Getting IPV4 address from a sockaddr structure

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.

score 70 · Answer 1 · answered Aug 14 '09 at 06:17

70

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.

answered Aug 14 '09 at 06:17

Emil H

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+1. Depending on platform, remember to check the family first. There may not be an IPV4 address to extract... – Steve Jessop Aug 14 '09 at 10:40
@SteveJessop How do we do the check btw? Is there any example code or something to look for? – Fatih Arslan Jul 13 '12 at 13:42
7

`if (their_sockaddr_ptr->sa_family == AF_INET) { struct sockaddr_in *their_inaddr_ptr = (struct sockaddr_in *)their_sockaddr_ptr; } else { /* not an IPv4 address */ }`, or similar. – Steve Jessop Jul 13 '12 at 14:32
@Emil How do you cast the entire sockaddr structure to sockaddr_in? Able to give an example? :D – mister Jul 27 '12 at 08:11
@mister If you have a question, start question, that's what SO is here for. Questions are first class citizen on SO. And the answer is, just cast it! `sockaddr` is only an "abstract" address, no address really ever is of that type, it just defines the common fields all socketaddr structs must have. `struct sockaddr * saddr = ...; if (saddr->sa_family == AF_INET) { struct sockaddr_in * saddr_in = (sockaddr_in *)saddr; ... }` – Mecki Aug 10 '16 at 10:35

score 43 · Answer 2 · edited May 15 '21 at 14:27

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);

Not sure why you're using malloc and free here, a static buffer would be the more appropriate solution. — FreelanceConsultant, May 15 '21 at 14:22

score 37 · Answer 3 · edited Nov 15 '16 at 02:37

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.

score 27 · Accepted Answer · edited Jun 09 '15 at 18:59

27

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];
};

edited Jun 09 '15 at 18:59

gomons

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answered Aug 14 '09 at 06:19

Amber

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This is a bit less than what you need to format as a string. – Preston L. Bannister Aug 01 '19 at 17:34
How can we ensure that the casting is safe? – eonil May 01 '20 at 20:58
@eonil the first member of all `sockaddr` is a `u_short` type tag. – Pedro Lamarão Aug 06 '20 at 15:30

score 5 · Answer 5 · answered Oct 02 '15 at 01:00

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;
}

score 1 · Answer 6 · answered Jul 19 '21 at 19:21

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.

score 0 · Answer 7 · answered May 07 '20 at 10:56

0

Type casting of sockaddr to sockaddr_in and retrieval of ipv4 using inet_ntoa

char * ip = inet_ntoa(((struct sockaddr_in *)sockaddr)->sin_addr);

answered May 07 '20 at 10:56

Aamir

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Getting IPV4 address from a sockaddr structure

7 Answers7

Using inet_ntoa()

Using inet_ntop()

Linked

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