Linux network troubleshooting
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Linux network troubleshooting. If your network 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.

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Linux network troubleshooting

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Linux network troubleshooting

Linux network troubleshooting

If your network is not working..


Quick troubleshoot

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


Linux network troubleshooting

Connection and ip-address seems to be ok. Interface has

received and sent some packages.

[[email protected] ~]# /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


Linux network troubleshooting

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.

[[email protected] ~]# 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


Linux network troubleshooting

[[email protected] ~]# /sbin/route -n

Kernel IP routing table

Destination Gateway Genmask Flags Metric Ref Use Iface

192.168.7.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1

172.24.16.0 0.0.0.0 255.255.252.0 U 0 0 0 eth0

169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth1

0.0.0.0 172.24.16.1 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

CentOS network configuration files

Exist in /etc/sysconfig –folder

[[email protected] ~]# 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 dhcp

Centos Interface configuration file (DHCP)

[[email protected] ~]# 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


Centos interface configuration file static

Centos Interface configuration file (static)

[[email protected] ~]# 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!


Ubuntu debian interface configuration file dhcp

Ubuntu/(Debian)Interface configuration file (DHCP)

Exist in /etc/network –folder

[email protected]:/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


Ubuntu debian interface configuration file static

Ubuntu/(Debian) Interface configuration file (static)

Exist in /etc/network –folder

[email protected]:/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 eth0iface eth0 inet staticaddress 172.24.16.240netmask 255.255.252.0gateway 172.24.16.1


Quick dns checkup

Quick DNS checkup

  • Name servers for machine are defined in /etc/resolv.conf –file.

  • [[email protected] 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.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


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