path: root/debian/README.Debian

OpenSSH for Debian
------------------

UPGRADE ISSUES
==============

Privilege Separation
--------------------

As of 3.3, openssh has employed privilege separation to reduce the
quantity of code that runs as root, thereby reducing the impact of
some security holes in sshd. This now also works properly with PAM.

Privilege separation is turned on by default, so, if you decide you
want it turned off, you need to add "UsePrivilegeSeparation no" to
/etc/ssh/sshd_config.

PermitRootLogin set to yes
--------------------------

This is now the default setting (in line with upstream), and people
who asked for an automatically-generated configuration file when
upgrading from potato (or on a new install) will have this setting in
their /etc/ssh/sshd_config file.

Should you wish to change this setting, edit /etc/ssh/sshd_config, and
change:
PermitRootLogin yes
to:
PermitRootLogin no

Having PermitRootLogin set to yes means that an attacker that knows
the root password can ssh in directly (without having to go via a user
account). If you set it to no, then they must compromise a normal user
account. In the vast majority of cases, this does not give added
security; remember that any account you su to root from is equivalent
to root - compromising this account gives an attacker access to root
easily. If you only ever log in as root from the physical console,
then you probably want to set this value to no.

As an aside, PermitRootLogin can also be set to "without-password" or
"forced-commands-only" - see sshd(8) for more details.

DO NOT FILE BUG REPORTS SAYING YOU THINK THIS DEFAULT IS INCORRECT!

The argument above is somewhat condensed; I have had this discussion
at great length with many people. If you think the default is
incorrect, and feel strongly enough to want to argue about it, then
send email to debian-ssh@lists.debian.org. I will close bug reports
claiming the default is incorrect.

X11 Forwarding
--------------

ssh's default for ForwardX11 has been changed to ``no'' because it has
been pointed out that logging into remote systems administered by
untrusted people is likely to open you up to X11 attacks, so you
should have to actively decide that you trust the remote machine's
root, before enabling X11.  I strongly recommend that you do this on a
machine-by-machine basis, rather than just enabling it in the default
host settings.

In order for X11 forwarding to work, you need to install xauth on the
server. In Debian this is in the xbase-clients package.

As of OpenSSH 3.1, the remote $DISPLAY uses localhost by default to reduce
the security risks of X11 forwarding. Look up X11UseLocalhost in
sshd_config(8) if this is a problem.

OpenSSH 3.8 invented ForwardX11Trusted, which when set to no causes the
ssh client to create an untrusted X cookie so that attacks on the
forwarded X11 connection can't become attacks on X clients on the remote
machine. However, this has some problems in implementation - notably a
very short timeout of the untrusted cookie - breaks large numbers of
existing setups, and generally seems immature. The Debian package
therefore sets the default for this option to "yes" (in ssh itself,
rather than in ssh_config).

Fallback to RSH
---------------

The default for this setting has been changed from Yes to No, for
security reasons, and to stop the delay attempting to rsh to machines
that don't offer the service.  Simply switch it back on in either
/etc/ssh/ssh_config or ~/.ssh/config for those machines that you need
it for.

Setgid ssh-agent and environment variables
------------------------------------------

As of version 1:3.5p1-1, ssh-agent is installed setgid to prevent ptrace()
attacks retrieving private key material. This has the side-effect of causing
glibc to remove certain environment variables which might have security
implications for set-id programs, including LD_PRELOAD, LD_LIBRARY_PATH, and
TMPDIR.

If you need to set any of these environment variables, you will need to do
so in the program exec()ed by ssh-agent. This may involve creating a small
wrapper script.

Symlink Hostname invocation
---------------------------

This version of ssh no longer includes support for invoking ssh with the
hostname as the name of the file run.  People wanting this support should
use the ssh-argv0 script.

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

OTHER ISSUES
============

/usr/bin/ssh not SUID
---------------------

Due to Debian bug #164325, RhostsRSAAuthentication can only be used if ssh
is SUID.  Until this is fixed, if that is a problem, use:

   dpkg-statoverride

or if that's also missing, use this:

   chown root.root /usr/bin/ssh
   chmod 04755 /usr/bin/ssh

Authorization Forwarding
------------------------

Similarly, root on a remote server could make use of your ssh-agent
(while you're logged into their machine) to obtain access to machines
which trust your keys.  This feature is therefore disabled by default.
You should only re-enable it for those hosts (in your ~/.ssh/config or
/etc/ssh/ssh_config) where you are confident that the remote machine
is not a threat.

Problems logging in with RSA authentication
-------------------------------------------

If you have trouble logging in with RSA authentication then the
problem is probably caused by the fact that you have your home
directory writable by group, as well as user (this is the default on
Debian systems).

Depending upon other settings on your system (i.e. other users being
in your group) this could open a security hole, so you will need to
make your home directory writable only by yourself.  Run this command,
as yourself:

  chmod g-w ~/

to remove group write permissions.  If you use ssh-copy-id to install your
keys, it does this for you.

-L option of ssh nonfree
------------------------

non-free ssh supported the usage of the option -L to use a non privileged
port for scp. This option will not be supported by scp from openssh.

Please use instead scp -o "UsePrivilegedPort=no" as documented in the
manpage to scp itself.

Problem logging in because of TCP-Wrappers
------------------------------------------

ssh is compiled with support for tcp-wrappers. So if you can no longer
log into your system, please check that /etc/hosts.allow and /etc/hosts.deny
are configured so that ssh is not blocked.

Kerberos support
----------------

ssh is now compiled with Kerberos support. Unfortunately, privilege
separation is incompatible with Kerberos support for SSH protocol 1 and
parts of the support for protocol 2; you may need to run kinit after logging
in.

Interoperability between scp and the ssh.com SSH server
-------------------------------------------------------

In version 2 and greater of the commercial SSH server produced by SSH
Communications Security, scp was changed to use SFTP (SSH2's file transfer
protocol) instead of the traditional rcp-over-ssh, thereby breaking
compatibility. The OpenSSH developers regard this as a bug in the ssh.com
server, and do not currently intend to change OpenSSH's scp to match.

Workarounds for this problem are to install scp1 on the server (scp2 will
fall back to it), to use sftp, or to use some other transfer mechanism such
as rsync-over-ssh or tar-over-ssh.

Running sshd from inittab
-------------------------

Some people find it useful to run the sshd server from inittab, to make sure
that it always stays running. To do this, stop sshd ('/etc/init.d/ssh
stop'), add the following line to /etc/inittab, and run 'telinit q':

  ss:2345:respawn:/usr/sbin/sshd -D

If you do this, note that you will need to stop sshd being started in the
normal way ('rm -f /etc/rc[2345].d/S16ssh') and that you will need to
restart this sshd manually on upgrades.

Per-connection sshd instances with systemd
------------------------------------------

If you want to reconfigure systemd to listen on port 22 itself and launch an
instance of sshd for each connection (inetd-style socket activation), then
you can run:

  systemctl stop ssh.service
  systemctl start ssh.socket

To make this permanent:

  systemctl disable ssh.service
  systemctl enable ssh.socket

This may be appropriate in environments where minimal footprint is critical
(e.g. cloud guests).  Be aware that this bypasses MaxStartups, and systemd's
MaxConnections cannot quite replace this as it cannot distinguish between
authenticated and unauthenticated connections; see
https://bugzilla.redhat.com/show_bug.cgi?id=963268 for more discussion.

-- 
Matthew Vernon <matthew@debian.org>
Colin Watson <cjwatson@debian.org>