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/*
*
* Copyright 2023 Google LLC
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*
*/
// Package fallback provides default implementations of fallback options when S2A fails.
package fallback
import (
"context"
"crypto/tls"
"fmt"
"net"
"google.golang.org/grpc/credentials"
"google.golang.org/grpc/grpclog"
)
const (
alpnProtoStrH2 = "h2"
alpnProtoStrHTTP = "http/1.1"
defaultHTTPSPort = "443"
)
// FallbackTLSConfigGRPC is a tls.Config used by the DefaultFallbackClientHandshakeFunc function.
// It supports GRPC use case, thus the alpn is set to 'h2'.
var FallbackTLSConfigGRPC = tls.Config{
MinVersion: tls.VersionTLS13,
ClientSessionCache: nil,
NextProtos: []string{alpnProtoStrH2},
}
// FallbackTLSConfigHTTP is a tls.Config used by the DefaultFallbackDialerAndAddress func.
// It supports the HTTP use case and the alpn is set to both 'http/1.1' and 'h2'.
var FallbackTLSConfigHTTP = tls.Config{
MinVersion: tls.VersionTLS13,
ClientSessionCache: nil,
NextProtos: []string{alpnProtoStrH2, alpnProtoStrHTTP},
}
// ClientHandshake establishes a TLS connection and returns it, plus its auth info.
// Inputs:
//
// targetServer: the server attempted with S2A.
// conn: the tcp connection to the server at address targetServer that was passed into S2A's ClientHandshake func.
// If fallback is successful, the `conn` should be closed.
// err: the error encountered when performing the client-side TLS handshake with S2A.
type ClientHandshake func(ctx context.Context, targetServer string, conn net.Conn, err error) (net.Conn, credentials.AuthInfo, error)
// DefaultFallbackClientHandshakeFunc returns a ClientHandshake function,
// which establishes a TLS connection to the provided fallbackAddr, returns the new connection and its auth info.
// Example use:
//
// transportCreds, _ = s2a.NewClientCreds(&s2a.ClientOptions{
// S2AAddress: s2aAddress,
// FallbackOpts: &s2a.FallbackOptions{ // optional
// FallbackClientHandshakeFunc: fallback.DefaultFallbackClientHandshakeFunc(fallbackAddr),
// },
// })
//
// The fallback server's certificate must be verifiable using OS root store.
// The fallbackAddr is expected to be a network address, e.g. example.com:port. If port is not specified,
// it uses default port 443.
// In the returned function's TLS config, ClientSessionCache is explicitly set to nil to disable TLS resumption,
// and min TLS version is set to 1.3.
func DefaultFallbackClientHandshakeFunc(fallbackAddr string) (ClientHandshake, error) {
var fallbackDialer = tls.Dialer{Config: &FallbackTLSConfigGRPC}
return defaultFallbackClientHandshakeFuncInternal(fallbackAddr, fallbackDialer.DialContext)
}
func defaultFallbackClientHandshakeFuncInternal(fallbackAddr string, dialContextFunc func(context.Context, string, string) (net.Conn, error)) (ClientHandshake, error) {
fallbackServerAddr, err := processFallbackAddr(fallbackAddr)
if err != nil {
if grpclog.V(1) {
grpclog.Infof("error processing fallback address [%s]: %v", fallbackAddr, err)
}
return nil, err
}
return func(ctx context.Context, targetServer string, conn net.Conn, s2aErr error) (net.Conn, credentials.AuthInfo, error) {
fbConn, fbErr := dialContextFunc(ctx, "tcp", fallbackServerAddr)
if fbErr != nil {
grpclog.Infof("dialing to fallback server %s failed: %v", fallbackServerAddr, fbErr)
return nil, nil, fmt.Errorf("dialing to fallback server %s failed: %v; S2A client handshake with %s error: %w", fallbackServerAddr, fbErr, targetServer, s2aErr)
}
tc, success := fbConn.(*tls.Conn)
if !success {
grpclog.Infof("the connection with fallback server is expected to be tls but isn't")
return nil, nil, fmt.Errorf("the connection with fallback server is expected to be tls but isn't; S2A client handshake with %s error: %w", targetServer, s2aErr)
}
tlsInfo := credentials.TLSInfo{
State: tc.ConnectionState(),
CommonAuthInfo: credentials.CommonAuthInfo{
SecurityLevel: credentials.PrivacyAndIntegrity,
},
}
if grpclog.V(1) {
grpclog.Infof("ConnectionState.NegotiatedProtocol: %v", tc.ConnectionState().NegotiatedProtocol)
grpclog.Infof("ConnectionState.HandshakeComplete: %v", tc.ConnectionState().HandshakeComplete)
grpclog.Infof("ConnectionState.ServerName: %v", tc.ConnectionState().ServerName)
}
conn.Close()
return fbConn, tlsInfo, nil
}, nil
}
// DefaultFallbackDialerAndAddress returns a TLS dialer and the network address to dial.
// Example use:
//
// fallbackDialer, fallbackServerAddr := fallback.DefaultFallbackDialerAndAddress(fallbackAddr)
// dialTLSContext := s2a.NewS2aDialTLSContextFunc(&s2a.ClientOptions{
// S2AAddress: s2aAddress, // required
// FallbackOpts: &s2a.FallbackOptions{
// FallbackDialer: &s2a.FallbackDialer{
// Dialer: fallbackDialer,
// ServerAddr: fallbackServerAddr,
// },
// },
// })
//
// The fallback server's certificate should be verifiable using OS root store.
// The fallbackAddr is expected to be a network address, e.g. example.com:port. If port is not specified,
// it uses default port 443.
// In the returned function's TLS config, ClientSessionCache is explicitly set to nil to disable TLS resumption,
// and min TLS version is set to 1.3.
func DefaultFallbackDialerAndAddress(fallbackAddr string) (*tls.Dialer, string, error) {
fallbackServerAddr, err := processFallbackAddr(fallbackAddr)
if err != nil {
if grpclog.V(1) {
grpclog.Infof("error processing fallback address [%s]: %v", fallbackAddr, err)
}
return nil, "", err
}
return &tls.Dialer{Config: &FallbackTLSConfigHTTP}, fallbackServerAddr, nil
}
func processFallbackAddr(fallbackAddr string) (string, error) {
var fallbackServerAddr string
var err error
if fallbackAddr == "" {
return "", fmt.Errorf("empty fallback address")
}
_, _, err = net.SplitHostPort(fallbackAddr)
if err != nil {
// fallbackAddr does not have port suffix
fallbackServerAddr = net.JoinHostPort(fallbackAddr, defaultHTTPSPort)
} else {
// FallbackServerAddr already has port suffix
fallbackServerAddr = fallbackAddr
}
return fallbackServerAddr, nil
}
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