Fix key principals not being used when identity files are being used (#11609)

lib/client/api.go

@@ -226,6 +226,10 @@ type Config struct {
 	// against Teleport client and obtaining credentials from elsewhere.
 	SkipLocalAuth bool
 
+	// UseKeyPrincipals forces the use of the username from the key principals rather than using
+	// the current user username.
+	UseKeyPrincipals bool
+
 	// Agent is used when SkipLocalAuth is true
 	Agent agent.Agent
 
@@ -2166,7 +2170,7 @@ func (tc *TeleportClient) getProxySSHPrincipal() string {
 		proxyPrincipal = tc.JumpHosts[0].Username
 	}
 	// see if we already have a signed key in the cache, we'll use that instead
-	if !tc.Config.SkipLocalAuth && tc.localAgent != nil {
+	if (!tc.Config.SkipLocalAuth || tc.UseKeyPrincipals) && tc.localAgent != nil {
 		signers, err := tc.localAgent.Signers()
 		if err != nil || len(signers) == 0 {
 			return proxyPrincipal

lib/client/api_test.go

@@ -17,12 +17,16 @@ limitations under the License.
 package client
 
 import (
+	"io"
 	"os"
 	"testing"
 
 	"github.com/gravitational/teleport/api/client/webclient"
 	"github.com/gravitational/teleport/lib/defaults"
 	"github.com/gravitational/teleport/lib/utils"
+	"github.com/gravitational/trace"
+	"golang.org/x/crypto/ssh"
+	"golang.org/x/crypto/ssh/agent"
 
 	"github.com/stretchr/testify/require"
 	"gopkg.in/check.v1"
@@ -499,6 +503,53 @@ func TestApplyProxySettings(t *testing.T) {
 	}
 }
 
+type mockAgent struct {
+	// Agent is embedded to avoid redeclaring all interface methods.
+	// Only the Signers method is implemented by testAgent.
+	agent.Agent
+	ValidPrincipals []string
+}
+
+type mockSigner struct {
+	ValidPrincipals []string
+}
+
+func (s *mockSigner) PublicKey() ssh.PublicKey {
+	return &ssh.Certificate{
+		ValidPrincipals: s.ValidPrincipals,
+	}
+}
+
+func (s *mockSigner) Sign(rand io.Reader, b []byte) (*ssh.Signature, error) {
+	return nil, trace.Errorf("mockSigner does not implement Sign")
+}
+
+// Signers implements agent.Agent.Signers.
+func (m *mockAgent) Signers() ([]ssh.Signer, error) {
+	return []ssh.Signer{&mockSigner{ValidPrincipals: m.ValidPrincipals}}, nil
+}
+
+func TestNewClient_UseKeyPrincipals(t *testing.T) {
+	cfg := &Config{
+		Username:         "xyz",
+		HostLogin:        "xyz",
+		WebProxyAddr:     "localhost",
+		SkipLocalAuth:    true,
+		UseKeyPrincipals: true, // causes VALID to be returned, as key was used
+		Agent:            &mockAgent{ValidPrincipals: []string{"VALID"}},
+		AuthMethods:      []ssh.AuthMethod{ssh.Password("xyz") /* placeholder authmethod */},
+	}
+	client, err := NewClient(cfg)
+	require.NoError(t, err)
+	require.Equal(t, "VALID", client.getProxySSHPrincipal(), "ProxySSHPrincipal mismatch")
+
+	cfg.UseKeyPrincipals = false // causes xyz to be returned as key was not used
+
+	client, err = NewClient(cfg)
+	require.NoError(t, err)
+	require.Equal(t, "xyz", client.getProxySSHPrincipal(), "ProxySSHPrincipal mismatch")
+}
+
 func TestParseSearchKeywords(t *testing.T) {
 	t.Parallel()
 

tool/tsh/tsh.go

@@ -1910,6 +1910,10 @@ func makeClient(cf *CLIConf, useProfileLogin bool) (*client.TeleportClient, erro
 	if cf.IdentityFileIn != "" {
 		// Ignore local authentication methods when identity file is provided
 		c.SkipLocalAuth = true
+		// Force the use of the certificate principals so Unix
+		// username does not get used when logging in
+		c.UseKeyPrincipals = hostLogin == ""
+
 		var (
 			key          *client.Key
 			identityAuth ssh.AuthMethod