My lo interface is down on boot. What can I do?

Phuong Nguyen asked:

My lo interface is down when the system boot. I can turn it up with ifconfig up lo but I wonder what could be the cause? Where can I look to debug such problem?

My answer:


  1. Make sure the network service is configured to start at boot.

    # chkconfig --list network
    network         0:off   1:off   2:on    3:on    4:on    5:on    6:off
    

    Runlevels 0, 1 and 6 should show as off (as they result in shutting down or restarting the system, or bringing it to single user mode) and the rest should be on.

    If it isn’t enabled, then you should enable it.

    # chkconfig network on
    
  2. Make sure the lo interface is configured correctly.

    Check /etc/sysconfig/network-scripts/ifcfg-lo and make sure it hasn’t been altered. On my CentOS 6.5 system it appears as:

    DEVICE=lo
    IPADDR=127.0.0.1
    NETMASK=255.0.0.0
    NETWORK=127.0.0.0
    # If you're having problems with gated making 127.0.0.0/8 a martian,
    # you can change this to something else (255.255.255.255, for example)
    BROADCAST=127.255.255.255
    ONBOOT=yes
    NAME=loopback
    

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