Avoid sending a trailing dot in SNI as a client

While an FQDN includes a trailing dot for the zero-length label of
the root, SNI explicitly does not contain it. Contrary to other TLS
implementations, our tlsext_sni_is_valid_hostname() rejects a trailing
dot. The result is that LibreSSL TLS servers encountering an SNI with
trailing dot abort the connection with an illegal_parameter alert.

This fixes an issue reported by danj in nc(1) and by sthen in ftp(1).
DNS cluebat from florian.

ok jsing

1 files changed, 19 insertions, 6 deletions

diff --git a/src/lib/libtls/tls_client.c b/src/lib/libtls/tls_client.c
index 04e44020ef..701bb6fc81 100644
--- a/src/lib/libtls/tls_client.c
+++ b/src/lib/libtls/tls_client.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: tls_client.c,v 1.45 2018/03/19 16:34:47 jsing Exp $ */
+/* $OpenBSD: tls_client.c,v 1.46 2021/06/01 20:14:17 tb Exp $ */
 /*
  * Copyright (c) 2014 Joel Sing <jsing@openbsd.org>
  *
@@ -279,6 +279,7 @@ static int
 tls_connect_common(struct tls *ctx, const char *servername)
 {
         union tls_addr addrbuf;
+        size_t servername_len;
         int rv = -1;
 
         if ((ctx->flags & TLS_CLIENT) == 0) {
@@ -291,6 +292,17 @@ tls_connect_common(struct tls *ctx, const char *servername)
                         tls_set_errorx(ctx, "out of memory");
                         goto err;
                 }
+
+                /*
+                 * If there's a trailing dot, remove it. While an FQDN includes
+                 * the terminating dot representing the zero-length label of
+                 * the root (RFC 8499, section 2), the SNI explicitly does not
+                 * include it (RFC 6066, section 3).
+                 */
+                servername_len = strlen(ctx->servername);
+                if (servername_len > 0 &&
+                    ctx->servername[servername_len - 1] == '.')
+                        ctx->servername[servername_len - 1] = '\0';
         }
 
         if ((ctx->ssl_ctx = SSL_CTX_new(SSLv23_client_method())) == NULL) {
@@ -306,7 +318,7 @@ tls_connect_common(struct tls *ctx, const char *servername)
                 goto err;
 
         if (ctx->config->verify_name) {
-                if (servername == NULL) {
+                if (ctx->servername == NULL) {
                         tls_set_errorx(ctx, "server name not specified");
                         goto err;
                 }
@@ -353,10 +365,11 @@ tls_connect_common(struct tls *ctx, const char *servername)
          * RFC4366 (SNI): Literal IPv4 and IPv6 addresses are not
          * permitted in "HostName".
          */
-        if (servername != NULL &&
-            inet_pton(AF_INET, servername, &addrbuf) != 1 &&
-            inet_pton(AF_INET6, servername, &addrbuf) != 1) {
-                if (SSL_set_tlsext_host_name(ctx->ssl_conn, servername) == 0) {
+        if (ctx->servername != NULL &&
+            inet_pton(AF_INET, ctx->servername, &addrbuf) != 1 &&
+            inet_pton(AF_INET6, ctx->servername, &addrbuf) != 1) {
+                if (SSL_set_tlsext_host_name(ctx->ssl_conn,
+                    ctx->servername) == 0) {
                         tls_set_errorx(ctx, "server name indication failure");
                         goto err;
                 }