Add configuration for cookie 'SameSite' value.

Values of 'lax' and 'strict' can improve and mitigate
some categories of cross-site traffic tampering.

Given that the nature of this proxy is often to proxy
private tools, this is useful to take advantage of.

See: https://www.owasp.org/index.php/SameSite

pkg/cookies/cookies.go

@@ -1,6 +1,7 @@
 package cookies
 
 import (
+	"fmt"
 	"net"
 	"net/http"
 	"strings"
@@ -12,7 +13,7 @@ import (
 
 // MakeCookie constructs a cookie from the given parameters,
 // discovering the domain from the request if not specified.
-func MakeCookie(req *http.Request, name string, value string, path string, domain string, httpOnly bool, secure bool, expiration time.Duration, now time.Time) *http.Cookie {
+func MakeCookie(req *http.Request, name string, value string, path string, domain string, httpOnly bool, secure bool, expiration time.Duration, now time.Time, sameSite http.SameSite) *http.Cookie {
 	if domain != "" {
 		host := req.Host
 		if h, _, err := net.SplitHostPort(host); err == nil {
@@ -31,11 +32,28 @@ func MakeCookie(req *http.Request, name string, value string, path string, domai
 		HttpOnly: httpOnly,
 		Secure:   secure,
 		Expires:  now.Add(expiration),
+		SameSite: sameSite,
 	}
 }
 
 // MakeCookieFromOptions constructs a cookie based on the given *options.CookieOptions,
 // value and creation time
 func MakeCookieFromOptions(req *http.Request, name string, value string, opts *options.CookieOptions, expiration time.Duration, now time.Time) *http.Cookie {
-	return MakeCookie(req, name, value, opts.CookiePath, opts.CookieDomain, opts.CookieHTTPOnly, opts.CookieSecure, expiration, now)
+	return MakeCookie(req, name, value, opts.CookiePath, opts.CookieDomain, opts.CookieHTTPOnly, opts.CookieSecure, expiration, now, ParseSameSite(opts.CookieSameSite))
+}
+
+// Parse a valid http.SameSite value from a user supplied string for use of making cookies.
+func ParseSameSite(v string) http.SameSite {
+	switch v {
+	case "lax":
+		return http.SameSiteLaxMode
+	case "strict":
+		return http.SameSiteStrictMode
+	case "none":
+		return http.SameSiteNoneMode
+	case "":
+		return http.SameSiteDefaultMode
+	default:
+		panic(fmt.Sprintf("Invalid value for SameSite: %s", v))
+	}
 }