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diff --git a/source/configuration/ipv6.rst b/source/configuration/ipv6.rst new file mode 100644 index 0000000..aeec21d --- /dev/null +++ b/source/configuration/ipv6.rst @@ -0,0 +1,47 @@ +Notes on IPv6 Handling in Rsyslog +================================= + +**Rsyslog fully\* supports sending and receiving syslog messages via +both IPv4 and IPv6.** IPv6 is natively supported for both UDP and TCP. +However, there are some options that control handling of IPv6 +operations. I thought it is is a good idea to elaborate a little about +them, so that you can probably find your way somewhat easier. + +First of all, you can restrict rsyslog to using IPv4 or IPv6 addresses +only by specifying the -4 or -6 command line option (now guess which one +does what...). If you do not provide any command line option, rsyslog +uses IPv4 and IPv6 addresses concurrently. In practice, that means the +listener binds to both addresses (provided they are configured). When +sending syslog messages, rsyslog uses IPv4 addresses when the receiver +can be reached via IPv4 and IPv6 addresses if it can be reached via +IPv6. If it can be reached on either IPv4 and v6, rsyslog leaves the +choice to the socket layer. The important point to know is that it uses +whatever connectivity is available to reach the destination. + +**There is one subtle difference between UDP and TCP.** With the new +IPv4/v6 ignorant code, rsyslog has potentially different ways to reach +destinations. The socket layer returns all of these paths in a sorted +array. For TCP, rsyslog loops through this array until a successful TCP +connect can be made. If that happens, the other addresses are ignored +and messages are sent via the successfully-connected socket. + +For UDP, there is no such definite success indicator. Sure, the socket +layer may detect some errors, but it may not notice other errors (due to +the unreliable nature of UDP). By default, the UDP sender also tries one +entry after the other in the sorted array of destination addresses. When +a send fails, the next address is tried. When the send function finally +succeeds, rsyslogd assumes the UDP packet has reached its final +destination. However, if rsyslogd is started with the "-A" (capital A!) +was given on the command line, rsyslogd will continue to send messages +until the end of the destination address array is reached. This may +result in duplicate messages, but it also provides some additional +reliability in case a message could not be received. You need to be sure +about the implications before applying this option. In general, it is +NOT recommended to use the -A option. + +**\***\ rsyslog does not support RFC 3195 over IPv6. The reason is that +the RFC 3195 library, `liblogging <http://www.liblogging.org/>`_, +supports IPv4, only. Currently, there are no plans to update either +rsyslog to another RFC 3195 stack or update liblogging. There is simply +no demand for 3195 solutions. + |