[RFH/PATCH] imap-send: support SNI (RFC4366)

[Junio C Hamano <gitster@pobox.com>]

To talk to a site that serves multiple names on a single IP address,
the client needs to ask for the specific hostname it wants to talk
to. Otherwise, the default certificate returned from the IP address
may not match that of the host we wanted to talk to.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---

 * I need help from people on this patch in two areas:

 (1) I only tested this patch by connecting to https://googlemail.com/ 
     with

     $ git -c imap.host=imaps://googlemail.com -c imap.port=443 imap-send <this-patch.txt

     as it is the only site I knew clients needs to talk SNI to get
     the right certificate to verify; of course the port does not
     talk imap, and the only thing that is tested by that approach is
     we successfully establish an SSL/TLS connection.  Without the
     patch, we fail to verify the certificate (we get a cert that is
     for another hostname that is hosted at the same IP address), and
     with the patch, we successfully get the right one.

     I would appreciate it if somebody knows an imap server that
     needs SNI and runs an end-to-end test against that server.

 (2) I do not know if everybody has SSL_set_tslext_host_name() macro
     defined, so this patch may be breaking build for people with
     different versions of OpenSSL.

 imap-send.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/imap-send.c b/imap-send.c
index 171c887..d9abd8b 100644
--- a/imap-send.c
+++ b/imap-send.c
@@ -370,6 +370,15 @@ static int ssl_socket_connect(struct imap_socket *sock, int use_tls_only, int ve
 		return -1;
 	}
 
+	/*
+	 * SNI (RFC4366)
+	 * OpenSSL does not document this function, but the implementation
+	 * returns 1 on success, 0 on failure after calling SSLerr().
+	 */
+	ret = SSL_set_tlsext_host_name(sock->ssl, server.host);
+	if (ret != 1)
+		warning("SSL_set_tslext_host_name(%s) failed.\n", server.host);
+
 	ret = SSL_connect(sock->ssl);
 	if (ret <= 0) {
 		socket_perror("SSL_connect", sock, ret);
-- 
1.8.2.rc0.106.ga6e4a61