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syslog-ng Premium Edition 7.0.20 - Administration Guide

Preface Introduction to syslog-ng The concepts of syslog-ng Installing syslog-ng The syslog-ng PE quick-start guide The syslog-ng PE configuration file Collecting log messages — sources and source drivers
How sources work default-network-drivers: Receive and parse common syslog messages internal: Collecting internal messages file: Collecting messages from text files wildcard-file: Collecting messages from multiple text files linux-audit: Collecting messages from Linux audit logs network: Collecting messages using the RFC3164 protocol (network() driver) office365: Fetching logs from Office 365 osquery: Collect and parse osquery result logs pipe: Collecting messages from named pipes program: Receiving messages from external applications python: writing server-style Python sources python-fetcher: writing fetcher-style Python sources snmptrap: Read Net-SNMP traps syslog: Collecting messages using the IETF syslog protocol (syslog() driver) system: Collecting the system-specific log messages of a platform systemd-journal: Collecting messages from the systemd-journal system log storage systemd-syslog: Collecting systemd messages using a socket tcp, tcp6, udp, udp6: Collecting messages from remote hosts using the BSD syslog protocol udp-balancer: Receiving UDP messages at very high rate unix-stream, unix-dgram: Collecting messages from UNIX domain sockets windowsevent: Collecting Windows event logs
Sending and storing log messages — destinations and destination drivers
elasticsearch2: Sending messages directly to Elasticsearch version 2.0 or higher (DEPRECATED) elasticsearch-http: Sending messages to Elasticsearch HTTP Event Collector file: Storing messages in plain-text files hdfs: Storing messages on the Hadoop Distributed File System (HDFS) http: Posting messages over HTTP kafka: Publishing messages to Apache Kafka logstore: Storing messages in encrypted files mongodb: Storing messages in a MongoDB database network: Sending messages to a remote log server using the RFC3164 protocol (network() driver) pipe: Sending messages to named pipes program: Sending messages to external applications python: writing custom Python destinations sentinel: Sending logs to the Microsoft Azure Sentinel cloud snmp: Sending SNMP traps smtp: Generating SMTP messages (email) from logs splunk-hec: Sending messages to Splunk HTTP Event Collector sql: Storing messages in an SQL database stackdriver: Sending logs to the Google Stackdriver cloud syslog: Sending messages to a remote logserver using the IETF-syslog protocol syslog-ng(): Forward logs to another syslog-ng node tcp, tcp6, udp, udp6: Sending messages to a remote log server using the legacy BSD-syslog protocol (tcp(), udp() drivers) unix-stream, unix-dgram: Sending messages to UNIX domain sockets usertty: Sending messages to a user terminal — usertty() destination Client-side failover
Routing messages: log paths, flags, and filters Global options of syslog-ng PE TLS-encrypted message transfer Advanced Log Transfer Protocol Reliability and minimizing the loss of log messages Manipulating messages parser: Parse and segment structured messages Processing message content with a pattern database Correlating log messages Enriching log messages with external data Monitoring statistics and metrics of syslog-ng Multithreading and scaling in syslog-ng PE Troubleshooting syslog-ng Best practices and examples The syslog-ng manual pages Glossary

Advanced Log Transfer Protocol

Logging using ALTP


Note that in earlier versions of syslog-ng Premium Edition, Advanced Log Transfer Protocol is called Reliable Log Transfer Protocol (RLTP).

The syslog-ng PE application can send and receive log messages in a reliable way over the TCP transport layer using the Advanced Log Transfer Protocol (ALTP). ALTP is a proprietary transport protocol that prevents message loss during connection breaks. The transport is used between syslog-ng PE hosts (for example, a client and a server, or a client-relay-server), and interoperates with the mechanisms of syslog-ng PE's flow-control and the reliable disk-buffer option, thus providing the best way to prevent message loss. The sender detects which messages the receiver has successfully received. If messages are lost during the transfer, the sender resends the missing messages, starting from the last successfully received message. Therefore, messages are not duplicated at the receiving end in case of a connection break (however, in failover mode, this is not completely ensured).

ALTP also allows for encrypted and non-encrypted connections to be received on the same port, using a single source driver.


Because of the communication overhead, the ALTP protocol is slower than other transport protocols, which might be a problem if you need to collect a high amount (over 200000 messages per second) of log messages on your log server. For performance details of syslog-ng PE see the syslog-ng Premium Edition Performance Guideline at the syslog-ng Premium Edition Documentation page.


Make sure that you have set the value of the log_msg_size() parameter large enough in your configuration. If its size is less than the size of the sent messages, it might result in disk fill-up and no incoming logs.


    In the following cases, it is possible to lose log messages even if you use ALTP:

  • If you use ALTP together with the non-reliable disk-buffer option, it is possible to lose logs.

  • When sending logs through a relay that is using the non-reliable disk-buffer option, it is possible to lose logs if the relay crashes.

  • When sending logs through a relay that is using the non-reliable disk-buffer option, it is possible that logs are duplicated if the relay crashes, or it is stopped.

  • If the underlying disk system of syslog-ng PE fails to write the log messages to the disk, but it does not return a write error, or some other hardware or operating-system error happens.

The ALTP protocol works on top of TCP, and can use STARTTLS for encryption. ALTP supports IPv4 and IPv6 addresses. Inside the ALTP message, the message can use any format, for example, RFC3164 (BSD-syslog) or RFC5424 (IETF-syslog). The default port of ALTP is 35514.

ALTP can be added to the configuration like a transport protocol within the syslog() driver and the network() driver.

How ALTP connections work

This procedure summarizes how two syslog-ng PE hosts (a sender and a receiver) communicate using the Advanced Log Transfer Protocol (ALTP).


The sender (also called the client) is the host that has ALTP configured in its destination driver. The receiver (also called the server) is the host that has ALTP configured in its source driver.

  1. The sender initiates the connection to the receiver.

  2. The sender and the receiver negotiate whether to encrypt the connection and to use compression or not.

  3. If the connection should be encrypted, the sender and the receiver perform authentication (as configured in the tls() options of their configuration).

  4. If the sender and the receiver have communicated earlier using ALTP, the receiver indicates which was the last message received from the sender.

  5. The sender starts sending messages in batches. Batch size depends on the batch-size() parameter of the sender.

    For optimal performance when sending messages to a syslog-ng PE server, make sure that the batch-size() is smaller than the window size set using the log-iw-size() option in the source of your server.

  6. When the receiver has successfully processed the messages in the batch, it sends an acknowledgment of the processed messages to the sender.

    What "successfully processed" means depends on the configuration of the receiver, for example, written to disk in a destination, forwarded to a remote destination using not ALTP, dropped because of filter settings, or written to the disk-buffer. (If the messages are forwarded using ALTP, see Using ALTP in a client-relay-server scenario.)

  7. After receiving the acknowledgment, the sender sends another batch of messages.

Using ALTP in a client-relay-server scenario


Note that in earlier versions of syslog-ng Premium Edition, Advanced Log Transfer Protocol is called Reliable Log Transfer Protocol (RLTP).

You can use ALTP between multiple syslog-ng PE hosts, for example, in a client-relay-server scenario. In this case, the communication described in How ALTP connections work applies both between the client and the relay, and the relay and the server. However, note the following points:

  • Unless you use disk-buffer on the relay, the relay waits for acknowledgment from the server before acknowledging the messages to the client. If you send the messages in large batches, and the server can process the messages slowly (or the network connection is slow), you might have to adjust the message-acknowledgment-timeout() on the client.

  • If you use reliable disk-buffer on the relay, the relay will acknowledge the messages when the messages are written to the disk-buffer. That way, the client does not have to wait for the server to acknowledge the messages.

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