path: root/lib/connect.c

/***************************************************************************
 *                                  _   _ ____  _
 *  Project                     ___| | | |  _ \| |
 *                             / __| | | | |_) | |
 *                            | (__| |_| |  _ <| |___
 *                             \___|\___/|_| \_\_____|
 *
 * Copyright (C) 1998 - 2021, Daniel Stenberg, <daniel@haxx.se>, et al.
 *
 * This software is licensed as described in the file COPYING, which
 * you should have received as part of this distribution. The terms
 * are also available at https://curl.se/docs/copyright.html.
 *
 * You may opt to use, copy, modify, merge, publish, distribute and/or sell
 * copies of the Software, and permit persons to whom the Software is
 * furnished to do so, under the terms of the COPYING file.
 *
 * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 * KIND, either express or implied.
 *
 ***************************************************************************/

#include "curl_setup.h"

#ifdef HAVE_NETINET_IN_H
#include <netinet/in.h> /* <netinet/tcp.h> may need it */
#endif
#ifdef HAVE_SYS_UN_H
#include <sys/un.h> /* for sockaddr_un */
#endif
#ifdef HAVE_LINUX_TCP_H
#include <linux/tcp.h>
#elif defined(HAVE_NETINET_TCP_H)
#include <netinet/tcp.h>
#endif
#ifdef HAVE_SYS_IOCTL_H
#include <sys/ioctl.h>
#endif
#ifdef HAVE_NETDB_H
#include <netdb.h>
#endif
#ifdef HAVE_FCNTL_H
#include <fcntl.h>
#endif
#ifdef HAVE_ARPA_INET_H
#include <arpa/inet.h>
#endif

#if (defined(HAVE_IOCTL_FIONBIO) && defined(NETWARE))
#include <sys/filio.h>
#endif
#ifdef NETWARE
#undef in_addr_t
#define in_addr_t unsigned long
#endif
#ifdef __VMS
#include <in.h>
#include <inet.h>
#endif

#include "urldata.h"
#include "sendf.h"
#include "if2ip.h"
#include "strerror.h"
#include "connect.h"
#include "select.h"
#include "url.h" /* for Curl_safefree() */
#include "multiif.h"
#include "sockaddr.h" /* required for Curl_sockaddr_storage */
#include "inet_ntop.h"
#include "inet_pton.h"
#include "vtls/vtls.h" /* for Curl_ssl_check_cxn() */
#include "progress.h"
#include "warnless.h"
#include "conncache.h"
#include "multihandle.h"
#include "version_win32.h"
#include "quic.h"
#include "socks.h"

/* The last 3 #include files should be in this order */
#include "curl_printf.h"
#include "curl_memory.h"
#include "memdebug.h"

static bool verifyconnect(curl_socket_t sockfd, int *error);

#if defined(__DragonFly__) || defined(HAVE_WINSOCK_H)
/* DragonFlyBSD and Windows use millisecond units */
#define KEEPALIVE_FACTOR(x) (x *= 1000)
#else
#define KEEPALIVE_FACTOR(x)
#endif

#if defined(HAVE_WINSOCK2_H) && !defined(SIO_KEEPALIVE_VALS)
#define SIO_KEEPALIVE_VALS    _WSAIOW(IOC_VENDOR,4)

struct tcp_keepalive {
  u_long onoff;
  u_long keepalivetime;
  u_long keepaliveinterval;
};
#endif

static void
tcpkeepalive(struct Curl_easy *data,
             curl_socket_t sockfd)
{
  int optval = data->set.tcp_keepalive?1:0;

  /* only set IDLE and INTVL if setting KEEPALIVE is successful */
  if(setsockopt(sockfd, SOL_SOCKET, SO_KEEPALIVE,
        (void *)&optval, sizeof(optval)) < 0) {
    infof(data, "Failed to set SO_KEEPALIVE on fd %d\n", sockfd);
  }
  else {
#if defined(SIO_KEEPALIVE_VALS)
    struct tcp_keepalive vals;
    DWORD dummy;
    vals.onoff = 1;
    optval = curlx_sltosi(data->set.tcp_keepidle);
    KEEPALIVE_FACTOR(optval);
    vals.keepalivetime = optval;
    optval = curlx_sltosi(data->set.tcp_keepintvl);
    KEEPALIVE_FACTOR(optval);
    vals.keepaliveinterval = optval;
    if(WSAIoctl(sockfd, SIO_KEEPALIVE_VALS, (LPVOID) &vals, sizeof(vals),
                NULL, 0, &dummy, NULL, NULL) != 0) {
      infof(data, "Failed to set SIO_KEEPALIVE_VALS on fd %d: %d\n",
            (int)sockfd, WSAGetLastError());
    }
#else
#ifdef TCP_KEEPIDLE
    optval = curlx_sltosi(data->set.tcp_keepidle);
    KEEPALIVE_FACTOR(optval);
    if(setsockopt(sockfd, IPPROTO_TCP, TCP_KEEPIDLE,
          (void *)&optval, sizeof(optval)) < 0) {
      infof(data, "Failed to set TCP_KEEPIDLE on fd %d\n", sockfd);
    }
#endif
#ifdef TCP_KEEPINTVL
    optval = curlx_sltosi(data->set.tcp_keepintvl);
    KEEPALIVE_FACTOR(optval);
    if(setsockopt(sockfd, IPPROTO_TCP, TCP_KEEPINTVL,
          (void *)&optval, sizeof(optval)) < 0) {
      infof(data, "Failed to set TCP_KEEPINTVL on fd %d\n", sockfd);
    }
#endif
#ifdef TCP_KEEPALIVE
    /* Mac OS X style */
    optval = curlx_sltosi(data->set.tcp_keepidle);
    KEEPALIVE_FACTOR(optval);
    if(setsockopt(sockfd, IPPROTO_TCP, TCP_KEEPALIVE,
          (void *)&optval, sizeof(optval)) < 0) {
      infof(data, "Failed to set TCP_KEEPALIVE on fd %d\n", sockfd);
    }
#endif
#endif
  }
}

static CURLcode
singleipconnect(struct Curl_easy *data,
                struct connectdata *conn,
                const struct Curl_addrinfo *ai, /* start connecting to this */
                int tempindex);          /* 0 or 1 among the temp ones */

/*
 * Curl_timeleft() returns the amount of milliseconds left allowed for the
 * transfer/connection. If the value is 0, there's no timeout (ie there's
 * infinite time left). If the value is negative, the timeout time has already
 * elapsed.
 *
 * If 'nowp' is non-NULL, it points to the current time.
 * 'duringconnect' is FALSE if not during a connect, as then of course the
 * connect timeout is not taken into account!
 *
 * @unittest: 1303
 */

#define TIMEOUT_CONNECT 1
#define TIMEOUT_MAXTIME 2

timediff_t Curl_timeleft(struct Curl_easy *data,
                         struct curltime *nowp,
                         bool duringconnect)
{
  unsigned int timeout_set = 0;
  timediff_t connect_timeout_ms = 0;
  timediff_t maxtime_timeout_ms = 0;
  timediff_t timeout_ms = 0;
  struct curltime now;

  /* The duration of a connect and the total transfer are calculated from two
     different time-stamps. It can end up with the total timeout being reached
     before the connect timeout expires and we must acknowledge whichever
     timeout that is reached first. The total timeout is set per entire
     operation, while the connect timeout is set per connect. */

  if(data->set.timeout > 0) {
    timeout_set = TIMEOUT_MAXTIME;
    maxtime_timeout_ms = data->set.timeout;
  }
  if(duringconnect) {
    timeout_set |= TIMEOUT_CONNECT;
    connect_timeout_ms = (data->set.connecttimeout > 0) ?
      data->set.connecttimeout : DEFAULT_CONNECT_TIMEOUT;
  }
  if(!timeout_set)
    /* no timeout  */
    return 0;

  if(!nowp) {
    now = Curl_now();
    nowp = &now;
  }

  if(timeout_set & TIMEOUT_MAXTIME) {
    maxtime_timeout_ms -= Curl_timediff(*nowp, data->progress.t_startop);
    timeout_ms = maxtime_timeout_ms;
  }

  if(timeout_set & TIMEOUT_CONNECT) {
    connect_timeout_ms -= Curl_timediff(*nowp, data->progress.t_startsingle);

    if(!(timeout_set & TIMEOUT_MAXTIME) ||
       (connect_timeout_ms < maxtime_timeout_ms))
      timeout_ms = connect_timeout_ms;
  }

  if(!timeout_ms)
    /* avoid returning 0 as that means no timeout! */
    return -1;

  return timeout_ms;
}

static CURLcode bindlocal(struct Curl_easy *data,
                          curl_socket_t sockfd, int af, unsigned int scope)
{
  struct connectdata *conn = data->conn;
  struct Curl_sockaddr_storage sa;
  struct sockaddr *sock = (struct sockaddr *)&sa;  /* bind to this address */
  curl_socklen_t sizeof_sa = 0; /* size of the data sock points to */
  struct sockaddr_in *si4 = (struct sockaddr_in *)&sa;
#ifdef ENABLE_IPV6
  struct sockaddr_in6 *si6 = (struct sockaddr_in6 *)&sa;
#endif

  struct Curl_dns_entry *h = NULL;
  unsigned short port = data->set.localport; /* use this port number, 0 for
                                                "random" */
  /* how many port numbers to try to bind to, increasing one at a time */
  int portnum = data->set.localportrange;
  const char *dev = data->set.str[STRING_DEVICE];
  int error;
#ifdef IP_BIND_ADDRESS_NO_PORT
  int on = 1;
#endif

  /*************************************************************
   * 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, scope, conn->scope_id, 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\n",
                dev, myhost, af);
          done = 1;
          break;
      }
    }
    if(!is_interface) {
      /*
       * This was not an interface, resolve the name as a host name
       * or IP number
       *
       * Temporarily force name resolution to use only the address type
       * of the connection. The resolve functions should really be changed
       * to take a type parameter instead.
       */
      unsigned char ipver = conn->ip_version;
      int rc;

      if(af == AF_INET)
        conn->ip_version = CURL_IPRESOLVE_V4;
#ifdef ENABLE_IPV6
      else if(af == AF_INET6)
        conn->ip_version = CURL_IPRESOLVE_V6;
#endif

      rc = Curl_resolv(data, dev, 0, FALSE, &h);
      if(rc == CURLRESOLV_PENDING)
        (void)Curl_resolver_wait_resolv(data, &h);
      conn->ip_version = ipver;

      if(h) {
        /* convert the resolved address, sizeof myhost >= INET_ADDRSTRLEN */
        Curl_printable_address(h->addr, myhost, sizeof(myhost));
        infof(data, "Name '%s' family %i resolved to '%s' family %i\n",
              dev, af, myhost, h->addr->ai_family);
        Curl_resolv_unlock(data, h);
        if(af != h->addr->ai_family) {
          /* bad IP version combo, signal the caller to try another address
             family if available */
          return CURLE_UNSUPPORTED_PROTOCOL;
        }
        done = 1;
      }
      else {
        /*
         * provided dev was no interface (or interfaces are not supported
         * e.g. solaris) no ip address and no domain we fail here
         */
        done = -1;
      }
    }

    if(done > 0) {
#ifdef ENABLE_IPV6
      /* IPv6 address */
      if(af == AF_INET6) {
#ifdef HAVE_SOCKADDR_IN6_SIN6_SCOPE_ID
        char *scope_ptr = strchr(myhost, '%');
        if(scope_ptr)
          *(scope_ptr++) = 0;
#endif
        if(Curl_inet_pton(AF_INET6, myhost, &si6->sin6_addr) > 0) {
          si6->sin6_family = AF_INET6;
          si6->sin6_port = htons(port);
#ifdef HAVE_SOCKADDR_IN6_SIN6_SCOPE_ID
          if(scope_ptr)
            /* The "myhost" string either comes from Curl_if2ip or from
               Curl_printable_address. The latter returns only numeric scope
               IDs and the former returns none at all.  So the scope ID, if
               present, is known to be numeric */
            si6->sin6_scope_id = atoi(scope_ptr);
#endif
        }
        sizeof_sa = sizeof(struct sockaddr_in6);
      }
      else
#endif
      /* IPv4 address */
      if((af == AF_INET) &&
         (Curl_inet_pton(AF_INET, myhost, &si4->sin_addr) > 0)) {
        si4->sin_family = AF_INET;
        si4->sin_port = htons(port);
        sizeof_sa = sizeof(struct sockaddr_in);
      }
    }

    if(done < 1) {
      /* errorbuf is set false so failf will overwrite any message already in
         the error buffer, so the user receives this error message instead of a
         generic resolve error. */
      data->state.errorbuf = FALSE;
      failf(data, "Couldn't bind to '%s'", dev);
      return CURLE_INTERFACE_FAILED;
    }
  }
  else {
    /* no device was given, prepare sa to match af's needs */
#ifdef ENABLE_IPV6
    if(af == AF_INET6) {
      si6->sin6_family = AF_INET6;
      si6->sin6_port = htons(port);
      sizeof_sa = sizeof(struct sockaddr_in6);
    }
    else
#endif
    if(af == AF_INET) {
      si4->sin_family = AF_INET;
      si4->sin_port = htons(port);
      sizeof_sa = sizeof(struct sockaddr_in);
    }
  }
#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\n", port);
      conn->bits.bound = TRUE;
      return CURLE_OK;
    }

    if(--portnum > 0) {
      infof(data, "Bind to local port %hu failed, trying next\n", port);
      port++; /* try next port */
      /* We re-use/clobber the port variable here below */
      if(sock->sa_family == AF_INET)
        si4->sin_port = ntohs(port);
#ifdef ENABLE_IPV6
      else
        si6->sin6_port = ntohs(port);
#endif
    }
    else
      break;
  }
  {
    char buffer[STRERROR_LEN];
    data->state.os_errno = error = SOCKERRNO;
    failf(data, "bind failed with errno %d: %s",
          error, Curl_strerror(error, buffer, sizeof(buffer)));
  }

  return CURLE_INTERFACE_FAILED;
}

/*
 * verifyconnect() returns TRUE if the connect really has happened.
 */
static bool verifyconnect(curl_socket_t sockfd, int *error)
{
  bool rc = TRUE;
#ifdef SO_ERROR
  int err = 0;
  curl_socklen_t errSize = sizeof(err);

#ifdef WIN32
  /*
   * In October 2003 we effectively nullified this function on Windows due to
   * problems with it using all CPU in multi-threaded cases.
   *
   * In May 2004, we bring it back to offer more info back on connect failures.
   * Gisle Vanem could reproduce the former problems with this function, but
   * could avoid them by adding this SleepEx() call below:
   *
   *    "I don't have Rational Quantify, but the hint from his post was
   *    ntdll::NtRemoveIoCompletion(). So I'd assume the SleepEx (or maybe
   *    just Sleep(0) would be enough?) would release whatever
   *    mutex/critical-section the ntdll call is waiting on.
   *
   *    Someone got to verify this on Win-NT 4.0, 2000."
   */

#ifdef _WIN32_WCE
  Sleep(0);
#else
  SleepEx(0, FALSE);
#endif

#endif

  if(0 != getsockopt(sockfd, SOL_SOCKET, SO_ERROR, (void *)&err, &errSize))
    err = SOCKERRNO;
#ifdef _WIN32_WCE
  /* Old WinCE versions don't support SO_ERROR */
  if(WSAENOPROTOOPT == err) {
    SET_SOCKERRNO(0);
    err = 0;
  }
#endif
#if defined(EBADIOCTL) && defined(__minix)
  /* Minix 3.1.x doesn't support getsockopt on UDP sockets */
  if(EBADIOCTL == err) {
    SET_SOCKERRNO(0);
    err = 0;
  }
#endif
  if((0 == err) || (EISCONN == err))
    /* we are connected, awesome! */
    rc = TRUE;
  else
    /* This wasn't a successful connect */
    rc = FALSE;
  if(error)
    *error = err;
#else
  (void)sockfd;
  if(error)
    *error = SOCKERRNO;
#endif
  return rc;
}

/* update tempaddr[tempindex] (to the next entry), makes sure to stick
   to the correct family */
static struct Curl_addrinfo *ainext(struct connectdata *conn,
                                    int tempindex,
                                    bool next) /* use next entry? */
{
  struct Curl_addrinfo *ai = conn->tempaddr[tempindex];
  if(ai && next)
    ai = ai->ai_next;
  while(ai && (ai->ai_family != conn->tempfamily[tempindex]))
    ai = ai->ai_next;
  conn->tempaddr[tempindex] = ai;
  return ai;
}

/* Used within the multi interface. Try next IP address, returns error if no
   more address exists or error */
static CURLcode trynextip(struct Curl_easy *data,
                          struct connectdata *conn,
                          int sockindex,
                          int tempindex)
{
  CURLcode result = CURLE_COULDNT_CONNECT;

  /* First clean up after the failed socket.
     Don't close it yet to ensure that the next IP's socket gets a different
     file descriptor, which can prevent bugs when the curl_multi_socket_action
     interface is used with certain select() replacements such as kqueue. */
  curl_socket_t fd_to_close = conn->tempsock[tempindex];
  conn->tempsock[tempindex] = CURL_SOCKET_BAD;

  if(sockindex == FIRSTSOCKET) {
    struct Curl_addrinfo *ai = conn->tempaddr[tempindex];

    while(ai) {
      if(ai) {
        result = singleipconnect(data, conn, ai, tempindex);
        if(result == CURLE_COULDNT_CONNECT) {
          ai = ainext(conn, tempindex, TRUE);
          continue;
        }
      }
      break;
    }
  }

  if(fd_to_close != CURL_SOCKET_BAD)
    Curl_closesocket(data, conn, fd_to_close);

  return result;
}

/* Copies connection info into the transfer handle to make it available when
   the transfer handle is no longer associated with the connection. */
void Curl_persistconninfo(struct Curl_easy *data, struct connectdata *conn,
                          char *local_ip, int local_port)
{
  memcpy(data->info.conn_primary_ip, conn->primary_ip, MAX_IPADR_LEN);
  if(local_ip && local_ip[0])
    memcpy(data->info.conn_local_ip, local_ip, MAX_IPADR_LEN);
  else
    data->info.conn_local_ip[0] = 0;
  data->info.conn_scheme = conn->handler->scheme;
  data->info.conn_protocol = conn->handler->protocol;
  data->info.conn_primary_port = conn->port;
  data->info.conn_local_port = local_port;
}

/* retrieves ip address and port from a sockaddr structure.
   note it calls Curl_inet_ntop which sets errno on fail, not SOCKERRNO. */
bool Curl_addr2string(struct sockaddr *sa, curl_socklen_t salen,
                      char *addr, int *port)
{
  struct sockaddr_in *si = NULL;
#ifdef ENABLE_IPV6
  struct sockaddr_in6 *si6 = NULL;
#endif
#if defined(HAVE_SYS_UN_H) && defined(AF_UNIX)
  struct sockaddr_un *su = NULL;
#else
  (void)salen;
#endif

  switch(sa->sa_family) {
    case AF_INET:
      si = (struct sockaddr_in *)(void *) sa;
      if(Curl_inet_ntop(sa->sa_family, &si->sin_addr,
                        addr, MAX_IPADR_LEN)) {
        unsigned short us_port = ntohs(si->sin_port);
        *port = us_port;
        return TRUE;
      }
      break;
#ifdef ENABLE_IPV6
    case AF_INET6:
      si6 = (struct sockaddr_in6 *)(void *) sa;
      if(Curl_inet_ntop(sa->sa_family, &si6->sin6_addr,
                        addr, MAX_IPADR_LEN)) {
        unsigned short us_port = ntohs(si6->sin6_port);
        *port = us_port;
        return TRUE;
      }
      break;
#endif
#if defined(HAVE_SYS_UN_H) && defined(AF_UNIX)
    case AF_UNIX:
      if(salen > (curl_socklen_t)sizeof(CURL_SA_FAMILY_T)) {
        su = (struct sockaddr_un*)sa;
        msnprintf(addr, MAX_IPADR_LEN, "%s", su->sun_path);
      }
      else
        addr[0] = 0; /* socket with no name */
      *port = 0;
      return TRUE;
#endif
    default:
      break;
  }

  addr[0] = '\0';
  *port = 0;
  errno = EAFNOSUPPORT;
  return FALSE;
}

/* retrieves the start/end point information of a socket of an established
   connection */
void Curl_conninfo_remote(struct Curl_easy *data,
                          struct connectdata *conn, curl_socket_t sockfd)
{
#ifdef HAVE_GETPEERNAME
  char buffer[STRERROR_LEN];
  struct Curl_sockaddr_storage ssrem;
  curl_socklen_t plen;
  int port;
  plen = sizeof(struct Curl_sockaddr_storage);
  memset(&ssrem, 0, sizeof(ssrem));
  if(getpeername(sockfd, (struct sockaddr*) &ssrem, &plen)) {
    int error = SOCKERRNO;
    failf(data, "getpeername() failed with errno %d: %s",
          error, Curl_strerror(error, buffer, sizeof(buffer)));
    return;
  }
  if(!Curl_addr2string((struct sockaddr*)&ssrem, plen,
                       conn->primary_ip, &port)) {
    failf(data, "ssrem inet_ntop() failed with errno %d: %s",
          errno, Curl_strerror(errno, buffer, sizeof(buffer)));
    return;
  }
#else
  (void)data;
  (void)conn;
  (void)sockfd;
#endif
}

/* retrieves the start/end point information of a socket of an established
   connection */
void Curl_conninfo_local(struct Curl_easy *data, curl_socket_t sockfd,
                         char *local_ip, int *local_port)
{
#ifdef HAVE_GETSOCKNAME
  char buffer[STRERROR_LEN];
  struct Curl_sockaddr_storage ssloc;
  curl_socklen_t slen;
  slen = sizeof(struct Curl_sockaddr_storage);
  memset(&ssloc, 0, sizeof(ssloc));
  if(getsockname(sockfd, (struct sockaddr*) &ssloc, &slen)) {
    int error = SOCKERRNO;
    failf(data, "getsockname() failed with errno %d: %s",
          error, Curl_strerror(error, buffer, sizeof(buffer)));
    return;
  }
  if(!Curl_addr2string((struct sockaddr*)&ssloc, slen,
                       local_ip, local_port)) {
    failf(data, "ssloc inet_ntop() failed with errno %d: %s",
          errno, Curl_strerror(errno, buffer, sizeof(buffer)));
    return;
  }
#else
  (void)data;
  (void)sockfd;
  (void)local_ip;
  (void)local_port;
#endif
}

/* retrieves the start/end point information of a socket of an established
   connection */
void Curl_updateconninfo(struct Curl_easy *data, struct connectdata *conn,
                         curl_socket_t sockfd)
{
  /* 'local_ip' and 'local_port' get filled with local's numerical
     ip address and port number whenever an outgoing connection is
     **established** from the primary socket to a remote address. */
  char local_ip[MAX_IPADR_LEN] = "";
  int local_port = -1;

  if(conn->transport == TRNSPRT_TCP) {
    if(!conn->bits.reuse && !conn->bits.tcp_fastopen) {
      Curl_conninfo_remote(data, conn, sockfd);
      Curl_conninfo_local(data, sockfd, local_ip, &local_port);
    }
  } /* end of TCP-only section */

  /* persist connection info in session handle */
  Curl_persistconninfo(data, conn, local_ip, local_port);
}

/* After a TCP connection to the proxy has been verified, this function does
   the next magic steps. If 'done' isn't set TRUE, it is not done yet and
   must be called again.

   Note: this function's sub-functions call failf()

*/
static CURLcode connect_SOCKS(struct Curl_easy *data, int sockindex,
                              bool *done)
{
  CURLcode result = CURLE_OK;
#ifndef CURL_DISABLE_PROXY
  CURLproxycode pxresult = CURLPX_OK;
  struct connectdata *conn = data->conn;
  if(conn->bits.socksproxy) {
    /* for the secondary socket (FTP), use the "connect to host"
     * but ignore the "connect to port" (use the secondary port)
     */
    const char * const host =
      conn->bits.httpproxy ?
      conn->http_proxy.host.name :
      conn->bits.conn_to_host ?
      conn->conn_to_host.name :
      sockindex == SECONDARYSOCKET ?
      conn->secondaryhostname : conn->host.name;
    const int port =
      conn->bits.httpproxy ? (int)conn->http_proxy.port :
      sockindex == SECONDARYSOCKET ? conn->secondary_port :
      conn->bits.conn_to_port ? conn->conn_to_port :
      conn->remote_port;
    switch(conn->socks_proxy.proxytype) {
    case CURLPROXY_SOCKS5:
    case CURLPROXY_SOCKS5_HOSTNAME:
      pxresult = Curl_SOCKS5(conn->socks_proxy.user, conn->socks_proxy.passwd,
                             host, port, sockindex, data, done);
      break;

    case CURLPROXY_SOCKS4:
    case CURLPROXY_SOCKS4A:
      pxresult = Curl_SOCKS4(conn->socks_proxy.user, host, port, sockindex,
                             data, done);
      break;

    default:
      failf(data, "unknown proxytype option given");
      result = CURLE_COULDNT_CONNECT;
    } /* switch proxytype */
    if(pxresult) {
      result = CURLE_PROXY;
      data->info.pxcode = pxresult;
    }
  }
  else
#else
    (void)data;
    (void)sockindex;
#endif /* CURL_DISABLE_PROXY */
    *done = TRUE; /* no SOCKS proxy, so consider us connected */

  return result;
}

/*
 * post_SOCKS() is called after a successful connect to the peer, which
 * *could* be a SOCKS proxy
 */
static void post_SOCKS(struct Curl_easy *data,
                       struct connectdata *conn,
                       int sockindex,
                       bool *connected)
{
  conn->bits.tcpconnect[sockindex] = TRUE;

  *connected = TRUE;
  if(sockindex == FIRSTSOCKET)
    Curl_pgrsTime(data, TIMER_CONNECT); /* connect done */
  Curl_updateconninfo(data, conn, conn->sock[sockindex]);
  Curl_verboseconnect(data, conn);
  data->info.numconnects++; /* to track the number of connections made */
}

/*
 * Curl_is_connected() checks if the socket has connected.
 */

CURLcode Curl_is_connected(struct Curl_easy *data,
                           struct connectdata *conn,
                           int sockindex,
                           bool *connected)
{
  CURLcode result = CURLE_OK;
  timediff_t allow;
  int error = 0;
  struct curltime now;
  int rc = 0;
  unsigned int i;

  DEBUGASSERT(sockindex >= FIRSTSOCKET && sockindex <= SECONDARYSOCKET);

  *connected = FALSE; /* a very negative world view is best */

  if(conn->bits.tcpconnect[sockindex]) {
    /* we are connected already! */
    *connected = TRUE;
    return CURLE_OK;
  }

  now = Curl_now();

  /* figure out how long time we have left to connect */
  allow = Curl_timeleft(data, &now, TRUE);

  if(allow < 0) {
    /* time-out, bail out, go home */
    failf(data, "Connection time-out");
    return CURLE_OPERATION_TIMEDOUT;
  }

  if(SOCKS_STATE(conn->cnnct.state)) {
    /* still doing SOCKS */
    result = connect_SOCKS(data, sockindex, connected);
    if(!result && *connected)
      post_SOCKS(data, conn, sockindex, connected);
    return result;
  }

  for(i = 0; i<2; i++) {
    const int other = i ^ 1;
    if(conn->tempsock[i] == CURL_SOCKET_BAD)
      continue;
    error = 0;
#ifdef ENABLE_QUIC
    if(conn->transport == TRNSPRT_QUIC) {
      result = Curl_quic_is_connected(data, conn, i, connected);
      if(!result && *connected) {
        /* use this socket from now on */
        conn->sock[sockindex] = conn->tempsock[i];
        conn->ip_addr = conn->tempaddr[i];
        conn->tempsock[i] = CURL_SOCKET_BAD;
        post_SOCKS(data, conn, sockindex, connected);
        connkeep(conn, "HTTP/3 default");
        return CURLE_OK;
      }
      if(result) {
        conn->tempsock[i] = CURL_SOCKET_BAD;
        error = SOCKERRNO;
      }
    }
    else
#endif
    {
#ifdef mpeix
      /* Call this function once now, and ignore the results. We do this to
         "clear" the error state on the socket so that we can later read it
         reliably. This is reported necessary on the MPE/iX operating
         system. */
      (void)verifyconnect(conn->tempsock[i], NULL);
#endif

      /* check socket for connect */
      rc = SOCKET_WRITABLE(conn->tempsock[i], 0);
    }

    if(rc == 0) { /* no connection yet */
      if(Curl_timediff(now, conn->connecttime) >=
         conn->timeoutms_per_addr[i]) {
        infof(data, "After %" CURL_FORMAT_TIMEDIFF_T
              "ms connect time, move on!\n", conn->timeoutms_per_addr[i]);
        error = ETIMEDOUT;
      }

      /* should we try another protocol family? */
      if(i == 0 && !conn->bits.parallel_connect &&
         (Curl_timediff(now, conn->connecttime) >=
          data->set.happy_eyeballs_timeout)) {
        conn->bits.parallel_connect = TRUE; /* starting now */
        trynextip(data, conn, sockindex, 1);
      }
    }
    else if(rc == CURL_CSELECT_OUT || conn->bits.tcp_fastopen) {
      if(verifyconnect(conn->tempsock[i], &error)) {
        /* we are connected with TCP, awesome! */

        /* use this socket from now on */
        conn->sock[sockindex] = conn->tempsock[i];
        conn->ip_addr = conn->tempaddr[i];
        conn->tempsock[i] = CURL_SOCKET_BAD;
#ifdef ENABLE_IPV6
        conn->bits.ipv6 = (conn->ip_addr->ai_family == AF_INET6)?TRUE:FALSE;
#endif

        /* close the other socket, if open */
        if(conn->tempsock[other] != CURL_SOCKET_BAD) {
          Curl_closesocket(data, conn, conn->tempsock[other]);
          conn->tempsock[other] = CURL_SOCKET_BAD;
        }

        /* see if we need to kick off any SOCKS proxy magic once we
           connected */
        result = connect_SOCKS(data, sockindex, connected);
        if(result || !*connected)
          return result;

        post_SOCKS(data, conn, sockindex, connected);

        return CURLE_OK;
      }
    }
    else if(rc & CURL_CSELECT_ERR) {
      (void)verifyconnect(conn->tempsock[i], &error);
    }

    /*
     * The connection failed here, we should attempt to connect to the "next
     * address" for the given host. But first remember the latest error.
     */
    if(error) {
      data->state.os_errno = error;
      SET_SOCKERRNO(error);
      if(conn->tempaddr[i]) {
        CURLcode status;
#ifndef CURL_DISABLE_VERBOSE_STRINGS
        char ipaddress[MAX_IPADR_LEN];
        char buffer[STRERROR_LEN];
        Curl_printable_address(conn->tempaddr[i], ipaddress,
                               sizeof(ipaddress));
        infof(data, "connect to %s port %ld failed: %s\n",
              ipaddress, conn->port,
              Curl_strerror(error, buffer, sizeof(buffer)));
#endif

        conn->timeoutms_per_addr[i] = conn->tempaddr[i]->ai_next == NULL ?
          allow : allow / 2;
        ainext(conn, i, TRUE);
        status = trynextip(data, conn, sockindex, i);
        if((status != CURLE_COULDNT_CONNECT) ||
           conn->tempsock[other] == CURL_SOCKET_BAD)
          /* the last attempt failed and no other sockets remain open */
          result = status;
      }
    }
  }

  if(result &&
     (conn->tempsock[0] == CURL_SOCKET_BAD) &&
     (conn->tempsock[1] == CURL_SOCKET_BAD)) {
    /* no more addresses to try */
    const char *hostname;
    char buffer[STRERROR_LEN];

    /* if the first address family runs out of addresses to try before the
       happy eyeball timeout, go ahead and try the next family now */
    result = trynextip(data, conn, sockindex, 1);
    if(!result)
      return result;

#ifndef CURL_DISABLE_PROXY
    if(conn->bits.socksproxy)
      hostname = conn->socks_proxy.host.name;
    else if(conn->bits.httpproxy)
      hostname = conn->http_proxy.host.name;
    else
#endif
      if(conn->bits.conn_to_host)
        hostname = conn->conn_to_host.name;
    else
      hostname = conn->host.name;

    failf(data, "Failed to connect to %s port %ld: %s",
          hostname, conn->port,
          Curl_strerror(error, buffer, sizeof(buffer)));

    Curl_quic_disconnect(data, conn, 0);
    Curl_quic_disconnect(data, conn, 1);

#ifdef WSAETIMEDOUT
    if(WSAETIMEDOUT == data->state.os_errno)
      result = CURLE_OPERATION_TIMEDOUT;
#elif defined(ETIMEDOUT)
    if(ETIMEDOUT == data->state.os_errno)
      result = CURLE_OPERATION_TIMEDOUT;
#endif
  }
  else
    result = CURLE_OK; /* still trying */

  return result;
}

static void tcpnodelay(struct Curl_easy *data, curl_socket_t sockfd)
{
#if defined(TCP_NODELAY)
  curl_socklen_t onoff = (curl_socklen_t) 1;
  int level = IPPROTO_TCP;
#if !defined(CURL_DISABLE_VERBOSE_STRINGS)
  char buffer[STRERROR_LEN];
#else
  (void) data;
#endif

  if(setsockopt(sockfd, level, TCP_NODELAY, (void *)&onoff,
                sizeof(onoff)) < 0)
    infof(data, "Could not set TCP_NODELAY: %s\n",
          Curl_strerror(SOCKERRNO, buffer, sizeof(buffer)));
#else
  (void)data;
  (void)sockfd;
#endif
}

#ifdef SO_NOSIGPIPE
/* The preferred method on Mac OS X (10.2 and later) to prevent SIGPIPEs when
   sending data to a dead peer (instead of relying on the 4th argument to send
   being MSG_NOSIGNAL). Possibly also existing and in use on other BSD
   systems? */
static void nosigpipe(struct Curl_easy *data,
                      curl_socket_t sockfd)
{
  int onoff = 1;
  if(setsockopt(sockfd, SOL_SOCKET, SO_NOSIGPIPE, (void *)&onoff,
                sizeof(onoff)) < 0) {
    char buffer[STRERROR_LEN];
    infof(data, "Could not set SO_NOSIGPIPE: %s\n",
          Curl_strerror(SOCKERRNO, buffer, sizeof(buffer)));
  }
}
#else
#define nosigpipe(x,y) Curl_nop_stmt
#endif

#ifdef USE_WINSOCK
/* When you run a program that uses the Windows Sockets API, you may
   experience slow performance when you copy data to a TCP server.

   https://support.microsoft.com/kb/823764

   Work-around: Make the Socket Send Buffer Size Larger Than the Program Send
   Buffer Size

   The problem described in this knowledge-base is applied only to pre-Vista
   Windows.  Following function trying to detect OS version and skips
   SO_SNDBUF adjustment for Windows Vista and above.
*/
#define DETECT_OS_NONE 0
#define DETECT_OS_PREVISTA 1
#define DETECT_OS_VISTA_OR_LATER 2

void Curl_sndbufset(curl_socket_t sockfd)
{
  int val = CURL_MAX_WRITE_SIZE + 32;
  int curval = 0;
  int curlen = sizeof(curval);

  static int detectOsState = DETECT_OS_NONE;

  if(detectOsState == DETECT_OS_NONE) {
    if(curlx_verify_windows_version(6, 0, PLATFORM_WINNT,
                                    VERSION_GREATER_THAN_EQUAL))
      detectOsState = DETECT_OS_VISTA_OR_LATER;
    else
      detectOsState = DETECT_OS_PREVISTA;
  }

  if(detectOsState == DETECT_OS_VISTA_OR_LATER)
    return;

  if(getsockopt(sockfd, SOL_SOCKET, SO_SNDBUF, (char *)&curval, &curlen) == 0)
    if(curval > val)
      return;

  setsockopt(sockfd, SOL_SOCKET, SO_SNDBUF, (const char *)&val, sizeof(val));
}
#endif

/*
 * singleipconnect()
 *
 * Note that even on connect fail it returns CURLE_OK, but with 'sock' set to
 * CURL_SOCKET_BAD. Other errors will however return proper errors.
 *
 * singleipconnect() connects to the given IP only, and it may return without
 * having connected.
 */
static CURLcode singleipconnect(struct Curl_easy *data,
                                struct connectdata *conn,
                                const struct Curl_addrinfo *ai,
                                int tempindex)
{
  struct Curl_sockaddr_ex addr;
  int rc = -1;
  int error = 0;
  bool isconnected = FALSE;
  curl_socket_t sockfd;
  CURLcode result;
  char ipaddress[MAX_IPADR_LEN];
  int port;
  bool is_tcp;
#ifdef TCP_FASTOPEN_CONNECT
  int optval = 1;
#endif
  char buffer[STRERROR_LEN];
  curl_socket_t *sockp = &conn->tempsock[tempindex];
  *sockp = CURL_SOCKET_BAD;

  result = Curl_socket(data, ai, &addr, &sockfd);
  if(result)
    return result;

  /* store remote address and port used in this connection attempt */
  if(!Curl_addr2string((struct sockaddr*)&addr.sa_addr, addr.addrlen,
                       ipaddress, &port)) {
    /* malformed address or bug in inet_ntop, try next address */
    failf(data, "sa_addr inet_ntop() failed with errno %d: %s",
          errno, Curl_strerror(errno, buffer, sizeof(buffer)));
    Curl_closesocket(data, conn, sockfd);
    return CURLE_OK;
  }
  infof(data, "  Trying %s:%d...\n", ipaddress, port);

#ifdef ENABLE_IPV6
  is_tcp = (addr.family == AF_INET || addr.family == AF_INET6) &&
    addr.socktype == SOCK_STREAM;
#else
  is_tcp = (addr.family == AF_INET) && addr.socktype == SOCK_STREAM;
#endif
  if(is_tcp && data->set.tcp_nodelay)
    tcpnodelay(data, sockfd);

  nosigpipe(data, sockfd);

  Curl_sndbufset(sockfd);

  if(is_tcp && data->set.tcp_keepalive)
    tcpkeepalive(data, sockfd);

  if(data->set.fsockopt) {
    /* activate callback for setting socket options */
    Curl_set_in_callback(data, true);
    error = data->set.fsockopt(data->set.sockopt_client,
                               sockfd,
                               CURLSOCKTYPE_IPCXN);
    Curl_set_in_callback(data, false);

    if(error == CURL_SOCKOPT_ALREADY_CONNECTED)
      isconnected = TRUE;
    else if(error) {
      Curl_closesocket(data, conn, sockfd); /* close the socket and bail out */
      return CURLE_ABORTED_BY_CALLBACK;
    }
  }

  /* possibly bind the local end to an IP, interface or port */
  if(addr.family == AF_INET
#ifdef ENABLE_IPV6
     || addr.family == AF_INET6
#endif
    ) {
    result = bindlocal(data, sockfd, addr.family,
                       Curl_ipv6_scope((struct sockaddr*)&addr.sa_addr));
    if(result) {
      Curl_closesocket(data, conn, sockfd); /* close socket and bail out */
      if(result == CURLE_UNSUPPORTED_PROTOCOL) {
        /* The address family is not supported on this interface.
           We can continue trying addresses */
        return CURLE_COULDNT_CONNECT;
      }
      return result;
    }
  }

  /* set socket non-blocking */
  (void)curlx_nonblock(sockfd, TRUE);

  conn->connecttime = Curl_now();
  if(conn->num_addr > 1) {
    Curl_expire(data, conn->timeoutms_per_addr[0], EXPIRE_DNS_PER_NAME);
    Curl_expire(data, conn->timeoutms_per_addr[1], EXPIRE_DNS_PER_NAME2);
  }

  /* Connect TCP and QUIC sockets */
  if(!isconnected && (conn->transport != TRNSPRT_UDP)) {
    if(conn->bits.tcp_fastopen) {
#if defined(CONNECT_DATA_IDEMPOTENT) /* Darwin */
#  if defined(HAVE_BUILTIN_AVAILABLE)
      /* while connectx function is available since macOS 10.11 / iOS 9,
         it did not have the interface declared correctly until
         Xcode 9 / macOS SDK 10.13 */
      if(__builtin_available(macOS 10.11, iOS 9.0, tvOS 9.0, watchOS 2.0, *)) {
        sa_endpoints_t endpoints;
        endpoints.sae_srcif = 0;
        endpoints.sae_srcaddr = NULL;
        endpoints.sae_srcaddrlen = 0;
        endpoints.sae_dstaddr = &addr.sa_addr;
        endpoints.sae_dstaddrlen = addr.addrlen;

        rc = connectx(sockfd, &endpoints, SAE_ASSOCID_ANY,
                      CONNECT_RESUME_ON_READ_WRITE | CONNECT_DATA_IDEMPOTENT,
                      NULL, 0, NULL, NULL);
      }
      else {
        rc = connect(sockfd, &addr.sa_addr, addr.addrlen);
      }
#  else
      rc = connect(sockfd, &addr.sa_addr, addr.addrlen);
#  endif /* HAVE_BUILTIN_AVAILABLE */
#elif defined(TCP_FASTOPEN_CONNECT) /* Linux >= 4.11 */
      if(setsockopt(sockfd, IPPROTO_TCP, TCP_FASTOPEN_CONNECT,
                    (void *)&optval, sizeof(optval)) < 0)
        infof(data, "Failed to enable TCP Fast Open on fd %d\n", sockfd);

      rc = connect(sockfd, &addr.sa_addr, addr.addrlen);
#elif defined(MSG_FASTOPEN) /* old Linux */
      if(conn->given->flags & PROTOPT_SSL)
        rc = connect(sockfd, &addr.sa_addr, addr.addrlen);
      else
        rc = 0; /* Do nothing */
#endif
    }
    else {
      rc = connect(sockfd, &addr.sa_addr, addr.addrlen);
    }

    if(-1 == rc)
      error = SOCKERRNO;
#ifdef ENABLE_QUIC
    else if(conn->transport == TRNSPRT_QUIC) {
      /* pass in 'sockfd' separately since it hasn't been put into the
         tempsock array at this point */
      result = Curl_quic_connect(data, conn, sockfd, tempindex,
                                 &addr.sa_addr, addr.addrlen);
      if(result)
        error = SOCKERRNO;
    }
#endif
  }
  else {
    *sockp = sockfd;
    return CURLE_OK;
  }

  if(-1 == rc) {
    switch(error) {
    case EINPROGRESS:
    case EWOULDBLOCK:
#if defined(EAGAIN)
#if (EAGAIN) != (EWOULDBLOCK)
      /* On some platforms EAGAIN and EWOULDBLOCK are the
       * same value, and on others they are different, hence
       * the odd #if
       */
    case EAGAIN:
#endif
#endif
      result = CURLE_OK;
      break;

    default:
      /* unknown error, fallthrough and try another address! */
      infof(data, "Immediate connect fail for %s: %s\n",
            ipaddress, Curl_strerror(error, buffer, sizeof(buffer)));
      data->state.os_errno = error;

      /* connect failed */
      Curl_closesocket(data, conn, sockfd);
      result = CURLE_COULDNT_CONNECT;
    }
  }

  if(!result)
    *sockp = sockfd;

  return result;
}

/*
 * TCP connect to the given host with timeout, proxy or remote doesn't matter.
 * There might be more than one IP address to try out. Fill in the passed
 * pointer with the connected socket.
 */

CURLcode Curl_connecthost(struct Curl_easy *data,
                          struct connectdata *conn,  /* context */
                          const struct Curl_dns_entry *remotehost)
{
  CURLcode result = CURLE_COULDNT_CONNECT;
  int i;
  timediff_t timeout_ms = Curl_timeleft(data, NULL, TRUE);

  if(timeout_ms < 0) {
    /* a precaution, no need to continue if time already is up */
    failf(data, "Connection time-out");
    return CURLE_OPERATION_TIMEDOUT;
  }

  conn->num_addr = Curl_num_addresses(remotehost->addr);
  conn->tempaddr[0] = conn->tempaddr[1] = remotehost->addr;
  conn->tempsock[0] = conn->tempsock[1] = CURL_SOCKET_BAD;

  /* Max time for the next connection attempt */
  conn->timeoutms_per_addr[0] =
    conn->tempaddr[0]->ai_next == NULL ? timeout_ms : timeout_ms / 2;
  conn->timeoutms_per_addr[1] =
    conn->tempaddr[1]->ai_next == NULL ? timeout_ms : timeout_ms / 2;

  conn->tempfamily[0] = conn->tempaddr[0]?
    conn->tempaddr[0]->ai_family:0;
#ifdef ENABLE_IPV6
  conn->tempfamily[1] = conn->tempfamily[0] == AF_INET6 ?
    AF_INET : AF_INET6;
#else
  conn->tempfamily[1] = AF_UNSPEC;
#endif
  ainext(conn, 1, FALSE); /* assigns conn->tempaddr[1] accordingly */

  DEBUGF(infof(data, "family0 == %s, family1 == %s\n",
               conn->tempfamily[0] == AF_INET ? "v4" : "v6",
               conn->tempfamily[1] == AF_INET ? "v4" : "v6"));

  /* get through the list in family order in case of quick failures */
  for(i = 0; (i < 2) && result; i++) {
    while(conn->tempaddr[i]) {
      result = singleipconnect(data, conn, conn->tempaddr[i], i);
      if(!result)
        break;
      ainext(conn, i, TRUE);
    }
  }
  if(result)
    return result;

  Curl_expire(data, data->set.happy_eyeballs_timeout,
              EXPIRE_HAPPY_EYEBALLS);

  return CURLE_OK;
}

struct connfind {
  long id_tofind;
  struct connectdata *found;
};

static int conn_is_conn(struct Curl_easy *data,
                        struct connectdata *conn, void *param)
{
  struct connfind *f = (struct connfind *)param;
  (void)data;
  if(conn->connection_id == f->id_tofind) {
    f->found = conn;
    return 1;
  }
  return 0;
}

/*
 * Used to extract socket and connectdata struct for the most recent
 * transfer on the given Curl_easy.
 *
 * The returned socket will be CURL_SOCKET_BAD in case of failure!
 */
curl_socket_t Curl_getconnectinfo(struct Curl_easy *data,
                                  struct connectdata **connp)
{
  DEBUGASSERT(data);

  /* this works for an easy handle:
   * - that has been used for curl_easy_perform()
   * - that is associated with a multi handle, and whose connection
   *   was detached with CURLOPT_CONNECT_ONLY
   */
  if((data->state.lastconnect_id != -1) && (data->multi_easy || data->multi)) {
    struct connectdata *c;
    struct connfind find;
    find.id_tofind = data->state.lastconnect_id;
    find.found = NULL;

    Curl_conncache_foreach(data, data->multi_easy?
                           &data->multi_easy->conn_cache:
                           &data->multi->conn_cache, &find, conn_is_conn);

    if(!find.found) {
      data->state.lastconnect_id = -1;
      return CURL_SOCKET_BAD;
    }

    c = find.found;
    if(connp)
      /* only store this if the caller cares for it */
      *connp = c;
    return c->sock[FIRSTSOCKET];
  }
  return CURL_SOCKET_BAD;
}

/*
 * Check if a connection seems to be alive.
 */
bool Curl_connalive(struct connectdata *conn)
{
  /* First determine if ssl */
  if(conn->ssl[FIRSTSOCKET].use) {
    /* use the SSL context */
    if(!Curl_ssl_check_cxn(conn))
      return false;   /* FIN received */
  }
/* Minix 3.1 doesn't support any flags on recv; just assume socket is OK */
#ifdef MSG_PEEK
  else if(conn->sock[FIRSTSOCKET] == CURL_SOCKET_BAD)
    return false;
  else {
    /* use the socket */
    char buf;
    if(recv((RECV_TYPE_ARG1)conn->sock[FIRSTSOCKET], (RECV_TYPE_ARG2)&buf,
            (RECV_TYPE_ARG3)1, (RECV_TYPE_ARG4)MSG_PEEK) == 0) {
      return false;   /* FIN received */
    }
  }
#endif
  return true;
}

/*
 * Close a socket.
 *
 * 'conn' can be NULL, beware!
 */
int Curl_closesocket(struct Curl_easy *data, struct connectdata *conn,
                     curl_socket_t sock)
{
  if(conn && conn->fclosesocket) {
    if((sock == conn->sock[SECONDARYSOCKET]) && conn->bits.sock_accepted)
      /* if this socket matches the second socket, and that was created with
         accept, then we MUST NOT call the callback but clear the accepted
         status */
      conn->bits.sock_accepted = FALSE;
    else {
      int rc;
      Curl_multi_closed(data, sock);
      Curl_set_in_callback(data, true);
      rc = conn->fclosesocket(conn->closesocket_client, sock);
      Curl_set_in_callback(data, false);
      return rc;
    }
  }

  if(conn)
    /* tell the multi-socket code about this */
    Curl_multi_closed(data, sock);

  sclose(sock);

  return 0;
}

/*
 * Create a socket based on info from 'conn' and 'ai'.
 *
 * 'addr' should be a pointer to the correct struct to get data back, or NULL.
 * 'sockfd' must be a pointer to a socket descriptor.
 *
 * If the open socket callback is set, used that!
 *
 */
CURLcode Curl_socket(struct Curl_easy *data,
                     const struct Curl_addrinfo *ai,
                     struct Curl_sockaddr_ex *addr,
                     curl_socket_t *sockfd)
{
  struct connectdata *conn = data->conn;
  struct Curl_sockaddr_ex dummy;

  if(!addr)
    /* if the caller doesn't want info back, use a local temp copy */
    addr = &dummy;

  /*
   * The Curl_sockaddr_ex structure is basically libcurl's external API
   * curl_sockaddr structure with enough space available to directly hold
   * any protocol-specific address structures. The variable declared here
   * will be used to pass / receive data to/from the fopensocket callback
   * if this has been set, before that, it is initialized from parameters.
   */

  addr->family = ai->ai_family;
  addr->socktype = (conn->transport == TRNSPRT_TCP) ? SOCK_STREAM : SOCK_DGRAM;
  addr->protocol = conn->transport != TRNSPRT_TCP ? IPPROTO_UDP :
    ai->ai_protocol;
  addr->addrlen = ai->ai_addrlen;

  if(addr->addrlen > sizeof(struct Curl_sockaddr_storage))
     addr->addrlen = sizeof(struct Curl_sockaddr_storage);
  memcpy(&addr->sa_addr, ai->ai_addr, addr->addrlen);

  if(data->set.fopensocket) {
   /*
    * If the opensocket callback is set, all the destination address
    * information is passed to the callback. Depending on this information the
    * callback may opt to abort the connection, this is indicated returning
    * CURL_SOCKET_BAD; otherwise it will return a not-connected socket. When
    * the callback returns a valid socket the destination address information
    * might have been changed and this 'new' address will actually be used
    * here to connect.
    */
    Curl_set_in_callback(data, true);
    *sockfd = data->set.fopensocket(data->set.opensocket_client,
                                    CURLSOCKTYPE_IPCXN,
                                    (struct curl_sockaddr *)addr);
    Curl_set_in_callback(data, false);
  }
  else
    /* opensocket callback not set, so simply create the socket now */
    *sockfd = socket(addr->family, addr->socktype, addr->protocol);

  if(*sockfd == CURL_SOCKET_BAD)
    /* no socket, no connection */
    return CURLE_COULDNT_CONNECT;

  if(conn->transport == TRNSPRT_QUIC) {
    /* QUIC sockets need to be nonblocking */
    (void)curlx_nonblock(*sockfd, TRUE);
  }

#if defined(ENABLE_IPV6) && defined(HAVE_SOCKADDR_IN6_SIN6_SCOPE_ID)
  if(conn->scope_id && (addr->family == AF_INET6)) {
    struct sockaddr_in6 * const sa6 = (void *)&addr->sa_addr;
    sa6->sin6_scope_id = conn->scope_id;
  }
#endif

#if defined(__linux__) && defined(IP_RECVERR)
  if(addr->socktype == SOCK_DGRAM) {
    int one = 1;
    switch(addr->family) {
    case AF_INET:
      (void)setsockopt(*sockfd, SOL_IP, IP_RECVERR, &one, sizeof(one));
      break;
    case AF_INET6:
      (void)setsockopt(*sockfd, SOL_IPV6, IPV6_RECVERR, &one, sizeof(one));
      break;
    }
  }
#endif

  return CURLE_OK;
}

/*
 * Curl_conncontrol() marks streams or connection for closure.
 */
void Curl_conncontrol(struct connectdata *conn,
                      int ctrl /* see defines in header */
#if defined(DEBUGBUILD) && !defined(CURL_DISABLE_VERBOSE_STRINGS)
                      , const char *reason
#endif
  )
{
  /* close if a connection, or a stream that isn't multiplexed. */
  /* This function will be called both before and after this connection is
     associated with a transfer. */
  bool closeit;
  DEBUGASSERT(conn);
#if defined(DEBUGBUILD) && !defined(CURL_DISABLE_VERBOSE_STRINGS)
  (void)reason; /* useful for debugging */
#endif
  closeit = (ctrl == CONNCTRL_CONNECTION) ||
    ((ctrl == CONNCTRL_STREAM) && !(conn->handler->flags & PROTOPT_STREAM));
  if((ctrl == CONNCTRL_STREAM) &&
     (conn->handler->flags & PROTOPT_STREAM))
    ;
  else if((bit)closeit != conn->bits.close) {
    conn->bits.close = closeit; /* the only place in the source code that
                                   should assign this bit */
  }
}

/* Data received can be cached at various levels, so check them all here. */
bool Curl_conn_data_pending(struct connectdata *conn, int sockindex)
{
  int readable;
  DEBUGASSERT(conn);

  if(Curl_ssl_data_pending(conn, sockindex) ||
     Curl_recv_has_postponed_data(conn, sockindex))
    return true;

  readable = SOCKET_READABLE(conn->sock[sockindex], 0);
  return (readable > 0 && (readable & CURL_CSELECT_IN));
}