How to tell an eth port has link? - Networking

This is a discussion on How to tell an eth port has link? - Networking ; There has to be a way to tell if the ethernet port has a cable plugged in. I see the kernel message go by telling me it has lost or established the link when I plug the cable in... But ...

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Thread: How to tell an eth port has link?

  1. How to tell an eth port has link?

    There has to be a way to tell if the ethernet port has a cable plugged in.

    I see the kernel message go by telling me it has lost or established the
    link when I plug the cable in... But I can't find anything that would
    tell me the status from userspace.

    I've been all through /proc and I don't see anything that would clue me
    in....

    So... How to tell if an ethX port has a valid hardware link? I need to
    test this before networking comes up so I know if I should run DHCP or
    not...

    --Yan

  2. Re: How to tell an eth port has link?

    On Mon, 02 Apr 2007 09:01:02 -0700, CptDondo wrote:

    > There has to be a way to tell if the ethernet port has a cable plugged in.


    Use ethtool and/or mii-tool. Depending on what type of card it is, one or
    the other should work.

    Regards, Ian

  3. Re: How to tell an eth port has link?

    CptDondo wrote:
    > There has to be a way to tell if the ethernet port has a cable plugged in.
    >
    > I see the kernel message go by telling me it has lost or established the
    > link when I plug the cable in... But I can't find anything that would
    > tell me the status from userspace.
    >
    > I've been all through /proc and I don't see anything that would clue me
    > in....
    >
    > So... How to tell if an ethX port has a valid hardware link? I need to
    > test this before networking comes up so I know if I should run DHCP or
    > not...
    >
    > --Yan



    Maybe you like to have ifplugd?

    --

    Tauno Voipio
    tauno voipio (at) iki fi

  4. Re: How to tell an eth port has link?

    CptDondo wrote:

    > I see the kernel message go by telling me it has lost or established the
    > link when I plug the cable in


    Is there a hook for this that can call a script, or trigger an initd event
    that can be used to call a script?

    I am curious, because on my network, if I have a power failure, the network
    hubs lose their connections, but the computers continue to run from a UPS
    supply. The loss of the network hubs is a sure signal to all computers that
    the power is out.

    I was hoping to hook a shutdown script to the loss of ethernet connection
    event, that shuts the computers down, if the outage lasts more than a couple
    of minutes.

    Regards,

    Mark.

    --
    Mark Hobley
    393 Quinton Road West
    QUINTON
    Birmingham
    B32 1QE

    Telephone: (0121) 247 1596
    International: 0044 121 247 1596

    Email: markhobley at hotpop dot donottypethisbit com

    http://markhobley.yi.org/


  5. Re: How to tell an eth port has link?

    Mark Hobley wrote:
    > CptDondo wrote:
    >
    >> I see the kernel message go by telling me it has lost or established the
    >> link when I plug the cable in

    >
    > Is there a hook for this that can call a script, or trigger an initd event
    > that can be used to call a script?
    >
    > I am curious, because on my network, if I have a power failure, the network
    > hubs lose their connections, but the computers continue to run from a UPS
    > supply. The loss of the network hubs is a sure signal to all computers that
    > the power is out.
    >
    > I was hoping to hook a shutdown script to the loss of ethernet connection
    > event, that shuts the computers down, if the outage lasts more than a couple
    > of minutes.
    >


    If I understand ifplugd correctly from reading the manpage, it should
    detect the other end losing power, as the link will drop.

    I have not gotten into ifplugd yet - I'm still hoping to get dhclient to
    do what I want - but it looks promising. For my purposes it's a bit of
    overkill...

    --Yan

  6. Re: How to tell an eth port has link?

    CptDondo wrote:
    > I have not gotten into ifplugd yet - I'm still hoping to get dhclient to
    > do what I want - but it looks promising. For my purposes it's a bit of
    > overkill...


    Yeah for mine too really, I don't want to configure the network, that is
    already done here, and my IP addresses are static, so I don't use dhclient. All
    I want to do is hook the cable dead event, possibly through initd, if it
    supports the incident.

    Regards,

    Mark.

    --
    Mark Hobley
    393 Quinton Road West
    QUINTON
    Birmingham
    B32 1QE

    Telephone: (0121) 247 1596
    International: 0044 121 247 1596

    Email: markhobley at hotpop dot donottypethisbit com

    http://markhobley.yi.org/


  7. Re: How to tell an eth port has link?

    Mark Hobley wrote:
    > CptDondo wrote:
    >> I have not gotten into ifplugd yet - I'm still hoping to get dhclient to
    >> do what I want - but it looks promising. For my purposes it's a bit of
    >> overkill...

    >
    > Yeah for mine too really, I don't want to configure the network, that is
    > already done here, and my IP addresses are static, so I don't use dhclient. All
    > I want to do is hook the cable dead event, possibly through initd, if it
    > supports the incident.


    I think what I am going to do is:

    on boot, run mii-tools to see if the cable is hooked up
    if it is, run dhclient {
    if dhclient gets an address {
    set up for networked database
    exit }
    }

    assume no network
    set up for local database

    I think you could run mii-tools/ethtools from a loop every so often
    (minute?) and see if the link is dead.

    --Yan

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