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.
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.
root@server1:~# netstatActive Internet connections (w/o servers)Proto Recv-Q Send-Q Local Address Foreign Address Statetcp 1 0 192.168.10.152:44480 hanger.canonical.c:http CLOSE_WAITtcp 0 0 192.168.10.152:42844 yukinko.canonical.:http ESTABLISHEDudp 0 0 localhost:57079 ubuntu:domain ESTABLISHEDudp 0 0 localhost:47407 ubuntu:domain ESTABLISHEDActive UNIX domain sockets (w/o servers)Proto RefCnt Flags Type State I-Node Pathunix 2 [ ] DGRAM 26445 /run/user/1000/systemd/notifyunix 2 [ ] DGRAM 22660 /run/user/108/systemd/notifyunix 9 [ ] DGRAM 15796 /run/systemd/journal/socketunix 2 [ ] DGRAM 15797 /run/systemd/journal/syslogunix 20 [ ] DGRAM 15798 /run/systemd/journal/dev-logunix 3 [ ] DGRAM 14516 /run/systemd/notifyunix 3 [ ] STREAM CONNECTED 27303unix 3 [ ] DGRAM 16955unix 3 [ ] STREAM CONNECTED 23403 @/tmp/dbus-0tx8y6vohaunix 3 [ ] STREAM CONNECTED 30750 /run/systemd/journal/stdoutunix 3 [ ] STREAM CONNECTED 23522 /run/systemd/journal/stdoutunix 3 [ ] STREAM CONNECTED 28047unix 3 [ ] STREAM CONNECTED 22005 /run/systemd/journal/stdoutunix 3 [ ] STREAM CONNECTED 28223 @/tmp/dbus-K7JvDcncxIunix 3 [ ] STREAM CONNECTED 22006 /run/systemd/journal/stdoutunix 3 [ ] DGRAM 28279unix 3 [ ] STREAM CONNECTED 27098 /var/run/dbus/system_bus_socketunix 3 [ ] STREAM CONNECTED 28737 /var/run/cups/cups.sockunix 3 [ ] STREAM CONNECTED 18372 /var/run/dbus/system_bus_socketunix 3 [ ] STREAM CONNECTED 29963unix 3 [ ] DGRAM 16958unix 3 [ ] STREAM CONNECTED 23372unix 3 [ ] STREAM CONNECTED 22015unix 2 [ ] DGRAM 18086unix 3 [ ] STREAM CONNECTED 30009unix 3 [ ] STREAM CONNECTED 23529 /var/run/dbus/system_bus_socketunix 3 [ ] STREAM CONNECTED 18188 /var/run/dbus/system_bus_socketunix 3 [ ] STREAM CONNECTED 28042unix 3 [ ] STREAM CONNECTED 19237 /run/systemd/journal/stdoutunix 3 [ ] STREAM CONNECTED 30014 @/tmp/dbus-K7JvDcncxIunix 3 [ ] STREAM CONNECTED 23289unix 3 [ ] STREAM CONNECTED 29849unix 3 [ ] STREAM CONNECTED 28050 @/com/ubuntu/upstart-session/1000/2169unix 3 [ ] STREAM CONNECTED 23332 @/tmp/.X11-unix/X0unix 3 [ ] STREAM CONNECTED 30806unix 2 [ ] DGRAM 22687unix 3 [ ] STREAM CONNECTED 27237unix 3 [ ] STREAM CONNECTED 28915unix 3 [ ] STREAM CONNECTED 26586unix 3 [ ] STREAM CONNECTED 24238 /run/systemd/journal/stdoutunix 2 [ ] DGRAM 22556unix 3 [ ] STREAM CONNECTED 27263unix 3 [ ] STREAM CONNECTED 30005 @/tmp/.X11-unix/X0unix 3 [ ] STREAM CONNECTED 27232unix 3 [ ] STREAM CONNECTED 27174 @/tmp/dbus-K7JvDcncxIunix 3 [ ] STREAM CONNECTED 18371 /var/run/dbus/system_bus_socketunix 3 [ ] STREAM CONNECTED 25098unix 3 [ ] STREAM CONNECTED 17382unix 3 [ ] STREAM CONNECTED 28751 /run/systemd/journal/stdoutunix 3 [ ] STREAM CONNECTED 23256unix 3 [ ] STREAM CONNECTED 19416 /run/systemd/journal/stdoutunix 3 [ ] STREAM CONNECTED 27276 @/tmp/dbus-K7JvDcncxIunix 3 [ ] STREAM CONNECTED 19418 /run/systemd/journal/stdoutunix 3 [ ] STREAM CONNECTED 28226 @/tmp/dbus-K7JvDcncxIunix 3 [ ] STREAM CONNECTED 23528unix 3 [ ] STREAM CONNECTED 29966unix 3 [ ] DGRAM 16957unix 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 | Description |
-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:Portu_str ESTAB 0 0 * 23433 * 27689u_str ESTAB 0 0 * 19210 * 19211u_str ESTAB 0 0 * 28948 * 30007u_str ESTAB 0 0 * 23378 * 25314u_str ESTAB 0 0 @/tmp/dbus-FSfvGgFOBN 23345 * 25283u_str ESTAB 0 0 * 30024 * 28207u_str ESTAB 0 0 * 27840 * 23551u_str ESTAB 0 0 @/tmp/dbus-FSfvGgFOBN 25388 * 23459u_str ESTAB 0 0 @/tmp/dbus-FSfvGgFOBN 29978 * 31023u_str ESTAB 0 0 @/tmp/.X11-unix/X0 30930 * 28156u_str ESTAB 0 0 /run/systemd/journal/stdout 28986* 31082u_str ESTAB 0 0 @/tmp/dbus-FSfvGgFOBN 27007 * 23483u_str ESTAB 0 0 /var/run/dbus/system_bus_socket 20007* 19011u_str ESTAB 0 0 * 28949 * 30010--More--
useful ss command | Description |
-l , --listening | Display only listening sockets |
-4 | for IPv4 |
-6 | 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).
root@server1:~# lsof | moreCOMMAND PID TID USER FD TYPE DEVICE SIZE/OFF NODE NAMEsystemd 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/systemdsystemd 1 root mem REG 8,1 18976 136484 /lib/x86_64-linux-gnu/libuuid.so.1.3.0systemd 1 root mem REG 8,1 262408 136292 /lib/x86_64-linux-gnu/libblkid.so.1.1.0systemd 1 root DEL REG 8,1 136324 /lib/x86_64-linux-gnu/libdl-2.23.sosystemd 1 root mem REG 8,1 456632 136429 /lib/x86_64-linux-gnu/libpcre.so.3.13.2systemd 1 root DEL REG 8,1 136300 /lib/x86_64-linux-gnu/libc-2.23.sosystemd 1 root DEL REG 8,1 136446 /lib/x86_64-linux-gnu/libpthread-2.23.sosystemd 1 root mem REG 8,1 286824 136376 /lib/x86_64-linux-gnu/libmount.so.1.1.0systemd 1 root mem REG 8,1 64144 136282 /lib/x86_64-linux-gnu/libapparmor.so.1.4.0systemd 1 root mem REG 8,1 92864 136363 /lib/x86_64-linux-gnu/libkmod.so.2.3.0systemd 1 root mem REG 8,1 117288 136290 /lib/x86_64-linux-gnu/libaudit.so.1.0.0systemd 1 root mem REG 8,1 55904 136416 /lib/x86_64-linux-gnu/libpam.so.0.83.1systemd 1 root mem REG 8,1 252152 136457 /lib/x86_64-linux-gnu/libseccomp.so.2.2.3
lsof useful commmands | Description |
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 :
root@server1:~# ping 8.8.8.8PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.64 bytes from 8.8.8.8: icmp_seq=1 ttl=128 time=244 ms64 bytes from 8.8.8.8: icmp_seq=2 ttl=128 time=239 ms^C--- 8.8.8.8 ping statistics ---2 packets transmitted, 2 received, 0% packet loss, time 1001msrtt min/avg/max/mdev = 239.179/241.812/244.446/2.679 msroot@server1:~# ping yahoo.comPING yahoo.com (98.139.180.180) 56(84) bytes of data.64 bytes from media-router-fp1.prod.media.vip.bf1.yahoo.com (98.139.180.180): icmp_seq=1 ttl=128 time=248 ms64 bytes from media-router-fp1.prod.media.vip.bf1.yahoo.com (98.139.180.180): icmp_seq=2 ttl=128 time=224 ms64 bytes from media-router-fp1.prod.media.vip.bf1.yahoo.com (98.139.180.180): icmp_seq=3 ttl=128 time=232 ms^C--- yahoo.com ping statistics ---3 packets transmitted, 3 received, 0% packet loss, time 2003msrtt 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 | Description |
-v | verbose mode |
-V | Show version and exit |
-c 5 | Stop after sending 5 ECHO_REQUEST packets. |
-i 0.9 | Increase / decrease pint time interval |
-O | 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 |
-R | Record and print route of how ECHO_REQUEST sent and ECHO_REPLY received |
-a | 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 ,==/^^^\ ./ \ )( )-( )/ _-""---""--- // \_/( ____\_.=|____Eroot@server1:~# ncThis is nc from the netcat-openbsd package. An alternative nc is availablein 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:
root@server1:~# nc -l 12345
send text stream from server2 to server1:
root@server2:~$ suPassword:root@server2:/home/payam# su -root@server2:~# nc 192.168.10.152 80Hello!I'm here to send some messages over port 80using netcat :)
and see the result on server1:
Hello!I'm here to send some messages over port 80using netcat :)
netcat useful switches | Description |
-4 | Forces nc to use IP v4 only |
-6 | Forces nc to use IP v6 only |
-v | Give more verbose output |
-n | Do not DNS Lookup for IP Addresses, Host Name or ports |
-k | allow server to continue listening after client disconnect |
-u | use udp |
-w | setting up time out |
also we can use nc for prt scanning although netcat is not the best tool for the job:
root@server2:~# nc -v 192.168.10.152 -z 12340-12346nc: connect to 192.168.10.152 port 12340 (tcp) failed: Connection refusednc: connect to 192.168.10.152 port 12341 (tcp) failed: Connection refusednc: connect to 192.168.10.152 port 12342 (tcp) failed: Connection refusednc: connect to 192.168.10.152 port 12343 (tcp) failed: Connection refusednc: connect to 192.168.10.152 port 12344 (tcp) failed: Connection refusedConnection to 192.168.10.152 12345 port [tcp/*] succeeded!nc: connect to 192.168.10.152 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.
root@server1:~# tcpdumptcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on ens33, link-type EN10MB (Ethernet), capture size 262144 bytes03:12:41.296807 IP economy.canonical.com.http > 192.168.10.152.60388: Flags [P.], seq 877174157:877175557, ack 3379990991, win 64240, length 1400: HTTP03:12:41.297065 IP 192.168.10.152.60388 > economy.canonical.com.http: Flags [.], ack 1400, win 65535, length 003:12:41.298574 IP economy.canonical.com.http > 192.168.10.152.60388: Flags [P.], seq 1400:2800, ack 1, win 64240, length 1400: HTTP03:12:41.298621 IP economy.canonical.com.http > 192.168.10.152.60388: Flags [P.], seq 2800:7000, ack 1, win 64240, length 4200: HTTP03:12:41.298650 IP 192.168.10.152.60388 > economy.canonical.com.http: Flags [.], ack 7000, win 65535, length 003:12:41.299625 IP economy.canonical.com.http > 192.168.10.152.60388: Flags [P.], seq 7000:8400, ack 1, win 64240, length 1400: HTTP03:12:41.300067 IP 192.168.10.152.60388 > economy.canonical.com.http: Flags [.], ack 8400, win 65535, length 003:12:41.798839 IP economy.canonical.com.http > 192.168.10.152.60388: Flags [P.], seq 92300:93600, ack 1, win 64240, length 1300: HTTP03:12:41.799351 IP 192.168.10.152.60388 > economy.canonical.com.http: Flags [.], ack 93600, win 65535, length 003:12:41.800018 IP economy.canonical.com.http > 192.168.10.152.60388: Flags [P.], seq 93600:96200, ack 1, win 64240, length 2600: HTTP03:12:41.800610 IP 192.168.10.152.60388 > economy.canonical.com.http: Flags [.], ack 96200, win 65535, length 003:12:41.801294 IP economy.canonical.com.http > 192.168.10.152.60388: Flags [P.], seq 96200:101600, ack 1, win 64240, length 5400: HTTP03:12:41.801393 IP 192.168.10.152.60388 > economy.canonical.com.http: Flags [.], ack 101600, win 65535, length 003:12:41.801538 IP economy.canonical.com.http > 192.168.10.152.60388: Flags [P.], seq 101600:103000, ack 1, win 64240, length 1400: HTTP03:12:41.801743 IP 192.168.10.152.60388 > economy.canonical.com.http: Flags [.], ack 103000, win 65535, length 003:12:41.807292 IP economy.canonical.com.http > 192.168.10.152.60388: Flags [P.], seq 103000:114200, ack 1, win 64240, length 11200: HTTP03:12:41.807493 IP 192.168.10.152.60388 > economy.canonical.com.http: Flags [.], ack 114200, win 65535, length 003:12:41.808278 IP economy.canonical.com.http > 192.168.10.152.60388: Flags [P.], seq 114200:118400, ack 1, win 64240, length 4200: HTTP03:12:41.808417 IP 192.168.10.152.60388 > economy.canonical.com.http: Flags [.], ack 118400, win 65535, length 003:12:41.809282 IP economy.canonical.com.http > 192.168.10.152.60388: Flags [P.], seq 118400:123500, ack 1, win 64240, length 5100: HTTP03:12:41.809402 IP 192.168.10.152.60388 > economy.canonical.com.http: Flags [.], ack 123500, win 65535, length 003:12:41.952442 IP economy.canonical.com.http > 192.168.10.152.60388: Flags [P.], seq 123500:124800, ack 1, win 64240, length 1300: HTTP03:12:41.952677 IP 192.168.10.152.60388 > economy.canonical.com.http: Flags [.], ack 124800, win 65535, length 003:12:41.961111 IP economy.canonical.com.http > 192.168.10.152.60388: Flags [P.], seq 124800:127400, ack 1, win 64240, length 2600: HTTP03:12:41.961181 IP 192.168.10.152.60388 > economy.canonical.com.http: Flags [.], ack 127400, win 65535, length 0^C27 packets captured71 packets received by filter44 packets dropped by kernel
watch some usefull tcpdum commands:
useful tcpdump command | Description |
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 192.168.10.150 | Capture packets from source IP |
tcpdump -i eth0 dst 209.85.144.139 | 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:
root@server1:~# 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 :
root@server1:~# nmap -v scanme.nmap.orgStarting Nmap 7.01 ( https://nmap.org ) at 2018-01-22 03:25 PSTInitiating Ping Scan at 03:25Scanning scanme.nmap.org (45.33.32.156) [4 ports]Completed Ping Scan at 03:25, 0.24s elapsed (1 total hosts)Initiating Parallel DNS resolution of 1 host. at 03:25Completed Parallel DNS resolution of 1 host. at 03:25, 0.35s elapsedInitiating SYN Stealth Scan at 03:25Scanning scanme.nmap.org (45.33.32.156) [1000 ports]Discovered open port 80/tcp on 45.33.32.156Discovered open port 22/tcp on 45.33.32.156Increasing send delay for 45.33.32.156 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 45.33.32.156 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 (45.33.32.156)Host is up (0.0018s latency).Other addresses for scanme.nmap.org (not scanned): 2600:3c01::f03c:91ff:fe18:bb2fNot shown: 987 filtered portsPORT STATE SERVICE21/tcp closed ftp22/tcp open ssh53/tcp closed domain80/tcp open http110/tcp closed pop3143/tcp closed imap443/tcp closed https587/tcp closed submission993/tcp closed imaps995/tcp closed pop3s3389/tcp closed ms-wbt-server8080/tcp closed http-proxy8291/tcp closed unknownRead data files from: /usr/bin/../share/nmapNmap done: 1 IP address (1 host up) scanned in 110.55 secondsRaw packets sent: 3033 (133.256KB) | Rcvd: 1500 (60.008KB)
The -v option enables verbose mode.
nmap Target selection | Description |
nmap 192.168.10.151 | scan a single IP |
nmap scanme.nmap.org | scan a host |
nmap 192.168.10.150-155 | scan a range of IPs |
nmap 192.168.10.0/24 | 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 | Dedscription |
nmap -p 22 192.168.10.151 | scan a single port |
nmap -p 1-100 192.168.10.151 | scan a range of port |
nmap -F 192.168.10.151 | Fast-scan 100 most common ports |
nmap -p- 192.168.10.151 | scan all 65535 ports |
Hmm. nmap has different techniques for scanning. But before explaining them we review some fundamentals.
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.
Event | Diagram |
| |
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 192.168.10.151
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 192.168.10.151
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 192.168.10.151
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 192.168.10.151
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 192.168.10.151
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 192.168.1.6 192.168.1.1
The idle scan technique (as mentioned above) is used to discover the open ports on 192.168.1.1 while it uses the zombie_host (192.168.1.6) 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.
