diff options
| -rw-r--r-- | usr.bin/ssh/sshd.8 | 180 |
1 files changed, 89 insertions, 91 deletions
diff --git a/usr.bin/ssh/sshd.8 b/usr.bin/ssh/sshd.8 index 56ad6a3e2e9..9c21d51f198 100644 --- a/usr.bin/ssh/sshd.8 +++ b/usr.bin/ssh/sshd.8 @@ -34,7 +34,7 @@ .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. .\" -.\" $OpenBSD: sshd.8,v 1.211 2006/01/12 22:20:00 jmc Exp $ +.\" $OpenBSD: sshd.8,v 1.212 2006/01/25 09:04:34 jmc Exp $ .Dd September 25, 1999 .Dt SSHD 8 .Os @@ -56,16 +56,14 @@ .Ek .Sh DESCRIPTION .Nm -(SSH Daemon) is the daemon program for +(OpenSSH Daemon) is the daemon program for .Xr ssh 1 . Together these programs replace rlogin and rsh, and provide secure encrypted communications between two untrusted hosts over an insecure network. -The programs are intended to be as easy to -install and use as possible. .Pp .Nm -is the daemon that listens for connections from clients. +listens for connections from clients. It is normally started at boot from .Pa /etc/rc . It forks a new @@ -73,97 +71,13 @@ daemon for each incoming connection. The forked daemons handle key exchange, encryption, authentication, command execution, and data exchange. -This implementation of -.Nm -supports both SSH protocol version 1 and 2 simultaneously. -.Nm -works as follows: -.Ss SSH protocol version 1 -Each host has a host-specific RSA key -(normally 2048 bits) used to identify the host. -Additionally, when -the daemon starts, it generates a server RSA key (normally 768 bits). -This key is normally regenerated every hour if it has been used, and -is never stored on disk. -.Pp -Whenever a client connects, the daemon responds with its public -host and server keys. -The client compares the -RSA host key against its own database to verify that it has not changed. -The client then generates a 256-bit random number. -It encrypts this -random number using both the host key and the server key, and sends -the encrypted number to the server. -Both sides then use this -random number as a session key which is used to encrypt all further -communications in the session. -The rest of the session is encrypted -using a conventional cipher, currently Blowfish or 3DES, with 3DES -being used by default. -The client selects the encryption algorithm -to use from those offered by the server. -.Pp -Next, the server and the client enter an authentication dialog. -The client tries to authenticate itself using -.Em rhosts -authentication combined with RSA host -authentication, RSA challenge-response authentication, or password -based authentication. -.Pp -System security is not improved unless -.Nm rshd , -.Nm rlogind , -and -.Nm rexecd -are disabled (thus completely disabling -.Xr rlogin -and -.Xr rsh -into the machine). -.Ss SSH protocol version 2 -Version 2 works similarly: -Each host has a host-specific key (RSA or DSA) used to identify the host. -However, when the daemon starts, it does not generate a server key. -Forward security is provided through a Diffie-Hellman key agreement. -This key agreement results in a shared session key. -.Pp -The rest of the session is encrypted using a symmetric cipher, currently -128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. -The client selects the encryption algorithm -to use from those offered by the server. -Additionally, session integrity is provided -through a cryptographic message authentication code -(hmac-sha1 or hmac-md5). -.Pp -Protocol version 2 provides a public key based -user (PubkeyAuthentication) or -client host (HostbasedAuthentication) authentication method, -conventional password authentication and challenge response based methods. -.Ss Command execution and data forwarding -If the client successfully authenticates itself, a dialog for -preparing the session is entered. -At this time the client may request -things like allocating a pseudo-tty, forwarding X11 connections, -forwarding TCP connections, or forwarding the authentication agent -connection over the secure channel. -.Pp -Finally, the client either requests a shell or execution of a command. -The sides then enter session mode. -In this mode, either side may send -data at any time, and such data is forwarded to/from the shell or -command on the server side, and the user terminal in the client side. -.Pp -When the user program terminates and all forwarded X11 and other -connections have been closed, the server sends command exit status to -the client, and both sides exit. .Pp .Nm can be configured using command-line options or a configuration file (by default -.Xr sshd_config 5 ) . -Command-line options override values specified in the +.Xr sshd_config 5 ) ; +command-line options override values specified in the configuration file. -.Pp .Nm rereads its configuration file when it receives a hangup signal, .Dv SIGHUP , @@ -313,6 +227,90 @@ USER@HOST pattern in or .Cm DenyUsers . .El +.Pp +This implementation of +.Nm +supports both SSH protocol version 1 and 2 simultaneously. +.Nm +works as follows: +.Ss SSH protocol version 1 +Each host has a host-specific RSA key +(normally 2048 bits) used to identify the host. +Additionally, when +the daemon starts, it generates a server RSA key (normally 768 bits). +This key is normally regenerated every hour if it has been used, and +is never stored on disk. +.Pp +Whenever a client connects, the daemon responds with its public +host and server keys. +The client compares the +RSA host key against its own database to verify that it has not changed. +The client then generates a 256-bit random number. +It encrypts this +random number using both the host key and the server key, and sends +the encrypted number to the server. +Both sides then use this +random number as a session key which is used to encrypt all further +communications in the session. +The rest of the session is encrypted +using a conventional cipher, currently Blowfish or 3DES, with 3DES +being used by default. +The client selects the encryption algorithm +to use from those offered by the server. +.Pp +Next, the server and the client enter an authentication dialog. +The client tries to authenticate itself using +.Em rhosts +authentication combined with RSA host +authentication, RSA challenge-response authentication, or password +based authentication. +.Pp +System security is not improved unless +.Nm rshd , +.Nm rlogind , +and +.Nm rexecd +are disabled (thus completely disabling +.Xr rlogin +and +.Xr rsh +into the machine). +.Ss SSH protocol version 2 +Version 2 works similarly: +Each host has a host-specific key (RSA or DSA) used to identify the host. +However, when the daemon starts, it does not generate a server key. +Forward security is provided through a Diffie-Hellman key agreement. +This key agreement results in a shared session key. +.Pp +The rest of the session is encrypted using a symmetric cipher, currently +128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. +The client selects the encryption algorithm +to use from those offered by the server. +Additionally, session integrity is provided +through a cryptographic message authentication code +(hmac-sha1 or hmac-md5). +.Pp +Protocol version 2 provides a public key based +user (PubkeyAuthentication) or +client host (HostbasedAuthentication) authentication method, +conventional password authentication and challenge response based methods. +.Ss Command execution and data forwarding +If the client successfully authenticates itself, a dialog for +preparing the session is entered. +At this time the client may request +things like allocating a pseudo-tty, forwarding X11 connections, +forwarding TCP connections, or forwarding the authentication agent +connection over the secure channel. +.Pp +Finally, the client either requests a shell or execution of a command. +The sides then enter session mode. +In this mode, either side may send +data at any time, and such data is forwarded to/from the shell or +command on the server side, and the user terminal in the client side. +.Pp +When the user program terminates and all forwarded X11 and other +connections have been closed, the server sends command exit status to +the client, and both sides exit. .Sh CONFIGURATION FILE .Nm reads configuration data from |