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Example Nmap output

Nmap Network Scanning

Finding an Organization's IP Addresses

Nmap automates many aspects of network scanning, but you still must tell it which networks to scan. I suppose you could specify -iR and hope Nmap hits your target company randomly, or you could try the brute force method of specifying 0.0.0.0/0 to scan the whole Internet. But either of those options could take months or years, and possibly get you into trouble. So it is important to carefully research target netblocks before scanning them. Even if you are conducting a legitimate penetration test and the client gave you a list of their netblocks, it is important to double check them. Clients sometimes have out-of-date records or simply write them down wrong. An authorization letter signed by your client won't help if you accidentally break into the wrong company.

In many cases, you start with only a company's domain name. This section demonstrates a few of the most common and effective ways to turn that into a list of netblocks owned, operated by, or affiliated with the target company. Typical Linux command-line utilities are demonstrated, but similar tools are available for other platforms.

At the ShmooCon conference in 2006, a fellow came up to me and complained that Nmap documentation specified many example ways to scan target.com. He noted that ICANN had reserved the domain name example.com for this purpose, and pressured me to revise the man page accordingly. While he was technically right, it was a strange thing to obsess about. His motivation became clear when he handed me his business card:

Figure 3.1. A business card explains everything

A business card explains everything

Apparently, many Nmap users copied examples straight from the man page and ran them without changing the target specifier. So target.com was flooded with scans and corresponding IDS alerts. In honor of that incident, the goal of this section is to determine IP ranges assigned to and used by Target Corporation.

DNS Tricks

The primary purpose of DNS is to resolve domain names into IP addresses, so it is a logical place to start. In Example 3.1, I use the Linux host command to query some common DNS record types.

Example 3.1. Using the host command to query common DNS record types

> host -t ns target.com
target.com name server ns4.target.com.
target.com name server ns3.target.com.
target.com name server ns1-auth.sprintlink.net.
target.com name server ns2-auth.sprintlink.net.
target.com name server ns3-auth.sprintlink.net.
> host -t a target.com
target.com has address 161.225.130.163
target.com has address 161.225.136.0
> host -t aaaa target.com
target.com has no AAAA record
> host -t mx target.com
target.com mail is handled by 50 smtp02.target.com.
target.com mail is handled by 5 smtp01.target.com.
> host -t soa target.com
target.com has SOA record extdns02.target.com. hostmaster.target.com.

Next I resolve the IP addresses for the hostnames above (using host again) and I try a few common subdomain names such as www.target.com and ftp.target.com. Starting with names like ns3.target.com and smtp01.target.com, I try changing the digits to find new machines. All of this leaves me with the following target.com names and addresses:

Table 3.1. First pass at listing target.com IPs

HostnameIP Addresses
ns3.target.com161.225.130.130
ns4.target.com161.225.136.136
ns5.target.com161.225.130.150
target.com161.225.136.0, 161.225.130.163
smtp01.target.com161.225.140.120
smtp02.target.com198.70.53.234, 198.70.53.235
extdns02.target.com172.17.14.69
www.target.com207.171.166.49

While a substantial hostname list can be generated in this manner, the mother lode of hostnames comes from a zone transfer. Most DNS servers now reject zone transfer requests, but it is worth a try because many still allow it. Be sure to try every DNS server you have found through domain NS records and port scanning corporate IP ranges. So far we have found seven Target nameservers: ns3.target.com, ns4.target.com, ns5.target.com, ns1-auth.sprintlink.net, ns2-auth.sprintlink.net, ns3-auth.sprintlink.net, and extdns02.target.com. Unfortunately, all of those servers either refused the transfer or did not support the TCP DNS connections required for a zone transfer. Example 3.2 shows a failed target.com zone transfer attempt using the common dig (domain information groper) tool[9], followed by a successful one against an unrelated organization (cpsr.org).

Example 3.2. Zone transfer failure and success

> dig @ns2-auth.sprintlink.net -t AXFR target.com
; <<>> DiG 9.5.0b3 <<>> @ns2-auth.sprintlink.net -t AXFR target.com

; Transfer failed.

> dig @ns2.eppi.com -t AXFR cpsr.org
; <<>> DiG 9.5.0b1 <<>> @ns2.eppi.com -t AXFR cpsr.org

cpsr.org             10800   IN      SOA   ns1.findpage.com. root.cpsr.org.
cpsr.org.            10800   IN      NS    ns.stimpy.net.
cpsr.org.            10800   IN      NS    ns1.findpage.com.
cpsr.org.            10800   IN      NS    ns2.eppi.com.
cpsr.org.            10800   IN      A     208.96.55.202
cpsr.org.            10800   IN      MX    0 smtp.electricembers.net.
diac.cpsr.org.       10800   IN      A     64.147.163.10
groups.cpsr.org.     10800   IN      NS    ns1.electricembers.net.
localhost.cpsr.org.  10800   IN      A     127.0.0.1
mail.cpsr.org.       10800   IN      A     209.209.81.73
peru.cpsr.org.       10800   IN      A     208.96.55.202
www.peru.cpsr.org.   10800   IN      A     208.96.55.202
[...]

A common mistake when gathering forward DNS results like these is assuming that all systems found under a domain name must be part of that organization's network and safe to scan. In fact, nothing prevents an organization from adding records pointing anywhere on the Internet. This is commonly done to outsource services to third parties while keeping the source domain name for branding. For example, www.target.com resolves to 207.171.166.49. Is this part of Target's network, or is it managed by a third party we might not want to scan? Three quick and easy tests are DNS reverse-resolution, traceroute, and whois against the relevant IP address registry. The first two steps can be done by Nmap, while the Linux whois command works well for the third. These tests against target.com are shown in Example 3.3 and Example 3.4.

Example 3.3. Nmap reverse-DNS and traceroute scan against www.target.com

# nmap -Pn -T4 --traceroute www.target.com

Starting Nmap ( http://nmap.org )
Nmap scan report for 166-49.amazon.com (207.171.166.49)
Not shown: 998 filtered ports
PORT    STATE SERVICE
80/tcp  open  http
443/tcp open  https

TRACEROUTE (using port 80/tcp)
HOP RTT    ADDRESS
[cut]
9   84.94  ae-2.ebr4.NewYork1.Level3.net (4.69.135.186)
10  87.91  ae-3.ebr4.Washington1.Level3.net (4.69.132.93)
11  94.80  ae-94-94.csw4.Washington1.Level3.net (4.69.134.190)
12  86.40  ae-21-69.car1.Washington3.Level3.net (4.68.17.7)
13  185.10 AMAZONCOM.car1.Washington3.Level3.net (4.71.204.18)
14  84.70  72.21.209.38
15  85.73  72.21.193.37
16  85.68  166-49.amazon.com (207.171.166.49)

Nmap done: 1 IP address (1 host up) scanned in 20.57 seconds

Example 3.4. Using whois to find owner of www.target.com IP address

> whois 207.171.166.49
[Querying whois.arin.net]
[whois.arin.net]

OrgName:    Amazon.com, Inc. 
OrgID:      AMAZON-4
Address:    605 5th Ave S
City:       SEATTLE
StateProv:  WA
PostalCode: 98104
Country:    US
[...]

In Example 3.3, the reverse DNS (two places) and interesting traceroute results are bolded. The Amazon.com domain name makes it highly likely that the web site is run by Amazon rather than Target itself. Then the whois results showing Amazon.com, Inc. as the IP space owner removes all doubt. The web site is Target branded, but displays Powered by Amazon.com at the bottom. If we were hired by Target to test their security, we would need separate permission from Amazon to touch this address space.

Web databases can also be used to find hostnames under a given domain. For example, Netcraft has a web site DNS search feature at http://searchdns.netcraft.com/?host. Typing .target.com in to the form brings 36 results, as shown in Figure 3.2. Their handy table shows the netblock owner too, which catches cases such as Amazon running www.target.com. We already knew about some of the discovered hosts, but we would have been unlikely to guess names such as sendasmoochie.target.com.

Figure 3.2. Netcraft finds 36 Target web servers

Netcraft finds 36 Target web servers

Google can also be used for this purpose with queries such as site:target.com.

Whois Queries Against IP Registries

After a set of initial seed IPs are discovered, they must be researched to ensure they belong to the company you expect and to determine what netblocks they are part of. A small company might have a tiny allocation of 1–16 IP addresses, while larger corporations often have thousands. This information is kept in regional databases, such as ARIN (American Registry for Internet Numbers) for North America and RIPE for Europe and the Middle East. Modern whois tools take an IP address and automatically query the appropriate registry.

Small and mid-sized companies normally don't have IP space allocated by the likes of ARIN. Instead, they are delegated netblocks from their ISPs. Sometimes you get this ISP information from IP queries. This generally leaves you with a big netblock and you don't know which portion of it is allocated to your target. Fortunately, many ISPs now subdelegate customer ranges using Shared Whois (SWIP) or Referral Whois (RWhois). If the ISP has done this, you learn the customer's exact netblock size.

One of the IP addresses previously discovered for target.com was 161.225.130.163. Example 3.5 demonstrates a whois query (automatically directed against ARIN) to determine the owner and IP allocation information for this IP.

Example 3.5. Using whois to find netblock containing 161.225.130.163

> whois 161.225.130.163
[Querying whois.arin.net]
[whois.arin.net]

OrgName:    Target Corporation 
OrgID:      TARGET-14
Address:    1000 Nicollet TPS 3165
City:       Minneapolis
StateProv:  MN
PostalCode: 55403
Country:    US

NetRange:   161.225.0.0 - 161.225.255.255 
CIDR:       161.225.0.0/16 
NetName:    TARGETNET
NetHandle:  NET-161-225-0-0-1
Parent:     NET-161-0-0-0-0
NetType:    Direct Assignment
NameServer: NS3.TARGET.COM
NameServer: NS4.TARGET.COM
Comment:    
RegDate:    1993-03-04
Updated:    2005-11-02

OrgTechHandle: DOMAI45-ARIN
OrgTechName:   Domainnames admin 
OrgTechPhone:  +1-612-696-2525
OrgTechEmail:  Domainnames.admin@target.com

Not surprisingly, Target owns a huge Class B netblock, covering all 65,536 IPs from 161.225.0.0 through 161.225.255.255. Since the OrgName is Target, this isn't a case where we are seeing results from their ISP.

The next step is to similarly look up all previously discovered IPs which don't fall within this range. Then you can begin with more advanced queries. The command whois -h whois.arin.net \? gives the ARIN query syntax. It would be nice if you could search for all netblocks matching a given address, OrgID, or OrgTechEmail, but IP registries generally don't allow that. However, many other helpful queries are allowed. For example, whois -h whois.arin.net @target.com shows all the ARIN contacts with email addresses at target.com. The query whois -h whois.arin.net "n target*" shows all the netblock handles starting with target. It is not case sensitive. Similarly, whois -h whois.arin.net "o target*" shows all of the organizational names starting with target. You can look up the address, phone number, and contact email associated with each entry to determine whether they are part of the company you wish to scan. Often they are 3rd parties who happen to have a similar name.

Internet Routing Information

The core routing protocol of the Internet is the Border Gateway Protocol (BGP). When scanning mid-sized and large organizations, BGP routing tables can help you find their IP subnets all over the world. For example, suppose you want to scan IP addresses belonging to Microsoft Corporation. A DNS lookup for microsoft.com provides the IP address 207.46.196.115. A whois query as discussed in the previous section shows that the whole 207.46.0.0/16 block belongs to Microsoft at their appropriate One Microsoft Way address in Redmond. That provides 65,536 IP addresses to scan, but BGP tables expose many more.

Entities such as Microsoft are assigned autonomous system (AS) numbers for routing purposes. A handy tool for determining the AS number advertised for a given IP address is available at http://asn.cymru.com/. Typing 207.46.0.0 into this form provides Microsoft's AS number 8075. Next, I want to find all of the IP prefixes which route to this AS. A handy tool for doing so is available at http://www.robtex.com/as/. Typing in AS8075 and hitting Go on that page leads to a summary screen showing 42 prefixes found. Those prefixes represent 339,456 IP addresses and can be enumerated by clicking the BGP tab.

While obtaining BGP information from canned web forms such as these is convenient, obtaining routing data from actual routers is more fun and may allow more powerful custom queries. Several organizations provide such a service. For an example, telnet to route-views.routeviews.org or visit http://routeviews.org. Of course these services provide read-only access to the data. If you need to manipulate global routing tables as part of a diabolical plan to take over the Internet, that is beyond the scope of this book.



[9] Nmap's dns-zone-transfer NSE script could have been used instead (see Chapter 9, Nmap Scripting Engine).

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