Open DHCP Window
1 Right click the host name and select "Set Predefined Options..."
2 Click "Add" button at "Predefined Option and Values" window. And then type as below. You can give any name, but data type and Code should be "String" and "252"
3 Add "AUTO-PROXY-CONFIG" to each scope options and the value should point your proxy.pac file (make link to proxy.dat or copy same file to proxy.pac)
4 Make a "proxy.pac" (text file) file and copy it into root directory of the local web server. Example "proxy.pac" file

function FindProxyForURL(url,host)
{
  if (isPlainHostName(host) || isInNet(host, "10.8.0.0", "255.255.255.0"))
    return "DIRECT";
  else
    return "PROXY 10.8.0.21:3128; DIRECT";
}

Syntax of the script file can be found in http://wp.netscape.com/eng/mozilla/2.0/relnotes/demo/proxy-live.html

Web Proxy Auto Discovery (WPAD)
Support: IE5.0 and above, Netscape
Does not support: Other than above browser
1 Prepare the wpad script file
Make a "wpad.dat" (text file) file and copy it into the root directory of the local web server. Example "wpad.dat" file

function FindProxyForURL(url,host)
{
  if (isPlainHostName(host) || isInNet(host, "10.8.0.0", "255.255.255.0"))
    return "DIRECT";
  else
    return "PROXY 10.8.0.21:3128; DIRECT";
}