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tcp.go
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// Copyright (C) 2022 Andrew Ayer
//
// Permission is hereby granted, free of charge, to any person obtaining a
// copy of this software and associated documentation files (the "Software"),
// to deal in the Software without restriction, including without limitation
// the rights to use, copy, modify, merge, publish, distribute, sublicense,
// and/or sell copies of the Software, and to permit persons to whom the
// Software is furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included
// in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
// THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
// OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
// ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
// OTHER DEALINGS IN THE SOFTWARE.
//
// Except as contained in this notice, the name(s) of the above copyright
// holders shall not be used in advertising or otherwise to promote the
// sale, use or other dealings in this Software without prior written
// authorization.
package main
import (
"fmt"
"net"
"strconv"
"syscall"
"time"
)
type TCPDialer struct {
Port int
Allowed []*net.IPNet
IPv6SourcePrefix net.IP
}
func (backend *TCPDialer) checkBackend(address string) error {
host, _, err := net.SplitHostPort(address)
if err != nil {
return err
}
ipaddress := net.ParseIP(host)
if ipaddress == nil {
return fmt.Errorf("%s is not a valid IP address", host)
}
for _, cidr := range backend.Allowed {
if cidr.Contains(ipaddress) {
return nil
}
}
return fmt.Errorf("%s is not an allowed backend", ipaddress)
}
func (backend *TCPDialer) bindIPv6(sock syscall.RawConn, clientConn ClientConn) error {
clientTCPAddress, isTCP := clientConn.RemoteAddr().(*net.TCPAddr)
if !isTCP {
return fmt.Errorf("client is not connected using TCP")
}
clientIPv4 := clientTCPAddress.IP.To4()
if clientIPv4 == nil {
return fmt.Errorf("client is not connected using IPv4")
}
sourceIPv6 := make(net.IP, 16)
copy(sourceIPv6[:12], backend.IPv6SourcePrefix)
copy(sourceIPv6[12:], clientIPv4)
var controlErr error
if err := sock.Control(func(fd uintptr) {
controlErr = syscall.SetsockoptInt(int(fd), syscall.SOL_IP, syscall.IP_FREEBIND, 1)
if controlErr != nil {
return
}
controlErr = syscall.Bind(int(fd), &syscall.SockaddrInet6{Addr: *(*[16]byte)(sourceIPv6)})
}); err != nil {
return err
}
return controlErr
}
func (backend *TCPDialer) port(clientConn ClientConn) (int, error) {
if backend.Port != 0 {
return backend.Port, nil
}
localTCPAddress, isTCP := clientConn.LocalAddr().(*net.TCPAddr)
if !isTCP {
return 0, fmt.Errorf("cannot determine backend port number because client is not connected using TCP")
}
return localTCPAddress.Port, nil
}
func (backend *TCPDialer) network() string {
if backend.IPv6SourcePrefix != nil {
return "tcp6"
} else {
return "tcp"
}
}
func (backend *TCPDialer) Dial(hostname string, protocols []string, clientConn ClientConn) (BackendConn, error) {
dialer := net.Dialer{
Timeout: 5 * time.Second,
Control: func(network string, address string, c syscall.RawConn) error {
if err := backend.checkBackend(address); err != nil {
return err
}
if backend.IPv6SourcePrefix != nil {
if err := backend.bindIPv6(c, clientConn); err != nil {
return err
}
}
return nil
},
}
if service := getSRVService(protocols); service != "" {
conn, err := dialSRV(dialer, backend.network(), hostname, service)
if err != nil {
return nil, err
}
return conn.(*net.TCPConn), nil
}
port, err := backend.port(clientConn)
if err != nil {
return nil, err
}
conn, err := dialer.Dial(backend.network(), net.JoinHostPort(hostname, strconv.Itoa(port)))
if err != nil {
return nil, err
}
return conn.(*net.TCPConn), nil
}