Linux, CCNA and MCSE Questions: Network Configuration and Troubleshooting

1. What is Network?

Combination of two more computers connected together to share their resources each other by means of communication like cable is called Network.

2. What is Networking?

It is a connection between two or more computers to communicate with each other.

3. what are the basic requirements for networking?

(a) NIC (Network Interface Card or controller)

(b) Media (nothing but cables)

(d) Protocol

(e) IP Addresses

4. Explain about NIC card?

A Network Interface Card or controller is hardware component that connects a computer to a computer network. Each NIC card will be having MAC (Media Access Controller) address to avoid conflicts between same NIC adapters. In Linux these NIC adapter is represented by the word "eth" . For example if two NIC cards are there in a system then it will be denoted as "eho","eth1", .....etc.,

5. What is media?

Media is nothing but cable to connect two or systems. Example : RJ 45, CAT 5 and CAT 6, ....etc.,

6. What is topology?

Topology is a design in which the computers in network will be connected to each other. Example for topologies are Bus, Ring, Star, Mesh, Tree topologies.

7. What is protocol?

A Network Protocol defines rules and conventions for communication between the network devices. Protocols are generally use packet switching techniques to send and receive messages in the form of packets.

Example for protocols are TCP/IP (Transmission Control Protocol and Internet Protocol), UPD (User Datagram Protocol) and HTTP (Hyper Text Transfer Protocol), ....etc.,

8. What are the differences between TCP/IP and UDP protocols?

TCP/IP	UDP
Transmission Control Protocol	User Datagram Protocol
It is connection oriented	It is connection less
Reliable	Non-Reliable
TCP Acknowledgement will be sent / received	No Acknowledgement
Slow communication	Fast communication
Protocol No. for TCP is 6	Protocol No. for UDP is 17
HTTP, FTP, SMTP, ....etc., uses TCP	DNS, DHCP, ....etc., uses UDP

9. What is an IP address?

Every Computer will be assigned an IP address to identify each one to communicate in the network. The IP address sub components are Classes of an IP address, Subnet masks and Gateway.

Classes of IP address :

The IP addresses are further divided into classes. The classes are A, B, C, D, E and the ranges are given below.

Class	Start	End	Default Subnet mask	Classless Inter Domain Routing
Class A	0.0.0.0	127.255.255.255	255.0.0.0	/8
Class B	128.0.0.0	191.255.255.255	255.255.0.0	/16
Class C	192.0.0.0	223.255.255.255	255.255.255.0	/24
Class D	224.0.0.0	239.255.255.255
Class E	240.0.0.0	255.255.255.255

10. What is loopback address?

A special IP number (127.0.0.1) is designated for the software loopback interface of a machine. 127.0.0.0 and 127.255.255.255 is also reserved for loopback and is used for internal testing on local machines.

11. What is multicasting?

Multicasting allows a single message to be sent to a group of recipients. Emailing and Teleconferencing are examples of multicasting. It uses the network infrastructure and standards to send messages.

12. What is subnet mask?

A subnet mask allows the users to identify which part of an IP address is reserved for the network and which part is available for host use.

13. What is Gateway?

A Gateway is the network point that provides entrance into another network. On the internet a node or stopping point can be either gateway node or a host (end point) node. Both the computers of internet users and the computer that serve the pages to users are host nodes. The computer that control traffic within your company's network or at our local internet service provider (ISP) are the gateway nodes.

14. What are important configuration files in network configuration?

# cat /etc/sysconfig/network (This file keeps the information about the hostname assigned to the system and if we want to change the hostname permanently, we need to change the hostname in this file)

# cat /etc/sysconfig/network-scripts/ (This directory keeps the configuration of network devices connected to the system. Examples are ifcfg-eht0, ifcfg-eth1, ifcfg-eth2, .....etc.,)

# cat /etc/hosts (This file is responsible for resolving hostname into IP address locally. ie., local DNS if DNS server is not available)

# cat /etc/resolve.conf (This file keeps the address of the DNS server to which the clients will be accessing to resolve IP address to hostname and hostname to IP address)

15. What are the differences between MAC and IP addresses?

MAC Address	IP Address
It is a permanent address. So we cannot change this address.	It is a temporary address. So, we can change this address any no. of times.
It stands for Media Access Control Address.	Internet Protocol address.
It is a physical address.	It is a logical address.
It is divided into 6 parts. --- : --- : --- : --- : --- : --- (each 8 bits. So, 8 X 6 = 48 bits	It is two types. IPV4 :(It is divided into 4 parts ) --- . --- . --- . --- (each 8 bits. So, 8 X 4 = 32 bits IPV6 : ( It is divided into 16 parts ) --- . --- . --- . --- . --- . --- . --- . --- . --- . --- . --- . --- . --- . --- . --- . --- (each 8 bits. So, 8 X 16 = 128 bits.
ifconfig (to see the MAC address)	# ifconfig (to see the IP address)

16. How many types of NIC cards available?

(a) eth0 (1st NIC card)

(b) eth1 (2nd NIC card)

(d) lo (loopback device name and IP address is 127.0.0.1)

# ifconfig (to see all the NIC devices connected to the system)

17. How many types of cable connections available?

(i) Cross cable (to connect two systems directly)

(ii) Straight cable (to connect more systems with the help of switch)

# ethtool (to check the network cable is connected or not)

# miitool (It is also used to check the network cable but it will not supports RHEL - 7 and only supports RHEL - 6 and it also works on physical system only not on virtual system)

18. In how many ways we can configure the network?

There are two ways to configure the network.

(a) Static Network.

(b) Dynamic Network.

Static Network :

In this way we assign the IP address and hostname manually. Once we configure the IP address, it will not change.

Dynamic Network :

In this way we assign the IP address and hostname dynamically. This means the IP address will change at every boot.

19. How to assign the static IP address to the NIC card?

In RHEL - 6 :

# setup

(Move the cursor to Network configuration and press Enter key)

(Move the cursor to Device configuration and press Enter key)

(Select the NIC adapter ie., eth0 and press Enter key)

(Assign the above IP address and other details as per our requirements and move the cursor to "OK" and press Enter key)

(Move the cursor to "Save" to save the changes in device configuration and press Enter key)

(Once again move the cursor to "Save & Quit" button and press Enter key)

(Finally move the cursor to "Quit" button and press Enter key to quit the utility)

(Then restart the network service and check for the IP address by # service network restart command)

(If the change is not reflected with the above service, then restart the network manager by

# service NetworkManager restart command)

# ifconfig (to see the IP address of the NIC card)

# ping < IP address > (to check whether the IP is pinging or not)

In RHEL - 7 :

# nmcli connection show (to see all the network connections)

# nmcli device show (to see the network details if already configured manually or dynamically)

# nmcli connection add con-name "System eth0" ifname eth0 type ethernet (to add the network connection)

# nmcli connection modify "System eth0" ipv4.addresses ' < IP address >/< netmask >< gateway > ' ipv4.dns < dns server IP address > ipv4.dns-search < domain name> ipv4.method (to assign IP address, gateway, dns, domain name and configure the network as static or manually)

# nmcli connection up "System eth0" (to up the connection)

# systemctl restart network (to restart the network service)

# systemctl enable network (to enable the network service)

# ifconfig (to see the IP address of the NIC card)

# ping < IP address > (to check whether the IP is pinging or not)

20. What are the differences between RHEL - 6 and RHEL - 7 network configuration files?

RHEL - 6	RHEL - 7
/etc/sysconfig/network-scripts is the directory which contains the NIC configuration information.	/etc/sysconfig/network-scripts is the directory which contains the NIC configuration information.
/etc/sysconfig/network-scripts/ifcfg- is the file which contains the NIC configuration details.	/etc/sysconfig/network-scripts/ifcfg- is the file which contains the NIC configuration details.
/etc/resolve.conf is the file which contains DNS server IP and domain name location.	/etc/resolve.conf is the file which contains DNS server IP and domain name location.
/etc/sysconfig/network is the hostname configuration file.	/etc/hostname is the hostname configuration file.
/etc/hosts is the file which contains the local DNS server IP address.	/etc/hosts is the file which contains the local DNS server IP address.

21. What are the differences between Dynamic and Static configuration information?

Dynamic configuration information	Static configuration information
Device =	Device =
HWADDR=02:8a:a6:30:45	HWADDR=02:8a:a6:30:45
Bootproto=DHCP	Bootproto=none (means static network)
Onboot=yes (yes means whenever we restart the system this connection will be activated and no means whenever we restart the system the connection will be deactivated)	Onboot=yes
Type=Ethernet	Type=Ethernet
Userctl=yes/no ----> If it is yes all normal users can disable the NIC card and If it is no except root user nobody can disable the NIC card.	Userctl=yes/no ----> If it is yes all normal users can disable the NIC card and If it is no except root user nobody can disable the NIC card.

22. How to set the hostname temporarily and permanently?

RHEL - 6 :

# hostname (to set the hostname temporarily)

# vim /etc/sysconfig/network (to set the hostname permanently)

HOSTNAME=

(save and exit this file)

# service network restart (to update the hostname in the network)

# chkconfig network on (to enable the connection at next reboot)

RHEL - 7 :

# hostname (to set the hostname temporarily)

# hostnamectl set-hostname (to set the hostname permanently)

# systemctl restart network (to update the hostname in the network)

# systemctl enable network (to enable the connection at next reboot)

23. How to troubleshoot if the NIC is notworking?

(a) First check the NIC card is present or not by # ifconfig command.

(b) If present thencheck the status of the NIC card is enabled or disabledby click on System menu on the status bar, then select Network Connections menu.

(d)Open /etc/sysconfig/network-scripts/ifcfg-eth0 file check Userctl=yes or no. If it is yes make it as no, then check Onboot= yes or no. If it is no make it as yes and save that file.

(e) If not present thencheck the status of the NIC card is enabled or disabled by click on System menu on the status bar, then select Network Connections menu.

(f) Click on IPV4 settings tab, select the device eth0 or any other and select Enable button, then Apply and OK.

(g) Using # setup (in RHEL - 6) or # nmcli (in RHEL - 7) commands assign the IP address to the system and restart the network service by # service network restart (in RHEL - 6) or # systemctl restart network (in RHEL - 7) commands and enable the service at next reboot by # chkconfig network on (in RHEL - 6) or # systemctl enable network (in RHEL - 7) commands.

(h) Then up the connection by # ifconfig eth0 up (in RHEL - 6) or # nmcli connection up commands.

(i) Even though it is not working may be the fault in NIC card. If so, contact the hardware vendor by taking the permissions from higher authorities.

24. What is bonding and how to configure bonding? (from RHEL - 6)

What is link aggregation or bridging or teaming and how to configure teaming? (from RHEL - 7)

Bonding or Teaming or Bridging:

Collection of multiple NIC cards and make them as single connection (virtual) NIC card is called bonding.

It is nothing but backup of NIC cards.

In RHEL - 6 it is called as Bonding or Bridging.

In RHEL - 7 it is called as Teaming or Link aggregation.

There are 3 types of backup in Bonding or Teaming.

(a) Mode 0 -----> Round Robbin

(b) Mode 1 -----> Activebackup

Mode 0 :

It provides load balancing and fault tolerance.

Data will be shared by both NIC cards in round robbin.

If one NIC card failed then another NIC card will be activated to communicate with the server

So, there is a load balancing and fault tolerance features.

Mode 1 :

Activebackup means only one NIC card is activated at a time and another one is in down state.

So, there is no load balancing.

But if one NIC card is failed then another NIC card will be activated automatically.

Mode 3 :

In this mode broadcasting is done.

In this the same data will be transferred through two NIC cards.

So there is no load balancing.

But if one NIC card is failed then second NIC card will be activated automatically.

So, all the 3 modes are supports only fault tolerance, but round robbin is the only one mode that provides load balancing.

Requirements to configure :

(i) Minimum two NIC cards.

(ii) One IP address.

(iii) Connection type is bond (in RHEL - 6) and team (in RHEL - 7) not the ethernet type.

Here no need to assign the IP addresses for two NIC cards and we are giving only one IP address to bond or team.

Bonding configuration : (in RHEL - 6)

(i) # vim /etc/sysconfig/network-scripts/ifcfg-bond0

DEVICE=bond0

IP ADDR=

TYPE=ethernet

NETMASK=255.225.225.0 or

ONBOOT=yes

BOOTPROTO=none

USERCTL=no

MASTER=yes

BONDING_OPTS="mode0 or mode1 or mode3 miimon=50" (Save and exit this file)

(ii) vim /etc/sysconfig/network-scripts/ifcfg-eth1

DEVICE=eth1

ONBOOT=yes

BOOTPROTO=none

USERCTL=no

MASTER=bond0

SLAVE=yes (Save and exit this file)

(iii) vim /etc/sysconfig/network-scripts/ifcfg-eth2

DEVICE=eth2

ONBOOT=yes

BOOTPROTO=none

USERCTL=no

MASTER=bond0

SLAVE=yes (Save and exit this file)

(iv) To add virtual NIC cards eth1 and eth2 :

# setup -----> Networking -----> Device configuration -----> New Device -----> eth1

Name : eth1

Device : eth1 (save and exit this setup)

# setup -----> Networking -----> Device configuration -----> New Device -----> eth2

Name : eth2

Device : eth2 (save and exit this setup)

(v) Adding bond0 connection :

# setup -----> Networking -----> Device configuration -----> New Device -----> bond0

Name : bond0

Device : bond0

IP address :

Netmask : 255.255.255.0

Default gateway : (save and exit this setup)

# ifdown bond0

# ifdown eth1

# ifdown eth2

# ifup bond0

# service NetworkManager stop

# service network restart

# chkconfig network on

# service NetworkManager restart

# cat /proc/net/bonding/bond0 (to check the bonding information)

# watch -n 1 cat /proc/net/bonding/bond0 (to check the bonding information for every 1 minute)

# echo "eth1" > /sys/devices/virtual/net/bond0/bonding/active_slave (to put the eth1 NIC in active state)

Teaming configuration :

(i) Add the team0 connection by

# nmcli connection add con-name team0 ifname team0 type team

config ' { "runner" : { "name" : "roundrobbin" or "activebackup" or "broadcasting" }} '

(ii) Add the two NIC cards one by one to the above created connection by

# nmcli connection add con-name port1 ifname eth1 type team-slave master team0

# nmcli connection add con-name port2 ifname eth2 type team-slave master team0

(iii) Assign the static IP address to the team0 connection by

#nmcli connection modify team0 ipv4.addresses / ipv4.method static

(iv) Up the connection by

# nmcli connection up team0

(v) To see the team0 connection up details by

# teamdctl team0 state

(vi) To check the connection communication by

# ping -I team0

(vii) To down the one NIC card in team0 by

# nmcli connection down port1

(viii) teamdctl team0 state (to check the team0 NIC card up or down details)

25. What is the difference between TCP and UDP protocol?
TCP is a connection oriented protocol and contain the information of sender as well as receiver.

Example : HTTP, FTP, Telnet

TCP is slower than UDP due to its error checking mechanism

UDP protocols are connection less packets have no information to where they are going. These type of ports are generally used for broadcasting.

For example : DNS, DHCP

UDP are faster

26. What are the benefits of NIC Teaming?

                (i)            Load balancing
                (ii)           Fault Tolerance
                (iii)          Failover
27.          Mention all the network configuration files you would check to configure your ethernet card?

                (i)            /etc/sysconfig/network-scripts/ifcfg-eth*
                (ii)           /etc/sysconfig/network
                (iii)          /etc/resolve.conf
                (iv) /etc/nsswitch.conf
28.          What is the use of /etc/resolve.conf?

It contains the details of nameserver, i.e., details of your DNS server which helps us connect to Internet.
29. What is the use of /etc/hosts file?

To map any hostname to its relevant IP address.
30. What is the command to check all the open ports of your machine?

#nmap localhost

31. What is the command to check all the open ports of remote machine?

# nmap
32. What is the command to check all the listening ports and services of your machine?

netstat
-ntulp

33. How can you make a service run automatically after boot?

# chkconfig on
34. What are the 6 run levels of linux? And how can you configure your script to run only when the system boots into GUI and not to any other runlevel?

                0             Power off
                1              Single user
                2              Multi user without network
                3              Multiuser with network
                4              Development purpose
                5              GUI
                6              Restart

                # chkconfig
--level 5 service_name on

                # chkconfig
--level 1234 service_name off

35. What is a 3 way handshake protocol? Give an example of it.

                SYN - system 1 sends SYN signal to remote system.
                SYN-ACK - remote system receives the syn signal and sends ack signal.
                ACK - system again receives ack signal from remote system and connection is established.
                For Example: When you ping to a machine you are sending a SYN signal which is ACK by the remote machine                then it sends a SYN ACK signal back to the host machine. Then the host machine receives SYN ACK and sends the       ACK signal back to confirm the same.
36.          What are the possible ways to check if your system is listening to port 67?

                # nmap localhost
| grep 67

                # netstat -ntulp
| grep 67

37. Explain about IPV6?

It's length is 128 bits. It's netmask is 64

# nmcli connection modify "System eth0" ipv6.addresses 2005:db8:0:1::a00:1/64 ipv6.method static

(to add the IPV6 version of IP address to the connection "System eth0" )

# nmcli connection modify "System eth0" ipv4.addresses '172.25.5.11/24 172.25.5.254' ipv4.dns

172.25.254.254 ipv4.dns-search example.com ipv4.method static ipv6. addresses 2005:ac18::45/64

ipv6.method static (to assign ipv4 and ipv6 IP addresses to "System eth0 connection)

# nmcli connection down "System eth0" (to down the "System eth0" connection)

# nmcli connection up "System eth0" (to up the "System eth0" connection)

38. How to troubleshoot if the network is not reaching?

(i) First check the network cable is connected or not by # ethtool command. if connected then check the IP address is assigned or not by # ifconfig command.

(ii) Then check the system uptime by # uptime command.

(iii) Then check the network services status by # service network status and # service NetworkManager status commands.

(iv) Then check the network service at Run Level by # Chkconfig --list network command.

(v) Then check whether the source network and destination network are in the same domain or not.

(v) Then finally check the routing table by # route -n command.

Other useful commands :

# ping (to check the pinging)

Normally the ping command pings continuously until a stop signal reaches by Ctrl + c, so to avoid continuous pinging by

# ping -c (to ping upto the specified no of times)

# ipcalc -m

Normally IP addresses are assigned by ISP (Internet Service Provider) and managed by IANA (Internet Assign Number Authority)

# ifconfig (to see or check all the NIC device names and IP addresses)

# ethtool (to check the network cable is connected or not)

# miitool (It is also used to check the network cable but it works on physical system not on virtual system and supports in RHEL - 6 only)

# ip addr show (to show all NIC devices present on the system)

# hostname (to see the hostname with fully qualified domain name)

# hostname -i (to see the IP address of the system)

# hostname -d (to check the domain name of the system)

# hostname -s (to check the hostname without domain name)

# netstat -r (to check the default gateway and routing table)

# route (to check the default gateway with routing table)

# ip route (to display the NIC device with default gateway)

# dig or # host or #nslookup (all are used to resolve the name to IP and IP to name)

# nslookup (to resolve IP to name)

# nslookup (to resolve name to IP)

# host (to resolve IP to name)

# host (to resolve name to IP address)

# dig -x (to resolve IP address to name)

# dig (to resolve name to IP address)

# nmcli (Network Manager Command Line Interface used to configure the network setup in RHEL - 7)

# setup (to setup the static network in RHEL - 2, 3, 4, 5 and 6)

# nmtui (to setup the static network in GUI mode for RHEL - 7)

# nmcli device show (It displays all the NIC devices network information of the system )

# nmcli device show eth0 (to see all the network devices information of the eth0)

# nmcli connection or nmcli connection show (to see all the network connection names)

# nmcli connection add con-name ifname type ethernet

(to create a new connection name for eth0)

# nmcli connection show --activate (it shows which connection is active currently)

# nmcli connection add con-name ifname type ethernet

(to add a connection name to NIC device)

# nmcli connection modify ipv4.addresses ' / '

ipv4.dns ipv4.dsn-search ipv4.method

(to modify the connection as static and assign the IP, gateway, dns IP, domain name)

# nmcli connection delete (to delete the specified connection)

# nmcli connection modify ipv4.method (to modify dynamic connection to static connection)

# nmcli connection up (to activate or up the specified connection)

# nmcli connection down (to disable or down the specified connection)

# nmcli connection show (to see the information about the specified NIC device)

# ping -I (to check the connection from NIC device to IP address)

# hostname (to set the hostname temporarily)

# hostnamectl set-hostname (to set the hostname permanently in RHEL - 7)

NOTE: Whenever we change any parameters in /etc/sysconfig/network-scripts/ifcfg- file, then we have to reload that file and again we have to up the connection (nothing but activate the connection by # nmcli connection reload command.

# nmcli connection reload (to reload the configuration of the connection if any changes on it and it reloads all configuration files)

# nmcli connection reload /etc/sysconfig/network-scripts/ifcfg- (to reload a single file)

# hostnamectl status (it displays full details of the hostname and works in RHEL - 7 only)

# nmcli networking off (to disable all the connections at a time)

# nmcli device status (to display all NIC device connections statuses)

# nmcli connection modify + ipv4.dns (to add a secondary dns server IP to the existing connection)

# netstat -ntulp (to check how many open ports are there in local system)

# ss -ntulp ( " " )

# nmap (to check how many open ports are there in remote system)

# tracepath (it displays the routing information)

# miitool (to check the network cable is connected or not)

# ethtool ( " " )

# ifconfig (to check the NIC card is enable or not)

# ifup (to enable or up the NIC card)

#ifdown (to disable or down the NIC card)

# route -n (to check the gateway)

# cat /etc/resolve.conf (to check the dns server information)

# cat /etc/sysconfig/network-scripts/ifcfg- (to see the NIC device information)

# hostname or cat /etc/sysconfig/network (to check the hostname in RHEL - 6)

# hostnamectl status or cat /etc/hostname (to check the hostname in RHEL - 7)

# ping (to check the connection communication)

# chkconfig --list (to list all the services which are running at boo time in RHEL - 6 & 7)

# systemctl list-unit-files (to list all the processes which are running at boot time in RHEL - 7)

# chkconfig --level (it will set the service at run level 3 when the system is booting)

# service --status-all (to see the list of all the processes which are currently running)

# ls /etc/init.d (is the location of all the services and deamons in RHEL - 6)

# ls /usr/lib/systemd/system (is the location of all the services and deamons in RHEL - 7)

# /etc/rc.local (is the last script to be run when the system is booting)

(If we enter as sshd stop at the last line of the script file then sshd will be stopped even though that sshd is enabled)

# service sshd status (to check the sshd status)

# service --service -all (to see the process ID of all the services)

# netstat -ntulp (to see all the services with port no., status, process ID and all open ports in local system, routing table and NIC device information)

-n -----> port no. (numeric no) -t ----->tcp protocol

-u -----> upd protocol -l -----> port is listening or not

-p -----> display the process ID

# netstat -r (to see all routing table information)

# netstat -i (to see all the NIC cards information)

# nmap (to see the network mapping ie., open ports list on remote system)

Note : By default this command will not available. So, first install the nmap package by # you install nmap -y

# nmap (to see all the services which are running in the specified remote system)

# nmap (to see the running services on specified remote systems)

# nmap 172.25.0.11 - 50 (to see the running service on 172.25.0.11 to 172.25.0.50 systems)

# nmap -p 80 (to see the http port is running or not on specified remote system)

# nmap -p 80 - 90 (to see port no's 80 to 90 are running or not on remote systems)

# nmap -sp 172.25.0.0/24 (to see all the systems which are in upstate ie., 172.25.0.1, 172.25.0.2,

(where s -- scan & p -- ping) 172.25.0.3, ......upto 172.25.0.254 systems)

Open a file, write all the systems IP addresses, save & exit the file. Example has given below,

# vim coss

172.25.2.50

172.25.3.50

172.25.4.50 ....etc., (save and exit this file)

# nmap -iL coss (to scan all the IP addresses by reading the coss file)(where -i ----> input, -L ----> list)

# nmap --iflist (to see all the routing table information in the network)

# nmap 172.25.0.10 - 20 --exclude 172.25.0.15 (to scan all the systems from 172.25.0.10 to 172.25.0.20 systems and excluding 172.25.0.15 system)

# nmcli connection show --active (to control the network connections)

# ip link (to check the network connection)

# ping -I eth1 (to check the 2nd NIC card connection)

Linux, CCNA and MCSE Questions

Monday, October 17, 2016

Network Configuration and Troubleshooting

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Linux, CCNA and MCSE Questions: User Managment