109.2. Basic network configuration

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Description: Candidates should be able to view, change and verify configuration settings on client hosts.

Key Knowledge Areas:

  • Manually and automatically configure network interfaces

  • Basic TCP/IP host configuration

  • Setting a default route

Terms and Utilities:

  • /etc/hostname

  • /etc/hosts

  • /etc/nsswitch.conf

  • ifconfig

  • ifup

  • ifdown

  • ip

  • route

  • ping

In this tutorial, we learn hat make network configurations persistent in linux.


ifconfig ( interface configuration) utility is used to configure, or view the configuration of, a network interface via command line interface(CentOS6).

ifconfig [...OPTIONS] [INTERFACE]

The “ifconfig” command with no option, displays current network configuration information:

[[email protected] ~]# ifconfig
eth0 Link encap:Ethernet HWaddr 00:0C:29:6D:D2:C5
inet addr: Bcast: Mask:
inet6 addr: fe80::20c:29ff:fe6d:d2c5/64 Scope:Link
RX packets:235444 errors:0 dropped:0 overruns:0 frame:0
TX packets:81562 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:309940884 (295.5 MiB) TX bytes:5027974 (4.7 MiB)
lo Link encap:Local Loopback
inet addr: Mask:
inet6 addr: ::1/128 Scope:Host
RX packets:2964 errors:0 dropped:0 overruns:0 frame:0
TX packets:2964 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:386271 (377.2 KiB) TX bytes:386271 (377.2 KiB)

the lo is the loopback adapter , with ip address which is used by operating system for its own internal communications.

ifconfig option



display all the interfaces available, even if they are down.


display a short list (like netstat -i)


be more verbose for some error conditions

We can use ifconfig to change network configurations, but this will require root access:

ifconfig command


ifconfig eth0

Assign a IP Address

ifconfig eth0 netmask

Assign a Netmask

ifconfig eth0 netmask

Assign a IP, Netmask

ifconfig can be used to turn an interface up and down ,again this will need root access:

  • ifconfig interface up: used to activate the driver for the given interface

  • ifconfig interface down: used to deactivate the driver for the given interface

If you're still using ifconfig, you're living in the past! ifconfig command is deprecated and replaced by ip command.

ifup , ifdown

Same as previous commands, ifup and ifdown are used to enable and disable an interface.

[[email protected] ~]# ifdown eth0
Device state: 3 (disconnected)
[[email protected] ~]# ifup eth0
Active connection state: activating
Active connection path: /org/freedesktop/NetworkManager/ActiveConnection/10
state: activated
Connection activated


One benefit of chopping ip addresses into classes and subnets is controlling broad casts.This happens using subnetmask. But what is two compuyers from two different networks wants to communicate with each other?

A gateway is a node or a router that acts as an access point to passes network data from local networks to remote networks.

the address which is used as a gateway to reach to other networks outside our local network is called default gateway. There are different ways to set gateway.

Network Configuration files

The bad news is that all configurations that we have done using ifconfig command are not persistent and got vanished whe system restarts ot the interface turns up and down. So we should find out a way to make our setting persistent and the only way we can achieve that by using network configuration files.

Unfortunately the network configuration files in linux are placed in different places and it depend on the distribution which we are talking about.

RedHat Based systems

In Redhat , CentOS, and fedora the files are located at /etc/sysconfig/network-scripts/ .

[[email protected] ~]# ls /etc/sysconfig/network-scripts/
ifcfg-eth0 ifdown-ippp ifdown-sit ifup-ib ifup-post init.ipv6-global
ifcfg-lo ifdown-ipv6 ifdown-tunnel ifup-ippp ifup-ppp net.hotplug
ifdown ifdown-isdn ifup ifup-ipv6 ifup-routes network-functions
ifdown-bnep ifdown-post ifup-aliases ifup-isdn ifup-sit network-functions-ipv6
ifdown-eth ifdown-ppp ifup-bnep ifup-plip ifup-tunnel
ifdown-ib ifdown-routes ifup-eth ifup-plusb ifup-wireless

and the default gateway is configured via file: /etc/sysconfig/network

[[email protected] Desktop]# cat /etc/sysconfig/network

lets take a look at eth0 configuration file:

[[email protected] ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0
NAME="System eth0"

Debian Based Systems

In debian based systems line Ubuntu , ... the main network configuration file location is /etc/network/interfaces and there is no separated file for gateway configuration :

auto lo
iface lo inte loopback
auto eth0
#ifconfig eth0 inet dhcp
iface eth0 inet static

there might be /etc/network/interfaces.d directory for configuration files.

ifdown and ifup commands use this configuration file.

DNS configuration file


As you have noticed DNS configuration is in the same file that interface configuration is located but there is another place in linux which contains DNS information /etc/resolv.conf :

[[email protected] Desktop]# cat /etc/resolv.conf
# Generated by NetworkManager

Again setting in this file are not permanent and it is not recommended to change this file by hand , except for temporary tests.


Hostname is the program that is used to either set or display the current host, domain or node name of the system (we are usiong centOS).

[[email protected] Desktop]# hostname
[[email protected] Desktop]# hostname centos6-1
[[email protected] Desktop]# hostname

The new hostname will appear if you open a new terminal but get vanished if you restart the system. To configure hostname permanently there are a couple of other places which should be changed

  1. /etc/hostname (Ubuntu) OR /etc/sysconfig/network (CentOS)

  2. /etc/hosts (both Ubuntu , CentOS)


/etc/hostname contains name of the machine and is one of the configuration files that should be modified in order to make a new hostname persistent in Debian based systems.(ubuntu16 here)

[email protected]:~# cat /etc/hostname


Another place which contains hostname and should be changed in RedHat based systems to have persistent hostname(CentOS6)

[[email protected] Desktop]# cat /etc/sysconfig/network
HOSTNAME=server1 ###<---- change it to centos6-1 in our ex


The /etc/hosts is an operating system file that translate hostnames or domain names to IP addresses , it do the same thing that DNS do. We can using for testing purposes or when DNS server is absent. Do not forget that it has a highr priority than DNS, (means that operating system first look inside /etc/hosts file to gain the ip address of a host, if it wasn't successful then it would query DNS Server).

[[email protected] Desktop]# cat /etc/hosts centos6-1 localhost localhost.localdomain localhost4 localhost4.localdomain4
::1 server1 localhost localhost.localdomain localhost6 localhost6.localdomain6

For making new hostname permanent it is another file which should be modified.


All network devices, whether they are hosts, routers, or other types of network nodes such as network attached printers, need to make decisions about where to route TCP/IP data packets. The routing table provides the configuration information required to make those decisions. the route command is used to view and make changes to the kernel routing table.

route command make temporary setting, use config files instead!

Running route command without any options displays the routing table entries:

[[email protected] Desktop]# route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface * U 1 0 0 eth0
default UG 0 0 0 eth0

This shows us how the system is currently configured. If a packet comes into the system and has a destination in the range through, then it is forwarded to the gateway *, which is — a special address which represents an invalid or non-existant destination. So, in this case, our system will not route these packets.

If the destination is not in this IP address range, it is forwarded to the default gateway (in this case,, and that system will determine how to forward the traffic on to the next step towards its destination.

By default route command displays the host name in its output. We can request it to display the numerical IP address using -n option:

[[email protected] Desktop]# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface U 1 0 0 eth0 UG 0 0 0 eth0

The following route add command will set the default gateway as

[[email protected] ~]# route add default gw
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface * U 0 0 0 eth0
link-local * U 1002 0 0 eth0
default UG 0 0 0 eth0
default UG 0 0 0 eth0

use route del for deleting:

[[email protected] ~]# route del default gw
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface * U 0 0 0 eth0
link-local * U 1002 0 0 eth0
default UG 0 0 0 eth0

Kernel maintains the routing cache information to route the packets faster. We can list the kernel’s routing cache information by using the -C flag.

netstat -rn also shows routing table.


This command replaces old good and now deprecated ifconfig command, however, ifconfig command is still works and available for most of the Linux distributions.

It can be used to assign and remove addresses , bring interfaces up or down, manipulate routing, and many more things.

ip [ OPTIONS ] OBJECT { COMMAND | help }

ip command example


ip address show

show all IP addresses associated on all network devices

ip address show eth0

view the information of any particular interface

ip addr add dev eth0

Assign a IP Address to Specific Interface

ip addr del dev eth0

Remove an IP Address

ip link show

Display Network Interface(s)

ip link set eth0 up

Enable Network Interface

ip link set eth0 down

Disable Network Interface

ip route show

Show routing table information

ip route add via dev eth0

Add static route

ip route del

Remove static route

All the above settings will be lost after a system restart. use config files instead.


The ping command is one of the most used utilities for troubleshooting, testing, and diagnosing network connectivity issues.

Ping works by sending one or more ICMP (Internet Control Message Protocol) Echo Request packages to a specified destination IP on the network and waits for a reply. When the destination receives the package, it will respond back with an ICMP echo reply.

[[email protected] ~]# ping -c3
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=128 time=45.0 ms
64 bytes from icmp_seq=2 ttl=128 time=47.0 ms
64 bytes from icmp_seq=3 ttl=128 time=40.2 ms
--- ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2043ms
rtt min/avg/max/mdev = 40.262/44.107/47.040/2.851 ms

With the ping command, we can determine whether a remote destination IP is active or inactive. You can also find the round-trip delay in communicating with the destination and check whether there is a packet loss.

ping command switch



Numeric output only.do not try to resolve hostname

-i interval

Wait interval seconds between sending each packet

-I interface

Set source address to specified interface address


Audible ping

for ipv6 environment use ping6 command.


This file determines where the system finds things like host names, passwords, and protocol numbers:

Here’s a snippet from a sample /etc/nsswitch.conf file

passwd: files nis
group: files nis
hosts: files dns myhostname

In this example, user information (the passwd and group services) come first from “files” (like /etc/passwd or /etc/group), and if no entries are found there, a query to an NIS server (configured elsewhere) will be used.

Host information first comes from /etc/hosts (files), then a DNS server (dns), and if neither of those work, at least a fallback of “myhostname” so that the local machine has some name.

The non-complexity comes in the “and if that doesn’t work” rule. When multiple services are listed, they’re tried in order, and a sevice either succeeds or fails. If it fails, the next is tried, etc.

that's all.




Consistent network device naming

Red Hat Enterprise Linux provides methods for consistent and predictable network device naming for network interfaces. These features change the name of network interfaces on a system in order to make locating and differentiating the interfaces easier.

Traditionally, network interfaces in Linux are enumerated as eth[0123…], but these names do not necessarily correspond to actual labels on the chassis. Modern server platforms with multiple network adapters can encounter non-deterministic and counter-intuitive naming of these interfaces. This affects both network adapters embedded on the motherboard (Lan-on-Motherboard, or LOM) and add-in (single and multiport) adapters.

In Red Hat Enterprise Linux, udev supports a number of different naming schemes. The default is to assign fixed names based on firmware, topology, and location information. This has the advantage that the names are fully automatic, fully predictable, that they stay fixed even if hardware is added or removed (no re-enumeration takes place), and that broken hardware can be replaced seamlessly. The disadvantage is that they are sometimes harder to read than the eth0 or wlan0 names traditionally used. For example: enp5s0.

  • for disabling that (how ever it is not recommanded Add both net.ifnames=0 and biosdevname=0 as kernel parameter values to the GRUB_CMDLINE_LINUX variable )