205.2. Advanced Network Configuration and Troubleshooting
  • 205.2 Advanced Network Configuration and Troubleshooting
Weight: 4
Description: Candidates should be able to configure a network device to implement various network authentication schemes. This objective includes configuring a multi-homed network device and resolving communication problems.
Key Knowledge Areas:
  • Utilities to manipulate routing tables
  • Utilities to configure and manipulate ethernet network interfaces
  • Utilities to analyze the status of the network devices
  • Utilities to monitor and analyze the TCP/IP traffic
Terms and Utilities:
  • ip
  • ifconfig
  • route
  • arp
  • ss
  • netstat
  • lsof
  • ping, ping6
  • nc
  • tcpdump
  • nmap
In this lesson we discuss about some other Network tools which help up in network trouble shooting.

What is socket?

In computer networking, and more definitely in software terms, a port is a logical entity which acts as a endpoint of communication to identify a given application or process on an Linux operating system. It is a 16-bit number (0 to 65535) which differentiates one application from another on end systems. Different categories of ports are:
  • 0-1023 – the Well Known Ports, also referred to as System Ports.
  • 1024-49151 – the Registered Ports, also known as User Ports.
  • 49152-65535 – the Dynamic Ports, also referred to as the Private Ports.
The two most popular Internet transport protocols, Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) and other less known protocols use port numbers for communication sessions.
A combination of an IP address, port and protocol such as TCP/UDP is known as a socket, and every service must have a unique socket.


netstat (network statistics)outputs network connections form local host perspective, routing, interface statistics, connections and multicast information. so its pretty handy.
[email protected]:~# netstat
Active Internet connections (w/o servers)
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp 1 0 hanger.canonical.c:http CLOSE_WAIT
tcp 0 0 yukinko.canonical.:http ESTABLISHED
udp 0 0 localhost:57079 ubuntu:domain ESTABLISHED
udp 0 0 localhost:47407 ubuntu:domain ESTABLISHED
Active UNIX domain sockets (w/o servers)
Proto RefCnt Flags Type State I-Node Path
unix 2 [ ] DGRAM 26445 /run/user/1000/systemd/notify
unix 2 [ ] DGRAM 22660 /run/user/108/systemd/notify
unix 9 [ ] DGRAM 15796 /run/systemd/journal/socket
unix 2 [ ] DGRAM 15797 /run/systemd/journal/syslog
unix 20 [ ] DGRAM 15798 /run/systemd/journal/dev-log
unix 3 [ ] DGRAM 14516 /run/systemd/notify
unix 3 [ ] STREAM CONNECTED 27303
unix 3 [ ] DGRAM 16955
unix 3 [ ] STREAM CONNECTED 23403 @/tmp/dbus-0tx8y6voha
unix 3 [ ] STREAM CONNECTED 30750 /run/systemd/journal/stdout
unix 3 [ ] STREAM CONNECTED 23522 /run/systemd/journal/stdout
unix 3 [ ] STREAM CONNECTED 28047
unix 3 [ ] STREAM CONNECTED 22005 /run/systemd/journal/stdout
unix 3 [ ] STREAM CONNECTED 28223 @/tmp/dbus-K7JvDcncxI
unix 3 [ ] STREAM CONNECTED 22006 /run/systemd/journal/stdout
unix 3 [ ] DGRAM 28279
unix 3 [ ] STREAM CONNECTED 27098 /var/run/dbus/system_bus_socket
unix 3 [ ] STREAM CONNECTED 28737 /var/run/cups/cups.sock
unix 3 [ ] STREAM CONNECTED 18372 /var/run/dbus/system_bus_socket
unix 3 [ ] STREAM CONNECTED 29963
unix 3 [ ] DGRAM 16958
unix 3 [ ] STREAM CONNECTED 23372
unix 3 [ ] STREAM CONNECTED 22015
unix 2 [ ] DGRAM 18086
unix 3 [ ] STREAM CONNECTED 30009
unix 3 [ ] STREAM CONNECTED 23529 /var/run/dbus/system_bus_socket
unix 3 [ ] STREAM CONNECTED 18188 /var/run/dbus/system_bus_socket
unix 3 [ ] STREAM CONNECTED 28042
unix 3 [ ] STREAM CONNECTED 19237 /run/systemd/journal/stdout
unix 3 [ ] STREAM CONNECTED 30014 @/tmp/dbus-K7JvDcncxI
unix 3 [ ] STREAM CONNECTED 23289
unix 3 [ ] STREAM CONNECTED 29849
unix 3 [ ] STREAM CONNECTED 28050 @/com/ubuntu/upstart-session/1000/2169
unix 3 [ ] STREAM CONNECTED 23332 @/tmp/.X11-unix/X0
unix 3 [ ] STREAM CONNECTED 30806
unix 2 [ ] DGRAM 22687
unix 3 [ ] STREAM CONNECTED 27237
unix 3 [ ] STREAM CONNECTED 28915
unix 3 [ ] STREAM CONNECTED 26586
unix 3 [ ] STREAM CONNECTED 24238 /run/systemd/journal/stdout
unix 2 [ ] DGRAM 22556
unix 3 [ ] STREAM CONNECTED 27263
unix 3 [ ] STREAM CONNECTED 30005 @/tmp/.X11-unix/X0
unix 3 [ ] STREAM CONNECTED 27232
unix 3 [ ] STREAM CONNECTED 27174 @/tmp/dbus-K7JvDcncxI
unix 3 [ ] STREAM CONNECTED 18371 /var/run/dbus/system_bus_socket
unix 3 [ ] STREAM CONNECTED 25098
unix 3 [ ] STREAM CONNECTED 17382
unix 3 [ ] STREAM CONNECTED 28751 /run/systemd/journal/stdout
unix 3 [ ] STREAM CONNECTED 23256
unix 3 [ ] STREAM CONNECTED 19416 /run/systemd/journal/stdout
unix 3 [ ] STREAM CONNECTED 27276 @/tmp/dbus-K7JvDcncxI
unix 3 [ ] STREAM CONNECTED 19418 /run/systemd/journal/stdout
unix 3 [ ] STREAM CONNECTED 28226 @/tmp/dbus-K7JvDcncxI
unix 3 [ ] STREAM CONNECTED 23528
unix 3 [ ] STREAM CONNECTED 29966
unix 3 [ ] DGRAM 16957
unix 3 [ ] STREAM CONNECTED 19739 /var/run/dbus/system_
netstat it self give us a log list of network information which might not be useful, so lets try some useful switches:
useful netstat command switches
-s , --statistics
gives summary by protocol type and message type
-i , --interfaces
Display a table of all network interfaces
-r , --route
shows routing table information
-a , --all
Show both listening and non-listening sockets
-t , --tcp
Enabales listening of tcp ports
-u , --udp
Enables listening of udp ports
-l , --listening
Prints only listening sockets
-p , --program
Shows PID and the name of associated program
-e , --extend
Shows additional information, use twice for more info
-n , --numeric
Shows numerical addresses
netstat supports ipv6.


The ss command is capable of showing more information than the netstat and is faster. The netstat command reads various /proc files to gather information. However this approach falls weak when there are lots of connections to display. This makes it slower.
The ss command gets its information directly from kernel space. The options used with the ss commands are very similar to netstat making it an easy replacement.
If we use the ss command without any arguments or options, it will return a complete list of TCP sockets with established connections:
Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port
u_str ESTAB 0 0 * 23433 * 27689
u_str ESTAB 0 0 * 19210 * 19211
u_str ESTAB 0 0 * 28948 * 30007
u_str ESTAB 0 0 * 23378 * 25314
u_str ESTAB 0 0 @/tmp/dbus-FSfvGgFOBN 23345 * 25
u_str ESTAB 0 0 * 30024 * 28207
u_str ESTAB 0 0 * 27840 * 23551
u_str ESTAB 0 0 @/tmp/dbus-FSfvGgFOBN 25388 * 23
u_str ESTAB 0 0 @/tmp/dbus-FSfvGgFOBN 29978 * 31
u_str ESTAB 0 0 @/tmp/.X11-unix/X0 30930 * 28156
u_str ESTAB 0 0 /run/systemd/journal/stdout 28986
* 31082
u_str ESTAB 0 0 @/tmp/dbus-FSfvGgFOBN 27007 * 23
u_str ESTAB 0 0 /var/run/dbus/system_bus_socket 20007
* 19011
u_str ESTAB 0 0 * 28949 * 30010
useful ss command
-l , --listening
Display only listening sockets
for IPv4
for IPv6
-a , --all
Display both listening and non-listening (for TCP this means established connections) sockets
-t , --tcp
Shows established or CONNECTED tcp connections
-u , --udp
Shows established or CONNECTED tcp connections
-s , --summary
Print summary statistics.
-o , --options
time information of each connection would be displayed
-n , --numeric
Do not try to resoleve service names


To get a list of files which are opened by users and associated processes with them we use lsof (LiSt Open Files).
[email protected]:~# lsof | more
systemd 1 root cwd DIR 8,1 4096 2 /
systemd 1 root rtd DIR 8,1 4096 2 /
systemd 1 root txt REG 8,1 1577232 135885 /lib/systemd/systemd
systemd 1 root mem REG 8,1 18976 136484 /lib/x86_64-linux-gnu/libuuid.so.1.3.0
systemd 1 root mem REG 8,1 262408 136292 /lib/x86_64-linux-gnu/libblkid.so.1.1.0
systemd 1 root DEL REG 8,1 136324 /lib/x86_64-linux-gnu/libdl-2.23.so
systemd 1 root mem REG 8,1 456632 136429 /lib/x86_64-linux-gnu/libpcre.so.3.13.2
systemd 1 root DEL REG 8,1 136300 /lib/x86_64-linux-gnu/libc-2.23.so
systemd 1 root DEL REG 8,1 136446 /lib/x86_64-linux-gnu/libpthread-2.23.so
systemd 1 root mem REG 8,1 286824 136376 /lib/x86_64-linux-gnu/libmount.so.1.1.0
systemd 1 root mem REG 8,1 64144 136282 /lib/x86_64-linux-gnu/libapparmor.so.1.4.0
systemd 1 root mem REG 8,1 92864 136363 /lib/x86_64-linux-gnu/libkmod.so.2.3.0
systemd 1 root mem REG 8,1 117288 136290 /lib/x86_64-linux-gnu/libaudit.so.1.0.0
systemd 1 root mem REG 8,1 55904 136416 /lib/x86_64-linux-gnu/libpam.so.0.83.1
systemd 1 root mem REG 8,1 252152 136457 /lib/x86_64-linux-gnu/libseccomp.so.2.2.3
lsof useful commmands
lsof -n
Do not tries to resolve IP Addresses to DNS
lsof /var/log/syslog
Shows which processes have opened specific file
lsof +D /var/log
List opened files under a Directory
lsof /home
List processes using a mount point
lsof -u payam
List files opened by a specific user
lsof -p 1357
all open files by specific process
lsof -c ssh
List opened files based on process names starting with ...
lsof -t /var/log/syslog
list process id of a process which opened /var/log/syslog
lsof -i
list all network connections


PING (Packet INternet Groper) command is the best way to test connectivity between two nodes. Whether it is Local Area Network (LAN) or Wide Area Network (WAN).
How does ping work? Ping use ICMP (Internet Control Message Protocol) to communicate to other devices.When a user pings a host on the Internet, a series of ICMP packets are sent to the host, which responds by sending packets in return. The user’s client is then able to compute the round trip time between two points on the Internet.
You can ping host name or ip address :
[email protected]:~# ping
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=128 time=244 ms
64 bytes from icmp_seq=2 ttl=128 time=239 ms
--- ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1001ms
rtt min/avg/max/mdev = 239.179/241.812/244.446/2.679 ms
[email protected]:~# ping yahoo.com
PING yahoo.com ( 56(84) bytes of data.
64 bytes from media-router-fp1.prod.media.vip.bf1.yahoo.com ( icmp_seq=1 ttl=128 time=248 ms
64 bytes from media-router-fp1.prod.media.vip.bf1.yahoo.com ( icmp_seq=2 ttl=128 time=224 ms
64 bytes from media-router-fp1.prod.media.vip.bf1.yahoo.com ( icmp_seq=3 ttl=128 time=232 ms
--- yahoo.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2003ms
rtt min/avg/max/mdev = 224.253/235.002/248.218/9.953 ms
In Linux ping command keep executing until we interrupt.
useful ping switches
verbose mode
Show version and exit
-c 5
Stop after sending 5 ECHO_REQUEST packets.
-i 0.9
Increase / decrease pint time interval
Report outstanding ECHO REPLAY before sending next packet
- w 10
Specify a timeout, in seconds, before ping exits regardless of how many packets have been sent or received
-s 100
Change the default packet size from 56 to 100
Record and print route of how ECHO_REQUEST sent and ECHO_REPLY received
Audible ping: Give beep when the peer is reachable


Regular ping command only works with IPv4 address. Use ping6 command to send ICMPv6 ECHO_REQUEST packets to network hosts from a host or gateway. Try to set IPv6 gateway and test:
ping6 ipv6.google.com


Netcat (also known as ‘nc’ or ‘Swiss Army knife’) is a networking utility used for reading or writing from TCP and UDP sockets using an easy interface. Netcat is used by Administrators, Developers and pen testers because of its features.
. .
} 6 6 {
==. Y ,==
/^^^\ .
/ \ )
( )-( )/ _
-""---""--- /
/ \_/
( ____
This is nc from the netcat-openbsd package. An alternative nc is available
in the netcat-traditional package.
usage: nc [-46bCDdhjklnrStUuvZz] [-I length] [-i interval] [-O length]
[-P proxy_username] [-p source_port] [-q seconds] [-s source]
[-T toskeyword] [-V rtable] [-w timeout] [-X proxy_protocol]
[-x proxy_address[:port]] [destination] [port]
With netcat (nc) we can setup kind of client server connection over a specific port. For demo lets start chatting between two servers, server1 starts listening on port 12345:
[email protected]:~# nc -l 12345
send text stream from server2 to server1:
[email protected]:/home/payam# su -
[email protected]:~# nc 80
I'm here to send some messages over port 80
using netcat :)
and see the result on server1:
I'm here to send some messages over port 80
using netcat :)
netcat useful switches
Forces nc to use IP v4 only
Forces nc to use IP v6 only
Give more verbose output
Do not DNS Lookup for IP Addresses, Host Name or ports
allow server to continue listening after client disconnect
use udp
setting up time out
also we can use nc for prt scanning although netcat is not the best tool for the job:
[email protected]:~# nc -v -z 12340-12346
nc: connect to port 12340 (tcp) failed: Connection refused
nc: connect to port 12341 (tcp) failed: Connection refused
nc: connect to port 12342 (tcp) failed: Connection refused
nc: connect to port 12343 (tcp) failed: Connection refused
nc: connect to port 12344 (tcp) failed: Connection refused
Connection to 12345 port [tcp/*] succeeded!
nc: connect to port 12346 (tcp) failed: Connection refused
-z says that nc should just scan for listening daemons, without sending any data to them. can not be used with -l option.


tcpdump is command-line packet sniffers and analyzer tool which is used to capture or filter TCP/IP packets which recieved or transfered over a network on a specific interface.
tcpdump also give us a option to save captured packets in a file in a pcap format. It is very useful for future analysis and also file can be viewed by tcpdump command or a open source GUI based tool like Wireshark.
[email protected]:~# tcpdump
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on ens33, link-type EN10MB (Ethernet), capture size 262144 bytes
03:12:41.296807 IP economy.canonical.com.http > Flags [P.], seq 877174157:877175557, ack 3379990991, win 64240, length 1400: HTTP
03:12:41.297065 IP > economy.canonical.com.http: Flags [.], ack 1400, win 65535, length 0
03:12:41.298574 IP economy.canonical.com.http > Flags [P.], seq 1400:2800, ack 1, win 64240, length 1400: HTTP
03:12:41.298621 IP economy.canonical.com.http > Flags [P.], seq 2800:7000, ack 1, win 64240, length 4200: HTTP
03:12:41.298650 IP > economy.canonical.com.http: Flags [.], ack 7000, win 65535, length 0
03:12:41.299625 IP economy.canonical.com.http > Flags [P.], seq 7000:8400, ack 1, win 64240, length 1400: HTTP
03:12:41.300067 IP > economy.canonical.com.http: Flags [.], ack 8400, win 65535, length 0
03:12:41.798839 IP economy.canonical.com.http > Flags [P.], seq 92300:93600, ack 1, win 64240, length 1300: HTTP
03:12:41.799351 IP > economy.canonical.com.http: Flags [.], ack 93600, win 65535, length 0
03:12:41.800018 IP economy.canonical.com.http > Flags [P.], seq 93600:96200, ack 1, win 64240, length 2600: HTTP
03:12:41.800610 IP > economy.canonical.com.http: Flags [.], ack 96200, win 65535, length 0
03:12:41.801294 IP economy.canonical.com.http > Flags [P.], seq 96200:101600, ack 1, win 64240, length 5400: HTTP
03:12:41.801393 IP > economy.canonical.com.http: Flags [.], ack 101600, win 65535, length 0
03:12:41.801538 IP economy.canonical.com.http > Flags [P.], seq 101600:103000, ack 1, win 64240, length 1400: HTTP
03:12:41.801743 IP > economy.canonical.com.http: Flags [.], ack 103000, win 65535, length 0
03:12:41.807292 IP economy.canonical.com.http > Flags [P.], seq 103000:114200, ack 1, win 64240, length 11200: HTTP
03:12:41.807493 IP > economy.canonical.com.http: Flags [.], ack 114200, win 65535, length 0
03:12:41.808278 IP economy.canonical.com.http > Flags [P.], seq 114200:118400, ack 1, win 64240, length 4200: HTTP
03:12:41.808417 IP > economy.canonical.com.http: Flags [.], ack 118400, win 65535, length 0
03:12:41.809282 IP economy.canonical.com.http > Flags [P.], seq 118400:123500, ack 1, win 64240, length 5100: HTTP
03:12:41.809402 IP > economy.canonical.com.http: Flags [.], ack 123500, win 65535, length 0
03:12:41.952442 IP economy.canonical.com.http > Flags [P.], seq 123500:124800, ack 1, win 64240, length 1300: HTTP
03:12:41.952677 IP > economy.canonical.com.http: Flags [.], ack 124800, win 65535, length 0
03:12:41.961111 IP economy.canonical.com.http > Flags [P.], seq 124800:127400, ack 1, win 64240, length 2600: HTTP
03:12:41.961181 IP > economy.canonical.com.http: Flags [.], ack 127400, win 65535, length 0
27 packets captured
71 packets received by filter
44 packets dropped by kernel
watch some usefull tcpdum commands:
useful tcpdump command
tcpdump -D
Display available interfaces
tcpdum -i ens33
Capture Packets from specific interface
tcpdump -c 10 -i ens33
Capture only N neumber of packets
tcpdump -w myfile.pcap -i ens33
Capture and save packets in a file
tcpdump -r myfile.pcap
Read captured packets file
tcpdump -n -i ens33
Capture IP address Packets
tcpdum -i ens33 tcp
Only TCP Packets are captured
tcpdump -i ens33 port 22
Capture packets from specific port
tcpdump -i eth0 src
Capture packets from source IP
tcpdump -i eth0 dst
Capture packets from destination IP
tcpdump -i ens33 ipv6
Sniffing for IPv6


nmap (Network Mapper) is a security scanner.It is used to discover hosts and services on a computer network, whith the goal of building a "map" of the network. To accomplish it, Nmap sends specially designed packets to the target host(s) and then analyzes the responses. Nmap can adapt to network conditions including latency and congestion during a scan. The Nmap user community continues to develop and refine the tool.
Nmap was originally written for Linux, but it has been ported to major operating systems, such as Windows, Solaris, HP-UX, etc. There is even a free and open source GUI called Zenmap.
Although usually used for port scanning, Nmap offers many additional features:
  • host discovery.
  • operating system detection.
  • service version detection.
  • network information about targets, such as DNS names, device types, and MAC addresses.
  • ability to scan for well-known vulnerabilities.
    nmap is not installed by default on most of linux distros and we need to install it:
[email protected]:~# apt install nmap
Before start scanning please note that Port scanning without authorization is sometimes against the provider's acceptable use policy (AUP).
Till the end of war Its better to test it on your own network and do not take the risk. nmap command with no option just prints its long list of switches and options. For our demonstartion we scan scanme.nmap.org :
[email protected]:~# nmap -v scanme.nmap.org
Starting Nmap 7.01 ( https://nmap.org ) at 2018-01-22 03:25 PST
Initiating Ping Scan at 03:25
Scanning scanme.nmap.org ( [4 ports]
Completed Ping Scan at 03:25, 0.24s elapsed (1 total hosts)
Initiating Parallel DNS resolution of 1 host. at 03:25
Completed Parallel DNS resolution of 1 host. at 03:25, 0.35s elapsed
Initiating SYN Stealth Scan at 03:25
Scanning scanme.nmap.org ( [1000 ports]
Discovered open port 80/tcp on
Discovered open port 22/tcp on
Increasing send delay for from 0 to 5 due to 11 out of 24 dropped probes since last increase.
SYN Stealth Scan Timing: About 12.63% done; ETC: 03:29 (0:03:34 remaining)
SYN Stealth Scan Timing: About 14.37% done; ETC: 03:32 (0:06:04 remaining)
Increasing send delay for from 5 to 10 due to 11 out of 11 dropped probes since last increase.
SYN Stealth Scan Timing: About 58.53% done; ETC: 03:28 (0:01:04 remaining)
Completed SYN Stealth Scan at 03:27, 109.38s elapsed (1000 total ports)
Nmap scan report for scanme.nmap.org (
Host is up (0.0018s latency).
Other addresses for scanme.nmap.org (not scanned): 2600:3c01::f03c:91ff:fe18:bb2f
Not shown: 987 filtered ports
21/tcp closed ftp
22/tcp open ssh
53/tcp closed domain
80/tcp open http
110/tcp closed pop3
143/tcp closed imap
443/tcp closed https
587/tcp closed submission
993/tcp closed imaps
995/tcp closed pop3s
3389/tcp closed ms-wbt-server
8080/tcp closed http-proxy
8291/tcp closed unknown
Read data files from: /usr/bin/../share/nmap
Nmap done: 1 IP address (1 host up) scanned in 110.55 seconds
Raw packets sent: 3033 (133.256KB) | Rcvd: 1500 (60.008KB)
The -v option enables verbose mode.
nmap Target selection
scan a single IP
nmap scanme.nmap.org
scan a host
scan a range of IPs
scan a subnet
nmap -iL myserverlist.txt
scan targets from a text file
nmap -6 [IP-V6-HERE]
enables IP v6 scanning
okey and ports:
nmap port selection
nmap -p 22
scan a single port
nmap -p 1-100
scan a range of port
nmap -F
Fast-scan 100 most common ports
nmap -p-
scan all 65535 ports
Hmm. nmap has different techniques for scanning. But before explaining them we review some fundamentals.

TCP 3-Way Handshake Diagram

Below is a very simplified diagram of the TCP 3-way handshake process. Have a look at the diagram on the right as you examine the list of events on the left.
after this small review we can talk about nmap different methods of scanning:
  • TCP SYN Scan (-sS)
It is a basic scan, and it is also called half-open scanning because this technique allows Nmap to get information from the remote host without the complete TCP handshake process, Nmap sends SYN packets to the destination, but it does not create any sessions, As a result, the target computer can’t create any log of the interaction because no session was initiated, making this feature an advantage of the TCP SYN scan.nmap -sT
  • TCP connect() scan (-sT)
This the default scanning technique used, if and only if the SYN scan is not an option, because the SYN scan requires root privilege. Unlike the TCP SYN scan, it completes the normal TCP three way handshake process and requires the system to call connect(), which is a part of the operating system. Keep in mind that this technique is only applicable to find out the TCP ports, not the UDP ports. nmap -sT
  • UDP Scan (-sU)
As the name suggests, this technique is used to find an open UDP port of the target machine. It does not require any SYN packet to be sent because it is targeting the UDP ports. But we can make the scanning more effective by using -sS along with –sU. UDP scans send the UDP packets to the target machine, and waits for a response—if an error message arrives saying the ICMP is unreachable, then it means that the port is closed; but if it gets an appropriate response, then it means that the port is open.
nmap -sU
  • FIN Scan (-sF)
Sometimes a normal TCP SYN scan is not the best solution because of the firewall. IDS and IPS scans might be deployed on the target machine, but a firewall will usually block the SYN packets. A FIN scan sends the packet only set with a FIN flag, so it is not required to complete the TCP handshaking.
The target computer is not able to create a log of this scan (again, an advantage of FIN). Just like a FIN scan, we can perform an xmas scan (-sX) and Null scan (-sN). The idea is same but there is a difference between each type of scan.
  • Ping Scan (-sP)
Ping scanning is unlike the other scan techniques because it is only used to find out whether the host is alive or not, it is not used to discover open ports. Ping scans require root access s ICMP packets can be sent, but if the user does not have administrator privilege, then the ping scan uses connect() call. nmap -sP
  • Version Detection (-sV)
Version detection is the right technique that is used to find out what software version is running on the target computer and on the respective ports. It is unlike the other scanning techniques because it is not used to detect the open ports, but it requires the information from open ports to detect the software version. In the first step of this scan technique, version detection uses the TCP SYN scan to find out which ports are open. nmap -sV
  • Idle Scan (-sI)
Idle scan is one of my favorite techniques, and it is an advance scan that provides complete anonymity while scanning. In idle scan, Nmap doesn’t send the packets from your real IP address—instead of generating the packets from the attacker machine, Nmap uses another host from the target network to send the packets. ex: nmap -sI
The idle scan technique (as mentioned above) is used to discover the open ports on while it uses the zombie_host ( to communicate with the target host. So this is an ideal technique to scan a target computer anonymously.
There are many other scanning techniques are available like FTP bounce, fragmentation scan, ... but that is enough for LPIC exam.
Copy link
On this page
What is socket?
TCP 3-Way Handshake Diagram