tftp(3)

NAME
tftp - Trivial FTP.

DESCRIPTION

Interface module for the tftp application.

  Overwiew

This is a complete implementation of the following IETF standards:

    • RFC 1350, The TFTP Protocol (revision 2).

    • RFC 2347, TFTP Option Extension.

    • RFC 2348, TFTP Blocksize Option.

    • RFC 2349, TFTP Timeout Interval and Transfer Size Options.

The only feature that not is implemented in this release is the "netascii" transfer mode.

The start function starts a daemon process which, listens for UDP packets on a port. When it receives a request for read or write it spawns a temporary server process which handles the actual transfer of the file. On the client side the read_file/3 and write_file/3 functions spawns a temporary client process which establishes contact with a TFTP daemon and performs the actual transfer of the file.

Most of the options are common for both the client and the server side, but some of them differs a little.

https://datatracker.ietf.org/doc/html/rfc1350 https://datatracker.ietf.org/doc/html/rfc2347 https://datatracker.ietf.org/doc/html/rfc2348 https://datatracker.ietf.org/doc/html/rfc2349

  Callbacks

A tftp callback module is to be implemented as a tftp behavior and export the functions listed in the following.

On the server side, the callback interaction starts with a call to open/5 with the registered initial callback state. open/5 is expected to open the (virtual) file. Then either function Module:read/1 or Module:write/2 is invoked repeatedly, once per transferred block. At each function call, the state returned from the previous call is obtained. When the last block is encountered, function Module:read/1 or Module:write/2 is expected to close the (virtual) file and return its last state. Function Module:abort/3 is only used in error situations. Function prepare/5 is not used on the server side.

On the client side, the callback interaction is the same, but it starts and ends a bit differently. It starts with a call to prepare/5 with the same arguments as open/5 takes. prepare/5 is expected to validate the TFTP options suggested by the user and to return the subset of them that it accepts. Then the options are sent to the server, which performs the same TFTP option negotiation procedure. The options that are accepted by the server are forwarded to function open/5 on the client side. On the client side, function open/5 must accept all option as-is or reject the transfer. Then the callback interaction follows the same pattern as described for the server side. When the last block is encountered in Module:read/1 or Module:write/2, the returned state is forwarded to the user and returned from read_file/3 or write_file/3.

If a callback (performing the file access in the TFTP server) takes too long time (more than the double TFTP time-out), the server aborts the connection and sends an error reply to the client. The server simply assumes that the client has given up.

If the TFTP server receives yet another request from the same client (same host and port) while it already has an active connection to the client, it ignores the new request if the request is equal to the first one (same filename and options). This implies that the (new) client will be served by the already ongoing connection on the server side. By not setting up yet another connection, in parallel with the ongoing one, the server consumes less resources.

DATA TYPES

access() :: read | write.

    Access mode.
connection_option() :: {atom(), term()} | option().

    All options most of them common to the client and server.

        • {debug, Level::none | error | warning | brief | normal | verbose | all}

        Controls the level of debug printouts. Default is none.

        • {host, Host::inet:hostname()} -

        The name or IP address of the host where the TFTP daemon resides. This option is only used by the client.

        • {port, Port::inet:port_number()}

        The TFTP port where the daemon listens. Defaults is the standardized number 69. On the server side, it can sometimes make sense to set it to 0, meaning that the daemon just picks a free port (which one is returned by function info/1).

        If a socket is connected already, option {udp, [{fd, integer()}]} can be used to pass the open file descriptor to gen_udp. This can be automated by using a command-line argument stating the prebound file descriptor number. For example, if the port is 69 and file descriptor 22 is opened by setuid_socket_wrap, the command-line argument "-tftpd_69 22" triggers the prebound file descriptor 22 to be used instead of opening port 69. The UDP option {udp, [{fd, 22}]} is automatically added. See init:get_argument/ about command-line arguments and gen_udp:open/2 about UDP options.

        • {port_policy, random | inet:port_number() | {range, Min::inet:port_number(), Max::inet:port_nuber()}

        Policy for the selection of the temporary port that is used by the server/client during the file transfer. Default is random, which is the standardized policy. With this policy a randomized free port is used. A single port or a range of ports can be useful if the protocol passes through a firewall.

        • {udp, Options::gen_udp:option}

        • {use_tsize, boolean()}

        Flag for automated use of option tsize. With this set to true, the write_file/3 client determines the filesize and sends it to the server as the standardized tsize option. A read_file/3 client acquires only a filesize from the server by sending a zero tsize.

        • {max_tsize, MaxTsize::pos_integer() | infinity}

        Threshold for the maximal filesize in bytes. The transfer is aborted if the limit is exceeded. Default is infinity.

        • {max_conn, MaxConn::pos_integer() | infinity}

        Threshold for the maximal number of active connections. The daemon rejects the setup of new connections if the limit is exceeded. Default is infinity.

        • TftpOption::option()

        Name and value of a TFTP option.

        • {reject, Feature:: access() | TftpKey::string()}

        Controls which features to reject. This is mostly useful for the server as it can restrict the use of certain TFTP options or read/write access.

        • {callback, {RegExp ::string(), Module::module(), InitialState :: term()}}

        Registration of a callback module. When a file is to be transferred, its local filename is matched to the regular expressions of the registered callbacks. The first matching callback is used during the transfer. See read_file/3 and write_file/3.

        The callback module must implement the tftp behavior, see callbacks.

        At the end of the list of callbacks there are always the default callbacks tftp_file and tftp_binary with the RegExp = "" and InitialState = [].

        The InitialState should be an option list, and the empty list should be accepted by any callback module. The tftp_file callback module accepts an InitialState = [{root_dir, Dir}] that restrict local file operations to files in Dir and subdirectories. All file names received in protocol requests, relative or absolute, are regarded as relative to this directory.

        WARNING: The default callback module configuration allows access to any file on any local filesystem that is readable or writable by the user running the Erlang VM. This can be a security vulnerability. It is therefore recommended to explicitly configure the tftp_file callback module to use the root_dir option.

        • {logger, module()}

        Callback module for customized logging of errors, warnings, and info messages. The callback module must implement the tftp_logger behavior. The default module is tftp_logger.

        • {max_retries, MaxRetries::non_neg_integer()}

        Threshold for the maximal number of retries. By default the server/client tries to resend a message up to five times when the time-out expires.
error_code() :: undef | enoent | eacces | enospc | badop | eexist | baduser | badopt | pos_integer().

    Error reason codes.
option() :: {string(), Value :: string()}.

    Specific TFTP protocol options
peer() :: {PeerType :: inet | inet6, PeerHost :: inet:ip_address(), PeerPort :: port()}.

    Information about the peer provided for callback.

FUNCTIONS

change_config(Pid, Options)

    Changes configuration a TFTP Server
info(Pid)

    Returns information about all TFTP server.
read_file(RemoteFilename, LocalFilename, Options) -> {ok, LastCallbackState} | {error, Reason} when RemoteFilename :: file:filename(), LocalFilename :: file:filename_all() | binary, Options :: [connection_option()], LastCallbackState :: term(), Reason :: term().

    Reads a (virtual) file RemoteFilename from a TFTP server.

    If LocalFilename is the atom binary, tftp_binary is used as callback module. It concatenates all transferred blocks and returns them as one single binary in LastCallbackState.

    If LocalFilename is a string and there are no registered callback modules, tftp_file is used as callback module. It writes each transferred block to the file named LocalFilename and returns the number of transferred bytes in LastCallbackState.

    If LocalFilename is a string and there are registered callback modules, LocalFilename is tested against the regexps of these and the callback module corresponding to the first match is used, or an error tuple is returned if no matching regexp is found.
start(Options) -> {ok, Pid} | {error, Reason} when Options :: [connection_option()], Pid :: pid(), Reason :: term().

    Starts a daemon process listening for UDP packets on a port.

    When it receives a request for read or write, it spawns a temporary server process handling the actual transfer of the (virtual) file.

    The request filename is matched against the regexps of the registered callback modules, and the first match selects the callback to handle the request.

    If there are no registered callback modules, tftp_file is used, with the initial state [].

    WARNING: The default callback module configuration allows access to any file on any local filesystem that is readable or writable by the user running the Erlang VM. This can be a security vulnerability. See the {callback,_} option at the start of this module reference for a remedy.
write_file(RemoteFilename, LocalFilename, Options) -> {ok, LastCallbackState} | {error, Reason} when RemoteFilename :: file:filename(), LocalFilename :: file:filename_all() | binary, Options :: [option()], LastCallbackState :: term(), Reason :: term().

    Writes a (virtual) file RemoteFilename to a TFTP server.

    If LocalFilename is a binary, tftp_binary is used as callback module. The binary is transferred block by block and the number of transferred bytes is returned in LastCallbackState.

    If LocalFilename is a string and there are no registered callback modules, tftp_file is used as callback module. It reads the file named LocalFilename block by block and returns the number of transferred bytes in LastCallbackState.

    If LocalFilename is a string and there are registered callback modules, LocalFilename is tested against the regexps of these and the callback module corresponding to the first match is used, or an error tuple is returned if no matching regexp is found.

CALLBACKS

abort/3

    Invoked when the file transfer is aborted.

    The callback function is expected to clean up its used resources after the aborted file transfer, such as closing open file descriptors and so on. The function is not invoked if any of the other callback functions returns an error, as it is expected that they already have cleaned up the necessary resources. However, it is invoked if the functions fail (crash).
open(Peer, Access, Filename, Mode, SuggestedOptions, State)

    Opens a file for read or write access.

    On the client side, where the open/5 call has been preceded by a call to prepare/5, all options must be accepted or rejected.

    On the server side, where there is no preceding prepare/5 call, no new options can be added, but those present in SuggestedOptions can be omitted or replaced with new values in AcceptedOptions.
prepare(Peer, Access, Filename, Mode, SuggestedOptions, InitialState)

    Prepares to open a file on the client side.

    No new options can be added, but those present in SuggestedOptions can be omitted or replaced with new values in AcceptedOptions.

    This is followed by a call to open/4 before any read/write access is performed. AcceptedOptions is sent to the server, which replies with the options that it accepts. These are then forwarded to open/4 as SuggestedOptions.
read(State)

    Reads a chunk from the file.

    The callback function is expected to close the file when the last file chunk is encountered. When an error is encountered, the callback function is expected to clean up after the aborted file transfer, such as closing open file descriptors, and so on. In both cases there will be no more calls to any of the callback functions.
write/2

    Writes a chunk to the file.

    The callback function is expected to close the file when the last file chunk is encountered. When an error is encountered, the callback function is expected to clean up after the aborted file transfer, such as closing open file descriptors, and so on. In both cases there will be no more calls to any of the callback functions.