dhcpv4, how to identify hosts by dhcp-client-identifier?

Thomas Markwalder tmark at isc.org
Mon Dec 5 11:48:46 UTC 2016

On 12/3/16 5:21 PM, Simon Hobson wrote:
> Robert Senger <robert.senger at lists.microscopium.de> wrote:
>> However, Windows and OS X and also *nix if the quotes are omitted, do
>> not send the dhcp-client-identifier as an ascii string, instead, those
>> operating systems send their dhcp-client-identifier in some binary
>> format.
> What you can do is simply specify a binary value to match against. So simply use (taking your example) 1:00:3a:63:b:1c:33 (note - no quotes) which means 7 bytes containing the values specified - ie 1, zero, 0x 3a, ...
> If you put quotes around 1:00:3a:63:b:1c:33 (ie use "1:00:3a:63:b:1c:33" in the config file) then that means 19 bytes containing 0x31 (1), 0x3a (:), 0x30 (0), 0x30 (0), 0x3a (:), 0x33 (3), and so on.
> If you look in the leases file, you'll see that the server can also work with a string with non-printable characters escaped - but that's not an easy format for humans to work with.
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As of 4.3.4, you can instruct both the server and client to output ID
values in hex:

       The lease-id-format parameter

         lease-id-format format;

         The format parameter must be either octal  or  hex.   This 
         governs  the format used to write certain values to lease
files. With
         the default format, octal, values are written as  quoted 
strings  in
         which  non-printable  characters are represented as octal
escapes - a
         backslash character followed by three octal  digits.   When 
the  hex
         format  is  specified,  values  are  written as an unquoted
series of
         pairs of hexadecimal digits, separated by colons.

         Currently, the values written out based on  lease-id-format 
are  the
         server-duid,  the  uid  (DHCPv4  leases),  and  the IAID_DUID
         leases).  Note the server automatically reads the  values  in 

This can make life easier when attempting to match values.  Note that
this only influences what one sees in the lease and log files and is
done for readability. The formats do not have to be the same between
server and client, although having them both output in hex is sort of
the point. 

Thomas Markwalder

ISC Software Engineering

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