query ip usage from dhcp server

turtlenodhead

is there anyway to query a dhcp server if an ip is already in use by other computer in the network. (under WinXP or linux)

RussS

Yup - called ping
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A couple good articles to read

http://www.unet.univie.ac.at/aix/cmds/aixcmds2/dadmin.htm

http://www.computerperformance.co.uk/w2k3/W2K3_DNS_Pure.htm

learningIT

If you have an honest CCNA you should already have known about PING. Even though that seems A+ish... I took a Cisco class and we learned ping when we did router labs and it was all over the curriculum...sometimes I wonder about you turtlehead.

turtlenodhead

yes, I know ping can check reachability.

I'm just so dumb to realize if an ip is not reachable = it's not taken.

Ten9t6

turtlenodhead wrote:

yes, I know ping can check reachability.

I'm just so dumb to realize if an ip is not reachable = it's not taken.

Actually, it can still be taken....It just may be filtering ICMP packets. Just a thought.

turtlenodhead

if I manually configured my host as follows:
ifconfig eth0 172.18.0.1 netmask 255.255.0.0

do the DHCP server know I take the IP.

Also how do I check if there is conflict IP on a LAN.
Right now I use tcpdump to check for 2 ARP reply from different MAC address.
Is there easier way?

ajs1976

If you statically assign an IP address to a machine that is also in your DHCP range, DHCP does not check first and you could end up with an IP conflict. There are a couple of ways around this.

1. Don't do let it happen. Plan out your IP usage.
2. Create an exclusion within your IP range.
3. Create a reservation for the machine. The IP isn't statically assigned to the PC, but a reservation is created in DHCP so whenever that PC requests an IP address, the DHCP server gives it the same address each time. The reservation is based on the MAC address of the DHCP client.

turtlenodhead

so... dhcp server do not periodically send broadcast to verify if host have try to take a static ip or a host have disconnected without releasing it's IP?

turtlenodhead

learningIT wrote:

If you have an honest CCNA you should already have known about PING. Even though that seems A+ish... I took a Cisco class and we learned ping when we did router labs and it was all over the curriculum...sometimes I wonder about you turtlehead.

A CCNA course cost more than 1 month of my salary. Don't put this kind of reply it's meanless.

reply such as "you are dumb", ""you should know this as a CCNA" is ok, but nothing over it.

Thanks to RussS, Ten9t6, Ajs1976, learned alot.

I know DHCP server would check for conflict before issueing an IP(at least in a LAN NT service pack 2 and beyond), but can a DHCP client actually query a DHCP server directly?

Webmaster

Not like a DNS client can query a DNS servers records, if that is what you mean. A DHCP client doesn't ask if an IP address is in use, it just asks for one (DHCP_Request) or to renew its current assigned address, and the server replies with an available IP address (DHCP_Offer). If the client accepts it, it returns an acknowledgement (DHCP_Ack), else it sends a Nack. Some systems can indeed be configured to ping an address before it assigns it. This is not a water proof solution though...