Configuring AxoSyslog relays

• 1: Configuring AxoSyslog on relay hosts
• 2: How relaying log messages works

1 - Configuring AxoSyslog on relay hosts

To configure AxoSyslog on a relay host, complete the following steps:

1. Install the AxoSyslog application on the host. For details on installing AxoSyslog on specific operating systems, see Install AxoSyslog.

2. Configure the network sources that collect the log messages sent by the clients.

3. Create a network destination that points to the AxoSyslog server.

4. Create a log statement connecting the network sources to the AxoSyslog server.

5. Configure the local sources that collect the log messages of the relay host.

6. Create a log statement connecting the local sources to the AxoSyslog server.

7. Enable the keep-hostname() and disable the chain-hostnames() options. (For details on how these options work, see Global options.)

   Note It is recommended to use these options on your AxoSyslog server as well.

8. Set filters and options (for example, TLS encryption) as necessary.

   Note

   By default, the AxoSyslog server treats the relayed messages as if they were created by the relay host, not the host that originally sent them to the relay. In order to use the original hostname on the AxoSyslog server, use the keep-hostname(yes) option both on the AxoSyslog relay and the AxoSyslog server. This option can be set individually for every source if needed.

   If you are relaying log messages and want to resolve IP addresses to hostnames, configure the first relay to do the name resolution.

Example: A simple configuration for relays

The following is a simple configuration file that collects local and incoming log messages and forwards them to a logserver using the IETF-syslog protocol.

    @version: 4.10.0
    @include "scl.conf"
    options {
        time-reap(30);
        mark-freq(10);
        keep-hostname(yes);
        chain-hostnames(no);
    };
    source s_local {
        system(); internal();
    };
    source s_network {
        syslog(transport(tcp));
    };
    destination d_syslog_tcp {
        syslog("192.168.1.5" transport("tcp") port(2010));
    };
    log {
        source(s_local); source(s_network);
        destination(d_syslog_tcp);
    };

2 - How relaying log messages works

Depending on your exact needs about relaying log messages, there are many scenarios and AxoSyslog options that influence how the log message will look like on the logserver. Some of the most common cases are summarized in the following example:

Consider the following example: client-host > relay > server, where the IP address of client-host is 192.168.1.2. The client-host device sends a syslog message to relay. Depending on the settings of relay, the following can happen.

• By default, the keep-hostname() option is disabled, so relay writes the IP address of the sender host (in this case, 192.168.1.2) to the HOST field of the syslog message, discarding any IP address or hostname that was originally in the message.

• If the keep-hostname() option is enabled on relay, but name resolution is disabled (the use-dns() option is set to no), relay uses the HOST field of the message as-is, which is probably 192.168.1.2.

• To resolve the 192.168.1.2 IP address to a hostname on relay using a DNS server, use the keep-hostname(no) and use-dns(yes) options. If the DNS server is properly configured and reverse DNS lookup is available for the 192.168.1.2 address, AxoSyslog will rewrite the HOST field of the log message to client-host.

  Note It is also possible to resolve IP addresses locally, without relying on the DNS server. For details on local name resolution, see Resolving hostnames locally.

• The above points apply to the AxoSyslog server (server) as well, so if relay is configured properly, use the keep-hostname(yes) option on server to retain the proper HOST field. Setting keep-hostname(no) on server would result in AxoSyslog rewriting the HOST field to the address of the host that sent the message to server, which is relay in this case.

• If you cannot or do not want to resolve the 192.168.1.2 IP address on relay, but want to store your log messages on server using the IP address of the original host (that is, client-host), you can enable the spoof-source() option on relay. However, spoof-source() works only under the following conditions:

  • The syslog-ng binary has been compiled with the --enable-spoof-source option.
  • The log messages are sent using the highly unreliable UDP transport protocol. (Extremely unrecommended.)