Find source address from multicast message in esp8266? - ip

Question

I'm using non-OS SDK v2.0 for esp8266.
I join a multicast group.
I then receive a message (that was sent to multicast).
I want to respond to the sender, but in struct espconn, udp.remote_ip is the address of the multicast group, not sender.
How do I get the ip address of the sender?
Edit:
Receive function has an void* arg argument that is casted to struct espconn.

Answer 1

The old method doesn't do the job anymore, but I found a way to find remote ip and port.
New code:
struct espconn* udp_ch;
remot_info *premot = NULL;
udp_ch = arg;
if (espconn_get_connection_info(udp_ch,&premot,0) == ESPCONN_OK){
os_printf("%d.%d.%d.%d:%d\n", premot->remote_ip[0], premot->remote_ip[1], premot->remote_ip[2], premot->remote_ip[3], premot->remote_port);
}
else{
os_printf("Get info fail\n");
}
This is exactly what I was searching for before. For now, as far as I can see, it works well.
OLD:
I found a way to find the ip, but I don't think it should be done this way. Until I find better, I will use this.
The first thing I did was print first 256 hex values, from void* arg.
I noticed that my address was occurring before a bunch of zeros.
On unicast, the starting position of 0s was 128.
I'm currently using this function:
uint32_t udp_get_addr(void* arg){
uint32_t adr = 0;
uint16_t pos;
uint8_t* data = (uint8_t*) arg;
//unicast?
for(pos = 128; pos<144; pos++){
if(data[pos] != 0){
adr = 1;
break;
}
}
//multicast
if(adr == 1)
pos = 172;
else
pos = 124;
adr = data[pos]<<24 | data[pos+1]<<16 | data[pos+2]<<8 | data[pos+3];
return adr;
}
I know this method is bad, and there is a number of things that can be changed for the better, but for now, this will do.
Edit2:
I needed the source port also. It is located 4 bytes before address. New functions that I currently use:
#define SRC_ADDR_U 120
#define SRC_ADDR_M 168
uint32_t udp_src_addr(void* arg, uint8_t isMulticast){
uint32_t res;
uint8_t* tmp = (uint8_t*) arg;
uint16_t pos;
if(isMulticast) pos = SRC_ADDR_M+4;
else pos = SRC_ADDR_U+4;
res = (tmp[pos+3] << 24) | (tmp[pos+2] << 16) | (tmp[pos+1] << 8) | tmp[pos];
return res;
}
uint16_t udp_src_port(void* arg, uint8_t isMulticast){
uint32_t res;
uint8_t* tmp = (uint8_t*) arg;
uint16_t pos;
if(isMulticast) pos = SRC_ADDR_M;
else pos = SRC_ADDR_U;
res = (tmp[pos+1] << 8) | tmp[pos];
return res;
}