// +build linux package dns // See: // * http://stackoverflow.com/questions/3062205/setting-the-source-ip-for-a-udp-socket and // * http://blog.powerdns.com/2012/10/08/on-binding-datagram-udp-sockets-to-the-any-addresses/ // // Why do we need this: When listening on 0.0.0.0 with UDP so kernel decides what is the outgoing // interface, this might not always be the correct one. This code will make sure the egress // packet's interface matched the ingress' one. import ( "net" "syscall" ) // setUDPSocketOptions4 prepares the v4 socket for sessions. func setUDPSocketOptions4(conn *net.UDPConn) error { file, err := conn.File() if err != nil { return err } if err := syscall.SetsockoptInt(int(file.Fd()), syscall.IPPROTO_IP, syscall.IP_PKTINFO, 1); err != nil { file.Close() return err } // Calling File() above results in the connection becoming blocking, we must fix that. // See https://github.com/miekg/dns/issues/279 err = syscall.SetNonblock(int(file.Fd()), true) if err != nil { file.Close() return err } file.Close() return nil } // setUDPSocketOptions6 prepares the v6 socket for sessions. func setUDPSocketOptions6(conn *net.UDPConn) error { file, err := conn.File() if err != nil { return err } if err := syscall.SetsockoptInt(int(file.Fd()), syscall.IPPROTO_IPV6, syscall.IPV6_RECVPKTINFO, 1); err != nil { file.Close() return err } err = syscall.SetNonblock(int(file.Fd()), true) if err != nil { file.Close() return err } file.Close() return nil } // getUDPSocketOption6Only return true if the socket is v6 only and false when it is v4/v6 combined // (dualstack). func getUDPSocketOptions6Only(conn *net.UDPConn) (bool, error) { file, err := conn.File() if err != nil { return false, err } // dual stack. See http://stackoverflow.com/questions/1618240/how-to-support-both-ipv4-and-ipv6-connections v6only, err := syscall.GetsockoptInt(int(file.Fd()), syscall.IPPROTO_IPV6, syscall.IPV6_V6ONLY) if err != nil { file.Close() return false, err } file.Close() return v6only == 1, nil } func getUDPSocketName(conn *net.UDPConn) (syscall.Sockaddr, error) { file, err := conn.File() if err != nil { return nil, err } defer file.Close() return syscall.Getsockname(int(file.Fd())) }