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Bug 1952411 - Sshd randomly drops client connections when a RekeyLimit timeout is specified and ClientAliveCountMax is zero
Summary: Sshd randomly drops client connections when a RekeyLimit timeout is specified...
Keywords:
Status: CLOSED WONTFIX
Alias: None
Product: Red Hat Enterprise Linux 8
Classification: Red Hat
Component: openssh
Version: 8.3
Hardware: All
OS: Linux
medium
medium
Target Milestone: beta
: ---
Assignee: Dmitry Belyavskiy
QA Contact: BaseOS QE Security Team
URL:
Whiteboard:
Depends On:
Blocks:
TreeView+ depends on / blocked
 
Reported: 2021-04-22 08:57 UTC by Eric Negaard
Modified: 2024-12-20 19:56 UTC (History)
9 users (show)

Fixed In Version:
Doc Type: If docs needed, set a value
Doc Text:
Clone Of:
Environment:
Last Closed: 2022-08-29 13:43:03 UTC
Type: Bug
Target Upstream Version:
Embargoed:
pm-rhel: mirror+

Attachments (Terms of Use)


Links
System ID Private Priority Status Summary Last Updated
Red Hat Bugzilla 1686065 1 low CLOSED SSH connections get closed when time-based rekeyring is used and ClientAliveMaxCount=0 2024-12-20 18:48:55 UTC

Description Eric Negaard 2021-04-22 08:57:43 UTC 
Description of problem:

When both a RekeyLimit timeout is specified and ClientAliveCountMax is set to zero, sshd disconnects sessions even when input has been detected from the client within the specified ClientAliveInterval.

Version-Release number of selected component (if applicable):

8.0p1

How reproducible:

Always

Steps to Reproduce:
1. Edit /etc/ssh/sshd_config, set RekeyLimit to "512M 120", ClientAliveCountMax to 0 and ClientAliveInterval to 60
2. Restart sshd
3. Create a connection to the server via ssh, and type "date" every 10 seconds

Actual results:

The server disconnects the client session even though input was received from the client within the last ClientAliveInterval  seconds. In the example below, the client produced input at 8:39:25 UTC and the server disconnected the session 15 seconds later at 8:39:40 UTC even though ClientAliveInterval is set to 60.

Client side:

...
$ date
Thu Apr 22 08:39:02 UTC 2021
$ date
Thu Apr 22 08:39:19 UTC 2021
$ date
Thu Apr 22 08:39:25 UTC 2021
$ Connection to SERVERNAME closed by remote host.
Connection to SERVERNAME closed.

Server side:

Apr 22 08:39:40 SERVERNAME sshd[1428458]: Timeout, client not responding from user USERNAME CLIENT-IP port 37998

Expected results:

It is expected that the server allows the connection to remain open as long as input is received from the client every ClientAliveInterval seconds.

Additional info:

This is a re-open of Bug #1686065 because the fix for that bug appears to be incomplete.
Comment 3 Dmitry Belyavskiy 2021-04-23 12:35:19 UTC 
Would you mind to test the proposed patch?
Comment 6 Alexander Sosedkin 2021-04-27 09:20:18 UTC 
Note: the meaning of setting ClientAliveCountMax to 0 used to be undefined.
In newer openssh versions it disables connection termination [1] altogether.

The documentation suggests that the disconnect timeout on inactivity
is a product of ClientAliveCountMax and ClientAliveInterval.

[1] https://github.com/openssh/openssh-portable/commit/69334996ae203c51c70bf01d414c918a44618f8e
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