I have added support in dhcrelay (daemon) to be able to fill the fields circuit_id and remote_id (option82) with custom messages. was based on latest dhcp-4.3.1 from ftp://ftp.isc.org/isc/dhcp/4.3.1/dhcp-4.3.1.tar.gz

1. build from the original source

    # mkdir -p /opt/isc-dhcp
    # cd /opt/isc-dhcp
    # wget https://raw.githubusercontent.com/jpereira/isc-dhp-relay/master/isc-dhcp-4.3.1_dhcrelay-custom-circuit_id-remote_id.patch 
    # wget ftp://ftp.isc.org/isc/dhcp/4.3.1/dhcp-4.3.1.tar.gz
    # tar xzf dhcp-4.3.1.tar.gz
    # cd dhcp-4.3.1
    # cat ../isc-dhcp-4.3.1_dhcrelay-custom-circuit_id-remote_id.patch | patch -p0
    # ./configure --prefix=/usr
    # make
    # make install
   

2. build with version already applied

    # git clone https://github.com/jpereira/isc-dhp-relay isc-dhcp-relay.git
    # cd isc-dhcp-relay.git/dhcp-4.3.1
    # ./configure --prefix=/usr
    # make
    # make install
   

3. example of relaying dhcp packets to 192.168.1.34, att: remember that the interface need to have a valid ip

    # dhcrelay -h
    Usage: dhcrelay [-4] [-d] [-q] [-a <circuit_id> <remote_id>] [-D]
                         [-A <length>] [-c <hops>] [-p <port>]
                         [-pf <pid-file>] [--no-pid]
                         [-m append|replace|forward|discard]
                         [-i interface0 [ ... -i interfaceN]
                         server0 [ ... serverN]
   
           dhcrelay -6   [-d] [-q] [-I] [-c <hops>] [-p <port>]
                         [-pf <pid-file>] [--no-pid]
                         [-s <subscriber-id>]
                         -l lower0 [ ... -l lowerN]
                         -u upper0 [ ... -u upperN]
           lower (client link): [address%]interface[#index]
           upper (server link): [address%]interface
   
    circuit_id/remote_id interpreted sequences are:
   
      %%  a single %
      %i  Its name of interfaceN that received the request
      %h  interfaceN hardware address
      %H  Client hardware address
      %I  DHCP relay agent IP Address
   
    # dhcrelay -i eth0 -a "example-circuitid;%i;%h;%I" "example-remoteid;%i;%h;%I" 192.168.1.34