Channel inactivity timeouts for sshd(8) #16
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djmdjm
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mfriedl
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Jan 2, 2023
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looks fine, one comment inline |
This API simplified poll()/ppoll() timeout calculation by remembering the next requested timeout and returning it in forms that can be used by either poll() or ppoll(). This gets the fiddly seconds/milliseconds/timespec math out of the mainloops, and makes adding more timeouts easier.
This will forcibly close an open channel by simulating read/write errors, draining the IO buffers and calling the detach function. Previously the detach function was only ever called during channel garbage collection, but there was no way to signal the user of a channel (e.g. session.c) that its channel was being closed deliberately (vs. by the usual state-machine logic). So this adds an extra "force" argument to the channel cleanup callback to indicate this condition.
These are now used by sshd_config:ChannelTimeouts to specify timeouts by channel type, so force them all to use a similar format without whitespace.
This sets an "extended" channel type after channel creation (e.g. "session:subsystem:sftp") that will be used for setting channel inactivity timeouts.
This adds a sshd_config ChannelTimeouts directive that allows channels that have not seen traffic in a configurable interval to be automatically closed. Different timeouts may be applied to session, X11, agent and TCP forwarding channels.
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These patches implement a channel inactivity timeout mechanism for
sshd, that allows different channel types to have different timeouts. Here is an excerpt from thesshd_config.5manpage:ChannelTimeouts
Specifies whether and how quickly sshd(8) should close inactive channels. Timeouts for specified as one or more
"type=interval"pairs separated by whitespace, where the"type"must be a channel type name (as described in the table below), optionally containing wildcard characters."interval"must be a value specified in seconds and may use any of the units documented in the TIME FORMATS section. For example,"session:*=5m"would cause all sessions to terminate after five minutes of inactivity.The available channel types include:
agent-connectionOpen connections to ssh-agent(1).
direct-tcpipdirect-streamlocal@openssh.comOpen TCP or Unix socket (respectively) connections that have been established from a ssh(1) local forwarding, i.e. LocalForward or DynamicForward.
forwarded-tcpipforwarded-streamlocal@openssh.comOpen TCP or Unix socket (respectively) connections that have been established to a sshd(8) listening on behalf of a ssh(1) remote forwarding, i.e. RemoteForward.
session:commandCommand execution sessions.
session:shellInteractive shell sessions.
session:subsystem:...Subsystem sessions, e.g. for sftp(1), which could be identified as
session:subsystem:sftp.x11-connectionOpen X11 forwarding sessions.
Note that, in all the above cases, terminating an inactive session does not guarantee to remove all resources associated with the session, e.g. shell processes or X11 clients relating to the session may continue to execute.
Moreover, terminating an inactive session does not prevent the client from requesting another session of the same type. In particular, expiring an inactive forwarding session does not prevent another identical forwarding from being subsequently created.
The default is not to expire channels of any type for inactivity.