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<title>Notes on IPv6 Handling in Rsyslog</title>
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<h1>Notes on IPv6 Handling in Rsyslog</h1>
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<p><b>Rsyslog fully* supports sending and receiving syslog messages via both
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IPv4 and IPv6.</b> IPv6 is natively supported for both UDP and TCP. However,
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there are some options that control handling of IPv6 operations. I thought it is
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is a good idea to elaborate a little about them, so that you can probably find
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your way somewhat easier.</p>
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<p>First of all, you can restrict rsyslog to using IPv4 or IPv6 addresses only
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by specifying the -4 or -6 command line option (now guess which one does
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what...). If you do not provide any command line option, rsyslog uses IPv4 and
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IPv6 addresses concurrently. In practice, that means the listener binds to both
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addresses (provided they are configured). When sending syslog messages, rsyslog
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uses IPv4 addresses when the receiver can be reached via IPv4 and IPv6 addresses
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if it can be reached via IPv6. If it can be reached on either IPv4 and v6,
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rsyslog leaves the choice to the socket layer. The important point to know is
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that it uses whatever connectivity is available to reach the destination.</p>
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<p><b>There is one subtle difference between UDP and TCP.</b> With the new
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IPv4/v6 ignorant code, rsyslog has potentially different ways to reach
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destinations. The socket layer returns all of these paths in a sorted array.
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For TCP, rsyslog loops through this array until a successful TCP connect can be
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made. If that happens, the other addresses are ignored and messages are sent via
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the successfully-connected socket.</p>
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<p>For UDP, there is no such definite success indicator. Sure, the socket layer
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may detect some errors, but it may not notice other errors (due to the
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unreliable nature of UDP). By default, the UDP sender also tries one entry after
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the other in the sorted array of destination addresses. When a send fails, the
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next address is tried. When the send function finally succeeds, rsyslogd assumes
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the UDP packet has reached its final destination. However, if rsyslogd is
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started with the "-A" (capital A!) was given on the command line, rsyslogd will
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continue to send messages until the end of the destination address array is
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reached. This may result in duplicate messages, but it also provides some
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additional reliability in case a message could not be received. You need to be
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sure about the implications before applying this option. In general, it is NOT
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recommended to use the -A option.</p>
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<p><i><b>*</b>rsyslog does not support RFC 3195 over IPv6. The reason is that
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the RFC 3195 library, <a href="http://www.liblogging.org/">liblogging</a>,
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supports IPv4, only. Currently, there are no plans to update either rsyslog to
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another RFC 3195 stack or update liblogging. There is simply no demand for 3195
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<p><font size="2">Last Updated: 2007-07-02<br>
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Copyright © 2007 by Rainer Gerhards, released under the GNU GPL V2 or later.</font></p>
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