Bit-struct and constructing ARP messages

Hello all.

I have been trying to write a ARP packet injector script for Ruby
and did quite a bit of research on how to construct ARP Reply datagrams
and what not, and now that I think I have an idea how to construct them
but also I’d like to transmit them to a host.

Courtesy of bit-struct I am able to craft and fill in the headers of an
IPv4 datagram with ease, but transmitting this information is not so
easy. What I’d like to know is is there some specific syntax using
Socket#new for just transmitting packets not to any specific TCP port?,
this is a snippet from the script:


class IP < BitStruct
unsigned :ip_v, 4, “Version”
unsigned :ip_hl, 4, “Header length”
unsigned :ip_tos, 8, “TOS”
unsigned :ip_len, 16, “Length”
unsigned :ip_id, 16, “ID”
unsigned :ip_off, 16, “Frag offset”
unsigned :ip_ttl, 8, “TTL”
unsigned :ip_p, 8, “Protocol”
unsigned :ip_sum, 16, “Checksum”
octets :ip_src, 32, “Source addr”
octets :ip_dst, 32, “Dest addr”
rest :body, “Body of message”

      note "     rest is application defined message body"

      initial_value.ip_v    = 4
      initial_value.ip_hl   = 5

end

def mk_dgram(src_ip,dst_ip,src_h,dst_h)
arp_msg = [‘1’, ‘0x800’, ‘6’, ‘4’, ‘2’, src_h, src_ip, dst_h,
dst_ip]

    ip = IP.new
    ip.ip_tos = 0
    ip.ip_len = 0
    ip.ip_id  = 0
    ip.ip_off = 0
    ip.ip_ttl = 255
    ip.ip_p   = 255
    ip.ip_sum = 0
    ip.ip_src = src_ip
    ip.ip_dst = dst_ip
    ip.body   = arp_msg.to_s
    ip.ip_len = ip.length
    puts ip.inspect_detailed

    # TRANSMISSION CODE WOULD GO HERE
    # .....

end

Now I’m stuck with transmitting this datagram, any idea how I would
implement this?. Also I’m just taking a guess there as how to craft an
ARP Message encapsulated in an IP datagram so if you know the correct
way of doing that too your feedback is much appreciated.

I apologize in advance if this was not an appropriate place to post this
subject .

You can achieve the goal using Array#pack / String#unpack by
hand. But there are already good libraries supporting C function
calling and type conversion in Ruby: ruby-dl21, ruby-dlx2.

HTH,

Gyoung-Yoon N. wrote:

You can achieve the goal using Array#pack / String#unpack by
hand. But there are already good libraries supporting C function
calling and type conversion in Ruby: ruby-dl21, ruby-dlx2.

HTH,

Array#pack / Array#unpack is useful for packaging the datagram yes, what
I want to know is what Class#method would I use to transmit an ARP
message contained in a datagram since it’s not going to any TCP port,
it’s just a broadcast arp message.

Sorry for misreading you post.

I’d like to say that I haven’t done such a thing before.
However, I just found that arping1 uses the libnet2 library,
which also has Ruby binding3.
Sorry for telling without doing.

Regards,

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