Make sure that SOCKS proxied server data are processed

There was a race condition where proxied server data could arrive appended
to the final SOCKS handshake response, causing the data to get skipped.

CHANGELOG

@@ -19,6 +19,10 @@ o [GH#2206] Nmap no longer produces cryptic message "Failed to convert
 o [Ncat][GH#2202] Use safety-checked versions of FD_* macros to abort early if
   number of connections exceeds FD_SETSIZE. [Pavel Zhukov]
 
+o [Ncat] Connections proxied via SOCKS4/SOCKS5 were intermittently dropping
+  server data sent right after the connection got established, such as port
+  banners. [Sami Pönkänen]
+
 o Nmap will now output a list of port numbers for each "ignored" state in the
   "extrareasons" element in XML output. The "All X ports" and "Not shown:" lines
   in normal output have been changed slightly to provide more detail. [Daniel Miller]

ncat/ncat_connect.c

@@ -552,6 +552,8 @@ static int do_proxy_socks4(void)
     union sockaddr_u addr;
     size_t sslen;
     int sd;
+    size_t remainderlen;
+    char* remainder;
 
     if (getaddrfamily(o.target) == 2) {
         loguser("Error: IPv6 addresses are not supported with Socks4.\n");
@@ -629,6 +631,10 @@ static int do_proxy_socks4(void)
         return -1;
     }
 
+    /* whatever is left in the buffer is part of the proxied connection */
+    remainder = socket_buffer_remainder(&stateful_buf, &remainderlen);
+    Write(STDOUT_FILENO, remainder, remainderlen);
+
     return sd;
 }
 
@@ -641,7 +647,7 @@ static int do_proxy_socks5(void)
     struct socket_buffer stateful_buf;
     struct socks5_connect socks5msg;
     uint16_t proxyport = htons(o.portno);
-    char socksbuf[8];
+    char socksbuf[4];
     int sd;
     size_t dstlen, targetlen;
     struct socks5_request socks5msg2;
@@ -653,6 +659,10 @@ static int do_proxy_socks5(void)
     void *addrbuf;
     size_t addrlen;
     char addrstr[INET6_ADDRSTRLEN];
+    size_t bndaddrlen;
+    char bndaddr[16 + 2]; /* IPv4/IPv6 address and port */
+    size_t remainderlen;
+    char* remainder;
 
     sd = do_connect(SOCK_STREAM);
     if (sd == -1) {
@@ -847,13 +857,18 @@ static int do_proxy_socks5(void)
         return -1;
     }
 
-    /* TODO just two bytes for now, need to read more for bind */
-    if (socket_buffer_readcount(&stateful_buf, socksbuf, 2) < 0) {
+    if (socket_buffer_readcount(&stateful_buf, socksbuf, 4) < 0) {
         loguser("Error: malformed request response from proxy.\n");
         close(sd);
         return -1;
     }
 
+    if (socksbuf[0] != SOCKS5_VERSION) {
+        loguser("Error: wrong SOCKS version in request response.\n");
+        close(sd);
+        return -1;
+    }
+
     switch(socksbuf[1]) {
         case 0:
             if (o.verbose)
@@ -897,6 +912,29 @@ static int do_proxy_socks5(void)
             return -1;
     }
 
+    switch (socksbuf[3]) {
+    case SOCKS5_ATYP_IPv4:
+        bndaddrlen = 4 + 2;
+        break;
+    case SOCKS5_ATYP_IPv6:
+        bndaddrlen = 16 + 2;
+        break;
+    default:
+        loguser("Error: invalid proxy bind address type.\n");
+        close(sd);
+        return -1;
+    }
+
+    if (socket_buffer_readcount(&stateful_buf, bndaddr, bndaddrlen) < 0) {
+        loguser("Error: malformed request response from proxy.\n");
+        close(sd);
+        return -1;
+    }
+
+    /* whatever is left in the buffer is part of the proxied connection */
+    remainder = socket_buffer_remainder(&stateful_buf, &remainderlen);
+    Write(STDOUT_FILENO, remainder, remainderlen);
+
     return(sd);
 }