Question

Referring to the Netfilter hook code at this page

The port to be checked against is declared as:

/* Port we want to drop packets on */
static const uint16_t port = 25;

The comparison is made as:

return (tcph->dest == port) ? NF_DROP : NF_ACCEPT;

In case variable port was of type int32, how can we convert it to uint16_t so that it can be checked against tcph->dest.

Thanks.

Answer 1

TCP ports are only 16 bit wide, so if your port-variable contains anything outside the range 0..65535, something is wrong anyway. Also, you should use ntohs to account for endianess differences.

So I suggest something like:

BUG_ON(port < 0 || port > 65535);
return (ntohs(tcph->dest) == (u16)port) ? NF_DROP : NF_ACCEPT;