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my Step_fin_bit = 0; # FIN FLAG BIT
# here we put together 2 reserved fields and the 6 flags to pack as binary.
my Stcp_codebi. ts '" Szero . hero . StcP_ur9_bi. t . Stcp_ocLbi t . Stcp_psh_bi t
Stcp_,·sLbi.t . Stcp_syn_bit . Step_fin_bit;
my Stcp_windowsize "" 124; I/. default window size
my Stcp_urgent_pointer z 0; # urgent poi.nter
# the followi.ng i.s not a tcp header per se. but a pseudo header
# used to calculate the tcp checksum. yes, its a pain in the ass.
my (Spseudo_tcp) = pack Co4 04 ( (
n n n
N N
H2 B8
n v n'.
Ssrc_host. Sdest_host, $zero, Sproto_ tcP.
Stcplength. Ssrc_port, Sdest_port.
Ssyn, Sack,
Shdr _"_reserved, Stcp_codebi. ts.
Stcp_windowsize, Szero. Stcp_urgent_pointer) j
my (Stcp_chksum) = &checkfro(Spseudo_tcp);
# PLAY WITH THE INNARDS OF THE IP PIECE HERE! !!
my Sip_version ::: "4" i # (nybble) tcp/ip version number (current is 4)
my Sip_hedlen = "5"; # (nybble) number of 32-bit words i.n ip header
my $ver _"_hlen ::: Sip_versi.on . Sip_hedlen; # we pack 2 nybbles together
my $i.p_tos = "00"; # (byte) ip type-of-servi.ce
my (Stotlength) = $teplength + 20; #tep + 20 byte ip hdr ##
## we'll pack totlength i.nto 2 bytes i.n the packet
my Si.p_fragment_id ::: 31337; # 2 bytes as well.
my Sip_3blt_flags = "010"; # ip fragmentation flags (3 bi.ts) (frag, do not frag)
my $ip_13bi. t_fragoffset = "0000000000000"; #fragment offset
my Sip_flags_n_frags ::: Sip_3bit_flags . Si.p_13biCfragoffsetj
my $ip_ tt 1 = 64 j # 64 seconds / hops
# we have proto_tcp from above.. my $proto_tcp = 6;
# we have hdr _checksum from above ...
# all source and destination infoz i.s passed to us (i. t
# gets set in parent routine)
# change Ssyn and Sack above in tcp section
# in fact, everything else i.n the packet is set above.
my (Shdr) = paek ('HZ H2 n n
B16 C2
n a4 a4
n n
N N
H2 B8
n v n' ,
$over _"_hlen, Sip_tos, $totlength, Sip_fragment_i.d,
Si.p_ flags_n_frags, $i.p_ ttl, $oproto_ tcp,
Shdr _cksum, $srchost, Sdest_host,
# end of i.p header, begi.n tcp header
$src_port, Sdest_port,
Ssyn, Sack,
Shdr _n_reserved,Step_eodebi ts,
Step_wi.ndows ize, $tcp_chksum, Stcp_urgent_pointer) i
return Shdr;
sub checkfro {
#dis sekzhun robbed from someplace else ... .
my (
Smsg # The message to checkfro
) = @-;
my CSlen_msg I
# length of the message
Snum_short, # The number of short words in the message
Sshort, # One short word
$chk # The checkfro
) ;
Slen_msg = length($msg);
Snum_short ::: S 1 en_ms9 I 2;
Sehk = 0;
foreach $short (unpack("SSnum_short", Smsg))
{
}
Schk += $short;
# Add some 1 ead
Schk += unpackC"C", substrC$ms9, Slen_msg 1, 1)) if Slen_msg % 2;
Schk = (Sehk » 16) + (Sehk & 0xffff); # bust out mah fro pie
return(-(($ehk » 16) + Sehk) & 0xffff); # spray some jheri
Fall 1998 2600 Magazine Page 9