Squid Web Cache wiki

Squid Web Cache documentation

🔗 Feature: Customizable Log Formats

🔗 Configuration Options

logformat option in squid.conf defines a named format for log output.

access_log option then uses the named format to write a given log file with its output about each request in that format.

🔗 Default Formats

The default formats are built-in to squid and do not need to be defined manually. They can be used simply by specifying the default format name on access_log lines.

🔗 squid

The native format for Squid

The format is:

time elapsed remotehost code/status bytes method URL rfc931 peerstatus/peerhost type

The native log file format logs more and different information than the common log file format: the request duration, some timeout information, the next upstream server address, and the content type.

There exist tools, which convert one file format into the other. Please mind that even though the log formats share most information, both formats contain information which is not part of the other format, and thus this part of the information is lost when converting. Especially converting back and forth is not possible without loss.

squid2common.pl is a conversion utility, which converts any of the squid log file formats into the old CERN proxy style output. There exist tools to analyse, evaluate and graph results from that format.

🔗 common

The Common Logfile Format is used by numerous HTTP servers. This format consists of the following seven fields:

remotehost rfc931 authuser [date] "method URL" status bytes

It is parsable by a variety of tools. The common format contains different information than the native log file format. The HTTP version is logged, which is not logged in native log file format.

🔗 Squid native access.log format in detail

We recommend that you use Squid’s native log format due to its greater amount of information made available for later analysis. The print format line for native access.log entries looks like this:

"%9d.%03d %6d %s %s/%03d %d %s %s %s %s%s/%s %s"

Therefore, an access.log entry usually consists of (at least) 10 columns separated by one ore more spaces:

  1. time A Unix timestamp as UTC seconds with a millisecond resolution. This is the time when Squid started to log the transaction, which normally happens at the end of a transaction lifecycle, after the entire request was received from and the entire response was sent to the HTTP client. To get the approximate transaction start time, subtract transaction duration (the second field) from this field, minding the different time units of those two fields. You can convert Unix timestamps into something more human readable using this short perl script:

       s/^\d+\.\d+/localtime $&/e;
  2. duration The elapsed time considers how many milliseconds the transaction busied the cache. It differs in interpretation between TCP and UDP:
    • For HTTP this is basically the time from having received the request to when Squid finishes sending the last byte of the response.
    • For ICP, this is the time between scheduling a reply and actually sending it.
    • Please note that the entries are logged after the reply finished being sent, not during the lifetime of the transaction.
  3. client address The IP address of the requesting instance, the client IP address. The client_netmask configuration option can distort the clients for data protection reasons, but it makes analysis more difficult. Often it is better to use one of the log file anonymizers.
  4. result codes This column is made up of two entries separated by a slash. This column encodes the transaction result:
    • The cache result of the request contains information on the kind of request, how it was satisfied, or in what way it failed. Please refer to Squid result codes for valid symbolic result codes. Several codes from older versions are no longer available, were renamed, or split. Especially the ERR_ codes do not seem to appear in the log file any more. Also refer to Squid result codes for details on the codes no longer available. The status part contains the HTTP result codes with some Squid specific extensions. Squid uses a subset of the RFC defined error codes for HTTP. Refer to section status codes for details of the status codes recognized.
  5. bytes The size is the amount of data delivered to the client. Mind that this does not constitute the net object size, as headers are also counted. Also, failed requests may deliver an error page, the size of which is also logged here.
  6. request method The request method to obtain an object. Please refer to section request-methods for available methods. If you turned off log_icp_queries in your configuration, you will not see (and thus unable to analyze) ICP exchanges. The PURGE method is only available, if you have an ACL for “method purge” enabled in your configuration file.
  7. URL This column contains the URL requested. Please note that the log file may contain whitespace for the URI. The default configuration for uri_whitespace denies or truncates whitespace, though.
  8. user The eighth column may contain the user identity for the requesting client. This may be sourced from one of HTTP authentication, an external ACL helper, TLS authentication, or IDENT lookup (RFC 931) - checked in that order with the first to present information displayed. If no user identity is available a “-“ will be logged.
  9. hierarchy code The hierarchy information consists of three items:
    • Any hierarchy tag may be prefixed with TIMEOUT_, if the timeout occurs waiting for all ICP replies to return from the neighbours. The timeout is either dynamic, if the icp_query_timeout was not set, or the time configured there has run up.
    • A code that explains how the request was handled, e.g. by forwarding it to a peer, or going straight to the source. Refer to Hierarchy Codes for details on hierarchy codes and removed hierarchy codes.
    • The IP address or hostname where the request (if a miss) was forwarded. For requests sent to origin servers, this is the origin server’s IP address. For requests sent to a neighbor cache, this is the neighbor’s hostname. NOTE: older versions of Squid would put the origin server hostname here.
  10. type The content type of the object as seen in the HTTP reply header. Please note that ICP exchanges usually don’t have any content type, and thus are logged “-“. Also, some weird replies have content types “:” or even empty ones.

There may be two more columns in the access.log, if the (debug) option log_mime_headers is enabled In this case, the HTTP request headers are logged between a “[" and a "]”, and the HTTP reply headers are also logged between “[" and "]”. All control characters like CR and LF are URL-escaped, but spaces are not escaped! Parsers should watch out for this.

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