Disable encrypt-then-mac where possible with DTLS and OpenSSL

There is pain here. Just don't bother.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

diff --git a/openssl-dtls.c b/openssl-dtls.c
index 9e3c5d4625cc3041fdc8963db69d4d6dfb5e34f3..646bf71c4da6cba8f98bb12ee676e0b852f0aea0 100644 (file)
--- a/openssl-dtls.c
+++ b/openssl-dtls.c
@@ -406,7 +406,17 @@ int start_dtls_handshake(struct openconnect_info *vpninfo, int dtls_fd)
                        cipher = "PSK";
                }
 #endif /* OPENSSL_NO_PSK */
-
+#ifdef SSL_OP_NO_ENCRYPT_THEN_MAC
+               /* I'm fairly sure I wasn't lying when I said I had tested
+                * https://github.com/openssl/openssl/commit/e23d5071ec4c7aa6bb2b
+                * against GnuTLS both with and without EtM in 2016.
+                * Nevertheless, in 2019 it seems to be failing to negotiate
+                * at least for DTLS1_BAD_VER against ocserv with GnuTLS 3.6.7.
+                * Just turn it off. Real Cisco servers don't do it for
+                * DTLS1_BAD_VER, and we should be using GCM ciphersuites in
+                * newer versions of DTLS anyway so it's irrelevant. */
+               SSL_CTX_set_options(vpninfo->dtls_ctx, SSL_OP_NO_ENCRYPT_THEN_MAC);
+#endif
                /* If we don't readahead, then we do short reads and throw
                   away the tail of data packets. */
                SSL_CTX_set_read_ahead(vpninfo->dtls_ctx, 1);