Question

Is there anyway to add and remove IP addresses from an interface (like loopback) in C?

I found ioctl and a few documents explaining how to do that (e.g. this link), however they are all for setting addresses not adding and removing?

Following the suggestions I ran strace for adding a new loopback and here is the results:

$ sudo strace ip addr add 1.2.3.4 dev lo
execve("/sbin/ip", ["ip", "addr", "add", "1.2.3.4", "dev", "lo"], [/* 17 vars */]) = 0
brk(0)                                  = 0x1bab000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f221ed04000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=109414, ...}) = 0
mmap(NULL, 109414, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f221ece9000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libdl.so.2", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\340\r\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=14768, ...}) = 0
mmap(NULL, 2109704, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f221e8e0000
mprotect(0x7f221e8e2000, 2097152, PROT_NONE) = 0
mmap(0x7f221eae2000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x2000) = 0x7f221eae2000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\200\30\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1811128, ...}) = 0
mmap(NULL, 3925208, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f221e521000
mprotect(0x7f221e6d6000, 2093056, PROT_NONE) = 0
mmap(0x7f221e8d5000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1b4000) = 0x7f221e8d5000
mmap(0x7f221e8db000, 17624, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f221e8db000
close(3)                                = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f221ece8000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f221ece7000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f221ece6000
arch_prctl(ARCH_SET_FS, 0x7f221ece7700) = 0
mprotect(0x7f221e8d5000, 16384, PROT_READ) = 0
mprotect(0x7f221eae2000, 4096, PROT_READ) = 0
mprotect(0x638000, 4096, PROT_READ)     = 0
mprotect(0x7f221ed06000, 4096, PROT_READ) = 0
munmap(0x7f221ece9000, 109414)          = 0
socket(PF_NETLINK, SOCK_RAW, 0)         = 3
setsockopt(3, SOL_SOCKET, SO_SNDBUF, [32768], 4) = 0
setsockopt(3, SOL_SOCKET, SO_RCVBUF, [1048576], 4) = 0
bind(3, {sa_family=AF_NETLINK, pid=0, groups=00000000}, 12) = 0
getsockname(3, {sa_family=AF_NETLINK, pid=6804, groups=00000000}, [12]) = 0
sendto(3, "\24\0\0\0\22\0\1\3\214;\367P\0\0\0\0\0\0\0\0", 20, 0, NULL, 0) = 20
recvmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"\344\3\0\0\20\0\2\0\214;\367P\224\32\0\0\0\0\4\3\1\0\0\0I\0\1\0\0\0\0\0"..., 16384}], msg_controllen=0, msg_flags=0}, 0) = 2000
brk(0)                                  = 0x1bab000
brk(0x1bcc000)                          = 0x1bcc000
recvmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"\24\0\0\0\3\0\2\0\214;\367P\224\32\0\0\0\0\0\0\1\0\0\0I\0\1\0\0\0\0\0"..., 16384}], msg_controllen=0, msg_flags=0}, 0) = 20
sendmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"(\0\0\0\24\0\5\6\215;\367P\0\0\0\0\2 \0\0\1\0\0\0\10\0\2\0\1\2\3\4"..., 40}], msg_controllen=0, msg_flags=0}, 0) = 40
recvmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"$\0\0\0\2\0\0\0\215;\367P\224\32\0\0\0\0\0\0(\0\0\0\24\0\5\6\215;\367P"..., 16384}], msg_controllen=0, msg_flags=0}, 0) = 36
exit_group(0)                           = ?

Answer 1

With some help from avahi project.
Here is the code:

$ cat netlink-test.cc

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <asm/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include <malloc.h>
#include <string.h>
#include <iostream>

using namespace std;

struct rtnl_handle
{
    int fd;
    struct sockaddr_nl local;
    struct sockaddr_nl peer;
    __u32 seq;
    __u32 dump;
};

typedef struct
{
    __u8 family;
    __u8 bytelen;
    __s16 bitlen;
    __u32 flags;
    __u32 data[8];
} inet_prefix;

/* This uses a non-standard parsing (ie not inet_aton, or inet_pton)
 * because of legacy choice to parse 10.8 as 10.8.0.0 not 10.0.0.8
 */
static int get_addr_ipv4(__u8 *ap, const char *cp)
{
    int i;

    for (i = 0; i < 4; i++) {
        unsigned long n;
        char *endp;

        n = strtoul(cp, &endp, 0);
        if (n > 255)
            return -1;      /* bogus network value */

        if (endp == cp) /* no digits */
            return -1;

        ap[i] = n;

        if (*endp == '\0')
            break;

        if (i == 3 || *endp != '.')
            return -1;      /* extra characters */
        cp = endp + 1;
    }

    return 1;
}

// This function is to open the netlink socket as the name suggests.
int netlink_open(struct rtnl_handle* rth)
{
    int addr_len;
    memset(rth, 0, sizeof(rth));

    // Creating the netlink socket of family NETLINK_ROUTE

    rth->fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
    if (rth->fd < 0)
    {
        perror("cannot open netlink socket");
        return -1;
    }
    memset(&rth->local, 0, sizeof(rth->local));
    rth->local.nl_family = AF_NETLINK;
    rth->local.nl_groups = 0;

    // Binding the netlink socket
    if (bind(rth->fd, (struct sockaddr*)&rth->local, sizeof(rth->local)) < 0)
    {
        perror("cannot bind netlink socket");
        return -1;
    }
    addr_len = sizeof(rth->local);
    if (getsockname(rth->fd, (struct sockaddr*)&rth->local, (socklen_t*) &addr_len) < 0)
    {
        perror("cannot getsockname");
        return -1;
    }
    if (addr_len != sizeof(rth->local))
    {
        fprintf(stderr, "wrong address lenght %d\n", addr_len);
        return -1;
    }
    if (rth->local.nl_family != AF_NETLINK)
    {
        fprintf(stderr, "wrong address family %d\n", rth->local.nl_family);
        return -1;
    }
    rth->seq = time(NULL);
    return 0;
}

// This function does the actual reading and writing to the netlink socket
int rtnl_talk(struct rtnl_handle *rtnl, struct nlmsghdr *n, pid_t peer,
        unsigned groups, struct nlmsghdr *answer)
{
    int status;
    struct nlmsghdr *h;
    struct sockaddr_nl nladdr;
    // Forming the iovector with the netlink packet.
    struct iovec iov = { (void*)n, n->nlmsg_len };
    char buf[8192];
    // Forming the message to be sent.
    struct msghdr msg = { (void*)&nladdr, sizeof(nladdr), &iov, 1, NULL, 0, 0 };
    // Filling up the details of the netlink socket to be contacted in the
    // kernel.
    memset(&nladdr, 0, sizeof(nladdr));
    nladdr.nl_family = AF_NETLINK;
    nladdr.nl_pid = peer;
    nladdr.nl_groups = groups;
    n->nlmsg_seq = ++rtnl->seq;
    if (answer == NULL)
        n->nlmsg_flags |= NLM_F_ACK;
    // Actual sending of the message, status contains success/failure
    status = sendmsg(rtnl->fd, &msg, 0);
    if (status < 0)
        return -1;
}




// This is the utility function for adding the parameters to the packet.
int addattr_l(struct nlmsghdr *n, int maxlen, int type, void *data, int alen)
{
    int len = RTA_LENGTH(alen);
    struct rtattr *rta;

    if (NLMSG_ALIGN(n->nlmsg_len) + len > maxlen)
        return -1;
    rta = (struct rtattr*)(((char*)n) + NLMSG_ALIGN(n->nlmsg_len));
    rta->rta_type = type;
    rta->rta_len = len;
    memcpy(RTA_DATA(rta), data, alen);
    n->nlmsg_len = NLMSG_ALIGN(n->nlmsg_len) + len;
    return 0;
}


int get_addr_1(inet_prefix *addr, const char *name, int family)
{
    memset(addr, 0, sizeof(*addr));

    if (strcmp(name, "default") == 0 ||
            strcmp(name, "all") == 0 ||
            strcmp(name, "any") == 0) {
        if (family == AF_DECnet)
            return -1;
        addr->family = family;
        addr->bytelen = (family == AF_INET6 ? 16 : 4);
        addr->bitlen = -1;
        return 0;
    }

    if (strchr(name, ':')) {
        addr->family = AF_INET6;
        if (family != AF_UNSPEC && family != AF_INET6)
            return -1;
        if (inet_pton(AF_INET6, name, addr->data) <= 0)
            return -1;
        addr->bytelen = 16;
        addr->bitlen = -1;
        return 0;
    }


    addr->family = AF_INET;
    if (family != AF_UNSPEC && family != AF_INET)
        return -1;

    if (get_addr_ipv4((__u8 *)addr->data, name) <= 0)
        return -1;

    addr->bytelen = 4;
    addr->bitlen = -1;
    return 0;
}

int get_prefix(inet_prefix *dst, char *arg, int family)
{
    int err;
    unsigned plen;

    memset(dst, 0, sizeof(*dst));

    if (strcmp(arg, "default") == 0 ||
            strcmp(arg, "any") == 0 ||
            strcmp(arg, "all") == 0) {
        if (family == AF_DECnet)
            return -1;
        dst->family = family;
        dst->bytelen = 0;
        dst->bitlen = 0;
        return 0;
    }

    err = get_addr_1(dst, arg, family);
    if (err == 0) {
        switch(dst->family) {
            case AF_INET6:
                dst->bitlen = 128;
                break;
            case AF_DECnet:
                dst->bitlen = 16;
                break;
            default:
            case AF_INET:
                dst->bitlen = 32;
        }
    }
    return err;
}


int add_IP_Address(char * IP, struct rtnl_handle * rth)
{

    inet_prefix lcl;
    // structure of the netlink packet. 
    struct {
        struct nlmsghdr     n;
        struct ifaddrmsg    ifa;
        char            buf[1024];
    } req;

    memset(&req, 0, sizeof(req));
    req.n.nlmsg_len = NLMSG_LENGTH(sizeof(struct ifaddrmsg));
    req.n.nlmsg_type = RTM_NEWADDR;
    req.n.nlmsg_flags = NLM_F_CREATE | NLM_F_EXCL | NLM_F_REQUEST;


//  req.n.nlmsg_type = RTM_DELADDR;
//  req.n.nlmsg_flags = NLM_F_REQUEST;

    req.ifa.ifa_family = AF_INET ;
    req.ifa.ifa_prefixlen = 32 ;
    req.ifa.ifa_index = 1 ; // get the loopback index
    req.ifa.ifa_scope = 0 ;

    get_prefix(&lcl, IP, req.ifa.ifa_family);
    if (req.ifa.ifa_family == AF_UNSPEC)
        req.ifa.ifa_family = lcl.family;
    addattr_l(&req.n, sizeof(req), IFA_LOCAL, &lcl.data, lcl.bytelen);

    if (rtnl_talk(rth, &req.n, 0, 0, NULL) < 0)
        return -2;
}

int main(int argc, char **argv)
{
    struct rtnl_handle * rth;
    netlink_open(rth);
    char * ip = "1.2.3.4";
    return add_IP_Address(ip,rth);
}

For deleting, you only need to uncomment:

    req.n.nlmsg_type = RTM_DELADDR;
    req.n.nlmsg_flags = NLM_F_REQUEST;

and comment:

    req.n.nlmsg_type = RTM_NEWADDR;
    req.n.nlmsg_flags = NLM_F_CREATE | NLM_F_EXCL | NLM_F_REQUEST;