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SSH

Defining Subconfigurations

Subconfiguration files can be used to specify configuration options that apply only to connections by specific users or from specific hosts. The subconfiguration files have the same basic format as the main configuration file and they are divided into two categories: host-specific and user-specific.

If parsing the subconfiguration files fails, the connection is terminated (for a host-specific configuration), or the access is denied by the server (for a user-specific configuration).

Most of the configuration options that work in the main file work also in these, but some do not, where it either does not make sense to set them (for example, ListenAddress and Port, which only affect the process listening to the port, and would not affect that behavior in any way in a subconfiguration file) or if it would be confusing (e.g. AllowUsers in a user-specific subconfiguration, and AllowHosts in a host-specific subconfiguration.).

The value for {Host,User}SpecificConfig keywords is a pattern-filename pair, separated by whitespace. With UserSpecificConfig, the pattern is of format user[%group][@host], where user is matched with the user name and UID, group is matched with the user's primary and any secondary groups, both group name and GID,and host is matched as described under option AllowHosts. With HostSpecificConfig, the pattern is host (as in UserSpecificConfig).

Unlike the main configuration file, the subconfiguration files may have configuration blocks, or stanzas, in them. The subconfiguration heading is interpreted identically to what is described above, that is with UserSpecificConfig the pattern is of format user[%group][@host], and with HostSpecificConfig the format is host.

[Note]Note

It is possible to mix these configuration files. This is not recommended, because any global settings in these files would be set multiple times (which would not do any harm per se, but might lead to behavior not intended by the administrator).

Subconfiguration files are very flexible and because of that, dangerous if the logic of the files is not carefully planned. You can, for example, specify different authentication methods for different users and different banner messages for people coming from certain hosts. There are a lot of possibilities here.

[Note]Note

Host-specific subconfiguration files are always read before the user-specific subconfiguration files. See the example file /opt/tectia/etc/sshd2_config.example and the host-specific and user-specific files in /opt/tectia/etc/subconfig.

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