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.

This module provides no access control. Firewall rules can be used to deny connections from certain hosts.

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. Host for incoming traffic will become deprecated 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.

When the ListenAddr (or similar) directive specifies a hostname, the module binds to the first IP address that is exposed by the system for that hostname. On most systems that support IPv6, this address will be an IPv6 address. This means that client applications and the systems they run on will also need to have IPv6 support enabled, and must be able to connect to the same IPv6 address. NXLog output modules achieve this requirement through failover. For third-party client applications, the configuration details are application-specific but they should have the ability to detect which IP address the server is listening on when using a hostname to connect.

For client applications that don’t support IPv6, to avoid the behavior described above the ListenAddr directive of the listening module may be set to an IPv4 address e.g. 0.0.0.0.

Alternatively, the server-side system may be configured to prioritize IPv4 addresses for the hostname specified by the ListenAddr directive, although this is a more complicated and potentially intrusive approach. On most Linux-based systems, this can be achieved through the /etc/gai.conf configuration file. On BSD-based systems, the ip6addrctl command can be used. Windows is more limited and can only be configured to prioritize IPv4 over IPv6 addresses for ALL hostnames resolved on the system, by setting the following registry value to 0x20:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip6\Parameters\DisabledComponents

For more information see the Microsoft documentation on Configuring IPv6 in Windows for advanced users.

This limitation will be addressed in a future release by making listening modules bind to all available IPv4/IPv6 addresses that a hostname resolves to.

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 will become deprecated 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.

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>