Hi @Ivan Pletnev ,
I faced the same problem. LwIP uses rand() function in tcp_init(). As Piranha pointed, unless srand() is called passing as parameter a "best-effort" system random value prior to call tcp_init(), you will get always the same TCP port initial value.
To avoid this, I feel that LwIP should call srand() itself during LWIP_init() process, but probably was thought that this might have side effects on random usage in application code, so is up to the application developer where and how to call srand().
I created a routine init_random_TCPport() to solve this issue, calling tcp_init() again after lwip_init(); and my own initialization to srand() using RTC value. I am not sure whether tcp_init() can be called as many times as you need or it might be dangerous. Looking at LwIP source code, it is not in my opinion.
#include "tcp_priv.h" // tcp_init()
// In order to get a different TCP port at every startup:
// · Set random seed using srand() with a "best-effort" random value from the system (systicks, RTC values, AC A/D value, ???)
// · Reinitialize global variable tcp_port in LwIP calling tcp_init()
static void init_random_TCPport(void)
{
//srand(HAL_GetTick()); // Not valid if init_random_TCPport(); is called at a deterministic point in code at each startup (quite often returns the same value)
RTCTime t = RTCGetTime(); // Use a value from RTC time instead. RTC value kept using an external battery in backup RAM.
srand((t.Minutes << 8) | t.Seconds);
tcp_init(); // Re-seed tcp_port value in LwIP
}
int main(void)
{
// some init code
MX_LWIP_Init(); // lwip_init();
// some code prior to network connections
init_random_TCPport();
// some code using network connections
}
Although this is working to me, I feel this is an ugly solution. Accessing a tcp_priv.h function in application code smells bad to me.... I would like to know how other community users solved this.
I was expecting from LwIP a second configuration parameter apart of LWIP_RANDOMIZE_INITIAL_LOCAL_PORTS, something like i.e. LWIP_RANDOMIZE_SEED that was setting srand() to a LwIP generated random value inside lwip_init(), but I haven't found anything like that yet....
Thanks!
