SANS ISC: DNS and DHCP Recon using Powershell - SANS Internet Storm Center

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I recently had a client pose an interesting problem.  They wanted to move all their thin clients to a separate VLAN.  In order to do that, I needed to identify which switch port each was on.  Since there were several device vendors involved, I couldn't use OUI portion of the MAC.  Fortunately, they were using only a few patterns in their thin client hostnames, so that gives me an "in".

Great you say, use "nmap -sn", sweep for the names, get the MAC addresses and map those to switch ports - easy right?  Yup, it would be, except that this won't tell me about any devices that are powered off at the time.  Which got me to thinking about DNS and DHCP - and how you could use these methods to "mine" Microsoft DHCP and DNS databases for Recon info in a much stealthier (and more complete) way than sweeping the network would be.

DNS Approach

We can get part of what we need out of DNS - first, let's dump DNS for all registered IPs:

$dns = Get-WmiObject -Class MicrosoftDNS_AType -NameSpace Root\MicrosoftDNS -ComputerName DC01  -Filter "DomainName = 'example.com'" -Credential (Get-Credential)

(the "Get-Credential" cmdlet will prompt you for credentials)

Yes, I know that there are get-dns cmdlets in the newest versions of powershell + OS combos (see references), but I haven't gotten to a cmdlet that does as nice a job as the WMI equivalent above ...

Look at what fields we have:

$dns | gm

   TypeName: System.Management.ManagementObject#Root\MicrosoftDNS\MicrosoftDNS_AType

Name                                 MemberType    Definition                                                
----                                 ----------    ----------                                                
PSComputerName                       AliasProperty PSComputerName = __SERVER                                 
CreateInstanceFromTextRepresentation Method        System.Management.ManagementBaseObject CreateInstanceFromTextRepresentation(System.String DnsServerName, System.Strin...
GetObjectByTextRepresentation        Method        System.Management.ManagementBaseObject GetObjectByTextRepresentation(System.String DnsServerName, System.String Conta...
Modify                               Method        System.Management.ManagementBaseObject Modify(System.UInt32 TTL, System.String IPAddress)                               
Caption                              Property      string Caption {get;set;}                                 
ContainerName                        Property      string ContainerName {get;set;}                           
Description                          Property      string Description {get;set;}                             
DnsServerName                        Property      string DnsServerName {get;set;}                           
DomainName                           Property      string DomainName {get;set;}                              
InstallDate                          Property      string InstallDate {get;set;}                             
IPAddress                            Property      string IPAddress {get;set;}                               
Name                                 Property      string Name {get;set;}                                    
OwnerName                            Property      string OwnerName {get;set;}                               
RecordClass                          Property      uint16 RecordClass {get;set;}                             
RecordData                           Property      string RecordData {get;set;}                              
Status                               Property      string Status {get;set;}                                  
TextRepresentation                   Property      string TextRepresentation {get;set;}                      
Timestamp                            Property      uint32 Timestamp {get;set;}                               
TTL                                  Property      uint32 TTL {get;set;}                                     
__CLASS                              Property      string __CLASS {get;set;}                                 
__DERIVATION                         Property      string[] __DERIVATION {get;set;}                          
__DYNASTY                            Property      string __DYNASTY {get;set;}                               
__GENUS                              Property      int __GENUS {get;set;}                                    
__NAMESPACE                          Property      string __NAMESPACE {get;set;}                             
__PATH                               Property      string __PATH {get;set;}                                  
__PROPERTY_COUNT                     Property      int __PROPERTY_COUNT {get;set;}                           
__RELPATH                            Property      string __RELPATH {get;set;}                               
__SERVER                             Property      string __SERVER {get;set;}                                
__SUPERCLASS                         Property      string __SUPERCLASS {get;set;}                            
ConvertFromDateTime                  ScriptMethod  System.Object ConvertFromDateTime();                      
ConvertToDateTime                    ScriptMethod  System.Object ConvertToDateTime();    

Let's just pull the system name and IP address:

$dns2 = $dns | Select-Object -property Ownername, ipaddress

or, more elegantly, do these two steps in one:

$dns2 = Get-WmiObject -Class MicrosoftDNS_AType -NameSpace Root\MicrosoftDNS -ComputerName DC01  -Filter "DomainName = 'example.com'" -Credential (Get-Credential)  | Select-Object -property Ownername, ipaddress

Next, winnow down to just the systems we want:

$ipsofinterest = $dns | where { ($_.Ownername -like "*TP*") -or ($_.Ownername -like "*THIN*") -or ($_.Ownername -like "*THP*") }
$ipsofinterest
Ownername                                              ipaddress                                             
---------                                              ---------                                             
THP-01.example.com                                        10.71.32.5                                            
THP-02.example.com                                        10.71.32.13                                           
THP-03.example.com                                        10.71.32.23                                           
THP-05.example.com                                        10.71.32.21                                           
THP-07.example.com                                        10.71.4.18                                            
THP-08.example.com                                        10.71.4.17  
....

  
Finally, what we really want is the MAC addresses and switch ports.  Ping the IP's, and while that's happening dump the mac address table on the switch to find which ports are involved:

$ipsofinterest | foreach { ping -n 2 $_.ipaddress }

Pinging 10.71.32.5 with 32 bytes of data:
Reply from 10.71.32.5: bytes=32 time=44ms TTL=123
Request timed out.

Ping statistics for 10.71.32.5:
    Packets: Sent = 2, Received = 1, Lost = 1 (50% loss),
Approximate round trip times in milli-seconds:
    Minimum = 44ms, Maximum = 44ms, Average = 44ms

This last thing is pretty cludgy though, you still need to get the ARP entry (from whatever subnet you are pinging), and relate that MAC back to the MACs on the switch - this started to sound like more work than I wanted to take on.  Plus it's totally counter to the stealthy approach we want to take in a penetration test.  Let's look at the DHCP database instead:

DHCP Approach

DHCP is more attractive for hosts that use DHCP - you'll get the hostname, the IP and the MAC address all in one go.

First, dump the scopes from the DHCP Server:

Get-DhcpServerv4Scope

ScopeId         SubnetMask      Name           State    StartRange      EndRange        LeaseDuration
-------         ----------      ----           -----    ----------      --------        -------------
10.71.33.0      255.255.255.0   Scope1         Active   10.71.33.17     10.71.33.139    8.00:00:00
10.71.34.0      255.255.255.0   Workstations   Active   10.71.34.10     10.71.34.250    01:00:00
10.71.35.0      255.255.255.0   Wireless       Active   10.71.35.50     10.71.35.200    8.00:00:00

Dump the leases for each ScopeId on the server:

Get-DhcpServerv4Scope | foreach { get-dhcpserverv4lease $_.ScopeId -allleases }

IPAddress       ScopeId         ClientId             HostName             AddressState         LeaseExpiryTime
---------       -------         --------             --------             ------------         ---------------
10.71.33.1      10.71.33.0      68-b5-99-e8-22-94    W-HOF-A30.example.com   ActiveReservation
10.71.33.2      10.71.33.0      b8-ac-6f-c9-9e-3b    W-HOF-A18.example.com   ActiveReservation
10.71.33.3      10.71.33.0      68-b5-99-e8-25-d2    w-hof-a12.example.com   InactiveReservation
10.71.33.4      10.71.33.0      f0-4d-a2-ab-f2-2a    W-HOF-A06.example.com   ActiveReservation                                     
10.71.33.5      10.71.33.0      f0-1f-af-66-46-7d    W-HOF-A25.example.com   ActiveReservation  
10.71.33.19     10.71.33.0      f0-4d-a2-ae-30-50    TPC-L19.example.com     Active               04 May 2016 4:29:16 PM
10.71.33.20     10.71.33.0      f0-4d-a2-ab-f2-20    W-HOF-L93.example.com   Active               04 May 2016 7:50:04 AM

Next, narrow down to just the hostnames of interest:

Get-DhcpServerv4Scope | foreach { get-dhcpserverv4lease $_.ScopeId -allleases } | where { ($_.hostname -like "*TP*") -or ($_.hostname -like "*THIN*") -or ($_.hostname -like "*THP*") }

Or, even better, do that and  just pull the fields we want ...

$targethosts = Get-DhcpServerv4Scope | foreach { get-dhcpserverv4lease $_.ScopeId -allleases } | where { ($_.hostname -like "*TP*") -or ($_.hostname -like "*THIN*") -or ($_.hostname -like "*THP*") } | select Hostname, IPAddress, Clientid

Hostname                         IPAddress                        Clientid
--------                         ---------                        --------                       
TPC-L08.example.com              10.71.33.17                      00-25-64-79-28-49   
W-HOF-THINPC20.example.com       10.71.33.18                      64-31-50-41-41-fc
TPC-L19.example.com              10.71.33.19                      f0-4d-a2-ae-30-50
 

Now we have the hostname, the IP and the MAC

For my ops problem, I'd pull the switch ports using some python fun or an SNMP tool

However, in a penetration test, you'd have much different uses for this data:

• umm... the reason that we were moving these thin clients to another vlan is because Thin Clients often have "IOT CLass" operating systems - in other words, linux OS's or embedded Windows OS, with much slower (or nonexistent) patch cycles.  So you could use exactly this to target thin clients.
• Or .. if you are perhaps targeting VM's, you could look for MAC addresses starting in 00-50
• If you were looking for something else with a known vulnerability, like say a printer or access point, you could look for the affected OUI(s)
• Maybe target hosts with names like "DC" - maybe look for an older one, maybe win2k3, maybe on a subnet you don't know about yet
• Other fun target strings?  "SQL" "COUCHDB" "NOSQL", log or syslog - you really can mine this database and only engage a single host.
• How about hostnames that include the names of system admins, or network admins?  Company Execs?  Social media is a great place to get this target data, or often you can find an "our executive team" page on the target company website.

How could you go one better?  I have found new target subnets using this approach (*everything* is in DNS!).  If you have an especially forward-thinking client, the DHCP cmdlets will work on IPv6 scopes by changing the "4" in the cmdlet to a "6".

Have you used an approach like this? If so, did you find anything good?  Or do you have a better cmdlet to get the DNS info?  Please, use our comment form to share your experiences ..

===========
references:
https://technet.microsoft.com/en-us/library/jj590751%28v=wps.630%29.aspx
https://technet.microsoft.com/en-us/library/jj649850.aspx

===============
Rob VandenBrink
Compugen