Linux network troubleshooting
If your network connection is not
working..
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
Connection and ip-address seems to be ok. Interface has
received and sent some packages.
[root@firewall ~]# /sbin/ifconfig
eth0
Link encap:Ethernet HWaddr 00:10:5A:39:AC:1D
inet addr:172.240.1.20 Bcast:172.240.1.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:7314123 errors:7 dropped:0 overruns:0 frame:7
TX packets:1894956 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:52529182 (50.0 MiB) TX bytes:234026392 (223.1 MiB)
Interrupt:11 Base address:0xd000
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.
[root@firewall ~]# ifconfig
eth0
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
[root@firewall ~]# /sbin/route -n
Kernel IP routing table
Destination Gateway
192.168.7.0 0.0.0.0
172.24.16.0 0.0.0.0
169.254.0.0 0.0.0.0
0.0.0.0
172.24.16.1
Genmask
Flags Metric Ref Use Iface
255.255.255.0 U 0
0
0 eth1
255.255.252.0 U 0
0
0 eth0
255.255.0.0 U 0
0
0 eth1
0.0.0.0
UG 0
0
0 eth0
Check that default GW exists! If you have two network adapters check
that there’s only one default GATEWAY!
CentOS network configuration files
Are located in /etc/sysconfig –folder
[root@firewall ~]# more /etc/sysconfig/network
NETWORKING=yes
HOSTNAME=firewall.datacom.intra.fi
GATEWAY=172.24.16.1
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!
CentOS Interface configuration file
(static)
[root@firewall ~]# more /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
BOOTPROTO=static
ONBOOT=yes
TYPE=Ethernet
HWADDR=00:50:8B:5D:EE:1A
IPADDR=172.24.16.240
NETMASK=255.255.252.0
NETWORK=172.24.16.0
BROADCAST=172.24.19.255
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!
CentOS
Interface configuration file (dynamic)
[root@firewall ~]# 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 eth0 192.168.7.220 netmask 255.255.255.0 broadcast 192.168.7.255
route add default gw 192.168.7.1
Ubuntu & Debian
Interface configuration file (static)
Are located in /etc/network –folder. Most of the sites in internet are
missing line auto eth0, which makes your system not to refetch
interface addresses from the file.
root@ubuntuserver:/home/test1# 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 172.24.16.240
netmask 255.255.252.0
gateway 172.24.16.1
Ubuntu & Debian
Interface configuration file (dynamic)
Are located in /etc/network –folder
root@ubuntuserver:/home/test1# 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
Quick DNS checkup
• Name servers for machine are defined in /etc/resolv.conf –file.
• [root@clump coursefiles]# more /etc/resolv.conf
search dc.turkuamk.fi
nameserver 193.166.135.5
nameserver 193.166.139.5
• Easy way to check that name servers work:
1) ping www.lukusali.nic.funet.fi
2) ping 193.166.3.7
• 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