Сделал

В общем для абонента с IP a.b.c.d и указывает шлюз по умолчанию a.b.c.1
Код:
/*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/
package bitel.billing.server.ext.dhcp;
import static bitel.billing.server.ext.dhcp.DHCPPacket.*;
import java.nio.channels.DatagramChannel;
/**
*
* @author lda
*/
public class DHCPPortDefaultGatewayRelayProcessor extends DHCPRelayProcessor{
public DHCPPortDefaultGatewayRelayProcessor( DHCPSetup setup, DatagramChannel channel )
{
super( setup, channel );
log.info("Start DHCPPortDefaultGatewayRelayProcessor");
}
@Override
protected void setOptions( DHCPGate gate, DHCPPacket response, DHCPIpAddress ip ){
super.setOptions(gate, response, ip);
byte[] route = new byte[4];
route[0] = response.yiaddr[0];
route[1] = response.yiaddr[1];
route[2] = response.yiaddr[2];
route[3] = 1;
response.setOption( OPTION_ROUTER, route );
}
}