Pushes AES-256-GCM when a connection client advertises IV_NCP=2, and
supports serving connections to clients with different data channel
cipher configuration simultaneously.
v2:
* Update manpage
* Add Changes.rst entry
Signed-off-by: Steffan Karger <stef...@karger.me>
---
Changes.rst | 11 +++++++++++
doc/openvpn.8 | 4 ++++
src/openvpn/init.c | 5 +++--
src/openvpn/push.c | 25 ++++++++++++++++++++++---
src/openvpn/ssl.c | 23 +++++++++++++----------
src/openvpn/ssl_common.h | 1 +
6 files changed, 54 insertions(+), 15 deletions(-)
diff --git a/Changes.rst b/Changes.rst
index 1ac3c2b..eb6018f 100644
--- a/Changes.rst
+++ b/Changes.rst
@@ -51,6 +51,13 @@ AEAD (GCM) data channel cipher support
bytes per packet for AES-128-GCM instead of 36 bytes per packet for
AES-128-CBC + HMAC-SHA1).
+Cipher negotiation
+ Data channel ciphers are now by default negotiated. If a client advertises
+ support for Negotiable Crypto Parameters (NCP), the server will choose a
+ cipher (by default AES-256-GCM) for the data channel, and tell the client
+ to use that cipher. Data channel cipher negotiation can be controlled
+ using --ncp-ciphers and --ncp-disable.
+
User-visible Changes
--------------------
@@ -106,6 +113,10 @@ User-visible Changes
- mbed TLS builds: minimum RSA key size is now 2048 bits. Shorter keys will
not be accepted, both local and from the peer.
+- Data channel cipher negotiation (see New features section) can override
+ ciphers configured in the config file. Use --ncp-disable if you don't want
+ that.
+
Maintainer-visible changes
--------------------------
diff --git a/doc/openvpn.8 b/doc/openvpn.8
index e349b77..5cb2885 100644
--- a/doc/openvpn.8
+++ b/doc/openvpn.8
@@ -4134,6 +4134,10 @@ Restrict the allowed ciphers to be negotiated to the
ciphers in
.B cipher_list
is a colon-separated list of ciphers, and defaults to
"AES-256-GCM:AES-128-GCM".
+
+For servers, the first cipher from
+.B cipher_list
+will be pushed to clients that support cipher negotiation.
.\"*********************************************************
.TP
.B \-\-ncp\-disable
diff --git a/src/openvpn/init.c b/src/openvpn/init.c
index c2e3b2e..5ed3f3c 100644
--- a/src/openvpn/init.c
+++ b/src/openvpn/init.c
@@ -2312,8 +2312,8 @@ do_init_crypto_tls (struct context *c, const unsigned int
flags)
/* In short form, unique datagram identifier is 32 bits, in long form 64
bits */
packet_id_long_form = cipher_kt_mode_ofb_cfb (c->c1.ks.key_type.cipher);
- /* Compute MTU parameters (postpone if we pull options) */
- if (c->options.pull)
+ /* Compute MTU parameters (postpone if we push/pull options) */
+ if (c->options.pull || c->options.mode == MODE_SERVER)
{
/* Account for worst-case crypto overhead before allocating buffers */
frame_add_to_extra_frame (&c->c2.frame, crypto_max_overhead());
@@ -2357,6 +2357,7 @@ do_init_crypto_tls (struct context *c, const unsigned int
flags)
to.renegotiate_packets = options->renegotiate_packets;
to.renegotiate_seconds = options->renegotiate_seconds;
to.single_session = options->single_session;
+ to.mode = options->mode;
to.pull = options->pull;
#ifdef ENABLE_PUSH_PEER_INFO
if (options->push_peer_info) /* all there is */
diff --git a/src/openvpn/push.c b/src/openvpn/push.c
index 4239e3e..f763e65 100644
--- a/src/openvpn/push.c
+++ b/src/openvpn/push.c
@@ -245,13 +245,28 @@ incoming_push_message (struct context *c, const struct
buffer *buffer)
if (!do_up (c, true, c->options.push_option_types_found))
{
msg (D_PUSH_ERRORS, "Failed to open tun/tap interface");
- register_signal (c, SIGUSR1, "do_up-failed");
- goto cleanup;
+ goto error;
}
}
event_timeout_clear (&c->c2.push_request_interval);
}
+ else if (status == PUSH_MSG_REQUEST)
+ {
+ if (c->options.mode == MODE_SERVER)
+ {
+ struct tls_session *session = &c->c2.tls_multi->session[TM_ACTIVE];
+ if (!tls_session_update_crypto_params (session, &c->options,
+ &c->c2.frame))
+ {
+ msg (D_TLS_ERRORS, "TLS Error: server generate_key_expansion
failed");
+ goto error;
+ }
+ }
+ }
+ goto cleanup;
+error:
+ register_signal (c, SIGUSR1, "process-push-msg-failed");
cleanup:
gc_free (&gc);
}
@@ -303,12 +318,16 @@ prepare_push_reply (struct options *o, struct tls_multi
*tls_multi)
/* Push cipher if client supports Negotiable Crypto Parameters */
optstr = peer_info ? strstr(peer_info, "IV_NCP=") : NULL;
- if (optstr)
+ if (optstr && o->ncp_enabled)
{
int ncp = 0;
int r = sscanf(optstr, "IV_NCP=%d", &ncp);
if ((r == 1) && (ncp == 2))
{
+ /* Push the first cipher from --ncp-ciphers to the client.
+ * TODO: actual negotiation, instead of server dictatorship. */
+ char *push_cipher = string_alloc(o->ncp_ciphers, &o->gc);
+ o->ciphername = strtok (push_cipher, ":");
push_option_fmt(o, M_USAGE, "cipher %s", o->ciphername);
}
}
diff --git a/src/openvpn/ssl.c b/src/openvpn/ssl.c
index 7ced41d..d5b52d7 100644
--- a/src/openvpn/ssl.c
+++ b/src/openvpn/ssl.c
@@ -1661,10 +1661,10 @@ tls_session_update_crypto_params(struct tls_session
*session,
bool ret = false;
struct key_state *ks = &session->key[KS_PRIMARY]; /* primary key */
- ASSERT (!session->opt->server);
ASSERT (ks->authenticated);
- if (0 != strcmp(options->ciphername, session->opt->config_ciphername) &&
+ if (!session->opt->server &&
+ 0 != strcmp(options->ciphername, session->opt->config_ciphername) &&
!item_in_list(options->ciphername, options->ncp_ciphers))
{
msg (D_TLS_ERRORS, "Error: pushed cipher not allowed - %s not in %s or
%s",
@@ -1691,12 +1691,13 @@ tls_session_update_crypto_params(struct tls_session
*session,
options->ce.tun_mtu_defined, options->ce.tun_mtu);
frame_print (frame, D_MTU_INFO, "Data Channel MTU parms");
+ const struct session_id *client_sid = session->opt->server ?
+ &ks->session_id_remote : &session->session_id;
+ const struct session_id *server_sid = !session->opt->server ?
+ &ks->session_id_remote : &session->session_id;
if (!generate_key_expansion (&ks->crypto_options.key_ctx_bi,
- &session->opt->key_type,
- ks->key_src,
- &session->session_id,
- &ks->session_id_remote,
- false))
+ &session->opt->key_type, ks->key_src, client_sid, server_sid,
+ session->opt->server))
{
msg (D_TLS_ERRORS, "TLS Error: server generate_key_expansion failed");
goto cleanup;
@@ -2057,10 +2058,12 @@ key_method_2_write (struct buffer *buf, struct
tls_session *session)
if (!push_peer_info (buf, session))
goto error;
- /*
- * generate tunnel keys if server
+ /* Generate tunnel keys if we're a TLS server.
+ * If we're a p2mp server, the first key generation is postponed until after
+ * the pull/push, so we can process pushed cipher directives.
*/
- if (session->opt->server)
+ if (session->opt->server &&
+ (session->opt->mode != MODE_SERVER || ks->key_id > 0))
{
if (ks->authenticated)
{
diff --git a/src/openvpn/ssl_common.h b/src/openvpn/ssl_common.h
index 6bfde6b..eb2ad6f 100644
--- a/src/openvpn/ssl_common.h
+++ b/src/openvpn/ssl_common.h
@@ -236,6 +236,7 @@ struct tls_options
#ifdef ENABLE_OCC
bool disable_occ;
#endif
+ int mode;
bool pull;
#ifdef ENABLE_PUSH_PEER_INFO
int push_peer_info_detail;
--
2.7.4
