Skip to content

Instantly share code, notes, and snippets.

@svagionitis

svagionitis/Makefile

Last active November 21, 2023 07:45
Show Gist options
  • Star 3 You must be signed in to star a gist
  • Fork 2 You must be signed in to fork a gist
  • Save svagionitis/18a714f7e71b5b2da5b15f06e372822f to your computer and use it in GitHub Desktop.
Save svagionitis/18a714f7e71b5b2da5b15f06e372822f to your computer and use it in GitHub Desktop.
Download ZIP
This small utility shows how to use the `getifaddrs` and get information for network interfaces.
Raw
get-gateway-ip.c
/* Based on https://www.linuxquestions.org/questions/linux-networking-3/howto-find-gateway-address-through-code-397078/ */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <net/if.h>
#include <linux/rtnetlink.h>
#include <unistd.h>
#include <arpa/inet.h>
#define BUFSIZE 8192
struct route_info {
struct in_addr dst_addr;
struct in_addr src_addr;
struct in_addr gw_addr;
char if_name[IF_NAMESIZE];
};
ssize_t
read_netlink_socket (int sock_fd, char *buffer, size_t buffer_sz,
uint32_t seq, uint32_t pid)
{
struct nlmsghdr *nl_hdr;
ssize_t read_len = 0, msg_len = 0;
do {
if ((size_t) msg_len > buffer_sz) {
perror ("No size in buffer");
return -1;
}
/* Recieve response from the kernel */
read_len = recv (sock_fd, buffer, buffer_sz - msg_len, 0);
if (read_len < 0) {
perror ("SOCK READ: ");
return -1;
}
nl_hdr = (struct nlmsghdr *)buffer;
/* Check if the header is valid */
if (!NLMSG_OK (nl_hdr, read_len)
|| (nl_hdr->nlmsg_type == NLMSG_ERROR)) {
perror ("Error in recieved packet");
return -1;
}
/* Check if the its the last message */
if (nl_hdr->nlmsg_type == NLMSG_DONE)
break;
else {
/* Else move the pointer to buffer appropriately */
buffer += read_len;
msg_len += read_len;
}
/* Check if its a multi part message */
if (!(nl_hdr->nlmsg_flags & NLM_F_MULTI))
/* return if its not */
break;
} while ((nl_hdr->nlmsg_seq != seq)
|| (nl_hdr->nlmsg_pid != pid));
return msg_len;
}
/* parse the route info returned */
int
parse_routes (struct nlmsghdr *nl_hdr, struct route_info *rt_info)
{
struct rtmsg *rt_msg;
struct rtattr *rt_attr;
int rt_len;
rt_msg = (struct rtmsg *)NLMSG_DATA (nl_hdr);
/* If the route is not for AF_INET or
* does not belong to main routing table then return. */
if ((rt_msg->rtm_family != AF_INET)
|| (rt_msg->rtm_table != RT_TABLE_MAIN))
return -1;
/* get the rtattr field */
rt_attr = (struct rtattr *)RTM_RTA (rt_msg);
rt_len = RTM_PAYLOAD (nl_hdr);
for (; RTA_OK (rt_attr,rt_len); rt_attr = RTA_NEXT (rt_attr,rt_len)) {
switch (rt_attr->rta_type) {
case RTA_OIF:
if_indextoname (*(int *)RTA_DATA (rt_attr), rt_info->if_name);
break;
case RTA_GATEWAY:
memcpy (&rt_info->gw_addr, RTA_DATA(rt_attr),
sizeof (rt_info->gw_addr));
break;
case RTA_PREFSRC:
memcpy (&rt_info->src_addr, RTA_DATA(rt_attr),
sizeof (rt_info->src_addr));
break;
case RTA_DST:
memcpy (&rt_info->dst_addr, RTA_DATA(rt_attr),
sizeof (rt_info->dst_addr));
break;
}
}
return 0;
}
// meat
int
get_gatewayip (char *gatewayip, socklen_t size)
{
int found_gatewayip = 0;
struct nlmsghdr *nl_msg;
struct route_info route_info;
char msg_buffer[BUFSIZE]; // pretty large buffer
int sock, len, msg_seq = 0;
/* Create Socket */
sock = socket (PF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE);
if (sock < 0) {
perror("Socket Creation: ");
return -1;
}
/* Initialize the buffer */
memset (msg_buffer, 0, sizeof (msg_buffer));
/* point the header and the msg structure pointers into the buffer */
nl_msg = (struct nlmsghdr *)msg_buffer;
/* Fill in the nlmsg header*/
nl_msg->nlmsg_len = NLMSG_LENGTH (sizeof (struct rtmsg)); // Length of message.
nl_msg->nlmsg_type = RTM_GETROUTE; // Get the routes from kernel routing table .
nl_msg->nlmsg_flags = NLM_F_DUMP | NLM_F_REQUEST; // The message is a request for dump.
nl_msg->nlmsg_seq = msg_seq++; // Sequence of the message packet.
nl_msg->nlmsg_pid = getpid(); // PID of process sending the request.
/* Send the request */
if (send (sock, nl_msg, nl_msg->nlmsg_len, 0) < 0) {
fprintf(stderr, "Write To Socket Failed...\n");
return -1;
}
/* Read the response */
len = read_netlink_socket (sock, msg_buffer, sizeof (msg_buffer),
msg_seq, getpid ());
if (len < 0) {
fprintf(stderr, "Read From Socket Failed...\n");
return -1;
}
close (sock);
/* Parse and print the response */
for (; NLMSG_OK (nl_msg, len); nl_msg = NLMSG_NEXT (nl_msg, len)) {
memset (&route_info, 0, sizeof(route_info));
if (parse_routes (nl_msg, &route_info) < 0)
continue; // don't check route_info if it has not been set up
// Check if default gateway
if (strstr ((char *)inet_ntoa (route_info.dst_addr), "0.0.0.0")) {
// copy it over
inet_ntop (AF_INET, &route_info.gw_addr, gatewayip, size);
found_gatewayip = 1;
printf ("ifname: %s\n", route_info.if_name);
break;
}
}
return found_gatewayip;
}
int main (int argc, char *argv[])
{
(void) argc;
(void) argv;
char gateway_ip[BUFSIZE];
get_gatewayip (gateway_ip, sizeof (gateway_ip));
printf ("GWIP: %s\n", gateway_ip);
return 0;
}
Raw
get-network-interfaces.c
/* Based on http://man7.org/linux/man-pages/man3/getifaddrs.3.html and
* https://www.linuxquestions.org/questions/linux-networking-3/howto-find-gateway-address-through-code-397078/ */
#if 0
#define _GNU_SOURCE /* To get defns of NI_MAXSERV and NI_MAXHOST */
#endif
#include <arpa/inet.h>
#include <sys/socket.h>
#include <netdb.h> /* definitions for NI_* */
#include <ifaddrs.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#ifndef __APPLE__
#include <linux/if_link.h> /* struct rtnl_link_stats */
#include <linux/rtnetlink.h> /* struct rtmsg, rtattr */
#endif
#include <net/if.h> /* definitions IFF_* */
/* Check http://opensource.apple.com/source/network_cmds/network_cmds-481.20.1/ifconfig.tproj/ifconfig.c */
#define IFFBITS \
"\020\1UP\2BROADCAST\3DEBUG\4LOOPBACK\5POINTOPOINT\6SMART\7RUNNING" \
"\10NOARP\11PROMISC\12ALLMULTI\13OACTIVE\14SIMPLEX\15LINK0\16LINK1\17LINK2" \
"\20MULTICAST"
#define BUFSIZE 8192
struct route_info {
struct in_addr dst_addr;
struct in_addr src_addr;
struct in_addr gw_addr;
char if_name[IF_NAMESIZE];
};
/*
* Print a value a la the %b format of the kernel's printf
*/
void
printb (const char *s,
unsigned v,
const char *bits)
{
int i, any = 0;
char c;
if (bits && *bits == 8)
printf ("%s=%o", s, v);
else
printf ("%s=0x%x", s, v);
bits++;
if (bits) {
putchar ('<');
while ((i = *bits++) != '\0') {
if (v & (1 << (i-1))) {
if (any)
putchar (',');
any = 1;
for (; (c = *bits) > 32; bits++)
putchar (c);
} else
for (; *bits > 32; bits++)
;
}
putchar ('>');
}
putchar ('\n');
}
void
print_network_interfaces (struct ifaddrs ifa) {
int family, s;
char host[NI_MAXHOST];
family = ifa.ifa_addr->sa_family;
/* Display interface name and family (including symbolic
form of the latter for the common families) */
fprintf (stdout, "%-8s %s (%d)\n",
ifa.ifa_name,
#ifndef __APPLE__
(family == AF_PACKET) ? "AF_PACKET" :
#else
(family == AF_LINK) ? "AF_LINK" :
#endif
(family == AF_INET) ? "AF_INET" :
(family == AF_INET6) ? "AF_INET6" : "???",
family);
printb ("\tflags", ifa.ifa_flags, IFFBITS);
/* For an AF_INET* interface address, display the address */
if (family == AF_INET || family == AF_INET6) {
s = getnameinfo (ifa.ifa_addr,
(family == AF_INET) ? sizeof (struct sockaddr_in) :
sizeof (struct sockaddr_in6),
host, NI_MAXHOST,
NULL, 0, NI_NUMERICHOST);
if (s != 0) {
fprintf (stderr, "*%s(): getnameinfo() failed: %s\n", __func__,
gai_strerror (s));
return;
}
fprintf (stdout, "\t\taddress: <%s>\n", host);
}
#ifndef __APPLE__
else if (family == AF_PACKET && ifa.ifa_data != NULL) {
struct rtnl_link_stats *stats = ifa.ifa_data;
fprintf (stdout, "\t\ttx_packets = %10u; rx_packets = %10u\n"
"\t\ttx_bytes = %10u; rx_bytes = %10u\n",
stats->tx_packets, stats->rx_packets,
stats->tx_bytes, stats->rx_bytes);
}
#endif
}
/** @brief Get a list of network interface names for a specific
* address family
*
* @param [out] ifa_names A list of network interface names.
* @param [out] ifa_names_sz The size of the above list.
* @param [in] sa_family The address family to filter the interfaces.
* @return 0 on success<br>
* -1 otherwise
*
* @note The loopback and not running interfaces are ignored.
* @note The list of names returned is allocated dynamically and needs
* to be free()d when not needed anymore.
*/
int
get_interface_names_by_family (char ***ifa_names,
size_t *ifa_names_sz,
int sa_family)
{
struct ifaddrs *ifaddr, *ifa;
char **tmp;
int ret = 0;
*ifa_names_sz = 0;
/* Get a linked list of network interfaces */
if (getifaddrs (&ifaddr) == -1) {
fprintf (stderr, "*%s(): error getting linked list of interfaces",
__func__);
return -1;
}
/* Walk through the linked list and find the
* interfaces that have the specific *family* and
* get the name */
for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {
/* No loopback interfaces or not running */
if (ifa->ifa_addr == NULL
|| ifa->ifa_flags & IFF_LOOPBACK
|| !(ifa->ifa_flags & IFF_RUNNING))
continue;
if (ifa->ifa_addr->sa_family == sa_family) {
*ifa_names_sz += 1;
tmp = realloc (*ifa_names, *ifa_names_sz * sizeof (char *));
if (tmp)
*ifa_names = tmp;
else {
fprintf (stderr, "*%s(): error realloc\n", __func__);
if (*ifa_names)
free (*ifa_names);
ret = -1;
goto out;
}
(*ifa_names)[*ifa_names_sz - 1] = strdup (ifa->ifa_name);
}
}
out:
freeifaddrs (ifaddr);
return ret;
}
/** @brief Get the network address from a specific network
* interface name
*
* @param [out] address The address to get.
* @param [in] ifa_name The interface name to get the address.
* @param [in] sa_family The address family to filter the interfaces.
* Currently only AF_INET and AF_INET6 are supported.
* @return 0 on success<br>
* -1 otherwise
*
* @note The loopback and not running interfaces are ignored.
* @note The address returned is allocated dynamically and needs
* to be free()d when not needed anymore.
*/
int
get_address_from_interface_name (char **address,
const char *ifa_name,
int sa_family)
{
struct ifaddrs *ifaddr, *ifa;
int ret = 0, s;
char ifa_address[NI_MAXHOST];
*address = NULL;
if (sa_family != AF_INET && sa_family != AF_INET6) {
fprintf (stderr, "*%s(): only AF_INET and AF_INET6 are supported\n",
__func__);
return -1;
}
/* Get a linked list of network interfaces */
if (getifaddrs (&ifaddr) == -1) {
fprintf (stderr, "*%s(): error getting linked list of interfaces\n",
__func__);
return -1;
}
/* Walk through the linked list and find the
* interface with the specific name */
for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {
/* No loopback interfaces or not running */
if (ifa->ifa_addr == NULL
|| ifa->ifa_flags & IFF_LOOPBACK
|| !(ifa->ifa_flags & IFF_RUNNING))
continue;
if (ifa->ifa_addr->sa_family == sa_family
&& !strcmp (ifa->ifa_name, ifa_name)) {
s = getnameinfo (ifa->ifa_addr,
(sa_family == AF_INET)
? sizeof (struct sockaddr_in)
: sizeof (struct sockaddr_in6),
ifa_address, NI_MAXHOST,
NULL, 0, NI_NUMERICHOST);
if (s != 0) {
fprintf (stderr, "*%s(): getnameinfo() failed: %s\n",
__func__, gai_strerror (s));
goto out;
}
*address = strdup (ifa_address);
break;
}
}
if (!*address) {
fprintf (stderr, "*%s(): interface %s not found.\n",
__func__, ifa_name);
ret = -1;
}
out:
freeifaddrs (ifaddr);
return ret;
}
static ssize_t
read_netlink_socket (int sock_fd, char *buffer, size_t buffer_sz,
uint32_t seq, uint32_t pid)
{
struct nlmsghdr *nl_hdr;
ssize_t read_len = 0, msg_len = 0;
do {
if ((size_t) msg_len > buffer_sz) {
fprintf (stderr, "*%s(): no size in buffer\n", __func__);
return -1;
}
/* Receive response from the kernel */
read_len = recv (sock_fd, buffer, buffer_sz - (size_t) msg_len, 0);
if (read_len < 0) {
fprintf (stderr, "*%s(): error receiving message from socket.\n",
__func__);
return -1;
}
/* Point the struct nlmsghdr to the buffer */
nl_hdr = (struct nlmsghdr *) buffer;
/* Check if the header is valid */
if (!NLMSG_OK (nl_hdr, read_len)
|| (nl_hdr->nlmsg_type == NLMSG_ERROR)) {
fprintf (stderr, "*%s(): error in received packet.\n", __func__);
return -1;
}
/* Check if the its the last message */
if (nl_hdr->nlmsg_type == NLMSG_DONE)
break;
else {
/* Else move the pointer to buffer appropriately */
buffer += read_len;
msg_len += read_len;
}
/* Check if its a multi part message */
if (!(nl_hdr->nlmsg_flags & NLM_F_MULTI))
/* return if its not */
break;
} while ((nl_hdr->nlmsg_seq != seq)
|| (nl_hdr->nlmsg_pid != pid));
return msg_len;
}
/* parse the route info returned */
static int
parse_routes (struct nlmsghdr *nl_hdr, struct route_info *rt_info)
{
struct rtmsg *rt_msg;
struct rtattr *rt_attr;
size_t rt_len;
rt_msg = (struct rtmsg *) NLMSG_DATA (nl_hdr);
/* If the route is not for AF_INET or
* does not belong to main routing table then return. */
if ((rt_msg->rtm_family != AF_INET)
|| (rt_msg->rtm_table != RT_TABLE_MAIN))
return -1;
/* get the rtattr field */
rt_attr = (struct rtattr *) RTM_RTA (rt_msg);
rt_len = RTM_PAYLOAD (nl_hdr);
for (; RTA_OK (rt_attr, rt_len); rt_attr = RTA_NEXT (rt_attr, rt_len)) {
switch (rt_attr->rta_type) {
case RTA_OIF:
if_indextoname (*(unsigned int *) RTA_DATA (rt_attr),
rt_info->if_name);
break;
case RTA_GATEWAY:
memcpy (&rt_info->gw_addr, RTA_DATA (rt_attr),
sizeof (rt_info->gw_addr));
break;
case RTA_PREFSRC:
memcpy (&rt_info->src_addr, RTA_DATA (rt_attr),
sizeof (rt_info->src_addr));
break;
case RTA_DST:
memcpy (&rt_info->dst_addr, RTA_DATA (rt_attr),
sizeof (rt_info->dst_addr));
break;
default:
break;
}
}
return 0;
}
/** @brief Get the default interface name
*
* @param [out] default_iface_name The name of the default interface
* @return 0 on success<br>
* -1 otherwise
*
* @note The name returned is allocated dynamically and needs
* to be free()d when not needed anymore.
*/
int
get_default_interface_name (char **default_iface_name)
{
struct nlmsghdr *nl_msg;
struct route_info route_info;
char msg_buffer[BUFSIZE];
int sock;
ssize_t len;
uint32_t msg_seq = 0;
/* Create socket */
sock = socket (PF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE);
if (sock < 0) {
fprintf (stderr, "*%s(): error creating socket.\n", __func__);
return -1;
}
/* Initialize the buffer */
memset (msg_buffer, 0, sizeof (msg_buffer));
/* Point the header and the msg structure pointers into the buffer */
nl_msg = (struct nlmsghdr *) msg_buffer;
/* Fill in the nlmsg header*/
/* Length of the message */
nl_msg->nlmsg_len = NLMSG_LENGTH (sizeof (struct rtmsg));
/* Get the routes from the kernel routing table */
nl_msg->nlmsg_type = RTM_GETROUTE;
/* The message is a request for dump */
nl_msg->nlmsg_flags = NLM_F_DUMP | NLM_F_REQUEST;
/* Sequence of the message packet */
nl_msg->nlmsg_seq = msg_seq++;
/* PID of the process sending the request */
nl_msg->nlmsg_pid = (uint32_t) getpid ();
/* Send the request */
if (send (sock, nl_msg, nl_msg->nlmsg_len, 0) < 0) {
fprintf (stderr, "*%s(): error sending the request.\n", __func__);
return -1;
}
/* Read the response */
len = read_netlink_socket (sock, msg_buffer, sizeof (msg_buffer),
msg_seq, nl_msg->nlmsg_pid);
if (len < 0) {
fprintf (stderr, "*%s(): error reading the response.\n", __func__);
return -1;
}
close (sock);
/* Parse and print the response */
for (; NLMSG_OK (nl_msg, len); nl_msg = NLMSG_NEXT (nl_msg, len)) {
memset (&route_info, 0, sizeof (route_info));
if (parse_routes (nl_msg, &route_info) < 0)
continue;
/* Check if it's the default interface */
if (strstr ((char *) inet_ntoa (route_info.dst_addr), "0.0.0.0")) {
*default_iface_name = strdup (route_info.if_name);
break;
}
}
return 0;
}
int main (int argc, char *argv[])
{
(void) argc;
(void) argv;
char **names = NULL;
int ret;
size_t names_sz;
ret = get_interface_names_by_family (&names, &names_sz, AF_INET);
if (ret == -1) {
fprintf (stderr, "*%s(): error getting network interfaces\n",
__func__);
return -1;
}
for (size_t i = 0;i<names_sz;i++)
printf ("%zu. Name: %s\n", i, names[i]);
for (size_t i = 0;i<names_sz;i++)
free (names[i]);
free (names);
fprintf (stdout, "* *** *\n");
char *adr = NULL;
const char *iface_name = "eth0";
ret = get_address_from_interface_name (&adr, iface_name, AF_INET);
if (ret == -1) {
fprintf (stderr, "*%s(): error getting address from interface %s\n",
__func__, iface_name);
return -1;
}
if (adr) {
printf ("Address for iface %s is %s\n", "eth0", adr);
free (adr);
}
fprintf (stdout, "* *** *\n");
int n;
struct ifaddrs *ifaddr, *ifa;
if (getifaddrs (&ifaddr) == -1) {
fprintf (stderr, "*%s(): getifaddrs", __func__);
return -1;
}
/* Walk through linked list, maintaining head pointer so we
can free list later */
for (ifa = ifaddr, n = 0; ifa != NULL; ifa = ifa->ifa_next, n++) {
if (ifa->ifa_addr == NULL)
continue;
print_network_interfaces (*ifa);
}
freeifaddrs (ifaddr);
fprintf (stdout, "* *** *\n");
char *if_name = NULL;
if (get_default_interface_name (&if_name) == -1) {
fprintf (stderr, "*%s(): error getting the default interface name.\n",
__func__);
return -1;
}
if (if_name) {
printf ("Default interface name: %s\n", if_name);
free (if_name);
}
return 0;
}
Raw
Makefile
all: get-network-interfaces get-gateway-ip
get-network-interfaces: get-network-interfaces.c
gcc -o get-network-interfaces -ggdb -O1 -Wall -W -ansi -pedantic -std=gnu99 get-network-interfaces.c
get-gateway-ip: get-gateway-ip.c
gcc -o get-gateway-ip -ggdb -O1 -Wall -W -ansi -pedantic -std=gnu99 get-gateway-ip.c
clean:
rm -rf get-network-interfaces
rm -rf get-gateway-ip
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment