UDP (im_udp)

This module accepts UDP datagrams on the configured address and port. UDP is the transport protocol of the legacy BSD Syslog as described in RFC 3164, so this module can be particularly useful to receive such messages from older devices that do not support other transports.

To examine the supported platforms, see the list of installer packages in the Available Modules chapter.

UDP is an unreliable transport protocol and does not guarantee delivery. Messages may not be received or may be truncated. It is recommended to use the TCP or SSL transport modules instead, if possible.

To reduce the likelihood of message loss, consider:

increasing the socket buffer size with SockBufSize,
raising the route priority by setting the Priority directive (to a low number such as 1), and
adding additional buffering by increasing the LogqueueSize or adding a pm_buffer instance.

This module does not provide access control. Firewall rules can be used to deny connections from certain hosts.

For parsing syslog messages, see the parse_syslog_bsd() procedure of the xm_syslog module.

Configuration

The im_udp module accepts the following directives in addition to the common module directives.

ListenAddr: The module accepts connections on the IP address or hostname and port defined here. The default address is localhost and the default port is 514. The port number can be defined by appending it at the end of the hostname or IP address using a colon as a separator (host:port). The port section of this directive and the Port directive are mutually exclusive. In case both are defined, the port number defined here takes precedence over a port defined in the Port directive. In case none of them is defined, the default port 514 is used.

To listen on multiple addresses or ports in a single module instance, this directive can be repeated multiple times. Both IPv4 and IPv6 addresses are supported. If a DNS name is used, the number of addresses or cnames should be kept below 16 to avoid potential issues caused by DNS response size limits.

Formerly called Host, this directive is now ListenAddr. The Host directive is deprecated and will be removed from NXLog EE 6.0.

When the Host directive is used with a hostname instead of an IP address, the the hostname will be resolved to an IP address for each new connection. If a resolver, e.g. DNS, returns multiple IP addresses, the module will connect to the first IP address. If a single output instance is configured with multiple Host directives or the resolver returns multiple addresses for a name, these hosts are accessed in failover mode. If a Host directive is configured with a hostname, the product performs a name lookup and establishes the connection to the first reachable address in the returned set of addresses. The module will remain connected to that address until it is stopped, or the connection is severed. DNS changes are therefore not picked up by the module without intervention. If the connection fails to the first address of the set, the module will attempt to connect to the next address, until it reaches the end of the set of addresses. Then it performs a lookup on the next Host directive, if so configured. Once all options are exhausted, the module will start over from the first Host directive, cycling through them again until connection can be reestablished.

Port: The module will listen for incoming connections on this port number. The default is port 514.

The Port directive is deprecated and will be removed from NXLog EE 6.0. After that, the port can only be defined in the ListenAddr directive.

ReusePort

This optional boolean directive enables synchronous listening on the same port by multiple module instances. Each module instance runs in its own thread, allowing NXLog to process incoming data simultaneously to take better advantage of multiprocessor systems. The default value is FALSE.

To enable synchronous listening, the configuration file should contain multiple im_udp module instances listening on the same port and the ReusePort directive set to TRUE, see the Examples section.

This directive is not supported on Windows platforms.

SockBufSize: This optional directive sets the socket buffer size (SO_RCVBUF) to the value specified. If not set, the operating system defaults are used. If UDP packet loss is occurring at the kernel level, setting this to a high value (such as 150000000) may help. On Windows systems, the default socket buffer size is extremely low, and using this option is highly recommended.

UseRecvmmsg: This boolean directive specifies that the recvmmsg() system call should be used, if available, to receive multiple messages per call to improve performance. The default is TRUE.

Fields

The following fields are used by im_udp.

$raw_event (type: string): The received string.

$MessageSourceAddress (type: string): The IP address of the remote host.

Examples

Pre-v5 syntax examples are included, they will become invalid with NXLog EE 6.0.

Example 1. Using the im_udp module

This configuration accepts log messages via UDP and writes them to a file.

nxlog.conf

<Input udp>
    Module        im_udp
    ListenAddr    192.168.1.1:514
</Input>

# Using the syntax prior to NXLog EE 5,
# where the port is defined in a separate directive.
#<Input udp>
#    Module       im_udp
#    Host         192.168.1.1
#    Port         514
#</Input>

<Output file>
    Module        om_file
    File          "tmp/output"
</Output>

<Route udp_to_file>
    Path          udp => file
</Route>

Example 2. Reusing a single port by multiple module instances

The configuration below provides two im_udp module instances to reuse port 514 via the ReusePort directive. Received messages are written to the /tmp/output file.

nxlog.conf

<Input udp_one>
    Module        im_udp
    ListenAddr    192.168.1.1:514
    ReusePort     TRUE
</Input>

<Input udp_two>
    Module        im_udp
    ListenAddr    192.168.1.1:514
    ReusePort     TRUE
</Input>

# Using the syntax prior to NXLog EE 5,
# where the port is defined in a separate directive.
#<Input udp_one>
#    Module       im_udp
#    Host         192.168.1.1
#    Port         514
#    ReusePort    TRUE
#</Input>
#
#<Input udp_two>
#    Module       im_udp
#    Host         192.168.1.1
#    Port         514
#    ReusePort    TRUE
#</Input>

<Output file>
    Module        om_file
    File          "tmp/output"
</Output>

<Route udp_to_file>
    Path          udp_one, udp_two => file
</Route>