Port Scanning

-Pn : Avoid host discovery phase.

Default scans

Nmap offers a lot of scanning options to specify the scanning technique. But if none were specified, nmap chooses a default scan type which depends solely on the privileges the user have. Here's a quick explanation:

Figure1

Discovering OPEN TCP Ports

└──╼ $nmap -Pn -n 10.10.10.10

Discovering OPEN UDP Ports (Much slower than TCP)

└──╼ $nmap -Pn -n -sU 10.10.10.10

• We do not receive any acknowledgment. Consequently, the timeout is much longer, making the whole UDP scan (-sU) much slower than the TCP scan (-sS).

• Also We often do not get a response back because Nmap sends empty datagrams to the scanned UDP ports, and we do not receive any response. So we cannot determine if the UDP packet has arrived at all or not. If the UDP port is open, we only get a response if the application is configured to do so.

UDP scans possible outputs.

Service detection:

└──╼ $sudo nmap 10.10.10.10 -p- -sV

Starting Nmap 7.80 ( https://nmap.org ) at 2020-06-15 20:00 CEST
Nmap scan report for 10.129.2.28
Host is up (0.013s latency).
Not shown: 65525 closed ports
PORT      STATE    SERVICE      VERSION
22/tcp    open     ssh          OpenSSH 7.6p1 Ubuntu 4ubuntu0.3 (Ubuntu Linux; protocol 2.0)
25/tcp    open     smtp         Postfix smtpd
80/tcp    open     http         Apache httpd 2.4.29 ((Ubuntu))
110/tcp   open     pop3         Dovecot pop3d
139/tcp   filtered netbios-ssn
143/tcp   open     imap         Dovecot imapd (Ubuntu)
445/tcp   filtered microsoft-ds
993/tcp   open     ssl/imap     Dovecot imapd (Ubuntu)
995/tcp   open     ssl/pop3     Dovecot pop3d
MAC Address: DE:AD:00:00:BE:EF (Intel Corporate)
Service Info: Host:  inlane; OS: Linux; CPE: cpe:/o:linux:linux_kernel

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 91.73 seconds

Primarily nmap prints out the banners of the scanned ports. If nmap cannot identify versions through banner grabbing, it performs a signature-based matching system which increases the duration.

Nmap can miss some information because sometimes it doesn't know how to handle some information.

It happens because, after a successful three-way handshake, the server often sends a banner for identification. This serves to let the client know which service it is working with. At the network level, this happens with a PSH flag in the TCP header. This mechanism is described in the first figure.

Banner grabbing for service detection:

└──╼ $ sudo nmap 10.10.10.10 -p- -sV -Pn -n --disable-arp-ping --packet-trace

Starting Nmap 7.80 ( https://nmap.org ) at 2020-06-16 20:10 CEST
<SNIP>
NSOCK INFO [0.4200s] nsock_trace_handler_callback() .. SNIP .. ESMTP 
Postfix (Ubuntu)..
It detected that it is Ubuntu

Service scan match (Probe NULL matched with NULL line 3104): 10.129.2.28:25 is smtp.  Version: |Postfix smtpd|||
NSOCK INFO [0.4200s] nsock_iod_delete(): nsock_iod_delete (IOD #1)
Nmap scan report for 10.129.2.28
Host is up (0.076s latency).

PORT   STATE SERVICE VERSION
25/tcp open  smtp    
MAC Address: DE:AD:00:00:BE:EF (Intel Corporate)
Service Info: Host:  inlane

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 0.47 seconds

Let's prepare the intercept network traffic using tcpdump

└──╼ $ sudo tcpdump -i eth0 host 10.10.10.10 and 1.1.1.1
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes

Now let's netcat to the target machine an look at the intercepted traffic in tcpdump output:

└──╼ $ nc -nv 10.10.10.10 25
Connection to 10.10.10.10 port 25 [tcp/*] succeeded!
220 inlane ESMTP Postfix (Ubuntu)

18:28:07.128564 IP 1.1.1.1.59618 > 10.10.10.10.smtp: Flags [S], seq 1798872233, win 65535, options [mss 1460,nop,wscale 6,nop,nop,TS val 331260178 ecr 0,sackOK,eol], length 0
18:28:07.255151 IP 10.10.10.10.smtp > 1.1.1.1.59618: Flags [S.], seq 1130574379, ack 1798872234, win 65160, options [mss 1460,sackOK,TS val 1800383922 ecr 331260178,nop,wscale 7], length 0
18:28:07.255281 IP 1.1.1.1.59618 > 10.10.10.10.smtp: Flags [.], ack 1, win 2058, options [nop,nop,TS val 331260304 ecr 1800383922], length 0
18:28:07.319306 IP 10.10.10.10.smtp > 1.1.1.1.59618: Flags [P.], seq 1:36, ack 1, win 510, options [nop,nop,TS val 1800383985 ecr 331260304], length 35: SMTP: 220 inlane ESMTP Postfix (Ubuntu)
18:28:07.319426 IP 1.1.1.1.59618 > 10.10.10.10.smtp: Flags [.], ack 36, win 2058, options [nop,nop,TS val 331260368 ecr 1800383985], length 0

1. SYN sent.

2. SYN/ACK received.

3. ACK sent.

4. PSH/ACK received. (Exactly where the confusion happens.)

5. ACK sent.