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SSH(1) BSD General Commands Manual SSH(1)

NAME

sssshh - OpenSSH SSH client (remote login program)

SYNOPSIS

sssshh [-11224466AAaaCCffggKKkkMMNNnnqqssTTttVVvvXXxxYY] [-bb bindaddress] [-cc cipherspec] [-DD

[bindaddress:]port] [-ee escapechar] [-FF configfile]

[-ii identityfile] [-LL [bindaddress:]port:host:hostport]

[-ll loginname] [-mm macspec] [-OO ctlcmd] [-oo option] [-pp port] [-RR

[bindaddress:]port:host:hostport] [-SS ctlpath]

[-ww localtun[:remotetun]] [user@]hostname [command]

DESCRIPTION

sssshh (SSH client) is a program for logging into a remote machine and for

executing commands on a remote machine. It is intended to replace rlogin and rsh, and provide secure encrypted communications between two untrusted hosts over an insecure network. X11 connections and arbitrary TCP ports can also be forwarded over the secure channel.

sssshh connects and logs into the specified hostname (with optional user

name). The user must prove his/her identity to the remote machine using one of several methods depending on the protocol version used (see below). If command is specified, it is executed on the remote host instead of a login shell. The options are as follows:

-11 Forces sssshh to try protocol version 1 only.

-22 Forces sssshh to try protocol version 2 only.

-44 Forces sssshh to use IPv4 addresses only.

-66 Forces sssshh to use IPv6 addresses only.

-AA Enables forwarding of the authentication agent connection. This

can also be specified on a per-host basis in a configuration

file. Agent forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the

agent's Unix-domain socket) can access the local agent through

the forwarded connection. An attacker cannot obtain key material from the agent, however they can perform operations on the keys that enable them to authenticate using the identities loaded into the agent.

-aa Disables forwarding of the authentication agent connection.

-bb bindaddress

Use bindaddress on the local machine as the source address of the connection. Only useful on systems with more than one address.

-CC Requests compression of all data (including stdin, stdout,

stderr, and data for forwarded X11 and TCP connections). The compression algorithm is the same used by gzip(1), and the ``level'' can be controlled by the CCoommpprreessssiioonnLLeevveell option for protocol version 1. Compression is desirable on modem lines and other slow connections, but will only slow down things on fast

networks. The default value can be set on a host-by-host basis

in the configuration files; see the CCoommpprreessssiioonn option.

-cc cipherspec

Selects the cipher specification for encrypting the session. Protocol version 1 allows specification of a single cipher. The supported values are ``3des'', ``blowfish'', and ``des''. 3des

(triple-des) is an encrypt-decrypt-encrypt triple with three dif-

ferent keys. It is believed to be secure. blowfish is a fast block cipher; it appears very secure and is much faster than

3des. des is only supported in the sssshh client for interoperabil-

ity with legacy protocol 1 implementations that do not support

the 3des cipher. Its use is strongly discouraged due to crypto-

graphic weaknesses. The default is ``3des''.

For protocol version 2, cipherspec is a comma-separated list of

ciphers listed in order of preference. The supported ciphers

are: 3des-cbc, aes128-cbc, aes192-cbc, aes256-cbc, aes128-ctr,

aes192-ctr, aes256-ctr, arcfour128, arcfour256, arcfour, blow-

fish-cbc, and cast128-cbc. The default is:

aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,

arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,

aes192-ctr,aes256-ctr

-DD [bindaddress:]port

Specifies a local ``dynamic'' application-level port forwarding.

This works by allocating a socket to listen to port on the local side, optionally bound to the specified bindaddress. Whenever a connection is made to this port, the connection is forwarded over the secure channel, and the application protocol is then used to determine where to connect to from the remote machine. Currently

the SOCKS4 and SOCKS5 protocols are supported, and sssshh will act

as a SOCKS server. Only root can forward privileged ports.

Dynamic port forwardings can also be specified in the configura-

tion file. IPv6 addresses can be specified with an alternative syntax:

[bindaddress/]port or by enclosing the address in square brack-

ets. Only the superuser can forward privileged ports. By default, the local port is bound in accordance with the GGaatteewwaayyPPoorrttss setting. However, an explicit bindaddress may be used to bind the connection to a specific address. The bindaddress of ``localhost'' indicates that the listening port

be bound for local use only, while an empty address or `*' indi-

cates that the port should be available from all interfaces.

-ee escapechar

Sets the escape character for sessions with a pty (default: `~'). The escape character is only recognized at the beginning of a line. The escape character followed by a dot (`.') closes the

connection; followed by control-Z suspends the connection; and

followed by itself sends the escape character once. Setting the character to ``none'' disables any escapes and makes the session fully transparent.

-FF configfile

Specifies an alternative per-user configuration file. If a con-

figuration file is given on the command line, the system-wide

configuration file (/etc/sshconfig) will be ignored. The

default for the per-user configuration file is ~/.ssh/config.

-ff Requests sssshh to go to background just before command execution.

This is useful if sssshh is going to ask for passwords or

passphrases, but the user wants it in the background. This

implies -nn. The recommended way to start X11 programs at a

remote site is with something like sssshh -ff hhoosstt xxtteerrmm.

If the EExxiittOOnnFFoorrwwaarrddFFaaiilluurree configuration option is set to

``yes'', then a client started with -ff will wait for all remote

port forwards to be successfully established before placing itself in the background.

-gg Allows remote hosts to connect to local forwarded ports.

-II smartcarddevice

Specify the device sssshh should use to communicate with a smartcard

used for storing the user's private RSA key. This option is only available if support for smartcard devices is compiled in (default is no support).

-ii identityfile

Selects a file from which the identity (private key) for RSA or

DSA authentication is read. The default is ~/.ssh/identity for

protocol version 1, and ~/.ssh/idrsa and ~/.ssh/iddsa for pro-

tocol version 2. Identity files may also be specified on a per-

host basis in the configuration file. It is possible to have

multiple -ii options (and multiple identities specified in config-

uration files).

-KK Enables GSSAPI-based authentication and forwarding (delegation)

of GSSAPI credentials to the server.

-kk Disables forwarding (delegation) of GSSAPI credentials to the

server.

-LL [bindaddress:]port:host:hostport

Specifies that the given port on the local (client) host is to be forwarded to the given host and port on the remote side. This works by allocating a socket to listen to port on the local side,

optionally bound to the specified bindaddress. Whenever a con-

nection is made to this port, the connection is forwarded over the secure channel, and a connection is made to host port hostport from the remote machine. Port forwardings can also be

specified in the configuration file. IPv6 addresses can be spec-

ified with an alternative syntax: [bindaddress/]port/host/hostport or by enclosing the address in square brackets. Only the superuser can forward privileged ports. By default, the local port is bound in accordance with the GGaatteewwaayyPPoorrttss setting. However, an explicit bindaddress may be used to bind the connection to a specific address. The bindaddress of ``localhost'' indicates that the listening port

be bound for local use only, while an empty address or `*' indi-

cates that the port should be available from all interfaces.

-ll loginname

Specifies the user to log in as on the remote machine. This also

may be specified on a per-host basis in the configuration file.

-MM Places the sssshh client into ``master'' mode for connection shar-

ing. Multiple -MM options places sssshh into ``master'' mode with

confirmation required before slave connections are accepted.

Refer to the description of CCoonnttrroollMMaasstteerr in sshconfig(5) for

details.

-mm macspec

Additionally, for protocol version 2 a comma-separated list of

MAC (message authentication code) algorithms can be specified in order of preference. See the MMAACCss keyword for more information.

-NN Do not execute a remote command. This is useful for just for-

warding ports (protocol version 2 only).

-nn Redirects stdin from /dev/null (actually, prevents reading from

stdin). This must be used when sssshh is run in the background. A

common trick is to use this to run X11 programs on a remote

machine. For example, sssshh -nn sshhaaddoowwss..ccss..hhuutt..ffii eemmaaccss && will

start an emacs on shadows.cs.hut.fi, and the X11 connection will

be automatically forwarded over an encrypted channel. The sssshh

program will be put in the background. (This does not work if

sssshh needs to ask for a password or passphrase; see also the -ff

option.)

-OO ctlcmd

Control an active connection multiplexing master process. When

the -OO option is specified, the ctlcmd argument is interpreted

and passed to the master process. Valid commands are: ``check'' (check that the master process is running) and ``exit'' (request the master to exit).

-oo option

Can be used to give options in the format used in the configura-

tion file. This is useful for specifying options for which there

is no separate command-line flag. For full details of the

options listed below, and their possible values, see

sshconfig(5).

AddressFamily BatchMode BindAddress ChallengeResponseAuthentication CheckHostIP Cipher Ciphers ClearAllForwardings Compression CompressionLevel ConnectionAttempts ConnectTimeout ControlMaster ControlPath DynamicForward EscapeChar ExitOnForwardFailure ForwardAgent ForwardX11 ForwardX11Trusted GatewayPorts GlobalKnownHostsFile GSSAPIAuthentication GSSAPIDelegateCredentials HashKnownHosts Host HostbasedAuthentication HostKeyAlgorithms HostKeyAlias HostName IdentityFile IdentitiesOnly KbdInteractiveDevices LocalCommand LocalForward LogLevel MACs NoHostAuthenticationForLocalhost NumberOfPasswordPrompts PasswordAuthentication PermitLocalCommand Port PreferredAuthentications Protocol ProxyCommand PubkeyAuthentication RekeyLimit RemoteForward RhostsRSAAuthentication RSAAuthentication SendEnv ServerAliveInterval ServerAliveCountMax SmartcardDevice StrictHostKeyChecking TCPKeepAlive Tunnel TunnelDevice UsePrivilegedPort User UserKnownHostsFile VerifyHostKeyDNS VisualHostKey XAuthLocation

-pp port

Port to connect to on the remote host. This can be specified on

a per-host basis in the configuration file.

-qq Quiet mode. Causes most warning and diagnostic messages to be

suppressed.

-RR [bindaddress:]port:host:hostport

Specifies that the given port on the remote (server) host is to be forwarded to the given host and port on the local side. This works by allocating a socket to listen to port on the remote

side, and whenever a connection is made to this port, the connec-

tion is forwarded over the secure channel, and a connection is made to host port hostport from the local machine. Port forwardings can also be specified in the configuration file. Privileged ports can be forwarded only when logging in as root on the remote machine. IPv6 addresses can be specified by enclosing the address in square braces or using an alternative syntax: [bindaddress/]host/port/hostport. By default, the listening socket on the server will be bound to the loopback interface only. This may be overriden by specifying

a bindaddress. An empty bindaddress, or the address `*', indi-

cates that the remote socket should listen on all interfaces. Specifying a remote bindaddress will only succeed if the

server's GGaatteewwaayyPPoorrttss option is enabled (see sshdconfig(5)).

-SS ctlpath

Specifies the location of a control socket for connection shar-

ing. Refer to the description of CCoonnttrroollPPaatthh and CCoonnttrroollMMaasstteerr

in sshconfig(5) for details.

-ss May be used to request invocation of a subsystem on the remote

system. Subsystems are a feature of the SSH2 protocol which

facilitate the use of SSH as a secure transport for other appli-

cations (eg. sftp(1)). The subsystem is specified as the remote command.

-TT Disable pseudo-tty allocation.

-tt Force pseudo-tty allocation. This can be used to execute arbi-

trary screen-based programs on a remote machine, which can be

very useful, e.g. when implementing menu services. Multiple -tt

options force tty allocation, even if sssshh has no local tty.

-VV Display the version number and exit.

-vv Verbose mode. Causes sssshh to print debugging messages about its

progress. This is helpful in debugging connection, authentica-

tion, and configuration problems. Multiple -vv options increase

the verbosity. The maximum is 3.

-ww localtun[:remotetun]

Requests tunnel device forwarding with the specified tun(4) devices between the client (localtun) and the server (remotetun). The devices may be specified by numerical ID or the keyword ``any'', which uses the next available tunnel device. If remotetun is not specified, it defaults to ``any''. See also

the TTuunnnneell and TTuunnnneellDDeevviiccee directives in sshconfig(5). If the

TTuunnnneell directive is unset, it is set to the default tunnel mode,

which is ``point-to-point''.

-XX Enables X11 forwarding. This can also be specified on a per-host

basis in a configuration file. X11 forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the user's X authorization database) can access the local X11 display through the forwarded connection. An attacker may then be able to perform activities such as keystroke monitoring. For this reason, X11 forwarding is subjected to X11 SECURITY

extension restrictions by default. Please refer to the sssshh -YY

option and the FFoorrwwaarrddXX1111TTrruusstteedd directive in sshconfig(5) for

more information.

-xx Disables X11 forwarding.

-YY Enables trusted X11 forwarding. Trusted X11 forwardings are not

subjected to the X11 SECURITY extension controls.

sssshh may additionally obtain configuration data from a per-user configura-

tion file and a system-wide configuration file. The file format and con-

figuration options are described in sshconfig(5).

sssshh exits with the exit status of the remote command or with 255 if an

error occurred. AAUUTTHHEENNTTIICCAATTIIOONN The OpenSSH SSH client supports SSH protocols 1 and 2. Protocol 2 is the

default, with sssshh falling back to protocol 1 if it detects protocol 2 is

unsupported. These settings may be altered using the PPrroottooccooll option in

sshconfig(5), or enforced using the -11 and -22 options (see above). Both

protocols support similar authentication methods, but protocol 2 is pre-

ferred since it provides additional mechanisms for confidentiality (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and

integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160). Protocol 1

lacks a strong mechanism for ensuring the integrity of the connection.

The methods available for authentication are: GSSAPI-based authentica-

tion, host-based authentication, public key authentication, challenge-

response authentication, and password authentication. Authentication methods are tried in the order specified above, though protocol 2 has a configuration option to change the default order: PPrreeffeerrrreeddAAuutthheennttiiccaattiioonnss.

Host-based authentication works as follows: If the machine the user logs

in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote machine, and the user names are the same on both sides, or if the files ~/.rhosts or ~/.shosts exist in the user's home directory on the remote machine and contain a line containing the name of the client machine and the name of the user on that machine, the user is considered for login. Additionally, the server must be able to verify the client's host key

(see the description of /etc/sshknownhosts and ~/.ssh/knownhosts,

below) for login to be permitted. This authentication method closes security holes due to IP spoofing, DNS spoofing, and routing spoofing. [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the rlogin/rsh protocol in general, are inherently insecure and should be disabled if security is desired.]

Public key authentication works as follows: The scheme is based on pub-

lic-key cryptography, using cryptosystems where encryption and decryption

are done using separate keys, and it is unfeasible to derive the decryp-

tion key from the encryption key. The idea is that each user creates a public/private key pair for authentication purposes. The server knows

the public key, and only the user knows the private key. sssshh implements

public key authentication protocol automatically, using either the RSA or DSA algorithms. Protocol 1 is restricted to using only RSA keys, but protocol 2 may use either. The HISTORY section of ssl(8) contains a brief discussion of the two algorithms.

The file ~/.ssh/authorizedkeys lists the public keys that are permitted

for logging in. When the user logs in, the sssshh program tells the server

which key pair it would like to use for authentication. The client proves that it has access to the private key and the server checks that the corresponding public key is authorized to accept the account.

The user creates his/her key pair by running ssh-keygen(1). This stores

the private key in ~/.ssh/identity (protocol 1), ~/.ssh/iddsa (protocol

2 DSA), or ~/.ssh/idrsa (protocol 2 RSA) and stores the public key in

~/.ssh/identity.pub (protocol 1), ~/.ssh/iddsa.pub (protocol 2 DSA), or

~/.ssh/idrsa.pub (protocol 2 RSA) in the user's home directory. The

user should then copy the public key to ~/.ssh/authorizedkeys in his/her

home directory on the remote machine. The authorizedkeys file corre-

sponds to the conventional ~/.rhosts file, and has one key per line,

though the lines can be very long. After this, the user can log in with-

out giving the password. The most convenient way to use public key authentication may be with an

authentication agent. See ssh-agent(1) for more information.

Challenge-response authentication works as follows: The server sends an

arbitrary "challenge" text, and prompts for a response. Protocol 2 allows multiple challenges and responses; protocol 1 is restricted to

just one challenge/response. Examples of challenge-response authentica-

tion include BSD Authentication (see login.conf(5)) and PAM (some non-

OpenBSD systems).

Finally, if other authentication methods fail, sssshh prompts the user for a

password. The password is sent to the remote host for checking; however, since all communications are encrypted, the password cannot be seen by someone listening on the network.

sssshh automatically maintains and checks a database containing identifica-

tion for all hosts it has ever been used with. Host keys are stored in

~/.ssh/knownhosts in the user's home directory. Additionally, the file

/etc/sshknownhosts is automatically checked for known hosts. Any new

hosts are automatically added to the user's file. If a host's identifi-

cation ever changes, sssshh warns about this and disables password authenti-

cation to prevent server spoofing or man-in-the-middle attacks, which

could otherwise be used to circumvent the encryption. The SSttrriiccttHHoossttKKeeyyCChheecckkiinngg option can be used to control logins to machines whose host key is not known or has changed. When the user's identity has been accepted by the server, the server either executes the given command, or logs into the machine and gives the user a normal shell on the remote machine. All communication with the remote command or shell will be automatically encrypted.

If a pseudo-terminal has been allocated (normal login session), the user

may use the escape characters noted below.

If no pseudo-tty has been allocated, the session is transparent and can

be used to reliably transfer binary data. On most systems, setting the escape character to ``none'' will also make the session transparent even if a tty is used. The session terminates when the command or shell on the remote machine exits and all X11 and TCP connections have been closed. EESSCCAAPPEE CCHHAARRAACCTTEERRSS

When a pseudo-terminal has been requested, sssshh supports a number of func-

tions through the use of an escape character. A single tilde character can be sent as ~~~~ or by following the tilde by a character other than those described below. The escape character must

always follow a newline to be interpreted as special. The escape charac-

ter can be changed in configuration files using the EEssccaappeeCChhaarr configura-

tion directive or on the command line by the -ee option.

The supported escapes (assuming the default `~') are: ~~.. Disconnect.

~~^^ZZ Background sssshh.

~~## List forwarded connections.

~~&& Background sssshh at logout when waiting for forwarded connection /

X11 sessions to terminate. ~~?? Display a list of escape characters. ~~BB Send a BREAK to the remote system (only useful for SSH protocol version 2 and if the peer supports it). ~~CC Open command line. Currently this allows the addition of port

forwardings using the -LL and -RR options (see above). It also

allows the cancellation of existing remote port-forwardings using

-KKRR[bindaddress:]port. !!command allows the user to execute a

local command if the PPeerrmmiittLLooccaallCCoommmmaanndd option is enabled in

sshconfig(5). Basic help is available, using the -hh option.

~~RR Request rekeying of the connection (only useful for SSH protocol version 2 and if the peer supports it). TTCCPP FFOORRWWAARRDDIINNGG Forwarding of arbitrary TCP connections over the secure channel can be specified either on the command line or in a configuration file. One possible application of TCP forwarding is a secure connection to a mail server; another is going through firewalls. In the example below, we look at encrypting communication between an IRC client and server, even though the IRC server does not directly support encrypted communications. This works as follows: the user connects to

the remote host using sssshh, specifying a port to be used to forward con-

nections to the remote server. After that it is possible to start the service which is to be encrypted on the client machine, connecting to the

same local port, and sssshh will encrypt and forward the connection.

The following example tunnels an IRC session from client machine ``127.0.0.1'' (localhost) to remote server ``server.example.com'':

$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10

$ irc -c '#users' -p 1234 pinky 127.0.0.1

This tunnels a connection to IRC server ``server.example.com'', joining

channel ``#users'', nickname ``pinky'', using port 1234. It doesn't mat-

ter which port is used, as long as it's greater than 1023 (remember, only root can open sockets on privileged ports) and doesn't conflict with any ports already in use. The connection is forwarded to port 6667 on the remote server, since that's the standard port for IRC services.

The -ff option backgrounds sssshh and the remote command ``sleep 10'' is

specified to allow an amount of time (10 seconds, in the example) to start the service which is to be tunnelled. If no connections are made

within the time specified, sssshh will exit.

XX1111 FFOORRWWAARRDDIINNGG If the FFoorrwwaarrddXX1111 variable is set to ``yes'' (or see the description of

the -XX, -xx, and -YY options above) and the user is using X11 (the DISPLAY

environment variable is set), the connection to the X11 display is auto-

matically forwarded to the remote side in such a way that any X11 pro-

grams started from the shell (or command) will go through the encrypted channel, and the connection to the real X server will be made from the local machine. The user should not manually set DISPLAY. Forwarding of X11 connections can be configured on the command line or in configuration files.

The DISPLAY value set by sssshh will point to the server machine, but with a

display number greater than zero. This is normal, and happens because

sssshh creates a ``proxy'' X server on the server machine for forwarding the

connections over the encrypted channel.

sssshh will also automatically set up Xauthority data on the server machine.

For this purpose, it will generate a random authorization cookie, store it in Xauthority on the server, and verify that any forwarded connections carry this cookie and replace it by the real cookie when the connection is opened. The real authentication cookie is never sent to the server machine (and no cookies are sent in the plain). If the FFoorrwwaarrddAAggeenntt variable is set to ``yes'' (or see the description of

the -AA and -aa options above) and the user is using an authentication

agent, the connection to the agent is automatically forwarded to the remote side. VVEERRIIFFYYIINNGG HHOOSSTT KKEEYYSS When connecting to a server for the first time, a fingerprint of the server's public key is presented to the user (unless the option SSttrriiccttHHoossttKKeeyyCChheecckkiinngg has been disabled). Fingerprints can be determined

using ssh-keygen(1):

$ ssh-keygen -l -f /etc/sshhostrsakey

If the fingerprint is already known, it can be matched and the key can be accepted or rejected. Because of the difficulty of comparing host keys just by looking at hex strings, there is also support to compare host keys visually, using random art. By setting the VViissuuaallHHoossttKKeeyy option to ``yes'', a small ASCII graphic gets displayed on every login to a server, no matter if the session itself is interactive or not. By learning the pattern a known server produces, a user can easily find out that the host key has changed when a completely different pattern is displayed. Because these patterns are not unambiguous however, a pattern that looks similar to the pattern remembered only gives a good probability that the host key is the same, not guaranteed proof. To get a listing of the fingerprints along with their random art for all known hosts, the following command line can be used:

$ ssh-keygen -lv -f ~/.ssh/knownhosts

If the fingerprint is unknown, an alternative method of verification is available: SSH fingerprints verified by DNS. An additional resource record (RR), SSHFP, is added to a zonefile and the connecting client is able to match the fingerprint with that of the key presented. In this example, we are connecting a client to a server, ``host.example.com''. The SSHFP resource records should first be added to the zonefile for host.example.com:

$ ssh-keygen -r host.example.com.

The output lines will have to be added to the zonefile. To check that the zone is answering fingerprint queries:

$ dig -t SSHFP host.example.com

Finally the client connects:

$ ssh -o "VerifyHostKeyDNS ask" host.example.com

[...] Matching host key fingerprint found in DNS. Are you sure you want to continue connecting (yes/no)?

See the VVeerriiffyyHHoossttKKeeyyDDNNSS option in sshconfig(5) for more information.

SSSSHH-BBAASSEEDD VVIIRRTTUUAALL PPRRIIVVAATTEE NNEETTWWOORRKKSS

sssshh contains support for Virtual Private Network (VPN) tunnelling using

the tun(4) network pseudo-device, allowing two networks to be joined

securely. The sshdconfig(5) configuration option PPeerrmmiittTTuunnnneell controls

whether the server supports this, and at what level (layer 2 or 3 traf-

fic). The following example would connect client network 10.0.50.0/24 with

remote network 10.0.99.0/24 using a point-to-point connection from

10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway to the remote network, at 192.168.1.15, allows it. On the client:

# ssh -f -w 0:1 192.168.1.15 true

# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252

# route add 10.0.99.0/24 10.1.1.2

On the server:

# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252

# route add 10.0.50.0/24 10.1.1.1

Client access may be more finely tuned via the /root/.ssh/authorizedkeys

file (see below) and the PPeerrmmiittRRoooottLLooggiinn server option. The following entry would permit connections on tun(4) device 1 from user ``jane'' and on tun device 2 from user ``john'', if PPeerrmmiittRRoooottLLooggiinn is set to

``forced-commands-only'':

tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane

tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john

Since an SSH-based setup entails a fair amount of overhead, it may be

more suited to temporary setups, such as for wireless VPNs. More perma-

nent VPNs are better provided by tools such as ipsecctl(8) and isakmpd(8). ENVIRONMENT

sssshh will normally set the following environment variables:

DISPLAY The DISPLAY variable indicates the location of the

X11 server. It is automatically set by sssshh to

point to a value of the form ``hostname:n'', where ``hostname'' indicates the host where the shell

runs, and `n' is an integer >= 1. sssshh uses this

special value to forward X11 connections over the secure channel. The user should normally not set DISPLAY explicitly, as that will render the X11 connection insecure (and will require the user to manually copy any required authorization cookies). HOME Set to the path of the user's home directory.

LOGNAME Synonym for USER; set for compatibility with sys-

tems that use this variable. MAIL Set to the path of the user's mailbox.

PATH Set to the default PATH, as specified when compil-

ing sssshh.

SSHASKPASS If sssshh needs a passphrase, it will read the

passphrase from the current terminal if it was run

from a terminal. If sssshh does not have a terminal

associated with it but DISPLAY and SSHASKPASS are set, it will execute the program specified by SSHASKPASS and open an X11 window to read the

passphrase. This is particularly useful when call-

ing sssshh from a .xsession or related script. (Note

that on some machines it may be necessary to redi-

rect the input from /dev/null to make this work.)

SSHAUTHSOCK Identifies the path of a UNIX-domain socket used to

communicate with the agent.

SSHCONNECTION Identifies the client and server ends of the con-

nection. The variable contains four space-sepa-

rated values: client IP address, client port num-

ber, server IP address, and server port number. SSHORIGINALCOMMAND This variable contains the original command line if a forced command is executed. It can be used to extract the original arguments. SSHTTY This is set to the name of the tty (path to the

device) associated with the current shell or com-

mand. If the current session has no tty, this variable is not set. TZ This variable is set to indicate the present time zone if it was set when the daemon was started

(i.e. the daemon passes the value on to new connec-

tions). USER Set to the name of the user logging in.

Additionally, sssshh reads ~/.ssh/environment, and adds lines of the format

``VARNAME=value'' to the environment if the file exists and users are

allowed to change their environment. For more information, see the

PPeerrmmiittUUsseerrEEnnvviirroonnmmeenntt option in sshdconfig(5).

FILES ~/.rhosts

This file is used for host-based authentication (see above). On

some machines this file may need to be world-readable if the

user's home directory is on an NFS partition, because sshd(8)

reads it as root. Additionally, this file must be owned by the user, and must not have write permissions for anyone else. The recommended permission for most machines is read/write for the user, and not accessible by others. ~/.shosts This file is used in exactly the same way as .rhosts, but allows

host-based authentication without permitting login with

rlogin/rsh.

~/.ssh/

This directory is the default location for all user-specific con-

figuration and authentication information. There is no general requirement to keep the entire contents of this directory secret, but the recommended permissions are read/write/execute for the user, and not accessible by others.

~/.ssh/authorizedkeys

Lists the public keys (RSA/DSA) that can be used for logging in as this user. The format of this file is described in the

sshd(8) manual page. This file is not highly sensitive, but the

recommended permissions are read/write for the user, and not accessible by others.

~/.ssh/config

This is the per-user configuration file. The file format and

configuration options are described in sshconfig(5). Because of

the potential for abuse, this file must have strict permissions: read/write for the user, and not accessible by others.

~/.ssh/environment

Contains additional definitions for environment variables; see ENVIRONMENT, above.

~/.ssh/identity

~/.ssh/iddsa

~/.ssh/idrsa

Contains the private key for authentication. These files contain

sensitive data and should be readable by the user but not acces-

sible by others (read/write/execute). sssshh will simply ignore a

private key file if it is accessible by others. It is possible to specify a passphrase when generating the key which will be used to encrypt the sensitive part of this file using 3DES.

~/.ssh/identity.pub

~/.ssh/iddsa.pub

~/.ssh/idrsa.pub

Contains the public key for authentication. These files are not sensitive and can (but need not) be readable by anyone.

~/.ssh/knownhosts

Contains a list of host keys for all hosts the user has logged into that are not already in the systemwide list of known host

keys. See sshd(8) for further details of the format of this

file.

~/.ssh/rc

Commands in this file are executed by sssshh when the user logs in,

just before the user's shell (or command) is started. See the

sshd(8) manual page for more information.

/etc/hosts.equiv

This file is for host-based authentication (see above). It

should only be writable by root. /etc/shosts.equiv This file is used in exactly the same way as hosts.equiv, but

allows host-based authentication without permitting login with

rlogin/rsh.

/etc/sshconfig

Systemwide configuration file. The file format and configuration

options are described in sshconfig(5).

/etc/sshhostkey

/etc/sshhostdsakey

/etc/sshhostrsakey

These three files contain the private parts of the host keys and

are used for host-based authentication. If protocol version 1 is

used, sssshh must be setuid root, since the host key is readable

only by root. For protocol version 2, sssshh uses ssh-keysign(8) to

access the host keys, eliminating the requirement that sssshh be

setuid root when host-based authentication is used. By default

sssshh is not setuid root.

/etc/sshknownhosts

Systemwide list of known host keys. This file should be prepared by the system administrator to contain the public host keys of

all machines in the organization. It should be world-readable.

See sshd(8) for further details of the format of this file.

/etc/sshrc

Commands in this file are executed by sssshh when the user logs in,

just before the user's shell (or command) is started. See the

sshd(8) manual page for more information.

scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),

tun(4), hosts.equiv(5), sshconfig(5), ssh-keysign(8), sshd(8)

The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006. The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006. The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006. The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006. The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006. Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 4255, 2006. Generic Message Exchange Authentication for the Secure Shell Protocol (SSH), RFC 4256, 2006. The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006. The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006. Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol, RFC 4345, 2006.

Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer

Protocol, RFC 4419, 2006. The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006. A. Perrig and D. Song, Hash Visualization: a New Technique to improve

Real-World Security, 1999, International Workshop on Cryptographic

Techniques and E-Commerce (CrypTEC '99).

AUTHORS

OpenSSH is a derivative of the original and free ssh 1.2.12 release by

Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo

de Raadt and Dug Song removed many bugs, re-added newer features and cre-

ated OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0. BSD December 21, 2019 BSD