certcheck.go

package main

import (
	"os"
	"log"
	"crypto/tls"
	"crypto/x509"
)

func vpc(rawCerts [][]byte, verifiedChains [][]*x509.Certificate) error {
	for ri, rawcert := range rawCerts {
		certs, err := x509.ParseCertificates(rawcert)
		if err != nil {
			continue
		}
		for i, cert := range(certs) {
			log.Println("index", ri, i)
			log.Println("Issuer", cert.Issuer)
			log.Println("Subject", cert.Subject)
		}
	}

	return nil
}

func main() {
	conf := &tls.Config {
		VerifyPeerCertificate: vpc,
		InsecureSkipVerify: true,
	}

	dest := "www.google.com:443"
	if len(os.Args) > 1 {
		dest = os.Args[1]
	}

	conn, err := tls.Dial("tcp", dest, conf)
	if err != nil {
		log.Println("Dial", err)
		return
	}
	conn.Close()
}

Thanks, that was a clear example, I used it here https://github.com/mau-network/mau/blob/a049629ba9d4be102faee140bbfa2b2381a6bf32/client.go#L50