dhcp ip in use, can it be revoked?

Discussion in 'Linux Networking' started by Ittay, Jan 17, 2005.

  1. Ittay

    Ittay Guest


    i have a client that requests an ip by dhcp, and then uses it to do
    NFS mounts. if the dhcp is configured to expire the IP after a while,
    what will happen? (assuming i don't have any daemon to renew the

    Ittay, Jan 17, 2005
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  2. Ittay

    prg Guest

    May depend on what you mean by "daemon". dhclient? dhcp server?

    The dhclient runs as a root daemon from the startup scripts. Can you
    gracefully stop it? Don't know. If I were admin of a net where
    someone tried such a thing (in order to keep an IP lease), I would
    refuse them _all_ future lease attempts till after a visit to the
    woodshed. Such IP hanky-panky might even be considered a fireable

    In normal operation the following sequence roughly proceeds

    Say the lease was issued for 10 hours.

    After 5 hours, the client will try to renew the lease. If the server
    is down or can't be contacted, the client just goes on about its way.
    But the clock keeps ticking ... 5 hours remaining.

    After 8.75 hours, the client will try again and if no lease offered,
    start worrrying ;-0

    After 9.5 hours (?), try once again and if no renewal ... You get the

    The above sequence is illustrative only of how the client tries to
    renew it's lease in a manner that maintains its IP and provides for
    temporary dhcp server troubles. See rebind below.

    A part of the lease the client receives includes certain date/time
    values. My current lease includes:
    /var/lib/dhclient-eth0-leases << a RH quirk
    lease {
    interface "eth0";
    fixed-address xxx.xxx.210.33;
    option subnet-mask xxx.xxx.248.0;
    option routers xxx.xxx.208.1;
    option dhcp-lease-time 54907;
    option ien116-name-servers xx.xxx.0.4,xx.xxx.0.5;
    option dhcp-message-type 5;
    option domain-name-servers xx.xxx.0.4,xx.xxx.0.5;
    option dhcp-server-identifier xx.xxx.3.45;
    option broadcast-address;
    renew 1 2005/1/17 21:36:05;
    rebind 2 2005/1/18 03:36:57; << begin trying to renew
    expire 2 2005/1/18 05:31:21; << here's the one that cuts you off

    $ man dhclient.conf
    The rebind statement defines the time at which the dhcp client should
    begin to try to contact any dhcp server in order to renew its lease.

    The expire statement defines the time at which the dhcp client must
    stop using a lease if it has not been able to contact a server in order
    to renew it.

    At expiry the client will check for any _unexpired_previous_ leases
    which it can attempt to renew.

    When all the leases are expired, the client begins all over, just as at
    boot time, attempting to acquire a lease.

    "If the client is unable to obtain a lease, the client's TCP/IP stack
    will not function and network errors will occur." See:
    or google:
    dhcp client lease expire

    The lesson: make sure a dhcp server is always available ;-)
    email above disabled
    prg, Jan 17, 2005
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