From mboxrd@z Thu Jan  1 00:00:00 1970
From: Michael Tremer <michael.tremer@ipfire.org>
To: development@lists.ipfire.org
Subject: Re: [PATCH 3/3] SSH: do not send spoofable TCP keep alive messages
Date: Tue, 19 Apr 2022 11:17:42 +0100
Message-ID: <B85C088F-5ADB-4C1A-8EE7-3F953AB0663E@ipfire.org>
In-Reply-To: <09688ff4-d262-d136-0d1f-9102732a5e0d@ipfire.org>
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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 channel=
s, 60 seconds is not a long time. There cannot be any security reason for kee=
ping 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 M=C3=BCller <peter.mueller(a)ipfire.org> wr=
ote:
>=20
> 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).
>=20
> 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.
>=20
> 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 =3D 300 seconds) will be used for both
> client and server configuration, as 60 seconds were found to be too
> short for unstable connectivity scenarios.
>=20
> Signed-off-by: Peter M=C3=BCller <peter.mueller(a)ipfire.org>
> ---
> config/ssh/ssh_config  | 12 ++++++++----
> config/ssh/sshd_config |  8 +++++---
> 2 files changed, 13 insertions(+), 7 deletions(-)
>=20
> 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 @@
>=20
> # 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 vulner=
able
>         UseRoaming no
>=20
>         # Only use secure crypto algorithms
> @@ -13,15 +13,19 @@ Host *
>         Ciphers chacha20-poly1305(a)openssh.com,aes256-gcm(a)openssh.com,ae=
s128-gcm(a)openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
>         MACs hmac-sha2-512-etm(a)openssh.com,hmac-sha2-256-etm(a)openssh.co=
m,umac-128-etm(a)openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128(a)openssh.c=
om
>=20
> -        # Always visualise server host keys (but helps to identify key bas=
ed MITM attacks)
> +        # Always visualise server host keys (helps to identify key based M=
ITM attacks)
>         VisualHostKey yes
>=20
>         # Use SSHFP (might work on some up-to-date networks) to look up hos=
t keys
>         VerifyHostKeyDNS yes
>=20
> -        # send keep-alive messages to connected server to avoid broken con=
nections
> +        # 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 S=
SH-based
> +	# keep alive messages enabled; there is no need to do things twice here
> +	TCPKeepAlive no
>=20
>         # 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
>=20
> -# 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
>=20
> -# 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 a=
lready,
> +# there is no need for this to be enabled as well
> +TCPKeepAlive no
>=20
> # Add support for SFTP
> Subsystem	sftp	/usr/lib/openssh/sftp-server
> --=20
> 2.34.1


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