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apparently someone on my LAN is running his own DHCP server (by mistake i assume) and broadcasting an address (yep it is 192.168.1.155) prevents me to connect to the 'real' DHCP server (195.249.186.1). how can i connect to my usual DHCP server? what commands shoul i use?

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2 Answers 2

up vote 4 down vote accepted

For example if the wrong dhcp server is 195.249.186.30 you can configure your dhcp client addng this line to dhclient.conf :

reject 195.249.186.30

reject ip-address; The reject statement causes the DHCP client to reject offers from servers who use the specified address as a server identifier. This can be used to avoid being configured by rogue or misconfigured DHCP servers, although it should be a last resort - better to track down the bad DHCP server and fix it.

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+1, better immediate solution to the problem than me. –  MDMarra Aug 16 '10 at 15:05
    
one other thing, how can i see all DHCP servers on my LAN (to find out if there are more rogue servers)? –  Infestor Aug 16 '10 at 15:20
    
For windows there's a program called DHCP Explorer that can discover DHCP server on your subnet –  aleroot Aug 16 '10 at 15:49

At the switch port level you can use DHCP snooping to ensure that only DHCP responses are received from trusted servers (this is a fairly basic feature on most non-consumer grade switches). The implementation of this will depend on your make and model of switches.

If your switches do not support this, you will need to trace down the DHCP server that is causing the issue and disable it. Since DHCP is a broadcast protocol, you are rather limited with how to manage it at any level higher than the switch ports.

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