TCP (im_tcp)

This module accepts TCP connections on the configured address and port. It can handle multiple simultaneous connections. The TCP transfer protocol provides more reliable log transmission than UDP. If security is a concern, consider using the im_ssl module instead.

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

Configuration

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

ListenAddr: The module will accept connections on this IP address or DNS hostname. For security, the default listen address is localhost (the localhost loopback address is not accessible from the outside). To receive logs from remote hosts, the address specified here must be accessible. The any address 0.0.0.0 is commonly used here.

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). IPv6 addresses must be enclosed in square brackets ([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 port is 514 if this directive is not specified.

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.

AllowIP

This optional directive may be used to specify a whitelist of IP addresses and/or networks that are allowed to connect. The directive can be specified more than once to add different IPs or networks to the whitelist. This directive is only active when the Listen or ListenAddr directives are present. In the absence of this directive, there is no restriction on the hosts which may connect to a listening module. The following formats may be used:

0.0.0.0 (IPv4 address)
0.0.0.0/32 (IPv4 network with subnet bits)
0.0.0.0/0.0.0.0 (IPv4 network with subnet address)
aa::1 (IPv6 address)
aa::12/64 (IPv6 network with subnet bits)

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_tcp 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.

ReuseAddr: This optional boolean directive allows multiple module instances to bind and reuse the same addresses. Each NXLog module can bind and reuse the same port without the operating system being completely sure the port is no longer in use. The default value is TRUE.

ExclusiveAddrUse

This optional boolean directive allows only one module instance to bind and use the specified address. The default value is FALSE.

To enable only one instance bind and use a specified port, the configuration file should contain directive EXCLUSIVEADDRUSE set to TRUE.

This directive is only supported on Windows platforms.

Fields

The following fields are used by im_tcp.

$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_tcp module

With this configuration, NXLog listens for TCP connections on port 1514 and writes the received log messages to a file.

nxlog.conf

<Input tcp>
    Module      im_tcp
    ListenAddr  0.0.0.0:1514
</Input>

# Using the syntax prior to NXLog EE 5,
# where the port is defined in a separate directive.
#<Input tcp>
#    Module  im_tcp
#    Host    0.0.0.0
#    Port    1514
#</Input>

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

<Route tcp_to_file>
    Path    tcp => file
</Route>

Example 2. Reusing a single port by multiple module instances

The configuration below provides two im_tcp module instances to reuse port 1514 via the ReusePort directive. Received messages are written to the /tmp/output file.

nxlog.conf

<Input tcp_one>
    Module      im_tcp
    ListenAddr  192.168.31.11:1514
    ReusePort   TRUE
</Input>

<Input tcp_two>
    Module      im_tcp
    ListenAddr  192.168.31.11:1514
    ReusePort   TRUE
</Input>

# Using the syntax prior to NXLog EE 5,
# where the port is defined in a separate directive.
#<Input tcp_one>
#    Module      im_tcp
#    Host        192.168.31.11
#    Port        1514
#    ReusePort   TRUE
#/Input>
#
#<Input tcp_two>
#    Module      im_tcp
#    Host        192.168.31.11
#    Port        1514
#    ReusePort   TRUE
#</Input>