TCP Connect Scan (-sT)

TCP connect scan is the default TCP scan type when SYN scan is not an option. This is the case when a user does not have raw packet privileges or is scanning IPv6 networks. Instead of writing raw packets as most other scan types do, Nmap asks the underlying operating system to establish a connection with the target machine and port by issuing the connect system call. This is the same high-level system call that web browsers, P2P clients, and most other network-enabled applications use to establish a connection. It is part of a programming interface known as the Berkeley Sockets API. Rather than read raw packet responses off the wire, Nmap uses this API to obtain status information on each connection attempt. This and the FTP bounce scan (the section called “TCP FTP Bounce Scan (-b)”) are the only scan types available to unprivileged users.

When SYN scan is available, it is usually a better choice. Nmap has less control over the high level connect call than with raw packets, making it less efficient. The system call completes connections to open target ports rather than performing the half-open reset that SYN scan does. Not only does this take longer and require more packets to obtain the same information, but target machines are more likely to log the connection. A decent IDS will catch either, but most machines have no such alarm system. Many services on your average Unix system will add a note to syslog, and sometimes a cryptic error message, when Nmap connects and then closes the connection without sending data. Truly pathetic services crash when this happens, though that is uncommon. An administrator who sees a bunch of connection attempts in her logs from a single system should know that she has been connect scanned.

Figure 5.5 shows a connect scan in action against open port 22 of Recall that this only required three packets in Figure 5.2, “SYN scan of open port 22”. The exact behavior against an open port depends on the platform Nmap runs on and the service listening at the other end, but this five packet example is typical.

Figure 5.5. Connect scan of open port 22
Connect scan of open port 22

The first two steps (SYN and SYN/ACK) are exactly the same as with a SYN scan. Then, instead of aborting the half-open connection with a RST packet, krad acknowledges the SYN/ACK with its own ACK packet, completing the connection. In this case, Scanme even had time to send its SSH banner string (SSH-1.99-OpenSSH_3.1p1\n) through the now-open connection. As soon as Nmap hears from its host OS that the connection was successful, it terminates the connection. TCP connections usually end with another handshake involving the FIN flag, but Nmap asks the host OS to terminate the connection immediately with a RST packet.

While this connect scan example took almost twice as many packets as a SYN scan, the bandwidth differences are rarely so substantial. The vast majority of ports in a large scan will be closed or filtered. The packet traces for those are the same as described for SYN scan in Figure 5.3, “SYN scan of closed port 113” and Figure 5.4, “SYN scan of filtered port 139”. Only open ports generate more network traffic.

The output of a connect scan doesn't differ significantly from a SYN scan. Example 5.3 shows a connect scan of Scanme. The -sT option could have been omitted since Nmap is being run from a non-privileged account so connect scan is the default type.

Example 5.3. Connect scan example
krad~> nmap -T4 -sT

Starting Nmap ( )
Nmap scan report for (
Not shown: 994 filtered ports
22/tcp  open   ssh
25/tcp  closed smtp
53/tcp  open   domain
70/tcp  closed gopher
80/tcp  open   http
113/tcp closed auth

Nmap done: 1 IP address (1 host up) scanned in 4.74 seconds