Add client TLS builder method to add trust anchors

As per #101, it is sometimes desirable to use standard web PKI roots for
gRPC clients. This commit adds a method to ClientTlsConfig to add the
trust roots from the system certificate store:

- OpenSSL uses `openssl-probe` to search the system for roots.
- Rustls uses `rustls-native-certs` to load the system roots.

Enabling the `openssl-roots` or `rustls-roots` feature for `tonic` in
`Cargo.toml` will add system roots by default when configuring a gRPC
client.

tonic/Cargo.toml

@@ -34,6 +34,8 @@ transport = [
 ]
 openssl = ["openssl1", "tokio-openssl", "tls"]
 rustls = ["tokio-rustls", "tls"]
+openssl-roots = ["openssl-probe"]
+rustls-roots = ["rustls-native-certs"]
 tls = []
 
 [[bench]]
@@ -73,9 +75,11 @@ tower-load = { version = "=0.3.0-alpha.2", optional = true }
 # openssl
 tokio-openssl = { version = "=0.4.0-alpha.6", optional = true }
 openssl1 = { package = "openssl", version = "0.10", optional = true }
+openssl-probe = { version = "0.1", optional = true }
 
 # rustls
 tokio-rustls = { version = "=0.12.0-alpha.5", optional = true }
+rustls-native-certs = { version = "0.1", optional = true }
 
 [dev-dependencies]
 static_assertions = "1.0"

tonic/src/transport/endpoint.rs

@@ -259,18 +259,27 @@ impl ClientTlsConfig {
     }
 
     /// Sets the domain name against which to verify the server's TLS certificate.
+    ///
+    /// This has no effect if `rustls_client_config` or `openssl_connector` is used to configure
+    /// Rustls or OpenSSL respectively.
     pub fn domain_name(&mut self, domain_name: impl Into<String>) -> &mut Self {
         self.domain = Some(domain_name.into());
         self
     }
 
     /// Sets the CA Certificate against which to verify the server's TLS certificate.
+    ///
+    /// This has no effect if `rustls_client_config` or `openssl_connector` is used to configure
+    /// Rustls or OpenSSL respectively.
     pub fn ca_certificate(&mut self, ca_certificate: Certificate) -> &mut Self {
         self.cert = Some(ca_certificate);
         self
     }
 
     /// Sets the client identity to present to the server.
+    ///
+    /// This has no effect if `rustls_client_config` or `openssl_connector` is used to configure
+    /// Rustls or OpenSSL respectively.
     pub fn identity(&mut self, identity: Identity) -> &mut Self {
         self.identity = Some(identity);
         self

tonic/src/transport/service/tls.rs

@@ -6,6 +6,8 @@ use openssl1::{
     ssl::{select_next_proto, AlpnError, SslAcceptor, SslConnector, SslMethod, SslVerifyMode},
     x509::{store::X509StoreBuilder, X509},
 };
+#[cfg(feature = "openssl-roots")]
+use openssl_probe;
 use std::{fmt, sync::Arc};
 use tokio::net::TcpStream;
 #[cfg(feature = "rustls")]
@@ -14,6 +16,8 @@ use tokio_rustls::{
     webpki::DNSNameRef,
     TlsAcceptor as RustlsAcceptor, TlsConnector as RustlsConnector,
 };
+#[cfg(feature = "rustls-roots")]
+use rustls_native_certs;
 
 /// h2 alpn in wire format for openssl.
 #[cfg(feature = "openssl")]
@@ -37,6 +41,8 @@ enum TlsError {
     CertificateParseError,
     #[cfg(feature = "rustls")]
     PrivateKeyParseError,
+    #[cfg(feature = "openssl-roots")]
+    TrustAnchorsConfigurationError(openssl1::error::ErrorStack),
 }
 
 #[derive(Clone)]
@@ -63,6 +69,15 @@ impl TlsConnector {
         let mut config = SslConnector::builder(SslMethod::tls())?;
         config.set_alpn_protos(ALPN_H2_WIRE)?;
 
+        #[cfg(feature = "openssl-roots")]
+        {
+            openssl_probe::init_ssl_cert_env_vars();
+            match config.cert_store_mut().set_default_paths() {
+                Ok(()) => (),
+                Err(e) => return Err(Box::new(TlsError::TrustAnchorsConfigurationError(e))),
+            };
+        }
+
         if let Some(cert) = cert {
             let ca = X509::from_pem(&cert.pem[..])?;
             config.cert_store_mut().add_cert(ca)?;
@@ -106,6 +121,11 @@ impl TlsConnector {
             config.set_single_client_cert(client_cert, client_key);
         }
 
+        #[cfg(feature = "rustls-roots")]
+        {
+            config.root_store = rustls_native_certs::load_native_certs()?;
+        }
+
         if let Some(cert) = ca_cert {
             let mut buf = std::io::Cursor::new(&cert.pem[..]);
             config.root_store.add_pem_file(&mut buf).unwrap();
@@ -336,6 +356,10 @@ impl fmt::Display for TlsError {
                 f,
                 "Error parsing TLS private key - no RSA or PKCS8-encoded keys found."
             ),
+            #[cfg(feature = "openssl-roots")]
+            TlsError::TrustAnchorsConfigurationError(stack) => {
+                f.write_fmt(format_args!("Error adding trust anchors - {}", stack))
+            }
         }
     }
 }