From 4b11c91ce4be62b2ba09be15fd24c4f5bb627925 Mon Sep 17 00:00:00 2001 From: Rainer Gerhards Date: Thu, 24 Nov 2005 17:16:55 +0000 Subject: documentation on syslog-protocol support added --- doc/syslog-protocol.html | 156 +++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 156 insertions(+) create mode 100644 doc/syslog-protocol.html (limited to 'doc') diff --git a/doc/syslog-protocol.html b/doc/syslog-protocol.html new file mode 100644 index 00000000..e5789ab8 --- /dev/null +++ b/doc/syslog-protocol.html @@ -0,0 +1,156 @@ + + +syslog-protocol support in rsyslog + + +

syslog-protocol support in rsyslog

Rsyslog provides a trial +implementation of the proposed + +syslog-protocol standard. The intention of this implementation is to +find out what inside syslog-protocol is causing problems during implementation. +As syslog-protocol is a standard under development, its support in rsyslog is +highly volatile. It may change from release to release. So while it provides +some advantages in the real world, users are cautioned against using it right +now. If you do, be prepared that you will probably need to update all of your +rsyslogds with each new release. If you try it anyhow, please provide feedback +as that would be most benefitial for us.

Currently supported message format

Due to recent discussion on syslog-protocol, we do not follow any specific +revision of the draft but rather the candidate ideas. The format supported +currently is:

<PRI>VERSION SP TIMESTAMP SP HOSTNAME SP APP-NAME SP PROCID SP MSGID SP [SD-ID]s +SP MSG

Field syntax and semantics are as defined in IETF I-D syslog-protocol-15.

Capabilities Implemented

receiving message in the supported format (see above)
sending messages in the supported format
relaying messages
receiving messages in either legacy or -protocol format and transforming + them into the other one
virtual availability of TAG, PROCID, APP-NAME, MSGID, SD-ID no matter if + the message was received via legacy format, API or syslog-protocol format (non-present + fields are being emulated with great success)
maximum message size is set via preprocessor #define

Findings

This lists what has been found during implementation:

The same receiver must be able to support both legacy and + syslog-protocol syslog messages. Anything else would be a big inconvenience + to users and would make deployment much harder. The detection must be done + automatically (see below on how easy that is).
NUL characters inside MSG cause the message to be truncated at + that point. This is probably a major point for many C-based implementations. + No measures have yet been taken against this. Modifying the code to "cleanly" + support NUL characters is non-trivial, even though rsyslogd already has some + byte-counted string library (but this is new and not yet available + everywhere).
character encoding in MSG: is is problematic to do the right + UTF-8 encoding. The reason is that we pick up the MSG from the local domain + socket (which got it from the syslog(3) API). The text obtained does not + include any encoding information, but it does include non US-ASCII + characters. It may also include any other encoding. Other than by guessing + based on the provided text, I have no way to find out what it is. In order + to make the syslogd do anything useful, I have now simply taken the message + as is and stuffed it into the MSG part. Please note that I think this will + be a route that other implementors would take, too.
A minimal parser is easy to implement. It took me roughly 2 hours to add + it to rsyslogd. This includes the time for restructering the code to be able + to parse both legacy syslog as well as syslog-protocol. The parser has some + restrictions, though
- STRUCTURED-DATA field is extracted, but not validated. Structured data + "[test ]]" is not caught as an error. Nor are any other errors caught. For + my needs with this syslogd, that level of structued data processing is + probably sufficient. I do not want to parse/validate it in all cases. This + is also a performance issue. I think other implementors could have the same + view. As such, we should not make validation a requirement.
- MSG is not further processed (e.g. Unicode not being validated)
- the other header fields are also extracted, but no validation is + performed right now. At least some validation should be easy to add (not + done this because it is a proof-of-concept and scheduled to change).
+
Universal access to all syslog fields (missing ones being emulated) was + also quite easy. It took me around another 2 hours to integrate emulation of + non-present fields into the code base.
The version at the start of the message makes it easy to detect if we + have legacy syslog or syslog-protocol. Do NOT move it to somewhere inside + the middle of the message, that would complicate things. It might not be + totally fail-safe to just rely on "1 " as the "cookie" for a syslog-protocol. + Eventually, it would be good to add some more uniqueness, e.g. "@#1 ".
I have no (easy) way to detect truncation if that happens on the UDP + stack. All I see is that I receive e.g. a 4K message. If the message was e.g. + 6K, I received two chunks. The first chunk (4K) is correctly detected as a + syslog-protocol message, the second (2K) as legacy syslog. I do not see what + we could do against this. This questions the usefulness of the TRUNCATE bit. + Eventually, I could look at the UDP headers and see that it is a fragment. I + have looked at a network sniffer log of the conversation. This looks like + two totally-independant messages were sent by the sender stack.
The maximum message size is currently being configured via a + preprocessor #define. It can easily be set to 2K or 4K, but more than 4K is + not possible because of UDP stack limitations. Eventually, this can be + worked around, but I have not done this yet.
rsyslogd can accept syslog-protocol formatted messages but is able to + relay them in legacy format. I find this a must in real-life deployments. + For this, I needed to do some field mapping so that APP-NAME/PROCID are + mapped into a TAG.
rsyslogd can also accept legacy syslog message and relay them in + syslog-protocol format. For this, I needed to apply some sub-parsing of the + TAG, which on most occasions provides correct results. There might be some + misinterpretations but I consider these to be mostly non-intrusive.
Messages received from the syslog API (the normal case under *nix) also + do not have APP-NAME and PROCID and I must parse them out of TAG as + described directly above. As such, this algorithm is absolutely vital to + make things work on *nix.
I have an issue with messages received via the syslog(3) API (or, to be + more precise, via the local domain socket this API writes to): These + messages contain a timestamp, but that timestamp does neither have the year + nor the high-resolution time. The year is no real issue, I just take the + year of the reception of that message. There is a very small window of + exposure for messages read from the log immediately after midnight Jan 1st. + The message in the domain socket might have been written immediately before + midnight in the old year. I think this is acceptable. However, I can not + assign a high-precision timestamp, at least it is somewhat off if I take the + timestamp from message reception on the local socket. An alternative might + be to ígnore the timestamp present and instead use that one when the message + is pulled from the local socket (I am talking about IPC, not the network - + just a reminder...). This is doable, but eventually not advisable. It looks + like this needs to be resolved via a configuration option.
rsyslogd already advertised its origin information on application + startup (in a syslog-protocol-14 compatible format). It is fairly easy to + include that with any message if desired (not currently done).
A big problem I noticed are malformed messages. In -syslog-protocol, we + recommend/require to discard malformed messages. However, in practice users + would like to see everything that the syslogd receives, even if it is in + error. For the first version, I have not included any error handling at all. + However, I think I would deliberately ignore any "discard" requirement. My + current point of view is that in my code I would eventually flag a message + as being invalid and allow the user to filter on this invalidness. So these + invalid messages could be redirected into special bins.
The error logging recommendations (those I insisted on;)) are not really + practical. My application has its own error logging philosophy and I will + not change this to follow a draft.

Conlusions/Suggestions

These are my personal conclusions and suggestions. Obviously, they must be +discussed ;)

NUL should be disallowd in MSG
As it is not possible to definitely know the character encoding of the + application-provided message, MSG should not be specified to use UTF-8 + exclusively. Instead, it is suggested that any encoding may be used but + UTF-8 is preferred. To detect UTF-8, the MSG should start with the UTF-8 + byte order mask of "EF BB BF" if it is UTF-8 encoded (see section 155.9 of + + http://www.unicode.org/versions/Unicode4.0.0/ch15.pdf)
Requirements to drop messages should be reconsidered. I guess I would + not be the only implementor ignoring them.
Logging requirements should be reconsidered and probably be removed.

+ + + -- cgit