[PATCH 1/3] OpenSSH: Update to 9.0p1

[PATCH 1/3] OpenSSH: Update to 9.0p1

[Peter Müller]

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Relevant changelog part, as retrieved from https://www.openssh.com/txt/release-9.0:

Changes since OpenSSH 8.9
=========================

This release is focused on bug fixing.

Potentially-incompatible changes
--------------------------------

This release switches scp(1) from using the legacy scp/rcp protocol
to using the SFTP protocol by default.

Legacy scp/rcp performs wildcard expansion of remote filenames (e.g.
"scp host:* .") through the remote shell. This has the side effect of
requiring double quoting of shell meta-characters in file names
included on scp(1) command-lines, otherwise they could be interpreted
as shell commands on the remote side.

This creates one area of potential incompatibility: scp(1) when using
the SFTP protocol no longer requires this finicky and brittle quoting,
and attempts to use it may cause transfers to fail. We consider the
removal of the need for double-quoting shell characters in file names
to be a benefit and do not intend to introduce bug-compatibility for
legacy scp/rcp in scp(1) when using the SFTP protocol.

Another area of potential incompatibility relates to the use of remote
paths relative to other user's home directories, for example -
"scp host:~user/file /tmp". The SFTP protocol has no native way to
expand a ~user path. However, sftp-server(8) in OpenSSH 8.7 and later
support a protocol extension "expand-path(a)openssh.com" to support
this.

In case of incompatibility, the scp(1) client may be instructed to use
the legacy scp/rcp using the -O flag.

New features
------------

 * ssh(1), sshd(8): use the hybrid Streamlined NTRU Prime + x25519 key
   exchange method by default ("sntrup761x25519-sha512(a)openssh.com").
   The NTRU algorithm is believed to resist attacks enabled by future
   quantum computers and is paired with the X25519 ECDH key exchange
   (the previous default) as a backstop against any weaknesses in
   NTRU Prime that may be discovered in the future. The combination
   ensures that the hybrid exchange offers at least as good security
   as the status quo.

   We are making this change now (i.e. ahead of cryptographically-
   relevant quantum computers) to prevent "capture now, decrypt
   later" attacks where an adversary who can record and store SSH
   session ciphertext would be able to decrypt it once a sufficiently
   advanced quantum computer is available.

 * sftp-server(8): support the "copy-data" extension to allow server-
   side copying of files/data, following the design in
   draft-ietf-secsh-filexfer-extensions-00. bz2948

 * sftp(1): add a "cp" command to allow the sftp client to perform
   server-side file copies.

Bugfixes
--------

 * ssh(1), sshd(8): upstream: fix poll(2) spin when a channel's output
   fd closes without data in the channel buffer. bz3405 and bz3411

 * sshd(8): pack pollfd array in server listen/accept loop. Could
   cause the server to hang/spin when MaxStartups > RLIMIT_NOFILE

 * ssh-keygen(1): avoid NULL deref via the find-principals and
   check-novalidate operations. bz3409 and GHPR#307 respectively.

 * scp(1): fix a memory leak in argument processing. bz3404

 * sshd(8): don't try to resolve ListenAddress directives in the sshd
   re-exec path. They are unused after re-exec and parsing errors
   (possible for example if the host's network configuration changed)
   could prevent connections from being accepted.

 * sshd(8): when refusing a public key authentication request from a
   client for using an unapproved or unsupported signature algorithm
   include the algorithm name in the log message to make debugging
   easier.

Portability
-----------

 * sshd(8): refactor platform-specific locked account check, fixing
   an incorrect free() on platforms with both libiaf and shadow
   passwords (probably only Unixware) GHPR#284,

 * ssh(1), sshd(8): Fix possible integer underflow in scan_scaled(3)
   parsing of K/M/G/etc quantities. bz#3401.

 * sshd(8): provide killpg implementation (mostly for Tandem NonStop)
   GHPR#301.

 * Check for missing ftruncate prototype. GHPR#301

 * sshd(8): default to not using sandbox when cross compiling. On most
   systems poll(2) does not work when the number of FDs is reduced
   with setrlimit, so assume it doesn't when cross compiling and we
   can't run the test.  bz#3398.

 * sshd(8): allow ppoll_time64 in seccomp sandbox. Should fix sandbox
   violations on some (at least i386 and armhf) 32bit Linux platforms.
   bz#3396.

 * Improve detection of -fzero-call-used-regs=all support in
   configure script.

Signed-off-by: Peter Müller <peter.mueller(a)ipfire.org>
---
 lfs/openssh | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/lfs/openssh b/lfs/openssh
index 8c958e9fe..302ae6041 100644
--- a/lfs/openssh
+++ b/lfs/openssh
@@ -1,7 +1,7 @@
 ###############################################################################
 #                                                                             #
 # IPFire.org - A linux based firewall                                         #
-# Copyright (C) 2007-2020  IPFire Team  <info(a)ipfire.org>                     #
+# Copyright (C) 2007-2022  IPFire Team  <info(a)ipfire.org>                     #
 #                                                                             #
 # This program is free software: you can redistribute it and/or modify        #
 # it under the terms of the GNU General Public License as published by        #
@@ -24,7 +24,7 @@
 
 include Config
 
-VER        = 8.9p1
+VER        = 9.0p1
 
 THISAPP    = openssh-$(VER)
 DL_FILE    = $(THISAPP).tar.gz
@@ -40,7 +40,7 @@ objects = $(DL_FILE)
 
 $(DL_FILE) = $(DL_FROM)/$(DL_FILE)
 
-$(DL_FILE)_BLAKE2 = 02934da7f7a2954141888e63e81e38fad4fb8558ddd1032de44f69684802c62771fdd7e9e470e0715059635999c8f9d2ab95f6351217e236573ead83a867f59b
+$(DL_FILE)_BLAKE2 = 49724a400951964d659d136908657940f79e150056728cc4dadf8ff8652a832f7fd46eebb47b15085e57fca4b00c77d1ec4dd1b056ea2bbcee89f54a121ed5e2
 
 install : $(TARGET)
 
-- 
2.34.1

[PATCH 2/3] SSH: Add sntrup761x25519-sha512@openssh.com key exchange to configurations

[Peter Müller]

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This algorithm was introduced in OpenSSH 9.0p1; also, align the
curve25519-sha256* key exchanges to keep things tidy.

Signed-off-by: Peter Müller <peter.mueller(a)ipfire.org>
---
 config/ssh/ssh_config  | 2 +-
 config/ssh/sshd_config | 5 ++---
 2 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/config/ssh/ssh_config b/config/ssh/ssh_config
index 2e2ee60c3..ee0954d5c 100644
--- a/config/ssh/ssh_config
+++ b/config/ssh/ssh_config
@@ -9,7 +9,7 @@ Host *
         UseRoaming no
 
         # Only use secure crypto algorithms
-        KexAlgorithms curve25519-sha256(a)libssh.org,diffie-hellman-group-exchange-sha256
+        KexAlgorithms sntrup761x25519-sha512(a)openssh.com,curve25519-sha256,curve25519-sha256(a)libssh.org,diffie-hellman-group-exchange-sha256
         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
 
diff --git a/config/ssh/sshd_config b/config/ssh/sshd_config
index bea5cee53..456556540 100644
--- a/config/ssh/sshd_config
+++ b/config/ssh/sshd_config
@@ -19,9 +19,8 @@ LoginGraceTime 30s
 # Limit maximum instanctes to prevent DoS
 MaxStartups 5
 
-# Only allow safe crypto algorithms (may break some _very_ outdated clients)
-# See also: https://stribika.github.io/2015/01/04/secure-secure-shell.html
-KexAlgorithms curve25519-sha256(a)libssh.org,diffie-hellman-group-exchange-sha256
+# Only allow safe crypto algorithms
+KexAlgorithms sntrup761x25519-sha512(a)openssh.com,curve25519-sha256,curve25519-sha256(a)libssh.org,diffie-hellman-group-exchange-sha256
 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
 
-- 
2.34.1

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

[Peter Müller]

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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

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

[Michael Tremer]

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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 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

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

[Peter Müller]

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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
> 

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

[Michael Tremer]

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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