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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 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 appendix B) and we will add documentation for it.
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 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 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 section 2.2 for more information on host keys.
This message, followed by ‘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 should be, but has found a different one.
(If the message instead talks about a ‘certified host key’, see instead section 10.3.)
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 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 section 2.2 for more information on host keys.
If you've configured PuTTY to trust at least one certification authority for signing host keys (see section 4.19.4), 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 different certification authority, PuTTY will present this variant of the host key prompt, preceded by ‘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.
By default, PuTTY only supports connecting to SSH servers that implement 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 ‘SSH protocol version’ setting (see section 4.17.4), or use the -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.
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 Blowfish, single-DES, and Arcfour encryption.
See section 4.20 for more information on this message.
(There are similar messages for other cryptographic primitives, such as host key algorithms.)
This message is produced by an OpenSSH (or 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 section 4.22.1); 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 MaxAuthTries in sshd_config.
This occurs when PuTTY tries to allocate more memory than the system can give it. This may happen for genuine reasons: if the computer really has run out of memory, or if you have configured an extremely large number of lines of scrollback in your terminal. PuTTY is not able to recover from running out of memory; it will terminate immediately after giving this error.
However, this error can also occur when memory is not running out at all, because PuTTY receives data in the wrong format. In SSH-2 and also in SFTP, the server sends the length of each message before the message itself; so PuTTY will receive the length, try to allocate space for the message, and then receive the rest of the message. If the length PuTTY receives is garbage, it will try to allocate a ridiculous amount of memory, and will terminate with an ‘Out of memory’ error.
This can happen in SSH-2, if PuTTY and the server have not enabled encryption in the same way (see question A.7.3 in the FAQ).
This can also happen in PSCP or PSFTP, if your login scripts on the server generate output: the client program will be expecting an SFTP message starting with a length, and if it receives some text from your login scripts instead it will try to interpret them as a message length. See question A.7.4 for details of this.
Any error beginning with the word ‘Internal’ should never occur. If it does, there is a bug in PuTTY by definition; please see appendix B and report it to us.
Similarly, any error message starting with ‘Assertion failed’ is a bug in PuTTY. Please report it to us, and include the exact text from the error message box.
Various forms of this error are printed in the PuTTY window, or written to the PuTTY Event Log (see section 3.1.3.1) 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 ‘foreign’ format (OpenSSH or ssh.com) directly into one of the PuTTY tools, in which case you need to import it into PuTTY's native format (*.PPK) using PuTTYgen – see section 8.2.15.
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).
Various forms of this error are printed in the PuTTY window, or written to the PuTTY Event Log (see section 3.1.3.1) 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 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.
Section 8.3 has some hints on server-side public key setup.
Various forms of this error are printed in the PuTTY window, or written to the PuTTY Event Log (see section 3.1.3.1) 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 section 4.27.14 and possibly section 4.27.15.
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 section 4.21.5 and section 4.21.6.
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 section 4.27.11, although you're very unlikely to encounter that one these days.
In this context MAC stands for 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.
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 ‘Miscomputes SSH-2 encryption keys’ (see section 4.27.13) or ‘Ignores SSH-2 maximum packet size’ (see section 4.27.5) on the Bugs panel.
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 section 3.4) 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 ‘fred’), and then used the Unix su command to become another user (typically ‘root’). The original user, ‘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 (‘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, 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 su to become. How you do this depends on your particular system; in fact many modern versions of su do it automatically.
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 section 4.18.2 for more about key re-exchange.)
(It can also occur if you are using keepalives in your connection. Other people have reported that keepalives fix this error for them. See section 4.14.1 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.
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 firewall or a NAT router which has timed the connection out. See question A.7.8 in the FAQ for more details. You may be able to improve the situation by using keepalives; see section 4.14.1 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.
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.
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.
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 section 4.18.2) or due to keepalives (section 4.14.1).
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.
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[PuTTY release 0.79]