Get IP, IPv6 and MAC addresses using ioctl and procfs on Linux in C

Getting IP, IPv6 and MAC addresses on Linux is no easy task if you do not use ifconfig. The information on the internet is not very good, and the process is quite complicated. I have spent most of yesterday writing the code, so I decided to describe it here.

The whole code can be found at the bottom.

Listing the interfaces

The first thing we need to do is get the names of network interfaces available on the system. We will parse sysfs. Interfaces are available as folders inside the /sys/class/net/ directory.

First we open the directory and check for errors:

d = opendir("/sys/class/net/");
if (d == NULL) {
	return -1;
}

Then we iterate over directory and ignore current (“.”) and parent (“..”) directory entries:

while (NULL != (de = readdir(d))) {
	if (strcmp(de->d_name, ".") == 0 || strcmp(de->d_name, "..") == 0) {
		continue;
	}

Finally we print the interface name, call functions that will display mac and ip info, close the folder and we are done.

	printf("Interface %s\\n", de->d_name);
	parse_ioctl(de->d_name);
	parse_inet6(de->d_name);
	printf("\\n");
}
closedir(d);

Get MAC, MTU and IPv4 info using ioctl

All calls to ioctl need ifreq struct:

struct ifreq {
    char    ifr_name[IFNAMSIZ];/* Interface name */
    union {
            struct sockaddr ifr_addr;
            struct sockaddr ifr_dstaddr;
            struct sockaddr ifr_broadaddr;
            struct sockaddr ifr_netmask;
            struct sockaddr ifr_hwaddr;
            short   ifr_flags;
            int     ifr_ifindex;
            int     ifr_metric;
            int     ifr_mtu;
            struct ifmap ifr_map;
            char    ifr_slave[IFNAMSIZ];
            char    ifr_newname[IFNAMSIZ];
            char *  ifr_data;
    };
};

First thing we need to do is define our function and variables.

void parse_ioctl(const char *ifname)
{
    int sock;
    struct ifreq ifr;
    struct sockaddr_in *ipaddr;
    char address[INET_ADDRSTRLEN];
    size_t ifnamelen;

Next we need to copy ifname to the ifreq struct that will be pased to ioctl. Because ifreq.ifr_name is a fixed length buffer we have to make sure it will fit.

    /* copy ifname to ifr object */
    ifnamelen = strlen(ifname);
    if (ifnamelen >= sizeof(ifr.ifr_name)) {
        return ;
    }
    memcpy(ifr.ifr_name, ifname, ifnamelen);
    ifr.ifr_name[ifnamelen] = \'\\0\';

For ioctl calls we also need an active socket number, so we have to create one:

    /* open socket */
    sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_IP);
    if (sock < 0) {
        return;
    }

First we call ioctl with SIOCGIFHWADDR to get mac address and print it out:

    /* process mac */
    if (ioctl(sock, SIOCGIFHWADDR, &ifr) != -1) {
        printf("Mac address: %02x:%02x:%02x:%02x:%02x:%02x\\n",
                (unsigned char)ifr.ifr_hwaddr.sa_data[0],
                (unsigned char)ifr.ifr_hwaddr.sa_data[1],
                (unsigned char)ifr.ifr_hwaddr.sa_data[2],
                (unsigned char)ifr.ifr_hwaddr.sa_data[3],
                (unsigned char)ifr.ifr_hwaddr.sa_data[4],
                (unsigned char)ifr.ifr_hwaddr.sa_data[5]);
    }

To get MTU we do the same but with SIOCGIFMTU:

    /* process mtu */
    if (ioctl(sock, SIOCGIFMTU, &ifr) != -1) {
        printf("MTU: %d\\n", ifr.ifr_mtu);
    }

We try to get IP address with SIOCGIFADDR. If there is an error we return, because IP configuration does not exist and we would not be able to get broadcast and netmask anyway. Then we process binary IP with inet_ntop to get a string we can print:

    /* die if cannot get address */
    if (ioctl(sock, SIOCGIFADDR, &ifr) == -1) {
        close(sock);
        return;
    }

    /* process ip */
    ipaddr = (struct sockaddr_in *)&ifr.ifr_addr;
    if (inet_ntop(AF_INET, &ipaddr->sin_addr, address, sizeof(address)) != NULL) {
        printf("Ip address: %s\\n", address);
    }

For broadcast and netmask we use SIOCGIFBRDADDR and SIOCGIFNETMASK respectively. We convert them to string with inet_ntop and print them out. Finally we close the opened socket:

    /* try to get broadcast */
    if (ioctl(sock, SIOCGIFBRDADDR, &ifr) != -1) {
        ipaddr = (struct sockaddr_in *)&ifr.ifr_broadaddr;
        if (inet_ntop(AF_INET, &ipaddr->sin_addr, address, sizeof(address)) != NULL) {
            printf("Broadcast: %s\\n", address);
        }
    }

    /* try to get mask */
    if (ioctl(sock, SIOCGIFNETMASK, &ifr) != -1) {
        ipaddr = (struct sockaddr_in *)&ifr.ifr_netmask;
        if (inet_ntop(AF_INET, &ipaddr->sin_addr, address, sizeof(address)) != NULL) {
            printf("Netmask: %s\\n", address);
        }
    }

    close(sock);
}

Parsing IPv6 from /proc/net/if_inet6

The ioctl calls do not support IPv6. Because of that we have to use another way. We will parse /proc/net/if_inet6.

First initialize function and open file, check for errors:

void parse_inet6(const char *ifname) {
    FILE *f;
    int ret, scope, prefix;
    unsigned char ipv6[16];
    char dname[IFNAMSIZ];
    char address[INET6_ADDRSTRLEN];
    char *scopestr;

    f = fopen("/proc/net/if_inet6", "r");
    if (f == NULL) {
        return;
    }

We will use fscanf to parse the file. The IPv6 in the file is stored in hexadecimal number. We cannot read it as one big number because endianness is a problem. We parse it in a unsigned char array 2 characters at a time, so fscanf becomes huge, but we save ourselves some trouble later on.

    while (19 == fscanf(f,
                        " %2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx %*x %x %x %*x %s",
                        &ipv6[0],
                        &ipv6[1],
                        &ipv6[2],
                        &ipv6[3],
                        &ipv6[4],
                        &ipv6[5],
                        &ipv6[6],
                        &ipv6[7],
                        &ipv6[8],
                        &ipv6[9],
                        &ipv6[10],
                        &ipv6[11],
                        &ipv6[12],
                        &ipv6[13],
                        &ipv6[14],
                        &ipv6[15],
                        &prefix,
                        &scope,
                        dname)) {

We only want the entries for current interface, so we skip if the name does not match. Than we try to convert IPv6 to string using inet_ntop.

        if (strcmp(ifname, dname) != 0) {
            continue;
        }
        if (inet_ntop(AF_INET6, ipv6, address, sizeof(address)) == NULL) {
            continue;
        }

We want scope to be in readable format, so we use switch to differentiate different values:

#define IPV6_ADDR_GLOBAL        0x0000U
#define IPV6_ADDR_LOOPBACK      0x0010U
#define IPV6_ADDR_LINKLOCAL     0x0020U
#define IPV6_ADDR_SITELOCAL     0x0040U
#define IPV6_ADDR_COMPATv4      0x0080U
        switch (scope) {
        case IPV6_ADDR_GLOBAL:
            scopestr = "Global";
            break;
        case IPV6_ADDR_LINKLOCAL:
            scopestr = "Link";
            break;
        case IPV6_ADDR_SITELOCAL:
            scopestr = "Site";
            break;
        case IPV6_ADDR_COMPATv4:
            scopestr = "Compat";
            break;
        case IPV6_ADDR_LOOPBACK:
            scopestr = "Host";
            break;
        default:
            scopestr = "Unknown";
        }

Finally we print the info, close the file and we are done.

        printf("IPv6 address: %s, prefix: %d, scope: %s\\n", address, prefix, scopestr);
    }
    fclose(f);
}

The whole code

#include <stdio.h>
#include <string.h>
#include <net/if.h>
#include <sys/ioctl.h>
#include <arpa/inet.h>
#include <dirent.h>

#define IPV6_ADDR_GLOBAL        0x0000U
#define IPV6_ADDR_LOOPBACK      0x0010U
#define IPV6_ADDR_LINKLOCAL     0x0020U
#define IPV6_ADDR_SITELOCAL     0x0040U
#define IPV6_ADDR_COMPATv4      0x0080U

void parse_inet6(const char *ifname) {
    FILE *f;
    int ret, scope, prefix;
    unsigned char ipv6[16];
    char dname[IFNAMSIZ];
    char address[INET6_ADDRSTRLEN];
    char *scopestr;

    f = fopen("/proc/net/if_inet6", "r");
    if (f == NULL) {
        return;
    }

    while (19 == fscanf(f,
                        " %2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx%2hhx %*x %x %x %*x %s",
                        &ipv6[0],
                        &ipv6[1],
                        &ipv6[2],
                        &ipv6[3],
                        &ipv6[4],
                        &ipv6[5],
                        &ipv6[6],
                        &ipv6[7],
                        &ipv6[8],
                        &ipv6[9],
                        &ipv6[10],
                        &ipv6[11],
                        &ipv6[12],
                        &ipv6[13],
                        &ipv6[14],
                        &ipv6[15],
                        &prefix,
                        &scope,
                        dname)) {

        if (strcmp(ifname, dname) != 0) {
            continue;
        }

        if (inet_ntop(AF_INET6, ipv6, address, sizeof(address)) == NULL) {
            continue;
        }

        switch (scope) {
        case IPV6_ADDR_GLOBAL:
            scopestr = "Global";
            break;
        case IPV6_ADDR_LINKLOCAL:
            scopestr = "Link";
            break;
        case IPV6_ADDR_SITELOCAL:
            scopestr = "Site";
            break;
        case IPV6_ADDR_COMPATv4:
            scopestr = "Compat";
            break;
        case IPV6_ADDR_LOOPBACK:
            scopestr = "Host";
            break;
        default:
            scopestr = "Unknown";
        }

        printf("IPv6 address: %s, prefix: %d, scope: %s\n", address, prefix, scopestr);
    }

    fclose(f);
}

void parse_ioctl(const char *ifname)
{
    int sock;
    struct ifreq ifr;
    struct sockaddr_in *ipaddr;
    char address[INET_ADDRSTRLEN];
    size_t ifnamelen;

    /* copy ifname to ifr object */
    ifnamelen = strlen(ifname);
    if (ifnamelen >= sizeof(ifr.ifr_name)) {
        return ;
    }
    memcpy(ifr.ifr_name, ifname, ifnamelen);
    ifr.ifr_name[ifnamelen] = '\0';

    /* open socket */
    sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_IP);
    if (sock < 0) {
        return;
    }

    /* process mac */
    if (ioctl(sock, SIOCGIFHWADDR, &ifr) != -1) {
        printf("Mac address: %02x:%02x:%02x:%02x:%02x:%02x\n",
                (unsigned char)ifr.ifr_hwaddr.sa_data[0],
                (unsigned char)ifr.ifr_hwaddr.sa_data[1],
                (unsigned char)ifr.ifr_hwaddr.sa_data[2],
                (unsigned char)ifr.ifr_hwaddr.sa_data[3],
                (unsigned char)ifr.ifr_hwaddr.sa_data[4],
                (unsigned char)ifr.ifr_hwaddr.sa_data[5]);
    }

    /* process mtu */
    if (ioctl(sock, SIOCGIFMTU, &ifr) != -1) {
        printf("MTU: %d\n", ifr.ifr_mtu);
    }

    /* die if cannot get address */
    if (ioctl(sock, SIOCGIFADDR, &ifr) == -1) {
        close(sock);
        return;
    }

    /* process ip */
    ipaddr = (struct sockaddr_in *)&ifr.ifr_addr;
    if (inet_ntop(AF_INET, &ipaddr->sin_addr, address, sizeof(address)) != NULL) {
        printf("Ip address: %s\n", address);
    }

    /* try to get broadcast */
    if (ioctl(sock, SIOCGIFBRDADDR, &ifr) != -1) {
        ipaddr = (struct sockaddr_in *)&ifr.ifr_broadaddr;
        if (inet_ntop(AF_INET, &ipaddr->sin_addr, address, sizeof(address)) != NULL) {
            printf("Broadcast: %s\n", address);
        }
    }

    /* try to get mask */
    if (ioctl(sock, SIOCGIFNETMASK, &ifr) != -1) {
        ipaddr = (struct sockaddr_in *)&ifr.ifr_netmask;
        if (inet_ntop(AF_INET, &ipaddr->sin_addr, address, sizeof(address)) != NULL) {
            printf("Netmask: %s\n", address);
        }
    }

    close(sock);
}

int main(void)
{
    DIR *d;
    struct dirent *de;

    d = opendir("/sys/class/net/");
    if (d == NULL) {
        return -1;
    }

    while (NULL != (de = readdir(d))) {
        if (strcmp(de->d_name, ".") == 0 || strcmp(de->d_name, "..") == 0) {
            continue;
        }

        printf("Interface %s\n", de->d_name);

        parse_ioctl(de->d_name);

        parse_inet6(de->d_name);

        printf("\n");
    }

    closedir(d);

    return 0;
}

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