diff options
Diffstat (limited to 'sshd.8')
| -rw-r--r-- | sshd.8 | 309 |
1 files changed, 175 insertions, 134 deletions
@@ -33,25 +33,23 @@ .\" (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 1.278 2014/11/15 14:41:03 bentley Exp $ -.Dd $Mdocdate: November 15 2014 $ +.\" $OpenBSD: sshd.8,v 1.312 2020/01/25 06:03:10 djm Exp $ +.Dd $Mdocdate: January 25 2020 $ .Dt SSHD 8 .Os .Sh NAME .Nm sshd -.Nd OpenSSH SSH daemon +.Nd OpenSSH daemon .Sh SYNOPSIS .Nm sshd .Bk -words .Op Fl 46DdeiqTt -.Op Fl b Ar bits .Op Fl C Ar connection_spec .Op Fl c Ar host_certificate_file .Op Fl E Ar log_file .Op Fl f Ar config_file .Op Fl g Ar login_grace_time .Op Fl h Ar host_key_file -.Op Fl k Ar key_gen_time .Op Fl o Ar option .Op Fl p Ar port .Op Fl u Ar len @@ -96,29 +94,28 @@ to use IPv4 addresses only. Forces .Nm to use IPv6 addresses only. -.It Fl b Ar bits -Specifies the number of bits in the ephemeral protocol version 1 -server key (default 1024). .It Fl C Ar connection_spec Specify the connection parameters to use for the .Fl T extended test mode. If provided, any .Cm Match -directives in the configuration file -that would apply to the specified user, host, and address will be set before -the configuration is written to standard output. -The connection parameters are supplied as keyword=value pairs. +directives in the configuration file that would apply are applied before the +configuration is written to standard output. +The connection parameters are supplied as keyword=value pairs and may be +supplied in any order, either with multiple +.Fl C +options or as a comma-separated list. The keywords are +.Dq addr, .Dq user , .Dq host , .Dq laddr , .Dq lport , and -.Dq addr . -All are required and may be supplied in any order, either with multiple -.Fl C -options or as a comma-separated list. +.Dq rdomain +and correspond to source address, user, resolved source host name, +local address, local port number and routing domain respectively. .It Fl c Ar host_certificate_file Specifies a path to a certificate file to identify .Nm @@ -169,39 +166,17 @@ This option must be given if is not run as root (as the normal host key files are normally not readable by anyone but root). The default is -.Pa /etc/ssh/ssh_host_key -for protocol version 1, and -.Pa /etc/ssh/ssh_host_dsa_key , -.Pa /etc/ssh/ssh_host_ecdsa_key . +.Pa /etc/ssh/ssh_host_ecdsa_key , .Pa /etc/ssh/ssh_host_ed25519_key and -.Pa /etc/ssh/ssh_host_rsa_key -for protocol version 2. +.Pa /etc/ssh/ssh_host_rsa_key . It is possible to have multiple host key files for -the different protocol versions and host key algorithms. +the different host key algorithms. .It Fl i Specifies that .Nm is being run from .Xr inetd 8 . -.Nm -is normally not run -from inetd because it needs to generate the server key before it can -respond to the client, and this may take tens of seconds. -Clients would have to wait too long if the key was regenerated every time. -However, with small key sizes (e.g. 512) using -.Nm -from inetd may -be feasible. -.It Fl k Ar key_gen_time -Specifies how often the ephemeral protocol version 1 server key is -regenerated (default 3600 seconds, or one hour). -The motivation for regenerating the key fairly -often is that the key is not stored anywhere, and after about an hour -it becomes impossible to recover the key for decrypting intercepted -communications even if the machine is cracked into or physically -seized. -A value of zero indicates that the key will never be regenerated. .It Fl o Ar option Can be used to give options in the format used in the configuration file. This is useful for specifying options for which there is no separate @@ -260,8 +235,7 @@ may also be used to prevent from making DNS requests unless the authentication mechanism or configuration requires it. Authentication mechanisms that may require DNS include -.Cm RhostsRSAAuthentication , -.Cm HostbasedAuthentication , +.Cm HostbasedAuthentication and using a .Cm from="pattern-list" option in a key file. @@ -272,53 +246,20 @@ or .Cm DenyUsers . .El .Sh AUTHENTICATION -The OpenSSH SSH daemon supports SSH protocols 1 and 2. -The default is to use protocol 2 only, -though this can be changed via the -.Cm Protocol -option in -.Xr sshd_config 5 . -Protocol 2 supports DSA, ECDSA, Ed25519 and RSA keys; -protocol 1 only supports RSA keys. -For both protocols, -each host has a host-specific key, -normally 2048 bits, +The OpenSSH SSH daemon supports SSH protocol 2 only. +Each host has a host-specific key, used to identify the host. -.Pp -Forward security for protocol 1 is provided through -an additional server key, -normally 768 bits, -generated when the server starts. -This key is normally regenerated every hour if it has been used, and -is never stored on disk. Whenever a client connects, the daemon responds with its public -host and server keys. +host key. 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 -For protocol 2, -forward security is provided through a Diffie-Hellman key agreement. +host key against its own database to verify that it has not changed. +Forward secrecy is provided through a Diffie-Hellman key agreement. This key agreement results in a shared session key. -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 rest of the session is encrypted using a symmetric cipher. 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-md5, hmac-sha1, umac-64, umac-128, hmac-ripemd160, -hmac-sha2-256 or hmac-sha2-512). +through a cryptographic message authentication code (MAC). .Pp Finally, the server and the client enter an authentication dialog. The client tries to authenticate itself using @@ -333,7 +274,7 @@ locked, listed in .Cm DenyUsers or its group is listed in .Cm DenyGroups -\&. The definition of a locked account is system dependant. Some platforms +\&. The definition of a locked account is system dependent. Some platforms have their own account database (eg AIX) and some modify the passwd field ( .Ql \&*LK\&* on Solaris and UnixWare, @@ -424,6 +365,8 @@ See below. .It Runs user's shell or command. +All commands are run under the user's login shell as specified in the +system password database. .El .Sh SSHRC If the file @@ -471,7 +414,7 @@ does not exist either, xauth is used to add the cookie. .Cm AuthorizedKeysFile specifies the files containing public keys for public key authentication; -if none is specified, the default is +if this option is not specified, the default is .Pa ~/.ssh/authorized_keys and .Pa ~/.ssh/authorized_keys2 . @@ -480,42 +423,48 @@ key (empty lines and lines starting with a .Ql # are ignored as comments). -Protocol 1 public keys consist of the following space-separated fields: -options, bits, exponent, modulus, comment. -Protocol 2 public key consist of: +Public keys consist of the following space-separated fields: options, keytype, base64-encoded key, comment. -The options field is optional; -its presence is determined by whether the line starts -with a number or not (the options field never starts with a number). -The bits, exponent, modulus, and comment fields give the RSA key for -protocol version 1; the -comment field is not used for anything (but may be convenient for the +The options field is optional. +The supported key types are: +.Pp +.Bl -item -compact -offset indent +.It +sk-ecdsa-sha2-nistp256@openssh.com +.It +ecdsa-sha2-nistp256 +.It +ecdsa-sha2-nistp384 +.It +ecdsa-sha2-nistp521 +.It +sk-ssh-ed25519@openssh.com +.It +ssh-ed25519 +.It +ssh-dss +.It +ssh-rsa +.El +.Pp +The comment field is not used for anything (but may be convenient for the user to identify the key). -For protocol version 2 the keytype is -.Dq ecdsa-sha2-nistp256 , -.Dq ecdsa-sha2-nistp384 , -.Dq ecdsa-sha2-nistp521 , -.Dq ssh-ed25519 , -.Dq ssh-dss -or -.Dq ssh-rsa . .Pp -Note that lines in this file are usually several hundred bytes long +Note that lines in this file can be several hundred bytes long (because of the size of the public key encoding) up to a limit of -8 kilobytes, which permits DSA keys up to 8 kilobits and RSA -keys up to 16 kilobits. +8 kilobytes, which permits RSA keys up to 16 kilobits. You don't want to type them in; instead, copy the -.Pa identity.pub , .Pa id_dsa.pub , .Pa id_ecdsa.pub , +.Pa id_ecdsa_sk.pub , .Pa id_ed25519.pub , +.Pa id_ed25519_sk.pub , or the .Pa id_rsa.pub file and edit it. .Pp .Nm -enforces a minimum RSA key modulus size for protocol 1 -and protocol 2 keys of 768 bits. +enforces a minimum RSA key modulus size of 1024 bits. .Pp The options (if present) consist of comma-separated option specifications. @@ -523,6 +472,10 @@ No spaces are permitted, except within double quotes. The following option specifications are supported (note that option keywords are case-insensitive): .Bl -tag -width Ds +.It Cm agent-forwarding +Enable authentication agent forwarding previously disabled by the +.Cm restrict +option. .It Cm cert-authority Specifies that the listed key is a certification authority (CA) that is trusted to validate signed certificates for user authentication. @@ -540,19 +493,27 @@ If an 8-bit clean channel is required, one must not request a pty or should specify .Cm no-pty . A quote may be included in the command by quoting it with a backslash. +.Pp This option might be useful to restrict certain public keys to perform just a specific operation. An example might be a key that permits remote backups but nothing else. Note that the client may specify TCP and/or X11 -forwarding unless they are explicitly prohibited. +forwarding unless they are explicitly prohibited, e.g. using the +.Cm restrict +key option. +.Pp The command originally supplied by the client is available in the .Ev SSH_ORIGINAL_COMMAND environment variable. Note that this option applies to shell, command or subsystem execution. -Also note that this command may be superseded by either a +Also note that this command may be superseded by a .Xr sshd_config 5 .Cm ForceCommand -directive or a command embedded in a certificate. +directive. +.Pp +If a command is specified and a forced-command is embedded in a certificate +used for authentication, then the certificate will be accepted only if the +two commands are identical. .It Cm environment="NAME=value" Specifies that the string is to be added to the environment when logging in using this key. @@ -563,9 +524,10 @@ Environment processing is disabled by default and is controlled via the .Cm PermitUserEnvironment option. -This option is automatically disabled if -.Cm UseLogin -is enabled. +.It Cm expiry-time="timespec" +Specifies a time after which the key will not be accepted. +The time may be specified as a YYYYMMDD date or a YYYYMMDDHHMM[SS] time +in the system time-zone. .It Cm from="pattern-list" Specifies that in addition to public key authentication, either the canonical name of the remote host or its IP address must be present in the @@ -603,20 +565,50 @@ Disables execution of .It Cm no-X11-forwarding Forbids X11 forwarding when this key is used for authentication. Any X11 forward requests by the client will return an error. +.It Cm permitlisten="[host:]port" +Limit remote port forwarding with the +.Xr ssh 1 +.Fl R +option such that it may only listen on the specified host (optional) and port. +IPv6 addresses can be specified by enclosing the address in square brackets. +Multiple +.Cm permitlisten +options may be applied separated by commas. +Hostnames may include wildcards as described in the PATTERNS section in +.Xr ssh_config 5 . +A port specification of +.Cm * +matches any port. +Note that the setting of +.Cm GatewayPorts +may further restrict listen addresses. +Note that +.Xr ssh 1 +will send a hostname of +.Dq localhost +if a listen host was not specified when the forwarding was requested, and +that this name is treated differently to the explicit localhost addresses +.Dq 127.0.0.1 +and +.Dq ::1 . .It Cm permitopen="host:port" -Limit local port forwarding with +Limit local port forwarding with the .Xr ssh 1 .Fl L -such that it may only connect to the specified host and port. +option such that it may only connect to the specified host and port. IPv6 addresses can be specified by enclosing the address in square brackets. Multiple .Cm permitopen options may be applied separated by commas. -No pattern matching is performed on the specified hostnames, -they must be literal domains or addresses. +No pattern matching or name lookup is performed on the +specified hostnames, they must be literal host names and/or addresses. A port specification of .Cm * matches any port. +.It Cm port-forwarding +Enable port forwarding previously disabled by the +.Cm restrict +option. .It Cm principals="principals" On a .Cm cert-authority @@ -628,12 +620,40 @@ This option is ignored for keys that are not marked as trusted certificate signers using the .Cm cert-authority option. +.It Cm pty +Permits tty allocation previously disabled by the +.Cm restrict +option. +.It Cm no-touch-required +Do not require demonstration of user presence +for signatures made using this key. +This option only makes sense for the FIDO authenticator algorithms +.Cm ecdsa-sk +and +.Cm ed25519-sk . +.It Cm restrict +Enable all restrictions, i.e. disable port, agent and X11 forwarding, +as well as disabling PTY allocation +and execution of +.Pa ~/.ssh/rc . +If any future restriction capabilities are added to authorized_keys files +they will be included in this set. .It Cm tunnel="n" Force a .Xr tun 4 device on the server. Without this option, the next available device will be used if the client requests a tunnel. +.It Cm user-rc +Enables execution of +.Pa ~/.ssh/rc +previously disabled by the +.Cm restrict +option. +.It Cm X11-forwarding +Permits X11 forwarding previously disabled by the +.Cm restrict +option. .El .Pp An example authorized_keys file: @@ -642,12 +662,20 @@ An example authorized_keys file: ssh-rsa AAAAB3Nza...LiPk== user@example.net from="*.sales.example.net,!pc.sales.example.net" ssh-rsa AAAAB2...19Q== john@example.net -command="dump /home",no-pty,no-port-forwarding ssh-dss +command="dump /home",no-pty,no-port-forwarding ssh-rsa AAAAC3...51R== example.net -permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss +permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-rsa +AAAAB5...21S== +permitlisten="localhost:8080",permitopen="localhost:22000" ssh-rsa AAAAB5...21S== tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== jane@example.net +restrict,command="uptime" ssh-rsa AAAA1C8...32Tv== +user@example.net +restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5== +user@example.net +no-touch-required sk-ecdsa-sha2-nistp256@openssh.com AAAAInN...Ko== +user@example.net .Ed .Sh SSH_KNOWN_HOSTS FILE FORMAT The @@ -657,11 +685,11 @@ and files contain host public keys for all known hosts. The global file should be prepared by the administrator (optional), and the per-user file is -maintained automatically: whenever the user connects from an unknown host, +maintained automatically: whenever the user connects to an unknown host, its key is added to the per-user file. .Pp Each line in these files contains the following fields: markers (optional), -hostnames, bits, exponent, modulus, comment. +hostnames, keytype, base64-encoded key, comment. The fields are separated by spaces. .Pp The marker is optional, but if it is present then it must be one of @@ -678,9 +706,23 @@ Hostnames is a comma-separated list of patterns and .Ql \&? act as -wildcards); each pattern in turn is matched against the canonical host -name (when authenticating a client) or against the user-supplied -name (when authenticating a server). +wildcards); each pattern in turn is matched against the host name. +When +.Nm sshd +is authenticating a client, such as when using +.Cm HostbasedAuthentication , +this will be the canonical client host name. +When +.Xr ssh 1 +is authenticating a server, this will be the host name +given by the user, the value of the +.Xr ssh 1 +.Cm HostkeyAlias +if it was specified, or the canonical server hostname if the +.Xr ssh 1 +.Cm CanonicalizeHostname +option was used. +.Pp A pattern may also be preceded by .Ql \&! to indicate negation: if the host name matches a negated @@ -702,9 +744,9 @@ character. Only one hashed hostname may appear on a single line and none of the above negation or wildcard operators may be applied. .Pp -Bits, exponent, and modulus are taken directly from the RSA host key; they +The keytype and base64-encoded key are taken directly from the host key; they can be obtained, for example, from -.Pa /etc/ssh/ssh_host_key.pub . +.Pa /etc/ssh/ssh_host_rsa_key.pub . The optional comment field continues to the end of the line, and is not used. .Pp Lines starting with @@ -743,8 +785,8 @@ Note that the lines in these files are typically hundreds of characters long, and you definitely don't want to type in the host keys by hand. Rather, generate them by a script, .Xr ssh-keyscan 1 -or by taking -.Pa /etc/ssh/ssh_host_key.pub +or by taking, for example, +.Pa /etc/ssh/ssh_host_rsa_key.pub and adding the host names at the front. .Xr ssh-keygen 1 also offers some basic automated editing for @@ -857,9 +899,12 @@ This file is for host-based authentication (see It should only be writable by root. .Pp .It Pa /etc/moduli -Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". +Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange" +key exchange method. The file format is described in .Xr moduli 5 . +If no usable groups are found in this file then fixed internal groups will +be used. .Pp .It Pa /etc/motd See @@ -880,8 +925,6 @@ This file is used in exactly the same way as but allows host-based authentication without permitting login with rlogin/rsh. .Pp -.It Pa /etc/ssh/ssh_host_key -.It Pa /etc/ssh/ssh_host_dsa_key .It Pa /etc/ssh/ssh_host_ecdsa_key .It Pa /etc/ssh/ssh_host_ed25519_key .It Pa /etc/ssh/ssh_host_rsa_key @@ -892,8 +935,6 @@ Note that .Nm does not start if these files are group/world-accessible. .Pp -.It Pa /etc/ssh/ssh_host_key.pub -.It Pa /etc/ssh/ssh_host_dsa_key.pub .It Pa /etc/ssh/ssh_host_ecdsa_key.pub .It Pa /etc/ssh/ssh_host_ed25519_key.pub .It Pa /etc/ssh/ssh_host_rsa_key.pub |