1. Linux Networking Commands

2. Commands Reviewed Ifconfig dmesg netstat ping route tcpdump wireshark
traceroute
nslookup
arp
dig

3. ifconfig
ifconfig is used to assign an address to a network interface and/or configure network interface parameters.
To determine if an interface has been recognized and configured on a system
To initially assign an IP address to an interface
to bring an interface up or down

4. View All Network Setting
The “ifconfig” command with no arguments will display all the active interfaces details.

5. Display Information of All Network Interfaces
ifconfig command with -a argument will display information of all active or inactive network interfaces on server.

6. ifconfig View Network Settings of Specific Interface
Enable an Network Interface

7. ifconfig Assign a IP Address to Network Interface
~]# ifconfig eth
Assign a netmask to Network Interface
~]# ifconfig eth0 netmask
Assign a Broadcast to Network Interface
~]# ifconfig eth0 broadcast
Assign all in one command
~]# ifconfig eth netmask broadcast
Change the MAC address of Network Interface
~]# ifconfig eth0 hw ether AA:BB:CC:DD:EE:FF

8. ifconfig Change MTU for an Network Interface
~]# ifconfig eth0 mtu 1000
Enable and disablePromiscuous Mode
~]# ifconfig eth0 promisc
~]# ifconfig eth0 –promisc
Add or remove New Alias to Network Interface
~]# ifconfig eth0:
~]# ifconfig eth0:0 down
Verify the newly created alias network interface address
~]# ifconfig eth0:0
eth0:0 Link encap:Ethernet HWaddr 00:01:6C:99:14:68 inet addr: Bcast: Mask: UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 Interrupt:17

9. See if the device has been found - use dmesg
The command 'dmesg', which is used to print kernel messages, is very useful in determining if a piece of hardware has been found, and if so, what the system is referring to it as.
Examples:
dmesg
dmesg | grep -i eth0
dmesg | grep -i usb
dmesg | grep -i Memory
dmesg | grep -i bluetooth
dmesg | grep -i tty
dmesg –c clear dmesg logs from boot time to till that time

10. ping Confirm that a remote host is online and responding.
ping is intended for use in network testing, measurement, and management.
It is unwise to use ping during normal operations or from automated scripts.

11. Decrease /Increase Ping Time Interval
$ ping -i 5 IP
# ping -i 0.1 IP
Note: Only super user can specify interval less than 0.2 seconds.
Check local interface
$ ping 0
$ ping localhost
$ ping
Send n packets and stop:
$ ping -c 5 google.com
Display the current version of ping program
$ ping -V

12. Ping 5. Flood the network Audible ping: Change Ping Packet Size
# ping -f localhost
Audible ping:
$ ping -a IP
Change Ping Packet Size
$ ping -s 100 localhost
Specify path for ping to send the packet
$ ping
Record and print route of how ECHO_REQUEST sent and ECHO_REPLY received
$ ping -R

13. Route command Display Existing Routes
if the destination is within the network range – , then the gateway is *, which is
By default route command displays the host name in its output. We can request it to display the numerical IP address using -n option as shown below.

14. Adding a Default Gateway List Kernel’s Routing Cache Information
$ route add default gw
List Kernel’s Routing Cache Information
Reject Routing to a Particular Host or Network
route add -host reject

15. Note traceroute Traceroute has lost some of
traceroute attempts tracing by launching UDP probe packets with a small TTL (time to live), then listening for an ICMP "time exceeded" reply from a gateway.
host is the destination hostname or the IP number of host to reach.
packetsize is the packet size in bytes of the probe datagram. Default is 38 bytes.
Note
Traceroute has lost some of
its effectiveness since most
ISP’s disallow it from
running on their networks

16. netstat List out all connections List only TCP or UDP connections
$ netstat –a
List only TCP or UDP connections
$ netstat –at
$ netstat –au
Disable reverse dns lookup for faster output: By default, the netstat command tries to find out the hostname of each ip address in the connection by doing a reverse dns lookup. This slows down the output.
$ netstat -ant

17. netstat Print statistic List out only listening connections
$ netstat -s
List out only listening connections
$ netstat –tnl
Get process name/pid and user id
$ sudo netstat –nlpt
Use the e option along with the p option to get the username too.
$ sudo netstat -ltpe

18. netstat -nr [root@localhost root]# netstat -nr Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
U eth0
U eth1
U lo
UG eth1

19. nslookup Query Internet domain name servers.
$ nslookup redhat.com
$ nslookup -query=mx redhat.com
nslookup -type=ns redhat.com
Query the SOA Record using -query=soa
provides the authoritative information about the domain, the address of the domain admin, the domain serial number, etc…
$ nslookup -type=soa redhat.com
View available DNS records using -query=any
$ nslookup -type=any google.com
Reverse DNS lookup
$ nslookup redhat.com ns1.redhat.com

20. arp -a [root@localhost root]# arp -a
gatewayout.tcp-ip.ca ( ) at 00:04:5A:DB:A1:C5 [ether] on eth1
basement.tcp-ip.ca ( ) at 00:10:E0:04:61:84 [ether] on eth0
just_a_node.tcp-ip.ca ( ) at 00:09:B7:13:AA:13 [ether] on eth1

21. dig - supercharged nslookup
root]# dig cs.senecac.on.ca
; <<>> DiG <<>> cs.senecac.on.ca
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 10483
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 6, ADDITIONAL: 4
;; QUESTION SECTION:
;cs.senecac.on.ca IN A
;; ANSWER SECTION:
cs.senecac.on.ca IN A

22. dig - better than nslookup
;; AUTHORITY SECTION:
senecac.on.ca IN NS ns.onet.on.ca.
senecac.on.ca IN NS ns2.senecac.on.ca.
senecac.on.ca IN NS hades.senecac.on.ca.
senecac.on.ca IN NS ittads.senecac.on.ca.
senecac.on.ca IN NS pulsar.senecac.on.ca.
senecac.on.ca IN NS nsprime.senecac.on.ca.
;; ADDITIONAL SECTION:
ns2.senecac.on.ca IN A
hades.senecac.on.ca IN A
ittads.senecac.on.ca IN A
pulsar.senecac.on.ca IN A
;; Query time: 2202 msec
;; SERVER: #53( )
;; WHEN: Sun Sep 29 16:38:
;; MSG SIZE rcvd: 238

23. tcpdump
tcpdump is a most powerful and widely used command-line packets sniffer or package analyzer tool which is used to capture or filter TCP/IP packets that received or transferred over a network on a specific interface.
# yum install tcpdump
$ apt-get install tcpdump
Capture Packets from Specific Interface
# tcpdump -i eth0
Capture Only N Number of Packets
# tcpdump -c 5 -i eth0

24. Display Available Interfaces Capture and Save Packets in a File
# tcpdump –D
Capture and Save Packets in a File
# tcpdump -w 0001.pcap -i eth0
Read Captured Packets File
# tcpdump -r 0001.pcap
Capture only TCP packets
# tcpdump -i eth0 tcp
Capture Packet from Specific Port
# tcpdump -i eth0 port 22
Capture Packets from destination /src IP
# tcpdump -i eth0 dst
# tcpdump -i eth0 src

25. Wireshark
Wireshark is a open source and freely available network analyzer tool which is shipped with most of the Linux distributions now a days.