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

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

[-- Attachment #1: Type: text/plain, Size: 3984 bytes --]

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.

-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.
> 
> 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