In a rare case of almost running out of leads to investigate I recently had a situation where clients in some subnets could not obtain an address via DHCP. The clients were connected to a network switch (or over Wi-Fi) which in turn was connected to a network firewall which functioned as the DHCP server. Giving the client a static IP in the correct subnet worked fine, so it was something purely DHCP related. Packet captures on the firewall showed a DHCP Discover and DHCP Offer but no DHCP Request making it to the firewall. Packet captures on the client showed the DHCP Request making it out but clearly no response.
To add to the oddity of it all for some VLANs DHCP was working perfectly which unless it was a super niche bug ruled out firmware related issues.
Solution: Turns out it was an errant configuration of DHCP Snooping configured on the switch between the client and the firewall, to add to the hilarity only some VLANs had it enabled while others didn’t. On this specific vendor (Ruckus) the DHCP Snooping feature isn’t shown when requesting the configuration for a specific VLAN and it was only once the entire configuration was reviewed that this was discovered to be the issue. Since DHCP Snooping wasn’t a required/desired feature it was simply turned off, in some situations you may however prefer to correctly enable it with the authorised upstream ports added to the configuration.