Tag Archives: netmask

Centos Network Configuration – on a Barebones from Command Line

Centos LogoCentos Network Configuration – is very simple on a full installation. However, I downloaded the Centos’ “Minimal Install” cd and used yum to install various packages I needed.

The simple way to do it if you have a standard installation is to use the Network Administration
Tool (system-config-network), which is a graphical interface to edit the configuration files. Since I haven’t installed this tool, I needed to edit the files manually.

How to Configure Network in Centos from Command Line:

A quick way to see if your interface works and if you can ping internal resources.
ifconfig eth0 192.168.0.2 netmask 255.255.255.0
Use your IP addresses.
Note that this is not persistent, at the first reboot, ot first network services restart this will be lost.

For a DHCP address assignment use:
ifdown eth0
dhclient eth0

To get a persistent centos network configuration use the following procedure:

Edit the network configuration file

vi /etc/sysconfig/network-scripts/ifcfg-eth0
Edit the configuration file so it contains your IP address configuration as follows:
DEVICE="eth0"
BOOTPROTO=none
HWADDR="00:00:00:00:00:00"
NM_CONTROLLED="yes"
ONBOOT="yes"
GATEWAY=192.168.0.1
NETMASK=255.255.255.0
IPADDR=192.168.0.2
PEERDNS=no
USERCTL=no

This configures your interface with the IP 192.168.0.2 and the netmask 255.255.255.0.
For more info about Centos’ network configuration look here: .

Set up Name Resolution

Name resolution configuration, is more consistent across various Linux distributions. Most of the Linux distros will have the configuration file at /etc/resolv.conf.
Edit this file and change according to your network:
vi /etc/resolv.conf
Modify the file so it contains pointers to your DNS servers like this:
nameserver 192.168.0.254
nameserver 192.168.0.253

How to – Debian Static IP Configuration

On a basic Debian machine without a graphical interface assigning the same IP address all the times can be achieved in two ways.

Static IP Address

To configure a static IP, (an IP that will never change), and not use DHCP you must edit the file /etc/networking/interfaces.
Insert the following code at the end of the file and don’t change anything else unless you know what you do:

# The first network card – this entry was created during the Debian installation
# (network, broadcast and gateway are optional)
#Private Interface
iface eth0 inet static
address 192.168.0.254
netmask 255.255.255.0
network 192.168.0.0
broadcast 192.168.0.255
## only use gateway if your machine is not multi-homed, (two network cards). You can only have a default route.
# gateway 192.168.0.1

In our case the IP of the Debian machine is 192.168.0.254. The gateway, (the router), is 192.168.0.1 and it is a standard Class C network.

To refresh the network configuration without restarting the server execute:
/etc/init.d/networking restart

If that doesn’t work reboot the machine (reboot or init 6).

For a second network card you should add at the end of the file another entry for your second card:
#External interface
iface eth1 inet static
address 1.1.2.2
netmask 255.255.255.0
network 1.1.2.0
gateway 1.1.2.254

Check the new configuration by issuing the command:
ifconfig

DHCP Reserved address

If you want to set this via DHCP you have to make a reservation into your DHCP server for your network card’s MAC address.
You can find your MAC address by using the command ifconfig.
The server will spit some information on the screen that looks like this:
eth0 Link encap:Ethernet HWaddr 00:33:ff:c4:2f:2b
inet addr:192.168.0.254 Bcast:192.168.10.255 Mask:255.255.255.0
inet6 addr: fe80::230:f4ff:fdd4:bf33/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:93373 errors:0 dropped:0 overruns:0 frame:0
TX packets:38320 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:76539317 (72.9 MiB) TX bytes:5551726 (5.2 MiB)
Interrupt:17 Base address:0x6000

The first line is the one you are interested in:
HWaddr 00:33:ff:c4:2f:2b
In your DHCP server make a reservation using 0033ffc42f2b as your MAC address. Note the removal of the colons in between.
Reboot the server and when the machine will try to renegociate its IP address the DHCP server will assign it the newly reserver address.

If you want to add a static route on your Debian machine edit your /etc/networking/interfaces file and add the following two lines at the end of your eth1, (eth0), configuration.
up route add -net 192.168.22.0 netmask 255.255.255.0 gw 192.168.100.254
down route del -net 192.168.22.0 netmask 255.255.255.0 gw 192.168.100.254
The two lines tell Debian to add a static route when the computer boots, and to remove the static route when it shuts down.

The parameters mean: 192.168.22.0 is the network you want to make your Debian machine aware of; 255.255.255.0 is the netmask of your added network, 192.168.100.254 is the gateway to that network.

Why would you need a static network? In our configuration example your default route is through your public network interface.
Any additional internal networks or VPN’s will not be available. The configuration above tells your Debian machine how to reach any VPN or networks not reachable via the default Network.

There is another change needed if you plan to configure this machine as a simple router. You need to enable IP forwarding, in other words allow the machine to forward traffic for its clients.
# nano /etc/sysctl.conf
Change the following line : net.ipv4.ip_forward = 0
to net.ipv4.ip_forward = 1

Reboot the machine to make the setting active, or issue the following command to make the kernel aware of the change:
# echo 1 > /proc/sys/net/ipv4/ip_forward

Why do you want your Debian machine to connect to other networks or VPN’s? If your machine is a proxy, or a gateway it needs to know where to route packets for its clients. Even if your remote networks or VPN’s have their own proxy, if you have a shared server in one of these networks you need to make it available for your users. It is easier to maintain a static route on one server than add it to all of the clients.

How to Troubleshoot Internet Connection Speed

Who didn’t have problems with the Internet speed at least once? OK, many of us, but as an IT professional I always meet and talk to the people who did have problems with their Internet Speed.
I hate to be run through standard procedure, which by the way I know by hearth, (I used to work in an ISP call center). You probably hate it too. If your problem is the Internet connection you will probably have to talk to an agent anyway, but, (with the help of my article), you will know from the start what’s wrong with your Internet connection.

Step 0 – Never Assume

First think I have learned in the Call Center was to never assume. Many times, when people call the Help Line they have a “pretty good idea” of what’s the problem. Usually they are wrong. Don’t do this mistake, do not presume or, do not presume without checking the facts. We hate the Call centers for making us do stupid things, check things that we don’t need to. The truth is that in most of the cases we overlook things and going by the book helps.
When we open up a browser and there is an error message “Page not found” we immediately assume that the connection is to blame. In fact many times this is just a temporary problem of our home page site. Try a different address to see if the connections is down or the home pages’s website is down. Try google.com for instance or ibm.com, or bing.com; if these don’t work it could be the connection

Step 1 – Try a reboot

Almost every time a reboot solves the problem. This might sound anecdotic but it actually works in many cases. Reboot the computer, reboot the modem, reboot the router behind the modem.

Isolate the problem
First of all we need to know that the problem is not in the network.

  • Use a computer that you are sure is not infected by any Mallware. Connect it directly to the DSL device, cable modem, or other type of modem.
  • Do not use the wireless; connect the computer with a network cable. This way you are testing the connection and nothing else.
  • As already mentioned, you need to make sure your computer works fine and is not infected. Use antivirus and antispyware if needed.
  • If you are not sure about the health of your computer you can always boot from a Live Linux CD. On the Live CD you will have a browser and other useful tools.
  • Once the system is up and you are connected to the Internet, test your connection with one of the free online services. Google-it and you will find many online DSL speed test websites. If the speed is OK then, the problem is in your computer, or your network, go to step 4.
  • Step 2 – Modem troubleshoot

    If the DSL speed test failed, (no connection or slow speed), you have a few things to try before calling the ISP
    Usually, for a DSL connection, the telephone line is shared with another device, a fax or a phone. Some of us have a separate line for DSL or they don’t even have a phone line, they only have a dry loop. If you don’t understand any of these you most likely don’t have it.

  • If you share the DSL line with another device you need to connect this second device through a DSL filter. I must stress: EVEN IF IT WORKED before without it, you must have the filter.
    DO NOT run the DSL line through a filter, you’ll only break your Internet Connection, only the phone or the fax, or Credit Card processing device, etc…
  • Check all the physical connections of the Modem Device, R6 or RJ11 or the RJ45 connectors and make sure that they are tight they don’t have the locking pins broken, etc…
  • For a DSL connection the length of the phone cable could be critical, so use the original cable, (the short one), provided with the modem at least for the testing period. The reason is because the length of the pair from the CO to your place can be at its length limits, it happens more often than you think. If you pass this limit your connection stability will be penalized. Another reason is because a long cable is more difficult to examine for defects than a short one.
    If needed, use a longer network cable to connect a device in the house. The network cable, (UTP – unshielded twisted pair), can go up to 100m (300 feet).
  • Some electric devices can interfere with the modem; avoid placing it near such devices.
  • Look at the modem’s lights. If the DSL light is off there is no service.
  • If there is no connection at all, (No DSL light), check the phone to see if you have a dial tone.
  • If the modem connects to a separate wall jack bring a phone and check if you have a dial tone, you might have a dial tone in the bedroom and not in the office.
  • Similarly check if there is TV service, if there is no TV service then most likely it is a bigger problem at your cable provider.
  • Look for the DSL filter as described above.
  • Check the LAN light on the modem. If the LAN LED is off your computer’s network adapter might be defective, or disabled, or the UTP cable that connects the computer with the modem might be defective. There is a chance that the modem’s LAN interface is defective.
  • If you have a solid DSL light, (check the modem’s manual, manufacturers have different signalling for established DSL connection), and you don’t have a connection check the password for the connection.
  • Step 3 – Connect to Your Modem’s Administration Interface

    All of the modern modems have a WEB Administration Interface. The admin Interface is usually accessible via a WEB browser. It lets you configure the authentication and other different settings.
    In order to administrate your modem you need the following:

  • The administrator’s password to access your router. If you don’t have it you can perform a hardware reset using the reset button located on the modem. You need to use a pin to press the button. This will reset all of your modem’s setting, including the admin password, to the factory defaults.
  • The IP address of the router. If the modem has its DHCP server started, (by default yes), it will lease your computer an IP address and provide it with other IP configuration settings such as: “Default Gateway”, netmask, and DNS. The important part is the gateway’s IP address as this is your modem’s internal interface and this is what you need to access your router’s web Interface. You can find all of this information by issuing the following command in a command prompt window with elevated rights, (admin credentials):
    ipconfig /all
    After issuing the command you will get a screen similar to this one. Note that the “Default Gateway” is the address of your modem.
    IPCONFIG
    In a web browser type: http://IPADDRESS where IPADDRES is your router.
    For instance if your router’s (gateway) IP address is 192.168.200.1 the address you are looking for is: http://192.168.200.1.
  • Once connected to the web interface you need to enter the authentication details. If you don’t have them you have to call your ISP which will make a reset after security verification.
  • If all of the previous steps did not solve the problem call your ISP. They might ask you to redo all of the steps above and maybe more. Be patient, they are just doing their job, they never assume, or they shouldn’t. The second they let themselves drawn into your speculation they make the same mistake as you do.

    Step 4 – Correct the problems behind the modem

    If the connection is OK the problem is behind the modem. This problem could be anything in your network.
    Some suggestions are:
    The router – many times the router can slow down your network. Try a reboot. If that doesn’t work try another model or another manufacturer. Do an upgrade of the firmware. Check the router’s configuration and try a default stripped down configuration.
    The computer – most of the times, the slow performance over Internet is actually the poor performance of a computer. This is a dense material that is treated in a separate article which you can find here: How to Refresh your Computer speed (coming soon).
    The network – many times a poor network will penalize al sorts of network traffic not only Internet. This makes the subject of another article here: How to Troubleshoot Your LAN, (Local Area Network). (coming soon)

    Ask me any technical question in the comments area so I can update the document or answer you directly on the comments. I would like to make this article as useful as possible.