NXLog User Guide

111.21. UDP with IP Spoofing (om_udpspoof)

This module sends log messages as UDP datagrams to the address and port specified and allows the source address in the UDP packet to be spoofed in order to make the packets appear as if they were sent from another host. This is particularly useful in situations where log data needs to be forwarded to another server and the server uses the client address to identify the data source. With IP spoofing the UDP packets will contain the IP address of the originating client that produced the message instead of the forwarding server.

This module is very similar to the om_udp module and can be used as a drop-in replacement. The SpoofAddress configuration directive can be used to set the address if necessary. The UDP datagram will be sent with the local IP address if the IP address to be spoofed is invalid. The source port in the UDP datagram will be set to the port number of the local connection (the port number is not spoofed).

The network input modules (im_udp, im_tcp, and im_ssl) all set the $MessageSourceAddress field, and this value will be used when sending the UDP datagrams (unless SpoofAddress is explicitly set to something else). This allows logs to be collected over reliable and secure transports (like SSL), while the om_udpspoof module is only used for forwarding to the destination server that requires spoofed UDP input.

111.21.1. Configuration

The om_udpspoof module accepts the following directives in addition to the common module directives. The Host directive is required.

Host: The module will send UDP datagrams to this IP address or DNS hostname.

Port: The module will send UDP packets to this port. The default port is 514 if this directive is not specified.

LocalPort: This optional directive specifies the local port number of the connection. If this is not specified a random high port number will be used which is not always ideal in firewalled network environments.

MTU: This directive can be used to specify the maximum transfer size of the IP data fragments. If this value exceeds the MTU size of the sending interface, an error may occur and the packet be dropped. The default MTU value is 1500.

SockBufSize: This optional directive sets the socket buffer size (SO_SNDBUF) to the value specified. If this is not set, the operating system default is used.

SpoofAddress

This directive is optional. The IP address rewrite takes place depending on how this directive is specified.

Directive not specified: The IP address stored in the $MessageSourceAddress field is used so the module should work automatically when the SpoofAddress directive is not specified.
Constant literal value: The literal value may be a string or an ip4addr type. For example, SpoofAddress '10.0.0.42' and SpoofAddress 10.0.0.42 are equivalent.
Expression: The expression specified here will be evaluated for each message to be sent. Normally this can be a field name, but anything is accepted which evaluates to a string or an ip4addr type. For example, SpoofAddress $MessageSourceAddress has the same effect as when SpoofAddress is not set.

111.21.2. Examples

Example 615. Simple Forwarding with IP Address Spoofing

The im_tcp module will accept log messages via TCP and will set the $MessageSourceAddress field for each event. This value will be used by om_udpspoof to set the UDP source address when sending the data to logserver via UDP.

nxlog.conf


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  <Input tcp>
    Module  im_tcp
    Host    0.0.0.0
    Port    1514
</Input>

<Output udpspoof>
    Module  om_udpspoof
    Host    logserver.example.com
    Port    1514
</Output>

<Route tcp_to_udpspoof>
    Path    tcp => udpspoof
</Route>

Example 616. Forwarding Log Messages with Spoofed IP Address from Multiple Sources

This configuration accepts log messages via TCP and UDP, and also reads messages from a file. Both im_tcp and im_udp set the $MessageSourceAddress field for incoming messages, and in both cases this is used to set $sourceaddr. The im_file module instance is configured to set the $sourceaddr field to 10.1.2.3 for all log messages. Finally, the om_udpspoof output module instance is configured to read the value of the $sourceaddr field for spoofing the UDP source address.

nxlog.conf


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  <Input tcp>
    Module          im_tcp
    Host            0.0.0.0
    Port            1514
    Exec            $sourceaddr = $MessageSourceAddress;
</Input>

<Input udp>
    Module          im_udp
    Host            0.0.0.0
    Port            1514
    Exec            $sourceaddr = $MessageSourceAddress;
</Input>

<Input file>
    Module          im_file
    File            '/var/log/myapp.log'
    Exec            $sourceaddr = 10.1.2.3;
</Input>

<Output udpspoof>
    Module          om_udpspoof
    SpoofAddress    $sourceaddr
    Host            10.0.0.1
    Port            1514
</Output>

<Route all_to_file>
    Path            tcp, udp, file => udpspoof
</Route>