What does ISIC option “-t” mean?

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As I already found out, ISIC tool has an option “-t” only for tcpsic or tcpsic6 and it means percentage of packets with wrong checksum.
In tcpsic case my router works fine.
But when Ive been testing my router with
isic -s rand -d -t 100 -p 1000000 -r 10
I’ve found a vulnerability on my router.
If I could know what -t does for isic I could figure out this problem on my router.

Let’s start by checking the man page. Sadly, it is ambiguous at best.

Percentage Options

These options are defined to all but esic. …


Available for tcpsic and tcpsic6 only, the percentage of TCP packets with bad TCP checksum.

So the -t option applies to isic per the “defined to all but esic” part. Yet it also does not apply, per the “for tcpsic and tcpsic6 only” part.

When faced with such ambiguity, check the source.

The source for tcpsic includes:

while((c = getopt(argc, argv, "hd:s:r:m:k:Dp:V:F:I:u:T:t:vx:")) != EOF){

case 't'

TCPCksm = atof(optarg);


And the source for isic includes:

while((c = getopt(argc, argv, "hd:I:s:r:m:k:Dp:V:F:vx:")) != EOF) {

and no case statement for -t.


  • The -t option is ignored for isic.
  • The part of the man page that says “These options are defined to all but esic” is incorrect.

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