DHCP requests with option 50 after adding a static mapping for the device.

ryan87 · Dec 24, 2016, 6:09 PM

Hi,

I've been running into this for a while now and I don't think the behavior is correct. I've only tested on Windows machines. I do the following:

Connect a device that uses DHCP and get an IP address
Find the MAC address and add a static DHCP mapping
Run ipconfig /release followed by ipconfig /renew

I always end up getting the old, non-static DHCP IP. For example, I tested on a Windows 10 VM with pfSense 2.3.2-RELEASE-p1 (amd64). After a fresh Windows 10 install, I was assigned the IP address 192.168.100.220. I logged in to pfSense and added a static DHCP mapping for the device. Then I release the IP:

11:40:55.633455 IP (tos 0x0, ttl 128, id 22267, offset 0, flags [none], proto UDP (17), length 328)
    192.168.100.220.68 > 192.168.100.254.67: [udp sum ok] BOOTP/DHCP, Request from 08:00:27:48:ce:81, length 300, xid 0xed9ed100, Flags [none] (0x0000)
          Client-IP 192.168.100.220
          Client-Ethernet-Address 08:00:27:48:ce:81
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Release
            Server-ID Option 54, length 4: 192.168.100.254
            Client-ID Option 61, length 7: ether 08:00:27:48:ce:81
            END Option 255, length 0
            PAD Option 0, length 0, occurs 41

Next I renew the IP. The request:

11:40:59.048937 IP (tos 0x0, ttl 128, id 15824, offset 0, flags [none], proto UDP (17), length 351)
    0.0.0.0.68 > 255.255.255.255.67: [udp sum ok] BOOTP/DHCP, Request from 08:00:27:48:ce:81, length 323, xid 0x52abb8af, Flags [none] (0x0000)
          Client-Ethernet-Address 08:00:27:48:ce:81
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Request
            Client-ID Option 61, length 7: ether 08:00:27:48:ce:81
            Requested-IP Option 50, length 4: 192.168.100.220
            Server-ID Option 54, length 4: 192.168.100.254
            Hostname Option 12, length 13: "netman-portal"
            FQDN Option 81, length 16: "netman-portal"
            Vendor-Class Option 60, length 8: "MSFT 5.0"
            Parameter-Request Option 55, length 13:
              Subnet-Mask, Default-Gateway, Domain-Name-Server, Domain-Name
              Router-Discovery, Static-Route, Vendor-Option, Netbios-Name-Server
              Netbios-Node, Netbios-Scope, Classless-Static-Route, Classless-Static-Route-Microsoft
              Option 252
            END Option 255, length 0

…and the reply:

11:40:59.049733 IP (tos 0x10, ttl 128, id 0, offset 0, flags [none], proto UDP (17), length 328)
    192.168.100.254.67 > 192.168.100.220.68: [udp sum ok] BOOTP/DHCP, Reply, length 300, xid 0x52abb8af, Flags [none] (0x0000)
          Your-IP 192.168.100.220
          Client-Ethernet-Address 08:00:27:48:ce:81
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: ACK
            Server-ID Option 54, length 4: 192.168.100.254
            Lease-Time Option 51, length 4: 7200
            Subnet-Mask Option 1, length 4: 255.255.255.0
            Default-Gateway Option 3, length 4: 192.168.100.254
            Domain-Name-Server Option 6, length 4: 192.168.100.254
            Domain-Name Option 15, length 17: "...[domain removed]..."
            END Option 255, length 0
            PAD Option 0, length 0, occurs 7

Why does the DHCP honor the Option 50 request? Shouldn't it ignore the request and assign the IP I've allocated via the static mapping?