Implement hostname extraction for HTTP and TLS

internal/host/host.go

@@ -0,0 +1,84 @@
+// Package host provides helper functions to extract hostname information from connections.
+package host
+
+import (
+	"bufio"
+	"context"
+	"crypto/tls"
+	"io"
+	"net"
+	"net/http"
+	"net/url"
+	"time"
+)
+
+type connWrapper struct {
+	r       io.Reader
+	connErr error
+}
+
+func (c *connWrapper) Read(b []byte) (n int, err error) {
+	n, err = c.r.Read(b)
+	if err != nil {
+		c.connErr = err
+	}
+	return n, err
+}
+
+func (c *connWrapper) Write(b []byte) (n int, err error) { return len(b), nil }
+
+func (c *connWrapper) LocalAddr() net.Addr { return &net.TCPAddr{} }
+
+func (c *connWrapper) RemoteAddr() net.Addr { return &net.TCPAddr{} }
+
+func (c *connWrapper) SetDeadline(t time.Time) error { return nil }
+
+func (c *connWrapper) SetReadDeadline(t time.Time) error { return nil }
+
+func (c *connWrapper) SetWriteDeadline(t time.Time) error { return nil }
+
+func (c *connWrapper) Close() error { return nil }
+
+// ExtractHTTPHost attempts to extract hostname information from an incoming HTTP connection.
+// If the extraction failed because the connection is not HTTP, or the HTTP
+// request is malformed, or the HTTP request doesn't contain a host header, it
+// will return "", nil.
+// This function only returns an error when the error arises from the
+// underlying IO (includes io.EOF).
+func ExtractHTTPHost(r io.Reader) (string, error) {
+	c := connWrapper{r: r}
+	req, err := http.ReadRequest(bufio.NewReader(&c))
+	if c.connErr != nil {
+		return "", c.connErr
+	}
+	if err != nil {
+		return "", nil
+	}
+	return (&url.URL{Host: req.Host}).Hostname(), nil
+}
+
+// ExtractSNI attempts to extract the SNI from the TLS ClientHello message of an
+// incoming TLS connection.
+// If the extraction failed because the connection is not TLS, or the TLS
+// handshake is malformed, it will return "", nil.
+// This function only returns an error when the error arises from the
+// underlying IO (includes io.EOF).
+func ExtractSNI(r io.Reader) (string, error) {
+	var sni string
+	c := connWrapper{r: r}
+	tlsConn := tls.Server(&c, &tls.Config{
+		GetConfigForClient: func(hello *tls.ClientHelloInfo) (*tls.Config, error) {
+			sni = hello.ServerName
+			return nil, context.Canceled
+		},
+	})
+	_ = tlsConn.Handshake()
+	_ = tlsConn.Close()
+	if c.connErr != nil {
+		return "", c.connErr
+	}
+	if sni != "" {
+		return sni, nil
+	}
+	return "", c.connErr
+}