Wireshark · Wireshark-dev: Re: [Wireshark-dev] Unable to add my dissector for tcp.dstport

[Guy Harris <guy@xxxxxxxxxxxx>]

On Jan 11, 2008, at 12:02 AM, Vikas Jain wrote:

> I am developing a dissector for my protocol that uses TCP as the  
> underlying layer and dissects messages that are destined for TCP  
> destination port of 50505.

What about messages sent *from* TCP source port 50505?

>     /* The following statement seems to cause an assertion when I  
> run the Wireshark executable. I get an assertion on line 697  
> packet.c file saying assertion failed on sub-dissectors */
>     dissector_add("tcp.dstport", 50505, foo_handle);

The assertion that fails is the assertion that there *exists* a  
dissector table named "tcp.dstport".  No such dissector table exists;  
there is no mechanism to request that your dissector be called for a  
particular destination port.

(It is not, and never will be, the case that there will exist a  
dissector table for every single named field in a protocol.)

> I presume that if keep tcp.port in my dissector_add invocation, my  
> dissector is going to be invoked even when tcp.srcport is 50505.

Yes.

> I want my dissector to be invoked only when a tcp destination port  
> is 50505.

Again, why only the destination port?