+static const char *alpn_client;
+static const char *alpn_server;
+static const char *alpn_expected;
+static unsigned char *alpn_selected;
+
+/* next_protos_parse parses a comma separated list of strings into a string
+ * in a format suitable for passing to SSL_CTX_set_next_protos_advertised.
+ * outlen: (output) set to the length of the resulting buffer on success.
+ * err: (maybe NULL) on failure, an error message line is written to this BIO.
+ * in: a NUL termianted string like "abc,def,ghi"
+ *
+ * returns: a malloced buffer or NULL on failure.
+ */
+static unsigned char *next_protos_parse(unsigned short *outlen, const char *in)
+ {
+ size_t len;
+ unsigned char *out;
+ size_t i, start = 0;
+
+ len = strlen(in);
+ if (len >= 65535)
+ return NULL;
+
+ out = OPENSSL_malloc(strlen(in) + 1);
+ if (!out)
+ return NULL;
+
+ for (i = 0; i <= len; ++i)
+ {
+ if (i == len || in[i] == ',')
+ {
+ if (i - start > 255)
+ {
+ OPENSSL_free(out);
+ return NULL;
+ }
+ out[start] = i - start;
+ start = i + 1;
+ }
+ else
+ out[i+1] = in[i];
+ }
+
+ *outlen = len + 1;
+ return out;
+ }
+
+static int cb_server_alpn(SSL *s, const unsigned char **out, unsigned char *outlen, const unsigned char *in, unsigned int inlen, void *arg)
+ {
+ unsigned char *protos;
+ unsigned short protos_len;
+
+ protos = next_protos_parse(&protos_len, alpn_server);
+ if (protos == NULL)
+ {
+ fprintf(stderr, "failed to parser ALPN server protocol string: %s\n", alpn_server);
+ abort();
+ }
+
+ if (SSL_select_next_proto((unsigned char**) out, outlen, protos, protos_len, in, inlen) !=
+ OPENSSL_NPN_NEGOTIATED)
+ {
+ OPENSSL_free(protos);
+ return SSL_TLSEXT_ERR_NOACK;
+ }
+
+ /* Make a copy of the selected protocol which will be freed in verify_alpn. */
+ alpn_selected = OPENSSL_malloc(*outlen);
+ memcpy(alpn_selected, *out, *outlen);
+ *out = alpn_selected;
+
+ OPENSSL_free(protos);
+ return SSL_TLSEXT_ERR_OK;
+ }
+
+static int verify_alpn(SSL *client, SSL *server)
+ {
+ const unsigned char *client_proto, *server_proto;
+ unsigned int client_proto_len = 0, server_proto_len = 0;
+ SSL_get0_alpn_selected(client, &client_proto, &client_proto_len);
+ SSL_get0_alpn_selected(server, &server_proto, &server_proto_len);
+
+ if (alpn_selected != NULL)
+ {
+ OPENSSL_free(alpn_selected);
+ alpn_selected = NULL;
+ }
+
+ if (client_proto_len != server_proto_len ||
+ memcmp(client_proto, server_proto, client_proto_len) != 0)
+ {
+ BIO_printf(bio_stdout, "ALPN selected protocols differ!\n");
+ goto err;
+ }
+
+ if (client_proto_len > 0 && alpn_expected == NULL)
+ {
+ BIO_printf(bio_stdout, "ALPN unexpectedly negotiated\n");
+ goto err;
+ }
+
+ if (alpn_expected != NULL &&
+ (client_proto_len != strlen(alpn_expected) ||
+ memcmp(client_proto, alpn_expected, client_proto_len) != 0))
+ {
+ BIO_printf(bio_stdout, "ALPN selected protocols not equal to expected protocol: %s\n", alpn_expected);
+ goto err;
+ }
+
+ return 0;
+
+err:
+ BIO_printf(bio_stdout, "ALPN results: client: '");
+ BIO_write(bio_stdout, client_proto, client_proto_len);
+ BIO_printf(bio_stdout, "', server: '");
+ BIO_write(bio_stdout, server_proto, server_proto_len);
+ BIO_printf(bio_stdout, "'\n");
+ BIO_printf(bio_stdout, "ALPN configured: client: '%s', server: '%s'\n", alpn_client, alpn_server);
+ return -1;
+ }
+