NAME
dhcping - send a DHCP request to DHCP server to see if it’s up and
running
SYNOPSIS
dhcping [-v] [-q] [-i] [-r] -t maxwait -c client-IP-address -s server-
IP-address -h client-hardware-address [-g gateway-IP-address]
DESCRIPTION
This command allows the system administrator to check if a remote DHCP
server is still functioning.
Options are:
-v Verbose, print some information.
-V Very verbose, print a lot of information.
-i Use DHCPINFORM packets.
-r Use DHCPREQUEST packets (default behaviour).
-q Quiet, print nothing on the screen.
-t maxwait
Maximum time to wait for an answer from the server in seconds.
Default is 3 seconds.
-c client-IP-address
Request this IP address. Note that this is also the IP address the
answer will be sent to.
-s server-IP-address
Send the DHCP packet to this IP address.
-h client-hardware-address
Use this hardware-address in the DHCP request. It can be up to
sixteen octets separated by colons (i.e. 01:02:03:04)
-g gateway-IP-address
Use this IP address for the gateway IP address in the DHCP packet.
This option is currently broken.
RETURN VALUES
If everything goes okay, it returns 0. If there went something wrong,
it returns 1.
SETUP
This program should be installed setuid root or ran by root only. See
SECURITY for more information.
On your DHCP server, add these lines to the dhcpd.conf:
host <your monitoring host FQDN> {
hardware ethernet <your monitor host mac address>;
fixed-address <your monitoring host IP address>;
}
Then try it:
$ dhcping -c your monitoring host IP address \
-s your DHCP server IP address \
-h your monitor host mac address
It will either respond with "no answer" or "Got answer from: your DHCP
server IP address"
The DHCP server logfile will give:
DHCPREQUEST for 192.168.1.1 from 00:20:18:56:29:8f via ed0
DHCPACK on 192.168.1.1 to 00:20:18:56:29:8f via ed0
DHCPRELEASE of 192.168.1.1 from 00:20:18:56:29:8f via ed0 (found)
Running in DHCPINFORM mode with -i:
If you see "DHCPINFORM from 192.168.1.1 via xl0: not authoritative for
subnet 192.168.1.0", you should add the authoritative statement to the
subnet, See dhcpd.conf(5) for details.
When running in very verbose mode, dhcping tries to dump all data of
the send and received DHCP packets. It will first dump the packet in
hex-format, then decodes the header and finally the options.
HOW IT WORKS
The client either sends a DHCPREQUEST or DHCPINFORM packet to the
server and waits for an answer. Then, if a DHCPREQUEST was send, it
will send a DHCPRELEASE back to the server.
SECURITY
This program is installed setuid root as it requires the privileges to
bind itself to port 68 (bootpc). Root privileges are dropped as soon as
the program has bound itself to that port.
BUGS
Currently (this may, or may not, change in the future) the ISC DHCP
daemon does not write leases with a fixed IP address in the
dhcpd.leases file.
DHCPINFORM packets can only be used on subnets the server is
authoritative for. If the monitoring script runs on a subnet the
server isn’t authoritative for, it should use the DHCPREQUEST packets.
I also experienced some problems with ISC DHCPD v2 servers, but that is
also in the README of it.
The -V option is still working, but shouldn’t be used for debugging of
the packets. Better use dhcpdump(8) for that, which is available on my
website. I wanted to remove it, but decided only to do it from the
documentation, not from the code. Maybe I’ll need it one day for
debugging.
AUTHOR
Edwin Groothuis, edwin@mavetju.org (http://www.mavetju.org)
SEE ALSO
dhcpd(8), dhclient(8), dhcpd.conf(5), dhcpdump(8)