Dora is an embeddable DHCP client and test tool. It can be used to obtain and manage DHCP
leases, probe subnets for DHCP servers, and query DHCP servers for supported options.

The command format is:

    dora [options] [command [server]] interface

For most commands, dora prints lease information to stdout on a single line, in form:

    address subnet broadcast router dnsserver domainname dhcpserver leaseseconds

For example:

    $ dora acquire eth0
    192.168.1.117 255.255.254.0 192.168.1.255 192.168.0.1 8.8.8.8 localdomain 192.168.1.254 86400

The invoking program acts as the DHCP client, and is responsible for parsing the response in
order to bring up the interface, set the default route, configure /etc/resolv.conf, and arrange
to run dora again to renew the lease at time T1 (50% of the lease time), rebind at time T2
(87.5% of the lease time), or acquire a new lease when the existing lease expires.  See the
dhcpcd.sh script for a trivial example.

Commands are:

    acquire - obtain a DHCP lease in the usual fashion, via broadcast. The interface must not be
    configured. This is the default if no command is specified at all.

    renew xx.xx.xx.xx - unicast a DHCP renew for the current configured address to the specified
    server, this is normally done when the lease is 50% expired (aka time T1).  If no response
    is received, the client tries to rebind at time T2.

    rebind - broadcast a DHCP renew for the current configured address, this is normally done
    when the lease is 87.5% expired (aka time T2). If no response is received, the client waits
    for the lease to expire then attempts to acquire a new lease.

    release xx.xx.xx.xx - unicast a DHCP release to the specified server address. The server
    won't respond, so this command exits immediately.

    inform - broadcast a request for local network information without creating a lease.  The
    interface is presumably configured with a static IP address.  Typically the resulting lease
    time will be 0, since no lease is issued.  However if the server does actually have a lease
    for the requesting IP it may return the remaining lease seconds, or it may not respond at
    all.

    probe - broadcast a DHCP discover and print all offers received before the timeout period.
    No lease is actually obtained, this is a way to find out if there are DHCP servers on the
    subnet.  If no offers are received, report an error.

For all commands, if the operation completes successfully then dora terminates with exit status
0.  If not, it prints a message to stderr and exits with non-zero status.

Options are:

    -c xx.xx.xx.xx - request client address

        For acquire or inform, this specifies an address to ask for, which the server is under
        no obligation to provide. For other operations, this will be used as the source address
        instead of the primary interface address, but note if the address is not actually
        assigned to the interface then the server's response may not be received.

    -f - force acquire even if interface already has an address

        Normally dora will refuse to request an IP address if the interface already has one, use
        -f to make it try anyway. This may invalidate the existing lease, if the result has a
        different IP address then the interface should be reconfigured.

    -h hostname - request specific hostname

        Some DHCP servers will register the specified hostname with the local DNS server, so
        that it will resolve to the assigned IP. Some won't, YMMV.

    -k - use UDP instead of raw IP

        This may be helpful in weird networking cases i.e. packets are being fragmented or the
        DHCP server is behind a gateway. However it will probably fail if there's a listener
        bound to the BOOTPC port or if the network interface is currently unconfigured.

    -m - output the address in CIDR notation, i.e. with appended netmask width

        The resulting address is therefore compatible with 'ip address add' for example.

    -o number - request DHCP option 1 to 254, can used multiple times, implies -x

        Dora always requests the subnet, router, dns server, domain name, broadcast address, and
        lease seconds from the server. This allows you to request other options as well, which
        the server may or may not respond to.

    -O - request all 254 possible DHCP options, implies -x

        This is simply shorthand for "-o1 -o2 -o3..." etc, an easy way to probe a server for all
        options it supports.

    -t number - transaction timeout seconds, default is 4

    -u number - max transaction attempts, default is 4

        These define how often and how many times to repeat a request if an ack is not received.
        Each attempt extends the timeout by about one second, plus/minus up to 1000 random
        milliseconds, so "-t 4 -u 4" will timeout after about 22 seconds, "-t 4 -u 5" will
        timeout after about 30 seconds, etc.

    -v - dump various status messages and other information to stderr

        Including packet dump, explanation as to why packets are rejected, etc.  This
        information always goes to stderr so as not to interfere with scripted output
        processing.

    -x - extended result report to stdout (instead of the single line)

        The extended report prints one DHCP option per line, in the order provided by the host,
        in the form:

            option description: value

        Where:

            option is the decimal option number, as defined at
            https://tools.ietf.org/html/rfc2132.

            description is arbitrary text from the RFC, guaranteed to not contain spaces.

            value is a stringified version of whatever was provided by the server for the
            option, such as an IP address, a decimal value, or an arbitrary string. Note that
            the value may contain whitespace, or may even be empty, but will not control
            characters or non-ASCII weirdness.

        Option code 0 is used to present the assigned IP address. This is an invalid RFC2132
        option, but allows all lines to have the same format.

        For example:

            $ ./dora -O inform eth0
            0 Address: 192.168.1.117
            53 DHCP_Response_Type: 5
            54 Server_Identifier: 192.168.1.254
            1 Subnet_Mask: 255.255.254.0
            28 Broadcast_Address: 192.168.1.255
            6 Domain_Name_Server: 8.8.8.8
            252 Unknown: 10
            44 NetBIOS_Over_TCP/IP_Name_Server: 192.168.1.253
            15 Domain_Name: localdomain
            3 Router: 192.168.0.1

        Note option 252 was reported by the server but is not defined by RF2132.  The server
        didn't specify lease seconds, therefore it doesn't appear in the extended output, but in
        the brief output it would appear as 0.

Dora performs privileged network operations, which usually means it has to be run as root.

To build the dora executable, install the usual developer tools and then just run 'make'.

For embedded applications 'make terse' removes help text, the -v option and the DHCP option text
(leaving only the ':' in extended output), and optimizes for size. This shrinks the executable
by about 50%.