https_custom_ip.go

package main

import (
    "context"
    "crypto/tls"
    "fmt"
    "net"
    "net/http"
    "time"
)

func main() {
    client := &http.Client{
        Transport: &http.Transport{
            // 1. Base dialer
            DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
                // addr is "hostname:port"
                host, port, err := net.SplitHostPort(addr)
                if err != nil {
                    return nil, err
                }
                // 2. If this is our FQDN, swap in the custom IP
                if host == "api.example.com" {
                    host = "203.0.113.42" // your override IP
                }
                d := &net.Dialer{
                    Timeout:   10 * time.Second,
                    KeepAlive: 30 * time.Second,
                }
                return d.DialContext(ctx, network, net.JoinHostPort(host, port))
            },
            // 3. Make sure TLS uses the real hostname for SNI & cert checks
            TLSClientConfig: &tls.Config{
                ServerName: "api.example.com",
            },
        },
    }

    req, _ := http.NewRequest("GET", "https://api.example.com/hello", nil)
    // 4. (Optional) ensure the HTTP Host header is correct
    req.Host = "api.example.com"

    resp, err := client.Do(req)
    if err != nil {
        panic(err)
    }
    defer resp.Body.Close()
    fmt.Println("Status:", resp.Status)
}

The code above is courtesy of chat gpt.