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

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

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Hello Michael,

thank you for your reply.

Context-based, I guess you meant "something more useful", didn't you? :-)

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 ;-) ).

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
>