Re: [PATCH 3/3] SSH: do not send spoofable TCP keep alive messages

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

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Oh, maybe I misread your first email. Sorry. 5 minutes > 60s. Cool.

> On 19 Apr 2022, at 11:40, Peter Müller <peter.mueller(a)ipfire.org> wrote:
> 
> Hello Michael,
> 
> thanks for your reply.
> 
>> Hello,
>> 
>> Thanks for this.
>> 
>> I would personally like a longer timeout than 60 seconds.
>> 
>> If a DSL modem loses sync, or DFS kicks in and the WiFi has to change channels, 60 seconds is not a long time. There cannot be any security reason for keeping it that low, so I would like to ask if there is any other reason that I missed.
> 
> Um, actually, this patch features a timeout of five minutes (10 seconds * 30 keep-alive's = 300
> seconds = 5 minutes) before a dangling SSH connection is being terminated. Or did I misunderstand
> you?
> 
> Thanks, and best regards,
> Peter Müller
> 
>> 
>> -Michael
>> 
>>> On 18 Apr 2022, at 21:40, 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. For usability reasons, a timeout of 5 minutes (10
>>> seconds * 30 keep alive messages = 300 seconds) will be used for both
>>> client and server configuration, as 60 seconds were found to be too
>>> short for unstable connectivity scenarios.
> 
> This was precisely your concern about the first attempt of this patch, which
> is why I raised this to 300 seconds instead of 60.
> 
>>> 
>>> Signed-off-by: Peter Müller <peter.mueller(a)ipfire.org>
>>> ---
>>> config/ssh/ssh_config | 12 ++++++++----
>>> config/ssh/sshd_config | 8 +++++---
>>> 2 files changed, 13 insertions(+), 7 deletions(-)
>>> 
>>> diff --git a/config/ssh/ssh_config b/config/ssh/ssh_config
>>> index ee0954d5c..85c069dda 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 +13,19 @@ 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
>>> + ServerAliveCountMax 30
>>> +
>>> +	# 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 456556540..76c9b3eb1 100644
>>> --- a/config/ssh/sshd_config
>>> +++ b/config/ssh/sshd_config
>>> @@ -46,11 +46,13 @@ 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 to connected clients to avoid broken connections
>>> ClientAliveInterval 10
>>> +ClientAliveCountMax 30
>>> 
>>> -# 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
>>> -- 
>>> 2.34.1