Re: [PATCH] SSH: do not send spoofable TCP keep alive messages

["Peter Müller" <peter.mueller@ipfire.org>]

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Hello Michael,

thanks for your reply.

Which timeout value would you suggest then?

Thanks, and best regards,
Peter Müller

> Hi,
> 
>> On 2 Apr 2021, at 20:27, Peter Müller <peter.mueller(a)ipfire.org> wrote:
>>
>> Hello Michael,
>>
>> thank you for your reply.
>>
>> Context-based, I guess you meant "something more useful", didn't you? :-)
> 
> Seems so. I struggle a lot with auto-correct.
> 
>> Well, if you like, we can leave 60 seconds here, but I would not go for a much
>> longer timeout. If a network issue takes longer than a minute, requiring a re-login
>> looks reasonable to me (it does for 30 seconds also, but hey ;-) ).
> 
> No, it kills whatever I am running and a 60 second break happens very quickly with a DSL reconnect or rebooting an access point somewhere. Why is that supposed to break the SSH session, too?
> 
>>
>> Thanks, and best regards,
>> Peter Müller
>>
>>
>>> Hello,
>>>
>>>> On 1 Feb 2021, at 18:06, Peter Müller <peter.mueller(a)ipfire.org> wrote:
>>>>
>>>> By default, both SSH server and client rely on TCP-based keep alive
>>>> messages to detect broken sessions, which can be spoofed rather easily
>>>> in order to keep a broken session opened (and vice versa).
>>>>
>>>> Since we rely on SSH-based keep alive messages, which are not vulnerable
>>>> to this kind of tampering, there is no need to double-check connections
>>>> via TCP keep alive as well.
>>>>
>>>> This patch thereof disables using TCP keep alive for both SSH client and
>>>> server scenario. Further, {Client,Server}AliveCountMax default to 3,
>>>> which is sufficient (3 * 10 sec. = broken SSH connections die after 30
>>>> seconds), so we can omit that option. 60 seconds won't have any
>>>> advantage here.
>>>
>>> Is there any considerable downside of increasing this to something more useless?
>>>
>>> I constantly run into broken SSH sessions because of smaller network hiccups (WiFi, VPNs, my crappy ISP, etc.). It would be useful to hold the connection for a little bit longer so that I can spend more time on fixing stuff instead of logging back in.
>>>
>>> -Michael
>>>
>>>>
>>>> Signed-off-by: Peter Müller <peter.mueller(a)ipfire.org>
>>>> ---
>>>> config/ssh/ssh_config  | 11 +++++++----
>>>> config/ssh/sshd_config |  7 ++++---
>>>> 2 files changed, 11 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/config/ssh/ssh_config b/config/ssh/ssh_config
>>>> index 2e2ee60c3..ab0967086 100644
>>>> --- a/config/ssh/ssh_config
>>>> +++ b/config/ssh/ssh_config
>>>> @@ -5,7 +5,7 @@
>>>>
>>>> # Set some basic hardening options for all connections
>>>> Host *
>>>> -        # Disable Roaming as it is known to be vulnerable
>>>> +        # Disable undocumented roaming feature as it is known to be vulnerable
>>>>        UseRoaming no
>>>>
>>>>        # Only use secure crypto algorithms
>>>> @@ -13,15 +13,18 @@ Host *
>>>>        Ciphers chacha20-poly1305(a)openssh.com,aes256-gcm(a)openssh.com,aes128-gcm(a)openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
>>>>        MACs hmac-sha2-512-etm(a)openssh.com,hmac-sha2-256-etm(a)openssh.com,umac-128-etm(a)openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128(a)openssh.com
>>>>
>>>> -        # Always visualise server host keys (but helps to identify key based MITM attacks)
>>>> +        # Always visualise server host keys (helps to identify key based MITM attacks)
>>>>        VisualHostKey yes
>>>>
>>>>        # Use SSHFP (might work on some up-to-date networks) to look up host keys
>>>>        VerifyHostKeyDNS yes
>>>>
>>>> -        # send keep-alive messages to connected server to avoid broken connections
>>>> +        # Send SSH-based keep alive messages to connected server to avoid broken connections
>>>>        ServerAliveInterval 10
>>>> -        ServerAliveCountMax 6
>>>> +
>>>> +	# Disable TCP keep alive messages since they can be spoofed and we have SSH-based
>>>> +	# keep alive messages enabled; there is no need to do things twice here
>>>> +	TCPKeepAlive no
>>>>
>>>>        # Ensure only allowed authentication methods are used
>>>>        PreferredAuthentications publickey,keyboard-interactive,password
>>>> diff --git a/config/ssh/sshd_config b/config/ssh/sshd_config
>>>> index bea5cee53..a9eb5ff14 100644
>>>> --- a/config/ssh/sshd_config
>>>> +++ b/config/ssh/sshd_config
>>>> @@ -47,11 +47,12 @@ AllowTcpForwarding no
>>>> AllowAgentForwarding no
>>>> PermitOpen none
>>>>
>>>> -# Detect broken sessions by sending keep-alive messages to clients via SSH connection
>>>> +# Send SSH-based keep alive messages every 10 seconds
>>>> ClientAliveInterval 10
>>>>
>>>> -# Close unresponsive SSH sessions which fail to answer keep-alive
>>>> -ClientAliveCountMax 6
>>>> +# Since TCP keep alive messages can be spoofed and we have the SSH-based already,
>>>> +# there is no need for this to be enabled as well
>>>> +TCPKeepAlive no
>>>>
>>>> # Add support for SFTP
>>>> Subsystem	sftp	/usr/lib/openssh/sftp-server
>>>> -- 
>>>> 2.26.2
>>>
>