[[!summary chatGPT: A blog post explaining how to execute custom scripts when a DHCP lease is passed to a client, with instructions and examples for setup in Gentoo Linux.]]
[[!meta date="2010-03-27 13:24"]]
[[!tag gentoo linux]]

[[!img media/internet_systems_consortium.jpg size=400x style="float: right"]]

# motivation

a friend of mine, **Andreas Korsten**, showed me how to execute custom scripts when a dhcp-lease is passed to a client. this is interesting stuff and since it seems **not to be documented**anywhere yet, i decided to blog it. it is probably of use for other admins out there - thanks to Andreas Korsten!

# concept


**idea: run a custom script when a lease is passed to the client.**in the example below every client in the netboot group will trigger 'custom logging' and additionally execute a script.


# ISC Dynamic Host Configuration Protocol


It is about: net-misc/dhcp-3.1.2_p1 (gentoo, portage), see [1] **No special useflags** were used: **+kernel_linux**-doc -minimal -selinux -static


# setup /etc/dhcp/dhcp.conf


    # vim: set noet ts=4 sw=4:
        
    allow booting;
    allow bootp;
    
    server-name "myServer";
    default-lease-time 3000;
    max-lease-time 6000;
    ddns-update-style none;
    
    subnet 10.0.0.0 netmask 255.255.255.0 {
     range 10.0.0.10 10.0.0.20;
     option subnet-mask 255.255.255.0;
     option domain-name-servers 10.0.0.1;
     option domain-name "myPool";
    
    group netboot {
     next-server 10.0.0.1;
     #server-identifier 10.0.0.1;
     #filename "pxelinux.0";
    
    #on commit { execute ("/tmp/dhcpcommit.sh", hardware , "fnord", host-decl-name, "foo", leased-address, "bar" ); }
     #on commit { execute ("/tmp/dhcpcommit.sh", host-decl-name ); }
     #on commit { execute ("/tmp/dhcpcommit.sh", leased-address ); }
    
    # helpful: https://lists.isc.org/pipermail/dhcp-users/2008-September/007167.html
     on commit { 28 set ClientIP = binary-to-ascii(10, 8, ".", leased-address); 29 set ClientMac = binary-to-ascii(16, 8, ":", substring(hardware, 1, 6)); 30 log(concat("Commit: IP: ", ClientIP, " Mac: ", ClientMac)); 31 execute("/tmp/dhcpcommit.sh", "commit", ClientIP, ClientMac); 32
     #if(execute("/root/scripts/dhcp-event", "commit", ClientIP, ClientMac) = 0) {
     #if(execute("/tmp/dhcpcommit.sh", "commit", ClientIP, ClientMac) = 0)
     #{
     # log(concat("Sent DHCP Commit Event For Client ", ClientIP));
     #}
     #} else {
     # log(concat("Error Sending DHCP Commit Event For Client ", ClientIP));
     #}
     }
    
    host router5 { hardware ethernet 00:40:ff:aa:b0:44; fixed-address 10.0.0.5; option host-name "router5"; }
     #include "/etc/dhcp/dhcpd.otherhosts.conf";
     }
    }



# the script



**you could send an email or jabber message or just do some advanced logging. consider: if you have a server-farm it might be interesting to see if a reboot actually worked**. the arguments to the bash script can be processed within the script. the order of the arguments is given by the dhcpd.conf file, see above.



# possible errors



always review the logs, in my case /var/log/syslog and since the dhcpd service on gentoo is running as user 'dhcp' and the script was not accessable for the user 'dhcp' this error could be found:


    
    debug: Mar 27 13:45:17 dhcpd: Commit: IP: 10.0.0.5 Mac: 0:40:ff:aa:b0:44
    debug: Mar 27 13:45:17 dhcpd: execute_statement argv[0] = /tmp/dhcpcommit.sh
    debug: Mar 27 13:45:17 dhcpd: execute_statement argv[1] = commit
    debug: Mar 27 13:45:17 dhcpd: execute_statement argv[2] = 10.0.0.5
    debug: Mar 27 13:45:17 dhcpd: execute_statement argv[3] = 0:40:ff:aa:b0:44
    err: Mar 27 13:45:17 dhcpd: Unable to execute /tmp/dhcpcommit.sh: Permission denied
    err: Mar 27 13:45:17 dhcpd: execute: /tmp/dhcpcommit.sh exit status 32512
    info: Mar 27 13:45:17 dhcpd: DHCPREQUEST for 10.0.0.5 (10.0.0.1) from 0:40:ff:aa:b0:44 via ath0
    info: Mar 27 13:45:17 dhcpd: DHCPACK on 10.0.0.5 to 0:40:ff:aa:b0:44 via ath0


right after i corrected the permission issue:

    
    debug: Mar 27 13:52:32 dhcpd: Commit: IP: 10.0.0.5 Mac: 0:40:ff:aa:b0:44
    debug: Mar 27 13:52:32 dhcpd: execute_statement argv[0] = /tmp/dhcpcommit.sh
    debug: Mar 27 13:52:32 dhcpd: execute_statement argv[1] = commit
    debug: Mar 27 13:52:32 dhcpd: execute_statement argv[2] = 10.0.0.5
    debug: Mar 27 13:52:32 dhcpd: execute_statement argv[3] = 0:40:ff:aa:b0:44
    info: Mar 27 13:52:32 dhcpd: DHCPREQUEST for 10.0.0.5 from 0:40:ff:aa:b0:44 via ath0
    info: Mar 27 13:52:32 dhcpd: DHCPACK on 10.0.0.5 to 00:40:ff:aa:b0:44 via ath0




# links


* [1] <http://www.isc.org/products/DHCP>