[u/mdw/putty] / winnet.c

/*
 * Windows networking abstraction.
 *
 * Due to this clean abstraction it was possible
 * to easily implement IPv6 support :)
 *
 * IPv6 patch 1 (27 October 2000) Jeroen Massar <jeroen@unfix.org>
 *  - Preliminary hacked IPv6 support.
 *    - Connecting to IPv6 address (eg fec0:4242:4242:100:2d0:b7ff:fe8f:5d42) works.
 *    - Connecting to IPv6 hostname (eg heaven.ipv6.unfix.org) works.
 *  - Compiles as either IPv4 or IPv6.
 *
 * IPv6 patch 2 (29 October 2000) Jeroen Massar <jeroen@unfix.org>
 *  - When compiled as IPv6 it also allows connecting to IPv4 hosts.
 *  - Added some more documentation.
 *
 * IPv6 patch 3 (18 November 2000) Jeroen Massar <jeroen@unfix.org>
 *  - It now supports dynamically loading the IPv6 resolver dll's.
 *    This way we should be able to distribute one (1) binary
 *    which supports both IPv4 and IPv6.
 *  - getaddrinfo() and getnameinfo() are loaded dynamicaly if possible.
 *  - in6addr_any is defined in this file so we don't need to link to wship6.lib
 *  - The patch is now more unified so that we can still
 *    remove all IPv6 support by undef'ing IPV6.
 *    But where it fallsback to IPv4 it uses the IPv4 code which is already in place...
 *  - Canonical name resolving works.
 *
 * IPv6 patch 4 (07 January 2001) Jeroen Massar <jeroen@unfix.org>
 *  - patch against CVS of today, will be submitted to the bugs list
 *    as a 'cvs diff -u' on Simon's request...
 *
 */

/*
 * Define IPV6 to have IPv6 on-the-fly-loading support.
 * This means that one doesn't have to have an IPv6 stack to use it.
 * But if an IPv6 stack is found it is used with a fallback to IPv4.
 */
/* #define IPV6 1 */

#ifdef IPV6
#include <winsock2.h>
#include <ws2tcpip.h>
#include <tpipv6.h>
#else
#include <winsock.h>
#endif
#include <windows.h>
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

#define DEFINE_PLUG_METHOD_MACROS
#include "putty.h"
#include "network.h"
#include "tree234.h"

struct Socket_tag {
    struct socket_function_table *fn;
    /* the above variable absolutely *must* be the first in this structure */
    char *error;
    SOCKET s;
    Plug plug;
    void *private_ptr;
    bufchain output_data;
    int writable;
    int frozen; /* this causes readability notifications to be ignored */
    int frozen_readable; /* this means we missed at least one readability
                          * notification while we were frozen */
    char oobdata[1];
    int sending_oob;
    int oobinline;
};

/*
 * We used to typedef struct Socket_tag *Socket.
 *
 * Since we have made the networking abstraction slightly more
 * abstract, Socket no longer means a tcp socket (it could mean
 * an ssl socket).  So now we must use Actual_Socket when we know
 * we are talking about a tcp socket.
 */
typedef struct Socket_tag *Actual_Socket;

struct SockAddr_tag {
    char *error;
    /* address family this belongs to, AF_INET for IPv4, AF_INET6 for IPv6. */
    int family;
    unsigned long address;             /* Address IPv4 style. */
#ifdef IPV6
    struct addrinfo *ai;               /* Address IPv6 style. */
#endif
};

static tree234 *sktree;

static int cmpfortree(void *av, void *bv)
{
    Actual_Socket a = (Actual_Socket) av, b = (Actual_Socket) bv;
    unsigned long as = (unsigned long) a->s, bs = (unsigned long) b->s;
    if (as < bs)
        return -1;
    if (as > bs)
        return +1;
    return 0;
}

static int cmpforsearch(void *av, void *bv)
{
    Actual_Socket b = (Actual_Socket) bv;
    unsigned long as = (unsigned long) av, bs = (unsigned long) b->s;
    if (as < bs)
        return -1;
    if (as > bs)
        return +1;
    return 0;
}

void sk_init(void)
{
    sktree = newtree234(cmpfortree);
}

char *winsock_error_string(int error)
{
    switch (error) {
      case WSAEACCES:
        return "Network error: Permission denied";
      case WSAEADDRINUSE:
        return "Network error: Address already in use";
      case WSAEADDRNOTAVAIL:
        return "Network error: Cannot assign requested address";
      case WSAEAFNOSUPPORT:
        return
            "Network error: Address family not supported by protocol family";
      case WSAEALREADY:
        return "Network error: Operation already in progress";
      case WSAECONNABORTED:
        return "Network error: Software caused connection abort";
      case WSAECONNREFUSED:
        return "Network error: Connection refused";
      case WSAECONNRESET:
        return "Network error: Connection reset by peer";
      case WSAEDESTADDRREQ:
        return "Network error: Destination address required";
      case WSAEFAULT:
        return "Network error: Bad address";
      case WSAEHOSTDOWN:
        return "Network error: Host is down";
      case WSAEHOSTUNREACH:
        return "Network error: No route to host";
      case WSAEINPROGRESS:
        return "Network error: Operation now in progress";
      case WSAEINTR:
        return "Network error: Interrupted function call";
      case WSAEINVAL:
        return "Network error: Invalid argument";
      case WSAEISCONN:
        return "Network error: Socket is already connected";
      case WSAEMFILE:
        return "Network error: Too many open files";
      case WSAEMSGSIZE:
        return "Network error: Message too long";
      case WSAENETDOWN:
        return "Network error: Network is down";
      case WSAENETRESET:
        return "Network error: Network dropped connection on reset";
      case WSAENETUNREACH:
        return "Network error: Network is unreachable";
      case WSAENOBUFS:
        return "Network error: No buffer space available";
      case WSAENOPROTOOPT:
        return "Network error: Bad protocol option";
      case WSAENOTCONN:
        return "Network error: Socket is not connected";
      case WSAENOTSOCK:
        return "Network error: Socket operation on non-socket";
      case WSAEOPNOTSUPP:
        return "Network error: Operation not supported";
      case WSAEPFNOSUPPORT:
        return "Network error: Protocol family not supported";
      case WSAEPROCLIM:
        return "Network error: Too many processes";
      case WSAEPROTONOSUPPORT:
        return "Network error: Protocol not supported";
      case WSAEPROTOTYPE:
        return "Network error: Protocol wrong type for socket";
      case WSAESHUTDOWN:
        return "Network error: Cannot send after socket shutdown";
      case WSAESOCKTNOSUPPORT:
        return "Network error: Socket type not supported";
      case WSAETIMEDOUT:
        return "Network error: Connection timed out";
      case WSAEWOULDBLOCK:
        return "Network error: Resource temporarily unavailable";
      case WSAEDISCON:
        return "Network error: Graceful shutdown in progress";
      default:
        return "Unknown network error";
    }
}

SockAddr sk_namelookup(char *host, char **canonicalname)
{
    SockAddr ret = smalloc(sizeof(struct SockAddr_tag));
    unsigned long a;
    struct hostent *h = NULL;
    char realhost[8192];

    /* Clear the structure and default to IPv4. */
    memset(ret, 0, sizeof(struct SockAddr_tag));
    ret->family = 0;                   /* We set this one when we have resolved the host. */
    *realhost = '\0';

    if ((a = inet_addr(host)) == (unsigned long) INADDR_NONE) {
#ifdef IPV6

        /* Try to get the getaddrinfo() function from wship6.dll */
        /* This way one doesn't need to have IPv6 dll's to use PuTTY and
         * it will fallback to IPv4. */
        typedef int (CALLBACK * FGETADDRINFO) (const char *nodename,
                                               const char *servname,
                                               const struct addrinfo *
                                               hints,
                                               struct addrinfo ** res);
        FGETADDRINFO fGetAddrInfo = NULL;

        HINSTANCE dllWSHIP6 = LoadLibrary("wship6.dll");
        if (dllWSHIP6)
            fGetAddrInfo = (FGETADDRINFO) GetProcAddress(dllWSHIP6,
                                                         "getaddrinfo");

        /*
         * Use fGetAddrInfo when it's available (which usually also
         * means IPv6 is installed...)
         */
        if (fGetAddrInfo) {
            /*debug(("Resolving \"%s\" with getaddrinfo()  (IPv4+IPv6 capable)...\n", host)); */
            if (fGetAddrInfo(host, NULL, NULL, &ret->ai) == 0)
                ret->family = ret->ai->ai_family;
        } else
#endif
        {
            /*
             * Otherwise use the IPv4-only gethostbyname...
             * (NOTE: we don't use gethostbyname as a
             * fallback!)
             */
            if (ret->family == 0) {
                /*debug(("Resolving \"%s\" with gethostbyname() (IPv4 only)...\n", host)); */
                if ( (h = gethostbyname(host)) )
                    ret->family = AF_INET;
            }
        }
        /*debug(("Done resolving...(family is %d) AF_INET = %d, AF_INET6 = %d\n", ret->family, AF_INET, AF_INET6)); */

        if (ret->family == 0) {
            DWORD err = WSAGetLastError();
            ret->error = (err == WSAENETDOWN ? "Network is down" :
                          err ==
                          WSAHOST_NOT_FOUND ? "Host does not exist" : err
                          == WSATRY_AGAIN ? "Host not found" :
#ifdef IPV6
                          fGetAddrInfo ? "getaddrinfo: unknown error" :
#endif
                          "gethostbyname: unknown error");
#ifdef DEBUG
            {
                LPVOID lpMsgBuf;
                FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER |
                              FORMAT_MESSAGE_FROM_SYSTEM |
                              FORMAT_MESSAGE_IGNORE_INSERTS, NULL, err,
                              MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
                              (LPTSTR) & lpMsgBuf, 0, NULL);
                /*debug(("Error %ld: %s (h=%lx)\n", err, lpMsgBuf, h)); */
                /* Free the buffer. */
                LocalFree(lpMsgBuf);
            }
#endif
        } else {
            ret->error = NULL;

#ifdef IPV6
            /* If we got an address info use that... */
            if (ret->ai) {
                typedef int (CALLBACK * FGETNAMEINFO)
                 (const struct sockaddr FAR * sa, socklen_t salen,
                  char FAR * host, size_t hostlen, char FAR * serv,
                  size_t servlen, int flags);
                FGETNAMEINFO fGetNameInfo = NULL;

                /* Are we in IPv4 fallback mode? */
                /* We put the IPv4 address into the a variable so we can further-on use the IPv4 code... */
                if (ret->family == AF_INET)
                    memcpy(&a,
                           (char *) &((SOCKADDR_IN *) ret->ai->
                                      ai_addr)->sin_addr, sizeof(a));

                /* Now let's find that canonicalname... */
                if ((dllWSHIP6)
                    && (fGetNameInfo =
                        (FGETNAMEINFO) GetProcAddress(dllWSHIP6,
                                                      "getnameinfo"))) {
                    if (fGetNameInfo
                        ((struct sockaddr *) ret->ai->ai_addr,
                         ret->family ==
                         AF_INET ? sizeof(SOCKADDR_IN) :
                         sizeof(SOCKADDR_IN6), realhost,
                         sizeof(realhost), NULL, 0, 0) != 0) {
                        strncpy(realhost, host, sizeof(realhost));
                    }
                }
            }
            /* We used the IPv4-only gethostbyname()... */
            else
#endif
            {
                memcpy(&a, h->h_addr, sizeof(a));
                /* This way we are always sure the h->h_name is valid :) */
                strncpy(realhost, h->h_name, sizeof(realhost));
            }
        }
#ifdef IPV6
        FreeLibrary(dllWSHIP6);
#endif
    } else {
        /*
         * This must be a numeric IPv4 address because it caused a
         * success return from inet_addr.
         */
        ret->family = AF_INET;
        strncpy(realhost, host, sizeof(realhost));
    }
    ret->address = ntohl(a);
    realhost[lenof(realhost)-1] = '\0';
    *canonicalname = smalloc(1+strlen(realhost));
    strcpy(*canonicalname, realhost);
    return ret;
}

void sk_addr_free(SockAddr addr)
{
    sfree(addr);
}

static Plug sk_tcp_plug(Socket sock, Plug p)
{
    Actual_Socket s = (Actual_Socket) sock;
    Plug ret = s->plug;
    if (p)
        s->plug = p;
    return ret;
}

static void sk_tcp_flush(Socket s)
{
    /*
     * We send data to the socket as soon as we can anyway,
     * so we don't need to do anything here.  :-)
     */
}

static void sk_tcp_close(Socket s);
static int sk_tcp_write(Socket s, char *data, int len);
static int sk_tcp_write_oob(Socket s, char *data, int len);
static char *sk_tcp_socket_error(Socket s);

extern char *do_select(SOCKET skt, int startup);

Socket sk_register(void *sock, Plug plug)
{
    static struct socket_function_table fn_table = {
        sk_tcp_plug,
        sk_tcp_close,
        sk_tcp_write,
        sk_tcp_write_oob,
        sk_tcp_flush,
        sk_tcp_socket_error
    };

    DWORD err;
    char *errstr;
    Actual_Socket ret;

    /*
     * Create Socket structure.
     */
    ret = smalloc(sizeof(struct Socket_tag));
    ret->fn = &fn_table;
    ret->error = NULL;
    ret->plug = plug;
    bufchain_init(&ret->output_data);
    ret->writable = 1;                 /* to start with */
    ret->sending_oob = 0;
    ret->frozen = 1;
    ret->frozen_readable = 0;

    ret->s = (SOCKET)sock;

    if (ret->s == INVALID_SOCKET) {
        err = WSAGetLastError();
        ret->error = winsock_error_string(err);
        return (Socket) ret;
    }

    ret->oobinline = 0;

    /* Set up a select mechanism. This could be an AsyncSelect on a
     * window, or an EventSelect on an event object. */
    errstr = do_select(ret->s, 1);
    if (errstr) {
        ret->error = errstr;
        return (Socket) ret;
    }

    add234(sktree, ret);

    return (Socket) ret;
}

Socket sk_new(SockAddr addr, int port, int privport, int oobinline,
              Plug plug)
{
    static struct socket_function_table fn_table = {
        sk_tcp_plug,
        sk_tcp_close,
        sk_tcp_write,
        sk_tcp_write_oob,
        sk_tcp_flush,
        sk_tcp_socket_error
    };

    SOCKET s;
#ifdef IPV6
    SOCKADDR_IN6 a6;
#endif
    SOCKADDR_IN a;
    DWORD err;
    char *errstr;
    Actual_Socket ret;
    short localport;

    /*
     * Create Socket structure.
     */
    ret = smalloc(sizeof(struct Socket_tag));
    ret->fn = &fn_table;
    ret->error = NULL;
    ret->plug = plug;
    bufchain_init(&ret->output_data);
    ret->writable = 1;                 /* to start with */
    ret->sending_oob = 0;
    ret->frozen = 0;
    ret->frozen_readable = 0;

    /*
     * Open socket.
     */
    s = socket(addr->family, SOCK_STREAM, 0);
    ret->s = s;

    if (s == INVALID_SOCKET) {
        err = WSAGetLastError();
        ret->error = winsock_error_string(err);
        return (Socket) ret;
    }

    ret->oobinline = oobinline;
    if (oobinline) {
        BOOL b = TRUE;
        setsockopt(s, SOL_SOCKET, SO_OOBINLINE, (void *) &b, sizeof(b));
    }

    /*
     * Bind to local address.
     */
    if (privport)
        localport = 1023;              /* count from 1023 downwards */
    else
        localport = 0;                 /* just use port 0 (ie winsock picks) */

    /* Loop round trying to bind */
    while (1) {
        int retcode;

#ifdef IPV6
        if (addr->family == AF_INET6) {
            memset(&a6, 0, sizeof(a6));
            a6.sin6_family = AF_INET6;
/*a6.sin6_addr      = in6addr_any; *//* == 0 */
            a6.sin6_port = htons(localport);
        } else
#endif
        {
            a.sin_family = AF_INET;
            a.sin_addr.s_addr = htonl(INADDR_ANY);
            a.sin_port = htons(localport);
        }
#ifdef IPV6
        retcode = bind(s, (addr->family == AF_INET6 ?
                           (struct sockaddr *) &a6 :
                           (struct sockaddr *) &a),
                       (addr->family ==
                        AF_INET6 ? sizeof(a6) : sizeof(a)));
#else
        retcode = bind(s, (struct sockaddr *) &a, sizeof(a));
#endif
        if (retcode != SOCKET_ERROR) {
            err = 0;
            break;                     /* done */
        } else {
            err = WSAGetLastError();
            if (err != WSAEADDRINUSE)  /* failed, for a bad reason */
                break;
        }

        if (localport == 0)
            break;                     /* we're only looping once */
        localport--;
        if (localport == 0)
            break;                     /* we might have got to the end */
    }

    if (err) {
        ret->error = winsock_error_string(err);
        return (Socket) ret;
    }

    /*
     * Connect to remote address.
     */
#ifdef IPV6
    if (addr->family == AF_INET6) {
        memset(&a, 0, sizeof(a));
        a6.sin6_family = AF_INET6;
        a6.sin6_port = htons((short) port);
        a6.sin6_addr =
            ((struct sockaddr_in6 *) addr->ai->ai_addr)->sin6_addr;
    } else
#endif
    {
        a.sin_family = AF_INET;
        a.sin_addr.s_addr = htonl(addr->address);
        a.sin_port = htons((short) port);
    }
    if ((
#ifdef IPV6
            connect(s, ((addr->family == AF_INET6) ?
                        (struct sockaddr *) &a6 : (struct sockaddr *) &a),
                    (addr->family == AF_INET6) ? sizeof(a6) : sizeof(a))
#else
            connect(s, (struct sockaddr *) &a, sizeof(a))
#endif
        ) == SOCKET_ERROR) {
        err = WSAGetLastError();
        ret->error = winsock_error_string(err);
        return (Socket) ret;
    }

    /* Set up a select mechanism. This could be an AsyncSelect on a
     * window, or an EventSelect on an event object. */
    errstr = do_select(s, 1);
    if (errstr) {
        ret->error = errstr;
        return (Socket) ret;
    }

    add234(sktree, ret);

    return (Socket) ret;
}

Socket sk_newlistener(int port, Plug plug, int local_host_only)
{
    static struct socket_function_table fn_table = {
        sk_tcp_plug,
        sk_tcp_close,
        sk_tcp_write,
        sk_tcp_write_oob,
        sk_tcp_flush,
        sk_tcp_socket_error
    };

    SOCKET s;
#ifdef IPV6
    SOCKADDR_IN6 a6;
#endif
    SOCKADDR_IN a;
    DWORD err;
    char *errstr;
    Actual_Socket ret;
    int retcode;
    int on = 1;

    /*
     * Create Socket structure.
     */
    ret = smalloc(sizeof(struct Socket_tag));
    ret->fn = &fn_table;
    ret->error = NULL;
    ret->plug = plug;
    bufchain_init(&ret->output_data);
    ret->writable = 0;                 /* to start with */
    ret->sending_oob = 0;
    ret->frozen = 0;
    ret->frozen_readable = 0;

    /*
     * Open socket.
     */
    s = socket(AF_INET, SOCK_STREAM, 0);
    ret->s = s;

    if (s == INVALID_SOCKET) {
        err = WSAGetLastError();
        ret->error = winsock_error_string(err);
        return (Socket) ret;
    }

    ret->oobinline = 0;


    setsockopt(s, SOL_SOCKET, SO_REUSEADDR, (const char *)&on, sizeof(on));


#ifdef IPV6
        if (addr->family == AF_INET6) {
            memset(&a6, 0, sizeof(a6));
            a6.sin6_family = AF_INET6;
            if (local_host_only)
                a6.sin6_addr = in6addr_loopback;
            else
                a6.sin6_addr = in6addr_any;
            a6.sin6_port = htons(port);
        } else
#endif
        {
            a.sin_family = AF_INET;
            if (local_host_only)
                a.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
            else
                a.sin_addr.s_addr = htonl(INADDR_ANY);
            a.sin_port = htons((short)port);
        }
#ifdef IPV6
        retcode = bind(s, (addr->family == AF_INET6 ?
                           (struct sockaddr *) &a6 :
                           (struct sockaddr *) &a),
                       (addr->family ==
                        AF_INET6 ? sizeof(a6) : sizeof(a)));
#else
        retcode = bind(s, (struct sockaddr *) &a, sizeof(a));
#endif
        if (retcode != SOCKET_ERROR) {
            err = 0;
        } else {
            err = WSAGetLastError();
        }

    if (err) {
        ret->error = winsock_error_string(err);
        return (Socket) ret;
    }


    if (listen(s, SOMAXCONN) == SOCKET_ERROR) {
        closesocket(s);
        ret->error = winsock_error_string(err);
        return (Socket) ret;
    }

    /* Set up a select mechanism. This could be an AsyncSelect on a
     * window, or an EventSelect on an event object. */
    errstr = do_select(s, 1);
    if (errstr) {
        ret->error = errstr;
        return (Socket) ret;
    }

    add234(sktree, ret);

    return (Socket) ret;
}

static void sk_tcp_close(Socket sock)
{
    extern char *do_select(SOCKET skt, int startup);
    Actual_Socket s = (Actual_Socket) sock;

    del234(sktree, s);
    do_select(s->s, 0);
    closesocket(s->s);
    sfree(s);
}

/*
 * The function which tries to send on a socket once it's deemed
 * writable.
 */
void try_send(Actual_Socket s)
{
    while (s->sending_oob || bufchain_size(&s->output_data) > 0) {
        int nsent;
        DWORD err;
        void *data;
        int len, urgentflag;

        if (s->sending_oob) {
            urgentflag = MSG_OOB;
            len = s->sending_oob;
            data = &s->oobdata;
        } else {
            urgentflag = 0;
            bufchain_prefix(&s->output_data, &data, &len);
        }

        nsent = send(s->s, data, len, urgentflag);
        noise_ultralight(nsent);
        if (nsent <= 0) {
            err = (nsent < 0 ? WSAGetLastError() : 0);
            if ((err < WSABASEERR && nsent < 0) || err == WSAEWOULDBLOCK) {
                /*
                 * Perfectly normal: we've sent all we can for the moment.
                 * 
                 * (Some WinSock send() implementations can return
                 * <0 but leave no sensible error indication -
                 * WSAGetLastError() is called but returns zero or
                 * a small number - so we check that case and treat
                 * it just like WSAEWOULDBLOCK.)
                 */
                s->writable = FALSE;
                return;
            } else if (nsent == 0 ||
                       err == WSAECONNABORTED || err == WSAECONNRESET) {
                /*
                 * FIXME. This will have to be done better when we
                 * start managing multiple sockets (e.g. SSH port
                 * forwarding), because if we get CONNRESET while
                 * trying to write a particular forwarded socket
                 * then it isn't necessarily the end of the world.
                 * Ideally I'd like to pass the error code back to
                 * somewhere the next select_result() will see it,
                 * but that might be hard. Perhaps I should pass it
                 * back to be queued in the Windows front end bit.
                 */
                fatalbox(winsock_error_string(err));
            } else {
                fatalbox(winsock_error_string(err));
            }
        } else {
            if (s->sending_oob) {
                if (nsent < len) {
                    memmove(s->oobdata, s->oobdata+nsent, len-nsent);
                    s->sending_oob = len - nsent;
                } else {
                    s->sending_oob = 0;
                }
            } else {
                bufchain_consume(&s->output_data, nsent);
            }
        }
    }
}

static int sk_tcp_write(Socket sock, char *buf, int len)
{
    Actual_Socket s = (Actual_Socket) sock;

    /*
     * Add the data to the buffer list on the socket.
     */
    bufchain_add(&s->output_data, buf, len);

    /*
     * Now try sending from the start of the buffer list.
     */
    if (s->writable)
        try_send(s);

    return bufchain_size(&s->output_data);
}

static int sk_tcp_write_oob(Socket sock, char *buf, int len)
{
    Actual_Socket s = (Actual_Socket) sock;

    /*
     * Replace the buffer list on the socket with the data.
     */
    bufchain_clear(&s->output_data);
    assert(len <= sizeof(s->oobdata));
    memcpy(s->oobdata, buf, len);
    s->sending_oob = len;

    /*
     * Now try sending from the start of the buffer list.
     */
    if (s->writable)
        try_send(s);

    return s->sending_oob;
}

int select_result(WPARAM wParam, LPARAM lParam)
{
    int ret, open;
    DWORD err;
    char buf[20480];                   /* nice big buffer for plenty of speed */
    Actual_Socket s;
    u_long atmark;

    /* wParam is the socket itself */
    s = find234(sktree, (void *) wParam, cmpforsearch);
    if (!s)
        return 1;                      /* boggle */

    if ((err = WSAGETSELECTERROR(lParam)) != 0) {
        /*
         * An error has occurred on this socket. Pass it to the
         * plug.
         */
        return plug_closing(s->plug, winsock_error_string(err), err, 0);
    }

    noise_ultralight(lParam);

    switch (WSAGETSELECTEVENT(lParam)) {
      case FD_READ:
        /* In the case the socket is still frozen, we don't even bother */
        if (s->frozen) {
            s->frozen_readable = 1;
            break;
        }

        /*
         * We have received data on the socket. For an oobinline
         * socket, this might be data _before_ an urgent pointer,
         * in which case we send it to the back end with type==1
         * (data prior to urgent).
         */
        if (s->oobinline) {
            atmark = 1;
            ioctlsocket(s->s, SIOCATMARK, &atmark);
            /*
             * Avoid checking the return value from ioctlsocket(),
             * on the grounds that some WinSock wrappers don't
             * support it. If it does nothing, we get atmark==1,
             * which is equivalent to `no OOB pending', so the
             * effect will be to non-OOB-ify any OOB data.
             */
        } else
            atmark = 1;

        ret = recv(s->s, buf, sizeof(buf), 0);
        noise_ultralight(ret);
        if (ret < 0) {
            err = WSAGetLastError();
            if (err == WSAEWOULDBLOCK) {
                break;
            }
        }
        if (ret < 0) {
            return plug_closing(s->plug, winsock_error_string(err), err,
                                0);
        } else if (0 == ret) {
            return plug_closing(s->plug, NULL, 0, 0);
        } else {
            return plug_receive(s->plug, atmark ? 0 : 1, buf, ret);
        }
        break;
      case FD_OOB:
        /*
         * This will only happen on a non-oobinline socket. It
         * indicates that we can immediately perform an OOB read
         * and get back OOB data, which we will send to the back
         * end with type==2 (urgent data).
         */
        ret = recv(s->s, buf, sizeof(buf), MSG_OOB);
        noise_ultralight(ret);
        if (ret <= 0) {
            fatalbox(ret == 0 ? "Internal networking trouble" :
                     winsock_error_string(WSAGetLastError()));
        } else {
            return plug_receive(s->plug, 2, buf, ret);
        }
        break;
      case FD_WRITE:
        {
            int bufsize_before, bufsize_after;
            s->writable = 1;
            bufsize_before = s->sending_oob + bufchain_size(&s->output_data);
            try_send(s);
            bufsize_after = s->sending_oob + bufchain_size(&s->output_data);
            if (bufsize_after < bufsize_before)
                plug_sent(s->plug, bufsize_after);
        }
        break;
      case FD_CLOSE:
        /* Signal a close on the socket. First read any outstanding data. */
        open = 1;
        do {
            ret = recv(s->s, buf, sizeof(buf), 0);
            if (ret < 0) {
                err = WSAGetLastError();
                if (err == WSAEWOULDBLOCK)
                    break;
                return plug_closing(s->plug, winsock_error_string(err),
                                    err, 0);
            } else {
                if (ret)
                    open &= plug_receive(s->plug, 0, buf, ret);
                else
                    open &= plug_closing(s->plug, NULL, 0, 0);
            }
        } while (ret > 0);
        return open;
       case FD_ACCEPT:
        {
            struct sockaddr isa;
            int addrlen = sizeof(struct sockaddr);
            SOCKET t;  /* socket of connection */

            memset(&isa, 0, sizeof(struct sockaddr));
            err = 0;
            t = accept(s->s,&isa,&addrlen);
            if (t == INVALID_SOCKET)
            {
                err = WSAGetLastError();
                if (err == WSATRY_AGAIN)
                    break;
            }

            if (plug_accepting(s->plug, (void*)t)) {
                closesocket(t);        /* denied or error */
            }
        }
    }

    return 1;
}

/*
 * Each socket abstraction contains a `void *' private field in
 * which the client can keep state.
 */
void sk_set_private_ptr(Socket sock, void *ptr)
{
    Actual_Socket s = (Actual_Socket) sock;
    s->private_ptr = ptr;
}

void *sk_get_private_ptr(Socket sock)
{
    Actual_Socket s = (Actual_Socket) sock;
    return s->private_ptr;
}

/*
 * Special error values are returned from sk_namelookup and sk_new
 * if there's a problem. These functions extract an error message,
 * or return NULL if there's no problem.
 */
char *sk_addr_error(SockAddr addr)
{
    return addr->error;
}
static char *sk_tcp_socket_error(Socket sock)
{
    Actual_Socket s = (Actual_Socket) sock;
    return s->error;
}

void sk_set_frozen(Socket sock, int is_frozen)
{
    Actual_Socket s = (Actual_Socket) sock;
    if (s->frozen == is_frozen)
        return;
    s->frozen = is_frozen;
    if (!is_frozen && s->frozen_readable) {
        char c;
        recv(s->s, &c, 1, MSG_PEEK);
    }
    s->frozen_readable = 0;
}

/*
 * For Plink: enumerate all sockets currently active.
 */
SOCKET first_socket(int *state)
{
    Actual_Socket s;
    *state = 0;
    s = index234(sktree, (*state)++);
    return s ? s->s : INVALID_SOCKET;
}

SOCKET next_socket(int *state)
{
    Actual_Socket s = index234(sktree, (*state)++);
    return s ? s->s : INVALID_SOCKET;
}