Keith Winstein
159 lines
6.3 KiB
Markdown
159 lines
6.3 KiB
Markdown
[](https://travis-ci.org/mobile-shell/mosh)
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Mosh: the mobile shell
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======================
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Mosh is a remote terminal application that supports intermittent
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connectivity, allows roaming, and provides speculative local echo
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and line editing of user keystrokes.
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It aims to support the typical interactive uses of SSH, plus:
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* Mosh keeps the session alive if the client goes to sleep and
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wakes up later, or temporarily loses its Internet connection.
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* Mosh allows the client and server to "roam" and change IP
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addresses, while keeping the connection alive. Unlike SSH, Mosh
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can be used while switching between Wi-Fi networks or from Wi-Fi
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to cellular data to wired Ethernet.
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* The Mosh client runs a predictive model of the server's behavior
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in the background and tries to guess intelligently how each
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keystroke will affect the screen state. When it is confident in
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its predictions, it will show them to the user while waiting for
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confirmation from the server. Most typing and uses of the left-
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and right-arrow keys can be echoed immediately.
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As a result, Mosh is usable on high-latency links, e.g. on a
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cellular data connection or spotty Wi-Fi. In distinction from
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previous attempts at local echo modes in other protocols, Mosh
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works properly with full-screen applications such as emacs, vi,
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alpine, and irssi, and automatically recovers from occasional
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prediction errors within an RTT. On high-latency links, Mosh
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underlines its predictions while they are outstanding and removes
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the underline when they are confirmed by the server.
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Mosh does not support X forwarding or the non-interactive uses of SSH,
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including port forwarding.
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Other features
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--------------
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* Mosh adjusts its frame rate so as not to fill up network queues
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on slow links, so "Control-C" always works within an RTT to halt
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a runaway process.
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* Mosh warns the user when it has not heard from the server
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in a while.
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* Mosh supports lossy links that lose a significant fraction
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of their packets.
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* Mosh handles some Unicode edge cases better than SSH and existing
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terminal emulators by themselves, but requires a UTF-8
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environment to run.
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* Mosh leverages SSH to set up the connection and authenticate
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users. Mosh does not contain any privileged (root) code.
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Getting Mosh
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------------
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[The Mosh web site](http://mosh.mit.edu/#getting) has information about
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packages for many operating systems, as well as instructions for building
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from source.
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Note that `mosh-client` receives an AES session key as an environment
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variable. If you are porting Mosh to a new operating system, please make
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sure that a running process's environment variables are not readable by other
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users. We have confirmed that this is the case on GNU/Linux, OS X, and
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FreeBSD.
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Usage
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-----
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The `mosh-client` binary must exist on the user's machine, and the
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`mosh-server` binary on the remote host.
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The user runs:
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$ mosh [user@]host
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If the `mosh-client` or `mosh-server` binaries live outside the user's
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`$PATH`, `mosh` accepts the arguments `--client=PATH` and `--server=PATH` to
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select alternate locations. More options are documented in the mosh(1) manual
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page.
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There are [more examples](http://mosh.mit.edu/#usage) and a
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[FAQ](http://mosh.mit.edu/#faq) on the Mosh web site.
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How it works
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------------
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The `mosh` program will SSH to `user@host` to establish the connection.
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SSH may prompt the user for a password or use public-key
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authentication to log in.
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From this point, `mosh` runs the `mosh-server` process (as the user)
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on the server machine. The server process listens on a high UDP port
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and sends its port number and an AES-128 secret key back to the
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client over SSH. The SSH connection is then shut down and the
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terminal session begins over UDP.
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If the client changes IP addresses, the server will begin sending
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to the client on the new IP address within a few seconds.
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To function, Mosh requires UDP datagrams to be passed between client
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and server. By default, `mosh` uses a port number between 60000 and
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61000, but the user can select a particular port with the -p option.
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Advice to distributors
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----------------------
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A note on compiler flags: Mosh is security-sensitive code. When making
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automated builds for a binary package, we recommend passing the option
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`--enable-compile-warnings=error` to `./configure`. On GNU/Linux with
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`g++` or `clang++`, the package should compile cleanly with
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`-Werror`. Please report a bug if it doesn't.
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Where available, Mosh builds with a variety of binary hardening flags
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such as `-fstack-protector-all`, `-D_FORTIFY_SOURCE=2`, etc. These
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provide proactive security against the possibility of a memory
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corruption bug in Mosh or one of the libraries it uses. For a full
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list of flags, search for `HARDEN` in `configure.ac`. The `configure`
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script detects which flags are supported by your compiler, and enables
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them automatically. To disable this detection, pass
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`--disable-hardening` to `./configure`. Please report a bug if you
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have trouble with the default settings; we would like as many users as
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possible to be running a configuration as secure as possible.
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Mosh ships with a default optimization setting of `-O2`. Some
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distributors have asked about changing this to `-Os` (which causes a
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compiler to prefer space optimizations to time optimizations). We have
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benchmarked with the included `src/examples/benchmark` program to test
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this. The results are that `-O2` is 40% faster than `-Os` with g++ 4.6
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on GNU/Linux, and 16% faster than `-Os` with clang++ 3.1 on Mac OS
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X. In both cases, `-Os` did produce a smaller binary (by up to 40%,
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saving almost 200 kilobytes on disk). While Mosh is not especially CPU
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intensive and mostly sits idle when the user is not typing, we think
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the results suggest that `-O2` (the default) is preferable.
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More info
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---------
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* Mosh Web site:
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<http://mosh.mit.edu>
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* `mosh-devel@mit.edu` mailing list:
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<http://mailman.mit.edu/mailman/listinfo/mosh-devel>
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* `mosh-users@mit.edu` mailing list:
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<http://mailman.mit.edu/mailman/listinfo/mosh-users>
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* `#mosh` channel on [Freenode IRC](http://freenode.net/)
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http://webchat.freenode.net/?channels=mosh
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