[Openvpn-devel,v2,2/4] Implement a function to calculate the default MTU

Message ID 20220624111318.3219982-2-arne@rfc2549.org
State Superseded
Headers show
Series
  • [Openvpn-devel,v2,1/4] Extract update_session_cipher into standalone function
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Commit Message

Arne Schwabe June 24, 2022, 11:13 a.m.
We could also just hardcode this value to 1420 but this approach does
not add much (complicated) code and it is a bit better than to have
a magic number to just be there.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
---
 src/openvpn/mtu.c                      | 22 ++++++++++++++++++++++
 src/openvpn/mtu.h                      | 14 ++++++++++++++
 tests/unit_tests/openvpn/test_crypto.c | 19 ++++++++++++++++++-
 3 files changed, 54 insertions(+), 1 deletion(-)

Comments

Frank Lichtenheld June 24, 2022, 12:18 p.m. | #1
On Fri, Jun 24, 2022 at 01:13:16PM +0200, Arne Schwabe wrote:
> We could also just hardcode this value to 1420 but this approach does
> not add much (complicated) code and it is a bit better than to have
> a magic number to just be there.
> 
[...]
> +/**
> + * Function to calculate the default MTU for Layer 3 VPNs. The function
> + * assumes that UDP packets should be a maximum of \c MTU_ENCAP_DEFAULT (1492)
> + * with a AEAD cipher. This default comes out to be 1420.
> + */

The result of the function is not always 1420.
E.g. if options.ce.fragment == true.

> +int
> +frame_calculate_default_mtu(struct options *o);
> +
> +
>  /*
>   * EXTENDED_SOCKET_ERROR_CAPABILITY functions -- print extra error info
>   * on socket errors, such as PMTU size.  As of 2003.05.11, only works

Patch

diff --git a/src/openvpn/mtu.c b/src/openvpn/mtu.c
index 59b917985..0cad9b1c9 100644
--- a/src/openvpn/mtu.c
+++ b/src/openvpn/mtu.c
@@ -205,6 +205,28 @@  calc_options_string_link_mtu(const struct options *o, const struct frame *frame)
     return payload + overhead;
 }
 
+int
+frame_calculate_default_mtu(struct options *o)
+{
+    struct options options = *o;
+
+    /* assume we have peer_id enabled */
+    options.use_peer_id = true;
+
+    /* We use IPv6+UDP here to have a consistent size for tun MTU no matter
+     * the combination of udp/tcp and IPv4/IPv6 */
+    int encap_overhead = datagram_overhead(AF_INET6, PROTO_UDP);
+
+    struct key_type kt;
+    init_key_type(&kt, "AES-256-GCM", "none", true, false);
+
+    size_t payload_overhead = frame_calculate_payload_overhead(0, &options, &kt, false);
+    size_t protocol_overhead = frame_calculate_protocol_header_size(&kt, &options, false);
+
+    return MTU_ENCAP_DEFAULT - encap_overhead - payload_overhead - protocol_overhead;
+
+}
+
 void
 frame_print(const struct frame *frame,
             int level,
diff --git a/src/openvpn/mtu.h b/src/openvpn/mtu.h
index 9db6cf26a..d643027d3 100644
--- a/src/openvpn/mtu.h
+++ b/src/openvpn/mtu.h
@@ -79,6 +79,10 @@ 
  */
 #define MSSFIX_DEFAULT     1492
 
+/* The default size we aim to reach to with our VPN packets by setting
+ * the MTU accordingly */
+#define MTU_ENCAP_DEFAULT   1492
+
 /*
  * Alignment of payload data such as IP packet or
  * ethernet frame.
@@ -260,6 +264,16 @@  void alloc_buf_sock_tun(struct buffer *buf,
                         const struct frame *frame,
                         const bool tuntap_buffer);
 
+
+/**
+ * Function to calculate the default MTU for Layer 3 VPNs. The function
+ * assumes that UDP packets should be a maximum of \c MTU_ENCAP_DEFAULT (1492)
+ * with a AEAD cipher. This default comes out to be 1420.
+ */
+int
+frame_calculate_default_mtu(struct options *o);
+
+
 /*
  * EXTENDED_SOCKET_ERROR_CAPABILITY functions -- print extra error info
  * on socket errors, such as PMTU size.  As of 2003.05.11, only works
diff --git a/tests/unit_tests/openvpn/test_crypto.c b/tests/unit_tests/openvpn/test_crypto.c
index 83572b827..ca595b0a5 100644
--- a/tests/unit_tests/openvpn/test_crypto.c
+++ b/tests/unit_tests/openvpn/test_crypto.c
@@ -477,6 +477,22 @@  test_mssfix_mtu_calculation(void **state)
     gc_free(&gc);
 }
 
+
+static void
+test_mtu_default_calculation(void **state)
+{
+    struct options o = {0};
+
+    /* common defaults */
+    o.ce.tun_mtu = 1400;
+    o.ce.mssfix = 1000;
+    o.replay = true;
+    o.ce.proto = PROTO_UDP;
+
+    size_t mtu = frame_calculate_default_mtu(&o);
+    assert_int_equal(1420, mtu);
+}
+
 int
 main(void)
 {
@@ -487,7 +503,8 @@  main(void)
         cmocka_unit_test(crypto_test_hmac),
         cmocka_unit_test(test_des_encrypt),
         cmocka_unit_test(test_occ_mtu_calculation),
-        cmocka_unit_test(test_mssfix_mtu_calculation)
+        cmocka_unit_test(test_mssfix_mtu_calculation),
+        cmocka_unit_test(test_mtu_default_calculation)
     };
 
 #if defined(ENABLE_CRYPTO_OPENSSL)