wolfSSL's Embeddable SSH Server wolfSSH Manual
wolfSSH is dependent on wolfCrypt, found as a part of wolfSSL. The following is the simplest configuration of wolfSSL to enable wolfSSH.
$ cd wolfssl
$ ./configure [OPTIONS] --enable-ssh
$ make check
$ sudo make install
On some systems the optional ldconfig command is needed after installing.
To use the key generation function in wolfSSH, wolfSSL will need to be
configured with keygen: --enable-keygen.
When using X.509 certificates for user authentication, wolfSSL must be
built with TLS enabled. wolfSSH uses wolfSSL's certificate manager system
for X.509, including OCSP lookups. To allow OCSP, add --enable-ocsp to the
wolfSSL configure.
If the bulk of wolfSSL code isn't desired, wolfSSL can be configured with
the crypto only option: --enable-cryptonly.
Additional build options for wolfSSL are located in chapter two. of the wolfSSH manual.
From the wolfSSH source directory run:
$ ./autogen.sh
$ ./configure --with-wolfssl=[/usr/local]
$ make
$ make check
The autogen.sh script only has to be run the first time after cloning
the repository. If you have already run it or are using code from a
source archive, you should skip it.
For building under Windows with Visual Studio, see the file "ide/winvs/README.md".
NOTE: On resource constrained devices the DEFAULT_WINDOW_SZ may need
to be set to a lower size. It can also be increased in desktop use cases
to help with large file transfers. By default channels are set to receive
up to 128kB of data before sending a channel window adjust message. An
example of setting a window size for new channels would be as follows
./configure CPPFLAGS="-DDEFAULT_WINDOW_SZ=16384"
For 32bit Linux platforms you can add support for files > 2GB by compling
with CFLAGS=-D_FILE_OFFSET_BITS=64.
The directory examples contains an echoserver that any client should
be able to connect to. From the terminal run:
$ ./examples/echoserver/echoserver -f
The option -f enables echo-only mode. From another terminal run:
$ ssh jill@localhost -p 22222
When prompted for a password, enter "upthehill". The server will send a canned banner to the client:
wolfSSH Example Echo Server
Characters typed into the client will be echoed to the screen by the server. If the characters are echoed twice, the client has local echo enabled. The echoserver isn't being a proper terminal so the CR/LF translation will not work as expected.
The following control characters will trigger special actions in the echoserver:
- CTRL-C: Terminate the connection.
- CTRL-E: Print out some session statistics.
- CTRL-F: Trigger a new key exchange.
After cloning the repository, be sure to make the testing private keys
read-only for the user, otherwise ssh will tell you to do it.
$ chmod 0600 ./keys/gretel-key-rsa.pem ./keys/hansel-key-rsa.pem \
./keys/gretel-key-ecc.pem ./keys/hansel-key-ecc.pem
Authentication against the example echoserver can be done with a password or public key. To use a password the command line:
$ ssh -p 22222 USER@localhost
Where the USER and password pairs are:
jill:upthehill
jack:fetchapail
To use public key authentication use the command line:
$ ssh -i ./keys/USER-key-TYPE.pem -p 22222 USER@localhost
Where the USER can be gretel or hansel, and TYPE is rsa or
ecc.
Keep in mind, the echoserver has several fake accounts in its
wsUserAuth() callback function. (jack, jill, hansel, and gretel) When
the shell support is enabled, those fake accounts will not work. They
don't exist in the system's passwd file. The users will authenticate,
but the server will err out because they don't exist in the system. You
can add your own username to the password or public key list in the
echoserver. That account will be logged into a shell started by the
echoserver with the privileges of the user running echoserver.
wolfSSH comes packaged with a few example tools for testing purposes and to demonstrate interoperability with other SSH implementations.
The echoserver is the workhorse of wolfSSH. It originally only allowed one to authenticate one of the canned account and would repeat the characters typed into it. When enabling shell support, it can spawn a user shell. It will need an actual user name on the machine and an updated user authentication callback function to validate the credentials. The echoserver can also handle SCP and SFTP connections.
The echoserver tool accepts the following command line options:
-1 exit after a single (one) connection
-e expect ECC public key from client
-E use ECC private key
-f echo input
-p <num> port to accept on, default 22222
-N use non-blocking sockets
-d <string> set the home directory for SFTP connections
-j <file> load in a public key to accept from peer
The client establishes a connection to an SSH server. In its simplest mode, it sends the string "Hello, wolfSSH!" to the server, prints the response, and then exits. With the pseudo terminal option, the client will be a real client.
The client tool accepts the following command line options:
-h <host> host to connect to, default 127.0.0.1
-p <num> port to connect on, default 22222
-u <username> username to authenticate as (REQUIRED)
-P <password> password for username, prompted if omitted
-e use sample ecc key for user
-i <filename> filename for the user's private key
-j <filename> filename for the user's public key
-x exit after successful connection without doing
read/write
-N use non-blocking sockets
-t use psuedo terminal
-c <command> executes remote command and pipe stdin/stdout
-a Attempt to use SSH-AGENT
The portfwd tool establishes a connection to an SSH server and sets up a listener for local port forwarding or requests a listener for remote port forwarding. After a connection, the tool terminates.
The portfwd tool accepts the following command line options:
-h <host> host to connect to, default 127.0.0.1
-p <num> port to connect on, default 22222
-u <username> username to authenticate as (REQUIRED)
-P <password> password for username, prompted if omitted
-F <host> host to forward from, default 0.0.0.0
-f <num> host port to forward from (REQUIRED)
-T <host> host to forward to, default to host
-t <num> port to forward to (REQUIRED)
The scpclient, wolfscp, establishes a connection to an SSH server and copies the specified files from or to the local machine.
The scpclient tool accepts the following command line options:
-H <host> host to connect to, default 127.0.0.1
-p <num> port to connect on, default 22222
-u <username> username to authenticate as (REQUIRED)
-P <password> password for username, prompted if omitted
-L <from>:<to> copy from local to server
-S <from>:<to> copy from server to local
The sftpclient, wolfsftp, establishes a connection to an SSH server and allows directory navigation, getting and putting files, making and removing directories, etc.
The sftpclient tool accepts the following command line options:
-h <host> host to connect to, default 127.0.0.1
-p <num> port to connect on, default 22222
-u <username> username to authenticate as (REQUIRED)
-P <password> password for username, prompted if omitted
-d <path> set the default local path
-N use non blocking sockets
-e use ECC user authentication
-l <filename> local filename
-r <filename> remote filename
-g put local filename as remote filename
-G get remote filename as local filename
wolfSSH includes server-side support for scp, which includes support for both copying files 'to' the server, and copying files 'from' the server. Both single file and recursive directory copy are supported with the default send and receive callbacks.
To compile wolfSSH with scp support, use the --enable-scp build option
or define WOLFSSH_SCP:
$ ./configure --enable-scp
$ make
For full API usage and implementation details, please see the wolfSSH User Manual.
The wolfSSH example server has been set up to accept a single scp request, and is compiled by default when compiling the wolfSSH library. To start the example server, run:
$ ./examples/server/server
Standard scp commands can be used on the client side. The following are a
few examples, where scp represents the ssh client you are using.
To copy a single file TO the server, using the default example user "jill":
$ scp -P 22222 <local_file> jill@127.0.0.1:<remote_path>
To copy the same single file TO the server, but with timestamp and in verbose mode:
$ scp -v -p -P 22222 <local_file> jill@127.0.0.1:<remote_path>
To recursively copy a directory TO the server:
$ scp -P 22222 -r <local_dir> jill@127.0.0.1:<remote_dir>
To copy a single file FROM the server to the local client:
$ scp -P 22222 jill@127.0.0.1:<remote_file> <local_path>
To recursively copy a directory FROM the server to the local client:
$ scp -P 22222 -r jill@127.0.0.1:<remote_dir> <local_path>
wolfSSH provides support for port forwarding. This allows the user to set up an encrypted tunnel to another server, where the SSH client listens on a socket and forwards connections on that socket to another socket on the server.
To compile wolfSSH with port forwarding support, use the --enable-fwd build
option or define WOLFSSH_FWD:
$ ./configure --enable-fwd
$ make
For full API usage and implementation details, please see the wolfSSH User Manual.
The portfwd example tool will create a "direct-tcpip" style channel. These directions assume you have OpenSSH's server running in the background with port forwarding enabled. This example forwards the port for the wolfSSL client to the server as the application. It assumes that all programs are run on the same machine in different terminals.
src/wolfssl$ ./examples/server/server
src/wolfssh$ ./examples/portfwd/portfwd -p 22 -u <username> \
-f 12345 -t 11111
src/wolfssl$ ./examples/client/client -p 12345
By default, the wolfSSL server listens on port 11111. The client is set to try to connect to port 12345. The portfwd logs in as user "username", opens a listener on port 12345 and connects to the server on port 11111. Packets are routed back and forth between the client and server. "Hello, wolfSSL!"
The source for portfwd provides an example on how to set up and use the port forwarding support in wolfSSH.
The echoserver will handle local and remote port forwarding. To connect with the ssh tool, using one of the following command lines. You can run either of the ssh command lines from anywhere:
src/wolfssl$ ./examples/server/server
src/wolfssh$ ./examples/echoserver/echoserver
anywhere 1$ ssh -p 22222 -L 12345:localhost:11111 jill@localhost
anywhere 2$ ssh -p 22222 -R 12345:localhost:11111 jill@localhost
src/wolfssl$ ./examples/client/client -p 12345
This will allow port forwarding between the wolfSSL client and server like in the previous example.
wolfSSH provides server and client side support for SFTP version 3. This allows the user to set up an encrypted connection for managing file systems.
To compile wolfSSH with SFTP support, use the --enable-sftp build option or
define WOLFSSH_SFTP:
$ ./configure --enable-sftp
$ make
For full API usage and implementation details, please see the wolfSSH User Manual.
The SFTP client created is located in the directory examples/sftpclient/ and the example echoserver acts as a SFTP server.
src/wolfssh$ ./examples/sftpclient/wolfsftp
A full list of supported commands can be seen with typing "help" after a connection.
wolfSSH sftp> help
Commands :
cd <string> change directory
chmod <mode> <path> change mode
get <remote file> <local file> pulls file(s) from server
ls list current directory
mkdir <dir name> creates new directory on server
put <local file> <remote file> push file(s) to server
pwd list current path
quit exit
rename <old> <new> renames remote file
reget <remote file> <local file> resume pulling file
reput <remote file> <local file> resume pushing file
<crtl + c> interrupt get/put cmd
An example of connecting to another system would be
src/wolfssh$ ./examples/sftpclient/wolfsftp -p 22 -u user -h 192.168.1.111
wolfSSH's example echoserver can now fork a shell for the user trying to log in. This currently has only been tested on Linux and macOS. The file echoserver.c must be modified to have the user's credentials in the user authentication callback, or the user authentication callback needs to be changed to verify the provided password.
To compile wolfSSH with shell support, use the --enable-shell build option
or define WOLFSSH_SHELL:
$ ./configure --enable-shell
$ make
To try out this functionality, you can use the example echoserver and client. In a terminal do the following to launch the server:
$ ./examples/echoserver/echoserver -P <user>:junk
And in another terminal do the following to launch the example client:
$ ./examples/client/client -t -u <user> -P junk
Note that <user> must be the user name of the current user that is logged in.
By default, the echoserver will try to start a shell. To use the echo testing
behavior, give the echoserver the command line option -f.
$ ./examples/echoserver/echoserver -f
To use the shell feature with wolfsshd add --enable-sshd to your configure
command and use the following command:
$ sudo ./apps/wolfsshd/wolfsshd -D -h keys/gretel-key-ecc.pem -p 11111
If it complains about a bad sshd_config file, simply copy it to another file
and remove the offending line that it complains about and use the -f command
line parameter to point to the new file.
You can then connect to the wolfsshd server with ssh:
$ ssh <user>@localhost -p 11111
Note that <user> must be the user name of the current user that is logged in.
wolfSSH now supports Curve25519 for key exchange. To enable this support simply compile wolfSSL with support for wolfssh and Curve25519.
$ cd wolfssl
$ ./configure --enable-wolfssh --enable-curve25519
After building and installing wolfSSL, you can simply configure with no options.
$ cd wolfssh
$ ./configure
The wolfSSH client and server will automatically negotiate using Curve25519.
$ ./examples/echoserver/echoserver -f
$ ./examples/client/client -u jill -P upthehill
wolfSSH now supports the post-quantum algorithm ML-KEM (also known as Kyber). It uses the KYBER512 parameter set and is hybridized with ECDHE over the P-256 ECC curve.
In order to use this key exchange you must build and install wolfSSL on your system. Here is an example of an effective configuration:
$ ./configure --enable-wolfssh --enable-experimental --enable-kyber
After that, simply configure and build wolfssh as usual:
$ ./configure
$ make all
The wolfSSH client and server will automatically negotiate using KYBER512 hybridized with ECDHE over the P-256 ECC curve.
$ ./examples/echoserver/echoserver -f
$ ./examples/client/client -u jill -P upthehill
On the client side, you will see the following output:
Server said: Hello, wolfSSH!
If you want to see inter-operability with OpenQauntumSafe's fork of OpenSSH, you can build and execute the fork while the echoserver is running. Download the release from here:
https://github.com/open-quantum-safe/openssh/archive/refs/tags/OQS-OpenSSH-snapshot-2021-08.tar.gz
The following is sufficient for build and execution:
$ tar xmvf openssh-OQS-OpenSSH-snapshot-2021-08.tar.gz
$ cd openssh-OQS-OpenSSH-snapshot-2021-08/
$ ./configure --with-liboqs-dir=/usr/local
$ make all
$ ./ssh -o"KexAlgorithms=ecdh-nistp256-kyber-512r3-sha256-d00@openquantumsafe.org" \
-o"PubkeyAcceptedAlgorithms +ssh-rsa" \
-o"HostkeyAlgorithms +ssh-rsa" \
jill@localhost -p 22222
NOTE: when prompted, enter the password which is "upthehill".
You can type a line of text and when you press enter, the line will be echoed back. Use CTRL-C to terminate the connection.
wolfSSH can accept X.509 certificates in place of just public keys when authenticating a user.
To compile wolfSSH with X.509 support, use the --enable-certs build option
or define WOLFSSH_CERTS:
$ ./configure --enable-certs CPPFLAGS=-DWOLFSSH_NO_FPKI
$ make
For this example, we are disabling the FPKI checking as the included certificate for "fred" does not have the required FPKI extensions. If the flag WOLFSSH_NO_FPKI is removed, you can see the certificate get rejected.
To provide a CA root certificate to validate a user's certificate, give the
echoserver the command line option -a.
$ ./examples/echoserver/echoserver -a ./keys/ca-cert-ecc.pem
The echoserver and client have a fake user named "fred" whose certificate will be used for authentication.
An example echoserver / client connection using the example certificate fred-cert.der would be:
$ ./examples/echoserver/echoserver -a ./keys/ca-cert-ecc.pem -K fred:./keys/fred-cert.der
$ ./examples/client/client -u fred -J ./keys/fred-cert.der -i ./keys/fred-key.der
wolfSSH comes with a server daemon and a command line shell tool. Check out the apps directory for more information.