[cam.pm] ICMP script without module Net::Ping

[zach]

hi..

I am trying to send a single icmp packet with this
code : http://www.perlmonks.org/?node_id=488226 .

Since i have no idea why this script wont work in my
LAN, i asked ny friend (in chat) to test it, and he
said the script's working fine.

So I re-checked all lines in this code, and seems its
fine, so i remade a script (shorter than before),
here's the code:

use Socket;
socket(SOCK,PF_INET,SOCK_RAW,getprotobyname('icmp'))
|| die "Error: $! \n"; 
$msg = pack("C2 n3",8,0,32127,0,1); 
print "the value of checksum is:", unpack('H4',
pack('n',32127)), "\n"; 
print "[And you should see this in your tcpdump -i lo
icmp -xxx]\n";
send(SOCK,$msg,0,sockaddr_in(0,inet_aton('localhost')))
|| die "Error: $! \n";

As you can see, there's no subroutine to calculate
checksum, yea, because im going to try it to LOCALHOST
and as far as i know, no 'valid checksum' needed for
localhost (so i add 32127 for checksum).

I ran this script, i got the response (echo reply) and
i can see '32127' or '0x7d7f' (in hex) in tcpdump,
here's tcpdump log :

01:55:23.930771 zacko > zacko: icmp: echo request (DF)
4500 001c 0000 4000 4001 3cdf 7f00 0001
7f00 0001 0800 [7d7f] 0000 0001

everything's work normal.
let's try to change the checksum value, change all
32127 (in code above) to 32128, well i still got the
response (echo reply), but when i look closer into
tcpdump log, i feel something wrong there, there's no
32128 or '0x7d80' (in hex) , here's tcpdump log :

02:09:11.912392 zacko > zacko: icmp: echo request (DF)
4500 001d 0000 4000 4001 3cde 7f00 0001
7f00 0001 0800 [7dc2] 8000 0000 01

There's no 0x7d80 but 7dc2 (and i think 7dc2 is
32194).
If ICMP_ECHO=8, SUBCODE=0, $pid=0, and $seq=1, and
then we pass these variables to subroutine checksum,
we'll get 63486, and of course if i send icmp packet
to another host, with checksum value=63486, i'll get
no response back (as long as checksum value > 32127).

Well, i have no idea, what's going on here, by the
way, i am using Redhat Kernel 2.4.20-8, libcap version
0.7.2

I wish, i can explain this problem in better english,
i'll ask my friend to translate this question :[




thanks, zac





		
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