Wireless problems often baffle me — so let’s postpone that for another post; right now we’ll focus on the wired network problems which is less common than the other one!

While using Ubuntu based systems, I often found that network manager just got into the way whenever I wanted to fix an internet connection. So, first thing to do is to kill the associated processes (kill -9) after you find the process ID by using top or htop (if installed). Or you can totally get rid of these packages (usually network-manager, network-manager-gnome in Gnome or knetworkmanager in KDE).

Then, edit /etc/network/interfaces file (you need to precede it by sudo) so that the lines for wired interface (usually eth0 — find this by issuing sudo ifconfig , first) look like the following:

auto eth0
iface eth0 inet dhcp


again assuming that we want to have a DHCP based connecton (which is the case most of the time and it’s, ofcourse, easier to fix!).

Then restart the network connection by issuing the following command:
sudo /etc/init.d/networking restart

And then bring eth0 first down, then up using
sudo ifdown eth0; sudo ifup eth0

This should be the first step toward troubleshooting the wired internet connection. The things starts getting just better if these steps fail ……well, let’s leave that for another post.

The differences here are pretty simple.  ‘ifconfig’ allows for actual modification of settings and state of a network adapter.  This will affect the current running configuration.

Setting IP information
ifconfig <interface> <ip addr> netmask <netmask>

Example
ifconfig eth1 10.1.1.200 netmask 255.255.255.0

Display currently up interfaces
# ifconfig
lo        Link encap:Local Loopback
inet addr:127.0.0.1  Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING  MTU:16436  Metric:1
RX packets:162 errors:0 dropped:0 overruns:0 frame:0
TX packets:162 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:85013 (83.0 KiB)  TX bytes:85013 (83.0 KiB)

Bring up an interface
# ifconfig eth1 up

Re-display up interfaces
# ifconfig
eth1      Link encap:Ethernet  HWaddr 00:50:56:A6:32:50
inet addr:10.1.1.200 Bcast:10.1.1.255  Mask:255.255.255.0
inet6 addr: fe80::250:56ff:fea6:3250/64 Scope:Link
UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:66 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0.0 b)  TX bytes:18956 (18.5 KiB)
Interrupt:185 Base address:0×1480

lo        Link encap:Local Loopback
inet addr:127.0.0.1  Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING  MTU:16436  Metric:1
RX packets:162 errors:0 dropped:0 overruns:0 frame:0
TX packets:162 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:85013 (83.0 KiB)  TX bytes:85013 (83.0 KiB)
‘ifconfig <interface> down’ will take the specified interface offline.

‘ifup’ will bring up the specified interface with the settings stored in ‘/etc/sysconfig/network-scripts/ifup-interfaceName’.  Below is an example of that file.

# cat /etc/sysconfig/network-scripts/ifcfg-eth1
# Advanced Micro Devices [AMD] 79c970 [PCnet32 LANCE]
DEVICE=eth0
BOOTPROTO=static
DHCPCLASS=
HWADDR=00:50:56:A6:6C:9B
IPADDR=10.0.0.155
NETMASK=255.255.255.0
ONBOOT=yes

‘ifdown <interface>’ will take the specified interface offline as well.

Note:  This is from a Centos / RedHat Linux server.  The stored network interface file might not be located under the sysconfig directory on other versions.  So basically, if you “fat finger” configuration information on an interface, you can just ifdown than ifup that interface to reload the previously saved settings.


#ifup eth0
#ifconfig eth0 up


both work the same way. ifup will configure network based on /etc/network/interfaces.
ifconfig use to configrue kernel resident network interfaces

1) ifconfig = allows the operating system to setup network interfaces and allow the user to view information about the configured network interfaces

Examples

ifconfig

View the network settings on the Ethernet adapter currently used.

ifconfig eth0

View the network settings on the first Ethernet adapter installed in the computer.

ifconfig -a

Display into on all network interfaces on server, active or inactive.

ifconfig eth0 down

If eth0 exists would take it down causing it cannot send or receive any information.

ifconfig eth0 up

If eth0 exists and in the down state would return it back to the up state allowing to to send and receive information.

ifconfig eth0 192.168.1.102 netmask 255.255.255.0 broadcast 192.168.1.255

Assign eth0 with the above values for IP, netmask and broadcast address.

.

2) ifup = bring a network interface up

ifup eth0 – bring up interface eth0

.

3) ifdown = bring a network interfacedown

ifdown eth2 – bring down interface eth2

.

4) ping = ping allows a user to ping another network IP address. This can help determine if the network is able to communicate with the network.

Example:

ping gmail.com – Would ping the gmailhost .com to see if it is alive.

ping gmail.com -c 1 – Would ping the hgmailost .com once and return to the command line

.

5) traceroute = print the route packets take to network host

Examples:

$ traceroute google.co.in — Trace the route for Google Server.

$ traceroute -v 192.168.12.1 – Show more details.

$ traceroute -n server — Print the HOP Address in numeric form.

$ traceroute -I server — Use ICMP ECHO instead of UDP datagrams.

$ traceroute -i eth1 server — Use eth1 for send the probe packets.

$ traceroute server 100 — Sent 100 Bytes packet instead of default 40 Bytes packet.

6) host = simple utility for performing DNS (Domain Name Service) lookups.  It is normally used to convert names to IP addresses and vice versa

Examples:

$ host google.com — Show the IP Address of Google.com.

$ host -v google.com — Show detailed output.

$ host -a google.com — Show more details.

$ host 127.0.0.1 — Do reverse lookup and Show the host name.

$ host -l mydomain.com — List all hosts available in MyDomain.

.

7) dig = DNS lookup utility

Ecamples:

dig yahoo.com

dig 66.94.234.13

dig -x 192.168.52.

dig -t ANY yahoo.com

Bien, hoy una curiosidad, un tanto Off-topic.
Aqui estan los planos de la red de mi casa y la de mi abuela.

¿A donde quiero llegar con todo esto? – Os preguntareis – Pues bien, aqui teneis un pequeño y básico manual sobre configuracion de redes en linux:

Editado: Este post habia salido demasiado largo así que lo he dividido en dos manuales, uno sobre ifconfig y otro sobre ifup.

how to renew ip address from dhcp server in linux workstation? in windows we will use ipconfig/release and ipconfig/renew. But what about our cute linux – you need to use command at shell dhclient eth0 where eth0 is interface . Put yours there.
or you can get using ifdown and ifup commands. ifdwon and ifup

Se state usando un portatile ASUS L5000D, sapete bene che il chipset della schede di rete senza fili 802.11g è l’odiatissimo Broadcom 4306, di cui non esistono specifiche pubbliche.

Il driver nativo bcm43xx, sebbene sia stato migliorato moltissimo per poter essere integrato nei kernel serie 2.6.17 (quando era sviluppato a solo non riusciva a riconoscere la nostra scheda, adesso ci riesce), sembra non funzionare benissimo con il WPA (in particolare, con wpa_supplicant: ma se si pensa che lo stramaledetto driver ASUS fornito con il PC nemmeno sa cosa sia…), mentre non ho testato la funzionalità WEP.

Dovremo quindi, almeno per un po’, continuare ad usare ndiswrapper e questo driver (attenzione: dovete avere un account su Ubuntu Forums per potervi accedere).

Non vi spiegherò come installare ndiswrapper (chiedete a Google piuttosto), ma vi spiegherò come porre rimedio a un problema lasciatoci in eredità da coloro che hanno assemblato il nostro portatile.

Vi sarete accorti che, anche in Windows, la scheda sembra “perdere” i parametri giusti di connessione dopo una serie di riavvii, o, peggio, quando si passa spesso da una rete ad un’altra. Il baco, signori, risiede nella scheda, e in Windows non ci si può far nulla, se non fare riavvii su riavvii e sperare che quello successivo sia quello buono.

In GNU/Linux (e presumibilmente anche su *BSD), invece, possiamo risolvere il problema molto velocemente. Infatti, noi abbiamo un controllo sul sottosistema NDIS (il protocollo di comunicazione utilizzato dai driver Windows) molto più esteso di quanto non sia quello disponibile a un utente Windows stesso. Signori, in pratica, è sufficiente fare

/sbin/iwpriv [device] ndis_reset

e poi riconfigurare la scheda per far funzionare tutto alla perfezione. :-d

Per integrare questi passaggi nella nostra distro Debian/Ubuntu ci serve soltanto:

1. Un /etc/network/interfacessimile a questo (supponendo eth1:

   auto eth1
   iface eth1 inet dhcp
   wireless-essid "Il mio SSID"
   wireless-channel 11
   wpa-driver wext
   wpa-conf /etc/wpa_supplicant.conf
   pre-up /sbin/iwpriv eth1 ndis_reset

2. Un /etc/rc.local (o un altro script di esecuzione locale) simile a questo:

   /sbin/iwpriv eth1 ndis_reset
   /sbin/ifdown eth1
   /sbin/ifup eth1

Sarete piacevolmente sorpresi, credo.