syslog-ng Open Source Edition 3.18 - Administration Guide

Troubleshooting syslog-ng

This chapter provides tips and guidelines about troubleshooting problems related to syslog-ng.

• As a general rule, first try to get logging the messages to a local file. Once this is working, you know that syslog-ng is running correctly and receiving messages, and you can proceed to forwarding the messages to the server.

• Always check the configuration files for any syntax errors on both the client and the server using the syslog-ng --syntax-only command.

• If the syslog-ng OSE server does not receive the messages, verify that the IP addresses and ports are correct in your sources and destinations. Also, check that the client and the server uses the same protocol (a common error is to send logs on UDP, but configure the server to receive logs on TCP.

  If the problem persist, use tcpdump or a similar packet sniffer tool on the client to verify that the messages are sent correctly, and on the server to verify that it receives the messages.

• To find message-routing problems, run syslog-ng OSE with the following command syslog-ng -Fevd. That way syslog-ng OSE will run in the foreground, and display debug messages about the messages that are processed.

• If syslog-ng is closing the connections for no apparent reason, be sure to check the log messages of syslog-ng. You might also want to run syslog-ng with the --verbose or --debug command-line options for more-detailed log messages. You can enable these messages without restarting syslog-ng using the syslog-ng-ctl verbose --set=on command. For details, see the syslog-ng-ctl man page at The syslog-ng control tool manual page.

• Build up encrypted connections step-by-step: first create a working unencrypted (for example TCP) connection, then add TLS encryption, and finally client authentication if needed.

• If you use the same driver and options in the destination of your syslog-ng OSE client and the source of your syslog-ng OSE server, everything should work as expected. Unfortunately there are some other combinations, that seem to be working, but result in losing parts of the messages. For details on the working combinations, see Things to consider when forwarding messages between syslog-ng OSE hosts.

• In case you experience a problem that is not covered in this guide, send it to our mailing list.

  To report bugs found in syslog-ng OSE, visit our GitHub issues page.

  Precompiled binary packages are available for free from various third-parties. See the list of precompiled syslog-ng OSE binary packages.

Possible causes of losing log messages

During the course of a message from the sending application to the final destination of the message, there are a number of locations where a message may be lost, even though syslog-ng does its best to avoid message loss. Usually losing messages can be avoided with careful planning and proper configuration of syslog-ng and the hosts running syslog-ng. The following list shows the possible locations where messages may be lost, and provides methods to minimize the risk of losing messages.

• Between the application and the syslog-ng client: Make sure to use an appropriate source to receive the logs from the application (for example from /dev/log). For example, use unix-stream instead of unix-dgram whenever possible.

• When syslog-ng is sending messages: If syslog-ng cannot send messages to the destination and the output buffer gets full, syslog-ng will drop messages.

  Use flags (flow-control) to avoid it (for details, see Configuring flow-control). For more information about the error caused by the missing flow-control, see Destination queue full.

  The number of dropped messages is displayed per destination in the log message statistics of syslog-ng (for details, see Statistics of syslog-ng).

• On the network: When transferring messages using the UDP protocol, messages may be lost without any notice or feedback — such is the nature of the UDP protocol. Always use the TCP protocol to transfer messages over the network whenever possible.

• In the socket receive buffer: When transferring messages using the UDP protocol, the UDP datagram (that is, the message) that reaches the receiving host placed in a memory area called the socket receive buffer. If the host receives more messages than it can process, this area overflows, and the kernel drops messages without letting syslog-ng know about it. Using TCP instead of UDP prevents this issue. If you must use the UDP protocol, increase the size of the receive buffer using the so-rcvbuf() option.

• When syslog-ng is receiving messages:

  • The receiving syslog-ng (for example the syslog-ng server or relay) may drop messages if the fifo of the destination file gets full. The number of dropped messages is displayed per destination in the log message statistics of syslog-ng (for details, see Statistics of syslog-ng).

• When the destination cannot handle large load: When syslog-ng is sending messages at a high rate into an SQL database, a file, or another destination, it is possible that the destination cannot handle the load, and processes the messages slowly. As a result, the buffers of syslog-ng fill up, syslog-ng cannot process the incoming messages, and starts to loose messages. For details, see the previous entry. Use the throttle parameter to avoid this problem.

• As a result of an unclean shutdown of the syslog-ng server: If the host running the syslog-ng server experiences an unclean shutdown, it takes time until the clients realize that the connection to the syslog-ng server is down. Messages that are put into the output TCP buffer of the clients during this period are not sent to the server.

• When syslog-ng OSE is writing messages into files: If syslog-ng OSE receives a signal (SIG) while writing log messages to file, the log message that is processed by the write call can be lost if the flush_lines parameter is higher than 1.

Creating syslog-ng core files

Purpose:

When syslog-ng crashes for some reason, it can create a core file that contains important troubleshooting information. To enable core files, complete the following procedure:

Collecting debugging information with strace, truss, or tusc

To properly troubleshoot certain situations, it can be useful to trace which system calls syslog-ng OSE performs. How this is performed depends on the platform running syslog-ng OSE. In general, note the following points:

• When syslog-ng OSE is started, a supervisor process might stay in the foreground, while the actual syslog-ng daemon goes to the background. Always trace the background process.

• Apart from the system calls, the time between two system calls can be important as well. Make sure that your tracing tool records the time information as well. For details on how to do that, refer to the manual page of your specific tool (for example, strace on Linux, or truss on Solaris and BSD).

• Run your tracing tool in verbose mode, and if possible, set it to print long output strings, so the messages are not truncated.

• When using strace, also record the output of lsof to see which files are accessed.

The following are examples for tracing system calls of syslog-ng on some platforms. The output is saved into the /tmp/syslog-ng-trace.txt file, sufficed with the PID of the related syslog-ng process.The path of the syslog-ng binary may be different for your installation, as distribution-specific packages may use different paths.

• Linux: strace -o /tmp/trace.txt -s256 -ff -ttT /opt/syslog-ng/sbin/syslog-ng -f /opt/syslog-ng/etc/syslog-ng.conf -Fdv

• HP-UX: tusc -f -o /tmp/syslog-ng-trace.txt -T /opt/syslog-ng/sbin/syslog-ng

• IBM AIX and Solaris: truss -f -o /tmp/syslog-ng-trace.txt -r all -w all -u libc:: /opt/syslog-ng/sbin/syslog-ng -d -d -d