Adding upstream version 7.0.14-dfsg.upstream/7.0.14-dfsg

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2524 insertions, 0 deletions

diff --git a/src/VBox/NetworkServices/NAT/pxtcp.c b/src/VBox/NetworkServices/NAT/pxtcp.c
new file mode 100644
index 00000000..716e9aff
--- /dev/null
+++ b/src/VBox/NetworkServices/NAT/pxtcp.c
@@ -0,0 +1,2524 @@
+/* $Id: pxtcp.c $ */
+/** @file
+ * NAT Network - TCP proxy.
+ */
+
+/*
+ * Copyright (C) 2013-2023 Oracle and/or its affiliates.
+ *
+ * This file is part of VirtualBox base platform packages, as
+ * available from https://www.virtualbox.org.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, in version 3 of the
+ * License.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <https://www.gnu.org/licenses>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+#define LOG_GROUP LOG_GROUP_NAT_SERVICE
+
+#include "winutils.h"
+
+#include "pxtcp.h"
+
+#include "proxy.h"
+#include "proxy_pollmgr.h"
+#include "pxremap.h"
+#include "portfwd.h"            /* fwspec */
+
+#ifndef RT_OS_WINDOWS
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <sys/ioctl.h>
+#ifdef RT_OS_SOLARIS
+#include <sys/filio.h>          /* FIONREAD is BSD'ism */
+#endif
+#include <stdlib.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <string.h>
+#include <poll.h>
+
+#include <err.h>                /* BSD'ism */
+#else
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+
+#include <iprt/stdint.h>
+#include "winpoll.h"
+#endif
+
+#include "lwip/opt.h"
+
+#include "lwip/sys.h"
+#include "lwip/tcpip.h"
+#include "lwip/netif.h"
+#include "lwip/tcp_impl.h"      /* XXX: to access tcp_abandon() */
+#include "lwip/icmp.h"
+#include "lwip/icmp6.h"
+
+/*
+ * Different OSes have different quirks in reporting POLLHUP for TCP
+ * sockets.
+ *
+ * Using shutdown(2) "how" values here would be more readable, but
+ * since SHUT_RD is 0, we can't use 0 for "none", unfortunately.
+ */
+#if defined(RT_OS_NETBSD) || defined(RT_OS_SOLARIS)
+# define HAVE_TCP_POLLHUP 0                     /* not reported */
+#elif defined(RT_OS_DARWIN) || defined(RT_OS_WINDOWS)
+# define HAVE_TCP_POLLHUP POLLIN                /* reported when remote closes */
+#else
+# define HAVE_TCP_POLLHUP (POLLIN|POLLOUT)      /* reported when both directions are closed */
+#endif
+
+
+/**
+ * Ring buffer for inbound data.  Filled with data from the host
+ * socket on poll manager thread.  Data consumed by scheduling
+ * tcp_write() to the pcb on the lwip thread.
+ *
+ * NB: There is actually third party present, the lwip stack itself.
+ * Thus the buffer doesn't have dual free vs. data split, but rather
+ * three-way free / send and unACKed data / unsent data split.
+ */
+struct ringbuf {
+    char *buf;
+    size_t bufsize;
+
+    /*
+     * Start of free space, producer writes here (up till "unacked").
+     */
+    volatile size_t vacant;
+
+    /*
+     * Start of sent but unacknowledged data.  The data are "owned" by
+     * the stack as it may need to retransmit.  This is the free space
+     * limit for producer.
+     */
+    volatile size_t unacked;
+
+    /*
+     * Start of unsent data, consumer reads/sends from here (up till
+     * "vacant").  Not declared volatile since it's only accessed from
+     * the consumer thread.
+     */
+    size_t unsent;
+};
+
+
+/**
+ */
+struct pxtcp {
+    /**
+     * Our poll manager handler.  Must be first, strong/weak
+     * references depend on this "inheritance".
+     */
+    struct pollmgr_handler pmhdl;
+
+    /**
+     * lwIP (internal/guest) side of the proxied connection.
+     */
+    struct tcp_pcb *pcb;
+
+    /**
+     * Host (external) side of the proxied connection.
+     */
+    SOCKET sock;
+
+    /**
+     * Socket events we are currently polling for.
+     */
+    int events;
+
+    /**
+     * Socket error.  Currently used to save connect(2) errors so that
+     * we can decide if we need to send ICMP error.
+     */
+    int sockerr;
+
+    /**
+     * Interface that we have got the SYN from.  Needed to send ICMP
+     * with correct source address.
+     */
+    struct netif *netif;
+
+    /**
+     * For tentatively accepted connections for which we are in
+     * process of connecting to the real destination this is the
+     * initial pbuf that we might need to build ICMP error.
+     *
+     * When connection is established this is used to hold outbound
+     * pbuf chain received by pxtcp_pcb_recv() but not yet completely
+     * forwarded over the socket.  We cannot "return" it to lwIP since
+     * the head of the chain is already sent and freed.
+     */
+    struct pbuf *unsent;
+
+    /**
+     * Guest has closed its side. Reported to pxtcp_pcb_recv() only
+     * once and we might not be able to forward it immediately if we
+     * have unsent pbuf.
+     */
+    int outbound_close;
+
+    /**
+     * Outbound half-close has been done on the socket.
+     */
+    int outbound_close_done;
+
+    /**
+     * External has closed its side.  We might not be able to forward
+     * it immediately if we have unforwarded data.
+     */
+    int inbound_close;
+
+    /**
+     * Inbound half-close has been done on the pcb.
+     */
+    int inbound_close_done;
+
+    /**
+     * On systems that report POLLHUP as soon as the final FIN is
+     * received on a socket we cannot continue polling for the rest of
+     * input, so we have to read (pull) last data from the socket on
+     * the lwIP thread instead of polling/pushing it from the poll
+     * manager thread.  See comment in pxtcp_pmgr_pump() POLLHUP case.
+     */
+    int inbound_pull;
+
+
+    /**
+     * When poll manager schedules delete we may not be able to delete
+     * a pxtcp immediately if not all inbound data has been acked by
+     * the guest: lwIP may need to resend and the data are in pxtcp's
+     * inbuf::buf.  We defer delete until all data are acked to
+     * pxtcp_pcb_sent().
+     */
+    int deferred_delete;
+
+    /**
+     * Ring-buffer for inbound data.
+     */
+    struct ringbuf inbuf;
+
+    /**
+     * lwIP thread's strong reference to us.
+     */
+    struct pollmgr_refptr *rp;
+
+
+    /*
+     * We use static messages to call functions on the lwIP thread to
+     * void malloc/free overhead.
+     */
+    struct tcpip_msg msg_delete;   /* delete pxtcp */
+    struct tcpip_msg msg_reset;    /* reset connection and delete pxtcp */
+    struct tcpip_msg msg_accept;   /* confirm accept of proxied connection */
+    struct tcpip_msg msg_outbound; /* trigger send of outbound data */
+    struct tcpip_msg msg_inbound;  /* trigger send of inbound data */
+#if HAVE_TCP_POLLHUP
+    struct tcpip_msg msg_inpull;   /* trigger pull of last inbound data */
+#endif
+};
+
+
+
+static struct pxtcp *pxtcp_allocate(void);
+static void pxtcp_free(struct pxtcp *);
+
+static void pxtcp_pcb_associate(struct pxtcp *, struct tcp_pcb *);
+static void pxtcp_pcb_dissociate(struct pxtcp *);
+
+/* poll manager callbacks for pxtcp related channels */
+static int pxtcp_pmgr_chan_add(struct pollmgr_handler *, SOCKET, int);
+static int pxtcp_pmgr_chan_pollout(struct pollmgr_handler *, SOCKET, int);
+static int pxtcp_pmgr_chan_pollin(struct pollmgr_handler *, SOCKET, int);
+#if !(HAVE_TCP_POLLHUP & POLLOUT)
+static int pxtcp_pmgr_chan_del(struct pollmgr_handler *, SOCKET, int);
+#endif
+static int pxtcp_pmgr_chan_reset(struct pollmgr_handler *, SOCKET, int);
+
+/* helper functions for sending/receiving pxtcp over poll manager channels */
+static ssize_t pxtcp_chan_send(enum pollmgr_slot_t, struct pxtcp *);
+static ssize_t pxtcp_chan_send_weak(enum pollmgr_slot_t, struct pxtcp *);
+static struct pxtcp *pxtcp_chan_recv(struct pollmgr_handler *, SOCKET, int);
+static struct pxtcp *pxtcp_chan_recv_strong(struct pollmgr_handler *, SOCKET, int);
+
+/* poll manager callbacks for individual sockets */
+static int pxtcp_pmgr_connect(struct pollmgr_handler *, SOCKET, int);
+static int pxtcp_pmgr_pump(struct pollmgr_handler *, SOCKET, int);
+
+/* get incoming traffic into ring buffer */
+static ssize_t pxtcp_sock_read(struct pxtcp *, int *);
+static ssize_t pxtcp_sock_recv(struct pxtcp *, IOVEC *, size_t); /* default */
+
+/* convenience functions for poll manager callbacks */
+static int pxtcp_schedule_delete(struct pxtcp *);
+static int pxtcp_schedule_reset(struct pxtcp *);
+static int pxtcp_schedule_reject(struct pxtcp *);
+
+/* lwip thread callbacks called via proxy_lwip_post() */
+static void pxtcp_pcb_delete_pxtcp(void *);
+static void pxtcp_pcb_reset_pxtcp(void *);
+static void pxtcp_pcb_accept_refuse(void *);
+static void pxtcp_pcb_accept_confirm(void *);
+static void pxtcp_pcb_write_outbound(void *);
+static void pxtcp_pcb_write_inbound(void *);
+#if HAVE_TCP_POLLHUP
+static void pxtcp_pcb_pull_inbound(void *);
+#endif
+
+/* tcp pcb callbacks */
+static err_t pxtcp_pcb_heard(void *, struct tcp_pcb *, struct pbuf *); /* global */
+static err_t pxtcp_pcb_accept(void *, struct tcp_pcb *, err_t);
+static err_t pxtcp_pcb_connected(void *, struct tcp_pcb *, err_t);
+static err_t pxtcp_pcb_recv(void *, struct tcp_pcb *, struct pbuf *, err_t);
+static err_t pxtcp_pcb_sent(void *, struct tcp_pcb *, u16_t);
+static err_t pxtcp_pcb_poll(void *, struct tcp_pcb *);
+static void pxtcp_pcb_err(void *, err_t);
+
+static err_t pxtcp_pcb_forward_outbound(struct pxtcp *, struct pbuf *);
+static void pxtcp_pcb_forward_outbound_close(struct pxtcp *);
+
+static ssize_t pxtcp_sock_send(struct pxtcp *, IOVEC *, size_t);
+
+static void pxtcp_pcb_forward_inbound(struct pxtcp *);
+static void pxtcp_pcb_forward_inbound_close(struct pxtcp *);
+DECLINLINE(int) pxtcp_pcb_forward_inbound_done(const struct pxtcp *);
+static void pxtcp_pcb_schedule_poll(struct pxtcp *);
+static void pxtcp_pcb_cancel_poll(struct pxtcp *);
+
+static void pxtcp_pcb_reject(struct tcp_pcb *, int, struct netif *, struct pbuf *);
+DECLINLINE(void) pxtcp_pcb_maybe_deferred_delete(struct pxtcp *);
+
+/* poll manager handlers for pxtcp channels */
+static struct pollmgr_handler pxtcp_pmgr_chan_add_hdl;
+static struct pollmgr_handler pxtcp_pmgr_chan_pollout_hdl;
+static struct pollmgr_handler pxtcp_pmgr_chan_pollin_hdl;
+#if !(HAVE_TCP_POLLHUP & POLLOUT)
+static struct pollmgr_handler pxtcp_pmgr_chan_del_hdl;
+#endif
+static struct pollmgr_handler pxtcp_pmgr_chan_reset_hdl;
+
+
+/**
+ * Init PXTCP - must be run when neither lwIP tcpip thread, nor poll
+ * manager threads haven't been created yet.
+ */
+void
+pxtcp_init(void)
+{
+    /*
+     * Create channels.
+     */
+#define CHANNEL(SLOT, NAME) do {                \
+        NAME##_hdl.callback = NAME;             \
+        NAME##_hdl.data = NULL;                 \
+        NAME##_hdl.slot = -1;                   \
+        pollmgr_add_chan(SLOT, &NAME##_hdl);    \
+    } while (0)
+
+    CHANNEL(POLLMGR_CHAN_PXTCP_ADD,     pxtcp_pmgr_chan_add);
+    CHANNEL(POLLMGR_CHAN_PXTCP_POLLIN,  pxtcp_pmgr_chan_pollin);
+    CHANNEL(POLLMGR_CHAN_PXTCP_POLLOUT, pxtcp_pmgr_chan_pollout);
+#if !(HAVE_TCP_POLLHUP & POLLOUT)
+    CHANNEL(POLLMGR_CHAN_PXTCP_DEL,     pxtcp_pmgr_chan_del);
+#endif
+    CHANNEL(POLLMGR_CHAN_PXTCP_RESET,   pxtcp_pmgr_chan_reset);
+
+#undef CHANNEL
+
+    /*
+     * Listen to outgoing connection from guest(s).
+     */
+    tcp_proxy_accept(pxtcp_pcb_heard);
+}
+
+
+/**
+ * Syntactic sugar for sending pxtcp pointer over poll manager
+ * channel.  Used by lwip thread functions.
+ */
+static ssize_t
+pxtcp_chan_send(enum pollmgr_slot_t slot, struct pxtcp *pxtcp)
+{
+    return pollmgr_chan_send(slot, &pxtcp, sizeof(pxtcp));
+}
+
+
+/**
+ * Syntactic sugar for sending weak reference to pxtcp over poll
+ * manager channel.  Used by lwip thread functions.
+ */
+static ssize_t
+pxtcp_chan_send_weak(enum pollmgr_slot_t slot, struct pxtcp *pxtcp)
+{
+    pollmgr_refptr_weak_ref(pxtcp->rp);
+    return pollmgr_chan_send(slot, &pxtcp->rp, sizeof(pxtcp->rp));
+}
+
+
+/**
+ * Counterpart of pxtcp_chan_send().
+ */
+static struct pxtcp *
+pxtcp_chan_recv(struct pollmgr_handler *handler, SOCKET fd, int revents)
+{
+    struct pxtcp *pxtcp;
+
+    pxtcp = (struct pxtcp *)pollmgr_chan_recv_ptr(handler, fd, revents);
+    return pxtcp;
+}
+
+
+/**
+ * Counterpart of pxtcp_chan_send_weak().
+ */
+static struct pxtcp *
+pxtcp_chan_recv_strong(struct pollmgr_handler *handler, SOCKET fd, int revents)
+{
+    struct pollmgr_refptr *rp;
+    struct pollmgr_handler *base;
+    struct pxtcp *pxtcp;
+
+    rp = (struct pollmgr_refptr *)pollmgr_chan_recv_ptr(handler, fd, revents);
+    base = (struct pollmgr_handler *)pollmgr_refptr_get(rp);
+    pxtcp = (struct pxtcp *)base;
+
+    return pxtcp;
+}
+
+
+/**
+ * Register pxtcp with poll manager.
+ *
+ * Used for POLLMGR_CHAN_PXTCP_ADD and by port-forwarding.  Since
+ * error handling is different in these two cases, we leave it up to
+ * the caller.
+ */
+int
+pxtcp_pmgr_add(struct pxtcp *pxtcp)
+{
+    int status;
+
+    LWIP_ASSERT1(pxtcp != NULL);
+#ifdef RT_OS_WINDOWS
+    LWIP_ASSERT1(pxtcp->sock != INVALID_SOCKET);
+#else
+    LWIP_ASSERT1(pxtcp->sock >= 0);
+#endif
+    LWIP_ASSERT1(pxtcp->pmhdl.callback != NULL);
+    LWIP_ASSERT1(pxtcp->pmhdl.data == (void *)pxtcp);
+    LWIP_ASSERT1(pxtcp->pmhdl.slot < 0);
+
+    status = pollmgr_add(&pxtcp->pmhdl, pxtcp->sock, pxtcp->events);
+    return status;
+}
+
+
+/**
+ * Unregister pxtcp with poll manager.
+ *
+ * Used for POLLMGR_CHAN_PXTCP_RESET and by port-forwarding (on error
+ * leg).
+ */
+void
+pxtcp_pmgr_del(struct pxtcp *pxtcp)
+{
+    LWIP_ASSERT1(pxtcp != NULL);
+
+    pollmgr_del_slot(pxtcp->pmhdl.slot);
+}
+
+
+/**
+ * POLLMGR_CHAN_PXTCP_ADD handler.
+ *
+ * Get new pxtcp from lwip thread and start polling its socket.
+ */
+static int
+pxtcp_pmgr_chan_add(struct pollmgr_handler *handler, SOCKET fd, int revents)
+{
+    struct pxtcp *pxtcp;
+    int status;
+
+    pxtcp = pxtcp_chan_recv(handler, fd, revents);
+    DPRINTF0(("pxtcp_add: new pxtcp %p; pcb %p; sock %d\n",
+              (void *)pxtcp, (void *)pxtcp->pcb, pxtcp->sock));
+
+    status = pxtcp_pmgr_add(pxtcp);
+    if (status < 0) {
+        (void) pxtcp_schedule_reset(pxtcp);
+    }
+
+    return POLLIN;
+}
+
+
+/**
+ * POLLMGR_CHAN_PXTCP_POLLOUT handler.
+ *
+ * pxtcp_pcb_forward_outbound() on the lwIP thread tried to send data
+ * and failed, it now requests us to poll the socket for POLLOUT and
+ * schedule pxtcp_pcb_forward_outbound() when sock is writable again.
+ */
+static int
+pxtcp_pmgr_chan_pollout(struct pollmgr_handler *handler, SOCKET fd, int revents)
+{
+    struct pxtcp *pxtcp;
+
+    pxtcp = pxtcp_chan_recv_strong(handler, fd, revents);
+    DPRINTF0(("pxtcp_pollout: pxtcp %p\n", (void *)pxtcp));
+
+    if (pxtcp == NULL) {
+        return POLLIN;
+    }
+
+    LWIP_ASSERT1(pxtcp->pmhdl.data == (void *)pxtcp);
+    LWIP_ASSERT1(pxtcp->pmhdl.slot > 0);
+
+    pxtcp->events |= POLLOUT;
+    pollmgr_update_events(pxtcp->pmhdl.slot, pxtcp->events);
+
+    return POLLIN;
+}
+
+
+/**
+ * POLLMGR_CHAN_PXTCP_POLLIN handler.
+ */
+static int
+pxtcp_pmgr_chan_pollin(struct pollmgr_handler *handler, SOCKET fd, int revents)
+{
+    struct pxtcp *pxtcp;
+
+    pxtcp = pxtcp_chan_recv_strong(handler, fd, revents);
+    DPRINTF2(("pxtcp_pollin: pxtcp %p\n", (void *)pxtcp));
+
+    if (pxtcp == NULL) {
+        return POLLIN;
+    }
+
+    LWIP_ASSERT1(pxtcp->pmhdl.data == (void *)pxtcp);
+    LWIP_ASSERT1(pxtcp->pmhdl.slot > 0);
+
+    if (pxtcp->inbound_close) {
+        return POLLIN;
+    }
+
+    pxtcp->events |= POLLIN;
+    pollmgr_update_events(pxtcp->pmhdl.slot, pxtcp->events);
+
+    return POLLIN;
+}
+
+
+#if !(HAVE_TCP_POLLHUP & POLLOUT)
+/**
+ * POLLMGR_CHAN_PXTCP_DEL handler.
+ *
+ * Schedule pxtcp deletion.  We only need this if host system doesn't
+ * report POLLHUP for fully closed tcp sockets.
+ */
+static int
+pxtcp_pmgr_chan_del(struct pollmgr_handler *handler, SOCKET fd, int revents)
+{
+    struct pxtcp *pxtcp;
+
+    pxtcp = pxtcp_chan_recv_strong(handler, fd, revents);
+    if (pxtcp == NULL) {
+        return POLLIN;
+    }
+
+    DPRINTF(("PXTCP_DEL: pxtcp %p; pcb %p; sock %d\n",
+             (void *)pxtcp, (void *)pxtcp->pcb, pxtcp->sock));
+
+    LWIP_ASSERT1(pxtcp->pmhdl.callback != NULL);
+    LWIP_ASSERT1(pxtcp->pmhdl.data == (void *)pxtcp);
+
+    LWIP_ASSERT1(pxtcp->inbound_close);       /* EOF read */
+    LWIP_ASSERT1(pxtcp->outbound_close_done); /* EOF sent */
+
+    pxtcp_pmgr_del(pxtcp);
+    (void) pxtcp_schedule_delete(pxtcp);
+
+    return POLLIN;
+}
+#endif  /* !(HAVE_TCP_POLLHUP & POLLOUT)  */
+
+
+/**
+ * POLLMGR_CHAN_PXTCP_RESET handler.
+ *
+ * Close the socket with RST and delete pxtcp.
+ */
+static int
+pxtcp_pmgr_chan_reset(struct pollmgr_handler *handler, SOCKET fd, int revents)
+{
+    struct pxtcp *pxtcp;
+
+    pxtcp = pxtcp_chan_recv_strong(handler, fd, revents);
+    if (pxtcp == NULL) {
+        return POLLIN;
+    }
+
+    DPRINTF0(("PXTCP_RESET: pxtcp %p; pcb %p; sock %d\n",
+              (void *)pxtcp, (void *)pxtcp->pcb, pxtcp->sock));
+
+    LWIP_ASSERT1(pxtcp->pmhdl.callback != NULL);
+    LWIP_ASSERT1(pxtcp->pmhdl.data == (void *)pxtcp);
+
+    pxtcp_pmgr_del(pxtcp);
+
+    proxy_reset_socket(pxtcp->sock);
+    pxtcp->sock = INVALID_SOCKET;
+
+    (void) pxtcp_schedule_reset(pxtcp);
+
+    return POLLIN;
+}
+
+
+static struct pxtcp *
+pxtcp_allocate(void)
+{
+    struct pxtcp *pxtcp;
+
+    pxtcp = (struct pxtcp *)malloc(sizeof(*pxtcp));
+    if (pxtcp == NULL) {
+        return NULL;
+    }
+
+    pxtcp->pmhdl.callback = NULL;
+    pxtcp->pmhdl.data = (void *)pxtcp;
+    pxtcp->pmhdl.slot = -1;
+
+    pxtcp->pcb = NULL;
+    pxtcp->sock = INVALID_SOCKET;
+    pxtcp->events = 0;
+    pxtcp->sockerr = 0;
+    pxtcp->netif = NULL;
+    pxtcp->unsent = NULL;
+    pxtcp->outbound_close = 0;
+    pxtcp->outbound_close_done = 0;
+    pxtcp->inbound_close = 0;
+    pxtcp->inbound_close_done = 0;
+    pxtcp->inbound_pull = 0;
+    pxtcp->deferred_delete = 0;
+
+    pxtcp->inbuf.bufsize = 64 * 1024;
+    pxtcp->inbuf.buf = (char *)malloc(pxtcp->inbuf.bufsize);
+    if (pxtcp->inbuf.buf == NULL) {
+        free(pxtcp);
+        return NULL;
+    }
+    pxtcp->inbuf.vacant = 0;
+    pxtcp->inbuf.unacked = 0;
+    pxtcp->inbuf.unsent = 0;
+
+    pxtcp->rp = pollmgr_refptr_create(&pxtcp->pmhdl);
+    if (pxtcp->rp == NULL) {
+        free(pxtcp->inbuf.buf);
+        free(pxtcp);
+        return NULL;
+    }
+
+#define CALLBACK_MSG(MSG, FUNC)                         \
+    do {                                                \
+        pxtcp->MSG.type = TCPIP_MSG_CALLBACK_STATIC;    \
+        pxtcp->MSG.sem = NULL;                          \
+        pxtcp->MSG.msg.cb.function = FUNC;              \
+        pxtcp->MSG.msg.cb.ctx = (void *)pxtcp;          \
+    } while (0)
+
+    CALLBACK_MSG(msg_delete, pxtcp_pcb_delete_pxtcp);
+    CALLBACK_MSG(msg_reset, pxtcp_pcb_reset_pxtcp);
+    CALLBACK_MSG(msg_accept, pxtcp_pcb_accept_confirm);
+    CALLBACK_MSG(msg_outbound, pxtcp_pcb_write_outbound);
+    CALLBACK_MSG(msg_inbound, pxtcp_pcb_write_inbound);
+#if HAVE_TCP_POLLHUP
+    CALLBACK_MSG(msg_inpull, pxtcp_pcb_pull_inbound);
+#endif
+
+#undef CALLBACK_MSG
+
+    return pxtcp;
+}
+
+
+/**
+ * Exported to fwtcp to create pxtcp for incoming port-forwarded
+ * connections.  Completed with pcb in pxtcp_pcb_connect().
+ */
+struct pxtcp *
+pxtcp_create_forwarded(SOCKET sock)
+{
+    struct pxtcp *pxtcp;
+
+    pxtcp = pxtcp_allocate();
+    if (pxtcp == NULL) {
+        return NULL;
+    }
+
+    pxtcp->sock = sock;
+    pxtcp->pmhdl.callback = pxtcp_pmgr_pump;
+    pxtcp->events = 0;
+
+    return pxtcp;
+}
+
+
+static void
+pxtcp_pcb_associate(struct pxtcp *pxtcp, struct tcp_pcb *pcb)
+{
+    LWIP_ASSERT1(pxtcp != NULL);
+    LWIP_ASSERT1(pcb != NULL);
+
+    pxtcp->pcb = pcb;
+
+    tcp_arg(pcb, pxtcp);
+
+    tcp_recv(pcb, pxtcp_pcb_recv);
+    tcp_sent(pcb, pxtcp_pcb_sent);
+    tcp_poll(pcb, NULL, 255);
+    tcp_err(pcb, pxtcp_pcb_err);
+}
+
+
+static void
+pxtcp_free(struct pxtcp *pxtcp)
+{
+    if (pxtcp->unsent != NULL) {
+        pbuf_free(pxtcp->unsent);
+    }
+    if (pxtcp->inbuf.buf != NULL) {
+        free(pxtcp->inbuf.buf);
+    }
+    free(pxtcp);
+}
+
+
+/**
+ * Counterpart to pxtcp_create_forwarded() to destruct pxtcp that
+ * fwtcp failed to register with poll manager to post to lwip thread
+ * for doing connect.
+ */
+void
+pxtcp_cancel_forwarded(struct pxtcp *pxtcp)
+{
+    LWIP_ASSERT1(pxtcp->pcb == NULL);
+    pxtcp_pcb_reset_pxtcp(pxtcp);
+}
+
+
+static void
+pxtcp_pcb_dissociate(struct pxtcp *pxtcp)
+{
+    if (pxtcp == NULL || pxtcp->pcb == NULL) {
+        return;
+    }
+
+    DPRINTF(("%s: pxtcp %p <-> pcb %p\n",
+             __func__, (void *)pxtcp, (void *)pxtcp->pcb));
+
+    /*
+     * We must have dissociated from a fully closed pcb immediately
+     * since lwip recycles them and we don't wan't to mess with what
+     * would be someone else's pcb that we happen to have a stale
+     * pointer to.
+     */
+    LWIP_ASSERT1(pxtcp->pcb->callback_arg == pxtcp);
+
+    tcp_recv(pxtcp->pcb, NULL);
+    tcp_sent(pxtcp->pcb, NULL);
+    tcp_poll(pxtcp->pcb, NULL, 255);
+    tcp_err(pxtcp->pcb, NULL);
+    tcp_arg(pxtcp->pcb, NULL);
+    pxtcp->pcb = NULL;
+}
+
+
+/**
+ * Lwip thread callback invoked via pxtcp::msg_delete
+ *
+ * Since we use static messages to communicate to the lwip thread, we
+ * cannot delete pxtcp without making sure there are no unprocessed
+ * messages in the lwip thread mailbox.
+ *
+ * The easiest way to ensure that is to send this "delete" message as
+ * the last one and when it's processed we know there are no more and
+ * it's safe to delete pxtcp.
+ *
+ * Poll manager handlers should use pxtcp_schedule_delete()
+ * convenience function.
+ */
+static void
+pxtcp_pcb_delete_pxtcp(void *ctx)
+{
+    struct pxtcp *pxtcp = (struct pxtcp *)ctx;
+
+    DPRINTF(("%s: pxtcp %p, pcb %p, sock %d%s\n",
+             __func__, (void *)pxtcp, (void *)pxtcp->pcb, pxtcp->sock,
+             (pxtcp->deferred_delete && !pxtcp->inbound_pull
+                  ? " (was deferred)" : "")));
+
+    LWIP_ASSERT1(pxtcp != NULL);
+    LWIP_ASSERT1(pxtcp->pmhdl.slot < 0);
+    LWIP_ASSERT1(pxtcp->outbound_close_done);
+    LWIP_ASSERT1(pxtcp->inbound_close); /* not necessarily done */
+
+
+    /*
+     * pxtcp is no longer registered with poll manager, so it's safe
+     * to close the socket.
+     */
+    if (pxtcp->sock != INVALID_SOCKET) {
+        closesocket(pxtcp->sock);
+        pxtcp->sock = INVALID_SOCKET;
+    }
+
+    /*
+     * We might have already dissociated from a fully closed pcb, or
+     * guest might have sent us a reset while msg_delete was in
+     * transit.  If there's no pcb, we are done.
+     */
+    if (pxtcp->pcb == NULL) {
+        pollmgr_refptr_unref(pxtcp->rp);
+        pxtcp_free(pxtcp);
+        return;
+    }
+
+    /*
+     * Have we completely forwarded all inbound traffic to the guest?
+     *
+     * We may still be waiting for ACKs.  We may have failed to send
+     * some of the data (tcp_write() failed with ERR_MEM).  We may
+     * have failed to send the FIN (tcp_shutdown() failed with
+     * ERR_MEM).
+     */
+    if (pxtcp_pcb_forward_inbound_done(pxtcp)) {
+        pxtcp_pcb_dissociate(pxtcp);
+        pollmgr_refptr_unref(pxtcp->rp);
+        pxtcp_free(pxtcp);
+    }
+    else {
+        DPRINTF2(("delete: pxtcp %p; pcb %p:"
+                  " unacked %d, unsent %d, vacant %d, %s - DEFER!\n",
+                  (void *)pxtcp, (void *)pxtcp->pcb,
+                  (int)pxtcp->inbuf.unacked,
+                  (int)pxtcp->inbuf.unsent,
+                  (int)pxtcp->inbuf.vacant,
+                  pxtcp->inbound_close_done ? "FIN sent" : "FIN is NOT sent"));
+
+        LWIP_ASSERT1(!pxtcp->deferred_delete);
+        pxtcp->deferred_delete = 1;
+    }
+}
+
+
+/**
+ * If we couldn't delete pxtcp right away in the msg_delete callback
+ * from the poll manager thread, we repeat the check at the end of
+ * relevant pcb callbacks.
+ */
+DECLINLINE(void)
+pxtcp_pcb_maybe_deferred_delete(struct pxtcp *pxtcp)
+{
+    if (pxtcp->deferred_delete && pxtcp_pcb_forward_inbound_done(pxtcp)) {
+        pxtcp_pcb_delete_pxtcp(pxtcp);
+    }
+}
+
+
+/**
+ * Poll manager callbacks should use this convenience wrapper to
+ * schedule pxtcp deletion on the lwip thread and to deregister from
+ * the poll manager.
+ */
+static int
+pxtcp_schedule_delete(struct pxtcp *pxtcp)
+{
+    /*
+     * If pollmgr_refptr_get() is called by any channel before
+     * scheduled deletion happens, let them know we are gone.
+     */
+    pxtcp->pmhdl.slot = -1;
+
+    /*
+     * Schedule deletion.  Since poll manager thread may be pre-empted
+     * right after we send the message, the deletion may actually
+     * happen on the lwip thread before we return from this function,
+     * so it's not safe to refer to pxtcp after this call.
+     */
+    proxy_lwip_post(&pxtcp->msg_delete);
+
+    /* tell poll manager to deregister us */
+    return -1;
+}
+
+
+/**
+ * Lwip thread callback invoked via pxtcp::msg_reset
+ *
+ * Like pxtcp_pcb_delete(), but sends RST to the guest before
+ * deleting this pxtcp.
+ */
+static void
+pxtcp_pcb_reset_pxtcp(void *ctx)
+{
+    struct pxtcp *pxtcp = (struct pxtcp *)ctx;
+    LWIP_ASSERT1(pxtcp != NULL);
+
+    DPRINTF0(("%s: pxtcp %p, pcb %p, sock %d\n",
+              __func__, (void *)pxtcp, (void *)pxtcp->pcb, pxtcp->sock));
+
+    if (pxtcp->sock != INVALID_SOCKET) {
+        proxy_reset_socket(pxtcp->sock);
+        pxtcp->sock = INVALID_SOCKET;
+    }
+
+    if (pxtcp->pcb != NULL) {
+        struct tcp_pcb *pcb = pxtcp->pcb;
+        pxtcp_pcb_dissociate(pxtcp);
+        tcp_abort(pcb);
+    }
+
+    pollmgr_refptr_unref(pxtcp->rp);
+    pxtcp_free(pxtcp);
+}
+
+
+
+/**
+ * Poll manager callbacks should use this convenience wrapper to
+ * schedule pxtcp reset and deletion on the lwip thread and to
+ * deregister from the poll manager.
+ *
+ * See pxtcp_schedule_delete() for additional comments.
+ */
+static int
+pxtcp_schedule_reset(struct pxtcp *pxtcp)
+{
+    pxtcp->pmhdl.slot = -1;
+    proxy_lwip_post(&pxtcp->msg_reset);
+    return -1;
+}
+
+
+/**
+ * Reject proxy connection attempt.  Depending on the cause (sockerr)
+ * we may just drop the pcb silently, generate an ICMP datagram or
+ * send TCP reset.
+ */
+static void
+pxtcp_pcb_reject(struct tcp_pcb *pcb, int sockerr,
+                 struct netif *netif,  struct pbuf *p)
+{
+    int reset = 0;
+
+    if (sockerr == ECONNREFUSED) {
+        reset = 1;
+    }
+    else if (p != NULL) {
+        struct netif *oif;
+
+        LWIP_ASSERT1(netif != NULL);
+
+        oif = ip_current_netif();
+        ip_current_netif() = netif;
+
+        if (PCB_ISIPV6(pcb)) {
+            if (sockerr == EHOSTDOWN) {
+                icmp6_dest_unreach(p, ICMP6_DUR_ADDRESS); /* XXX: ??? */
+            }
+            else if (sockerr == EHOSTUNREACH
+                     || sockerr == ENETDOWN
+                     || sockerr == ENETUNREACH)
+            {
+                icmp6_dest_unreach(p, ICMP6_DUR_NO_ROUTE);
+            }
+        }
+        else {
+            if (sockerr == EHOSTDOWN
+                || sockerr == EHOSTUNREACH
+                || sockerr == ENETDOWN
+                || sockerr == ENETUNREACH)
+            {
+                icmp_dest_unreach(p, ICMP_DUR_HOST);
+            }
+        }
+
+        ip_current_netif() = oif;
+    }
+
+    tcp_abandon(pcb, reset);
+}
+
+
+/**
+ * Called from poll manager thread via pxtcp::msg_accept when proxy
+ * failed to connect to the destination.  Also called when we failed
+ * to register pxtcp with poll manager.
+ *
+ * This is like pxtcp_pcb_reset_pxtcp() but is more discriminate in
+ * how this unestablished connection is terminated.
+ */
+static void
+pxtcp_pcb_accept_refuse(void *ctx)
+{
+    struct pxtcp *pxtcp = (struct pxtcp *)ctx;
+
+    DPRINTF0(("%s: pxtcp %p, pcb %p, sock %d: %R[sockerr]\n",
+              __func__, (void *)pxtcp, (void *)pxtcp->pcb,
+              pxtcp->sock, pxtcp->sockerr));
+
+    LWIP_ASSERT1(pxtcp != NULL);
+    LWIP_ASSERT1(pxtcp->sock == INVALID_SOCKET);
+
+    if (pxtcp->pcb != NULL) {
+        struct tcp_pcb *pcb = pxtcp->pcb;
+        pxtcp_pcb_dissociate(pxtcp);
+        pxtcp_pcb_reject(pcb,  pxtcp->sockerr, pxtcp->netif, pxtcp->unsent);
+    }
+
+    pollmgr_refptr_unref(pxtcp->rp);
+    pxtcp_free(pxtcp);
+}
+
+
+/**
+ * Convenience wrapper for poll manager connect callback to reject
+ * connection attempt.
+ *
+ * Like pxtcp_schedule_reset(), but the callback is more discriminate
+ * in how this unestablished connection is terminated.
+ */
+static int
+pxtcp_schedule_reject(struct pxtcp *pxtcp)
+{
+    pxtcp->msg_accept.msg.cb.function = pxtcp_pcb_accept_refuse;
+    pxtcp->pmhdl.slot = -1;
+    proxy_lwip_post(&pxtcp->msg_accept);
+    return -1;
+}
+
+
+/**
+ * Global tcp_proxy_accept() callback for proxied outgoing TCP
+ * connections from guest(s).
+ */
+static err_t
+pxtcp_pcb_heard(void *arg, struct tcp_pcb *newpcb, struct pbuf *syn)
+{
+    LWIP_UNUSED_ARG(arg);
+
+    return pxtcp_pcb_accept_outbound(newpcb, syn,
+               PCB_ISIPV6(newpcb), &newpcb->local_ip, newpcb->local_port);
+}
+
+
+err_t
+pxtcp_pcb_accept_outbound(struct tcp_pcb *newpcb, struct pbuf *p,
+                          int is_ipv6, ipX_addr_t *dst_addr, u16_t dst_port)
+{
+    struct pxtcp *pxtcp;
+    ipX_addr_t mapped_dst_addr;
+    int sdom;
+    SOCKET sock;
+    ssize_t nsent;
+    int sockerr = 0;
+
+    /*
+     * TCP first calls accept callback when it receives the first SYN
+     * and "tentatively accepts" new proxied connection attempt.  When
+     * proxy "confirms" the SYN and sends SYN|ACK and the guest
+     * replies with ACK the accept callback is called again, this time
+     * with the established connection.
+     */
+    LWIP_ASSERT1(newpcb->state == SYN_RCVD_0);
+    tcp_accept(newpcb, pxtcp_pcb_accept);
+    tcp_arg(newpcb, NULL);
+
+    tcp_setprio(newpcb, TCP_PRIO_MAX);
+
+    pxremap_outbound_ipX(is_ipv6, &mapped_dst_addr, dst_addr);
+
+    sdom = is_ipv6 ? PF_INET6 : PF_INET;
+    sock = proxy_connected_socket(sdom, SOCK_STREAM,
+                                  &mapped_dst_addr, dst_port);
+    if (sock == INVALID_SOCKET) {
+        sockerr = SOCKERRNO();
+        goto abort;
+    }
+
+    pxtcp = pxtcp_allocate();
+    if (pxtcp == NULL) {
+        proxy_reset_socket(sock);
+        goto abort;
+    }
+
+    /* save initial datagram in case we need to reply with ICMP */
+    if (p != NULL) {
+        pbuf_ref(p);
+        pxtcp->unsent = p;
+        pxtcp->netif = ip_current_netif();
+    }
+
+    pxtcp_pcb_associate(pxtcp, newpcb);
+    pxtcp->sock = sock;
+
+    pxtcp->pmhdl.callback = pxtcp_pmgr_connect;
+    pxtcp->events = POLLOUT;
+
+    nsent = pxtcp_chan_send(POLLMGR_CHAN_PXTCP_ADD, pxtcp);
+    if (nsent < 0) {
+        pxtcp->sock = INVALID_SOCKET;
+        proxy_reset_socket(sock);
+        pxtcp_pcb_accept_refuse(pxtcp);
+        return ERR_ABRT;
+    }
+
+    return ERR_OK;
+
+  abort:
+    DPRINTF0(("%s: pcb %p, sock %d: %R[sockerr]\n",
+              __func__, (void *)newpcb, sock, sockerr));
+    pxtcp_pcb_reject(newpcb, sockerr, ip_current_netif(), p);
+    return ERR_ABRT;
+}
+
+
+/**
+ * tcp_proxy_accept() callback for accepted proxied outgoing TCP
+ * connections from guest(s).  This is "real" accept with three-way
+ * handshake completed.
+ */
+static err_t
+pxtcp_pcb_accept(void *arg, struct tcp_pcb *pcb, err_t error)
+{
+    struct pxtcp *pxtcp = (struct pxtcp *)arg;
+
+    LWIP_UNUSED_ARG(pcb);       /* used only in asserts */
+    LWIP_UNUSED_ARG(error);     /* always ERR_OK */
+
+    LWIP_ASSERT1(pxtcp != NULL);
+    LWIP_ASSERT1(pxtcp->pcb = pcb);
+    LWIP_ASSERT1(pcb->callback_arg == pxtcp);
+
+    /* send any inbound data that are already queued */
+    pxtcp_pcb_forward_inbound(pxtcp);
+    return ERR_OK;
+}
+
+
+/**
+ * Initial poll manager callback for proxied outgoing TCP connections.
+ * pxtcp_pcb_accept() sets pxtcp::pmhdl::callback to this.
+ *
+ * Waits for connect(2) to the destination to complete.  On success
+ * replaces itself with pxtcp_pmgr_pump() callback common to all
+ * established TCP connections.
+ */
+static int
+pxtcp_pmgr_connect(struct pollmgr_handler *handler, SOCKET fd, int revents)
+{
+    struct pxtcp *pxtcp;
+    RT_NOREF(fd);
+
+    pxtcp = (struct pxtcp *)handler->data;
+    LWIP_ASSERT1(handler == &pxtcp->pmhdl);
+    LWIP_ASSERT1(fd == pxtcp->sock);
+    LWIP_ASSERT1(pxtcp->sockerr == 0);
+
+    if (revents & POLLNVAL) {
+        pxtcp->sock = INVALID_SOCKET;
+        pxtcp->sockerr = ETIMEDOUT;
+        return pxtcp_schedule_reject(pxtcp);
+    }
+
+    /*
+     * Solaris and NetBSD don't report either POLLERR or POLLHUP when
+     * connect(2) fails, just POLLOUT.  In that case we always need to
+     * check SO_ERROR.
+     */
+#if defined(RT_OS_SOLARIS) || defined(RT_OS_NETBSD)
+# define CONNECT_CHECK_ERROR POLLOUT
+#else
+# define CONNECT_CHECK_ERROR (POLLERR | POLLHUP)
+#endif
+
+    /*
+     * Check the cause of the failure so that pxtcp_pcb_reject() may
+     * behave accordingly.
+     */
+    if (revents & CONNECT_CHECK_ERROR) {
+        socklen_t optlen = (socklen_t)sizeof(pxtcp->sockerr);
+        int status;
+        SOCKET s;
+
+        status = getsockopt(pxtcp->sock, SOL_SOCKET, SO_ERROR,
+                            (char *)&pxtcp->sockerr, &optlen);
+        if (RT_UNLIKELY(status == SOCKET_ERROR)) { /* should not happen */
+            DPRINTF(("%s: sock %d: SO_ERROR failed: %R[sockerr]\n",
+                     __func__, fd, SOCKERRNO()));
+            pxtcp->sockerr = ETIMEDOUT;
+        }
+        else {
+            /* don't spam this log on successful connect(2) */
+            if ((revents & (POLLERR | POLLHUP)) /* we were told it's failed */
+                || pxtcp->sockerr != 0)         /* we determined it's failed */
+            {
+                DPRINTF(("%s: sock %d: connect: %R[sockerr]\n",
+                         __func__, fd, pxtcp->sockerr));
+            }
+
+            if ((revents & (POLLERR | POLLHUP))
+                && RT_UNLIKELY(pxtcp->sockerr == 0))
+            {
+                /* if we're told it's failed, make sure it's marked as such */
+                pxtcp->sockerr = ETIMEDOUT;
+            }
+        }
+
+        if (pxtcp->sockerr != 0) {
+            s = pxtcp->sock;
+            pxtcp->sock = INVALID_SOCKET;
+            closesocket(s);
+            return pxtcp_schedule_reject(pxtcp);
+        }
+    }
+
+    if (revents & POLLOUT) { /* connect is successful */
+        /* confirm accept to the guest */
+        proxy_lwip_post(&pxtcp->msg_accept);
+
+        /*
+         * Switch to common callback used for all established proxied
+         * connections.
+         */
+        pxtcp->pmhdl.callback = pxtcp_pmgr_pump;
+
+        /*
+         * Initially we poll for incoming traffic only.  Outgoing
+         * traffic is fast-forwarded by pxtcp_pcb_recv(); if it fails
+         * it will ask us to poll for POLLOUT too.
+         */
+        pxtcp->events = POLLIN;
+        return pxtcp->events;
+    }
+
+    /* should never get here */
+    DPRINTF0(("%s: pxtcp %p, sock %d: unexpected revents 0x%x\n",
+              __func__, (void *)pxtcp, fd, revents));
+    return pxtcp_schedule_reset(pxtcp);
+}
+
+
+/**
+ * Called from poll manager thread via pxtcp::msg_accept when proxy
+ * connected to the destination.  Finalize accept by sending SYN|ACK
+ * to the guest.
+ */
+static void
+pxtcp_pcb_accept_confirm(void *ctx)
+{
+    struct pxtcp *pxtcp = (struct pxtcp *)ctx;
+    err_t error;
+
+    LWIP_ASSERT1(pxtcp != NULL);
+    if (pxtcp->pcb == NULL) {
+        return;
+    }
+
+    /* we are not going to reply with ICMP, so we can drop initial pbuf */
+    if (pxtcp->unsent != NULL) {
+        pbuf_free(pxtcp->unsent);
+        pxtcp->unsent = NULL;
+    }
+
+    error = tcp_proxy_accept_confirm(pxtcp->pcb);
+
+    /*
+     * If lwIP failed to enqueue SYN|ACK because it's out of pbufs it
+     * abandons the pcb.  Retrying that is not very easy, since it
+     * would require keeping "fractional state".  From guest's point
+     * of view there is no reply to its SYN so it will either resend
+     * the SYN (effetively triggering full connection retry for us),
+     * or it will eventually time out.
+     */
+    if (error == ERR_ABRT) {
+        pxtcp->pcb = NULL;      /* pcb is gone */
+        pxtcp_chan_send_weak(POLLMGR_CHAN_PXTCP_RESET, pxtcp);
+    }
+
+    /*
+     * else if (error != ERR_OK): even if tcp_output() failed with
+     * ERR_MEM - don't give up, that SYN|ACK is enqueued and will be
+     * retransmitted eventually.
+     */
+}
+
+
+/**
+ * Entry point for port-forwarding.
+ *
+ * fwtcp accepts new incoming connection, creates pxtcp for the socket
+ * (with no pcb yet) and adds it to the poll manager (polling for
+ * errors only).  Then it calls this function to construct the pcb and
+ * perform connection to the guest.
+ */
+void
+pxtcp_pcb_connect(struct pxtcp *pxtcp, const struct fwspec *fwspec)
+{
+    struct sockaddr_storage ss;
+    socklen_t sslen;
+    struct tcp_pcb *pcb;
+    ipX_addr_t src_addr, dst_addr;
+    u16_t src_port, dst_port;
+    int status;
+    err_t error;
+
+    LWIP_ASSERT1(pxtcp != NULL);
+    LWIP_ASSERT1(pxtcp->pcb == NULL);
+    LWIP_ASSERT1(fwspec->stype == SOCK_STREAM);
+
+    pcb = tcp_new();
+    if (pcb == NULL) {
+        goto reset;
+    }
+
+    tcp_setprio(pcb, TCP_PRIO_MAX);
+    pxtcp_pcb_associate(pxtcp, pcb);
+
+    sslen = sizeof(ss);
+    status = getpeername(pxtcp->sock, (struct sockaddr *)&ss, &sslen);
+    if (status  == SOCKET_ERROR) {
+        goto reset;
+    }
+
+    /* nit: compares PF and AF, but they are the same everywhere */
+    LWIP_ASSERT1(ss.ss_family == fwspec->sdom);
+
+    status = fwany_ipX_addr_set_src(&src_addr, (const struct sockaddr *)&ss);
+    if (status == PXREMAP_FAILED) {
+        goto reset;
+    }
+
+    if (ss.ss_family == PF_INET) {
+        const struct sockaddr_in *peer4 = (const struct sockaddr_in *)&ss;
+
+        src_port = peer4->sin_port;
+
+        memcpy(&dst_addr.ip4, &fwspec->dst.sin.sin_addr, sizeof(ip_addr_t));
+        dst_port = fwspec->dst.sin.sin_port;
+    }
+    else { /* PF_INET6 */
+        const struct sockaddr_in6 *peer6 = (const struct sockaddr_in6 *)&ss;
+        ip_set_v6(pcb, 1);
+
+        src_port = peer6->sin6_port;
+
+        memcpy(&dst_addr.ip6, &fwspec->dst.sin6.sin6_addr, sizeof(ip6_addr_t));
+        dst_port = fwspec->dst.sin6.sin6_port;
+    }
+
+    /* lwip port arguments are in host order */
+    src_port = ntohs(src_port);
+    dst_port = ntohs(dst_port);
+
+    error = tcp_proxy_bind(pcb, ipX_2_ip(&src_addr), src_port);
+    if (error != ERR_OK) {
+        goto reset;
+    }
+
+    error = tcp_connect(pcb, ipX_2_ip(&dst_addr), dst_port,
+                        /* callback: */ pxtcp_pcb_connected);
+    if (error != ERR_OK) {
+        goto reset;
+    }
+
+    return;
+
+  reset:
+    pxtcp_chan_send_weak(POLLMGR_CHAN_PXTCP_RESET, pxtcp);
+}
+
+
+/**
+ * Port-forwarded connection to guest is successful, pump data.
+ */
+static err_t
+pxtcp_pcb_connected(void *arg, struct tcp_pcb *pcb, err_t error)
+{
+    struct pxtcp *pxtcp = (struct pxtcp *)arg;
+
+    LWIP_ASSERT1(error == ERR_OK); /* always called with ERR_OK */
+    LWIP_UNUSED_ARG(error);
+
+    LWIP_ASSERT1(pxtcp != NULL);
+    LWIP_ASSERT1(pxtcp->pcb == pcb);
+    LWIP_ASSERT1(pcb->callback_arg == pxtcp);
+    LWIP_UNUSED_ARG(pcb);
+
+    DPRINTF0(("%s: new pxtcp %p; pcb %p; sock %d\n",
+              __func__, (void *)pxtcp, (void *)pxtcp->pcb, pxtcp->sock));
+
+    /* ACK on connection is like ACK on data in pxtcp_pcb_sent() */
+    pxtcp_chan_send_weak(POLLMGR_CHAN_PXTCP_POLLIN, pxtcp);
+
+    return ERR_OK;
+}
+
+
+/**
+ * tcp_recv() callback.
+ */
+static err_t
+pxtcp_pcb_recv(void *arg, struct tcp_pcb *pcb, struct pbuf *p, err_t error)
+{
+    struct pxtcp *pxtcp = (struct pxtcp *)arg;
+
+    LWIP_ASSERT1(error == ERR_OK); /* always called with ERR_OK */
+    LWIP_UNUSED_ARG(error);
+
+    LWIP_ASSERT1(pxtcp != NULL);
+    LWIP_ASSERT1(pxtcp->pcb == pcb);
+    LWIP_ASSERT1(pcb->callback_arg == pxtcp);
+    LWIP_UNUSED_ARG(pcb);
+
+
+    /*
+     * Have we done sending previous batch?
+     */
+    if (pxtcp->unsent != NULL) {
+        if (p != NULL) {
+            /*
+             * Return an error to tell TCP to hold onto that pbuf.
+             * It will be presented to us later from tcp_fasttmr().
+             */
+            return ERR_WOULDBLOCK;
+        }
+        else {
+            /*
+             * Unlike data, p == NULL indicating orderly shutdown is
+             * NOT presented to us again
+             */
+            pxtcp->outbound_close = 1;
+            return ERR_OK;
+        }
+    }
+
+
+    /*
+     * Guest closed?
+     */
+    if (p == NULL) {
+        pxtcp->outbound_close = 1;
+        pxtcp_pcb_forward_outbound_close(pxtcp);
+        return ERR_OK;
+    }
+
+
+    /*
+     * Got data, send what we can without blocking.
+     */
+    return pxtcp_pcb_forward_outbound(pxtcp, p);
+}
+
+
+/**
+ * Guest half-closed its TX side of the connection.
+ *
+ * Called either immediately from pxtcp_pcb_recv() when it gets NULL,
+ * or from pxtcp_pcb_forward_outbound() when it finishes forwarding
+ * previously unsent data and sees pxtcp::outbound_close flag saved by
+ * pxtcp_pcb_recv().
+ */
+static void
+pxtcp_pcb_forward_outbound_close(struct pxtcp *pxtcp)
+{
+    struct tcp_pcb *pcb;
+
+    LWIP_ASSERT1(pxtcp != NULL);
+    LWIP_ASSERT1(pxtcp->outbound_close);
+    LWIP_ASSERT1(!pxtcp->outbound_close_done);
+
+    pcb = pxtcp->pcb;
+    LWIP_ASSERT1(pcb != NULL);
+
+    DPRINTF(("outbound_close: pxtcp %p; pcb %p %s\n",
+             (void *)pxtcp, (void *)pcb, tcp_debug_state_str(pcb->state)));
+
+
+    /* set the flag first, since shutdown() may trigger POLLHUP */
+    pxtcp->outbound_close_done = 1;
+    shutdown(pxtcp->sock, SHUT_WR); /* half-close the socket */
+
+#if !(HAVE_TCP_POLLHUP & POLLOUT)
+    /*
+     * We need to nudge poll manager manually, since OS will not
+     * report POLLHUP.
+     */
+    if (pxtcp->inbound_close) {
+        pxtcp_chan_send_weak(POLLMGR_CHAN_PXTCP_DEL, pxtcp);
+    }
+#endif
+
+
+    /* no more outbound data coming to us */
+    tcp_recv(pcb, NULL);
+
+    /*
+     * If we have already done inbound close previously (active close
+     * on the pcb), then we must not hold onto a pcb in TIME_WAIT
+     * state since those will be recycled by lwip when it runs out of
+     * free pcbs in the pool.
+     *
+     * The test is true also for a pcb in CLOSING state that waits
+     * just for the ACK of its FIN (to transition to TIME_WAIT).
+     */
+    if (pxtcp_pcb_forward_inbound_done(pxtcp)) {
+        pxtcp_pcb_dissociate(pxtcp);
+    }
+}
+
+
+/**
+ * Forward outbound data from pcb to socket.
+ *
+ * Called by pxtcp_pcb_recv() to forward new data and by callout
+ * triggered by POLLOUT on the socket to send previously unsent data.
+ *
+ * (Re)scehdules one-time callout if not all data are sent.
+ */
+static err_t
+pxtcp_pcb_forward_outbound(struct pxtcp *pxtcp, struct pbuf *p)
+{
+    struct pbuf *qs, *q;
+    size_t qoff;
+    size_t forwarded;
+    int sockerr;
+
+    LWIP_ASSERT1(pxtcp->unsent == NULL || pxtcp->unsent == p);
+
+    forwarded = 0;
+    sockerr = 0;
+
+    q = NULL;
+    qoff = 0;
+
+    qs = p;
+    while (qs != NULL) {
+        IOVEC iov[8];
+        const size_t iovsize = sizeof(iov)/sizeof(iov[0]);
+        size_t fwd1;
+        ssize_t nsent;
+        size_t i;
+
+        fwd1 = 0;
+        for (i = 0, q = qs; i < iovsize && q != NULL; ++i, q = q->next) {
+            LWIP_ASSERT1(q->len > 0);
+            IOVEC_SET_BASE(iov[i], q->payload);
+            IOVEC_SET_LEN(iov[i], q->len);
+            fwd1 += q->len;
+        }
+
+        /*
+         * TODO: This is where application-level proxy can hook into
+         * to process outbound traffic.
+         */
+        nsent = pxtcp_sock_send(pxtcp, iov, i);
+
+        if (nsent == (ssize_t)fwd1) {
+            /* successfully sent this chain fragment completely */
+            forwarded += nsent;
+            qs = q;
+        }
+        else if (nsent >= 0) {
+            /* successfully sent only some data */
+            forwarded += nsent;
+
+            /* find the first pbuf that was not completely forwarded */
+            qoff = nsent;
+            for (i = 0, q = qs; i < iovsize && q != NULL; ++i, q = q->next) {
+                if (qoff < q->len) {
+                    break;
+                }
+                qoff -= q->len;
+            }
+            LWIP_ASSERT1(q != NULL);
+            LWIP_ASSERT1(qoff < q->len);
+            break;
+        }
+        else {
+            sockerr = -nsent;
+
+            /*
+             * Some errors are really not errors - if we get them,
+             * it's not different from getting nsent == 0, so filter
+             * them out here.
+             */
+            if (proxy_error_is_transient(sockerr)) {
+                sockerr = 0;
+            }
+            q = qs;
+            qoff = 0;
+            break;
+        }
+    }
+
+    if (forwarded > 0) {
+        DPRINTF2(("forward_outbound: pxtcp %p, pcb %p: sent %d bytes\n",
+                  (void *)pxtcp, (void *)pxtcp->pcb, (int)forwarded));
+        tcp_recved(pxtcp->pcb, (u16_t)forwarded);
+    }
+
+    if (q == NULL) { /* everything is forwarded? */
+        LWIP_ASSERT1(sockerr == 0);
+        LWIP_ASSERT1(forwarded == p->tot_len);
+
+        pxtcp->unsent = NULL;
+        pbuf_free(p);
+        if (pxtcp->outbound_close) {
+            pxtcp_pcb_forward_outbound_close(pxtcp);
+        }
+    }
+    else {
+        if (q != p) {
+            /* free forwarded pbufs at the beginning of the chain */
+            pbuf_ref(q);
+            pbuf_free(p);
+        }
+        if (qoff > 0) {
+            /* advance payload pointer past the forwarded part */
+            pbuf_header(q, -(s16_t)qoff);
+        }
+        pxtcp->unsent = q;
+        DPRINTF2(("forward_outbound: pxtcp %p, pcb %p: kept %d bytes\n",
+                  (void *)pxtcp, (void *)pxtcp->pcb, (int)q->tot_len));
+
+        /*
+         * Have sendmsg() failed?
+         *
+         * Connection reset will be detected by poll and
+         * pxtcp_schedule_reset() will be called.
+         *
+         * Otherwise something *really* unexpected must have happened,
+         * so we'd better abort.
+         */
+        if (sockerr != 0 && sockerr != ECONNRESET) {
+            struct tcp_pcb *pcb = pxtcp->pcb;
+            DPRINTF2(("forward_outbound: pxtcp %p, pcb %p: %R[sockerr]\n",
+                      (void *)pxtcp, (void *)pcb, sockerr));
+
+            pxtcp_pcb_dissociate(pxtcp);
+
+            tcp_abort(pcb);
+
+            /* call error callback manually since we've already dissociated */
+            pxtcp_pcb_err((void *)pxtcp, ERR_ABRT);
+            return ERR_ABRT;
+        }
+
+        /* schedule one-shot POLLOUT on the socket */
+        pxtcp_chan_send_weak(POLLMGR_CHAN_PXTCP_POLLOUT, pxtcp);
+    }
+    return ERR_OK;
+}
+
+
+#if !defined(RT_OS_WINDOWS)
+static ssize_t
+pxtcp_sock_send(struct pxtcp *pxtcp, IOVEC *iov, size_t iovlen)
+{
+    struct msghdr mh;
+    ssize_t nsent;
+
+#ifdef MSG_NOSIGNAL
+    const int send_flags = MSG_NOSIGNAL;
+#else
+    const int send_flags = 0;
+#endif
+
+    memset(&mh, 0, sizeof(mh));
+
+    mh.msg_iov = iov;
+    mh.msg_iovlen = iovlen;
+
+    nsent = sendmsg(pxtcp->sock, &mh, send_flags);
+    if (nsent < 0) {
+        nsent = -SOCKERRNO();
+    }
+
+    return nsent;
+}
+#else /* RT_OS_WINDOWS */
+static ssize_t
+pxtcp_sock_send(struct pxtcp *pxtcp, IOVEC *iov, size_t iovlen)
+{
+    DWORD nsent;
+    int status;
+
+    status = WSASend(pxtcp->sock, iov, (DWORD)iovlen, &nsent,
+                     0, NULL, NULL);
+    if (status == SOCKET_ERROR) {
+        return -SOCKERRNO();
+    }
+
+    return nsent;
+}
+#endif /* RT_OS_WINDOWS */
+
+
+/**
+ * Callback from poll manager (on POLLOUT) to send data from
+ * pxtcp::unsent pbuf to socket.
+ */
+static void
+pxtcp_pcb_write_outbound(void *ctx)
+{
+    struct pxtcp *pxtcp = (struct pxtcp *)ctx;
+    LWIP_ASSERT1(pxtcp != NULL);
+
+    if (pxtcp->pcb == NULL) {
+        return;
+    }
+
+    pxtcp_pcb_forward_outbound(pxtcp, pxtcp->unsent);
+}
+
+
+/**
+ * Common poll manager callback used by both outgoing and incoming
+ * (port-forwarded) connections that has connected socket.
+ */
+static int
+pxtcp_pmgr_pump(struct pollmgr_handler *handler, SOCKET fd, int revents)
+{
+    struct pxtcp *pxtcp;
+    int status;
+    int sockerr;
+    RT_NOREF(fd);
+
+    pxtcp = (struct pxtcp *)handler->data;
+    LWIP_ASSERT1(handler == &pxtcp->pmhdl);
+    LWIP_ASSERT1(fd == pxtcp->sock);
+
+    if (revents & POLLNVAL) {
+        pxtcp->sock = INVALID_SOCKET;
+        return pxtcp_schedule_reset(pxtcp);
+    }
+
+    if (revents & POLLERR) {
+        socklen_t optlen = (socklen_t)sizeof(sockerr);
+
+        status = getsockopt(pxtcp->sock, SOL_SOCKET, SO_ERROR,
+                            (char *)&sockerr, &optlen);
+        if (status == SOCKET_ERROR) { /* should not happen */
+            DPRINTF(("sock %d: POLLERR: SO_ERROR failed: %R[sockerr]\n",
+                     fd, SOCKERRNO()));
+        }
+        else {
+            DPRINTF0(("sock %d: POLLERR: %R[sockerr]\n", fd, sockerr));
+        }
+        return pxtcp_schedule_reset(pxtcp);
+    }
+
+    if (revents & POLLOUT) {
+        pxtcp->events &= ~POLLOUT;
+        proxy_lwip_post(&pxtcp->msg_outbound);
+    }
+
+    if (revents & POLLIN) {
+        ssize_t nread;
+        int stop_pollin;
+
+        nread = pxtcp_sock_read(pxtcp, &stop_pollin);
+        if (nread < 0) {
+            sockerr = -(int)nread;
+            DPRINTF0(("sock %d: POLLIN: %R[sockerr]\n", fd, sockerr));
+            return pxtcp_schedule_reset(pxtcp);
+        }
+
+        if (stop_pollin) {
+            pxtcp->events &= ~POLLIN;
+        }
+
+        if (nread > 0) {
+            proxy_lwip_post(&pxtcp->msg_inbound);
+#if !HAVE_TCP_POLLHUP
+            /*
+             * If host does not report POLLHUP for closed sockets
+             * (e.g. NetBSD) we should check for full close manually.
+             */
+            if (pxtcp->inbound_close && pxtcp->outbound_close_done) {
+                LWIP_ASSERT1((revents & POLLHUP) == 0);
+                return pxtcp_schedule_delete(pxtcp);
+            }
+#endif
+        }
+    }
+
+#if !HAVE_TCP_POLLHUP
+    LWIP_ASSERT1((revents & POLLHUP) == 0);
+#else
+    if (revents & POLLHUP) {
+        DPRINTF(("sock %d: HUP\n", fd));
+
+#if HAVE_TCP_POLLHUP == POLLIN
+        /*
+         * XXX: OSX reports POLLHUP once more when inbound is already
+         * half-closed (which has already been reported as a "normal"
+         * POLLHUP, handled below), the socket is polled for POLLOUT
+         * (guest sends a lot of data that we can't push out fast
+         * enough), and remote sends a reset - e.g. an http client
+         * that half-closes after request and then aborts the transfer.
+         *
+         * It really should have been reported as POLLERR, but it
+         * seems OSX never reports POLLERR for sockets.
+         */
+#if defined(RT_OS_DARWIN)
+        {
+            socklen_t optlen = (socklen_t)sizeof(sockerr);
+
+            status = getsockopt(pxtcp->sock, SOL_SOCKET, SO_ERROR,
+                                (char *)&sockerr, &optlen);
+            if (status == SOCKET_ERROR) { /* should not happen */
+                DPRINTF(("sock %d: POLLHUP: SO_ERROR failed: %R[sockerr]\n",
+                         fd, SOCKERRNO()));
+                sockerr = ECONNRESET;
+            }
+            else if (sockerr != 0) {
+                DPRINTF0(("sock %d: POLLHUP: %R[sockerr]\n", fd, sockerr));
+            }
+
+            if (sockerr != 0) { /* XXX: should have been POLLERR */
+                return pxtcp_schedule_reset(pxtcp);
+            }
+        }
+#endif  /* RT_OS_DARWIN */
+
+        /*
+         * Remote closed inbound.
+         */
+        if (!pxtcp->outbound_close_done) {
+            /*
+             * We might still need to poll for POLLOUT, but we can not
+             * poll for POLLIN anymore (even if not all data are read)
+             * because we will be spammed by POLLHUP.
+             */
+            pxtcp->events &= ~POLLIN;
+            if (!pxtcp->inbound_close) {
+                /* the rest of the input has to be pulled */
+                proxy_lwip_post(&pxtcp->msg_inpull);
+            }
+        }
+        else
+#endif
+        /*
+         * Both directions are closed.
+         */
+        {
+            LWIP_ASSERT1(pxtcp->outbound_close_done);
+
+            if (pxtcp->inbound_close) {
+                /* there's no unread data, we are done */
+                return pxtcp_schedule_delete(pxtcp);
+            }
+            else {
+                /* pull the rest of the input first (deferred_delete) */
+                pxtcp->pmhdl.slot = -1;
+                proxy_lwip_post(&pxtcp->msg_inpull);
+                return -1;
+            }
+            /* NOTREACHED */
+        }
+
+    }
+#endif  /* HAVE_TCP_POLLHUP */
+
+    return pxtcp->events;
+}
+
+
+/**
+ * Read data from socket to ringbuf.  This may be used both on lwip
+ * and poll manager threads.
+ *
+ * Flag pointed to by pstop is set when further reading is impossible,
+ * either temporary when buffer is full, or permanently when EOF is
+ * received.
+ *
+ * Returns number of bytes read.  NB: EOF is reported as 1!
+ *
+ * Returns zero if nothing was read, either because buffer is full, or
+ * if no data is available (EWOULDBLOCK, EINTR &c).
+ *
+ * Returns -errno on real socket errors.
+ */
+static ssize_t
+pxtcp_sock_read(struct pxtcp *pxtcp, int *pstop)
+{
+    IOVEC iov[2];
+    size_t iovlen;
+    ssize_t nread;
+
+    const size_t sz = pxtcp->inbuf.bufsize;
+    size_t beg, lim, wrnew;
+
+    *pstop = 0;
+
+    beg = pxtcp->inbuf.vacant;
+    IOVEC_SET_BASE(iov[0], &pxtcp->inbuf.buf[beg]);
+
+    /* lim is the index we can NOT write to */
+    lim = pxtcp->inbuf.unacked;
+    if (lim == 0) {
+        lim = sz - 1;           /* empty slot at the end */
+    }
+    else if (lim == 1 && beg != 0) {
+        lim = sz;               /* empty slot at the beginning */
+    }
+    else {
+        --lim;
+    }
+
+    if (beg == lim) {
+        /*
+         * Buffer is full, stop polling for POLLIN.
+         *
+         * pxtcp_pcb_sent() will re-enable POLLIN when guest ACKs
+         * data, freeing space in the ring buffer.
+         */
+        *pstop = 1;
+        return 0;
+    }
+
+    if (beg < lim) {
+        /* free space in one chunk */
+        iovlen = 1;
+        IOVEC_SET_LEN(iov[0], lim - beg);
+    }
+    else {
+        /* free space in two chunks */
+        iovlen = 2;
+        IOVEC_SET_LEN(iov[0], sz - beg);
+        IOVEC_SET_BASE(iov[1], &pxtcp->inbuf.buf[0]);
+        IOVEC_SET_LEN(iov[1], lim);
+    }
+
+    /*
+     * TODO: This is where application-level proxy can hook into to
+     * process inbound traffic.
+     */
+    nread = pxtcp_sock_recv(pxtcp, iov, iovlen);
+
+    if (nread > 0) {
+        wrnew = beg + nread;
+        if (wrnew >= sz) {
+            wrnew -= sz;
+        }
+        pxtcp->inbuf.vacant = wrnew;
+        DPRINTF2(("pxtcp %p: sock %d read %d bytes\n",
+                  (void *)pxtcp, pxtcp->sock, (int)nread));
+        return nread;
+    }
+    else if (nread == 0) {
+        *pstop = 1;
+        pxtcp->inbound_close = 1;
+        DPRINTF2(("pxtcp %p: sock %d read EOF\n",
+                 (void *)pxtcp, pxtcp->sock));
+        return 1;
+    }
+    else {
+        int sockerr = -nread;
+
+        if (proxy_error_is_transient(sockerr)) {
+            /* haven't read anything, just return */
+            DPRINTF2(("pxtcp %p: sock %d read cancelled\n",
+                      (void *)pxtcp, pxtcp->sock));
+            return 0;
+        }
+        else {
+            /* socket error! */
+            DPRINTF0(("pxtcp %p: sock %d read: %R[sockerr]\n",
+                      (void *)pxtcp, pxtcp->sock, sockerr));
+            return -sockerr;
+        }
+    }
+}
+
+
+#if !defined(RT_OS_WINDOWS)
+static ssize_t
+pxtcp_sock_recv(struct pxtcp *pxtcp, IOVEC *iov, size_t iovlen)
+{
+    struct msghdr mh;
+    ssize_t nread;
+
+    memset(&mh, 0, sizeof(mh));
+
+    mh.msg_iov = iov;
+    mh.msg_iovlen = iovlen;
+
+    nread = recvmsg(pxtcp->sock, &mh, 0);
+    if (nread < 0) {
+        nread = -SOCKERRNO();
+    }
+
+    return nread;
+}
+#else /* RT_OS_WINDOWS */
+static ssize_t
+pxtcp_sock_recv(struct pxtcp *pxtcp, IOVEC *iov, size_t iovlen)
+{
+    DWORD flags;
+    DWORD nread;
+    int status;
+
+    flags = 0;
+    status = WSARecv(pxtcp->sock, iov, (DWORD)iovlen, &nread,
+                     &flags, NULL, NULL);
+    if (status == SOCKET_ERROR) {
+        return -SOCKERRNO();
+    }
+
+    return (ssize_t)nread;
+}
+#endif /* RT_OS_WINDOWS */
+
+
+/**
+ * Callback from poll manager (pxtcp::msg_inbound) to trigger output
+ * from ringbuf to guest.
+ */
+static void
+pxtcp_pcb_write_inbound(void *ctx)
+{
+    struct pxtcp *pxtcp = (struct pxtcp *)ctx;
+    LWIP_ASSERT1(pxtcp != NULL);
+
+    if (pxtcp->pcb == NULL) {
+        return;
+    }
+
+    pxtcp_pcb_forward_inbound(pxtcp);
+}
+
+
+/**
+ * tcp_poll() callback
+ *
+ * We swtich it on when tcp_write() or tcp_shutdown() fail with
+ * ERR_MEM to prevent connection from stalling.  If there are ACKs or
+ * more inbound data then pxtcp_pcb_forward_inbound() will be
+ * triggered again, but if neither happens, tcp_poll() comes to the
+ * rescue.
+ */
+static err_t
+pxtcp_pcb_poll(void *arg, struct tcp_pcb *pcb)
+{
+    struct pxtcp *pxtcp = (struct pxtcp *)arg;
+    LWIP_UNUSED_ARG(pcb);
+
+    DPRINTF2(("%s: pxtcp %p; pcb %p\n",
+              __func__, (void *)pxtcp, (void *)pxtcp->pcb));
+
+    pxtcp_pcb_forward_inbound(pxtcp);
+
+    /*
+     * If the last thing holding up deletion of the pxtcp was failed
+     * tcp_shutdown() and it succeeded, we may be the last callback.
+     */
+    pxtcp_pcb_maybe_deferred_delete(pxtcp);
+
+    return ERR_OK;
+}
+
+
+static void
+pxtcp_pcb_schedule_poll(struct pxtcp *pxtcp)
+{
+    tcp_poll(pxtcp->pcb, pxtcp_pcb_poll, 0);
+}
+
+
+static void
+pxtcp_pcb_cancel_poll(struct pxtcp *pxtcp)
+{
+    tcp_poll(pxtcp->pcb, NULL, 255);
+}
+
+
+/**
+ * Forward inbound data from ring buffer to the guest.
+ *
+ * Scheduled by poll manager thread after it receives more data into
+ * the ring buffer (we have more data to send).
+
+ * Also called from tcp_sent() callback when guest ACKs some data,
+ * increasing pcb->snd_buf (we are permitted to send more data).
+ *
+ * Also called from tcp_poll() callback if previous attempt to forward
+ * inbound data failed with ERR_MEM (we need to try again).
+ */
+static void
+pxtcp_pcb_forward_inbound(struct pxtcp *pxtcp)
+{
+    struct tcp_pcb *pcb;
+    size_t sndbuf;
+    size_t beg, lim, sndlim;
+    size_t toeob, tolim;
+    size_t nsent;
+    err_t error;
+
+    LWIP_ASSERT1(pxtcp != NULL);
+    pcb = pxtcp->pcb;
+    if (pcb == NULL) {
+        return;
+    }
+
+    if (/* __predict_false */ pcb->state < ESTABLISHED) {
+        /*
+         * If we have just confirmed accept of this connection, the
+         * pcb is in SYN_RCVD state and we still haven't received the
+         * ACK of our SYN.  It's only in SYN_RCVD -> ESTABLISHED
+         * transition that lwip decrements pcb->acked so that that ACK
+         * is not reported to pxtcp_pcb_sent().  If we send something
+         * now and immediately close (think "daytime", e.g.)  while
+         * still in SYN_RCVD state, we will move directly to
+         * FIN_WAIT_1 and when our confirming SYN is ACK'ed lwip will
+         * report it to pxtcp_pcb_sent().
+         */
+        DPRINTF2(("forward_inbound: pxtcp %p; pcb %p %s - later...\n",
+                  (void *)pxtcp, (void *)pcb, tcp_debug_state_str(pcb->state)));
+        return;
+    }
+
+
+    beg = pxtcp->inbuf.unsent;  /* private to lwip thread */
+    lim = pxtcp->inbuf.vacant;
+
+    if (beg == lim) {
+        if (pxtcp->inbound_close && !pxtcp->inbound_close_done) {
+            pxtcp_pcb_forward_inbound_close(pxtcp);
+            tcp_output(pcb);
+            return;
+        }
+
+        /*
+         * Else, there's no data to send.
+         *
+         * If there is free space in the buffer, producer will
+         * reschedule us as it receives more data and vacant (lim)
+         * advances.
+         *
+         * If buffer is full when all data have been passed to
+         * tcp_write() but not yet acknowledged, we will advance
+         * unacked on ACK, freeing some space for producer to write to
+         * (then see above).
+         */
+        return;
+    }
+
+    sndbuf = tcp_sndbuf(pcb);
+    if (sndbuf == 0) {
+        /*
+         * Can't send anything now.  As guest ACKs some data, TCP will
+         * call pxtcp_pcb_sent() callback and we will come here again.
+         */
+        return;
+    }
+
+    nsent = 0;
+
+    /*
+     * We have three limits to consider:
+     * - how much data we have in the ringbuf
+     * - how much data we are allowed to send
+     * - ringbuf size
+     */
+    toeob = pxtcp->inbuf.bufsize - beg;
+    if (lim < beg) {            /* lim wrapped */
+        if (sndbuf < toeob) {   /* but we are limited by sndbuf */
+            /* so beg is not going to wrap, treat sndbuf as lim */
+            lim = beg + sndbuf; /* ... and proceed to the simple case */
+        }
+        else { /* we are limited by the end of the buffer, beg will wrap */
+            u8_t maybemore;
+            if (toeob == sndbuf || lim == 0) {
+                maybemore = 0;
+            }
+            else {
+                maybemore = TCP_WRITE_FLAG_MORE;
+            }
+
+            Assert(toeob == (u16_t)toeob);
+            error = tcp_write(pcb, &pxtcp->inbuf.buf[beg], (u16_t)toeob, maybemore);
+            if (error != ERR_OK) {
+                goto writeerr;
+            }
+            nsent += toeob;
+            pxtcp->inbuf.unsent = 0; /* wrap */
+
+            if (maybemore) {
+                beg = 0;
+                sndbuf -= toeob;
+            }
+            else {
+                /* we are done sending, but ... */
+                goto check_inbound_close;
+            }
+        }
+    }
+
+    LWIP_ASSERT1(beg < lim);
+    sndlim = beg + sndbuf;
+    if (lim > sndlim) {
+        lim = sndlim;
+    }
+    tolim = lim - beg;
+    if (tolim > 0) {
+        error = tcp_write(pcb, &pxtcp->inbuf.buf[beg], (u16_t)tolim, 0);
+        if (error != ERR_OK) {
+            goto writeerr;
+        }
+        nsent += tolim;
+        pxtcp->inbuf.unsent = lim;
+    }
+
+  check_inbound_close:
+    if (pxtcp->inbound_close && pxtcp->inbuf.unsent == pxtcp->inbuf.vacant) {
+        pxtcp_pcb_forward_inbound_close(pxtcp);
+    }
+
+    DPRINTF2(("forward_inbound: pxtcp %p, pcb %p: sent %d bytes\n",
+              (void *)pxtcp, (void *)pcb, (int)nsent));
+    tcp_output(pcb);
+    pxtcp_pcb_cancel_poll(pxtcp);
+    return;
+
+  writeerr:
+    if (error == ERR_MEM) {
+        if (nsent > 0) {    /* first write succeeded, second failed */
+            DPRINTF2(("forward_inbound: pxtcp %p, pcb %p: sent %d bytes only\n",
+                      (void *)pxtcp, (void *)pcb, (int)nsent));
+            tcp_output(pcb);
+        }
+        DPRINTF(("forward_inbound: pxtcp %p, pcb %p: ERR_MEM\n",
+                 (void *)pxtcp, (void *)pcb));
+        pxtcp_pcb_schedule_poll(pxtcp);
+    }
+    else {
+        DPRINTF(("forward_inbound: pxtcp %p, pcb %p: %s\n",
+                 (void *)pxtcp, (void *)pcb, proxy_lwip_strerr(error)));
+
+        /* XXX: We shouldn't get ERR_ARG.  Check ERR_CONN conditions early? */
+        LWIP_ASSERT1(error == ERR_MEM);
+    }
+}
+
+
+static void
+pxtcp_pcb_forward_inbound_close(struct pxtcp *pxtcp)
+{
+    struct tcp_pcb *pcb;
+    err_t error;
+
+    LWIP_ASSERT1(pxtcp != NULL);
+    LWIP_ASSERT1(pxtcp->inbound_close);
+    LWIP_ASSERT1(!pxtcp->inbound_close_done);
+    LWIP_ASSERT1(pxtcp->inbuf.unsent == pxtcp->inbuf.vacant);
+
+    pcb = pxtcp->pcb;
+    LWIP_ASSERT1(pcb != NULL);
+
+    DPRINTF(("inbound_close: pxtcp %p; pcb %p: %s\n",
+             (void *)pxtcp, (void *)pcb, tcp_debug_state_str(pcb->state)));
+
+    error = tcp_shutdown(pcb, /*RX*/ 0, /*TX*/ 1);
+    if (error != ERR_OK) {
+        DPRINTF(("inbound_close: pxtcp %p; pcb %p:"
+                 " tcp_shutdown: error=%s\n",
+                 (void *)pxtcp, (void *)pcb, proxy_lwip_strerr(error)));
+        pxtcp_pcb_schedule_poll(pxtcp);
+        return;
+    }
+
+    pxtcp_pcb_cancel_poll(pxtcp);
+    pxtcp->inbound_close_done = 1;
+
+
+    /*
+     * If we have already done outbound close previously (passive
+     * close on the pcb), then we must not hold onto a pcb in LAST_ACK
+     * state since those will be deleted by lwip when that last ack
+     * comes from the guest.
+     *
+     * NB: We do NOT check for deferred delete here, even though we
+     * have just set one of its conditions, inbound_close_done.  We
+     * let pcb callbacks that called us do that.  It's simpler and
+     * cleaner that way.
+     */
+    if (pxtcp->outbound_close_done && pxtcp_pcb_forward_inbound_done(pxtcp)) {
+        pxtcp_pcb_dissociate(pxtcp);
+    }
+}
+
+
+/**
+ * Check that all forwarded inbound data is sent and acked, and that
+ * inbound close is scheduled (we aren't called back when it's acked).
+ */
+DECLINLINE(int)
+pxtcp_pcb_forward_inbound_done(const struct pxtcp *pxtcp)
+{
+    return (pxtcp->inbound_close_done /* also implies that all data forwarded */
+            && pxtcp->inbuf.unacked == pxtcp->inbuf.unsent);
+}
+
+
+/**
+ * tcp_sent() callback - guest acknowledged len bytes.
+ *
+ * We can advance inbuf::unacked index, making more free space in the
+ * ringbuf and wake up producer on poll manager thread.
+ *
+ * We can also try to send more data if we have any since pcb->snd_buf
+ * was increased and we are now permitted to send more.
+ */
+static err_t
+pxtcp_pcb_sent(void *arg, struct tcp_pcb *pcb, u16_t len)
+{
+    struct pxtcp *pxtcp = (struct pxtcp *)arg;
+    size_t unacked;
+
+    LWIP_ASSERT1(pxtcp != NULL);
+    LWIP_ASSERT1(pxtcp->pcb == pcb);
+    LWIP_ASSERT1(pcb->callback_arg == pxtcp);
+    LWIP_UNUSED_ARG(pcb);       /* only in assert */
+
+    DPRINTF2(("%s: pxtcp %p; pcb %p: +%d ACKed:"
+              " unacked %d, unsent %d, vacant %d\n",
+              __func__, (void *)pxtcp, (void *)pcb, (int)len,
+              (int)pxtcp->inbuf.unacked,
+              (int)pxtcp->inbuf.unsent,
+              (int)pxtcp->inbuf.vacant));
+
+    if (/* __predict_false */ len == 0) {
+        /* we are notified to start pulling */
+        LWIP_ASSERT1(!pxtcp->inbound_close);
+        LWIP_ASSERT1(pxtcp->inbound_pull);
+
+        unacked = pxtcp->inbuf.unacked;
+    }
+    else {
+        /*
+         * Advance unacked index.  Guest acknowledged the data, so it
+         * won't be needed again for potential retransmits.
+         */
+        unacked = pxtcp->inbuf.unacked + len;
+        if (unacked > pxtcp->inbuf.bufsize) {
+            unacked -= pxtcp->inbuf.bufsize;
+        }
+        pxtcp->inbuf.unacked = unacked;
+    }
+
+    /* arrange for more inbound data */
+    if (!pxtcp->inbound_close) {
+        if (!pxtcp->inbound_pull) {
+            /* wake up producer, in case it has stopped polling for POLLIN */
+            pxtcp_chan_send_weak(POLLMGR_CHAN_PXTCP_POLLIN, pxtcp);
+#ifdef RT_OS_WINDOWS
+            /**
+             * We have't got enought room in ring buffer to read atm,
+             * but we don't want to lose notification from WSAW4ME when
+             * space would be available, so we reset event with empty recv
+             */
+            recv(pxtcp->sock, NULL, 0, 0);
+#endif
+        }
+        else {
+            ssize_t nread;
+            int stop_pollin;    /* ignored */
+
+            nread = pxtcp_sock_read(pxtcp, &stop_pollin);
+
+            if (nread < 0) {
+                int sockerr = -(int)nread;
+                LWIP_UNUSED_ARG(sockerr);
+                DPRINTF0(("%s: sock %d: %R[sockerr]\n",
+                          __func__, pxtcp->sock, sockerr));
+
+#if HAVE_TCP_POLLHUP == POLLIN /* see counterpart in pxtcp_pmgr_pump() */
+                /*
+                 * It may still be registered with poll manager for POLLOUT.
+                 */
+                pxtcp_chan_send_weak(POLLMGR_CHAN_PXTCP_RESET, pxtcp);
+                return ERR_OK;
+#else
+                /*
+                 * It is no longer registered with poll manager so we
+                 * can kill it directly.
+                 */
+                pxtcp_pcb_reset_pxtcp(pxtcp);
+                return ERR_ABRT;
+#endif
+            }
+        }
+    }
+
+    /* forward more data if we can */
+    if (!pxtcp->inbound_close_done) {
+        pxtcp_pcb_forward_inbound(pxtcp);
+
+        /*
+         * NB: we might have dissociated from a pcb that transitioned
+         * to LAST_ACK state, so don't refer to pcb below.
+         */
+    }
+
+
+    /* have we got all the acks? */
+    if (pxtcp->inbound_close                          /* no more new data */
+        && pxtcp->inbuf.unsent == pxtcp->inbuf.vacant /* all data is sent */
+        && unacked == pxtcp->inbuf.unsent)            /* ... and is acked */
+    {
+        char *buf;
+
+        DPRINTF(("%s: pxtcp %p; pcb %p; all data ACKed\n",
+                 __func__, (void *)pxtcp, (void *)pxtcp->pcb));
+
+        /* no more retransmits, so buf is not needed */
+        buf = pxtcp->inbuf.buf;
+        pxtcp->inbuf.buf = NULL;
+        free(buf);
+
+        /* no more acks, so no more callbacks */
+        if (pxtcp->pcb != NULL) {
+            tcp_sent(pxtcp->pcb, NULL);
+        }
+
+        /*
+         * We may be the last callback for this pcb if we have also
+         * successfully forwarded inbound_close.
+         */
+        pxtcp_pcb_maybe_deferred_delete(pxtcp);
+    }
+
+    return ERR_OK;
+}
+
+
+#if HAVE_TCP_POLLHUP
+/**
+ * Callback from poll manager (pxtcp::msg_inpull) to switch
+ * pxtcp_pcb_sent() to actively pull the last bits of input.  See
+ * POLLHUP comment in pxtcp_pmgr_pump().
+ *
+ * pxtcp::sock is deregistered from poll manager after this callback
+ * is scheduled.
+ */
+static void
+pxtcp_pcb_pull_inbound(void *ctx)
+{
+    struct pxtcp *pxtcp = (struct pxtcp *)ctx;
+    LWIP_ASSERT1(pxtcp != NULL);
+
+    if (pxtcp->pcb == NULL) {
+        DPRINTF(("%s: pxtcp %p: PCB IS GONE\n", __func__, (void *)pxtcp));
+        pxtcp_pcb_reset_pxtcp(pxtcp);
+        return;
+    }
+
+    pxtcp->inbound_pull = 1;
+    if (pxtcp->pmhdl.slot < 0) {
+        DPRINTF(("%s: pxtcp %p: pcb %p (deferred delete)\n",
+                 __func__, (void *)pxtcp, (void *)pxtcp->pcb));
+        pxtcp->deferred_delete = 1;
+    }
+    else {
+        DPRINTF(("%s: pxtcp %p: pcb %p\n",
+                 __func__, (void *)pxtcp, (void *)pxtcp->pcb));
+    }
+
+    pxtcp_pcb_sent(pxtcp, pxtcp->pcb, 0);
+}
+#endif  /* HAVE_TCP_POLLHUP */
+
+
+/**
+ * tcp_err() callback.
+ *
+ * pcb is not passed to this callback since it may be already
+ * deallocated by the stack, but we can't do anything useful with it
+ * anyway since connection is gone.
+ */
+static void
+pxtcp_pcb_err(void *arg, err_t error)
+{
+    struct pxtcp *pxtcp = (struct pxtcp *)arg;
+    LWIP_ASSERT1(pxtcp != NULL);
+
+    /*
+     * ERR_CLSD is special - it is reported here when:
+     *
+     * . guest has already half-closed
+     * . we send FIN to guest when external half-closes
+     * . guest acks that FIN
+     *
+     * Since connection is closed but receive has been already closed
+     * lwip can only report this via tcp_err.  At this point the pcb
+     * is still alive, so we can peek at it if need be.
+     *
+     * The interesting twist is when the ACK from guest that akcs our
+     * FIN also acks some data.  In this scenario lwip will NOT call
+     * tcp_sent() callback with the ACK for that last bit of data but
+     * instead will call tcp_err with ERR_CLSD right away.  Since that
+     * ACK also acknowledges all the data, we should run some of
+     * pxtcp_pcb_sent() logic here.
+     */
+    if (error == ERR_CLSD) {
+        struct tcp_pcb *pcb = pxtcp->pcb; /* still alive */
+
+        DPRINTF2(("ERR_CLSD: pxtcp %p; pcb %p:"
+                  " pcb->acked %d;"
+                  " unacked %d, unsent %d, vacant %d\n",
+                  (void *)pxtcp, (void *)pcb,
+                  pcb->acked,
+                  (int)pxtcp->inbuf.unacked,
+                  (int)pxtcp->inbuf.unsent,
+                  (int)pxtcp->inbuf.vacant));
+
+        LWIP_ASSERT1(pxtcp->pcb == pcb);
+        LWIP_ASSERT1(pcb->callback_arg == pxtcp);
+
+        if (pcb->acked > 0) {
+            pxtcp_pcb_sent(pxtcp, pcb, pcb->acked);
+        }
+        return;
+    }
+
+    DPRINTF0(("tcp_err: pxtcp=%p, error=%s\n",
+              (void *)pxtcp, proxy_lwip_strerr(error)));
+
+    pxtcp->pcb = NULL;          /* pcb is gone */
+    if (pxtcp->deferred_delete) {
+        pxtcp_pcb_reset_pxtcp(pxtcp);
+    }
+    else {
+        pxtcp_chan_send_weak(POLLMGR_CHAN_PXTCP_RESET, pxtcp);
+    }
+}