Encrypting Syslog Traffic with TLS (SSL)

Written by Rainer Gerhards (2008-06-17)

• Overview
• Sample Scenario
• Setting up the CA
• Generating Machine Certificates
• Setting up the Central Server
• Setting up syslog Clients
• Setting up the UDP syslog relay
• Wrapping it all up
• Frequently seen Error Messages

Error Messages

This page covers error message you may see when setting up rsyslog with TLS. Please note that many of the message stem back to the TLS library being used. In those cases, there is not always a good explanation available in rsyslog alone.

A single error typically results in two or more message being emitted: (at least) one is the actual error cause, followed by usually one message with additional information (like certificate contents). In a typical system, these message should immediately follow each other in your log. Kepp in mind that they are reported as syslog.err, so you need to capture these to actually see errors (the default rsyslog.conf's shipped by many systems will do that, recording them e.g. in /etc/messages).

certificate invalid

Sample: not permitted to talk to peer, certificate invalid: insecure algorithm

This message may occur during connection setup. It indicates that the remote peer's certificate can not be accepted. The reason for this is given in the message part that is shown in red. Please note that this red part directly stems back to the TLS library, so rsyslog does acutally not have any more information about the reason.

With GnuTLS, the following reasons have been seen in practice:

insecure algorith

The certificate contains information on which encryption algorithms are to be used. This information is entered when the certificate is created. Some older alogrithms are no longer secure and the TLS library does not accept them. Thus the connection request failed. The cure is to use a certificate with sufficiently secure alogorithms.

Please note that noi encryption algorithm is totally secure. It only is secure based on our current knowledge AND on computing power available. As computers get more and more powerful, previously secure algorithms become insecure over time. As such, algorithms considered secure today may not be accepted by the TLS library in the future.

So in theory, after a system upgrade, a connection request may fail with the "insecure algorithm" failure without any change in rsyslog configuration or certificates. This could be caused by a new perception of the TLS library of what is secure and what not.

GnuTLS error -64

Sample: unexpected GnuTLS error -64 in nsd_gtls.c:517: Error while reading file.

This error points to an encoding error witht the pem file in question. It means "base 64 encoding error". From my experience, it can be caused by a couple of things, some of them not obvious:

• You specified a wrong file, which is not actually in .pem format
• The file was incorrectly generated
• I think I have also seen this when I accidently swapped private key files and certificate files. So double-check the type of file you are using.
• It may even be a result of an access (permission) problem. In theory, that should lead to another error, but in practice it sometimes seems to lead to this -64 error.

info on invalid cert

Sample: info on invalid cert: peer provided 1 certificate(s). Certificate 1 info: certificate valid from Wed Jun 18 11:45:44 2008 to Sat Jun 16 11:45:53 2018; Certificate public key: RSA; DN: C=US,O=Sample Corp,OU=Certs,L=Somehwere,ST=CA,CN=somename; Issuer DN: C=US,O=Sample Corp,OU=Certs,L=Somewhere,ST=CA,CN=somename,EMAIL=xxx@example.com; SAN:DNSname: machine.example.net;

This is not an error message in itself. It always follows the actual error message and tells you what is seen in the peer's certificate. This is done to give you a chance to evaluate the certificate and better understand why the initial error message was issued.

Please note that you can NOT diagnose problems based on this message alone. It follows in a number of error cases and does not pinpoint any problems by itself.

Copyright

Copyright (c) 2008 Rainer Gerhards and Adiscon.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license can be viewed at http://www.gnu.org/copyleft/fdl.html.