Date: Thu, 22 Mar 2018 02:08:15 +0700 From: Eugene Grosbein <eugen@grosbein.net> To: "Ronald F. Guilmette" <rfg@tristatelogic.com>, FreeBSD Net <freebsd-net@freebsd.org> Subject: Re: Raw Sockets: Two Questions Message-ID: <5AB2AD9F.6040600@grosbein.net> In-Reply-To: <3559.1521655704@segfault.tristatelogic.com> References: <3559.1521655704@segfault.tristatelogic.com>
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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]);
...
}
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