From 765575e41f366135fe32ce888cfcc396617747c3 Mon Sep 17 00:00:00 2001 From: Rainer Gerhards Date: Tue, 23 Sep 2008 13:26:57 +0200 Subject: added doc on malformed messages, cause and how to work-around, to the doc set --- doc/syslog_parsing.html | 196 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 196 insertions(+) create mode 100644 doc/syslog_parsing.html (limited to 'doc/syslog_parsing.html') diff --git a/doc/syslog_parsing.html b/doc/syslog_parsing.html new file mode 100644 index 00000000..57da6657 --- /dev/null +++ b/doc/syslog_parsing.html @@ -0,0 +1,196 @@ + +syslog parsing in rsyslog + + +

syslog parsing in rsyslog

Written by Rainer Gerhards +(2008-09-23)

We regularly receive messages asking why rsyslog +parses this or that message incorrectly. Of course, it turns out that rsyslog does +the right thing, but the message sender does not. And also of course, this is not even +of the slightest help to the end user experiencing the problem ;). So I thought I write this +paper. It describes the problem source and shows potential solutions (aha!). +

Syslog Standardization

+The syslog protocol has not been standardized until relatively recently.The first document "smelling" a bit +like a standard is RFC 3164, which dates back +to August 2001. The problem is that this document is no real standard. It has assigned "informational" +status by the IETF which means it provides some hopefully +useful information but does not demand anything. It is impossible to "comply" to an informational +document. This, of course, doesn't stop marketing guys from telling they comply to RFC3164 and +it also does not stop some techs to tell you "this and that does not comply to RFC3164, so it is +<anybody else but them>'s fault". +

Then, there is RFC3195, which is +a real standard. In it's section 3 it makes (a somewhat questionable) reference to (informational) +RFC 3164 which may be interpreted in a way that RFC3195 standardizes the format layed out +in RFC 3164 by virtue of referencing them. So RFC3195 seems to extend its standardization +domain to the concepts layed out in RFC 3164 (which is why I tend to find that refrence +questionable). In that sense, RFC3195 standardizes the format informationally described in +RFC3164, Section 4. But it demands it only for the scope of RFC3195, which is syslog over +BEEP - and NOT syslog over UDP. So one may argue whether or not the RFC3164 format could +be considered a standard for any non-BEEP (including UDP) syslog, too. In the strict view +I tend to have, it does not. Refering to the RFC3195 context usually does not help, +because there are virtually no RFC3195 implementations available (at this time, +I would consider this RFC a failure). +

Now let's for a short moment assume that RFC3195 would somehow be able to demand +RFC3164 format for non-BEEP syslog. So we could use RFC3164 format as a standard. But does +that really help? Let's cite RFC 3164, right at the begining of section 4 (actually, this +is the first sentence): +

+   The payload of any IP packet that has a UDP destination port of 514
+   MUST be treated as a syslog message. 
++

Think a bit about it: this means that whatever is send to port 514 must be considered +a valid syslog message. No format at all is demanded. So if "this is junk" is sent to +UDP port 514 - voila, we have a valid message (interestingly, it is no longer a syslog +message if it is sent to port 515 ;)). You may now argue that I am overdoing. So let's +cite RFC 3164, Section 5.4, Example 2: +

+  Example 2
+
+        Use the BFG!
+
+   While this is a valid message, it has extraordinarily little useful
+   information.
+

As you can see, RFC3164 explicitely states that no format at all is required. +

Now a side-note is due: all of this does not mean that the RFC3164 authors +did not know what they were doing. No, right the contrary is true: RFC3164 mission +is to describe what has been seen in practice as syslog messages and the +conclusion is quite right that there is no common understanding on the +message format. This is also the reason why RFC3164 is an informational document: +it provides useful information, but does not precisely specify anything. +

After all of this bashing, I now have to admit that RFC3164 has some format +recommendations layed out in section 4. The format described has quite some +value in it and implementors recently try to follow it. This format is usually meant +when someone tells you that a software is "RFC3164 compliant" or expects "RFC3164 compliant messages". +I also have to admit that rsyslog also uses this format and, in the sense outlined here, +expects messages received to be "RFC3164 compliant" (knowingly that such a beast does not +exist - I am simply lying here ;)). +

Please note that there is some relief of the situation in reach. There is a new normative +syslog RFC series upcoming, and it specifies a standard message format. At the time of +this writing, the main documents are sitting in the RFC editor queue waiting for a transport +mapping to be completed. I personally expect them to be assigned RFC numbers in 2009. +

Practical Format Requirements

From a practical point of view, the message format expected (and generated by +default in legacy mode) is: +


+<PRI>TIMESTAMP SP HOST SP TAG MSG(Freetext)
+

SP is the ASCII "space" character and the definition of the rest of the fields +can be taken from RFC3164. Please note that there also is a lot of confusion on what +syntax and semantics the TAG actually has. This format is called "legacy syslog" because +it is not well specified (as you know by now) and has been "inherited from the real world". +

Rsyslog offers two parsers: one for the upcoming RFC series and one for legacy format. We +concentrate on the later. That parser applies some logic to detect missing hostnames, +is able to handle various ways the TIMESTAMP is typically malformed. In short it applies +a lot of guesswork in trying to figure out what a message really means. I am sure the +guessing algorithm can be improved, and I am always trying that when I see new malformed +messages (and there is an ample set of them...). However, this finds its limits where +it is not possible to differentiate between two entities which could be either. +For example, look at this message: +


+<144>Tue Sep 23 11:40:01 taghost sample message
+

Does it contain a hostname? Mabye. The value "taghost" is a valid hostname. Of course, it is +also a valid tag. If it is a hostname, the tag's value is "sample" and the msg value is "message". +Or is the hostname missing, the tag is "taghost" and msg is "sample message"? As a human, I tend +to say the later interpretation is correct. But that's hard to tell the message parser (and, no, I do +not intend to apply artificial intelligence just to guess what the hostname value is...). +

One approach is to configure the parser so that it never expects hostnames. This becomes problematic +if you receive messages from multiple devices. Over time, I may implement parser conditionals, +but this is not yet available and I am not really sure if it is needed comlexity... +

Things like this, happen. Even more scary formats happen in practice. Even from mainstream +vendors. For example, I was just asked about this message (which, btw, finally made me +write this article here): +


+"<130> [ERROR] iapp_socket_task.c 399: iappSocketTask: iappRecvPkt returned error"
+

If you compare it with the format RFC3164 "suggests", you'll quickly notice that +the message is "a bit" malformed. Actually, even my human intelligence is not sufficient +to guess if there is a TAG or not (is "[ERROR]" a tag or part of the message). I may not be +the smartest guy, but don't expect me to program a parser that is smarter than me. +

To the best of my konwledge, these vendor's device's syslog format can be configured, so it +would proabably be a good idea to include a (sufficiently well-formed) timestamp, +the sending hostname and (maybe?) a tag to make this message well parseable. +I will also once again take this sample and see if we can apply some guesswork. +For example, "[" can not be part of a well-formed TIMESTAMP, so logic can conclude +there is not TIMESTAMP. Also, "[" can not be used inside a valid hostname, so +logic can conclude that the message contains no hostname. Even if I implement this +logic (which I will probably do), this is a partial solution: it is impossible to +guess if there is a tag or not (honestly!). And, even worse, it is a solution only for +those set of messages that can be handled by the logic described. Now consider this +hypothetical message: +


+"<130> [ERROR] host.example.net 2008-09-23 11-40-22 PST iapp_socket_task.c 399: iappSocketTask: iappRecvPkt returned error"
+

Obviously, it requires additional guesswork. If we iterate over all the cases, we +can very quickly see that it is impossible to guess everything correct. In the example above +we can not even surely tell if PST should be a timezone or some other message property. +

A potential solution is to generate a parser-table based parser, but this requires +considerable effort and also has quite some runtime overhead. I try to avoid this for +now (but I may do it, especially if someone sponsors this work ;)). Side-note: if you want +to be a bit scared about potential formats, you may want to have a look at my paper +"On the Nature of Syslog Data". +

Work-Around

The number one work-around is to configure your devices so that they emit +(sufficiently) well-formed messages. You should by now know what these look +like. +

If that cure is not available, there are some things you can do in rsyslog to +handle the situation. First of all, be sure to read about +rsyslog.conf format +and the property replacer and properties specifically. +You need to understand that everything is configured in rsyslog. And that the message is parsed +into properties. There are also properties available which do not stem back directly to parsing. +Most importantly, %fromhost% property holds the name of the system rsyslog received +the message from. In non-relay cases, this can be used instead of hostname. In relay cases, +there is no cure other than to either fix the orginal sender or at least one of the +relays in front of the rsyslog instance in question. Similarly, you can use %timegenerated% +instead of %timereported%. Timegenerated is the time the message hit rsyslog for the first +time. For non-relayed, locally connected peers, Timegenerated should be a very close approximation +of the actual time a message was formed at the sender (depending, of course, on potential +internal queueing inside the sender). +Also, you may use the +%rawmsg% property together with the several extraction modes the property replacer supports. +Rawmsg contains the message as it is received from the remote peer. In a sense, you can +implement a post-parser with this method. +

To use these properties, you need to define your own templates and assign them. Details +can be found in the above-quoted documentation. Just let's do a quick example. Let's say +you have the horrible message shown above and can not fix the sending device for +some good reason. In rsyslog.conf, you used to say: +


+*.* /var/log/somefile
+

Of course, things do not work out well with that ill-formed message. So you decide +to dump the rawmsg to the file and pull the remote host and time of message generation +from rsyslog's internal properties (which, btw, is clever, because otherwise there is no +indication of these two properties...). So you need to define a template for that and +make sure the template is used with your file logging action. This is how it may look: +


+$template, MalfromedMsgFormater,"%timegenerated% %fromhost% %rawmsg:::drop-last-lf%\n"
+*.* /var/log/somefile;MalformedMsgFormatter
+

This will make your log much nicer, but not look perfect. Experiment a bit +with the available properties and replacer extraction options to fine-tune it +to your needs. +

Wrap-Up

Syslog message format is not sufficiently standardized. There exists a weak +"standard" format, which is used by a good number of implementations. However, there +exist many others, including mainstream vendor implementations, which have a +(sometimes horribly) different format. Rsyslog tries to deal with anomalies but +can not guess right in all instances. If possible, the sender should be configured +to submit well-formed messages. If that is not possible, you can work around these +issues with rsyslog's property replacer and template system. +

I hope this is a useful guide. You may also have a look at the +rsyslog troubleshooting guide for further help and places where +to ask questions. +

[manual index] [rsyslog site]

+ -- cgit