Tuesday, November 29, 2011

Trusted Properties in rsyslog

Today, I implemented "trusted (syslog) properties" inside rsyslog's imuxsock module. The term "trusted" refers to the fact that these properties can not be faked by the logging application, creating an additional layer of log integrity protection. The idea is rooted in the journald proposal, where they are called "metadata" and "trusted fields". Actually I liked the idea implied by "trusted", but thought "property" would be a better name than "field".

The concept is not totally new. Actually, for some month rsyslog can patch the PID field of the syslog TAG with the correct pid, so that this cannot be mangled with. This was based on an idea from Lennart Poettering, which I found nice and implemented quickly (I met him at Linux Tag 2010 in N├╝rnberg, Germany where we discussed this and other things). The core idea is to use SCM_CREDENTIALS so that the OS itself records pid, gid and uid. With the new feature, this is taken one step further. Now, we also query the /proc virtual file system for additional information like the location of the logging application's binary. Undoubtedly, this provides some extra protection against faked messages. On the downside, it has some obvious overhead. A simple and immediate solution to this is to use rsyslog's omfile in zip mode. In journald, overhead is tried to avoid via a proprietary binary format, its event log, which provides compression features (but for syslog transmission the journald event log obviously needs to be decompressed as well). Some restrictions exist with trusted properties, some obvious, some less obvious (see the trusted property doc for details; it also has the list of currently supported properties).

The current implementation is in experimental status. Based on feedback, some specifics may be changed in future versions. Also, the current implementation does not try to be standards-compliant. This will probably also change in the future. I hope that the new capability is useful to the logging community. As a side-note, the new feature, implemented in one morning, also shows that it often is easy to extend existing technology instead of writing everything new from scratch ;)

The actual release announcement will go out either today or tomorrow. The code is available via the v5-devel git branch right now.

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