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

[Simon Hobson]

Ryan McCain wrote:
>I found a few posts in the archives but am a bit confused.  I don't 
>know what the 0 and 9 are for and used the example I found as a 
>template.  I am on the right track if I want all DHCP clients that 
>connect w/ the UID containing ASYNC be assigned IP addresses between 
>10.120.5.100-108 only??
>
>
>class "DialUp" {
>     match if substring(option vendor-class-identifier, 0, 9) = "ASYNC";
>         log (info, " Matched Dialup Rule");
>         range 10.120.5.100 10.120.5.108
>}

See (IIRC) 'man dhcp-eval' for info on expressions - in this case, 
the two numbers are start and length ...


Ryan McCain wrote:
>I might have figured it out.  Unfortunately there is no one here to 
>test due to the holidays:
>
>class "DialUp" {
>     match if substring(option vendor-class-identifier, 0, 9) = "ASYNC";
>         log (info, " Matched Dialup Rule");
>}
>
>subnet 10.116.6.0 netmask 255.255.255.0 {
>     pool {
>         allow members of "DialUp";
>         range 10.116.6.1 10.116.6.8;
>         option routers 10.116.6.1;
>         }
>}
>
><SNIP>
>
>#VPN
>subnet 10.116.6.0 netmask 255.255.255.0 {
>   range 10.116.6.9 10.116.6.179 ;
>   option routers 10.116.6.1;
>
>...
>
>..Will this reserve IP addresses 10.116.6.1 - 10.116.6.8 for DHCP 
>clients with a UID that contains ASYNC?

No, for two reasons.

Your expressions says to compare the first (starting at offset ZERO) 
NINE bytes of the UID with the string "ASYNC". This will only match 
if the UID is exactly "ASYNC". *See below for why that would be very 
bad !

The substring function doesn't search for a string, it returns a 
specific selection of the input. If your UIDs are of a fixed format 
then you would be OK, eg :

ASYNC1234, ASYNC5678, etc would all be matchable with substring 0,5
12ASYNC345, 67ASYNC890, etc would all be matchable with substring 3,5

But if you have a mix, eg ASYNC123, 45ASYNC67, 8ASYNC9012, then the 
string appears in different places - you would have to test for the 
different options :

match if <expression>
   or <expression>
   or <expression>
   or ...


* Just in case you weren't aware, the UID must be unique to each 
client - if present it is used as the primary key and if (for 
example) you decided to make all dialup users have the UID of "ASYNC" 
then to the DHCP server they would be the same client - and would get 
the same address. This does happen, we once had a query from someone 
who decided to identify all his Macs by setting their Client ID to 
"MAC" and wondered why they all got the same address !