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

[Michael Tremer <michael.tremer@ipfire.org>]

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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
>>