4.2 TCP Port Scanning

Accessible TCP ports can be identified by port scanning target IP addresses. The following nine different types of TCP port scanning are used in the wild by both attackers and security consultants:

Standard scanning methods

Vanilla connect( ) scanning
Half-open SYN flag scanning

Stealth TCP scanning methods

Inverse TCP flag scanning
ACK flag probe scanning
TCP fragmentation scanning

Third-party and spoofed TCP scanning methods

FTP bounce scanning
Proxy bounce scanning
Sniffer-based spoofed scanning
IP ID header scanning

What follows is a technical breakdown for each TCP port scanning type, along with details of Windows and Unix-based tools that can perform scanning.

4.2.1 Standard Scanning Methods

Standard scanning methods, such as vanilla and half-open SYN scanning, are extremely simple direct techniques used to identify accessible TCP ports and services accurately. These scanning methods are reliable but are easily logged and identified. Vanilla connect( ) scanning

TCP connect( ) port scanning is the most simple type of probe to launch. There is no stealth whatsoever involved in this form of scanning because a full TCP/IP connection is established with TCP port one of the target host, then incrementally through ports two, three, four, and so on.

TCP/IP's reliability as a protocol, vanilla port scanning is a very accurate way to determine which TCP services are accessible on a given target host. Figures Figure 4-2 and Figure 4-3 show the various TCP packets and their flags, as they are sent and received by the attacker and the host he is scanning.

In Figure 4-2, the attacker first sends a SYN probe packet to the port he wishes to test. Upon receiving a packet from the port with the SYN and ACK flags set, he knows that the port is open. The attacker completes the three-way handshake by sending an ACK packet back.

Figure 4-2. A vanilla TCP scan result when a port is open

If, however, the target port is closed, the attacker receives an RST/ACK packet directly back, as shown in Figure 4-3.

Figure 4-3. A vanilla TCP scan result when a port is closed

As before, the attacker sends a SYN probe packet, but the target server responds with an RST/ACK. Standard connect( ) scanning in this way is a reliable way to identify accessible TCP network services. The downside is that the scanning type is extremely simple and hence easily identified and logged. Tools that perform connect( ) TCP scanning

nmap can perform a TCP connect( ) port scan using the -sT flag. Other very simple scanners exist; one such as pscan.c, which is available as source code from many sites, including Packet Storm (http://www.packetstormsecurity.org).

For Windows, Foundstone's SuperScan is an excellent port-scanning utility with good functionality. It's available from http://www.foundstone.com/knowledge/scanning.html.

When performing a full assessment exercise, every TCP port from 0 to 65535 should be checked. For speed reasons, tools such as SuperScan and nmap have internal lists of only some 1,500 common ports to check; thus they often miss all kinds of interesting services that can be found on high ports?for example, Check Point SVN web services on TCP port 18264. Half-open SYN flag scanning

Usually, a three-way handshake is initiated to synchronize a connection between two hosts; the client sends a SYN packet to the server, which responds with SYN and ACK if the port is open, and the client then sends an ACK to complete the handshake.

In the case of half-open SYN port scanning when a port is found to be listening, an RST packet is sent as the third part of the handshake. Sending an RST packet in this way abruptly resets the TCP connection, and because you have not completed the three-way handshake, the connection attempt often isn't logged on the target host.

Most intrusion detection systems (IDS) and other security programs, such as portsentry, can easily detect and prevent half-open SYN port-scanning attempts. In cases where stealth is required, other techniques are recommended, such as FIN or TTL-based scanning, or even using a utility such as fragroute, to fragment outbound probe packets.

Figures Figure 4-4 and Figure 4-5 outline the packets sent between the two hosts when launching a SYN port scan and finding either an open and a closed port.

In Figure 4-5, a SYN probe packet is sent to the target port; a SYN/ACK packet is received indicating that the port is open. Normally at this stage, a connect( ) scanner sends an ACK packet to establish the connection, but this is half-open scanning so instead, a RST packet is sent to tear down the connection.

Figure 4-4. A half-open SYN scan result when a port is closed

Figure 4-4 shows that when a closed port is found, a RST/ACK packet is received, and nothing happens (as before in Figure 4-3). The benefit of half-open scanning is that a true three-way TCP handshake is never completed, and the connection doesn't appear to be established.

Figure 4-5. A half-open SYN scan result when a port is open

Nowadays, all IDS and personal firewall systems can identify SYN port scans (although they often mislabel them as SYN flood attacks due to the number of probe packets). SYN scanning is fast and reliable, although it requires raw access to network sockets and, therefore, privileged access to Unix and Windows hosts. Tools that perform half-open SYN scanning

nmap can perform a SYN port scan under both Unix and Windows environments using the -sS flag. Many other Unix half-open port scanners exist, including strobe, which is available in source form from many sites including Packet Storm (http://www.packetstormsecurity.org).

The -T flag can be used within nmap to change the timing policy used when scanning. Networks protected by commercial firewalls (NetScreen, WatchGuard, and Check Point in particular) will sometimes drop SYN probes if nmap is sending the packets out too quickly, nmap's actions resemble a SYN flood denial of service attack. I have found that by setting the timing policy to -T Sneaky, it's often possible to glean accurate results against hosts protected by firewalls with SYN flood protection enabled.

A second SYN port scanner worth mentioning is the scanrand component of the Paketto Keiretsu suite by Dan Kaminsky. Paketto Keiretsu contains a number of useful networking utilities that are available at http://www.doxpara.com/read.php/code/paketto.html. The scanrand tool is very well designed, with distinct SYN probing and background listening components so that you can launch the quickest possible scans. Inverse SYN cookies (using the HMAC SHA1 hashing algorithm) tag outgoing probe packets, so that false positive results become nonexistent (because the listening component registers only SYN/ACK responses with the correct cryptographic cookies). Example 4-3 shows scanrand identifying open ports on a local network in less than one second.

Example 4-3. Using scanrand to quickly scan the local network
# scanrand

  UP:    [01]   0.003s

  UP:   [01]   0.017s

  UP:   [01]   0.021s

  UP:   [01]   0.024s

  UP:   [01]   0.024s

  UP:    [01]   0.047s

  UP:    [01]   0.058s

  UP:    [01]   0.058s

  UP:    [01]   0.077s

  UP:    [01]   0.077s

  UP:   [01]   0.088s

  UP:   [01]   0.089s

  UP:   [01]   0.090s

  UP:   [01]   0.097s

  UP:   [01]   0.098s

  UP:   [01]   0.099s

  UP:   [01]   0.099s

  UP:   [01]   0.127s

  UP:  [01]   0.147s

  UP:  [01]   0.147s

  UP:  [01]   0.156s

  UP:  [01]   0.157s

  UP:   [01]   0.182s

Due to the way scanrand sends a deluge of SYN probes and then listens for positive SYN/ACK responses, the order in which the open ports are displayed will look a little odd. On the positive side, scanrand is lightning fast; it allows specific ports (e.g., common backdoors) to be identified in seconds even across large networks, as opposed to minutes or hours with a bulkier tool such as nmap.

4.2.2 Stealth TCP Scanning Methods

Stealth scanning methods involve idiosyncrasies in the way TCP/IP stacks of target hosts process and respond to packets with strange bits set or other features. Such techniques aren't effective at accurately mapping the open ports of some operating systems but do provide a degree of stealth and are sometimes not logged. Inverse TCP flag scanning

Security mechanisms such as firewalls and IDS usually detect SYN packets being sent to sensitive ports of target hosts. Programs are also available to log half-open SYN flag scan attempts, including synlogger and courtney. Probe packets with strange TCP flags set can sometimes pass through filters undetected, depending on the security mechanisms deployed.

Using half-open SYN flags to probe a target is known as an inverted technique because responses are sent back only by closed ports. RFC 793 states that if a port is closed on a host, an RST/ACK packet should be sent to reset the connection. To take advantage of this feature, attackers send TCP probe packets with various TCP flags set.

A TCP probe packet is sent to each port of the target host. Three types of probe packet flag configurations are normally used:

  • A FIN probe with the FIN TCP flag set

  • An XMAS probe with the FIN, URG, and PUSH TCP flags set

  • A NULL probe with no TCP flags set

Figures Figure 4-6 and Figure 4-7 depict the probe packets and responses generated by the target host if the target port is found to be open or closed.

Figure 4-6. An inverse TCP scan result when a port is open
Figure 4-7. An inverse TCP scan result when a port is closed

The RFC standard states that, if no response is seen from the target port, the port is open, or the server is down. This scanning method isn't necessarily the most accurate, but it is stealthy; it sends garbage to each port that usually won't be picked up.

For all closed ports on the target host, RST/ACK packets are received. However, some operating platforms (such as those in the Microsoft Windows family) disregard the RFC 793 standard, so no RST/ACK response is seen when an attempt is made to connect to a closed port. Hence, this technique is effective against most Unix-based operating systems. Tools that perform inverse TCP flag scanning

nmap can perform an inverse TCP flag port scan under both Unix and Windows environments, using the following flags:


For a scan with only the FIN flag set on probe packets


For a NULL scan with no TCP flags set on probe packets


For an Xmas tree scan with all TCP flags set

vscan is another Windows tool you can use to perform inverse TCP flag scanning. The utility doesn't require installation of WinPcap network drivers; instead it uses raw sockets within Winsock 2 (present in Windows 2000, XP, and 2003). vscan is available at http://host.deluxnetwork.com/~vsniff/vscan.zip. ACK flag probe scanning

A stealthy technique documented by Uriel Maimon in Phrack Magazine, Issue 49, is that of identifying open TCP ports by sending ACK probe packets and analyzing the header information of the RST packets received from the target host. This technique exploits vulnerabilities within the BSD derived TCP/IP stack and is therefore only effective against certain operating systems and platforms. There are two main ACK scanning techniques that involve:

  • Analysis of the time-to-live (TTL) field of received packets

  • Analysis of the WINDOW field of received packets

These techniques can also check filtering systems and complicated networks to understand the processes packets go through on the target network. For example, the TTL value can be used as a marker of how many systems the packet has hopped through. The firewalk filter assessment tool works in a similar fashion, available from http://www.packetfactory.net/projects/firewalk/. Analysis of the TTL field of received packets

To analyze the TTL field data of received RST packets, an attacker first sends thousands of crafted ACK packets to different TCP ports, as shown in Figure 4-8.

Figure 4-8. ACK probe packets are sent to various ports

Here is a log of the first four RST packets received using the hping2 utility:

1: host port 20: F:RST -> ttl: 70 win: 0

2: host port 21: F:RST -> ttl: 70 win: 0

3: host port 22: F:RST -> ttl: 40 win: 0

4: host port 23: F:RST -> ttl: 70 win: 0

By analyzing the TTL value of each packet, an attacker can easily see that the value returned by port 22 is 40, whereas the other ports return a value of 70. This suggests that port 22 is open on the target host because the TTL value returned is smaller than the TTL boundary value of 64. Analysis of the WINDOW field of received packets

To analyze the WINDOW field data of received RST packets, an attacker sends thousands of the same crafted ACK packets to different TCP ports (as shown in Figure 4-8). Here is a log of the first four RST packets received, again using the hping2 utility:

1: host port 20: F:RST -> ttl: 64 win: 0

2: host port 21: F:RST -> ttl: 64 win: 0

3: host port 22: F:RST -> ttl: 64 win: 512

4: host port 23: F:RST -> ttl: 64 win: 0

Notice that the TTL value for each packet is 64, meaning that TTL analysis of the packets isn't effective in identifying open ports on this host. However, by analyzing the WINDOW values, the attacker finds that the third packet has a non-zero value, indicating an open port.

The advantage of using ACK flag probe scanning is that detection is difficult (for both IDS and host-based systems, such as personal firewalls). The disadvantage is that this scanning type relies on TCP/IP stack implementation bugs, which are prominent in BSD-derived systems but not in many other modern platforms. Tools that perform ACK flag probe scanning

nmap supports ACK flag probe scanning, with the -sA and -sW flags to analyze the TTL and WINDOW values respectively. See the nmap manpage for more detailed information.

hping2 can also sample TTL and WINDOW values, but this can prove highly time consuming in most cases. The tool is more useful for analyzing low-level responses, as opposed to port scanning in this fashion. hping2 is available from http://www.eaglenet.org/antirez/hping2.html and http://www.hping.org.

4.2.3 Third-Party and Spoofed TCP Scanning Methods

Third-party port scanning methods allow for probes to be effectively bounced through vulnerable servers to hide the true source of the network scanning. An additional benefit of using a third-party technique in this way is that insight into firewall configuration can be gained by potentially bouncing scans through trusted hosts that are vulnerable. FTP bounce scanning

Hosts running outdated FTP services can relay numerous TCP attacks, including port scanning. There is a flaw in the way many FTP servers handle connections using the PORT command (see RFC 959 or technical description of the PORT feature) that allows for data to be sent to user-specified hosts and ports. In their default configurations, the FTP services running on the following platforms are affected:

  • FreeBSD 2.1.7 and earlier

  • HP-UX 10.10 and earlier

  • Solaris 2.6/SunOS 5.6 and earlier

  • SunOS 4.1.4 and earlier

  • SCO OpenServer 5.0.4 and earlier

  • SCO UnixWare 2.1 and earlier

  • IBM AIX 4.3 and earlier

  • Caldera Linux 1.2 and earlier

  • Red Hat Linux 4.2 and earlier

  • Slackware 3.3 and earlier

  • Any Linux distribution running WU-FTP 2.4.2-BETA-16 or earlier

The FTP bounce attack can have a far more devastating effect if a writable directory exists because a series of commands or other data can be entered into a file and then relayed via the PORT command to a specified port of a target host. For example, someone can upload a spam email message to a vulnerable FTP server and then send this email message to the SMTP port of a target mail server. Figure 4-9 shows the parties involved in FTP bounce scanning.

Figure 4-9. FTP bounce port scanning

The following occurs when performing an FTP bounce scan:

  1. The attacker connects to the FTP control port (TCP port 21) of the vulnerable FTP server that she is going to bounce her attack through and enters passive mode, forcing the FTP server to send data using DTP (data transfer process) to a specific port of a specific host:

    227 Entering Passive Mode (64,12,168,246,56,185).
  2. A PORT command is issued, with an argument passed to the FTP service telling it to attempt a connection to a specific TCP port of the target server; for example, TCP port 23 of

    PORT 144,51,17,230,0,23
    200 PORT command successful.
  3. After issuing the PORT command, a LIST command is sent. The FTP server then attempts to create a connection with the target host defined in the PORT command issued previously:

    150 Opening ASCII mode data connection for file list
    226 Transfer complete.

    If a 226 response is seen, then the port on the target host is open. If, however, a 425 response is seen, the connection has been refused:

    425 Can't build data connection: Connection refused Tools that perform FTP bounce port scanning

nmap for both Unix and Windows can effectively perform an FTP bounce port scan, using the -P0 and -b flags in the following manner:

nmap -P0 -b username:password@ftp-server:port <target host> Proxy bounce scanning

Attackers bounce TCP attacks through open proxy servers. Depending on the level of poor configuration, the server will sometimes allow a full-blown TCP port scan to be relayed. Using proxy servers to perform bounce port scanning in this fashion is often time consuming, so many attackers prefer to abuse open proxy servers more efficiently by bouncing actual attacks through to target networks.

ppscan.c, a publicly available Unix-based tool to bounce port scans, can be found in source form at:

http://www.phreak.org/archives/exploits/unix/network-scanners/ppscan.c Sniffer-based spoofed scanning

An innovative half-open SYN TCP port scanning method was realized when jsbach published his spoofscan Unix-based scanner in 1998. The spoofscan tool is run as root on a given host to perform a stealthy port scan. The key feature that makes this scanner so innovative is that it places the host network card into promiscuous mode and then sniffs for responses on the local network segment.

The following unique benefits are immediately realized when using a sniffer-based spoofing port scanner:

  • If you have superuser access to a machine on the same physical network segment as the target host or a firewall protecting a target host, you can spoof TCP probes from other IP addresses to identify trusted hosts and to gain insight into the firewall policy (by spoofing scans from trusted office hosts, for example). Accurate results will be retrieved because of the background sniffing process, which monitors the local network segment for responses to your spoofed probes.

  • If you have access to a large shared network segment, you can spoof scans from hosts you don't have access to or that don't exist (such as unused IP addresses within your local network segment), to effectively port scan remote networks in a distributed and stealthy fashion.

The beauty of this method is that the attacker is abusing his access to the local network segment. Such techniques can even be carried out to good effect in switched network environments using ARP redirect spoofing and other techniques. spoofscan is available at http://examples.oreilly.com/networksa/tools/spoofscan.c. IP ID header scanning

IP ID header scanning (also known as idle or dumb scanning) is an obscure scanning technique that involves abusing implementation peculiarities within the TCP/IP stack of most operating systems. Three hosts are involved:

  • The host, from which the scan is launched

  • The target host, which will be scanned

  • A zombie or idle host, which is an Internet-based server that is queried with spoofed port scanning against the target host to identify open ports from the perspective of the zombie host

IP ID header scanning is extraordinarily stealthy due to its blind nature. Determined attackers will often use this type of scan to map out IP-based trust relationships between machines, such as firewalls and VPN gateways.

The listing returned by the scan shows open ports from the perspective of the zombie host, so you can try scanning a target using various zombies you think might be trusted (such as hosts at remote offices or DMZ machines). Figure 4-10 depicts the process undertaken during an IP ID header scan.

Figure 4-10. IP ID header scanning and the parties involved

hping2 was originally used in a manual fashion to perform such low-level TCP scanning, which was time consuming and tricky to undertake against an entire network of hosts. A white paper that fully discusses using the tool to perform IP ID header scanning by hand is available from http://www.kyuzz.org/antirez/papers/dumbscan.html.

nmap supports such IP ID header scanning with the option:

-sI <zombie host[:probe port]>

Example 4-4 shows how nmap uses this functionality to scan through

Example 4-4. Using nmap to perform IP ID header scanning
# nmap -P0 -sI

Starting nmap 3.45 ( www.insecure.org/nmap/ )

Idlescan using zombie; Class: Incremental

Interesting ports on  (

(The 1582 ports scanned but not shown below are in state: closed)

Port       State       Service

25/tcp     open        smtp

53/tcp     open        domain

80/tcp     open        http

88/tcp     open        kerberos-sec

135/tcp    open        loc-srv

139/tcp    open        netbios-ssn

389/tcp    open        ldap

443/tcp    open        https

445/tcp    open        microsoft-ds

464/tcp    open        kpasswd5

593/tcp    open        http-rpc-epmap

636/tcp    open        ldapssl

1026/tcp   open        LSA-or-nterm

1029/tcp   open        ms-lsa

1033/tcp   open        netinfo

3268/tcp   open        globalcatLDAP

3269/tcp   open        globalcatLDAPssl

3372/tcp   open        msdtc

3389/tcp   open        ms-term-serv

Nmap run completed -- 1 IP address (1 host up)

If nmap is run without the -P0 flag when performing third-party scanning, the source IP address of the attacker's host performs ICMP and TCP pinging of the target hosts before starting to scan; this can appear in firewall and IDS audit logs of security-conscious organizations.

vscan is another Windows tool that can perform the same inverse IP ID scanning. As discussed earlier, the vscan utility doesn't require installation of WinPcap network drivers. Instead, it uses raw sockets within Winsock 2 (present in Windows 2000, XP, and 2003). vscan is available at http://host.deluxnetwork.com/~vsniff/vscan.zip.

Figure 4-11 shows the vscan utility in use, along with its options and functionality.

Figure 4-11. vscan used to launch an IP ID header scan