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Command line clients

The apache-sshd.zip distribution provides Windows/Linux scripts that use the MINA SSHD code base to implement the common ssh, scp, sftp commands. The clients accept most useful switches from the original commands they mimic, where the -o Option=Value arguments can be used to configure the client/server in addition to the system properties mechanism. For more details, consult the main methods code in the respective SshClientMain, SftpCommandMain and ScpClientMain classes. The code also includes SshKeyScanMain that is a simple implementation for ssh-keyscan(1).

The distribution also includes also an sshd script that can be used to launch a server instance - see SshServerMain#main for activation command line arguments and options.

In order to use this CLI code as part of another project, one needs to include the sshd-cli module:

    <dependency>
        <groupId>org.apache.sshd</groupId>
        <artifactId>sshd-cli</artifactId>
        <version>...same version as the core...</version>
    </dependency>

In general, the CLI clients accept most of their Linux counterpart arguments. Furthermore, one can use the -o Option=Value argument in order to provide internal SSHD code configurations (in addition to the ones specified as system properties via -Dprop=value JVM option. Note: not all options listed in ssh_config or sshd_config are supported, some of the -o Option=Value options have extra or special meaning, or are new altogether. Here are a few worth mentioning:

Ciphers

Comma-separated list of allowed/supported ciphers in their order of preference.

MACs

Comma-separated list of allowed/supported message authentication code algorithms in their order of preference.

KexAlgorithms

Comma-separated list of allowed/supported key exchange algorithms in their order of preference.

HostKeyAlgorithms

Comma-separated list of allowed/supported signature algorithms in their order of preference.

Compression

Whether to use compression, and if so which.

LogLevel

The verbosity level that is used when logging messages - Note: this is not the same as the internal logging configuration but rather an extra verbosity level of the CLI code itself - instructing it what extra data to display in STDOUT/STDERR. Each specific CLI (scp, sftp, ssh, sshd) has its own interpretation of this value.

PreferredAuthentications

The preferred user authentications factory names and their order:

# Allow only public key authentication
-o PreferredAuthentications=publickey

# Prefer keyboard-interactive BEFORE publickey
-o PreferredAuthentications=keyboard-interactive,publickey

ShellFactory

One can use it specify a non-default shell factory - including disabling it altogether - or add the SCP shell to an existing one:

# Disable shell entirely
-o ShellFactory=none

# Add the SCP shell to the default factory
-o ShellFactory=+scp

# Use ONLY the SCP shell
-o ShellFactory=scp

# Use a custom factory
-o ShellFactory=com.demo.MyShellFactory

# Add the SCP shell to a custom factory
-o ShellFactory=scp+com.demo.MyShellFactory

Subsystem

Can be used to specify built-in or custom subsystems to use in the server - or disable them altogether:

# Disable all subsystems
-o Subsystem=none

# Use the built-in SFTP subsystem
-o Subsystem=sftp

# Use one or more custom subsystems
-o Subsystem=Sub1,Sub2,Sub3

Note: Subsystems are automatically detected via ServiceLoader#load(SubsystemFactory.class) call - the option value simply states which ones to use - according to their logical name.

Server/Client heartbeat

Controlled by a combination of the ServerAliveInterval, ClientAliveInterval, ClientAliveUseNullPackets and ClientAliveReplyWait properties.

Host keys and certificate

HostKey and HostCertificate properties - enable specifying multiple paths to key files/certificates.

Banner / VisualHostKey

Controls the server's banner display.

AllowTcpForwarding / AllowAgentForwarding / X11Forwarding

Control server forwarding capabilities.

SftpCommandMain

A CLI client reminiscent of sftp(1). By default uses an internal SftpClientFactory. This can be overridden as follows:

  1. Provide a -o SftpClientFactory=XXX command line argument where the option specifies the fully-qualified name of the class that implements this interface.

  2. Add a META-INF\services\org.apache.sshd.sftp.client.SftpClientFactory file containing the fully-qualified name of the class that implements this interface. Note: if more than one such instance is detected an exception is thrown.

Note: The specified class(es) must be public and contain a public no-args constructor.

The CLI client provides a few extra "commands" that can be used to view metadata information about the current session

  • session - Show current SSH session details - including ID, client/server identification line, peer, etc..
  • kex - Show KEX details - client proposal, server one and negotiated parameters.
  • info - General details about the SFTP protocol - e.g., supported extensions by the server.
  • version - The negotiated SFTP protocol version.
  • help - List all available commands.
  • exit - Quit the SFTP session

SshClientMain

A CLI client compatible with the ssh(1) command line options, with a few extra options:

  • -io - select a specific IoServiceFactoryFactory:
    java -cp ... org.apache.sshd.cli.client.SshClientMain -io <value>

Where value can be:

* One of the default builtin values (NIO2, MINA, NETTY)

* A fully qualified class name implementing this interface

If no specific value provided NIO2 is used.
  • -w <password> - provide a password as part of the command instead of waiting to be prompted.
    java -cp ... org.apache.sshd.cli.client.SshClientMain -l <login> -w <password> ...host...
  • SetEnv/SendEnv - can be used to send specific environment variables to the server when executing a command or opening a shell. Example:
    java -cp ... org.apache.sshd.cli.client.SshClientMain -o SetEnv=X=7,Y=8

    # Can also be used as separate options

    java -cp ... org.apache.sshd.cli.client.SshClientMain -o SetEnv=X=7 -o SetEnv=Y=8
  • RequestTTY - can be no, yes or auto (default). If auto the CLI client will attempt to initialize the PTY options according to the O/S. In addition to the auto-detected PTY modes, one can override them by using the PtyMode option:
    java -cp ... org.apache.sshd.cli.client.SshClientMain -o PtyMode=VINTR,TTY_OP_ISPEED=4200

    # Can also be used as separate options

    java -cp ... org.apache.sshd.cli.client.SshClientMain -o PtyMode=VINTR -o PtyMode=TTY_OP_ISPEED=4200

Any option that does not have a specific value specified for it is assumed to use 1 - therefore, in order to disable an option one must use -o PtyMode=WHATEVER=0.

ScpCommandMain

Reminiscent of the scp(1) CLI client - including support for "3-way" copy (a.k.a. remote-to-remote) option:

scp -p -r -3 user1@server1:source user2@server2:destination

In this context, it is worth mentioning that the CLI also supports URI locations having the format scp://[user@]host[:port][/path]

# If port is omitted then 22 is assumed
scp -p scp://user1@server1:2222/source/file /home/user2/destination

# Note: same effect can be achieved with -P option

scp -p -P 2222 user1@server1:source/file /home/user2/destination

# the URI is better suited for remote-to-remote transfers
scp -p -r -3 scp://user1@server1:2222/source scp://user2@server2:3333/destination

SshServerMain

Command line SSH daemon

  • Port - by default the SSH server sets up to list on port 8000 in order to avoid conflicts with any running SSH O/S daemon. This can be modified by providing a -p NNNN or -o Port=NNNN command line option.

  • Subsystem(s) - the server automatically detects subsystems using the Java ServiceLoader mechanism.

This can be overwritten as follows (in this order):

  1. Provide a org.apache.sshd.server.subsystem.SubsystemFactory system property containing comma-separated fully-qualified names of classes implementing this interface. The implementations must be public and have a public no-args constructor for instantiating them. The order of the provided subsystems will be according to their order in the specified list.

  2. Provide a -o Subsystem=xxx,yyy command line argument where value is a comma-separated list of the name(s) of the auto-detected factories via the ServiceLoader mechanism. The special value none may be used to indicate that no subsystem is to be configured. Note: no specific order is provided when subsystems are auto-detected and/or filtered.

  • Shell - unless otherwise instructed, the default SSH server uses an internal shell (see InteractiveProcessShellFactory). The shell can be overridden or disabled by specifying a -o ShellFactory=XXX option where the value can either be none to specify that no shell is to be used, or the fully-qualified name of a class that implements the ShellFactory interface. The implementation must be public and have a public no-args constructor for instantiating it.

Note: A special value of scp can be used to use the built-in ScpShell instead of the interactive one (reminder: the SCP "shell" is a limited shell that provides a good enough functionality for WinScp).