TelnetAppender
The TelnetAppender listens for incoming TCP connections and streams rendered log events to
every connected client, so that a running application’s log can be watched over a socket with a
telnet client.
Unlike every other appender, it does not write to a destination you configure: it accepts
connections from clients that reach it.
It is intended for diagnostic use on trusted networks; see Intended use and trust model.
At most 20 clients may be connected at the same time; further connection attempts are answered with a message and closed.
The following example configures the appender to listen on port 8023.
<appender name="TelnetAppender" type="log4net.Appender.TelnetAppender">
<listenAddress value="127.0.0.1" />
<port value="8023" />
<sendTimeoutMillis value="5000" />
<layout type="log4net.Layout.PatternLayout">
<conversionPattern value="%date %-5level %logger - %message%newline" />
</layout>
</appender>Settings
port-
The TCP port to listen on. The default is
23, the telnet port. listenAddress-
The local address to accept connections on. The default is
127.0.0.1, the machine the application runs on, which is what diagnostic use usually needs.The stream is unauthenticated and unencrypted, so watching it from another machine is opt-in: set this to
0.0.0.0for every IPv4 interface, or::for every IPv6 one, and keep untrusted parties away from the port. An IPv6 address may be given instead, and the listening socket follows its family. sendTimeoutMillis-
How long, in milliseconds, a write to a client may block before that client is treated as dead and disconnected. The default is
5000.Clients are written to from a background thread, so this no longer delays the application: it is how long a client that stopped reading holds up the stream before it is dropped.
0blocks indefinitely and is not recommended. sendQueueSize-
How many rendered events may wait to be written to the clients. The default is
500.A full queue drops the event from the telnet stream, counted and reported once. Only the telnet view is lost; every other appender still receives the event.
enqueueTimeoutMillis-
How long, in milliseconds, a logging call may wait for room in the send queue. The default is
50.This is the only delay a client can impose on the application. While the queue stays full it caps logging at 20 events per second;
0drops immediately instead of waiting.
Intended use and trust model
This appender is a diagnostic tool for trusted networks. It is meant for watching the log of a running application during development or while investigating a problem, not as a general-purpose logging destination.
Like every other appender destination, the connecting client is trusted: by enabling the appender the operator declares that whoever can reach the port is allowed to read the application’s log. The appender therefore performs no authentication of its own.
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The connection is unauthenticated and unencrypted. There is no option to require a credential or to enable TLS. Any client that can reach the port receives the full rendered log stream, including whatever the
layout renders: user names, session identifiers, request parameters, stack traces.
The appender listens on
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