If a client reconnects on a soft-restart from the same port (due to --bind
in use on the client), both sides will handle this as a "reconnect" and
not a "full new connect" internally, re-using existing crypto context.
The client will still ask the server for pushed options, and the server
code to handle this refuses to do NCP if a key has already been negotiated
(because there is no way to *change* the cipher after that) - which ends
up in "the client uses the non-negotiated cipher from the config file,
while the server uses the previously-negotiated NCP cipher", and nothing
works.
The easy workaround: if we find us in the situation that we think NCP
has already been done, just re-push "cipher o->ciphername" with the
current cipher for this client context.
All credits for this go to Stefan Behrens <sbehr...@giantdisaster.de>
who found and diagnosed the issue in trac #887, came up with a first
patch to solve the issue quite similar to this (simplified) one, and
helped testing.
Trac: #887
---
src/openvpn/push.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)
diff --git a/src/openvpn/push.c b/src/openvpn/push.c
index 8c3104e..bcef0ef 100644
--- a/src/openvpn/push.c
+++ b/src/openvpn/push.c
@@ -372,15 +372,17 @@ prepare_push_reply(struct context *c, struct gc_arena *gc,
/* Push cipher if client supports Negotiable Crypto Parameters */
if (tls_peer_info_ncp_ver(peer_info) >= 2 && o->ncp_enabled)
{
- /* if we have already created our key, we cannot change our own
- * cipher, so disable NCP and warn = explain why
+ /* if we have already created our key, we cannot *change* our own
+ * cipher -> so log the fact and push the "what we have now" cipher
+ * (so the client is always told what we expect it to use)
*/
const struct tls_session *session = &tls_multi->session[TM_ACTIVE];
if (session->key[KS_PRIMARY].crypto_options.key_ctx_bi.initialized)
{
msg( M_INFO, "PUSH: client wants to negotiate cipher (NCP), but "
"server has already generated data channel keys, "
- "ignoring client request" );
+ "re-sending previously negotiated cipher '%s'",
+ o->ciphername );
}
else
{
@@ -388,8 +390,8 @@ prepare_push_reply(struct context *c, struct gc_arena *gc,
* 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(gc, push_list, M_USAGE, "cipher %s",
o->ciphername);
}
+ push_option_fmt(gc, push_list, M_USAGE, "cipher %s", o->ciphername);
}
else if (o->ncp_enabled)
{
--
2.10.2
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