[1/2] sshd_config: Do not set defaults explicitly

  In order to keep configurations as small as possible and to make them
easier to read/audit, this patch omits all default configuration in the
OpenSSH server configuration file.

Further, it mentions where to refer for the full documentation.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
---
 config/ssh/sshd_config | 62 ++++++++++++++++----------------------------------
 1 file changed, 20 insertions(+), 42 deletions(-)
  

Hello,

I am not 100% sure if I like this change.

Simply because of defaults changing in software. Although we generally want to follow upstream this might break installations.

Do we not see this as a danger?

I would prefer to have shorter configurations, but not breaking production is more important.

-Michael

> On 20 Jan 2020, at 20:04, Peter Müller <peter.mueller@ipfire.org> wrote:
> 
> In order to keep configurations as small as possible and to make them
> easier to read/audit, this patch omits all default configuration in the
> OpenSSH server configuration file.
> 
> Further, it mentions where to refer for the full documentation.
> 
> Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
> ---
> config/ssh/sshd_config | 62 ++++++++++++++++----------------------------------
> 1 file changed, 20 insertions(+), 42 deletions(-)
> 
> diff --git a/config/ssh/sshd_config b/config/ssh/sshd_config
> index a248c4906..bea5cee53 100644
> --- a/config/ssh/sshd_config
> +++ b/config/ssh/sshd_config
> @@ -1,81 +1,59 @@
> -# ultra-secure OpenSSH server configuration
> +# OpenSSH server configuration file for IPFire
> +#
> +# The full documentation is available at: https://man.openbsd.org/sshd_config
> +#
> 
> -# only allow version 2 of SSH protocol
> +# Only allow version 2 of SSH protocol
> Protocol 2
> 
> -# listen on port 22 by default
> +# Listen on port 22 by default
> Port 22
> 
> -# listen on these interfaces and protocols
> -AddressFamily any
> +# Listen on every interface and IPv4 only
> +AddressFamily inet
> ListenAddress 0.0.0.0
> 
> -# limit authentication thresholds
> +# Limit authentication timeout to 30 seconds
> LoginGraceTime 30s
> -MaxAuthTries 6
> 
> -# limit maximum instanctes to prevent DoS
> +# Limit maximum instanctes to prevent DoS
> MaxStartups 5
> 
> -# ensure proper logging
> -SyslogFacility AUTH
> -LogLevel INFO
> -
> -# enforce permission checks before a login is accepted
> -# (prevents damage because of hacked systems with world-writeable
> -# home directories or similar)
> -StrictModes yes
> -
> -# only allow safe crypto algorithms (may break some _very_ outdated clients)
> -# see also: https://stribika.github.io/2015/01/04/secure-secure-shell.html
> +# 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@libssh.org,diffie-hellman-group-exchange-sha256
> Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
> MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com
> 
> -# enable data compression after successful login only
> -Compression delayed
> -
> -# only allow cryptographically safe SSH host keys (adjust paths if needed)
> +# Only allow cryptographically safe SSH host keys (adjust paths if needed)
> HostKey /etc/ssh/ssh_host_ed25519_key
> HostKey /etc/ssh/ssh_host_ecdsa_key
> HostKey /etc/ssh/ssh_host_rsa_key
> 
> -# only allow login via public key by default
> +# Only allow login via public key by default
> PubkeyAuthentication yes
> PasswordAuthentication no
> ChallengeResponseAuthentication no
> -PermitEmptyPasswords no
> 
> -# permit root login as there is no other user in IPFire 2.x
> +# Permit root login as there is no other user in IPFire 2.x
> PermitRootLogin yes
> 
> -# ignore user ~/.rhost* files
> -IgnoreRhosts yes
> -
> -# ignore user known hosts file
> +# Ignore user ~/.ssh/known_hosts file
> IgnoreUserKnownHosts yes
> 
> -# ignore user environments
> -PermitUserEnvironment no
> -
> -# do not allow any kind of forwarding (provides only low security)
> +# Do not allow any kind of forwarding (provides only low security);
> # some of them might need to be re-enabled if SSH server is a jump platform
> -X11Forwarding no
> AllowTcpForwarding no
> AllowAgentForwarding no
> -PermitTunnel no
> -GatewayPorts no
> PermitOpen none
> 
> -# detect broken sessions by sending keep-alive messages to
> -# clients (both via TCP and SSH)
> -TCPKeepAlive yes
> +# Detect broken sessions by sending keep-alive messages to clients via SSH connection
> ClientAliveInterval 10
> 
> -# close unresponsive SSH sessions which fail to answer keep-alive
> +# Close unresponsive SSH sessions which fail to answer keep-alive
> ClientAliveCountMax 6
> 
> -# add support for SFTP
> +# Add support for SFTP
> Subsystem	sftp	/usr/lib/openssh/sftp-server
> 
> # EOF
> -- 
> 2.16.4
  

Hello Michael,

thanks for your reply.

Since the Unbound configuration patch (https://patchwork.ipfire.org/patch/2710/)
is based on the same motivation than this one, I am a bit surprised by your Reviewed-by
tag on it. :-)

Breaking productive environment is a latter weak point to me, as keeping configuration
statements/directives removed in the upstream may or may not cause the same effect.
Worse, there may be good reasons for changing upstream defaults which we should track
closely due to security considerations. On the other hands, relying on third parties
is a bad idea when it comes to security - not to mention personal aversions against
relying something or someone in general...

In the end, I figure it is less worse to have a shorter configuration which takes less
disk space and is easier to review and audit. You are right, the downside of this is
to rely on other people's opinion of security and interoperability.

Thanks, and best regards,
Peter Müller

> Hello,
> 
> I am not 100% sure if I like this change.
> 
> Simply because of defaults changing in software. Although we generally want to follow upstream this might break installations.
> 
> Do we not see this as a danger?
> 
> I would prefer to have shorter configurations, but not breaking production is more important.
> 
> -Michael
> 
>> On 20 Jan 2020, at 20:04, Peter Müller <peter.mueller@ipfire.org> wrote:
>>
>> In order to keep configurations as small as possible and to make them
>> easier to read/audit, this patch omits all default configuration in the
>> OpenSSH server configuration file.
>>
>> Further, it mentions where to refer for the full documentation.
>>
>> Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
>> ---
>> config/ssh/sshd_config | 62 ++++++++++++++++----------------------------------
>> 1 file changed, 20 insertions(+), 42 deletions(-)
>>
>> diff --git a/config/ssh/sshd_config b/config/ssh/sshd_config
>> index a248c4906..bea5cee53 100644
>> --- a/config/ssh/sshd_config
>> +++ b/config/ssh/sshd_config
>> @@ -1,81 +1,59 @@
>> -# ultra-secure OpenSSH server configuration
>> +# OpenSSH server configuration file for IPFire
>> +#
>> +# The full documentation is available at: https://man.openbsd.org/sshd_config
>> +#
>>
>> -# only allow version 2 of SSH protocol
>> +# Only allow version 2 of SSH protocol
>> Protocol 2
>>
>> -# listen on port 22 by default
>> +# Listen on port 22 by default
>> Port 22
>>
>> -# listen on these interfaces and protocols
>> -AddressFamily any
>> +# Listen on every interface and IPv4 only
>> +AddressFamily inet
>> ListenAddress 0.0.0.0
>>
>> -# limit authentication thresholds
>> +# Limit authentication timeout to 30 seconds
>> LoginGraceTime 30s
>> -MaxAuthTries 6
>>
>> -# limit maximum instanctes to prevent DoS
>> +# Limit maximum instanctes to prevent DoS
>> MaxStartups 5
>>
>> -# ensure proper logging
>> -SyslogFacility AUTH
>> -LogLevel INFO
>> -
>> -# enforce permission checks before a login is accepted
>> -# (prevents damage because of hacked systems with world-writeable
>> -# home directories or similar)
>> -StrictModes yes
>> -
>> -# only allow safe crypto algorithms (may break some _very_ outdated clients)
>> -# see also: https://stribika.github.io/2015/01/04/secure-secure-shell.html
>> +# 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@libssh.org,diffie-hellman-group-exchange-sha256
>> Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
>> MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com
>>
>> -# enable data compression after successful login only
>> -Compression delayed
>> -
>> -# only allow cryptographically safe SSH host keys (adjust paths if needed)
>> +# Only allow cryptographically safe SSH host keys (adjust paths if needed)
>> HostKey /etc/ssh/ssh_host_ed25519_key
>> HostKey /etc/ssh/ssh_host_ecdsa_key
>> HostKey /etc/ssh/ssh_host_rsa_key
>>
>> -# only allow login via public key by default
>> +# Only allow login via public key by default
>> PubkeyAuthentication yes
>> PasswordAuthentication no
>> ChallengeResponseAuthentication no
>> -PermitEmptyPasswords no
>>
>> -# permit root login as there is no other user in IPFire 2.x
>> +# Permit root login as there is no other user in IPFire 2.x
>> PermitRootLogin yes
>>
>> -# ignore user ~/.rhost* files
>> -IgnoreRhosts yes
>> -
>> -# ignore user known hosts file
>> +# Ignore user ~/.ssh/known_hosts file
>> IgnoreUserKnownHosts yes
>>
>> -# ignore user environments
>> -PermitUserEnvironment no
>> -
>> -# do not allow any kind of forwarding (provides only low security)
>> +# Do not allow any kind of forwarding (provides only low security);
>> # some of them might need to be re-enabled if SSH server is a jump platform
>> -X11Forwarding no
>> AllowTcpForwarding no
>> AllowAgentForwarding no
>> -PermitTunnel no
>> -GatewayPorts no
>> PermitOpen none
>>
>> -# detect broken sessions by sending keep-alive messages to
>> -# clients (both via TCP and SSH)
>> -TCPKeepAlive yes
>> +# Detect broken sessions by sending keep-alive messages to clients via SSH connection
>> ClientAliveInterval 10
>>
>> -# close unresponsive SSH sessions which fail to answer keep-alive
>> +# Close unresponsive SSH sessions which fail to answer keep-alive
>> ClientAliveCountMax 6
>>
>> -# add support for SFTP
>> +# Add support for SFTP
>> Subsystem	sftp	/usr/lib/openssh/sftp-server
>>
>> # EOF
>> -- 
>> 2.16.4
>
  

Hi,

> On 3 Feb 2020, at 17:24, Peter Müller <peter.mueller@ipfire.org> wrote:
> 
> Hello Michael,
> 
> thanks for your reply.
> 
> Since the Unbound configuration patch (https://patchwork.ipfire.org/patch/2710/)
> is based on the same motivation than this one, I am a bit surprised by your Reviewed-by
> tag on it. :-)

Yeah I could not remember if I wrote this email and waiting for a reply kind of timed out :)

> Breaking productive environment is a latter weak point to me, as keeping configuration
> statements/directives removed in the upstream may or may not cause the same effect.
> Worse, there may be good reasons for changing upstream defaults which we should track
> closely due to security considerations. On the other hands, relying on third parties
> is a bad idea when it comes to security - not to mention personal aversions against
> relying something or someone in general...
> 
> In the end, I figure it is less worse to have a shorter configuration which takes less
> disk space and is easier to review and audit. You are right, the downside of this is
> to rely on other people's opinion of security and interoperability.

Disk space? We are saying bytes. As long as we are shipping half a gigabyte of firmware for outdated SCSI controllers, this won’t really help you much.

-Michael

P.S. I suppose we should just go ahead with this then:

Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>

> 
> Thanks, and best regards,
> Peter Müller
> 
>> Hello,
>> 
>> I am not 100% sure if I like this change.
>> 
>> Simply because of defaults changing in software. Although we generally want to follow upstream this might break installations.
>> 
>> Do we not see this as a danger?
>> 
>> I would prefer to have shorter configurations, but not breaking production is more important.
>> 
>> -Michael
>> 
>>> On 20 Jan 2020, at 20:04, Peter Müller <peter.mueller@ipfire.org> wrote:
>>> 
>>> In order to keep configurations as small as possible and to make them
>>> easier to read/audit, this patch omits all default configuration in the
>>> OpenSSH server configuration file.
>>> 
>>> Further, it mentions where to refer for the full documentation.
>>> 
>>> Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
>>> ---
>>> config/ssh/sshd_config | 62 ++++++++++++++++----------------------------------
>>> 1 file changed, 20 insertions(+), 42 deletions(-)
>>> 
>>> diff --git a/config/ssh/sshd_config b/config/ssh/sshd_config
>>> index a248c4906..bea5cee53 100644
>>> --- a/config/ssh/sshd_config
>>> +++ b/config/ssh/sshd_config
>>> @@ -1,81 +1,59 @@
>>> -# ultra-secure OpenSSH server configuration
>>> +# OpenSSH server configuration file for IPFire
>>> +#
>>> +# The full documentation is available at: https://man.openbsd.org/sshd_config
>>> +#
>>> 
>>> -# only allow version 2 of SSH protocol
>>> +# Only allow version 2 of SSH protocol
>>> Protocol 2
>>> 
>>> -# listen on port 22 by default
>>> +# Listen on port 22 by default
>>> Port 22
>>> 
>>> -# listen on these interfaces and protocols
>>> -AddressFamily any
>>> +# Listen on every interface and IPv4 only
>>> +AddressFamily inet
>>> ListenAddress 0.0.0.0
>>> 
>>> -# limit authentication thresholds
>>> +# Limit authentication timeout to 30 seconds
>>> LoginGraceTime 30s
>>> -MaxAuthTries 6
>>> 
>>> -# limit maximum instanctes to prevent DoS
>>> +# Limit maximum instanctes to prevent DoS
>>> MaxStartups 5
>>> 
>>> -# ensure proper logging
>>> -SyslogFacility AUTH
>>> -LogLevel INFO
>>> -
>>> -# enforce permission checks before a login is accepted
>>> -# (prevents damage because of hacked systems with world-writeable
>>> -# home directories or similar)
>>> -StrictModes yes
>>> -
>>> -# only allow safe crypto algorithms (may break some _very_ outdated clients)
>>> -# see also: https://stribika.github.io/2015/01/04/secure-secure-shell.html
>>> +# 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@libssh.org,diffie-hellman-group-exchange-sha256
>>> Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
>>> MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com
>>> 
>>> -# enable data compression after successful login only
>>> -Compression delayed
>>> -
>>> -# only allow cryptographically safe SSH host keys (adjust paths if needed)
>>> +# Only allow cryptographically safe SSH host keys (adjust paths if needed)
>>> HostKey /etc/ssh/ssh_host_ed25519_key
>>> HostKey /etc/ssh/ssh_host_ecdsa_key
>>> HostKey /etc/ssh/ssh_host_rsa_key
>>> 
>>> -# only allow login via public key by default
>>> +# Only allow login via public key by default
>>> PubkeyAuthentication yes
>>> PasswordAuthentication no
>>> ChallengeResponseAuthentication no
>>> -PermitEmptyPasswords no
>>> 
>>> -# permit root login as there is no other user in IPFire 2.x
>>> +# Permit root login as there is no other user in IPFire 2.x
>>> PermitRootLogin yes
>>> 
>>> -# ignore user ~/.rhost* files
>>> -IgnoreRhosts yes
>>> -
>>> -# ignore user known hosts file
>>> +# Ignore user ~/.ssh/known_hosts file
>>> IgnoreUserKnownHosts yes
>>> 
>>> -# ignore user environments
>>> -PermitUserEnvironment no
>>> -
>>> -# do not allow any kind of forwarding (provides only low security)
>>> +# Do not allow any kind of forwarding (provides only low security);
>>> # some of them might need to be re-enabled if SSH server is a jump platform
>>> -X11Forwarding no
>>> AllowTcpForwarding no
>>> AllowAgentForwarding no
>>> -PermitTunnel no
>>> -GatewayPorts no
>>> PermitOpen none
>>> 
>>> -# detect broken sessions by sending keep-alive messages to
>>> -# clients (both via TCP and SSH)
>>> -TCPKeepAlive yes
>>> +# Detect broken sessions by sending keep-alive messages to clients via SSH connection
>>> ClientAliveInterval 10
>>> 
>>> -# close unresponsive SSH sessions which fail to answer keep-alive
>>> +# Close unresponsive SSH sessions which fail to answer keep-alive
>>> ClientAliveCountMax 6
>>> 
>>> -# add support for SFTP
>>> +# Add support for SFTP
>>> Subsystem	sftp	/usr/lib/openssh/sftp-server
>>> 
>>> # EOF
>>> -- 
>>> 2.16.4
>>