1. Linux network troubleshooting
If your network is not working..

2. Quick troubleshoot Check that network cable is connected and one of
NIC’s leds is lit. Also check that computer is powered on!
If possible go to switch/hub where your machine is connected and
check that connection led is lit.
Is network leds seem to be ok, restart network with following command:
service network restart (works with CentOS, fedora core and redhat)
/etc/init.d/network restart (should work with other distributions also)
Check what ifconfig tells you. If you’re not root user, you can check
ifconfig with following command:
/sbin/ifconfig

3. Connection and ip-address seems to be ok. Interface has
received and sent some packages.
~]# /sbin/ifconfig
eth Link encap:Ethernet HWaddr 00:10:5A:39:AC:1D
inet addr: Bcast: Mask:
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets: errors:7 dropped:0 overruns:0 frame:7
TX packets: errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes: (50.0 MiB) TX bytes: (223.1 MiB)
Interrupt:11 Base address:0xd000

4. If your ip-address isn’t shown, you most probably have DHCP
enabled and DHCP-server is not giving you any address. If you don’t
get ip-address Linux might interface disabled. You can use
dhclient command manually to restart dhcp search.
~]# ifconfig
eth Link encap:Ethernet HWaddr 00:10:5A:39:AC:1D
UP BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:14 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0.0 MiB) TX bytes:0 (0.1 MiB)
Interrupt:11 Base address:0xd000

5. [root@firewall ~]# /sbin/route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
U eth1
U eth0
U eth1
UG eth0
Check that default GW exists! If you have two network adapters check
that there’s only one default GATEWAY!

6. CentOS network configuration files
Exist in /etc/sysconfig –folder
~]# more /etc/sysconfig/network
NETWORKING=yes
HOSTNAME=firewall.datacom.intra.fi
GATEWAY=
The default GATEWAY can be set in network file or ifcfg-ethX file. It’s
easier to troubleshoot if you have gateway set in one file only. If you
have two network adapters check that there’s only one default
GATEWAY!

7. Centos Interface configuration file (DHCP)
~]# more /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
BOOTPROTO=dhcp
HWADDR=00:50:8B:5D:EE:1A
ONBOOT=yes
TYPE=Ethernet
Dhcp gets all addresses automatically, only thing you need to set is
BOOTPROTO. If your network still doesn’t work, you can add
network configuration manually with following commands:
ifconfig eth netmask broadcast
route add default gw

8. Centos Interface configuration file (static)
~]# more /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
BOOTPROTO=static
ONBOOT=yes
TYPE=Ethernet
HWADDR=00:50:8B:5D:EE:1A
IPADDR=
NETMASK=
NETWORK=
BROADCAST=
Static configuration for interface eth0. You must define at least IPADDR
and NETMASK. If you have other than basic class C network specify
also BROADCAST and NETWORK addresses. Remember to change
BOOTPROTO to static if you’re setting static address manually!

9. Ubuntu/(Debian) Interface configuration file (DHCP)
Exist in /etc/network –folder
more /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The primary network interface
auto eth0
iface eth0 inet dhcp

10. Ubuntu/(Debian) Interface configuration file (static)
Exist in /etc/network –folder
more /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The primary network interface
auto eth0 iface eth0 inet static address netmask gateway

11. Quick DNS checkup
Name servers for machine are defined in /etc/resolv.conf –file.
coursefiles]# more /etc/resolv.conf
search dc.turkuamk.fi
nameserver
nameserver
Easy way to check that name servers work:
1) ping
2) ping
If both pings works, then name servers are working
If only the last ping works, then you have some problems with your name server configuration
If both pings fail, check ifconfig and default route