Java implementation of xmodem, ymodem, and kermit
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Jermit is a Java implementation of several serial file transfer protocols. General summary:

  • Xmodem and Ymodem are fully supported. Flavors include vanilla, relaxed, CRC-16, 1K, 1K/G, and G.

  • Kermit is supported in both vanilla and streaming modes. (Full duplex sliding windows are estimated to be in around 3Q 2018.)

  • Zmodem is in, but still very rough. Use it only on clean lines (TCP, error-corrected serial) for now.

  • Uploads and downloads occur between and a local file.

  • Two interfaces are provided so far:

    1. Command-line utilities that can be drop-in replacements for rx/sx/rz/rb/sb/sz.

    2. A new 'jermit' command that transfers across with a Swing-based transfer file dialog window that resembles Qodem.


Serial file transfer protocols -- primarily Xmodem, Ymodem, Zmodem, and Kermit -- are still useful today:

  • Communicating with existing serial and dialup systems such as bulletin board systems (BBS), embedded devices, and PLCs.

  • Uploading new firmware to switches, routers, and other embedded devices.

  • Transferring files over an existing interactive ssh session.

  • Transferring files across noisy and unreliable links, for example 3-wire RS-232 to an embedded system.

  • Transferring files over UDP to avoid TCP connection overhead.

Programmers seeking to use the serial file transfer protocols in their applications have few good choices that are Open Source / Free Software, especially if they are writing in languages other than C. (And for those that do write in C, most of the existing implementations (rzsz, lrzsz, ckermit, gkermit) were written in the era of "C as portable assembly language" rather than "C as object-oriented modules like the Linux kernel".)

Jermit is intended to provided a good baseline implementation of these protocols, in a modern environment with easy-to-read source, and liberally licensed for any use. The protocols are designed to be flexible building blocks: the remote side can be any InputStream + OutputStream, and the local "file" can be file, byte buffer, or a custom interface that provides a few file primitives. These can thus be easily incorporated into other APIs. For example one could use a KermitURLConnection to download from a C-Kermit Internet Kermit Server using an address like "kermit://" . It is hoped that this code is very obvious in what it does, such that it would be very straightforward to transliterate these protocols into other languages (C++, C#, Go, Rust, Pascal, etc.).


This program is licensed under the MIT License. See the file LICENSE for the full license text.


This library is still in development. The eventual intent is to invoke Jermit code in one of these general ways:

  • Create a Receiver or Sender class, and call its run() method on a new Thread. See the jermit.tests.{protocol} classes for examples.

  • Use one of the command-line interfaces. A GUI transfer window is already available, as are drop-in replacements for the (l)rzsz utilities. A replacement for ckermit with readline-like support is also in plan.

  • It is planned to provide a heirarchy of SerialURLConnection's (XmodemURLConnection, KermitURLConnection, etc.) that can be treated like a HttpURLConnetion. The connections will support both UDP and TCP transports.

System Properties

The following properties control features of Jermit:


If true, use streaming mode (do not send ACKs during file data packets transfer). Default: true. Note that if streaming is enabled, full duplex sliding windows (not yet implemented) is automatically disabled.


If true, when sending files convert filenames to Kermit's "common form": only numbers and letters, only one '.' in the filename, '.' cannot be the first or last character, and all uppercase. Default: false.


If true, support the RESEND option to resume file transfers. Default: true.


If true, support long packets (up to 9k). Default: true.

If true, treat all downloaded files as though they are binary (do not convert line endings). Default: true.


If true, treat all uploaded files as though they are binary (do not advertise as ASCII in the Attributes packet). Default: true.


If true, escape control characters. This will make file transfers slower but may be necessary for Zmodem to work at all over the link. Default: false.

If true, issue a ZCHALLENGE at the beginning of a download session. ZCHALLENGE was meant to improve security but some Zmodem senders do not support it. Its security benefits are dubious. Default: false.


If true, use a 32-bit CRC. Default: true.

Known Issues / Arbitrary Decisions

Some arbitrary design decisions had to be made when either the obviously expected behavior did not happen or when a specification was ambiguous. This section describes such issues.

  • See docs/ for general discussion of supported protocols.

  • Kermit defaults to streaming enabled. This is fast and appropriate for TCP links. However, it will fail on unreliable links. Pass -Djermit.kermit.streaming=false to the JVM for those cases.


Many tasks remain before calling this version 1.0. See docs/ for the complete list of tasks.

See Also

These protocols are very loosely based on those developed for the Qodem terminal emulator, taking advantage of the testing performed for that project. Qodem's other offshoot is a VT100/xterm-like Java terminal in the Jexer jexer.tterminal.ECMA48 class: combining Jermit and Jexer one could rather quickly put together a terminal emulator that could both pass vttest and transfer files.