On Tue, 2019-10-15 at 12:03 +0100, David Woodhouse wrote:
> 
> Something like this... and instead of calling SSL_set1_host(ssl, host)
> your own code now has to call
> SSL_set_ex_data(ssl, ssl_target_idx, strdup(host));

Here's how I tested that in the OpenSSL tree in userspace, FWIW...

diff --git a/demos/bio/sconnect.c b/demos/bio/sconnect.c
index 7e46bf0ad8..1fd2829a79 100644
--- a/demos/bio/sconnect.c
+++ b/demos/bio/sconnect.c
@@ -25,11 +25,40 @@
 #define HOSTPORT "localhost:4433"
 #define CAFILE "root.pem"
 
+static int ssl_target_idx;
+
+void ssl_target_free(void *parent, void *ptr, CRYPTO_EX_DATA *ad,
+		    int idx, long argl, void *argp)
+{
+	printf("Freeing %s\n", ptr);
+	free(ptr);
+}
+
+int ssl_target_dup(CRYPTO_EX_DATA *to, const CRYPTO_EX_DATA *from,
+		   void *from_d, int idx, long argl, void *argp)
+{
+	/* strdup it */
+	return 0;
+}
+
+int app_verify_callback(X509_STORE_CTX *ctx, void *dummy)
+{
+	SSL *ssl = X509_STORE_CTX_get_ex_data(ctx, SSL_get_ex_data_X509_STORE_CTX_idx());
+	char *hostname = SSL_get_ex_data(ssl, ssl_target_idx);
+	X509_VERIFY_PARAM *vpm = X509_STORE_CTX_get0_param(ctx);
+
+	printf("Setting %s\n", hostname);
+	if (hostname && !X509_VERIFY_PARAM_set1_ip_asc(vpm, hostname))
+		X509_VERIFY_PARAM_set1_host(vpm, hostname, 0);
+
+	return X509_verify_cert(ctx);
+}
+
 int main(int argc, char *argv[])
 {
     const char *hostport = HOSTPORT;
     const char *CAfile = CAFILE;
-    char *hostname;
+    const char *hostname;
     char *cp;
     BIO *out = NULL;
     char buf[1024 * 10], *p;
@@ -43,10 +72,6 @@ int main(int argc, char *argv[])
     if (argc > 2)
         CAfile = argv[2];
 
-    hostname = OPENSSL_strdup(hostport);
-    if ((cp = strchr(hostname, ':')) != NULL)
-        *cp = 0;
-
 #ifdef WATT32
     dbug_init();
     sock_init();
@@ -54,6 +79,8 @@ int main(int argc, char *argv[])
 
     ssl_ctx = SSL_CTX_new(TLS_client_method());
 
+    SSL_CTX_set_cert_verify_callback(ssl_ctx, app_verify_callback, NULL);
+
     /* Enable trust chain verification */
     SSL_CTX_set_verify(ssl_ctx, SSL_VERIFY_PEER, NULL);
     SSL_CTX_load_verify_locations(ssl_ctx, CAfile, NULL);
@@ -62,9 +89,6 @@ int main(int argc, char *argv[])
     ssl = SSL_new(ssl_ctx);
     SSL_set_connect_state(ssl);
 
-    /* Enable peername verification */
-    if (SSL_set1_host(ssl, hostname) <= 0)
-        goto err;
 
     /* Use it inside an SSL BIO */
     ssl_bio = BIO_new(BIO_f_ssl());
@@ -73,6 +97,13 @@ int main(int argc, char *argv[])
     /* Lets use a connect BIO under the SSL BIO */
     out = BIO_new(BIO_s_connect());
     BIO_set_conn_hostname(out, hostport);
+
+    /* The BIO has parsed the host:port and even IPv6 literals in [] */
+    hostname = BIO_get_conn_hostname(out);
+
+    if (hostname)
+	    SSL_set_ex_data(ssl, ssl_target_idx, strdup(hostname));
+
     BIO_set_nbio(out, 1);
     out = BIO_push(ssl_bio, out);