconds after the first DHCPACK.

Do you have any guess on what might be causing the client to send the DHCPDISCOVER again after 4 seconds? Could it be that the DHCPACK is not reaching the client?

insomniacslk commented 2 months ago

Could it be that the DHCPACK is not reaching the client?

Usually, yes, it depends on the client. You should verify that the server is sending the reply and that it's making its way back to the client. Using tcpdump on the server should be enough

sebastiaopamplona commented 2 months ago

Using tcpdump on the server should be enough

Thanks for the tip; I re-ran my tests and the ACK that I suspect is being dropped is showing on the pcap from tcpdump on the server:

suspected-ack-drop

I knew which IP was going to be leased to which mac, so I ran ping on the side to see when the IP is actually picked up (because I know eventually it will be picked up):

2024-04-20T13:28:33,304819126+00:00 64 bytes from 10.175.137.17: icmp_seq=304 ttl=64 time=0.101 ms
2024-04-20T13:28:32,280775642+00:00 64 bytes from 10.175.137.17: icmp_seq=303 ttl=64 time=0.120 ms
2024-04-20T13:28:31,256329346+00:00 64 bytes from 10.175.137.17: icmp_seq=302 ttl=64 time=0.096 ms
2024-04-20T13:28:30,232135206+00:00 64 bytes from 10.175.137.17: icmp_seq=301 ttl=64 time=0.093 ms
2024-04-20T13:28:29,208345975+00:00 64 bytes from 10.175.137.17: icmp_seq=300 ttl=64 time=0.096 ms
2024-04-20T13:28:28,185493237+00:00 64 bytes from 10.175.137.17: icmp_seq=299 ttl=64 time=0.109 ms
2024-04-20T13:28:16,925263459+00:00 From 10.175.136.253 icmp_seq=287 Destination Host Unreachable
2024-04-20T13:28:16,923468680+00:00 From 10.175.136.253 icmp_seq=286 Destination Host Unreachable
2024-04-20T13:28:16,921300666+00:00 From 10.175.136.253 icmp_seq=285 Destination Host Unreachable
2024-04-20T13:28:13,856348517+00:00 From 10.175.136.253 icmp_seq=284 Destination Host Unreachable
2024-04-20T13:28:13,854944842+00:00 From 10.175.136.253 icmp_seq=283 Destination Host Unreachable
2024-04-20T13:28:13,853319968+00:00 From 10.175.136.253 icmp_seq=282 Destination Host Unreachable

Around the timestamp of the first ACK (13:28:19) the IP is still not pingable and the client sends another DISCOVER (let's call this retry DISCOVER) 4 seconds after the last message (DHCPREQUEST at 13:28:19).

When I had a look at the logs of my DHCP server, I noticed retry DISCOVER had num seconds: 4:

2024-04-20T13:43:11.31803562Z  -->> DHCPACK 0.0.0.0 01:01:01:01:01:01 10.175.137.17 renewal DHCPv4 Message | opcode: BootReply | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x2d878c1f | num seconds: 0 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 10.175.137.17 | server IP: 10.175.136.253 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Subnet Mask: fffff800
2024-04-20T13:43:11.315012534Z <<-- DHCPREQUEST 10.175.137.17 01:01:01:01:01:01 <nil>  DHCPv4 Message | opcode: BootRequest | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x2d878c1f | num seconds: 0 | flags: Unicast (0x00) | client IP: 10.175.137.17 | your IP: 0.0.0.0 | server IP: 0.0.0.0 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    DHCP Message Type: REQUEST
2024-04-20T13:33:37.314792609Z -->> DHCPACK 0.0.0.0 01:01:01:01:01:01 10.175.137.17 renewal DHCPv4 Message | opcode: BootReply | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x2d878c1f | num seconds: 0 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 10.175.137.17 | server IP: 10.175.136.253 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Subnet Mask: fffff800
2024-04-20T13:33:37.31196775Z  <<-- DHCPREQUEST 0.0.0.0 01:01:01:01:01:01 10.175.137.17  DHCPv4 Message | opcode: BootRequest | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x2d878c1f | num seconds: 0 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 0.0.0.0 | server IP: 0.0.0.0 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Requested IP Address: 10.175.137.17
2024-04-20T13:33:37.311804205Z -->> DHCPOFFER 0.0.0.0 01:01:01:01:01:01 10.175.137.17  DHCPv4 Message | opcode: BootReply | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x2d878c1f | num seconds: 0 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 10.175.137.17 | server IP: 10.175.136.253 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Subnet Mask: fffff800
2024-04-20T13:33:37.310294661Z <<-- DHCPDISCOVER 0.0.0.0 01:01:01:01:01:01  DHCPv4 Message | opcode: BootRequest | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x2d878c1f | num seconds: 0 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 0.0.0.0 | server IP: 0.0.0.0 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    DHCP Message Type: DISCOVER
2024-04-20T13:28:28.140622434Z -->> DHCPACK 0.0.0.0 01:01:01:01:01:01 10.175.137.17 renewal DHCPv4 Message | opcode: BootReply | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x45eba95d | num seconds: 0 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 10.175.137.17 | server IP: 10.175.136.253 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Subnet Mask: fffff800
2024-04-20T13:28:28.137630163Z <<-- DHCPREQUEST 0.0.0.0 01:01:01:01:01:01 10.175.137.17  DHCPv4 Message | opcode: BootRequest | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x45eba95d | num seconds: 18 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 0.0.0.0 | server IP: 0.0.0.0 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Requested IP Address: 10.175.137.17
2024-04-20T13:28:24.623792011Z -->> DHCPOFFER 0.0.0.0 01:01:01:01:01:01 10.175.137.17  DHCPv4 Message | opcode: BootReply | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x45eba95d | num seconds: 0 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 10.175.137.17 | server IP: 10.175.136.253 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Subnet Mask: fffff800
2024-04-20T13:28:24.622407292Z <<-- DHCPDISCOVER 0.0.0.0 01:01:01:01:01:01  DHCPv4 Message | opcode: BootRequest | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x45eba95d | num seconds: 10 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 0.0.0.0 | server IP: 0.0.0.0 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    DHCP Message Type: DISCOVER
2024-04-20T13:28:22.895196511Z -->> DHCPOFFER 0.0.0.0 01:01:01:01:01:01 10.175.137.17  DHCPv4 Message | opcode: BootReply | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x45eba95d | num seconds: 0 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 10.175.137.17 | server IP: 10.175.136.253 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Subnet Mask: fffff800

(!2) (retry DISCOVER) num seconds is 4, which is the time since the last message (DHCPREQUEST at 13:28:19)
2024-04-20T13:28:22.893984593Z <<-- DHCPDISCOVER 0.0.0.0 01:01:01:01:01:01  DHCPv4 Message | opcode: BootRequest | hwtype: Ethernet | hopcount: 0 | transaction ID: 0x45eba95d | num seconds: 4 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 0.0.0.0 | server IP: 0.0.0.0 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    DHCP Message Type: DISCOVER

(!1) i suspect this DHCPACK did not reach the client
2024-04-20T13:28:19.077922738Z -->> DHCPACK 0.0.0.0 01:01:01:01:01:01 10.175.137.17 renewal DHCPv4 Message | opcode: BootReply | hwtype: Ethernet | hopcount: 0 | transaction ID: 0xc3bf6768 | num seconds: 0 | flags: Broadcast (0x8000) | client IP: 0.0.0.0 | your IP: 10.175.137.17 | server IP: 10.175.136.253 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Subnet Mask: fffff800
2024-04-20T13:28:19.074780638Z <<-- DHCPREQUEST 0.0.0.0 01:01:01:01:01:01 10.175.137.17  DHCPv4 Message | opcode: BootRequest | hwtype: Ethernet | hopcount: 0 | transaction ID: 0xc3bf6768 | num seconds: 0 | flags: Broadcast (0x8000) | client IP: 0.0.0.0 | your IP: 0.0.0.0 | server IP: 0.0.0.0 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Requested IP Address: 10.175.137.17
2024-04-20T13:28:15.518372064Z -->> DHCPOFFER 0.0.0.0 01:01:01:01:01:01 10.175.137.17  DHCPv4 Message | opcode: BootReply | hwtype: Ethernet | hopcount: 0 | transaction ID: 0xc3bf6768 | num seconds: 0 | flags: Broadcast (0x8000) | client IP: 0.0.0.0 | your IP: 10.175.137.17 | server IP: 10.175.136.253 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Subnet Mask: fffff800

2024-04-20T13:28:15.517139138Z <<-- DHCPDISCOVER 0.0.0.0 01:01:01:01:01:01  DHCPv4 Message | opcode: BootRequest | hwtype: Ethernet | hopcount: 0 | transaction ID: 0xc3bf6768 | num seconds: 0 | flags: Broadcast (0x8000) | client IP: 0.0.0.0 | your IP: 0.0.0.0 | server IP: 0.0.0.0 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    DHCP Message Type: DISCOVER
2024-04-20T13:23:07.595853391Z -->> DHCPACK 0.0.0.0 01:01:01:01:01:01 10.175.137.17 renewal DHCPv4 Message | opcode: BootReply | hwtype: Ethernet | hopcount: 0 | transaction ID: 0xa43b020b | num seconds: 0 | flags: Unicast (0x00) | client IP: 0.0.0.0 | your IP: 10.175.137.17 | server IP: 10.175.136.253 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    Subnet Mask: fffff800
2024-04-20T13:23:07.592484438Z <<-- DHCPREQUEST 10.175.137.17 01:01:01:01:01:01 <nil>  DHCPv4 Message | opcode: BootRequest | hwtype: Ethernet | hopcount: 0 | transaction ID: 0xa43b020b | num seconds: 0 | flags: Unicast (0x00) | client IP: 10.175.137.17 | your IP: 0.0.0.0 | server IP: 0.0.0.0 | gateway IP: 0.0.0.0 | client MAC: 01:01:01:01:01:01 | server hostname:  | bootfile name:   options:    DHCP Message Type: REQUEST

It looks to me like this num seconds: 4 matches the time between the last DHCP message the client received.

Could it be that my only option is to figure out a way to run tcpdump on the client, to make sure the ACK got there? Or is there a flaw in my analysis 🤔