Squid Log Files

The logs are a valuable source of information about Squid workloads and performance. The logs record not only access information, but also system configuration errors and resource consumption (e.g. memory, disk space). There are several log file maintained by Squid. Some have to be explicitly activated during compile time, others can safely be deactivated during run-time.

There are a few basic points common to all log files. The time stamps logged into the log files are usually UTC seconds unless stated otherwise. The initial time stamp usually contains a millisecond extension.

squid.out

If you run your Squid from the RunCache script, a file squid.out contains the Squid startup times, and also all fatal errors, e.g. as produced by an assert() failure. If you are not using RunCache, you will not see such a file.

• RunCache has been obsoleted since Squid-2.6. Modern Squid run as daemons usually log this output to the system syslog facility or if run manually to stdout for the account which operates the master daemon process.

cache.log

The cache.log file contains the debug and error messages that Squid generates. If you start your Squid using the -s command line option, a copy of certain messages will go into your syslog facilities. It is a matter of personal preferences to use a separate file for the squid log data.

From the area of automatic log file analysis, the cache.log file does not have much to offer. You will usually look into this file for automated error reports, when programming Squid, testing new features, or searching for reasons of a perceived misbehavior, etc.

useragent.log

The user agent log file is only maintained, if

• you configured the compile time --enable-useragent-log option, and

• you pointed the useragent_log configuration option to a file.

From the user agent log file you are able to find out about distribution of browsers of your clients. Using this option in conjunction with a loaded production squid might not be the best of all ideas.

store.log

The store.log file covers the objects currently kept on disk or removed ones. As a kind of transaction log it is usually used for debugging purposes. A definitive statement, whether an object resides on your disks is only possible after analyzing the complete log file. The release (deletion) of an object may be logged at a later time than the swap out (save to disk).

The store.log file may be of interest to log file analysis which looks into the objects on your disks and the time they spend there, or how many times a hot object was accessed. The latter may be covered by another log file, too. With knowledge of the cache_dir configuration option, this log file allows for a URL to filename mapping without recursing your cache disks. However, the Squid developers recommend to treat store.log primarily as a debug file, and so should you, unless you know what you are doing.

The print format for a store log entry (one line) consists of thirteen space-separated columns, compare with the storeLog() function in file src/store_log.c:

9ld.%03d %-7s %02d %08X %s %4d %9ld %9ld %9ld %s %ld/%ld %s %s

1. time The timestamp when the line was logged in UTC with a millisecond fraction.

2. action The action the object was sumitted to, compare with src/store_log.c:

   • CREATE Seems to be unused.

   • RELEASE The object was removed from the cache (see also file number below).

   • SWAPOUT The object was saved to disk.

   • SWAPIN The object existed on disk and was read into memory.

3. dir number The cache_dir number this object was stored into, starting at 0 for your first cache_dir line.

4. file number The file number for the object storage file. Please note that the path to this file is calculated according to your cache_dir configuration. A file number of FFFFFFFF indicates "memory only" objects. Any action code for such a file number refers to an object which existed only in memory, not on disk. For instance, if a RELEASE code was logged with file number FFFFFFFF, the object existed only in memory, and was released from memory.

5. hash The hash value used to index the object in the cache. Squid currently uses MD5 for the hash value.

6. status The HTTP reply status code.

7. datehdr The value of the HTTP Date reply header.

8. lastmod The value of the HTTP Last-Modified reply header.

9. expires The value of the HTTP "Expires: " reply header.

10. type The HTTP Content-Type major value, or "unknown" if it cannot be determined.

11. sizes This column consists of two slash separated fields:

    • The advertised content length from the HTTP Content-Length reply header.

    • The size actually read.
      • If the advertised (or expected) length is missing, it will be set to zero. If the advertised length is not zero, but not equal to the real length, the object will be realeased from the cache.

12. method The request method for the object, e.g. GET.

13. key The key to the object, usually the URL.

    • The datehdr, lastmod, and expires values are all expressed in UTC seconds. The actual values are parsed from the HTTP reply headers. An unparsable header is represented by a value of -1, and a missing header is represented by a value of -2.

access.log

Most log file analysis program are based on the entries in access.log.

Squid-2.7 and Squid-3.2 allow the administrators to configure their logfile format and log output method with great flexibility. Previous versions offered a much more limited functionality.

Squid result codes

The TCP_ codes refer to requests on the HTTP port (usually 3128). The UDP_ codes refer to requests on the ICP port (usually 3130). If ICP logging was disabled using the log_icp_queries option, no ICP replies will be logged.

The following result codes were taken from a Squid-2, compare with the log_type enum in src/enums.h:

TCP_HIT A valid copy of the requested object was in the cache.

TCP_MISS The requested object was not in the cache.

TCP_REFRESH_HIT The requested object was cached but STALE. The IMS query for the object resulted in "304 not modified".

TCP_REFRESH_FAIL_HIT The requested object was cached but STALE. The IMS query failed and the stale object was delivered.

TCP_REFRESH_MISS The requested object was cached but STALE. The IMS query returned the new content.

TCP_CLIENT_REFRESH_MISS The client issued a "no-cache" pragma, or some analogous cache control command along with the request. Thus, the cache has to refetch the object.

TCP_IMS_HIT The client issued an IMS request for an object which was in the cache and fresh.

TCP_SWAPFAIL_MISS The object was believed to be in the cache, but could not be accessed.

TCP_NEGATIVE_HIT Request for a negatively cached object, e.g. "404 not found", for which the cache believes to know that it is inaccessible. Also refer to the explainations for negative_ttl in your squid.conf file.

TCP_MEM_HIT A valid copy of the requested object was in the cache and it was in memory, thus avoiding disk accesses.

TCP_DENIED Access was denied for this request.

TCP_OFFLINE_HIT The requested object was retrieved from the cache during offline mode. The offline mode never validates any object, see offline_mode in squid.conf file.

TCP_STALE_HIT The object was cached and served stale. This is usually caused by stale-while-revalidate or stale-if-error.

TCP_ASYNC_HIT A background request (e.g., one started by stale-while-revalidate) resulted in a refresh hit.

TCP_ASYNC_MISS A background request (e.g., one started by stale-while-revalidate) resulted in a miss; i.e., the cached object (if any) was updated).

UDP_HIT A valid copy of the requested object was in the cache.

UDP_MISS The requested object is not in this cache.

UDP_DENIED Access was denied for this request.

UDP_INVALID An invalid request was received.

UDP_MISS_NOFETCH During "-Y" startup, or during frequent failures, a cache in hit only mode will return either UDP_HIT or this code. Neighbours will thus only fetch hits.

NONE Seen with cachemgr requests and errors, usually when the transaction fails before being classified into one of the above outcomes.

The following code suffixes are specific to Squid3:

_ABORTED suffix means that the connection with HTTP client was closed or otherwise failed prematurely. This includes half-closed client sockets when half_closed_clients in squid.conf is off.

_TIMEDOUT suffix means that the transaction timed out while writing the response to the HTTP client (i.e., the client was not reading or stopped reading Squid's response).

The following codes are no longer available in Squid-2:

ERR_* Errors are now contained in the status code.

TCP_CLIENT_REFRESH See: TCP_CLIENT_REFRESH_MISS.

TCP_SWAPFAIL See: TCP_SWAPFAIL_MISS.

TCP_IMS_MISS Deleted, now replaced with TCP_IMS_HIT.

UDP_HIT_OBJ Refers to an old version that would send cache hits in ICP replies. No longer implemented.

UDP_RELOADING See: UDP_MISS_NOFETCH.

HTTP status codes

These are taken from RFC 2616 and verified for Squid. Squid uses almost all codes except 307 (Temporary Redirect), and 416 (Request Range Not Satisfiable). Extra codes include 000 for a result code being unavailable, and 600 to signal an invalid header, a proxy error. Also, some definitions were added as for RFC 2518 (WebDAV). Yes, there are really two entries for status code 424:

 000 Used mostly with UDP traffic.
 100 Continue
 101 Switching Protocols
*102 Processing
 200 OK
 201 Created
 202 Accepted
 203 Non-Authoritative Information
 204 No Content
 205 Reset Content
 206 Partial Content
*207 Multi Status
 300 Multiple Choices
 301 Moved Permanently
 302 Moved Temporarily
 303 See Other
 304 Not Modified
 305 Use Proxy
[307 Temporary Redirect]
 400 Bad Request
 401 Unauthorized
 402 Payment Required
 403 Forbidden
 404 Not Found
 405 Method Not Allowed
 406 Not Acceptable
 407 Proxy Authentication Required
 408 Request Timeout
 409 Conflict
 410 Gone
 411 Length Required
 412 Precondition Failed
 413 Request Entity Too Large
 414 Request URI Too Large
 415 Unsupported Media Type
[416 Request Range Not Satisfiable]
 417 Expectation Failed
*424 Locked
*424 Failed Dependency
*433 Unprocessable Entity
 500 Internal Server Error
 501 Not Implemented
 502 Bad Gateway
 503 Service Unavailable
 504 Gateway Timeout
 505 HTTP Version Not Supported
*507 Insufficient Storage
 600 Squid header parsing error

Request methods

Squid recognizes several request methods as defined in RFC 2616. Newer versions of Squid (2.2.STABLE5 and above) also recognize RFC 2518 "HTTP Extensions for Distributed Authoring -- WEBDAV" extensions.

 method    defined    cachabil.  meaning
 --------- ---------- ---------- -------------------------------------------
 GET       HTTP/0.9   possibly   object retrieval and simple searches.
 HEAD      HTTP/1.0   possibly   metadata retrieval.
 POST      HTTP/1.0   CC or Exp. submit data (to a program).
 PUT       HTTP/1.1   never      upload data (e.g. to a file).
 DELETE    HTTP/1.1   never      remove resource (e.g. file).
 TRACE     HTTP/1.1   never      appl. layer trace of request route.
 OPTIONS   HTTP/1.1   never      request available comm. options.
 CONNECT   HTTP/1.1r3 never      tunnel SSL connection.
 ICP_QUERY Squid      never      used for ICP based exchanges.
 PURGE     Squid      never      remove object from cache.
 PROPFIND  rfc2518    ?          retrieve properties of an object.
 PROPATCH  rfc2518    ?          change properties of an object.
 MKCOL     rfc2518    never      create a new collection.
 COPY      rfc2518    never      create a duplicate of src in dst.
 MOVE      rfc2518    never      atomically move src to dst.
 LOCK      rfc2518    never      lock an object against modifications.
 UNLOCK    rfc2518    never      unlock an object.

Hierarchy Codes

The following hierarchy codes are used with Squid-2:

NONE For TCP HIT, TCP failures, cachemgr requests and all UDP requests, there is no hierarchy information.

DIRECT The object was fetched from the origin server.

SIBLING_HIT The object was fetched from a sibling cache which replied with UDP_HIT.

PARENT_HIT The object was requested from a parent cache which replied with UDP_HIT.

DEFAULT_PARENT No ICP queries were sent. This parent was chosen because it was marked "default" in the config file.

SINGLE_PARENT The object was requested from the only parent appropriate for the given URL.

FIRST_UP_PARENT The object was fetched from the first parent in the list of parents.

NO_PARENT_DIRECT The object was fetched from the origin server, because no parents existed for the given URL.

FIRST_PARENT_MISS The object was fetched from the parent with the fastest (possibly weighted) round trip time.

CLOSEST_PARENT_MISS This parent was chosen, because it included the the lowest RTT measurement to the origin server. See also the closest-only peer configuration option.

CLOSEST_PARENT The parent selection was based on our own RTT measurements.

CLOSEST_DIRECT Our own RTT measurements returned a shorter time than any parent.

NO_DIRECT_FAIL The object could not be requested because of a firewall configuration, see also never_direct and related material, and no parents were available.

SOURCE_FASTEST The origin site was chosen, because the source ping arrived fastest.

ROUNDROBIN_PARENT No ICP replies were received from any parent. The parent was chosen, because it was marked for round robin in the config file and had the lowest usage count.

CACHE_DIGEST_HIT The peer was chosen, because the cache digest predicted a hit. This option was later replaced in order to distinguish between parents and siblings.

CD_PARENT_HIT The parent was chosen, because the cache digest predicted a hit.

CD_SIBLING_HIT The sibling was chosen, because the cache digest predicted a hit.

NO_CACHE_DIGEST_DIRECT This output seems to be unused?

CARP The peer was selected by CARP.

ANY_PARENT part of src/peer_select.c:hier_strings[].

INVALID CODE part of src/peer_select.c:hier_strings[].

Almost any of these may be preceded by 'TIMEOUT_' if the two-second (default) timeout occurs waiting for all ICP replies to arrive from neighbors, see also the icp_query_timeout configuration option.

The following hierarchy codes were removed from Squid-2:

code                  meaning
--------------------  -------------------------------------------------
PARENT_UDP_HIT_OBJ    hit objects are not longer available.
SIBLING_UDP_HIT_OBJ   hit objects are not longer available.
SSL_PARENT_MISS       SSL can now be handled by squid.
FIREWALL_IP_DIRECT    No special logging for hosts inside the firewall.
LOCAL_IP_DIRECT       No special logging for local networks.

swap.state

This file has a rather unfortunate history which has led to it often being called the swap log. It is in fact a journal of the cache index with a record of every cache object written to disk. It is read when Squid starts up to "reload" the cache quickly.

If you remove this file when squid is NOT running, you will effectively wipe out your cache index of contents. Squid can rebuild it from the original files, but that procedure can take a long time as every file in the cache must be fully scanned for meta data.

If you remove this file while squid IS running, you can easily recreate it. The safest way is to simply shutdown the running process:

% squid -k shutdown

This will disrupt service, but at least you will have your swap log back. Alternatively, you can tell squid to rotate its log files. This also causes a clean swap log to be written.

% squid -k rotate

By default the swap.state file is stored in the top-level of each cache_dir. You can move the logs to a different location with the cache_swap_state option.

The file is a binary format that includes MD5 checksums, and StoreEntry fields. Please see the Programmers' Guide for information on the contents and format of that file.

Which log files can I delete safely?

You should never delete access.log, store.log, or cache.log while Squid is running. With Unix, you can delete a file when a process has the file opened. However, the filesystem space is not reclaimed until the process closes the file.

If you accidentally delete swap.state while Squid is running, you can recover it by following the instructions in the previous questions. If you delete the others while Squid is running, you can not recover them.

The correct way to maintain your log files is with Squid's "rotate" feature. You should rotate your log files at least once per day. The current log files are closed and then renamed with numeric extensions (.0, .1, etc). If you want to, you can write your own scripts to archive or remove the old log files. If not, Squid will only keep up to logfile_rotate versions of each log file. The logfile rotation procedure also writes a clean swap.state file, but it does not leave numbered versions of the old files.

If you set logfile_rotate to 0, Squid simply closes and then re-opens the logs. This allows third-party logfile management systems, such as newsyslog, to maintain the log files.

To rotate Squid's logs, simple use this command:

squid -k rotate

For example, use this cron entry to rotate the logs at midnight:

0 0 * * * /usr/local/squid/bin/squid -k rotate

How can I disable Squid's log files?

To disable access.log:

access_log none

To disable store.log:

cache_store_log none

To disable cache.log:

cache_log /dev/null

What is the maximum size of access.log?

Squid does not impose a size limit on its log files. Some operating systems have a maximum file size limit, however. If a Squid log file exceeds the operating system's size limit, Squid receives a write error and shuts down. You should regularly rotate Squid's log files so that they do not become very large.

My log files get very big!

You need to rotate your log files with a cron job. For example:

0 0 * * * /usr/local/squid/bin/squid -k rotate

When logging debug information into cache.log it can easily become extremely large and when a long access.log traffic history is required (ie by law in some countries) storing large cache.log for that time is not reasonable. From Squid-3.2 cache.log can be rotated with an individual cap set by debug_options } option to store fewer of these large files in the .0 to .N series of backups. The default is to store the same number as with access.log and set in the logfile_rotate directive.

I want to use another tool to maintain the log files.

If you set logfile_rotate to 0, Squid simply closes and then re-opens the logs. This allows third-party logfile management systems, such as newsyslog or logrotate, to maintain the log files.

Squid-2.7 and Squid-3.2 and later also provide modular logging outputs which provide flexibility for sending log data to alternative logging systems.

Managing log files

The preferred log file for analysis is the access.log file in native format. For long term evaluations, the log file should be obtained at regular intervals. Squid offers an easy to use API for rotating log files, in order that they may be moved (or removed) without disturbing the cache operations in progress. The procedures were described above.

Depending on the disk space allocated for log file storage, it is recommended to set up a cron job which rotates the log files every 24, 12, or 8 hour. You will need to set your logfile_rotate to a sufficiently large number. During a time of some idleness, you can safely transfer the log files to your analysis host in one burst.

Before transport, the log files can be compressed during off-peak time. On the analysis host, the log file are concatenated into one file, so one file for 24 hours is the yield. Also note that with log_icp_queries enabled, you might have around 1 GB of uncompressed log information per day and busy cache. Look into you cache manager info page to make an educated guess on the size of your log files.

The EU project DESIRE developed some some basic rules to obey when handling and processing log files:

• Respect the privacy of your clients when publishing results.
• Keep logs unavailable unless anonymized. Most countries have laws on privacy protection, and some even on how long you are legally allowed to keep certain kinds of information.

• Rotate and process log files at least once a day. Even if you don't process the log files, they will grow quite large, see My log files get very big above here. If you rely on processing the log files, reserve a large enough partition solely for log files.

• Keep the size in mind when processing. It might take longer to process log files than to generate them!
• Limit yourself to the numbers you are interested in. There is data beyond your dreams available in your log file, some quite obvious, others by combination of different views. Here are some examples for figures to watch:
  • The hosts using your cache.
  • The elapsed time for HTTP requests - this is the latency the user sees. Usually, you will want to make a distinction for HITs and MISSes and overall times. Also, medians are preferred over averages.
  • The requests handled per interval (e.g. second, minute or hour).

Why do I get ERR_NO_CLIENTS_BIG_OBJ messages so often?

This message means that the requested object was in "Delete Behind" mode and the user aborted the transfer. An object will go into "Delete Behind" mode if

• It is larger than maximum_object_size

• It is being fetched from a neighbor which has the proxy-only option set.

What does ERR_LIFETIME_EXP mean?

This means that a timeout occurred while the object was being transferred. Most likely the retrieval of this object was very slow (or it stalled before finishing) and the user aborted the request. However, depending on your settings for quick_abort, Squid may have continued to try retrieving the object. Squid imposes a maximum amount of time on all open sockets, so after some amount of time the stalled request was aborted and logged win an ERR_LIFETIME_EXP message.

Retrieving "lost" files from the cache

"I've been asked to retrieve an object which was accidentally destroyed at the source for recovery. So, how do I figure out where the things are so I can copy them out and strip off the headers?""

The following method applies only to the Squid-1.1 versions:

Use grep to find the named object (URL) in the cache.log file. The first field in this file is an integer file number.

Then, find the file fileno-to-pathname.pl from the "scripts" directory of the Squid source distribution. The usage is

perl fileno-to-pathname.pl [-c squid.conf]

file numbers are read on stdin, and pathnames are printed on stdout.

Can I use store.log to figure out if a response was cachable?

Sort of. You can use store.log to find out if a particular response was cached.

Cached responses are logged with the SWAPOUT tag. Uncached responses are logged with the RELEASE tag.

However, your analysis must also consider that when a cached response is removed from the cache (for example due to cache replacement) it is also logged in store.log with the RELEASE tag. To differentiate these two, you can look at the filenumber (3rd) field. When an uncachable response is released, the filenumber is FFFFFFFF (-1). Any other filenumber indicates a cached response was released.

Can I pump the squid access.log directly into a pipe?

Several people have asked for this, usually to feed the log into some kind of external database, or to analyze them in real-time.

The answer is No. Well, yes, sorta. Using a pipe directly opens up a whole load of possible problems.

There are several alternatives which are much safer to setup and use. The basic capabilities present are :

since Squid-2.6:

• logging to system syslog

since Squid-2.7:

• logging to an external service via UDP packets
• logging through IPC to a custom local daemon

since Squid-3.2:

• logging to an external service via TCP streams

See the Log Modules feature for technical details on setting up a daemon or other output modules.

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