net/tlsdial: call out firewalls blocking Tailscale in health warnings (#13840)

Updates tailscale/tailscale#13839

Adds a new blockblame package which can detect common MITM SSL certificates used by network appliances. We use this in `tlsdial` to display a dedicated health warning when we cannot connect to control, and a network appliance MITM attack is detected.

Signed-off-by: Andrea Gottardo <andrea@gottardo.me>

net/tlsdial/blockblame/blockblame_test.go

@@ -0,0 +1,54 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package blockblame
+
+import (
+	"crypto/x509"
+	"encoding/pem"
+	"testing"
+)
+
+const controlplaneDotTailscaleDotComPEM = `
+-----BEGIN CERTIFICATE-----
+MIIDkzCCAxqgAwIBAgISA2GOahsftpp59yuHClbDuoduMAoGCCqGSM49BAMDMDIx
+CzAJBgNVBAYTAlVTMRYwFAYDVQQKEw1MZXQncyBFbmNyeXB0MQswCQYDVQQDEwJF
+NjAeFw0yNDEwMTIxNjE2NDVaFw0yNTAxMTAxNjE2NDRaMCUxIzAhBgNVBAMTGmNv
+bnRyb2xwbGFuZS50YWlsc2NhbGUuY29tMFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcD
+QgAExfraDUc1t185zuGtZlnPDtEJJSDBqvHN4vQcXSzSTPSAdDYHcA8fL5woU2Kg
+jK/2C0wm/rYy2Rre/ulhkS4wB6OCAhswggIXMA4GA1UdDwEB/wQEAwIHgDAdBgNV
+HSUEFjAUBggrBgEFBQcDAQYIKwYBBQUHAwIwDAYDVR0TAQH/BAIwADAdBgNVHQ4E
+FgQUpArnpDj8Yh6NTgMOZjDPx0TuLmcwHwYDVR0jBBgwFoAUkydGmAOpUWiOmNbE
+QkjbI79YlNIwVQYIKwYBBQUHAQEESTBHMCEGCCsGAQUFBzABhhVodHRwOi8vZTYu
+by5sZW5jci5vcmcwIgYIKwYBBQUHMAKGFmh0dHA6Ly9lNi5pLmxlbmNyLm9yZy8w
+JQYDVR0RBB4wHIIaY29udHJvbHBsYW5lLnRhaWxzY2FsZS5jb20wEwYDVR0gBAww
+CjAIBgZngQwBAgEwggEDBgorBgEEAdZ5AgQCBIH0BIHxAO8AdgDgkrP8DB3I52g2
+H95huZZNClJ4GYpy1nLEsE2lbW9UBAAAAZKBujCyAAAEAwBHMEUCIQDHMgUaL4H9
+ZJa090ZOpBeEVu3+t+EF4HlHI1NqAai6uQIgeY/lLfjAXfcVgxBHHR4zjd0SzhaP
+TREHXzwxzN/8blkAdQDPEVbu1S58r/OHW9lpLpvpGnFnSrAX7KwB0lt3zsw7CAAA
+AZKBujh8AAAEAwBGMEQCICQwhMk45t9aiFjfwOC/y6+hDbszqSCpIv63kFElweUy
+AiAqTdkqmbqUVpnav5JdWkNERVAIlY4jqrThLsCLZYbNszAKBggqhkjOPQQDAwNn
+ADBkAjALyfgAt1XQp1uSfxy4GapR5OsmjEMBRVq6IgsPBlCRBfmf0Q3/a6mF0pjb
+Sj4oa+cCMEhZk4DmBTIdZY9zjuh8s7bXNfKxUQS0pEhALtXqyFr+D5dF7JcQo9+s
+Z98JY7/PCA==
+-----END CERTIFICATE-----`
+
+func TestVerifyCertificateOurControlPlane(t *testing.T) {
+	p, _ := pem.Decode([]byte(controlplaneDotTailscaleDotComPEM))
+	if p == nil {
+		t.Fatalf("failed to extract certificate bytes for controlplane.tailscale.com")
+		return
+	}
+	cert, err := x509.ParseCertificate(p.Bytes)
+	if err != nil {
+		t.Fatalf("failed to parse certificate: %v", err)
+		return
+	}
+	m, found := VerifyCertificate(cert)
+	if found {
+		t.Fatalf("expected to not get a result for the controlplane.tailscale.com certificate")
+	}
+	if m != nil {
+		t.Fatalf("expected nil manufacturer for controlplane.tailscale.com certificate")
+	}
+}