\C{errors} Common \i{error messages}

This chapter lists a number of common error messages which PuTTY and
its associated tools can produce, and explains what they mean in
more detail.

We do not attempt to list \e{all} error messages here: there are
many which should never occur, and some which should be
self-explanatory. If you get an error message which is not listed in
this chapter and which you don't understand, report it to us as a
bug (see \k{feedback}) and we will add documentation for it.

\H{errors-hostkey-absent} \q{The host key is not cached for this
server}

This error message occurs when PuTTY connects to a new SSH server.
Every server identifies itself by means of a host key; once PuTTY
knows the host key for a server, it will be able to detect if a
malicious attacker redirects your connection to another machine.

If you see this message, it means that PuTTY has not seen this host
key before, and has no way of knowing whether it is correct or not.
You should attempt to verify the host key by other means, such as
asking the machine's administrator.

If you see this message and you know that your installation of PuTTY
\e{has} connected to the same server before, it may have been
recently upgraded to SSH protocol version 2. SSH protocols 1 and 2
use separate host keys, so when you first use \i{SSH-2} with a server
you have only used SSH-1 with before, you will see this message
again. You should verify the correctness of the key as before.

See \k{gs-hostkey} for more information on host keys.

\H{errors-hostkey-wrong} \q{WARNING - POTENTIAL SECURITY BREACH!}

This message, followed by \q{The server's host key does not match
the one PuTTY has cached for this server}, means that PuTTY has
connected to the SSH server before, knows what its host key
\e{should} be, but has found a different one.

(If the message instead talks about a \q{certified host key}, see
instead \k{errors-cert-mismatch}.)

This may mean that a malicious attacker has replaced your server
with a different one, or has redirected your network connection to
their own machine. On the other hand, it may simply mean that the
administrator of your server has accidentally changed the key while
upgrading the SSH software; this \e{shouldn't} happen but it is
unfortunately possible.

You should contact your server's administrator and see whether they
expect the host key to have changed. If so, verify the new host key
in the same way as you would if it was new.

See \k{gs-hostkey} for more information on host keys.

\H{errors-cert-mismatch} \q{This server presented a certified host key
which was signed by a different certification authority ...}

If you've configured PuTTY to trust at least one
\I{certificate}certification authority for signing host keys (see
\k{config-ssh-kex-cert}), then it will ask the SSH server to send it
any available certified host keys. If the server sends back a
certified key signed by a \e{different} certification authority, PuTTY
will present this variant of the host key prompt, preceded by
\q{WARNING - POTENTIAL SECURITY BREACH!}

One reason why this can happen is a deliberate attack. Just like an
ordinary man-in-the-middle attack which substitutes a wrong host key,
a particularly ambitious attacker might substitute an entire wrong
certification authority, and hope that you connect anyway.

But it's also possible in some situations that this error might arise
legitimately. For example, if your organisation's IT department has
just rolled out a new CA key which you haven't yet entered in PuTTY's
configuration, or if your CA configuration involves two overlapping
domains, or something similar.

So, unfortunately, you'll have to work out what to do about it
yourself: make an exception for this specific case, or abandon this
connection and install a new CA key before trying again (if you're
really sure you trust the CA), or edit your configuration in some
other way, or just stop trying to use this server.

If you're convinced that this particular server is legitimate even
though the CA is not one you trust, PuTTY will let you cache the
certified host key, treating it in the same way as an uncertified one.
Then that particular certificate will be accepted for future
connections to this specific server, even though other certificates
signed by the same CA will still be rejected.

\H{errors-ssh-protocol} \q{SSH protocol version 2 required by our
configuration but remote only provides (old, insecure) SSH-1}

By default, PuTTY only supports connecting to SSH servers that
implement \i{SSH protocol version 2}. If you see this message, the
server you're trying to connect to only supports the older SSH-1
protocol.

If the server genuinely only supports SSH-1, then you need to either
change the \q{SSH protocol version} setting (see \k{config-ssh-prot}),
or use the \c{\-1} command-line option; in any case, you should not
treat the resulting connection as secure.

You might start seeing this message with new versions of PuTTY (from
0.68 onwards) where you didn't before, because it used to be possible
to configure PuTTY to automatically fall back from SSH-2 to SSH-1.
This is no longer supported, to prevent the possibility of a downgrade
attack.

\H{errors-cipher-warning} \q{The first cipher supported by the server is
... below the configured warning threshold}

This occurs when the SSH server does not offer any ciphers which you
have configured PuTTY to consider strong enough. By default, PuTTY
puts up this warning only for \i{Blowfish}, \ii{single-DES}, and
\i{Arcfour} encryption.

See \k{config-ssh-encryption} for more information on this message.

(There are similar messages for other cryptographic primitives, such
as host key algorithms.)

\H{errors-toomanyauth} \q{Remote side sent disconnect message type 2
(protocol error): "Too many authentication failures for root"}

This message is produced by an \i{OpenSSH} (or \i{Sun SSH}) server if it
receives more failed authentication attempts than it is willing to
tolerate.

This can easily happen if you are using Pageant and have a
large number of keys loaded into it, since these servers count each
offer of a public key as an authentication attempt. This can be worked
around by specifying the key that's required for the authentication in
the PuTTY configuration (see \k{config-ssh-privkey}); PuTTY will ignore
any other keys Pageant may have, but will ask Pageant to do the
authentication, so that you don't have to type your passphrase.

On the server, this can be worked around by disabling public-key
authentication or (for Sun SSH only) by increasing \c{MaxAuthTries} in
\c{sshd_config}.

\H{errors-internal} \q{\ii{Internal error}}, \q{\ii{Internal fault}},
\q{\ii{Assertion failed}}

Any error beginning with the word \q{Internal} should \e{never}
occur. If it does, there is a bug in PuTTY by definition; please see
\k{feedback} and report it to us.

Similarly, any error message starting with \q{Assertion failed} is a
bug in PuTTY. Please report it to us, and include the exact text
from the error message box.

\H{errors-cant-load-key} \q{Unable to use key file},
\q{Couldn't load private key}, \q{Couldn't load this key}

Various forms of this error are printed in the PuTTY window, or
written to the PuTTY Event Log (see \k{using-eventlog}) when trying
public-key authentication, or given by Pageant when trying to load a
private key.

If you see one of these messages, it often indicates that you've tried
to load a key of an inappropriate type into PuTTY, Plink, PSCP, PSFTP,
or Pageant.

You may have tried to load an SSH-2 key in a \q{foreign}
format (OpenSSH or \cw{ssh.com}) directly into one of the PuTTY tools,
in which case you need to import it into PuTTY's native format
(\c{*.PPK}) using PuTTYgen \dash see \k{puttygen-conversions}.

Alternatively, you may have specified a key that's inappropriate for
the connection you're making. The SSH-2 and the old SSH-1 protocols
require different private key formats, and a SSH-1 key can't be used
for a SSH-2 connection (or vice versa).

\H{errors-refused} \q{Server refused our key},
\q{Server refused our public key}, \q{Key refused}

Various forms of this error are printed in the PuTTY window, or
written to the PuTTY Event Log (see \k{using-eventlog}) when trying
public-key authentication.

If you see one of these messages, it means that PuTTY has sent a
public key to the server and offered to authenticate with it, and
the server has refused to accept authentication. This usually means
that the server is not configured to accept this key to authenticate
this user.

This is almost certainly not a problem with PuTTY. If you see this
type of message, the first thing you should do is check your
\e{server} configuration carefully. Common errors include having
the wrong permissions or ownership set on the public key or the
user's home directory on the server. Also, read the PuTTY Event Log;
the server may have sent diagnostic messages explaining exactly what
problem it had with your setup.

\K{pubkey-gettingready} has some hints on server-side public key
setup.

\H{errors-access-denied} \q{Access denied}, \q{Authentication refused}

Various forms of this error are printed in the PuTTY window, or
written to the PuTTY Event Log (see \k{using-eventlog}) during
authentication.

If you see one of these messages, it means that the server has refused
all the forms of authentication PuTTY has tried and it has no further
ideas.

It may be worth checking the Event Log for diagnostic messages from
the server giving more detail.

This error can be caused by buggy SSH-1 servers that fail to cope with
the various strategies we use for camouflaging passwords in transit.
Upgrade your server, or use the workarounds described in
\k{config-ssh-bug-ignore1} and possibly \k{config-ssh-bug-plainpw1}.

\H{errors-no-auth} \q{No supported authentication methods available}

This error indicates that PuTTY has run out of ways to authenticate
you to an SSH server.  This may be because PuTTY has TIS or
keyboard-interactive authentication disabled, in which case see
\k{config-ssh-tis} and \k{config-ssh-ki}.

\H{errors-crc} \q{Incorrect \i{MAC} received on packet} or
\q{Incorrect \i{CRC} received on packet}

This error occurs when PuTTY decrypts an SSH packet and its checksum
is not correct. This probably means something has gone wrong in the
encryption or decryption process. It's difficult to tell from this
error message whether the problem is in the client, in the server,
or in between.

In particular, if the network is corrupting data at the TCP level, it
may only be obvious with cryptographic protocols such as SSH, which
explicitly check the integrity of the transferred data and complain
loudly if the checks fail. Corruption of protocols without integrity
protection (such as HTTP) will manifest in more subtle failures (such
as misdisplayed text or images in a web browser) which may not be
noticed.

Occasionally this has been caused by server bugs. An example is the
bug described at \k{config-ssh-bug-hmac2}, although you're very
unlikely to encounter that one these days.

In this context MAC stands for \ii{Message Authentication Code}. It's a
cryptographic term, and it has nothing at all to do with Ethernet
MAC (Media Access Control) addresses, or with the Apple computer.

\H{errors-garbled} \q{Incoming packet was garbled on decryption}

This error occurs when PuTTY decrypts an SSH packet and the
decrypted data makes no sense. This probably means something has
gone wrong in the encryption or decryption process. It's difficult
to tell from this error message whether the problem is in the client,
in the server, or in between.

If you get this error, one thing you could try would be to fiddle with
the setting of \q{Miscomputes SSH-2 encryption keys} (see
\k{config-ssh-bug-derivekey2}) or \q{Ignores SSH-2 maximum packet
size} (see \k{config-ssh-bug-maxpkt2}) on the Bugs panel.

\H{errors-x11-proxy} \q{PuTTY X11 proxy: \e{various errors}}

This family of errors are reported when PuTTY is doing X forwarding.
They are sent back to the X application running on the SSH server,
which will usually report the error to the user.

When PuTTY enables X forwarding (see \k{using-x-forwarding}) it
creates a virtual X display running on the SSH server. This display
requires authentication to connect to it (this is how PuTTY prevents
other users on your server machine from connecting through the PuTTY
proxy to your real X display). PuTTY also sends the server the
details it needs to enable clients to connect, and the server should
put this mechanism in place automatically, so your X applications
should just work.

A common reason why people see one of these messages is because they
used SSH to log in as one user (let's say \q{fred}), and then used
the Unix \c{su} command to become another user (typically \q{root}).
The original user, \q{fred}, has access to the X authentication data
provided by the SSH server, and can run X applications which are
forwarded over the SSH connection. However, the second user
(\q{root}) does not automatically have the authentication data
passed on to it, so attempting to run an X application as that user
often fails with this error.

If this happens, \e{it is not a problem with PuTTY}. You need to
arrange for your X authentication data to be passed from the user
you logged in as to the user you used \c{su} to become. How you do
this depends on your particular system; in fact many modern versions
of \c{su} do it automatically.

\H{errors-connaborted} \q{Network error: Software caused connection
abort}

This is a generic error produced by the Windows network code when it
kills an established connection for some reason. For example, it might
happen if you pull the network cable out of the back of an
Ethernet-connected computer, or if Windows has any other similar
reason to believe the entire network has become unreachable.

Windows also generates this error if it has given up on the machine
at the other end of the connection ever responding to it. If the
network between your client and server goes down and your client
then tries to send some data, Windows will make several attempts to
send the data and will then give up and kill the connection. In
particular, this can occur even if you didn't type anything, if you
are using SSH-2 and PuTTY attempts a key re-exchange. (See
\k{config-ssh-kex-rekey} for more about key re-exchange.)

(It can also occur if you are using keepalives in your connection.
Other people have reported that keepalives \e{fix} this error for
them. See \k{config-keepalive} for a discussion of the pros and cons
of keepalives.)

We are not aware of any reason why this error might occur that would
represent a bug in PuTTY. The problem is between you, your Windows
system, your network and the remote system.

\H{errors-connreset} \q{Network error: Connection reset by peer}

This error occurs when the machines at each end of a network
connection lose track of the state of the connection between them.
For example, you might see it if your SSH server crashes, and
manages to reboot fully before you next attempt to send data to it.

However, the most common reason to see this message is if you are
connecting through a \i{firewall} or a \i{NAT router} which has timed the
connection out. See \k{faq-idleout} in the FAQ for more details. You
may be able to improve the situation by using keepalives; see
\k{config-keepalive} for details on this.

Note that Windows can produce this error in some circumstances without
seeing a connection reset from the server, for instance if the
connection to the network is lost.

\H{errors-connrefused} \q{Network error: Connection refused}

This error means that the network connection PuTTY tried to make to
your server was rejected by the server. Usually this happens because
the server does not provide the service which PuTTY is trying to
access.

Check that you are connecting with the correct protocol (SSH, Telnet,
etc), and check that the port number is correct. If that
fails, consult the administrator of your server.

This error can also be caused by a firewall in between you and the
server, which rejects the connection and sends back the same type of
error packet as the real server would have sent.

\H{errors-conntimedout} \q{Network error: Connection timed out}

This error means that the network connection PuTTY tried to make to
your server received no response at all from the server. Usually
this happens because the server machine is completely isolated from
the network, or because it is turned off.

Check that you have correctly entered the host name or IP address of
your server machine. If that fails, consult the administrator of
your server.

\i{Unix} also generates this error when it tries to send data down a
connection and contact with the server has been completely lost
during a connection. (There is a delay of minutes before Unix gives
up on receiving a reply from the server.) This can occur if you type
things into PuTTY while the network is down, but it can also occur
if PuTTY decides of its own accord to send data: due to a repeat key
exchange in SSH-2 (see \k{config-ssh-kex-rekey}) or due to
keepalives (\k{config-keepalive}).

\H{errors-cannotassignaddress} \q{Network error: Cannot assign requested
address}

This means that the operating system rejected the parameters of the
network connection PuTTY tried to make, usually without actually
trying to connect to anything, because they were simply invalid.

A common way to provoke this error is to accidentally try to connect
to port 0, which is not a valid port number.
