DHCP via wrong network

  • I plugged a cable into a wrong switch port and some clients ended up with dhcp leases from the wrong network. The gateway and dns assigned by dhcp were useless. Things got back on track once I plugged the cable into a proper port.

    Is there a way dhcpd not to dhcpoffer to clients via wrong network?

    Looking at the opt1 dhcpd log entries looks like this in pseudo code

    dhcpdiscover client mac via opt1
    dhcpnak on ip of wrong subnet  to client mac via opt1
    dhcprequest for ip of wrong subnet from client mac via opt1:wrong network
    dhcpoffer on ip from opt1 pool to client mac(client hostname) via opt1

  • LAYER 8 Global Moderator

    A dhcp can be set to nak or not a request to renew an IP that it is not authoritative for..

    So your saying you don't want your dhcpd to nak requests for IPs that are wrong?  This can cause problems with clients that are moving from a different network - this can be a major issue with clients moving between wifi networks, and trying to renew their old IP.  Depending on the client if they do not get a renewal or a NAK that this is not the right network for that IP range they could continue to use that old IP until the lease fully expires and they send out a discover to get the IP on this network.

  • The dhcp bleed out problem started when I accidentally connected untagged vlan 100 port to untagged vlan 200 port. Maybe I should try tagged ports to prevent dhcp requests going to a foreign subnet.

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