libcurl の CURLOPT_INTERFACE のソースコードを読んでみた話

#include <curl/curl.h>
 
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_INTERFACE, char *interface);

/*
 * Do not make Curl_vsetopt() static: it is called from
 * packages/OS400/ccsidcurl.c.
 */
CURLcode Curl_vsetopt(struct Curl_easy *data, CURLoption option, va_list param)
{

  case CURLOPT_INTERFACE:
    /*
     * Set what interface or address/hostname to bind the socket to when
     * performing an operation and thus what from-IP your connection will use.
     */
    result = Curl_setstropt(&data->set.str[STRING_DEVICE],
                            va_arg(param, char *));
    break;

static CURLcode bindlocal(struct Curl_easy *data,
                          curl_socket_t sockfd, int af, unsigned int scope)
{

  const char *dev = data->set.str[STRING_DEVICE];
  int error;

  /*************************************************************
   * Select device to bind socket to
   *************************************************************/
  if(!dev && !port)
    /* no local kind of binding was requested */
    return CURLE_OK;

  memset(&sa, 0, sizeof(struct Curl_sockaddr_storage));

  if(dev && (strlen(dev)<255) ) {
    char myhost[256] = "";
    int done = 0; /* -1 for error, 1 for address found */
    bool is_interface = FALSE;
    bool is_host = FALSE;
    static const char *if_prefix = "if!";
    static const char *host_prefix = "host!";

    if(strncmp(if_prefix, dev, strlen(if_prefix)) == 0) {
      dev += strlen(if_prefix);
      is_interface = TRUE;
    }
    else if(strncmp(host_prefix, dev, strlen(host_prefix)) == 0) {
      dev += strlen(host_prefix);
      is_host = TRUE;
    }

  /* interface */
    if(!is_host) {
#ifdef SO_BINDTODEVICE
      /* I am not sure any other OSs than Linux that provide this feature,
       * and at the least I cannot test. --Ben
       *
       * This feature allows one to tightly bind the local socket to a
       * particular interface.  This will force even requests to other
       * local interfaces to go out the external interface.
       *
       *
       * Only bind to the interface when specified as interface, not just
       * as a hostname or ip address.
       *
       * interface might be a VRF, eg: vrf-blue, which means it cannot be
       * converted to an IP address and would fail Curl_if2ip. Simply try
       * to use it straight away.
       */
      if(setsockopt(sockfd, SOL_SOCKET, SO_BINDTODEVICE,
                    dev, (curl_socklen_t)strlen(dev) + 1) == 0) {
        /* This is typically "errno 1, error: Operation not permitted" if
         * you're not running as root or another suitable privileged
         * user.
         * If it succeeds it means the parameter was a valid interface and
         * not an IP address. Return immediately.
         */
        return CURLE_OK;
      }
#endif

      switch(Curl_if2ip(af,
#ifdef ENABLE_IPV6
                        scope, conn->scope_id,
#endif
                        dev, myhost, sizeof(myhost))) {
        case IF2IP_NOT_FOUND:
          if(is_interface) {
            /* Do not fall back to treating it as a host name */
            failf(data, "Couldn't bind to interface '%s'", dev);
            return CURLE_INTERFACE_FAILED;
          }
          break;
        case IF2IP_AF_NOT_SUPPORTED:
          /* Signal the caller to try another address family if available */
          return CURLE_UNSUPPORTED_PROTOCOL;
        case IF2IP_FOUND:
          is_interface = TRUE;
          /*
           * We now have the numerical IP address in the 'myhost' buffer
           */
          infof(data, "Local Interface %s is ip %s using address family %i",
                dev, myhost, af);
          done = 1;
          break;
      }

#if defined(HAVE_GETIFADDRS)

if2ip_result_t Curl_if2ip(int af,
#ifdef ENABLE_IPV6
                          unsigned int remote_scope,
                          unsigned int local_scope_id,
#endif
                          const char *interf,
                          char *buf, int buf_size)
{
  struct ifaddrs *iface, *head;
  if2ip_result_t res = IF2IP_NOT_FOUND;

#if defined(ENABLE_IPV6) && \
    !defined(HAVE_SOCKADDR_IN6_SIN6_SCOPE_ID)
  (void) local_scope_id;
#endif

  if(getifaddrs(&head) >= 0) {
    for(iface = head; iface != NULL; iface = iface->ifa_next) {
      if(iface->ifa_addr) {
        if(iface->ifa_addr->sa_family == af) {
          if(strcasecompare(iface->ifa_name, interf)) {
            void *addr;
            const char *ip;
            char scope[12] = "";
            char ipstr[64];

#ifdef IP_BIND_ADDRESS_NO_PORT
  (void)setsockopt(sockfd, SOL_IP, IP_BIND_ADDRESS_NO_PORT, &on, sizeof(on));
#endif
  for(;;) {
    if(bind(sockfd, sock, sizeof_sa) >= 0) {
      /* we succeeded to bind */
      struct Curl_sockaddr_storage add;
      curl_socklen_t size = sizeof(add);
      memset(&add, 0, sizeof(struct Curl_sockaddr_storage));
      if(getsockname(sockfd, (struct sockaddr *) &add, &size) < 0) {
        char buffer[STRERROR_LEN];
        data->state.os_errno = error = SOCKERRNO;
        failf(data, "getsockname() failed with errno %d: %s",
              error, Curl_strerror(error, buffer, sizeof(buffer)));
        return CURLE_INTERFACE_FAILED;
      }
      infof(data, "Local port: %hu", port);
      conn->bits.bound = TRUE;
      return CURLE_OK;
    }