socks: Replace magic "10" for socks header with macro

So that it is easier to check that we indeed
have reserved this prior to assuming we have.

Github: openvpn-private-issues#4
Change-Id: I0aca7e7d9aa190541f11745cf72193cb6b39540a
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1467
Message-Id: <20260112171122.3994-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg35214.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

src/openvpn/init.c

@@ -2927,7 +2927,7 @@ frame_finalize_options(struct context *c, const struct options *o)
     headroom += 4;
 
     /* socks proxy header */
-    headroom += 10;
+    headroom += SOCKS_UDPv4_HEADROOM;
 
     /* compression header and fragment header (part of the encrypted payload) */
     headroom += 1 + 1;

src/openvpn/mtu.c

@@ -66,11 +66,11 @@ frame_calculate_protocol_header_size(const struct key_type *kt, const struct opt
 
     bool tlsmode = options->tls_server || options->tls_client;
 
-    /* A socks proxy adds 10 byte of extra header to each packet
+    /* A socks proxy adds extra header to each packet
      * (we only support Socks with IPv4, this value is different for IPv6) */
     if (options->ce.socks_proxy_server && proto_is_udp(options->ce.proto))
     {
-        header_size += 10;
+        header_size += SOCKS_UDPv4_HEADROOM;
     }
 
     /* TCP stream based packets have a 16 bit length field */

src/openvpn/mtu.h

@@ -94,6 +94,11 @@
  */
 #define PAYLOAD_ALIGN 4
 
+/*
+ * How many bytes we prepend for a SOCKS UDP proxy.
+ * This only handles IPv4 right now.
+ */
+#define SOCKS_UDPv4_HEADROOM 10
 
 /**************************************************************************/
 /**

src/openvpn/socks.c

@@ -434,7 +434,7 @@ error:
 }
 
 /*
- * Remove the 10 byte socks5 header from an incoming
+ * Remove the socks5 header from an incoming
  * UDP packet, setting *from to the source address.
  *
  * Run after UDP read.
@@ -444,7 +444,7 @@ socks_process_incoming_udp(struct buffer *buf, struct link_socket_actual *from)
 {
     int atyp;
 
-    if (BLEN(buf) < 10)
+    if (BLEN(buf) < SOCKS_UDPv4_HEADROOM)
     {
         goto error;
     }
@@ -471,7 +471,7 @@ error:
 }
 
 /*
- * Add a 10 byte socks header prior to UDP write.
+ * Add a socks header prior to UDP write.
  * *to is the destination address.
  *
  * Run before UDP write.
@@ -481,11 +481,11 @@ int
 socks_process_outgoing_udp(struct buffer *buf, const struct link_socket_actual *to)
 {
     /*
-     * Get a 10 byte subset buffer prepended to buf --
+     * Get a subset buffer prepended to buf --
      * we expect these bytes will be here because
      * we always allocate space for these bytes
      */
-    struct buffer head = buf_sub(buf, 10, true);
+    struct buffer head = buf_sub(buf, SOCKS_UDPv4_HEADROOM, true);
 
     /* crash if not enough headroom in buf */
     ASSERT(buf_defined(&head));
@@ -496,5 +496,5 @@ socks_process_outgoing_udp(struct buffer *buf, const struct link_socket_actual *
     buf_write(&head, &to->dest.addr.in4.sin_addr, sizeof(to->dest.addr.in4.sin_addr));
     buf_write(&head, &to->dest.addr.in4.sin_port, sizeof(to->dest.addr.in4.sin_port));
 
-    return 10;
+    return SOCKS_UDPv4_HEADROOM;
 }