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.

Configuration

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

Optional 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.

When the Host directive is used with a hostname instead of an IP address, 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 the connection can be re-established.

Port

The module will listen for incoming connections on this port number. The default port is 514 if this directive is not specified.

AllowIP

This optional directive can be used to allow IP addresses and/or networks to connect. The directive can be set multiple times to add different IPs or networks to allow. This directive is only active when the ListenAddr directive is present. In the absence of this directive, the BlockIP directive is considered. If both AllowIP and BlockIP are absent, then hosts are not restricted from connecting to a listening module.

The following formats may be used for the AllowIP directive:

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)

BlockIP

This optional directive can be used to deny IP addresses and/or networks to connect. The directive can be set multiple times to add different IPs or networks to deny. This directive is only active when the ListenAddr directive is present. In the absence of this directive, the AllowIP directive is considered. If both AllowIP and BlockIP are absent, then hosts are not restricted from connecting to a listening module.

The following formats may be used for the BlockIP directive:

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)

ExclusiveAddrUse

This optional boolean directive specifies whether the module instance should exclusively bind to the specified port. The default value is FALSE; multiple module instances can bind to the same port.

This directive is only supported on Windows platforms.

ReuseAddr

This optional boolean directive determines whether the module instance should forcibly bind to a port already in use. The default value is TRUE; multiple instances can listen on the same port and process data simultaneously.

ReusePort

This optional boolean directive specifies whether multiple im_tcp module instances can listen on the same port. When you enable this directive, multiple instances run in a separate thread, allowing NXLog Agent to process incoming logs simultaneously. See the examples below. The default value is FALSE; multiple instances cannot bind to the same port.

This directive is not supported on Windows platforms.

TCP connections are kept alive by keep-alive packets. This feature is enabled by default and cannot be disabled.

Fields

The following fields are used by im_tcp.

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

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

Examples

Example 1. Using the im_tcp module

With this configuration, NXLog Agent 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>

<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>