dhcp.dhcp_build lease_time set to 1 second #2197
Description
The broadcast-dhcp-discover.nse tends to return very low lease times from DHCP servers.
dhcpd3 (configured default lease time 86400s)
| broadcast-dhcp-discover:
| Response 1 of 1:
| IP Offered: 10.1.1.192
| DHCP Message Type: DHCPOFFER
| Server Identifier: 10.1.1.1
| IP Address Lease Time: 5m00s
| Subnet Mask: 255.255.255.0
| Router: 10.1.1.1
| Time Server: 10.1.1.9
| Domain Name Server: 10.1.1.9, 10.1.3.3
| NTP Servers: 10.1.1.9
| Renewal Time Value: 2m30s
|_ Rebinding Time Value: 4m22s
Windows Server 2016 (configured default lease time 8 days)
| broadcast-dhcp-discover:
| Response 1 of 1:
| IP Offered: 10.1.1.90
| DHCP Message Type: DHCPOFFER
| Subnet Mask: 255.255.255.0
| Renewal Time Value: 0s
| Rebinding Time Value: 0s
| IP Address Lease Time: 1s
|_ Server Identifier: 10.1.1.126
Sometimes windows ignores the requested lease time.
Windows Server 2016 (configured default lease time 8 days)
| broadcast-dhcp-discover:
| Response 1 of 1:
| IP Offered: 10.1.1.90
| DHCP Message Type: DHCPOFFER
| Subnet Mask: 255.255.255.0
| Renewal Time Value: 4d00h00m00s
| Rebinding Time Value: 7d00h00m00s
| IP Address Lease Time: 8d00h00m00s
|_ Server Identifier: 10.1.1.126
dhcp.dhcp_build crafts a DHCPREQUEST packet that requests a 1 second lease time. If this line is commented out the request does not specify the lease and the server configured default lease is sent in the DHCPOFFER reply
packet = packet .. string.pack(">BBI4", 0x33, 4, lease_time or 1) -- Lease time