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main.go
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main.go
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package main
import (
"bufio"
"bytes"
"crypto/ecdsa"
"crypto/rsa"
"crypto/tls"
"crypto/x509"
"flag"
"fmt"
"io/ioutil"
"log"
"net"
"net/http"
"net/url"
"os"
"strings"
"sync"
"time"
"golang.org/x/crypto/ocsp"
)
const appName = "cert-checker"
func main() {
var (
domain = flag.String("domain", "", "domain to validate")
interval = flag.Duration("interval", time.Minute, "interval to validate domain (optional)")
papertrailAddr = flag.String("papertrail", "", "papertrail destination address logsN.papertrailapp.com:XXXXX (optional)")
domains = flag.String("domains", "", "text file contain domains to validate, see sample (optional)")
)
flag.Parse()
log.SetPrefix(fmt.Sprintf("[%s] ", appName))
log.Printf("Checking domain certificate every %s", *interval)
if *papertrailAddr != "" {
log.Printf("Certificate failures will be logged to papertrail at %s", *papertrailAddr)
}
if *domain == "" && *domains == "" {
log.Fatalf("You must provide a domain to validate, or domains file")
}
for {
domainsToCheck := getDomainsToCheck(*domain, *domains)
runValidation(domainsToCheck, *papertrailAddr)
time.Sleep(*interval)
}
}
func getDomainsToCheck(domain, domains string) []string {
var domainsList []string
if domains != "" {
domainsList = getDomains(domains)
}
if domain != "" {
domainsList = append(domainsList, domain)
}
var domainsToCheck []string
seen := make(map[string]bool)
for _, domainV := range domainsList {
v := sanitizeDomain(domainV)
if v == "" {
continue
}
if seen[v] {
continue
}
seen[v] = true
domainsToCheck = append(domainsToCheck, v)
}
return domainsToCheck
}
const maxConcurrentRequests = 100
func runValidation(domainsToCheck []string, papertrailAddr string) {
var wg sync.WaitGroup
maxReqs := make(chan bool, maxConcurrentRequests)
for _, domain := range domainsToCheck {
wg.Add(1)
maxReqs <- true
go func(domain string) {
defer func() {
<-maxReqs
wg.Done()
}()
if err := validateDomain(domain); err != nil {
log.Printf(err.Error())
if err := paperlog(papertrailAddr, err.Error()); err != nil {
log.Printf(err.Error())
}
} else {
log.Printf("Domain %s is valid", domain)
}
}(domain)
}
wg.Wait()
for i := 0; i < cap(maxReqs); i++ {
maxReqs <- true
}
}
func sanitizeDomain(domain string) string {
domain = strings.TrimSpace(domain)
domain = strings.ToLower(domain)
if domain == "" {
return ""
}
if !strings.HasPrefix(domain, "http") {
domain = fmt.Sprintf("https://%s", domain)
}
parsedURL, err := url.Parse(domain)
if err != nil {
return domain
}
return parsedURL.Hostname()
}
// getDomains reads a file containing a list of domains to validate and returns a slice of domains
func getDomains(domainsFile string) []string {
file, err := os.Open(domainsFile)
if err != nil {
log.Fatalf("failed to open domains file: %v", err)
}
defer func(file *os.File) {
err := file.Close()
if err != nil {
log.Printf("failed to close domains file: %v", err)
}
}(file)
var domains []string
scanner := bufio.NewScanner(file)
for scanner.Scan() {
domains = append(domains, scanner.Text())
}
if err := scanner.Err(); err != nil {
log.Fatalf("failed to read domains file: %v", err)
}
return domains
}
func validateDomain(domain string) error {
ips, err := net.LookupIP(domain)
if err != nil {
return fmt.Errorf("Failed to resolve domain %s: %v\n", domain, err)
}
var errs []string
for _, ip := range ips {
if ip.To4() == nil {
// log.Printf("Skipping IPv6 address %s for domain %s, only IPv4 addresses are supported", ip.String(), domain)
continue
}
if err := checkCertificate(domain, ip.String()); err != nil {
errs = append(errs, err.Error())
}
}
if errs != nil {
return fmt.Errorf("%+v", errs)
}
return nil
}
func checkCertificate(domain, ip string) error {
address := fmt.Sprintf("%s:443", ip)
conn, err := tls.Dial("tcp", address, &tls.Config{
ServerName: domain,
InsecureSkipVerify: true,
})
if err != nil {
return fmt.Errorf("failed to connect %s - %s: %v\n", domain, address, err)
}
defer func(conn *tls.Conn) {
err := conn.Close()
if err != nil {
log.Printf("failed to close tls connection: %v", err)
}
}(conn)
connState := conn.ConnectionState()
certs := connState.PeerCertificates
if len(certs) == 0 {
return fmt.Errorf("no certificates found for %s - %s\n", domain, address)
}
roots := x509.NewCertPool()
intermediates := x509.NewCertPool()
for i, cert := range certs {
if i == 0 {
continue // Skip the leaf certificate
}
if cert.IsCA {
roots.AddCert(cert)
} else {
intermediates.AddCert(cert)
}
}
opts := x509.VerifyOptions{
DNSName: domain,
Intermediates: intermediates,
Roots: roots,
}
// Certificate chain validation
chains, err := certs[0].Verify(opts)
if err != nil {
return fmt.Errorf("certificate verification failed for %s - %s: %v", domain, address, err)
}
// OCSP check for revoked certificates in the chain
if err := checkOCSPForChain(chains); err != nil {
return fmt.Errorf("OCSP revocation check failed for %s - %s: %v", domain, address, err)
}
// Certificate expiration
now := time.Now()
if now.Before(certs[0].NotBefore) || now.After(certs[0].NotAfter) {
return fmt.Errorf("certificate for %s - %s is expired or not yet valid", domain, address)
}
// Key usage and extended key usage
if certs[0].KeyUsage&x509.KeyUsageDigitalSignature == 0 {
return fmt.Errorf("certificate for %s - %s does not have the Digital Signature key usage", domain, address)
}
if len(certs[0].ExtKeyUsage) == 0 || !containsExtKeyUsage(certs[0].ExtKeyUsage, x509.ExtKeyUsageServerAuth) {
return fmt.Errorf("certificate for %s - %s is not valid for server authentication", domain, address)
}
// Basic constraints
//if certs[0].BasicConstraintsValid {
// return fmt.Errorf("certificate for %s - %s should not have basic constraints", domain, address)
//}
// Signature algorithm and key strength
if certs[0].PublicKeyAlgorithm == x509.UnknownPublicKeyAlgorithm {
return fmt.Errorf("certificate for %s - %s has an unknown public key algorithm", domain, address)
}
if !isKeyStrengthValid(certs[0]) {
return fmt.Errorf("certificate for %s - %s has insufficient key strength", domain, address)
}
return nil
}
func checkOCSPForChain(chains [][]*x509.Certificate) error {
for _, chain := range chains {
for i := 0; i < len(chain)-1; i++ {
issuer := chain[i+1]
cert := chain[i]
ocspReq, err := ocsp.CreateRequest(cert, issuer, &ocsp.RequestOptions{})
if err != nil {
return fmt.Errorf("failed to create OCSP request: %v", err)
}
ocspRes, err := getOCSPResponse(ocspReq, cert, issuer)
if err != nil {
return fmt.Errorf("failed to get OCSP response: %v", err)
}
if ocspRes.Status != ocsp.Good {
return fmt.Errorf("certificate status is not good: %v", ocspRes.Status)
}
}
}
return nil
}
func getOCSPResponse(req []byte, cert *x509.Certificate, issuer *x509.Certificate) (*ocsp.Response, error) {
// This function assumes that the OCSP responder URL is present in the certificate
// and sends the request to the first responder URL found.
if len(cert.OCSPServer) == 0 {
return nil, fmt.Errorf("no OCSP responder URL found in the certificate")
}
url := cert.OCSPServer[0]
httpClient := &http.Client{
Timeout: 10 * time.Second,
}
httpReq, err := http.NewRequest(http.MethodPost, url, bytes.NewBuffer(req))
if err != nil {
if err, ok := err.(net.Error); ok && err.Timeout() {
return nil, fmt.Errorf("network timeout while contacting OCSP server: %w", err)
} else {
return nil, fmt.Errorf("network error while contacting OCSP server: %w", err)
}
}
httpReq.Header.Add("Content-Type", "application/ocsp-request")
httpReq.Header.Add("Accept", "application/ocsp-response")
httpRes, err := httpClient.Do(httpReq)
if err != nil {
return nil, fmt.Errorf("failed to get HTTP response: %v", err)
}
defer httpRes.Body.Close()
if httpRes.StatusCode != http.StatusOK {
return nil, fmt.Errorf("OCSP server returned a non-200 status code: %d", httpRes.StatusCode)
}
respBytes, err := ioutil.ReadAll(httpRes.Body)
if err != nil {
return nil, fmt.Errorf("failed to read OCSP response body: %v", err)
}
ocspRes, err := ocsp.ParseResponse(respBytes, issuer)
if err != nil {
return nil, fmt.Errorf("failed to parse OCSP response: %v", err)
}
return ocspRes, nil
}
func containsExtKeyUsage(extKeyUsages []x509.ExtKeyUsage, extKeyUsage x509.ExtKeyUsage) bool {
for _, usage := range extKeyUsages {
if usage == extKeyUsage {
return true
}
}
return false
}
func isKeyStrengthValid(cert *x509.Certificate) bool {
// This function assumes the public key algorithm is already known
// and checks the key strength depending on the algorithm.
minRSAKeySize := 2048
minECKeySize := 256
switch pubKey := cert.PublicKey.(type) {
case *rsa.PublicKey:
if pubKey.N.BitLen() >= minRSAKeySize {
return true
}
case *ecdsa.PublicKey:
if pubKey.Curve.Params().BitSize >= minECKeySize {
return true
}
}
return false
}
func paperlog(papertrailAddr, message string) error {
if papertrailAddr == "" {
return nil
}
hostname, err := os.Hostname()
if err != nil {
hostname = fmt.Sprintf("%s-host", appName)
}
timestamp := time.Now().UTC().Format(time.RFC3339)
logMessage := fmt.Sprintf("<22>1 %s %s %s - - %s", timestamp, hostname, appName, message)
// Send the message to Papertrail using UDP
conn, err := net.Dial("udp", papertrailAddr)
if err != nil {
return fmt.Errorf("failed to connect to Papertrail: %v", err)
}
defer func(conn net.Conn) {
err := conn.Close()
if err != nil {
log.Printf("failed to close connection to Papertrail: %v", err)
}
}(conn)
_, err = conn.Write([]byte(logMessage))
if err != nil {
return fmt.Errorf("failed to send log message to Papertrail: %v", err)
}
return nil
}