AF_NETLINK (netlink) sockets using boost::asio

Question

I'm writing multicast client/server application based on this and this; which work great.

However, I would also need to do something when the number of active network interfaces in the computer changes, something like what the program in the example section of this page does.

I guess I should use the tools in boost::asio::local, but I am unsure whether I should use boost::asio::local::datagram_protocol or boost::asio::local::stream_protocol or...

An example of how to do something as similar as possible would be really helpful. Thanks.

Answer 1

As you noticed there's a little bit extra code that has to get written to do this.

Netlink uses its own Address Family (PF_NETLINK) and isn't TCP or UDP. Instead it uses RAW sockets. Since it is a private family (not PF_INET or PF_INET6) we need to define it ourselves.

I used a basic unix local socket as a template to implement this (see the bottom for test program). If you don't want to copy paste all the code, I also put it on github (http://github.com/gille/boost_netlink):

template <typename Protocol>
class nl_endpoint
{
private:
    sockaddr_nl sockaddr;
public:
    /// The protocol type associated with the endpoint.
    typedef Protocol protocol_type;
    typedef boost::asio::detail::socket_addr_type data_type;        

    /// Default constructor.
    nl_endpoint()
    {
        sockaddr.nl_family = PF_NETLINK;
        sockaddr.nl_groups = 0;
        sockaddr.nl_pid = getpid(); 
    }

    /// Construct an endpoint using the specified path name.
    nl_endpoint(int group, int pid=getpid())
    {
        sockaddr.nl_family = PF_NETLINK;
        sockaddr.nl_groups = group;
        sockaddr.nl_pid = pid;
    }

    /// Copy constructor.
    nl_endpoint(const nl_endpoint& other)
    {
        sockaddr = other.sockaddr;
    }

    /// Assign from another endpoint.
    nl_endpoint& operator=(const nl_endpoint& other)
    {
        sockaddr = other.sockaddr;
        return *this;
    }

    /// The protocol associated with the endpoint.
    protocol_type protocol() const
    {
        return protocol_type();
    }

    /// Get the underlying endpoint in the native type.
    data_type* data()
    {
        return &sockaddr;
    }

    /// Get the underlying endpoint in the native type.
    const data_type* data() const
    {
        return (struct sockaddr*)&sockaddr;
    }

    /// Get the underlying size of the endpoint in the native type.
    std::size_t size() const
    {
        return sizeof(sockaddr);
    }

    /// Set the underlying size of the endpoint in the native type.
    void resize(std::size_t size)
    {
    /* nothing we can do here */
    }

    /// Get the capacity of the endpoint in the native type.
    std::size_t capacity() const
    {
        return sizeof(sockaddr);
    }

    /// Compare two endpoints for equality.
    friend bool operator==(const nl_endpoint<Protocol>& e1,
               const nl_endpoint<Protocol>& e2)
    {
        return e1.sockaddr == e2.sockaddr;
    }

    /// Compare two endpoints for inequality.
    friend bool operator!=(const nl_endpoint<Protocol>& e1,
               const nl_endpoint<Protocol>& e2)
    {
        return !(e1.sockaddr == e2.sockaddr);
    }

    /// Compare endpoints for ordering.
    friend bool operator<(const nl_endpoint<Protocol>& e1,
              const nl_endpoint<Protocol>& e2)
    {
        return e1.sockaddr < e2.sockaddr;
    }

    /// Compare endpoints for ordering.
    friend bool operator>(const nl_endpoint<Protocol>& e1,
              const nl_endpoint<Protocol>& e2)
    {
        return e2.sockaddr < e1.sockaddr;
    }

    /// Compare endpoints for ordering.
    friend bool operator<=(const nl_endpoint<Protocol>& e1,
               const nl_endpoint<Protocol>& e2)
    {
        return !(e2 < e1);
    }

    /// Compare endpoints for ordering.
    friend bool operator>=(const nl_endpoint<Protocol>& e1,
               const nl_endpoint<Protocol>& e2)
    {
        return !(e1 < e2);
    }
};

The protocol:

class nl_protocol
{
private:
    int proto; 
public:
    nl_protocol() {
        proto = 0;
    }
    nl_protocol(int proto) {
        this->proto = proto;
    }
    /// Obtain an identifier for the type of the protocol.
    int type() const
    {
        return SOCK_RAW;
    }
    /// Obtain an identifier for the protocol.
    int protocol() const
    {
        return proto;
    }
    /// Obtain an identifier for the protocol family.
    int family() const
    {
        return PF_NETLINK;
    }

    typedef nl_endpoint<nl_protocol> endpoint;
    typedef boost::asio::basic_raw_socket<nl_protocol> socket;
};

Then from here all we need to do is create a raw socket and read it:

void handle_netlink(struct nlmsghdr *nlm);

int main(int argc, char* argv[])
{
    try
    {
        boost::asio::io_service io_service;
        boost::asio::basic_raw_socket<nl_protocol> s(io_service ); 

        s.open(nl_protocol(NETLINK_ROUTE));
        s.bind(nl_endpoint<nl_protocol>(RTMGRP_LINK)); 

        char buffer[max_length];
        int bytes;

        while((bytes=s.receive(boost::asio::buffer(buffer, max_length)))) {
             struct nlmsghdr *nlm;

            for (nlm = (struct nlmsghdr *)buffer;
              NLMSG_OK(nlm, (size_t)bytes);
              nlm = NLMSG_NEXT(nlm, bytes))
            {
            handle_netlink(nlm);
            }
        }
    }
    catch (std::exception& e)
    {
        std::cerr << "Exception: " << e.what() << "\n";
    }        
    return 0;
}

/* This code just prints out what interface went up or down */    
void handle_netlink(struct nlmsghdr *nlm) {
    int len;
    char ifname[IF_NAMESIZE + 1];
    ifinfomsg *ifi;
    rtattr *rta;

    if (nlm->nlmsg_type == RTM_NEWLINK) {
        len = nlm->nlmsg_len - sizeof(*nlm);
        if ((size_t)len < sizeof(*ifi)) {
            errno = EBADMSG;
            return;
        }
        ifi = (ifinfomsg*)NLMSG_DATA(nlm);
        if (ifi->ifi_flags & IFF_LOOPBACK)
            return;

        rta = (rtattr *) ((char *)ifi + NLMSG_ALIGN(sizeof(*ifi)));
        len = NLMSG_PAYLOAD(nlm, sizeof(*ifi));
        ifname[0] = '\0';
        while (RTA_OK(rta, len)) {
            switch (rta->rta_type) {
                case IFLA_IFNAME:
                strncpy(ifname, (char*)RTA_DATA(rta), sizeof(ifname));
                break;
                default:
                    break;
            }
            rta = RTA_NEXT(rta, len);       
        }
    }
    if (nlm->nlmsg_type == RTM_NEWLINK)
        len = ifi->ifi_change == ~0U ? 1 : 0;

    std::cout << "Interface " << ifname << " changed status, now: ";
    if((ifi->ifi_flags&IFF_LOWER_UP)==IFF_LOWER_UP)
        std::cout << " Up" << std::endl;
    else
        std::cout << " Down" << std::endl;    
}