remove vLAN tags for DHCP discover script

[kbucheli]

The broadcast-dhcp-discover script fails to report/dissect DHCP offers if the packet has a vLAN tag:

[root@lx-test-01 ~]# nmap --script /usr/share/nmap/scripts/broadcast-dhcp-discover.nse 
Starting Nmap 7.92 ( https://nmap.org ) at 2024-04-18 13:09 CEST
WARNING: No targets were specified, so 0 hosts scanned.
Nmap done: 0 IP addresses (0 hosts up) scanned in 10.20 seconds
[root@lx-test-01 ~]#

Whereas with this fix the packet is properly dissected by removing the tag if one is found:

[root@lx-test-01 ~]# nmap --script /root/nmap/scripts/broadcast-dhcp-discover.nse
Starting Nmap 7.92 ( https://nmap.org ) at 2024-04-18 13:10 CEST
Pre-scan script results:
| broadcast-dhcp-discover: 
|   Response 1 of 1: 
|     Interface: enp98s0
|     IP Offered: 129.129.240.213
|     DHCP Message Type: DHCPOFFER
|     Server Identifier: 129.129.190.11
|     Subnet Mask: 255.255.255.0
|     Router: 129.129.240.1
|     Domain Name Server: 129.129.190.11, 129.129.230.11
|     Domain Name: psi.ch
|_    IP Address Lease Time: 1h00m00s
WARNING: No targets were specified, so 0 hosts scanned.
Nmap done: 0 IP addresses (0 hosts up) scanned in 10.20 seconds
[root@lx-test-01 ~]# 

And it still works fine on networks where there is no vLAN tag:

[root@lx-test-02 ~]# nmap --script /root/nmap/scripts/broadcast-dhcp-discover.nse 
Starting Nmap 7.92 ( https://nmap.org ) at 2024-04-18 13:15 CEST
Pre-scan script results:
| broadcast-dhcp-discover: 
|   Response 1 of 2: 
|     Interface: ens192
|     IP Offered: 10.129.160.247
|     DHCP Message Type: DHCPOFFER
|     Server Identifier: 129.129.190.11
|     Subnet Mask: 255.255.255.0
|     Router: 10.129.160.1
|     Domain Name Server: 129.129.190.11, 129.129.230.11
|     Domain Name: psi.ch
|     IP Address Lease Time: 1h00m00s
|   Response 2 of 2: 
|     Interface: ens192
|     IP Offered: 10.129.160.247
|     DHCP Message Type: DHCPOFFER
|     Server Identifier: 129.129.190.11
|     Subnet Mask: 255.255.255.0
|     Router: 10.129.160.1
|     Domain Name Server: 129.129.190.11, 129.129.230.11
|     Domain Name: psi.ch
|_    IP Address Lease Time: 1h00m00s
WARNING: No targets were specified, so 0 hosts scanned.
Nmap done: 0 IP addresses (0 hosts up) scanned in 10.14 seconds
[root@lx-test-02 ~]#