Raw Sockets: Two Questions

[Julian Elischer]

On 22/3/18 3:08 am, Eugene Grosbein wrote:
> 22.03.2018 1:08, Ronald F. Guilmette wrote:
>
>> OK, so, if I have understood all that has been said in this thread so
>> far, then I would assert that, from the perspective of a simple-minded
>> and naive end user (e.g. me), the assertion that I originally quoted
>> -is- in fact correct, i.e. one -cannot- just simply do sendto/recvfrom
>> (and expect to get back responses) if the raw packets that one sends out
>> happen to be, for example, well formed TCP or UDP packets.
> Not exactly. You can't use raw sockets to receive but that does not mean
> you cannot use sendto/recvfrom (or similar calls) at all: there are
> libpcap, libdnet and NETGRAPH allowing to send requests and receive answers.
>
>> If I have correctly understood Matt Joras, there -are- ways to get hold
>> of such reply packets, under FreeBSD, but those require getting a bit more
>> "under the hood" in order to actually get hold of them... more than just
>> a simple recvfrom on the RAW socket.
> Why should you concentrate on RAW sockets?
>
> I have small perl script that sends manually crafted PPPoE frames
> and receives replies using simple libpcap interface:
>
> use Net::Pcap qw(:DEFAULT :functions);
>
> use constant V8021Q                     => 0x8100;
> use constant ETHERTYPE_PPPOEDISC        => 0x8863;
> use constant PPPOE_VER                  => 1;
> use constant PPPOE_TYPE                 => 1;
> use constant PADO_CODE                  => 7;
> use constant PADI_CODE                  => 9;
> use constant TAG_END_OF_LIST            => 0x0000;
> use constant TAG_SERVICE_NAME           => 0x0101;
> use constant TAG_AC_NAME                => 0x0102;
> use constant TAG_HOST_UNIQ              => 0x0103;
>
> $packet =
>      # Ethernet header: dst MAC, src MAC, TYPE
>      ether_aton('ff:ff:ff:ff:ff:ff') . $bmac . pack('n', ETHERTYPE_PPPOEDISC) .
>      # PPPoE PADI: VER, TYPE, CODE, SESSION_ID=0
>      pack('C', (PPPOE_VER<<4) + PPPOE_TYPE) . pack('C', PADI_CODE) . pack('n', 0)
>      # LENGTH, tags
>      pack('n', $tlen) . $tags;
> # zero padding upto 60 bytes ethernet frame length (without checksum)
> $packet .= pack('a' . (40-$tlen) , '') if $tlen < 40;
>
> err("cannot open interface $interface: $err")
>      unless $pcap = pcap_open_live($interface, $snaplen, 0, 0, \$err);
> err("could not send PADI")
>    if pcap_sendpacket($pcap, $packet) != 0;
>
> $filter = "ether proto " . ETHERTYPE_PPPOEDISC . " and ether dst $mac";
> err("cannot compile filter: $filter")
>    if pcap_compile($pcap, \$bpf, $filter, 1, 0) < 0;
> pcap_setfilter($pcap, $bpf);
>
> $ec = 0;
> while($ec == 0) {
>    $ec = pcap_loop($pcap, -1, \&callback, undef);
> }
> pcap_close($pcap);
> exit(0);
>
> sub callback($$$) {
>    return if $_[1]->{'len'} < 20;        # sanity check: short frame
>    my ($dst, $src, $ftype, $ftag, $fp) = unpack('a6a6na4a*' , $_[2]);
>    ...
> }
why not just use the ng-pppoe node? :-)
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