upstream commit

Add 'reverse' dynamic forwarding which combines dynamic
forwarding (-D) with remote forwarding (-R) where the remote-forwarded port
expects SOCKS-requests.

The SSH server code is unchanged and the parsing happens at the SSH
clients side. Thus the full SOCKS-request is sent over the forwarded
channel and the client parses c->output. Parsing happens in
channel_before_prepare_select(), _before_ the select bitmask is
computed in the pre[] handlers, but after network input processing
in the post[] handlers.

help and ok djm@

Upstream-ID: aa25a6a3851064f34fe719e0bf15656ad5a64b89

6 files changed, 346 insertions, 118 deletions

diff --git a/channels.c b/channels.c
index 89b7d3486..8ef37c453 100644
--- a/channels.c
+++ b/channels.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: channels.c,v 1.371 2017/09/19 12:10:30 millert Exp $ */
+/* $OpenBSD: channels.c,v 1.372 2017/09/21 19:16:53 markus Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -209,6 +209,8 @@ static const char *channel_rfwd_bind_host(const char *listen_host);
 /* non-blocking connect helpers */
 static int connect_next(struct channel_connect *);
 static void channel_connect_ctx_free(struct channel_connect *);
+static Channel *rdynamic_connect_prepare(struct ssh *, char *, char *);
+static int rdynamic_connect_finish(struct ssh *, Channel *);
 
 /* Setup helper */
 static void channel_handler_init(struct ssh_channels *sc);
@@ -282,6 +284,8 @@ channel_lookup(struct ssh *ssh, int id)
         case SSH_CHANNEL_LARVAL:
         case SSH_CHANNEL_CONNECTING:
         case SSH_CHANNEL_DYNAMIC:
+        case SSH_CHANNEL_RDYNAMIC_OPEN:
+        case SSH_CHANNEL_RDYNAMIC_FINISH:
         case SSH_CHANNEL_OPENING:
         case SSH_CHANNEL_OPEN:
         case SSH_CHANNEL_ABANDONED:
@@ -671,6 +675,7 @@ channel_still_open(struct ssh *ssh)
                 case SSH_CHANNEL_CLOSED:
                 case SSH_CHANNEL_AUTH_SOCKET:
                 case SSH_CHANNEL_DYNAMIC:
+                case SSH_CHANNEL_RDYNAMIC_OPEN:
                 case SSH_CHANNEL_CONNECTING:
                 case SSH_CHANNEL_ZOMBIE:
                 case SSH_CHANNEL_ABANDONED:
@@ -681,6 +686,7 @@ channel_still_open(struct ssh *ssh)
                         continue;
                 case SSH_CHANNEL_OPENING:
                 case SSH_CHANNEL_OPEN:
+                case SSH_CHANNEL_RDYNAMIC_FINISH:
                 case SSH_CHANNEL_X11_OPEN:
                 case SSH_CHANNEL_MUX_CLIENT:
                 case SSH_CHANNEL_MUX_PROXY:
@@ -707,6 +713,8 @@ channel_find_open(struct ssh *ssh)
                 switch (c->type) {
                 case SSH_CHANNEL_CLOSED:
                 case SSH_CHANNEL_DYNAMIC:
+                case SSH_CHANNEL_RDYNAMIC_OPEN:
+                case SSH_CHANNEL_RDYNAMIC_FINISH:
                 case SSH_CHANNEL_X11_LISTENER:
                 case SSH_CHANNEL_PORT_LISTENER:
                 case SSH_CHANNEL_RPORT_LISTENER:
@@ -772,6 +780,8 @@ channel_open_message(struct ssh *ssh)
                 case SSH_CHANNEL_OPENING:
                 case SSH_CHANNEL_CONNECTING:
                 case SSH_CHANNEL_DYNAMIC:
+                case SSH_CHANNEL_RDYNAMIC_OPEN:
+                case SSH_CHANNEL_RDYNAMIC_FINISH:
                 case SSH_CHANNEL_OPEN:
                 case SSH_CHANNEL_X11_OPEN:
                 case SSH_CHANNEL_MUX_PROXY:
@@ -1124,8 +1134,7 @@ channel_pre_mux_client(struct ssh *ssh,
 
 /* try to decode a socks4 header */
 static int
-channel_decode_socks4(struct ssh *ssh, Channel *c,
-    fd_set *readset, fd_set *writeset)
+channel_decode_socks4(Channel *c, struct sshbuf *input, struct sshbuf *output)
 {
         const u_char *p;
         char *host;
@@ -1141,11 +1150,11 @@ channel_decode_socks4(struct ssh *ssh, Channel *c,
 
         debug2("channel %d: decode socks4", c->self);
 
-        have = sshbuf_len(c->input);
+        have = sshbuf_len(input);
         len = sizeof(s4_req);
         if (have < len)
                 return 0;
-        p = sshbuf_ptr(c->input);
+        p = sshbuf_ptr(input);
 
         need = 1;
         /* SOCKS4A uses an invalid IP address 0.0.0.x */
@@ -1170,15 +1179,15 @@ channel_decode_socks4(struct ssh *ssh, Channel *c,
         }
         if (found < need)
                 return 0;
-        if ((r = sshbuf_get(c->input, &s4_req.version, 1)) != 0 ||
-            (r = sshbuf_get(c->input, &s4_req.command, 1)) != 0 ||
-            (r = sshbuf_get(c->input, &s4_req.dest_port, 2)) != 0 ||
-            (r = sshbuf_get(c->input, &s4_req.dest_addr, 4)) != 0) {
+        if ((r = sshbuf_get(input, &s4_req.version, 1)) != 0 ||
+            (r = sshbuf_get(input, &s4_req.command, 1)) != 0 ||
+            (r = sshbuf_get(input, &s4_req.dest_port, 2)) != 0 ||
+            (r = sshbuf_get(input, &s4_req.dest_addr, 4)) != 0) {
                 debug("channels %d: decode socks4: %s", c->self, ssh_err(r));
                 return -1;
         }
-        have = sshbuf_len(c->input);
-        p = sshbuf_ptr(c->input);
+        have = sshbuf_len(input);
+        p = sshbuf_ptr(input);
         if (memchr(p, '\0', have) == NULL) {
                 error("channel %d: decode socks4: user not nul terminated",
                     c->self);
@@ -1188,7 +1197,7 @@ channel_decode_socks4(struct ssh *ssh, Channel *c,
         debug2("channel %d: decode socks4: user %s/%d", c->self, p, len);
         len++; /* trailing '\0' */
         strlcpy(username, p, sizeof(username));
-        if ((r = sshbuf_consume(c->input, len)) != 0) {
+        if ((r = sshbuf_consume(input, len)) != 0) {
                 fatal("%s: channel %d: consume: %s", __func__,
                     c->self, ssh_err(r));
         }
@@ -1198,8 +1207,8 @@ channel_decode_socks4(struct ssh *ssh, Channel *c,
                 host = inet_ntoa(s4_req.dest_addr);
                 c->path = xstrdup(host);
         } else {                                /* SOCKS4A: two strings */
-                have = sshbuf_len(c->input);
-                p = sshbuf_ptr(c->input);
+                have = sshbuf_len(input);
+                p = sshbuf_ptr(input);
                 if (memchr(p, '\0', have) == NULL) {
                         error("channel %d: decode socks4a: host not nul "
                             "terminated", c->self);
@@ -1215,7 +1224,7 @@ channel_decode_socks4(struct ssh *ssh, Channel *c,
                         return -1;
                 }
                 c->path = xstrdup(p);
-                if ((r = sshbuf_consume(c->input, len)) != 0) {
+                if ((r = sshbuf_consume(input, len)) != 0) {
                         fatal("%s: channel %d: consume: %s", __func__,
                             c->self, ssh_err(r));
                 }
@@ -1234,7 +1243,7 @@ channel_decode_socks4(struct ssh *ssh, Channel *c,
         s4_rsp.command = 90;                    /* cd: req granted */
         s4_rsp.dest_port = 0;                   /* ignored */
         s4_rsp.dest_addr.s_addr = INADDR_ANY;   /* ignored */
-        if ((r = sshbuf_put(c->output, &s4_rsp, sizeof(s4_rsp))) != 0) {
+        if ((r = sshbuf_put(output, &s4_rsp, sizeof(s4_rsp))) != 0) {
                 fatal("%s: channel %d: append reply: %s", __func__,
                     c->self, ssh_err(r));
         }
@@ -1251,8 +1260,7 @@ channel_decode_socks4(struct ssh *ssh, Channel *c,
 #define SSH_SOCKS5_SUCCESS      0x00
 
 static int
-channel_decode_socks5(struct ssh *ssh, Channel *c,
-    fd_set *readset, fd_set *writeset)
+channel_decode_socks5(Channel *c, struct sshbuf *input, struct sshbuf *output)
 {
         /* XXX use get/put_u8 instead of trusting struct padding */
         struct {
@@ -1268,10 +1276,10 @@ channel_decode_socks5(struct ssh *ssh, Channel *c,
         int r;
 
         debug2("channel %d: decode socks5", c->self);
-        p = sshbuf_ptr(c->input);
+        p = sshbuf_ptr(input);
         if (p[0] != 0x05)
                 return -1;
-        have = sshbuf_len(c->input);
+        have = sshbuf_len(input);
         if (!(c->flags & SSH_SOCKS5_AUTHDONE)) {
                 /* format: ver | nmethods | methods */
                 if (have < 2)
@@ -1291,17 +1299,16 @@ channel_decode_socks5(struct ssh *ssh, Channel *c,
                             c->self);
                         return -1;
                 }
-                if ((r = sshbuf_consume(c->input, nmethods + 2)) != 0) {
+                if ((r = sshbuf_consume(input, nmethods + 2)) != 0) {
                         fatal("%s: channel %d: consume: %s", __func__,
                             c->self, ssh_err(r));
                 }
                 /* version, method */
-                if ((r = sshbuf_put_u8(c->output, 0x05)) != 0 ||
-                    (r = sshbuf_put_u8(c->output, SSH_SOCKS5_NOAUTH)) != 0) {
+                if ((r = sshbuf_put_u8(output, 0x05)) != 0 ||
+                    (r = sshbuf_put_u8(output, SSH_SOCKS5_NOAUTH)) != 0) {
                         fatal("%s: channel %d: append reply: %s", __func__,
                             c->self, ssh_err(r));
                 }
-                FD_SET(c->sock, writeset);
                 c->flags |= SSH_SOCKS5_AUTHDONE;
                 debug2("channel %d: socks5 auth done", c->self);
                 return 0;                               /* need more */
@@ -1338,19 +1345,19 @@ channel_decode_socks5(struct ssh *ssh, Channel *c,
                 need++;
         if (have < need)
                 return 0;
-        if ((r = sshbuf_consume(c->input, sizeof(s5_req))) != 0) {
+        if ((r = sshbuf_consume(input, sizeof(s5_req))) != 0) {
                 fatal("%s: channel %d: consume: %s", __func__,
                     c->self, ssh_err(r));
         }
         if (s5_req.atyp == SSH_SOCKS5_DOMAIN) {
                 /* host string length */
-                if ((r = sshbuf_consume(c->input, 1)) != 0) {
+                if ((r = sshbuf_consume(input, 1)) != 0) {
                         fatal("%s: channel %d: consume: %s", __func__,
                             c->self, ssh_err(r));
                 }
         }
-        if ((r = sshbuf_get(c->input, &dest_addr, addrlen)) != 0 ||
-            (r = sshbuf_get(c->input, &dest_port, 2)) != 0) {
+        if ((r = sshbuf_get(input, &dest_addr, addrlen)) != 0 ||
+            (r = sshbuf_get(input, &dest_port, 2)) != 0) {
                 debug("channel %d: parse addr/port: %s", c->self, ssh_err(r));
                 return -1;
         }
@@ -1380,9 +1387,9 @@ channel_decode_socks5(struct ssh *ssh, Channel *c,
         s5_rsp.atyp = SSH_SOCKS5_IPV4;
         dest_port = 0;                          /* ignored */
 
-        if ((r = sshbuf_put(c->output, &s5_rsp, sizeof(s5_rsp))) != 0 ||
-            (r = sshbuf_put_u32(c->output, ntohl(INADDR_ANY))) != 0 ||
-            (r = sshbuf_put(c->output, &dest_port, sizeof(dest_port))) != 0)
+        if ((r = sshbuf_put(output, &s5_rsp, sizeof(s5_rsp))) != 0 ||
+            (r = sshbuf_put_u32(output, ntohl(INADDR_ANY))) != 0 ||
+            (r = sshbuf_put(output, &dest_port, sizeof(dest_port))) != 0)
                 fatal("%s: channel %d: append reply: %s", __func__,
                     c->self, ssh_err(r));
         return 1;
@@ -1434,10 +1441,10 @@ channel_pre_dynamic(struct ssh *ssh, Channel *c,
         /* XXX sshbuf_peek_u8? */
         switch (p[0]) {
         case 0x04:
-                ret = channel_decode_socks4(ssh, c, readset, writeset);
+                ret = channel_decode_socks4(c, c->input, c->output);
                 break;
         case 0x05:
-                ret = channel_decode_socks5(ssh, c, readset, writeset);
+                ret = channel_decode_socks5(c, c->input, c->output);
                 break;
         default:
                 ret = -1;
@@ -1449,6 +1456,8 @@ channel_pre_dynamic(struct ssh *ssh, Channel *c,
                 debug2("channel %d: pre_dynamic: need more", c->self);
                 /* need more */
                 FD_SET(c->sock, readset);
+                if (sshbuf_len(c->output))
+                        FD_SET(c->sock, writeset);
         } else {
                 /* switch to the next state */
                 c->type = SSH_CHANNEL_OPENING;
@@ -1456,6 +1465,81 @@ channel_pre_dynamic(struct ssh *ssh, Channel *c,
         }
 }
 
+/* simulate read-error */
+static void
+rdynamic_close(struct ssh *ssh, Channel *c)
+{
+        c->type = SSH_CHANNEL_OPEN;
+        chan_read_failed(ssh, c);
+        sshbuf_reset(c->input);
+        chan_ibuf_empty(ssh, c);
+        sshbuf_reset(c->output);
+        chan_write_failed(ssh, c);
+}
+
+/* reverse dynamic port forwarding */
+static void
+channel_before_prepare_select_rdynamic(struct ssh *ssh, Channel *c)
+{
+        const u_char *p;
+        u_int have, len;
+        int r, ret;
+
+        have = sshbuf_len(c->output);
+        debug2("channel %d: pre_rdynamic: have %d", c->self, have);
+        /* sshbuf_dump(c->output, stderr); */
+        /* EOF received */
+        if (c->flags & CHAN_EOF_RCVD) {
+                if ((r = sshbuf_consume(c->output, have)) != 0) {
+                        fatal("%s: channel %d: consume: %s",
+                            __func__, c->self, ssh_err(r));
+                }
+                rdynamic_close(ssh, c);
+                return;
+        }
+        /* check if the fixed size part of the packet is in buffer. */
+        if (have < 3)
+                return;
+        /* try to guess the protocol */
+        p = sshbuf_ptr(c->output);
+        switch (p[0]) {
+        case 0x04:
+                /* switch input/output for reverse forwarding */
+                ret = channel_decode_socks4(c, c->output, c->input);
+                break;
+        case 0x05:
+                ret = channel_decode_socks5(c, c->output, c->input);
+                break;
+        default:
+                ret = -1;
+                break;
+        }
+        if (ret < 0) {
+                rdynamic_close(ssh, c);
+        } else if (ret == 0) {
+                debug2("channel %d: pre_rdynamic: need more", c->self);
+                /* send socks request to peer */
+                len = sshbuf_len(c->input);
+                if (len > 0 && len < c->remote_window) {
+                        if ((r = sshpkt_start(ssh, SSH2_MSG_CHANNEL_DATA)) != 0 ||
+                            (r = sshpkt_put_u32(ssh, c->remote_id)) != 0 ||
+                            (r = sshpkt_put_stringb(ssh, c->input)) != 0 ||
+                            (r = sshpkt_send(ssh)) != 0) {
+                                fatal("%s: channel %i: rdynamic: %s", __func__,
+                                    c->self, ssh_err(r));
+                        }
+                        if ((r = sshbuf_consume(c->input, len)) != 0) {
+                                fatal("%s: channel %d: consume: %s",
+                                    __func__, c->self, ssh_err(r));
+                        }
+                        c->remote_window -= len;
+                }
+        } else if (rdynamic_connect_finish(ssh, c) < 0) {
+                /* the connect failed */
+                rdynamic_close(ssh, c);
+        }
+}
+
 /* This is our fake X11 server socket. */
 static void
 channel_post_x11_listener(struct ssh *ssh, Channel *c,
@@ -1699,14 +1783,15 @@ static void
 channel_post_connecting(struct ssh *ssh, Channel *c,
     fd_set *readset, fd_set *writeset)
 {
-        int err = 0, sock, r;
+        int err = 0, sock, isopen, r;
         socklen_t sz = sizeof(err);
 
         if (!FD_ISSET(c->sock, writeset))
                 return;
         if (!c->have_remote_id)
                 fatal(":%s: channel %d: no remote id", __func__, c->self);
-
+        /* for rdynamic the OPEN_CONFIRMATION has been sent already */
+        isopen = (c->type == SSH_CHANNEL_RDYNAMIC_FINISH);
         if (getsockopt(c->sock, SOL_SOCKET, SO_ERROR, &err, &sz) < 0) {
                 err = errno;
                 error("getsockopt SO_ERROR failed");
@@ -1716,14 +1801,21 @@ channel_post_connecting(struct ssh *ssh, Channel *c,
                     c->self, c->connect_ctx.host, c->connect_ctx.port);
                 channel_connect_ctx_free(&c->connect_ctx);
                 c->type = SSH_CHANNEL_OPEN;
-                if ((r = sshpkt_start(ssh,
-                    SSH2_MSG_CHANNEL_OPEN_CONFIRMATION)) != 0 ||
-                    (r = sshpkt_put_u32(ssh, c->remote_id)) != 0 ||
-                    (r = sshpkt_put_u32(ssh, c->self)) != 0 ||
-                    (r = sshpkt_put_u32(ssh, c->local_window)) != 0 ||
-                    (r = sshpkt_put_u32(ssh, c->local_maxpacket)) != 0) {
-                        fatal("%s: channel %i: confirm: %s", __func__,
-                            c->self, ssh_err(r));
+                if (isopen) {
+                        /* no message necessary */
+                } else {
+                        if ((r = sshpkt_start(ssh,
+                            SSH2_MSG_CHANNEL_OPEN_CONFIRMATION)) != 0 ||
+                            (r = sshpkt_put_u32(ssh, c->remote_id)) != 0 ||
+                            (r = sshpkt_put_u32(ssh, c->self)) != 0 ||
+                            (r = sshpkt_put_u32(ssh, c->local_window)) != 0 ||
+                            (r = sshpkt_put_u32(ssh, c->local_maxpacket))
+                            != 0)
+                                fatal("%s: channel %i: confirm: %s", __func__,
+                                    c->self, ssh_err(r));
+                        if ((r = sshpkt_send(ssh)) != 0)
+                                fatal("%s: channel %i: %s", __func__, c->self,
+                                    ssh_err(r));
                 }
         } else {
                 debug("channel %d: connection failed: %s",
@@ -1739,22 +1831,27 @@ channel_post_connecting(struct ssh *ssh, Channel *c,
                 error("connect_to %.100s port %d: failed.",
                     c->connect_ctx.host, c->connect_ctx.port);
                 channel_connect_ctx_free(&c->connect_ctx);
-                if ((r = sshpkt_start(ssh, SSH2_MSG_CHANNEL_OPEN_FAILURE)) != 0 ||
-                    (r = sshpkt_put_u32(ssh, c->remote_id)) != 0 ||
-                    (r = sshpkt_put_u32(ssh, SSH2_OPEN_CONNECT_FAILED)) != 0) {
-                        fatal("%s: channel %i: failure: %s", __func__,
-                            c->self, ssh_err(r));
-                }
-                if ((datafellows & SSH_BUG_OPENFAILURE) == 0 &&
-                    ((r = sshpkt_put_cstring(ssh, strerror(err))) != 0 ||
-                    (r = sshpkt_put_cstring(ssh, "")) != 0)) {
-                        fatal("%s: channel %i: failure: %s", __func__,
-                            c->self, ssh_err(r));
+                if (isopen) {
+                        rdynamic_close(ssh, c);
+                } else {
+                        if ((r = sshpkt_start(ssh,
+                            SSH2_MSG_CHANNEL_OPEN_FAILURE)) != 0 ||
+                            (r = sshpkt_put_u32(ssh, c->remote_id)) != 0 ||
+                            (r = sshpkt_put_u32(ssh, SSH2_OPEN_CONNECT_FAILED))
+                            != 0)
+                                fatal("%s: channel %i: failure: %s", __func__,
+                                    c->self, ssh_err(r));
+                        if ((datafellows & SSH_BUG_OPENFAILURE) == 0 &&
+                            ((r = sshpkt_put_cstring(ssh, strerror(err))) != 0 ||
+                            (r = sshpkt_put_cstring(ssh, "")) != 0))
+                                fatal("%s: channel %i: failure: %s", __func__,
+                                    c->self, ssh_err(r));
+                        if ((r = sshpkt_send(ssh)) != 0)
+                                fatal("%s: channel %i: %s", __func__, c->self,
+                                    ssh_err(r));
+                        chan_mark_dead(ssh, c);
                 }
-                chan_mark_dead(ssh, c);
         }
-        if ((r = sshpkt_send(ssh)) != 0)
-                fatal("%s: channel %i: %s", __func__, c->self, ssh_err(r));
 }
 
 static int
@@ -2187,6 +2284,7 @@ channel_handler_init(struct ssh_channels *sc)
         pre[SSH_CHANNEL_AUTH_SOCKET] =          &channel_pre_listener;
         pre[SSH_CHANNEL_CONNECTING] =           &channel_pre_connecting;
         pre[SSH_CHANNEL_DYNAMIC] =              &channel_pre_dynamic;
+        pre[SSH_CHANNEL_RDYNAMIC_FINISH] =      &channel_pre_connecting;
         pre[SSH_CHANNEL_MUX_LISTENER] =         &channel_pre_listener;
         pre[SSH_CHANNEL_MUX_CLIENT] =           &channel_pre_mux_client;
 
@@ -2199,6 +2297,7 @@ channel_handler_init(struct ssh_channels *sc)
         post[SSH_CHANNEL_AUTH_SOCKET] =         &channel_post_auth_listener;
         post[SSH_CHANNEL_CONNECTING] =          &channel_post_connecting;
         post[SSH_CHANNEL_DYNAMIC] =             &channel_post_open;
+        post[SSH_CHANNEL_RDYNAMIC_FINISH] =     &channel_post_connecting;
         post[SSH_CHANNEL_MUX_LISTENER] =        &channel_post_mux_listener;
         post[SSH_CHANNEL_MUX_CLIENT] =          &channel_post_mux_client;
 
@@ -2280,6 +2379,27 @@ channel_handler(struct ssh *ssh, int table,
 }
 
 /*
+ * Create sockets before allocating the select bitmasks.
+ * This is necessary for things that need to happen after reading
+ * the network-input but before channel_prepare_select().
+ */
+static void
+channel_before_prepare_select(struct ssh *ssh)
+{
+        struct ssh_channels *sc = ssh->chanctxt;
+        Channel *c;
+        u_int i, oalloc;
+
+        for (i = 0, oalloc = sc->channels_alloc; i < oalloc; i++) {
+                c = sc->channels[i];
+                if (c == NULL)
+                        continue;
+                if (c->type == SSH_CHANNEL_RDYNAMIC_OPEN)
+                        channel_before_prepare_select_rdynamic(ssh, c);
+        }
+}
+
+/*
  * Allocate/update select bitmasks and add any bits relevant to channels in
  * select bitmasks.
  */
@@ -2289,6 +2409,8 @@ channel_prepare_select(struct ssh *ssh, fd_set **readsetp, fd_set **writesetp,
 {
         u_int n, sz, nfdset;
 
+        channel_before_prepare_select(ssh); /* might update channel_max_fd */
+
         n = MAXIMUM(*maxfdp, ssh->chanctxt->channel_max_fd);
 
         nfdset = howmany(n+1, NFDBITS);
@@ -2794,6 +2916,8 @@ channel_input_data(int type, u_int32_t seq, struct ssh *ssh)
 
         /* Ignore any data for non-open channels (might happen on close) */
         if (c->type != SSH_CHANNEL_OPEN &&
+            c->type != SSH_CHANNEL_RDYNAMIC_OPEN &&
+            c->type != SSH_CHANNEL_RDYNAMIC_FINISH &&
             c->type != SSH_CHANNEL_X11_OPEN)
                 return 0;
 
@@ -3032,7 +3156,7 @@ channel_input_window_adjust(int type, u_int32_t seq, struct ssh *ssh)
         if ((c = channel_lookup(ssh, id)) == NULL) {
                 logit("Received window adjust for non-open channel %d.", id);
                 return 0;
-        }
+        }
 
         if (channel_proxy_upstream(c, type, seq, ssh))
                 return 0;
@@ -3939,21 +4063,18 @@ channel_connect_ctx_free(struct channel_connect *cctx)
 }
 
 /*
- * Return CONNECTING channel to remote host:port or local socket path,
+ * Return connecting socket to remote host:port or local socket path,
  * passing back the failure reason if appropriate.
  */
-static Channel *
-connect_to_reason(struct ssh *ssh, const char *name, int port,
-    char *ctype, char *rname, int *reason, const char **errmsg)
+static int
+connect_to_helper(struct ssh *ssh, const char *name, int port, int socktype,
+    char *ctype, char *rname, struct channel_connect *cctx,
+    int *reason, const char **errmsg)
 {
         struct addrinfo hints;
         int gaierr;
         int sock = -1;
         char strport[NI_MAXSERV];
-        struct channel_connect cctx;
-        Channel *c;
-
-        memset(&cctx, 0, sizeof(cctx));
 
         if (port == PORT_STREAMLOCAL) {
                 struct sockaddr_un *sunaddr;
@@ -3961,7 +4082,7 @@ connect_to_reason(struct ssh *ssh, const char *name, int port,
 
                 if (strlen(name) > sizeof(sunaddr->sun_path)) {
                         error("%.100s: %.100s", name, strerror(ENAMETOOLONG));
-                        return (NULL);
+                        return -1;
                 }
 
                 /*
@@ -3974,18 +4095,18 @@ connect_to_reason(struct ssh *ssh, const char *name, int port,
                 ai->ai_addr = (struct sockaddr *)(ai + 1);
                 ai->ai_addrlen = sizeof(*sunaddr);
                 ai->ai_family = AF_UNIX;
-                ai->ai_socktype = SOCK_STREAM;
+                ai->ai_socktype = socktype;
                 ai->ai_protocol = PF_UNSPEC;
                 sunaddr = (struct sockaddr_un *)ai->ai_addr;
                 sunaddr->sun_family = AF_UNIX;
                 strlcpy(sunaddr->sun_path, name, sizeof(sunaddr->sun_path));
-                cctx.aitop = ai;
+                cctx->aitop = ai;
         } else {
                 memset(&hints, 0, sizeof(hints));
                 hints.ai_family = ssh->chanctxt->IPv4or6;
-                hints.ai_socktype = SOCK_STREAM;
+                hints.ai_socktype = socktype;
                 snprintf(strport, sizeof strport, "%d", port);
-                if ((gaierr = getaddrinfo(name, strport, &hints, &cctx.aitop))
+                if ((gaierr = getaddrinfo(name, strport, &hints, &cctx->aitop))
                     != 0) {
                         if (errmsg != NULL)
                                 *errmsg = ssh_gai_strerror(gaierr);
@@ -3993,32 +4114,46 @@ connect_to_reason(struct ssh *ssh, const char *name, int port,
                                 *reason = SSH2_OPEN_CONNECT_FAILED;
                         error("connect_to %.100s: unknown host (%s)", name,
                             ssh_gai_strerror(gaierr));
-                        return NULL;
+                        return -1;
                 }
         }
 
-        cctx.host = xstrdup(name);
-        cctx.port = port;
-        cctx.ai = cctx.aitop;
+        cctx->host = xstrdup(name);
+        cctx->port = port;
+        cctx->ai = cctx->aitop;
 
-        if ((sock = connect_next(&cctx)) == -1) {
+        if ((sock = connect_next(cctx)) == -1) {
                 error("connect to %.100s port %d failed: %s",
                     name, port, strerror(errno));
-                channel_connect_ctx_free(&cctx);
-                return NULL;
+                return -1;
         }
-        c = channel_new(ssh, ctype, SSH_CHANNEL_CONNECTING, sock, sock, -1,
-            CHAN_TCP_WINDOW_DEFAULT, CHAN_TCP_PACKET_DEFAULT, 0, rname, 1);
-        c->connect_ctx = cctx;
-        return c;
+
+        return sock;
 }
 
 /* Return CONNECTING channel to remote host:port or local socket path */
 static Channel *
-connect_to(struct ssh *ssh, const char *name, int port,
+connect_to(struct ssh *ssh, const char *host, int port,
     char *ctype, char *rname)
 {
-        return connect_to_reason(ssh, name, port, ctype, rname, NULL, NULL);
+        struct channel_connect cctx;
+        Channel *c;
+        int sock;
+
+        memset(&cctx, 0, sizeof(cctx));
+        sock = connect_to_helper(ssh, host, port, SOCK_STREAM, ctype, rname,
+            &cctx, NULL, NULL);
+        if (sock == -1) {
+                channel_connect_ctx_free(&cctx);
+                return NULL;
+        }
+        c = channel_new(ssh, ctype, SSH_CHANNEL_CONNECTING, sock, sock, -1,
+            CHAN_TCP_WINDOW_DEFAULT, CHAN_TCP_PACKET_DEFAULT, 0, rname, 1);
+        c->host_port = port;
+        c->path = xstrdup(host);
+        c->connect_ctx = cctx;
+
+        return c;
 }
 
 /*
@@ -4038,6 +4173,9 @@ channel_connect_by_listen_address(struct ssh *ssh, const char *listen_host,
                 if (open_listen_match_tcpip(fp, listen_host, listen_port, 1)) {
                         if (fp->downstream)
                                 return fp->downstream;
+                        if (fp->port_to_connect == 0)
+                                return rdynamic_connect_prepare(ssh,
+                                    ctype, rname);
                         return connect_to(ssh,
                             fp->host_to_connect, fp->port_to_connect,
                             ctype, rname);
@@ -4075,7 +4213,10 @@ channel_connect_to_port(struct ssh *ssh, const char *host, u_short port,
     char *ctype, char *rname, int *reason, const char **errmsg)
 {
         struct ssh_channels *sc = ssh->chanctxt;
+        struct channel_connect cctx;
+        Channel *c;
         u_int i, permit, permit_adm = 1;
+        int sock;
         ForwardPermission *fp;
 
         permit = sc->all_opens_permitted;
@@ -4107,7 +4248,22 @@ channel_connect_to_port(struct ssh *ssh, const char *host, u_short port,
                         *reason = SSH2_OPEN_ADMINISTRATIVELY_PROHIBITED;
                 return NULL;
         }
-        return connect_to_reason(ssh, host, port, ctype, rname, reason, errmsg);
+
+        memset(&cctx, 0, sizeof(cctx));
+        sock = connect_to_helper(ssh, host, port, SOCK_STREAM, ctype, rname,
+            &cctx, reason, errmsg);
+        if (sock == -1) {
+                channel_connect_ctx_free(&cctx);
+                return NULL;
+        }
+
+        c = channel_new(ssh, ctype, SSH_CHANNEL_CONNECTING, sock, sock, -1,
+            CHAN_TCP_WINDOW_DEFAULT, CHAN_TCP_PACKET_DEFAULT, 0, rname, 1);
+        c->host_port = port;
+        c->path = xstrdup(host);
+        c->connect_ctx = cctx;
+
+        return c;
 }
 
 /* Check if connecting to that path is permitted and connect. */
@@ -4174,6 +4330,54 @@ channel_send_window_changes(struct ssh *ssh)
         }
 }
 
+/* Return RDYNAMIC_OPEN channel: channel allows SOCKS, but is not connected */
+static Channel *
+rdynamic_connect_prepare(struct ssh *ssh, char *ctype, char *rname)
+{
+        Channel *c;
+        int r;
+
+        c = channel_new(ssh, ctype, SSH_CHANNEL_RDYNAMIC_OPEN, -1, -1, -1,
+            CHAN_TCP_WINDOW_DEFAULT, CHAN_TCP_PACKET_DEFAULT, 0, rname, 1);
+        c->host_port = 0;
+        c->path = NULL;
+
+        /*
+         * We need to open the channel before we have a FD,
+         * so that we can get SOCKS header from peer.
+         */
+        if ((r = sshpkt_start(ssh, SSH2_MSG_CHANNEL_OPEN_CONFIRMATION)) != 0 ||
+            (r = sshpkt_put_u32(ssh, c->remote_id)) != 0 ||
+            (r = sshpkt_put_u32(ssh, c->self)) != 0 ||
+            (r = sshpkt_put_u32(ssh, c->local_window)) != 0 ||
+            (r = sshpkt_put_u32(ssh, c->local_maxpacket)) != 0) {
+                fatal("%s: channel %i: confirm: %s", __func__,
+                    c->self, ssh_err(r));
+        }
+        return c;
+}
+
+/* Return CONNECTING socket to remote host:port or local socket path */
+static int
+rdynamic_connect_finish(struct ssh *ssh, Channel *c)
+{
+        struct channel_connect cctx;
+        int sock;
+
+        memset(&cctx, 0, sizeof(cctx));
+        sock = connect_to_helper(ssh, c->path, c->host_port, SOCK_STREAM, NULL,
+            NULL, &cctx, NULL, NULL);
+        if (sock == -1)
+                channel_connect_ctx_free(&cctx);
+        else {
+                /* similar to SSH_CHANNEL_CONNECTING but we've already sent the open */
+                c->type = SSH_CHANNEL_RDYNAMIC_FINISH;
+                c->connect_ctx = cctx;
+                channel_register_fds(ssh, c, sock, sock, -1, 0, 1, 0);
+        }
+        return sock;
+}
+
 /* -- X11 forwarding */
 
 /*
diff --git a/channels.h b/channels.h
index d1cf5dc6a..126b04345 100644
--- a/channels.h
+++ b/channels.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: channels.h,v 1.129 2017/09/12 06:35:32 djm Exp $ */
+/* $OpenBSD: channels.h,v 1.130 2017/09/21 19:16:53 markus Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -57,7 +57,9 @@
 #define SSH_CHANNEL_UNIX_LISTENER       18      /* Listening on a domain socket. */
 #define SSH_CHANNEL_RUNIX_LISTENER      19      /* Listening to a R-style domain socket. */
 #define SSH_CHANNEL_MUX_PROXY           20      /* proxy channel for mux-slave */
-#define SSH_CHANNEL_MAX_TYPE            21
+#define SSH_CHANNEL_RDYNAMIC_OPEN       21      /* reverse SOCKS, parsing request */
+#define SSH_CHANNEL_RDYNAMIC_FINISH     22      /* reverse SOCKS, finishing connect */
+#define SSH_CHANNEL_MAX_TYPE            23
 
 #define CHANNEL_CANCEL_PORT_STATIC      -1
 
diff --git a/readconf.c b/readconf.c
index 4f38b27cf..f63894f9c 100644
--- a/readconf.c
+++ b/readconf.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: readconf.c,v 1.278 2017/09/03 23:33:13 djm Exp $ */
+/* $OpenBSD: readconf.c,v 1.279 2017/09/21 19:16:53 markus Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -836,6 +836,7 @@ process_config_line_depth(Options *options, struct passwd *pw, const char *host,
         char **cpptr, fwdarg[256];
         u_int i, *uintptr, max_entries = 0;
         int r, oactive, negated, opcode, *intptr, value, value2, cmdline = 0;
+        int remotefwd, dynamicfwd;
         LogLevel *log_level_ptr;
         SyslogFacility *log_facility_ptr;
         long long val64;
@@ -1255,31 +1256,36 @@ parse_keytypes:
                         fatal("%.200s line %d: Missing port argument.",
                             filename, linenum);
 
-                if (opcode == oLocalForward ||
-                    opcode == oRemoteForward) {
-                        arg2 = strdelim(&s);
-                        if (arg2 == NULL || *arg2 == '\0')
-                                fatal("%.200s line %d: Missing target argument.",
-                                    filename, linenum);
+                remotefwd = (opcode == oRemoteForward);
+                dynamicfwd = (opcode == oDynamicForward);
 
-                        /* construct a string for parse_forward */
-                        snprintf(fwdarg, sizeof(fwdarg), "%s:%s", arg, arg2);
-                } else if (opcode == oDynamicForward) {
-                        strlcpy(fwdarg, arg, sizeof(fwdarg));
+                if (!dynamicfwd) {
+                        arg2 = strdelim(&s);
+                        if (arg2 == NULL || *arg2 == '\0') {
+                                if (remotefwd)
+                                        dynamicfwd = 1;
+                                else
+                                        fatal("%.200s line %d: Missing target "
+                                            "argument.", filename, linenum);
+                        } else {
+                                /* construct a string for parse_forward */
+                                snprintf(fwdarg, sizeof(fwdarg), "%s:%s", arg,
+                                    arg2);
+                        }
                 }
+                if (dynamicfwd)
+                        strlcpy(fwdarg, arg, sizeof(fwdarg));
 
-                if (parse_forward(&fwd, fwdarg,
-                    opcode == oDynamicForward ? 1 : 0,
-                    opcode == oRemoteForward ? 1 : 0) == 0)
+                if (parse_forward(&fwd, fwdarg, dynamicfwd, remotefwd) == 0)
                         fatal("%.200s line %d: Bad forwarding specification.",
                             filename, linenum);
 
                 if (*activep) {
-                        if (opcode == oLocalForward ||
-                            opcode == oDynamicForward)
-                                add_local_forward(options, &fwd);
-                        else if (opcode == oRemoteForward)
+                        if (remotefwd) {
                                 add_remote_forward(options, &fwd);
+                        } else {
+                                add_local_forward(options, &fwd);
+                        }
                 }
                 break;
 
diff --git a/ssh.1 b/ssh.1
index 3aacec415..2ab1697f9 100644
--- a/ssh.1
+++ b/ssh.1
@@ -33,8 +33,8 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.\" $OpenBSD: ssh.1,v 1.383 2017/06/09 06:43:01 djm Exp $
-.Dd $Mdocdate: June 9 2017 $
+.\" $OpenBSD: ssh.1,v 1.384 2017/09/21 19:16:53 markus Exp $
+.Dd $Mdocdate: September 21 2017 $
 .Dt SSH 1
 .Os
 .Sh NAME
@@ -592,21 +592,30 @@ Causes most warning and diagnostic messages to be suppressed.
 .Ar remote_socket : local_socket
 .Sm on
 .Xc
+.It Fl R Xo
+.Sm off
+.Oo Ar bind_address : Oc
+.Ar port
+.Sm on
+.Xc
 Specifies that connections to the given TCP port or Unix socket on the remote
-(server) host are to be forwarded to the given host and port, or Unix socket,
-on the local side.
+(server) host are to be forwarded to the local side.
+.Pp
 This works by allocating a socket to listen to either a TCP
 .Ar port
 or to a Unix socket on the remote side.
 Whenever a connection is made to this port or Unix socket, the
 connection is forwarded over the secure channel, and a connection
-is made to either
+is made from the local machine to either an explicit destination specified by
 .Ar host
 port
 .Ar hostport ,
 or
 .Ar local_socket ,
-from the local machine.
+or, if no explicit destination was specified,
+.Nm
+will act as a SOCKS 4/5 proxy and forward connections to the destinations
+requested by the remote SOCKS client.
 .Pp
 Port forwardings can also be specified in the configuration file.
 Privileged ports can be forwarded only when
diff --git a/ssh.c b/ssh.c
index ecc50f37e..ae37432bd 100644
--- a/ssh.c
+++ b/ssh.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh.c,v 1.463 2017/09/12 06:32:07 djm Exp $ */
+/* $OpenBSD: ssh.c,v 1.464 2017/09/21 19:16:53 markus Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -868,7 +868,8 @@ main(int ac, char **av)
                         break;
 
                 case 'R':
-                        if (parse_forward(&fwd, optarg, 0, 1)) {
+                        if (parse_forward(&fwd, optarg, 0, 1) ||
+                            parse_forward(&fwd, optarg, 1, 1)) {
                                 add_remote_forward(&options, &fwd);
                         } else {
                                 fprintf(stderr,
diff --git a/ssh_config.5 b/ssh_config.5
index ca5a41103..eab8dd01c 100644
--- a/ssh_config.5
+++ b/ssh_config.5
@@ -33,8 +33,8 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.\" $OpenBSD: ssh_config.5,v 1.255 2017/09/04 06:34:43 jmc Exp $
-.Dd $Mdocdate: September 4 2017 $
+.\" $OpenBSD: ssh_config.5,v 1.256 2017/09/21 19:16:53 markus Exp $
+.Dd $Mdocdate: September 21 2017 $
 .Dt SSH_CONFIG 5
 .Os
 .Sh NAME
@@ -1298,13 +1298,19 @@ accept the tokens described in the
 section.
 .It Cm RemoteForward
 Specifies that a TCP port on the remote machine be forwarded over
-the secure channel to the specified host and port from the local machine.
+the secure channel.
+The remote port may either be fowarded to a specified host and port
+from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote
+client to connect to arbitrary destinations from the local machine.
 The first argument must be
 .Sm off
 .Oo Ar bind_address : Oc Ar port
 .Sm on
-and the second argument must be
-.Ar host : Ns Ar hostport .
+If forwarding to a specific destination then the second argument must be
+.Ar host : Ns Ar hostport ,
+otherwise if no destination argument is specified then the remote forwarding
+will be established as a SOCKS proxy.
+.Pp
 IPv6 addresses can be specified by enclosing addresses in square brackets.
 Multiple forwardings may be specified, and additional
 forwardings can be given on the command line.