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282 lines
10 KiB
Go
282 lines
10 KiB
Go
// Copyright 2020 Google LLC.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// cba.go (certificate-based access) contains utils for implementing Device Certificate
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// Authentication according to https://google.aip.dev/auth/4114 and Default Credentials
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// for Google Cloud Virtual Environments according to https://google.aip.dev/auth/4115.
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//
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// The overall logic for DCA is as follows:
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// 1. If both endpoint override and client certificate are specified, use them as is.
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// 2. If user does not specify client certificate, we will attempt to use default
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// client certificate.
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// 3. If user does not specify endpoint override, we will use defaultMtlsEndpoint if
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// client certificate is available and defaultEndpoint otherwise.
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//
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// Implications of the above logic:
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// 1. If the user specifies a non-mTLS endpoint override but client certificate is
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// available, we will pass along the cert anyway and let the server decide what to do.
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// 2. If the user specifies an mTLS endpoint override but client certificate is not
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// available, we will not fail-fast, but let backend throw error when connecting.
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//
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// If running within Google's cloud environment, and client certificate is not specified
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// and not available through DCA, we will try mTLS with credentials held by
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// the Secure Session Agent, which is part of Google's cloud infrastructure.
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//
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// We would like to avoid introducing client-side logic that parses whether the
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// endpoint override is an mTLS url, since the url pattern may change at anytime.
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//
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// This package is not intended for use by end developers. Use the
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// google.golang.org/api/option package to configure API clients.
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// Package internal supports the options and transport packages.
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package internal
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import (
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"context"
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"crypto/tls"
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"net"
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"net/url"
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"os"
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"strings"
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"github.com/google/s2a-go"
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"github.com/google/s2a-go/fallback"
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"google.golang.org/api/internal/cert"
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"google.golang.org/grpc/credentials"
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)
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const (
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mTLSModeAlways = "always"
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mTLSModeNever = "never"
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mTLSModeAuto = "auto"
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// Experimental: if true, the code will try MTLS with S2A as the default for transport security. Default value is false.
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googleAPIUseS2AEnv = "EXPERIMENTAL_GOOGLE_API_USE_S2A"
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)
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// getClientCertificateSourceAndEndpoint is a convenience function that invokes
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// getClientCertificateSource and getEndpoint sequentially and returns the client
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// cert source and endpoint as a tuple.
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func getClientCertificateSourceAndEndpoint(settings *DialSettings) (cert.Source, string, error) {
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clientCertSource, err := getClientCertificateSource(settings)
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if err != nil {
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return nil, "", err
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}
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endpoint, err := getEndpoint(settings, clientCertSource)
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if err != nil {
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return nil, "", err
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}
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return clientCertSource, endpoint, nil
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}
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type transportConfig struct {
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clientCertSource cert.Source // The client certificate source.
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endpoint string // The corresponding endpoint to use based on client certificate source.
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s2aAddress string // The S2A address if it can be used, otherwise an empty string.
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s2aMTLSEndpoint string // The MTLS endpoint to use with S2A.
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}
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func getTransportConfig(settings *DialSettings) (*transportConfig, error) {
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clientCertSource, endpoint, err := getClientCertificateSourceAndEndpoint(settings)
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if err != nil {
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return &transportConfig{
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clientCertSource: nil, endpoint: "", s2aAddress: "", s2aMTLSEndpoint: "",
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}, err
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}
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defaultTransportConfig := transportConfig{
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clientCertSource: clientCertSource,
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endpoint: endpoint,
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s2aAddress: "",
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s2aMTLSEndpoint: "",
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}
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// Check the env to determine whether to use S2A.
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if !isGoogleS2AEnabled() {
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return &defaultTransportConfig, nil
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}
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// If client cert is found, use that over S2A.
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// If MTLS is not enabled for the endpoint, skip S2A.
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if clientCertSource != nil || !mtlsEndpointEnabledForS2A() {
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return &defaultTransportConfig, nil
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}
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s2aMTLSEndpoint := settings.DefaultMTLSEndpoint
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// If there is endpoint override, honor it.
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if settings.Endpoint != "" {
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s2aMTLSEndpoint = endpoint
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}
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s2aAddress := GetS2AAddress()
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if s2aAddress == "" {
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return &defaultTransportConfig, nil
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}
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return &transportConfig{
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clientCertSource: clientCertSource,
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endpoint: endpoint,
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s2aAddress: s2aAddress,
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s2aMTLSEndpoint: s2aMTLSEndpoint,
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}, nil
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}
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func isGoogleS2AEnabled() bool {
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return strings.ToLower(os.Getenv(googleAPIUseS2AEnv)) == "true"
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}
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// getClientCertificateSource returns a default client certificate source, if
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// not provided by the user.
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//
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// A nil default source can be returned if the source does not exist. Any exceptions
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// encountered while initializing the default source will be reported as client
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// error (ex. corrupt metadata file).
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//
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// Important Note: For now, the environment variable GOOGLE_API_USE_CLIENT_CERTIFICATE
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// must be set to "true" to allow certificate to be used (including user provided
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// certificates). For details, see AIP-4114.
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func getClientCertificateSource(settings *DialSettings) (cert.Source, error) {
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if !isClientCertificateEnabled() {
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return nil, nil
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} else if settings.ClientCertSource != nil {
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return settings.ClientCertSource, nil
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} else {
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return cert.DefaultSource()
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}
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}
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func isClientCertificateEnabled() bool {
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useClientCert := os.Getenv("GOOGLE_API_USE_CLIENT_CERTIFICATE")
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// TODO(andyrzhao): Update default to return "true" after DCA feature is fully released.
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return strings.ToLower(useClientCert) == "true"
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}
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// getEndpoint returns the endpoint for the service, taking into account the
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// user-provided endpoint override "settings.Endpoint".
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//
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// If no endpoint override is specified, we will either return the default endpoint or
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// the default mTLS endpoint if a client certificate is available.
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//
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// You can override the default endpoint choice (mtls vs. regular) by setting the
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// GOOGLE_API_USE_MTLS_ENDPOINT environment variable.
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//
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// If the endpoint override is an address (host:port) rather than full base
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// URL (ex. https://...), then the user-provided address will be merged into
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// the default endpoint. For example, WithEndpoint("myhost:8000") and
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// WithDefaultEndpoint("https://foo.com/bar/baz") will return "https://myhost:8080/bar/baz"
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func getEndpoint(settings *DialSettings, clientCertSource cert.Source) (string, error) {
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if settings.Endpoint == "" {
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mtlsMode := getMTLSMode()
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if mtlsMode == mTLSModeAlways || (clientCertSource != nil && mtlsMode == mTLSModeAuto) {
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return settings.DefaultMTLSEndpoint, nil
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}
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return settings.DefaultEndpoint, nil
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}
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if strings.Contains(settings.Endpoint, "://") {
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// User passed in a full URL path, use it verbatim.
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return settings.Endpoint, nil
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}
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if settings.DefaultEndpoint == "" {
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// If DefaultEndpoint is not configured, use the user provided endpoint verbatim.
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// This allows a naked "host[:port]" URL to be used with GRPC Direct Path.
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return settings.Endpoint, nil
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}
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// Assume user-provided endpoint is host[:port], merge it with the default endpoint.
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return mergeEndpoints(settings.DefaultEndpoint, settings.Endpoint)
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}
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func getMTLSMode() string {
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mode := os.Getenv("GOOGLE_API_USE_MTLS_ENDPOINT")
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if mode == "" {
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mode = os.Getenv("GOOGLE_API_USE_MTLS") // Deprecated.
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}
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if mode == "" {
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return mTLSModeAuto
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}
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return strings.ToLower(mode)
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}
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func mergeEndpoints(baseURL, newHost string) (string, error) {
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u, err := url.Parse(fixScheme(baseURL))
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if err != nil {
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return "", err
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}
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return strings.Replace(baseURL, u.Host, newHost, 1), nil
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}
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func fixScheme(baseURL string) string {
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if !strings.Contains(baseURL, "://") {
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return "https://" + baseURL
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}
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return baseURL
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}
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// GetGRPCTransportConfigAndEndpoint returns an instance of credentials.TransportCredentials, and the
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// corresponding endpoint to use for GRPC client.
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func GetGRPCTransportConfigAndEndpoint(settings *DialSettings) (credentials.TransportCredentials, string, error) {
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config, err := getTransportConfig(settings)
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if err != nil {
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return nil, "", err
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}
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defaultTransportCreds := credentials.NewTLS(&tls.Config{
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GetClientCertificate: config.clientCertSource,
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})
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if config.s2aAddress == "" {
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return defaultTransportCreds, config.endpoint, nil
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}
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var fallbackOpts *s2a.FallbackOptions
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// In case of S2A failure, fall back to the endpoint that would've been used without S2A.
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if fallbackHandshake, err := fallback.DefaultFallbackClientHandshakeFunc(config.endpoint); err == nil {
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fallbackOpts = &s2a.FallbackOptions{
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FallbackClientHandshakeFunc: fallbackHandshake,
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}
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}
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s2aTransportCreds, err := s2a.NewClientCreds(&s2a.ClientOptions{
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S2AAddress: config.s2aAddress,
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FallbackOpts: fallbackOpts,
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})
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if err != nil {
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// Use default if we cannot initialize S2A client transport credentials.
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return defaultTransportCreds, config.endpoint, nil
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}
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return s2aTransportCreds, config.s2aMTLSEndpoint, nil
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}
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// GetHTTPTransportConfigAndEndpoint returns a client certificate source, a function for dialing MTLS with S2A,
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// and the endpoint to use for HTTP client.
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func GetHTTPTransportConfigAndEndpoint(settings *DialSettings) (cert.Source, func(context.Context, string, string) (net.Conn, error), string, error) {
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config, err := getTransportConfig(settings)
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if err != nil {
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return nil, nil, "", err
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}
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if config.s2aAddress == "" {
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return config.clientCertSource, nil, config.endpoint, nil
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}
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var fallbackOpts *s2a.FallbackOptions
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// In case of S2A failure, fall back to the endpoint that would've been used without S2A.
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if fallbackURL, err := url.Parse(config.endpoint); err == nil {
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if fallbackDialer, fallbackServerAddr, err := fallback.DefaultFallbackDialerAndAddress(fallbackURL.Hostname()); err == nil {
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fallbackOpts = &s2a.FallbackOptions{
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FallbackDialer: &s2a.FallbackDialer{
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Dialer: fallbackDialer,
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ServerAddr: fallbackServerAddr,
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},
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}
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}
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}
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dialTLSContextFunc := s2a.NewS2ADialTLSContextFunc(&s2a.ClientOptions{
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S2AAddress: config.s2aAddress,
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FallbackOpts: fallbackOpts,
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})
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return nil, dialTLSContextFunc, config.s2aMTLSEndpoint, nil
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}
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// mtlsEndpointEnabledForS2A checks if the endpoint is indeed MTLS-enabled, so that we can use S2A for MTLS connection.
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var mtlsEndpointEnabledForS2A = func() bool {
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// TODO(xmenxk): determine this via discovery config.
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return true
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}
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