An apache log format that allow access logs (but not error logs) to be output in JSON format. I found this here: http://untergeek.com/2012/10/11/getting-apache-to-output-json-for-logstash/ -- but modified it for my purposes a good bit.

apache-json-log-format

# Access Logs
LogFormat "{ \
            \"@vips\":[\"%v\"], \
            \"@source\":\"%v%U%q\", \
            \"@source_host\": \"%v\", \
            \"@source_path\": \"%f\", \
            \"@tags\":[\"Apache\",\"Access\"], \
            \"@message\": \"%h %l %u %t \\\"%r\\\" %>s %b\", \
            \"@fields\": { \
                \"timestamp\": \"%{%Y-%m-%dT%H:%M:%S%z}t\", \
                \"clientip\": \"%a\", \
                \"duration\": %D, \
                \"status\": %>s, \
                \"request\": \"%U%q\", \
                \"urlpath\": \"%U\", \
                \"urlquery\": \"%q\", \
                \"method\": \"%m\", \
                \"referer\": \"%{Referer}i\", \
                \"user-agent\": \"%{User-agent}i\", \
                \"bytes\": %B \
                }  \
           }" ls_apache_json
 
 
# The catch-all
CustomLog "||/usr/local/bin/udpclient.pl 127.0.0.1 5001" ls_apache_json

Clever postprocessing of the non-JSON \x and \v escapes!

However, I think this one is wrong:

s/\v/\u0013/g

because \v is 13 in octal, but b in hex, so it should be:

s/\v/\u000b/g

Also beware that this will wrongly convert already escaped things like literal \x which should stay as is, as opposed to \x.

Quite right regarding \v

For \x, I think the right thing to do would be to read in strings of contiguous \xXX\xYY..., unescape them to raw bytes, and attempt to treat them as UTF-8. If that works, then output them as Unicode characters (encoded in UTF-8), and if not, then output each byte in \u00xx form.

Literal \x should not appear without already being escaped to \x,, but yes, care is needed.

In my deployment, I'm now using filebeat to send these logs to a logstash instance, so I'm toying with the idea of creating a filter plugin (I'm imagining calling this logstash-filter-reescape-c-to-json) which should do a good job of this, ready for actual JSON parsing.

The most common requirement for this is for when path names legitimately contain characters outside the US-ASCII range. Notably Chinese. It will help to clean up the error signal too, which could be useful later as a signal of attack.

Speaking of which, if any of the unescaped sequences mention dangerous characters such as \0 then I could set a tag, such as dangerous_escape_encountered

Small correction, the sed customlog command needs to be spawned with a shell (|$ vs |) for the >> redirect to have meaning.

Corrected line (with \v fix as well):

CustomLog "|$/bin/sed -e s/\\v/\\ u000b/g -e s/\\x/\\u00/ >> /var/log/httpd/access_log" myformat

Doc reference: https://httpd.apache.org/docs/2.4/logs.html#piped

Actually, even simpler is to use the new feature of Filebeat that handles JSON decode errors automatically:

  - type: log
    enabled: true
    paths:
      - /var/log/mylog.json
    json:
      keys_under_root: true
      add_error_key: true

If parsing fails, you'll get an entry with fields (among others):

{
  "error.message": "Error decoding JSON: invalid character 'o' in literal false (expecting 'a')",
  "error.type": "json",
  "message": "fooooo"
}

Then you can set up CustomLog to write directly to a file, bypassing sed.

Small correction, the sed customlog command needs to be spawned with a shell (|$ vs |) for the >> redirect to have meaning.

Seems to me that parts of the access_log contain user input from untrusted sources on the Internet. Is it really safe to pipe that through sed on the web server? The sed command will be running as the web server user and this might create opportunities for command injection. Likewise that's like one sed invocation per web request. Surely this will perform badly on a highly loaded server. Apache goes to great lengths to be scalable, but if we invoke a whole unix process on each and every log line, I think that would significantly hurt scalability.

Likewise that's like one sed invocation per web request. Surely this will perform badly on a highly loaded server.

@pacohope Luckily Apache 2.4 (and < 2.4 when using the || form) starts the filtering process once at Apache startup time, and pushes data through that one always-running process, so no, it doesn't add much overhead assuming the filter program is efficient, and it doesn't respawn anew for each request:

https://httpd.apache.org/docs/2.4/logs.html#piped