SSH(1)

NAME

ssh, sshnet, scp, sshserve − secure login and file copy from/to Unix or Plan 9

SYNOPSIS

ssh [ -CfiImPpRrw ] [ -A authlist ] [ -c cipherlist ] [ -[lu] user ] [user@]host [ cmd [ args ... ]]

sshnet [ -A authlist ] [ -c cipherlist ] [ -m mtpt ] [ -s service ] [user@]host

scp [host:]file [host:]file
scp [host:]file ... [host:]dir

aux/sshserve [ -p ] address

DESCRIPTION

Ssh allows authenticated login over an encrypted channel to hosts that support the ssh protocol (see the RFC listed below for encryption and authentication details).

Ssh takes the host name of the machine to connect to as its mandatory argument. It may be specified as a domain name or an IP address. Normally, login is attempted using the user name from /dev/user.

Command-line options are:

-C force input to be read in cooked mode: “line at a time” with local echo.

-f enable agent forwarding. With this flag, ssh uses SSH’s agent forwarding protocol to allow programs running on the remote server to interact with factotum(4) to perform RSA authentication.

-i force interactive mode. In interactive mode, ssh prompts for passwords and confirmations of new host keys when necessary. (In non-interactive mode, password requests are rejected and unrecognized host keys are cause for disconnecting.) By default, ssh runs in interactive mode only when its input file descriptor is /dev/cons.

-I force non-interactive mode.

-m disable the control-\ menu, described below.

-p force pseudoterminal request. The ssh protocol, grounded in Unix tradition, differentiates between connections that request controlling pseudoterminals and those that do not. By default, ssh requests a pseudoterminal only when no command is given.

-P force no pseudoterminal request.

-r strip carriage returns.

-R put the allocated pseudoterminal, if any, in raw mode.

-w notify the remote side whenever the window changes size.

-[lu] user
specify user name. This option is deprecated in favor of the user@hostname syntax.

-A authlist
specify an ordered space-separated list of authentication protocols to try. The full set of authentication protocols is rsa (RSA using factotum(4) to moderate key usage), password (use a password gathered from factotum), and tis (challenge-response). The default list is all three in that order.

-c cipherlist
specify an ordered space-separated list of allowed ciphers to use when encrypting the channel. The full set of ciphers is des (standard DES), 3des (a somewhat doubtful variation on triple DES), blowfish (Bruce Schneier’s Blowfish), rc4 (RC4), and none (no encryption). The default cipher list is blowfish rc4 3des.

The control−\ character is a local escape, as in con(1). It prompts with >>>. Legitimate responses to the prompt are

q Exit.

. Return from the escape.

!cmd Run the command with the network connection as its standard input and standard output. Standard error will go to the screen.

r Toggle printing of carriage returns.

If no command is specified, a login session is started on the remote host. Otherwise, the command is executed with its arguments.

Ssh establishes a connection with an ssh daemon on the remote host. The daemon sends to ssh its RSA public host key and session key. Using these, ssh sends a session key which, presumably, only the daemon can decipher. After this, both sides start encrypting their data with this session key.

When the daemon’s host key has been received, ssh looks it up in $home/lib/keyring and in /sys/lib/ssh/keyring. If the key is found there, and it matches the received key, ssh is satisfied. If not, ssh reports this and offers to add the key to $home/lib/keyring.

Over the encrypted channel, ssh attempts to convince the daemon to accept the call using the listed authentication protocols (see the -A option above).

The preferred way to authenticate is a netkey-style challenge/response or via a SecurID token. Ssh users on other systems than Plan 9 should enable TIS_Authentication.

When the connection is authenticated, the given command line, (by default, a login shell) is executed on the remote host. The SSH protocol allows clients to make outgoing TCP calls via the server. Sshnet establishes an SSH connection and, rather than execute a remote command, presents the remote server’s TCP stack as a network stack (see the discussion of TCP in ip(3)) mounted at mtpt (default /net), optionally posting a 9P service descriptor for the new file system as /srv/service. The -A and -c arguments are as in ssh. Scp uses ssh to copy files from one host to another. A remote file is identified by a host name, a colon and a file name (no spaces). Scp can copy files from remote hosts and to remote hosts. Sshserve is the server that services ssh calls from remote hosts. The -A and -c options set valid authentication methods and ciphers as in ssh, except that there is no rsa authentication method. Unlike in ssh, the list is not ordered: the server presents a set and the client makes the choice. The default sets are tis and blowfish rc4 3des. By default, users start with the namespace defined in /lib/namespace. Users in group noworld in /adm/users start with the namespace defined in /lib/namespace.noworld. Sshserve does not provide the TCP forwarding functionality used by sshnet, because many Unix clients present this capability in an insecure manner.

Sshserve requires that factotum(4) hold the host key, identified by having attributes proto=rsa service=sshserve. To generate a host key:

auth/rsagen -t ’service=sshserve’ >/mnt/factotum/ctl

To extract the public part of the host key in the form used by SSH key rings:

grep ’service=sshserve’ /mnt/factotum/ctl | auth/rsa2ssh

FILES

/sys/lib/ssh/keyring
System key ring file containing public keys for remote ssh clients and servers.

/usr/user/lib/keyring
Personal key ring file containing public keys for remote ssh clients and servers.

SOURCE

/sys/src/cmd/ssh

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