How does DHCPD determine what IP address to assign and...

Glenn Satchell Glenn.Satchell at
Thu Jan 3 12:24:59 UTC 2008

>Date: Wed, 02 Jan 2008 14:01:03 -0600
>From: "Ryan McCain" <Ryan.McCain at>
>To: <dhcp-users at>
>Subject: Re: RE: How does DHCPD determine what IP address to assign and...
>class "DialUp" {
>    match if (substring(suffix(dhcp-client-identifier,7),0,5)="Async")
>                   or
>(substring(suffix(dhcp-client-identifier,6),0,5)="Async") ; 
>            log (info, " Matched Dialup Rule");
>                        }

ok Ryan, let's break it down. I assume you have looked at the dhcp-eval
man page to get the descriptions for substring() and suffix().

Let's look at the first evaluation and start with the assumption that
option dhcp-client-identifier is the string

suffix(option dhcp-client-identifier, 7) is the last 7 chars from out
string, in this case TUVWXYZ.

substring("TUVWXYZ", 0, 5) is the string starting at position 0 and is
5 chars long, in this case TUVWX.

Now we are determining membership of the class if this final substring
is equal to "Async".

The second line uses a suffix of length 6. So it would evaluate the
suffix bit to be the last 6 chars, UVWXYZ. Doing a substring of that
gives UVWXY.

Now in your case to match the option dhcp-client-ientifier must be
something like


to log this, in the global section, ie outside the class and subnet put
something like

log(concat("dhcp-client-identifier is ", option

Looking at packet captures, global log entries, etc, does the string
look like that pattern? Often using a tool like wireshark will give
better results because there may be unprintable binary chars in the
string, especially sometimes a trailing null byte which you can't see
but gets counted in the suffix...


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