Getting IPV4 address from a sockaddr structure - c

Question

How can I extract an IP address into a string? I can't find a reference that tells me how char sa_data[14] is encoded.

Answer 1

Just cast the entire sockaddr structure to a sockaddr_in. Then you can use:
char *ip = inet_ntoa(their_addr.sin_addr)
To retrieve the standard ip representation.

Answer 2

inet_ntoa() works for IPv4; inet_ntop() works for both IPv4 and IPv6.
Given an input struct sockaddr *res, here are two snippets of code (tested on macOS):
Using inet_ntoa()
#include <arpa/inet.h>
struct sockaddr_in *addr_in = (struct sockaddr_in *)res;
char *s = inet_ntoa(addr_in->sin_addr);
printf("IP address: %s\n", s);
Using inet_ntop()
#include <arpa/inet.h>
#include <stdlib.h>
// obviously INET6_ADDRSTRLEN is expected to be larger
// than INET_ADDRSTRLEN, but this may be required in case
// if for some unexpected reason IPv6 is not supported, and
// INET6_ADDRSTRLEN is defined as 0
// but this is not very likely and I am aware of no cases of
// this in practice (editor)
char s[INET6_ADDRSTRLEN > INET_ADDRSTRLEN ? INET6_ADDRSTRLEN : INET_ADDRSTRLEN]
= '\0';
switch(res->sa_family) {
case AF_INET: {
struct sockaddr_in *addr_in = (struct sockaddr_in *)res;
////char s[INET_ADDRSTRLEN] = '\0';
// this is large enough to include terminating null
inet_ntop(AF_INET, &(addr_in->sin_addr), s, INET_ADDRSTRLEN);
break;
}
case AF_INET6: {
struct sockaddr_in6 *addr_in6 = (struct sockaddr_in6 *)res;
////char s[INET6_ADDRSTRLEN] = '\0';
// not sure if large enough to include terminating null?
inet_ntop(AF_INET6, &(addr_in6->sin6_addr), s, INET6_ADDRSTRLEN);
break;
}
default:
break;
}
printf("IP address: %s\n", s);

Answer 3

Emil's answer is correct, but it's my understanding that inet_ntoa is deprecated and that instead you should use inet_ntop. If you are using IPv4, cast your struct sockaddr to sockaddr_in. Your code will look something like this:
struct addrinfo *res; // populated elsewhere in your code
struct sockaddr_in *ipv4 = (struct sockaddr_in *)res->ai_addr;
char ipAddress[INET_ADDRSTRLEN];
inet_ntop(AF_INET, &(ipv4->sin_addr), ipAddress, INET_ADDRSTRLEN);
printf("The IP address is: %s\n", ipAddress);
Take a look at this great resource for more explanation, including how to do this for IPv6 addresses.

Answer 4

Once sockaddr cast to sockaddr_in, it becomes this:
struct sockaddr_in {
u_short sin_family;
u_short sin_port;
struct in_addr sin_addr;
char sin_zero[8];
};

Answer 5

You can use getnameinfo for Windows and for Linux.
Assuming you have a good (i.e. it's members have appropriate values) sockaddr* called pSockaddr:
char clienthost[NI_MAXHOST]; //The clienthost will hold the IP address.
char clientservice[NI_MAXSERV];
int theErrorCode = getnameinfo(pSockaddr, sizeof(*pSockaddr), clienthost, sizeof(clienthost), clientservice, sizeof(clientservice), NI_NUMERICHOST|NI_NUMERICSERV);
if( theErrorCode != 0 )
{
//There was an error.
cout << gai_strerror(e1) << endl;
}else{
//Print the info.
cout << "The ip address is = " << clienthost << endl;
cout << "The clientservice = " << clientservice << endl;
}

Answer 6

The following program resolves a given domain:
$ gcc a.c
$ ./a.out google.com
AF_INET: 216.58.214.238
AF_INET6: 2a00:1450:400d:803::200e
$ ./a.out google.com af_inet
AF_INET: 216.58.214.238
a.c:
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <arpa/inet.h>
int
main(int argc, char *argv[])
{
struct addrinfo hints, *res, *cres;
int r;
char s[INET6_ADDRSTRLEN];
memset(&hints, 0, sizeof(hints));
if (argc > 2)
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_DGRAM;
r = getaddrinfo(argv[1], NULL, &hints, &res);
if (r) {
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(r));
exit(EXIT_FAILURE);
}
for (cres = res; cres; cres = cres->ai_next) {
switch (cres->ai_family) {
case AF_INET:
inet_ntop(AF_INET, &((struct sockaddr_in *)cres->ai_addr)->sin_addr, s, INET6_ADDRSTRLEN);
printf("AF_INET: %s\n", s);
break;
case AF_INET6:
inet_ntop(AF_INET6, &((struct sockaddr_in6 *)cres->ai_addr)->sin6_addr, s, INET6_ADDRSTRLEN);
printf("AF_INET6: %s\n", s);
break;
}
}
freeaddrinfo(res);
}
Another example can be found here.

Answer 7

Type casting of sockaddr to sockaddr_in and retrieval of ipv4 using inet_ntoa
char * ip = inet_ntoa(((struct sockaddr_in *)sockaddr)->sin_addr);