10 As of 3.3, openssh has employed privilege separation to reduce the
11 quantity of code that runs as root, thereby reducing the impact of
12 some security holes in sshd. This now also works properly with PAM.
14 Privilege separation is turned on by default, so, if you decide you
15 want it turned off, you need to add "UsePrivilegeSeparation no" to
18 PermitRootLogin set to yes
19 --------------------------
21 This is now the default setting (in line with upstream), and people
22 who asked for an automatically-generated configuration file when
23 upgrading from potato (or on a new install) will have this setting in
24 their /etc/ssh/sshd_config file.
26 Should you wish to change this setting, edit /etc/ssh/sshd_config, and
32 Having PermitRootLogin set to yes means that an attacker that knows
33 the root password can ssh in directly (without having to go via a user
34 account). If you set it to no, then they must compromise a normal user
35 account. In the vast majority of cases, this does not give added
36 security; remember that any account you su to root from is equivalent
37 to root - compromising this account gives an attacker access to root
38 easily. If you only ever log in as root from the physical console,
39 then you probably want to set this value to no.
41 As an aside, PermitRootLogin can also be set to "without-password" or
42 "forced-commands-only" - see sshd(8) for more details.
44 DO NOT FILE BUG REPORTS SAYING YOU THINK THIS DEFAULT IS INCORRECT!
46 The argument above is somewhat condensed; I have had this discussion
47 at great length with many people. If you think the default is
48 incorrect, and feel strongly enough to want to argue about it, then
49 send email to debian-ssh@lists.debian.org. I will close bug reports
50 claiming the default is incorrect.
52 SSH now uses protocol 2 by default
53 ----------------------------------
55 This means all your keyfiles you used for protocol version 1 need to
56 be re-generated. The server keys are done automatically, but for RSA
57 authentication, please read the ssh-keygen manpage.
59 If you have an automatically generated configuration file, and decide
60 at a later stage that you do want to support protocol version 1 (not
61 recommended, but note that the ssh client shipped with Debian potato
62 only supported protocol version 1), then you need to do the following:
64 Change /etc/ssh/sshd_config such that:
69 HostKey /etc/ssh/ssh_host_key
71 If you do not already have an RSA1 host key in /etc/ssh/ssh_host_key,
72 you will need to generate one. To do so, run this command as root:
74 ssh-keygen -f /etc/ssh/ssh_host_key -N '' -t rsa1
76 As of openssh-server 1:4.1p1-2, the option to support protocol version 1
77 is no longer available via debconf. You must edit the configuration file
83 ssh's default for ForwardX11 has been changed to ``no'' because it has
84 been pointed out that logging into remote systems administered by
85 untrusted people is likely to open you up to X11 attacks, so you
86 should have to actively decide that you trust the remote machine's
87 root, before enabling X11. I strongly recommend that you do this on a
88 machine-by-machine basis, rather than just enabling it in the default
91 In order for X11 forwarding to work, you need to install xauth on the
92 server. In Debian this is in the xbase-clients package.
94 As of OpenSSH 3.1, the remote $DISPLAY uses localhost by default to reduce
95 the security risks of X11 forwarding. Look up X11UseLocalhost in
96 sshd_config(8) if this is a problem.
98 OpenSSH 3.8 invented ForwardX11Trusted, which when set to no causes the
99 ssh client to create an untrusted X cookie so that attacks on the
100 forwarded X11 connection can't become attacks on X clients on the remote
101 machine. However, this has some problems in implementation - notably a
102 very short timeout of the untrusted cookie - breaks large numbers of
103 existing setups, and generally seems immature. The Debian package
104 therefore sets the default for this option to "yes" (in ssh itself,
105 rather than in ssh_config).
110 The default for this setting has been changed from Yes to No, for
111 security reasons, and to stop the delay attempting to rsh to machines
112 that don't offer the service. Simply switch it back on in either
113 /etc/ssh/ssh_config or ~/.ssh/config for those machines that you need
116 Setgid ssh-agent and environment variables
117 ------------------------------------------
119 As of version 1:3.5p1-1, ssh-agent is installed setgid to prevent ptrace()
120 attacks retrieving private key material. This has the side-effect of causing
121 glibc to remove certain environment variables which might have security
122 implications for set-id programs, including LD_PRELOAD, LD_LIBRARY_PATH, and
125 If you need to set any of these environment variables, you will need to do
126 so in the program exec()ed by ssh-agent. This may involve creating a small
129 Symlink Hostname invocation
130 ---------------------------
132 This version of ssh no longer includes support for invoking ssh with the
133 hostname as the name of the file run. People wanting this support should
134 use the ssh-argv0 script.
136 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
141 /usr/bin/ssh not SUID
142 ---------------------
144 Due to Debian bug #164325, RhostsRSAAuthentication can only be used if ssh
145 is SUID. Until this is fixed, if that is a problem, use:
149 or if that's also missing, use this:
151 chown root.root /usr/bin/ssh
152 chmod 04755 /usr/bin/ssh
154 Authorization Forwarding
155 ------------------------
157 Similarly, root on a remote server could make use of your ssh-agent
158 (while you're logged into their machine) to obtain access to machines
159 which trust your keys. This feature is therefore disabled by default.
160 You should only re-enable it for those hosts (in your ~/.ssh/config or
161 /etc/ssh/ssh_config) where you are confident that the remote machine
164 Problems logging in with RSA authentication
165 -------------------------------------------
167 If you have trouble logging in with RSA authentication then the
168 problem is probably caused by the fact that you have your home
169 directory writable by group, as well as user (this is the default on
172 Depending upon other settings on your system (i.e. other users being
173 in your group) this could open a security hole, so you will need to
174 make your home directory writable only by yourself. Run this command,
179 to remove group write permissions. If you use ssh-copy-id to install your
180 keys, it does this for you.
182 -L option of ssh nonfree
183 ------------------------
185 non-free ssh supported the usage of the option -L to use a non privileged
186 port for scp. This option will not be supported by scp from openssh.
188 Please use instead scp -o "UsePrivilegedPort=no" as documented in the
189 manpage to scp itself.
191 Problem logging in because of TCP-Wrappers
192 ------------------------------------------
194 ssh is compiled with support for tcp-wrappers. So if you can no longer
195 log into your system, please check that /etc/hosts.allow and /etc/hosts.deny
196 are configured so that ssh is not blocked.
201 ssh is now compiled with Kerberos support. Unfortunately, privilege
202 separation is incompatible with Kerberos support for SSH protocol 1 and
203 parts of the support for protocol 2; you may need to run kinit after logging
206 Interoperability between scp and the ssh.com SSH server
207 -------------------------------------------------------
209 In version 2 and greater of the commercial SSH server produced by SSH
210 Communications Security, scp was changed to use SFTP (SSH2's file transfer
211 protocol) instead of the traditional rcp-over-ssh, thereby breaking
212 compatibility. The OpenSSH developers regard this as a bug in the ssh.com
213 server, and do not currently intend to change OpenSSH's scp to match.
215 Workarounds for this problem are to install scp1 on the server (scp2 will
216 fall back to it), to use sftp, or to use some other transfer mechanism such
217 as rsync-over-ssh or tar-over-ssh.
219 Running sshd from inittab
220 -------------------------
222 Some people find it useful to run the sshd server from inittab, to make sure
223 that it always stays running. To do this, stop sshd ('/etc/init.d/ssh
224 stop'), add the following line to /etc/inittab, and run 'telinit q':
226 ss:2345:respawn:/usr/sbin/sshd -D
228 If you do this, note that you will need to stop sshd being started in the
229 normal way ('rm -f /etc/rc[2345].d/S16ssh') and that you will need to
230 restart this sshd manually on upgrades.
233 Matthew Vernon <matthew@debian.org>
234 Colin Watson <cjwatson@debian.org>