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Brian
Asked: 2011-05-25 09:07:54 +0800 CST

modify dhcpd.conf via script to convert whole subnet to DHCP reservations

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We have a hodgepodge of systems that are either static IP addresses, or DHCP reservations. (We don't hand out any addresses except reserved ones). Some servers have multiple Nics, and ILO cards. Its getting annoying to have to scan the subnet to find free IP's to be able to use, especially because a server might be offline for a few mintues when I perform the scan. Any listing seems to be forgotten about quickly..

I would like to create a reservation for every single IP address in my subnets. That would make things quite a bit easier to manage/view. (and afterwards, look at setting up DHCP to do DNS updates too)

I have run the following command:

sudo arp-scan -I eth0 10.10.10.0/24

And have a list of all IP's and Mac Addresses. I am about to get a list of all host/dns names as well. (and if they don't have a hostname, I will call it ip-last octet, so for 10.10.10.100, "ip-100".). That is easy to do with some work in Open Office Calcl

Is there a way to automagically add reservations to the dhcpd.conf file?

I really don't want to hand edit this, as there are hundreds (multiple subnets).

is there an easy way I can call a command to create a reservation, or import a list from CSV? I could find many ways to do this with Windows DHCP, using Net Sh, but not with my CentOS based DHCP server.

I would rather use an existing way, then have to write my own tool.

TL:DR - I need a way to modify Cento's DHCPd from the command line, like you can in Windows with Netsh

centos dhcp isc-dhcp

3 Answers

  1. Best Answer

    I suppose you are using ISC DHCPd. In this case just add 

    host client_name {
    hardware ethernet 00:00:de:ad:be:ef;
    fixed-address 10.10.10.100;
}

    to your scope in /etc/dhcpd.conf.

    Of course, you would not "hand-edit" this but rather create a short script that would spit out the necessary host definitions per scope. Something like that would do:

    #!/bin/bash
function hostdef {
        echo host $1 {
        echo -e \\thardware ethernet $2\;
        echo -e \\tfixed-address $3\;
        echo }
        echo
}

cat $1 | while read name mac ip; do hostdef $name $mac $ip; done

    calling it with a flat file with contents like this:

    ip-100 00:00:de:ad:be:ef 10.10.10.100
ip-101 00:00:0b:ad:be:ef 10.10.10.101
ip-102 00:00:0b:ad:de:ed 10.10.10.102
ip-103 00:00:de:af:be:ef 10.10.10.103

    would generate the necessary lines to paste into your config file or write the output to a separate file which you would include in your dhcpd.conf configuration like this:

    include "/etc/dhcpd-reservations-10-10-10.conf";
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  2. Have a look at omshell it may do what you want.It allows you to create new host objects which reserve the ip address fot a particular MAC address.

    To get omapi listening on your dhcp server

    creat a key to secure access

    dnssec-keygen -a HMAC-MD5 -b 512 -n HOST omapi_key

    which generates files like 

    Komapi_key.+157+24472.key  
Komapi_key.+157+24472.private

    I used the contents of the Key: line from the private file

    fzg+LkcuXNcnFL1GqwuGv/OafeWEszUjn3dZV0FJkXyPyrOSjqNUKII0NSqEWN8W9/BEakGf9aa/GSV44C0zNQ==

    Edit yout dhcp.conf file and add

    omapi-port 9999;
key omapi_key {
algorithm HMAC-MD5;
secret fzg+LkcuXNcnFL1GqwuGv/OafeWEszUjn3dZV0FJkXyPyrOSjqNUKII0NSqEWN8W9/BEakGf9aa/GSV44C0zNQ==;

    }; omapi-key omapi_key;

    using the key generated above.

    Start dhcpd 

    /usr/sbin/dhcpd

    now you can use omshell to add hosts

    /usr/bin/omshell
> port 9999
> server 192.168.200.207
> key omapikey fzg+LkcuXNcnFL1GqwuGv/OafeWEszUjn3dZV0FJkXyPyrOSjqNUKII0NSqEWN8W9/BEakGf9aa/GSV44C0zNQ==
> connect
obj: <null>
> new host
obj: host

    Now you can add host entries as explained in the man page.

    • 1

  3. If you're doing anything with OMshell, per Iain's suggestion, bear in mind that those changes are transient. If you create a new lease object for a given MAC address via OMAPI, the object will be lost the next time the server is restarted.

    You need to do both - OMAPI to add the reservation to the running server immediately and a script that updates dhcpd.conf for posterity.

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