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Pro OpenSSH. Authors. Michael Stahnke. Book Configuring OpenSSH. Front Matter. Pages PDF · The File Structure of OpenSSH. Pages PDF. CHAPTER 1 Legacy Protocols: Why Replace Telnet, FTP, rsh, rcp, and rlogin with SSH? 3. Foundations of Information Security. 3. Analysis of Legacy Protocols . Pro Openssh extron pro series control product network ports and licenses - 1 pro series control product network ports and licenses this guide contains.
Compression Specifies whether compression is enabled after the user has authenticated successfully. The argument must be yes, delayed a legacy synonym for yes or no. DenyGroups This keyword can be followed by a list of group name patterns, separated by spaces. Login is disallowed for users whose primary group or supplementary group list matches one of the patterns. Only group names are valid; a numerical group ID is not recog- nized.
DenyUsers This keyword can be followed by a list of user name patterns, separated by spaces.
Login is disallowed for user names that match one of the patterns. Only user names are valid; a numeri- cal user ID is not recognized. This option overrides all other forwarding- related options and may simplify restricted configurations. ExposeAuthInfo Writes a temporary file containing a list of authentication meth- ods and public credentials e.
The default is no. FingerprintHash Specifies the hash algorithm used when logging key fingerprints. Valid options are: md5 and sha The default is sha The command is invoked by using the user's login shell with the -c option.
This applies to shell, command, or subsystem execution. It is most useful inside a Match block. Specifying a command of internal-sftp will force the use of an in-process SFTP server that requires no support files when used with ChrootDirectory. The default is none. GatewayPorts Specifies whether remote hosts are allowed to connect to ports forwarded for the client. By default, sshd 8 binds remote port forwardings to the loopback address.
This prevents other remote hosts from connecting to forwarded ports.
GatewayPorts can be used to specify that sshd should allow remote port forwardings to bind to non-loopback addresses, thus allowing other hosts to con- nect. The argument may be no to force remote port forwardings to be available to the local host only, yes to force remote port forwardings to bind to the wildcard address, or clientspecified to allow the client to select the address to which the forwarding is bound.
If set to yes then the client must authenticate against the host service on the current hostname. If set to no then the client may authenticate against any service key stored in the machine's default store. This facility is provided to assist with operation on multi homed machines. HostbasedAcceptedKeyTypes Specifies the key types that will be accepted for hostbased authentication as a list of comma-separated patterns. The default for this option is: ecdsa-sha2-nistpcert-v01 openssh.
A setting of yes means that sshd 8 uses the name supplied by the client rather than attempting to resolve the name from the TCP connection itself. HostCertificate Specifies a file containing a public host certificate. The cer- tificate's public key must match a private host key already spec- ified by HostKey.
The default behaviour of sshd 8 is not to load any certificates. It is possible to have multiple host key files. It is also pos- sible to specify public host key files instead. In this case operations on the private key will be delegated to an ssh-agent 1.
HostKeyAlgorithms Specifies the host key algorithms that the server offers. IgnoreRhosts Specifies that. Accepted values are af11, af12, af13, af21, af22, af23, af31, af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3, cs4, cs5, cs6, cs7, ef, lowdelay, throughput, reliability, a numeric value, or none to use the operating system default. This option may take one or two arguments, separated by whitespace. If one argu- ment is specified, it is used as the packet class uncondition- ally.
If two values are specified, the first is automatically selected for interactive sessions and the second for non-interac- tive sessions. The default is af21 Low-Latency Data for inter- active sessions and cs1 Lower Effort for non-interactive ses- sions. KbdInteractiveAuthentication Specifies whether to allow keyboard-interactive authentication.
The argument to this keyword must be yes or no. The default is to use whatever value ChallengeResponseAuthentication is set to by default yes. To use this option, the server needs a Kerberos servtab which allows the verification of the KDC's identity. KerberosTicketCleanup Specifies whether to automatically destroy the user's ticket cache file on logout. Multiple algorithms must be comma-separated.
The supported algorithms are: curvesha curvesha libssh. ListenAddress Specifies the local addresses sshd 8 should listen on. If port is not specified, sshd will listen on the address and all Port options specified. The default is to listen on all local addresses on the current default routing domain. Multiple ListenAddress options are per- mitted. For more information on routing domains, see rdomain 4.
LoginGraceTime The server disconnects after this time if the user has not suc- cessfully logged in. If the value is 0, there is no time limit.
The default is seconds. LogLevel Gives the verbosity level that is used when logging messages from sshd 8.
The default is INFO. The MAC algorithm is used for data integrity protection. The algorithms that contain "-etm" calculate the MAC after encryption encrypt-then-mac.
These are considered safer and their use recommended. The supported MACs are: hmac-md5 hmac-md hmac-sha1 hmac-sha hmac-sha hmac-sha umac openssh. Match Introduces a conditional block. If all of the criteria on the Match line are satisfied, the keywords on the following lines override those set in the global section of the config file, until either another Match line or the end of the file.
If a keyword appears in multiple Match blocks that are satisfied, only the first instance of the keyword is applied. The arguments to Match are one or more criteria-pattern pairs or the single token All which matches all criteria.
Note that the mask length pro- vided must be consistent with the address - it is an error to specify a mask length that is too long for the address or one with bits set in this host portion of the address. For example, Only a subset of keywords may be used on the lines following a Match keyword.
MaxAuthTries Specifies the maximum number of authentication attempts permitted per connection. Once the number of failures reaches half this value, additional failures are logged.
The default is 6. MaxSessions Specifies the maximum number of open shell, login or subsystem e. Multiple sessions may be established by clients that support connection multiplexing. Setting MaxSessions to 1 will effectively disable session multiplexing, whereas setting it to 0 will prevent all shell, login and subsystem sessions while still permitting for- warding. The default is MaxStartups Specifies the maximum number of concurrent unauthenticated con- nections to the SSH daemon.
Additional connections will be dropped until authentication succeeds or the LoginGraceTime expires for a connection. Alternatively, random early drop can be enabled by specifying the three colon separated values start:rate:full e. The probability increases linearly and all connec- tion attempts are refused if the number of unauthenticated con- nections reaches full PasswordAuthentication Specifies whether password authentication is allowed.
See also UsePAM. PermitEmptyPasswords When password authentication is allowed, it specifies whether the server allows login to accounts with empty password strings.
The listen specification must be one of the fol- lowing forms: PermitListen port PermitListen host:port Multiple permissions may be specified by separating them with whitespace. An argument of any can be used to remove all restrictions and permit any listen requests. An argument of none can be used to prohibit all listen requests. By default all port forwarding listen requests are permitted. Note that the GatewayPorts option may further restrict which addresses may be listened on.
An argument of any can be used to remove all restrictions and permit any forwarding requests. An argument of none can be used to prohibit all forwarding requests. By default all port forwarding requests are permitted. PermitRootLogin Specifies whether root can log in using ssh 1. The argument must be yes, prohibit-password, forced-commands-only, or no.
If this option is set to prohibit-password or its deprecated alias, without-password , password and keyboard-interactive authentication are disabled for root. If this option is set to forced-commands-only, root login with public key authentication will be allowed, but only if the command option has been specified which may be useful for taking remote backups even if root login is normally not allowed.
All other authentication methods are disabled for root. If this option is set to no, root is not allowed to log in. PermitTTY Specifies whether pty 4 allocation is permitted. PermitTunnel Specifies whether tun 4 device forwarding is allowed. The argu- ment must be yes, point-to-point layer 3 , ethernet layer 2 , or no.
Specifying yes permits both point-to-point and ethernet. Independent of this setting, the permissions of the selected tun 4 device must allow access to the user. Port Specifies the port number that sshd 8 listens on. Multiple options of this type are permitted. See also ListenAddress. PrintLastLog Specifies whether sshd 8 should print the date and time of the last user login when a user logs in interactively. PubkeyAcceptedKeyTypes Specifies the key types that will be accepted for public key authentication as a list of comma-separated patterns.
PubkeyAuthentication Specifies whether public key authentication is allowed.
RekeyLimit Specifies the maximum amount of data that may be transmitted before the session key is renegotiated, optionally followed a maximum amount of time that may pass before the session key is renegotiated. The default value for RekeyLimit is default none, which means that rekeying is per- formed after the cipher's default amount of data has been sent or received and no time based rekeying is done. RevokedKeys Specifies revoked public keys file, or none to not use one.
Keys listed in this file will be refused for public key authentica- tion. Note that if this file is not readable, then public key authentication will be refused for all users. RDomain Specifies an explicit routing domain that is applied after authentication has completed. The user session, as well and any forwarded or listening IP sockets, will be bound to this rdomain 4.
The environment value may be quoted e. Environment variables set by SetEnv override the default environ- ment and any variables specified by the user via AcceptEnv or PermitUserEnvironment. StreamLocalBindMask Sets the octal file creation mode mask umask used when creating a Unix-domain socket file for local or remote port forwarding.
This option is only used for port forwarding to a Unix-domain socket file. The default value is , which creates a Unix-domain socket file that is readable and writable only by the owner. Note that not all operating systems honor the file mode on Unix-domain socket files.
StreamLocalBindUnlink Specifies whether to remove an existing Unix-domain socket file for local or remote port forwarding before creating a new one. If the socket file already exists and StreamLocalBindUnlink is not enabled, sshd will be unable to forward the port to the Unix- domain socket file.
Better security, for example, comes through Diffie—Hellman key exchange and strong integrity checking via message authentication codes. Version 1. From this version, a "portability" branch was formed to port OpenSSH to other operating systems. OSSH meanwhile has become obsolete.
Uses[ edit ] Example of tunneling an X11 application over SSH: the user 'josh' has SSHed from the local machine 'foofighter' to the remote machine 'tengwar' to run xeyes. Some of the applications below may require features that are only available or compatible with specific SSH clients or servers.
For login to a shell on a remote host replacing Telnet and rlogin For executing a single command on a remote host replacing rsh For setting up automatic passwordless login to a remote server for example, using OpenSSH  In combination with rsync to back up, copy and mirror files efficiently and securely For forwarding or tunneling a port not to be confused with a VPN , which routes packets between different networks, or bridges two broadcast domains into one.
For using as a full-fledged encrypted VPN. Note that only OpenSSH server and client supports this feature. For forwarding X from a remote host possible through multiple intermediate hosts For browsing the web through an encrypted proxy connection with SSH clients that support the SOCKS protocol. For securely mounting a directory on a remote server as a filesystem on a local computer using SSHFS. For automated remote monitoring and management of servers through one or more of the mechanisms discussed above.
For development on a mobile or embedded device that supports SSH. For securing file transfer protocols. File transfer protocols[ edit ] The Secure Shell protocols are used in several file transfer mechanisms. Generally runs over an SSH connection.
Files transferred over shell protocol a. Diagram of the SSH-2 binary packet. This layer handles initial key exchange as well as server authentication, and sets up encryption, compression and integrity verification.
It exposes to the upper layer an interface for sending and receiving plaintext packets with sizes of up to 32, bytes each more can be allowed by the implementation. The transport layer also arranges for key re-exchange, usually after 1 GB of data has been transferred or after 1 hour has passed, whichever occurs first.
The user authentication layer RFC This layer handles client authentication and provides a number of authentication methods. Authentication is client-driven: when one is prompted for a password, it may be the SSH client prompting, not the server. The server merely responds to the client's authentication requests. Widely used user-authentication methods include the following: password: a method for straightforward password authentication, including a facility allowing a password to be changed.
Not all programs implement this method. Used by some OpenSSH configurations when PAM is the underlying host-authentication provider to effectively provide password authentication, sometimes leading to inability to log in with a client that supports just the plain password authentication method.
The connection layer RFC This layer defines the concept of channels, channel requests and global requests using which SSH services are provided. A single SSH connection can host multiple channels simultaneously, each transferring data in both directions.Joux, G. Hiltgen, S.
Bellare, T. The argument to this keyword must be yes or no. Only one optional stream decrypted. In the second anticipated, and did not influence the behaviour of the server phase, the attacker can exploit his knowledge of these 14 in our attack in any other way. If set to no then the client may authenticate against any service key stored in the machine's default store.
In all versions of SSH it is important to verify unknown public keys , i.
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