Add a configuration option for TCP keepalives (SO_KEEPALIVE), default off.

[u/mdw/putty] / unix / uxnet.c

/*
 * Unix networking abstraction.
 */

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
#include <netdb.h>
#include <sys/un.h>

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

#ifndef X11_UNIX_PATH
# define X11_UNIX_PATH "/tmp/.X11-unix/X"
#endif

#define ipv4_is_loopback(addr) (inet_netof(addr) == IN_LOOPBACKNET)

struct Socket_tag {
    struct socket_function_table *fn;
    /* the above variable absolutely *must* be the first in this structure */
    const char *error;
    int s;
    Plug plug;
    void *private_ptr;
    bufchain output_data;
    int connected;
    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 */
    int localhost_only;                /* for listening sockets */
    char oobdata[1];
    int sending_oob;
    int oobpending;                    /* is there OOB data available to read? */
    int oobinline;
    int pending_error;                 /* in case send() returns error */
    int listener;
};

/*
 * 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 {
    const char *error;
    /*
     * Which address family this address belongs to. AF_INET for
     * IPv4; AF_INET6 for IPv6; AF_UNSPEC indicates that name
     * resolution has not been done and a simple host name is held
     * in this SockAddr structure.
     */
    int family;
#ifdef IPV6
    struct addrinfo *ai;               /* Address IPv6 style. */
#else
    unsigned long address;             /* Address IPv4 style. */
#endif
    char hostname[512];                /* Store an unresolved host name. */
};

static tree234 *sktree;

static void uxsel_tell(Actual_Socket s);

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

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

void sk_cleanup(void)
{
    Actual_Socket s;
    int i;

    if (sktree) {
        for (i = 0; (s = index234(sktree, i)) != NULL; i++) {
            close(s->s);
        }
    }
}

const char *error_string(int error)
{
    return strerror(error);
}

SockAddr sk_namelookup(const char *host, char **canonicalname)
{
    SockAddr ret = snew(struct SockAddr_tag);
#ifdef IPV6
    struct addrinfo hints;
    int err;
#else
    unsigned long a;
    struct hostent *h = NULL;
#endif
    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';
    ret->error = NULL;

#ifdef IPV6
    hints.ai_flags = AI_CANONNAME;
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = 0;
    hints.ai_protocol = 0;
    hints.ai_addrlen = 0;
    hints.ai_addr = NULL;
    hints.ai_canonname = NULL;
    hints.ai_next = NULL;
    err = getaddrinfo(host, NULL, NULL, &ret->ai);
    if (err != 0) {
        ret->error = gai_strerror(err);
        return ret;
    }
    ret->family = ret->ai->ai_family;
    *realhost = '\0';
    if (ret->ai->ai_canonname != NULL)
        strncat(realhost, ret->ai->ai_canonname, sizeof(realhost) - 1);
    else
        strncat(realhost, host, sizeof(realhost) - 1);
#else
    if ((a = inet_addr(host)) == (unsigned long) INADDR_NONE) {
        /*
         * 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;
        }
        if (ret->family == 0) {
                ret->error = (h_errno == HOST_NOT_FOUND ||
                    h_errno == NO_DATA ||
                    h_errno == NO_ADDRESS ? "Host does not exist" :
                    h_errno == TRY_AGAIN ?
                    "Temporary name service failure" :
                    "gethostbyname: unknown error");
                return ret;
        }
        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));
    } 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);
#endif
    realhost[lenof(realhost)-1] = '\0';
    *canonicalname = snewn(1+strlen(realhost), char);
    strcpy(*canonicalname, realhost);
    return ret;
}

SockAddr sk_nonamelookup(const char *host)
{
    SockAddr ret = snew(struct SockAddr_tag);
    ret->error = NULL;
    ret->family = AF_UNSPEC;
    strncpy(ret->hostname, host, lenof(ret->hostname));
    ret->hostname[lenof(ret->hostname)-1] = '\0';
    return ret;
}

void sk_getaddr(SockAddr addr, char *buf, int buflen)
{

    if (addr->family == AF_UNSPEC) {
        strncpy(buf, addr->hostname, buflen);
        buf[buflen-1] = '\0';
    } else {
#ifdef IPV6
        if (getnameinfo(addr->ai->ai_addr, addr->ai->ai_addrlen, buf, buflen,
                        NULL, 0, NI_NUMERICHOST) != 0) {
            buf[0] = '\0';
            strncat(buf, "<unknown>", buflen - 1);
        }
#else
        struct in_addr a;
        assert(addr->family == AF_INET);
        a.s_addr = htonl(addr->address);
        strncpy(buf, inet_ntoa(a), buflen);
        buf[buflen-1] = '\0';
#endif
    }
}

int sk_hostname_is_local(char *name)
{
    return !strcmp(name, "localhost");
}

int sk_address_is_local(SockAddr addr)
{

    if (addr->family == AF_UNSPEC)
        return 0;                      /* we don't know; assume not */
    else {
#ifdef IPV6
        if (addr->family == AF_INET)
            return ipv4_is_loopback(
                ((struct sockaddr_in *)addr->ai->ai_addr)->sin_addr);
        else if (addr->family == AF_INET6)
            return IN6_IS_ADDR_LOOPBACK(
                &((struct sockaddr_in6 *)addr->ai->ai_addr)->sin6_addr);
        else
            return 0;
#else
        struct in_addr a;
        assert(addr->family == AF_INET);
        a.s_addr = htonl(addr->address);
        return ipv4_is_loopback(a);
#endif
    }
}

int sk_addrtype(SockAddr addr)
{
    return (addr->family == AF_INET ? ADDRTYPE_IPV4 :
#ifdef IPV6
            addr->family == AF_INET6 ? ADDRTYPE_IPV6 :
#endif
            ADDRTYPE_NAME);
}

void sk_addrcopy(SockAddr addr, char *buf)
{

#ifdef IPV6
    if (addr->family == AF_INET)
        memcpy(buf, &((struct sockaddr_in *)addr->ai->ai_addr)->sin_addr,
               sizeof(struct in_addr));
    else if (addr->family == AF_INET6)
        memcpy(buf, &((struct sockaddr_in6 *)addr->ai->ai_addr)->sin6_addr,
               sizeof(struct in6_addr));
    else
        assert(FALSE);
#else
    struct in_addr a;

    assert(addr->family == AF_INET);
    a.s_addr = htonl(addr->address);
    memcpy(buf, (char*) &a.s_addr, 4);
#endif
}

void sk_addr_free(SockAddr addr)
{

#ifdef IPV6
    if (addr->ai != NULL)
        freeaddrinfo(addr->ai);
#endif
    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, const char *data, int len);
static int sk_tcp_write_oob(Socket s, const char *data, int len);
static void sk_tcp_set_private_ptr(Socket s, void *ptr);
static void *sk_tcp_get_private_ptr(Socket s);
static void sk_tcp_set_frozen(Socket s, int is_frozen);
static const char *sk_tcp_socket_error(Socket s);

static struct socket_function_table tcp_fn_table = {
    sk_tcp_plug,
    sk_tcp_close,
    sk_tcp_write,
    sk_tcp_write_oob,
    sk_tcp_flush,
    sk_tcp_set_private_ptr,
    sk_tcp_get_private_ptr,
    sk_tcp_set_frozen,
    sk_tcp_socket_error
};

Socket sk_register(OSSocket sockfd, Plug plug)
{
    Actual_Socket ret;

    /*
     * Create Socket structure.
     */
    ret = snew(struct Socket_tag);
    ret->fn = &tcp_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->localhost_only = 0;           /* unused, but best init anyway */
    ret->pending_error = 0;
    ret->oobpending = FALSE;
    ret->listener = 0;

    ret->s = sockfd;

    if (ret->s < 0) {
        ret->error = error_string(errno);
        return (Socket) ret;
    }

    ret->oobinline = 0;

    uxsel_tell(ret);
    add234(sktree, ret);

    return (Socket) ret;
}

Socket sk_new(SockAddr addr, int port, int privport, int oobinline,
              int nodelay, int keepalive, Plug plug)
{
    int s;
#ifdef IPV6
    struct sockaddr_in6 a6;
#endif
    struct sockaddr_in a;
    struct sockaddr_un au;
    const struct sockaddr *sa;
    int err;
    Actual_Socket ret;
    short localport;
    int fl, salen;

    /*
     * Create Socket structure.
     */
    ret = snew(struct Socket_tag);
    ret->fn = &tcp_fn_table;
    ret->error = NULL;
    ret->plug = plug;
    bufchain_init(&ret->output_data);
    ret->connected = 0;                /* to start with */
    ret->writable = 0;                 /* to start with */
    ret->sending_oob = 0;
    ret->frozen = 0;
    ret->frozen_readable = 0;
    ret->localhost_only = 0;           /* unused, but best init anyway */
    ret->pending_error = 0;
    ret->oobpending = FALSE;
    ret->listener = 0;

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

    if (s < 0) {
        ret->error = error_string(errno);
        return (Socket) ret;
    }

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

    if (nodelay) {
        int b = TRUE;
        setsockopt(s, IPPROTO_TCP, TCP_NODELAY, (void *) &b, sizeof(b));
    }

    if (keepalive) {
        int b = TRUE;
        setsockopt(s, SOL_SOCKET, SO_KEEPALIVE, (void *) &b, sizeof(b));
    }

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

    /* BSD IP stacks need sockaddr_in zeroed before filling in */
    memset(&a,'\0',sizeof(struct sockaddr_in));
#ifdef IPV6
    memset(&a6,'\0',sizeof(struct sockaddr_in6));
#endif

    /* We don't try to bind to a local address for UNIX domain sockets.  (Why
     * do we bother doing the bind when localport == 0 anyway?) */
    if(addr->family != AF_UNIX) {
        /* Loop round trying to bind */
        while (1) {
            int retcode;

#ifdef IPV6
            if (addr->family == AF_INET6) {
                /* XXX use getaddrinfo to get a local address? */
                a6.sin6_family = AF_INET6;
                a6.sin6_addr = in6addr_any;
                a6.sin6_port = htons(localport);
                retcode = bind(s, (struct sockaddr *) &a6, sizeof(a6));
            } else
#endif
            {
                assert(addr->family == AF_INET);
                a.sin_family = AF_INET;
                a.sin_addr.s_addr = htonl(INADDR_ANY);
                a.sin_port = htons(localport);
                retcode = bind(s, (struct sockaddr *) &a, sizeof(a));
            }
            if (retcode >= 0) {
                err = 0;
                break;                 /* done */
            } else {
                err = errno;
                if (err != EADDRINUSE) /* 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 = error_string(err);
            return (Socket) ret;
        }
    }

    /*
     * Connect to remote address.
     */
    switch(addr->family) {
#ifdef IPV6
      case AF_INET:
        /* XXX would be better to have got getaddrinfo() to fill in the port. */
        ((struct sockaddr_in *)addr->ai->ai_addr)->sin_port =
            htons(port);
        sa = (const struct sockaddr *)addr->ai->ai_addr;
        salen = addr->ai->ai_addrlen;
        break;
      case AF_INET6:
        ((struct sockaddr_in *)addr->ai->ai_addr)->sin_port =
            htons(port);
        sa = (const struct sockaddr *)addr->ai->ai_addr;
        salen = addr->ai->ai_addrlen;
        break;
#else
      case AF_INET:
        a.sin_family = AF_INET;
        a.sin_addr.s_addr = htonl(addr->address);
        a.sin_port = htons((short) port);
        sa = (const struct sockaddr *)&a;
        salen = sizeof a;
        break;
#endif
      case AF_UNIX:
        assert(port == 0);      /* to catch confused people */
        assert(strlen(addr->hostname) < sizeof au.sun_path);
        memset(&au, 0, sizeof au);
        au.sun_family = AF_UNIX;
        strcpy(au.sun_path, addr->hostname);
        sa = (const struct sockaddr *)&au;
        salen = sizeof au;
        break;

      default:
        assert(0 && "unknown address family");
    }

    fl = fcntl(s, F_GETFL);
    if (fl != -1)
        fcntl(s, F_SETFL, fl | O_NONBLOCK);

    if ((connect(s, sa, salen)) < 0) {
        if ( errno != EINPROGRESS ) {
            ret->error = error_string(errno);
            return (Socket) ret;
        }
    } else {
        /*
         * If we _don't_ get EWOULDBLOCK, the connect has completed
         * and we should set the socket as connected and writable.
         */
        ret->connected = 1;
        ret->writable = 1;
    }

    uxsel_tell(ret);
    add234(sktree, ret);

    sk_addr_free(addr);

    return (Socket) ret;
}

Socket sk_newlistener(char *srcaddr, int port, Plug plug, int local_host_only)
{
    int s;
#ifdef IPV6
#if 0
    struct sockaddr_in6 a6;
#endif
    struct addrinfo hints, *ai;
    char portstr[6];
#endif
    struct sockaddr_in a;
    int err;
    Actual_Socket ret;
    int retcode;
    int on = 1;

    /*
     * Create Socket structure.
     */
    ret = snew(struct Socket_tag);
    ret->fn = &tcp_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;
    ret->localhost_only = local_host_only;
    ret->pending_error = 0;
    ret->oobpending = FALSE;
    ret->listener = 1;

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

    if (s < 0) {
        ret->error = error_string(errno);
        return (Socket) ret;
    }

    ret->oobinline = 0;

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

    /* BSD IP stacks need sockaddr_in zeroed before filling in */
    memset(&a,'\0',sizeof(struct sockaddr_in));
#ifdef IPV6
#if 0
    memset(&a6,'\0',sizeof(struct sockaddr_in6));
#endif
    hints.ai_flags = AI_NUMERICHOST;
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = 0;
    hints.ai_protocol = 0;
    hints.ai_addrlen = 0;
    hints.ai_addr = NULL;
    hints.ai_canonname = NULL;
    hints.ai_next = NULL;
    sprintf(portstr, "%d", port);
    if (srcaddr != NULL && getaddrinfo(srcaddr, portstr, &hints, &ai) == 0)
        retcode = bind(s, ai->ai_addr, ai->ai_addrlen);
    else
#if 0
    {
        /*
         * FIXME: Need two listening sockets, in principle, one for v4
         * and one for v6
         */
        if (local_host_only)
            a6.sin6_addr = in6addr_loopback;
        else
            a6.sin6_addr = in6addr_any;
        a6.sin6_port = htons(port);
    } else
#endif
#endif
    {
        int got_addr = 0;
        a.sin_family = AF_INET;

        /*
         * Bind to source address. First try an explicitly
         * specified one...
         */
        if (srcaddr) {
            a.sin_addr.s_addr = inet_addr(srcaddr);
            if (a.sin_addr.s_addr != INADDR_NONE) {
                /* Override localhost_only with specified listen addr. */
                ret->localhost_only = ipv4_is_loopback(a.sin_addr);
                got_addr = 1;
            }
        }

        /*
         * ... and failing that, go with one of the standard ones.
         */
        if (!got_addr) {
            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);
        retcode = bind(s, (struct sockaddr *) &a, sizeof(a));
    }

    if (retcode >= 0) {
        err = 0;
    } else {
        err = errno;
    }

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


    if (listen(s, SOMAXCONN) < 0) {
        close(s);
        ret->error = error_string(errno);
        return (Socket) ret;
    }

    uxsel_tell(ret);
    add234(sktree, ret);

    return (Socket) ret;
}

static void sk_tcp_close(Socket sock)
{
    Actual_Socket s = (Actual_Socket) sock;

    uxsel_del(s->s);
    del234(sktree, s);
    close(s->s);
    sfree(s);
}

int sk_getxdmdata(void *sock, unsigned long *ip, int *port)
{
    Actual_Socket s = (Actual_Socket) sock;
    struct sockaddr_in addr;
    socklen_t addrlen;

    /*
     * We must check that this socket really _is_ an Actual_Socket.
     */
    if (s->fn != &tcp_fn_table)
        return 0;                      /* failure */

    addrlen = sizeof(addr);
    if (getsockname(s->s, (struct sockaddr *)&addr, &addrlen) < 0)
        return 0;
    switch(addr.sin_family) {
      case AF_INET:
        *ip = ntohl(addr.sin_addr.s_addr);
        *port = ntohs(addr.sin_port);
        break;
      case AF_UNIX:
        /*
         * For a Unix socket, we return 0xFFFFFFFF for the IP address and
         * our current pid for the port. Bizarre, but such is life.
         */
        *ip = ntohl(0xFFFFFFFF);
        *port = getpid();
        break;

        /* XXX IPV6 */

      default:
        return 0;
    }

    return 1;
}

/*
 * 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;
        int 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 ? errno : 0);
            if (err == EWOULDBLOCK) {
                /*
                 * Perfectly normal: we've sent all we can for the moment.
                 */
                s->writable = FALSE;
                return;
            } else if (nsent == 0 ||
                       err == ECONNABORTED || err == ECONNRESET) {
                /*
                 * If send() returns CONNABORTED or CONNRESET, we
                 * unfortunately can't just call plug_closing(),
                 * because it's quite likely that we're currently
                 * _in_ a call from the code we'd be calling back
                 * to, so we'd have to make half the SSH code
                 * reentrant. Instead we flag a pending error on
                 * the socket, to be dealt with (by calling
                 * plug_closing()) at some suitable future moment.
                 */
                s->pending_error = err;
                return;
            } else {
                /* We're inside the Unix frontend here, so we know
                 * that the frontend handle is unnecessary. */
                logevent(NULL, error_string(err));
                fatalbox("%s", 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);
            }
        }
    }
    uxsel_tell(s);
}

static int sk_tcp_write(Socket sock, const 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);

    /*
     * Update the select() status to correctly reflect whether or
     * not we should be selecting for write.
     */
    uxsel_tell(s);

    return bufchain_size(&s->output_data);
}

static int sk_tcp_write_oob(Socket sock, const 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);

    /*
     * Update the select() status to correctly reflect whether or
     * not we should be selecting for write.
     */
    uxsel_tell(s);

    return s->sending_oob;
}

static int net_select_result(int fd, int event)
{
    int ret;
    int err;
    char buf[20480];                   /* nice big buffer for plenty of speed */
    Actual_Socket s;
    u_long atmark;

    /* Find the Socket structure */
    s = find234(sktree, &fd, cmpforsearch);
    if (!s)
        return 1;                      /* boggle */

    noise_ultralight(event);

    switch (event) {
      case 4:                          /* exceptional */
        if (!s->oobinline) {
            /*
             * On a non-oobinline socket, this 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) {
                const char *str = (ret == 0 ? "Internal networking trouble" :
                                   error_string(errno));
                /* We're inside the Unix frontend here, so we know
                 * that the frontend handle is unnecessary. */
                logevent(NULL, str);
                fatalbox("%s", str);
            } else {
                return plug_receive(s->plug, 2, buf, ret);
            }
            break;
        }

        /*
         * If we reach here, this is an oobinline socket, which
         * means we should set s->oobpending and then deal with it
         * when we get called for the readability event (which
         * should also occur).
         */
        s->oobpending = TRUE;
        break;
      case 1:                          /* readable; also acceptance */
        if (s->listener) {
            /*
             * On a listening socket, the readability event means a
             * connection is ready to be accepted.
             */
            struct sockaddr_in isa;
            int addrlen = sizeof(struct sockaddr_in);
            int t;  /* socket of connection */

            memset(&isa, 0, sizeof(struct sockaddr_in));
            err = 0;
            t = accept(s->s,(struct sockaddr *)&isa,(socklen_t *) &addrlen);
            if (t < 0) {
                break;
            }

            if (s->localhost_only && !ipv4_is_loopback(isa.sin_addr)) {
                close(t);              /* someone let nonlocal through?! */
            } else if (plug_accepting(s->plug, t)) {
                close(t);              /* denied or error */
            }
            break;
        }

        /*
         * If we reach here, this is not a listening socket, so
         * readability really means readability.
         */

        /* 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 && s->oobpending) {
            atmark = 1;
            if (ioctl(s->s, SIOCATMARK, &atmark) == 0 && atmark)
                s->oobpending = FALSE; /* clear this indicator */
        } else
            atmark = 1;

        ret = recv(s->s, buf, s->oobpending ? 1 : sizeof(buf), 0);
        noise_ultralight(ret);
        if (ret < 0) {
            if (errno == EWOULDBLOCK) {
                break;
            }
        }
        if (ret < 0) {
            return plug_closing(s->plug, error_string(errno), errno, 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 2:                          /* writable */
        if (!s->connected) {
            /*
             * select() reports a socket as _writable_ when an
             * asynchronous connection is completed.
             */
            s->connected = s->writable = 1;
            uxsel_tell(s);
            break;
        } else {
            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;
    }

    return 1;
}

/*
 * Deal with socket errors detected in try_send().
 */
void net_pending_errors(void)
{
    int i;
    Actual_Socket s;

    /*
     * This might be a fiddly business, because it's just possible
     * that handling a pending error on one socket might cause
     * others to be closed. (I can't think of any reason this might
     * happen in current SSH implementation, but to maintain
     * generality of this network layer I'll assume the worst.)
     * 
     * So what we'll do is search the socket list for _one_ socket
     * with a pending error, and then handle it, and then search
     * the list again _from the beginning_. Repeat until we make a
     * pass with no socket errors present. That way we are
     * protected against the socket list changing under our feet.
     */

    do {
        for (i = 0; (s = index234(sktree, i)) != NULL; i++) {
            if (s->pending_error) {
                /*
                 * An error has occurred on this socket. Pass it to the
                 * plug.
                 */
                plug_closing(s->plug, error_string(s->pending_error),
                             s->pending_error, 0);
                break;
            }
        }
    } while (s);
}

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

static void *sk_tcp_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.
 */
const char *sk_addr_error(SockAddr addr)
{
    return addr->error;
}
static const char *sk_tcp_socket_error(Socket sock)
{
    Actual_Socket s = (Actual_Socket) sock;
    return s->error;
}

static void sk_tcp_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;
    uxsel_tell(s);
}

static void uxsel_tell(Actual_Socket s)
{
    int rwx = 0;
    if (!s->connected)
        rwx |= 2;                      /* write == connect */
    if (s->connected && !s->frozen)
        rwx |= 1 | 4;                  /* read, except */
    if (bufchain_size(&s->output_data))
        rwx |= 2;                      /* write */
    if (s->listener)
        rwx |= 1;                      /* read == accept */
    uxsel_set(s->s, rwx, net_select_result);
}

int net_service_lookup(char *service)
{
    struct servent *se;
    se = getservbyname(service, NULL);
    if (se != NULL)
        return ntohs(se->s_port);
    else
        return 0;
}

SockAddr platform_get_x11_unix_address(int displaynum, char **canonicalname)
{
    SockAddr ret = snew(struct SockAddr_tag);
    int n;

    memset(ret, 0, sizeof *ret);
    ret->family = AF_UNIX;
    n = snprintf(ret->hostname, sizeof ret->hostname,
                 "%s%d", X11_UNIX_PATH, displaynum);
    if(n < 0)
        ret->error = "snprintf failed";
    else if(n >= sizeof ret->hostname)
        ret->error = "X11 UNIX name too long";
    else
        *canonicalname = dupstr(ret->hostname);
    return ret;
}