🔗 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.

🔗 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.

🔗 Squid Error Messages

Error messages come in several forms. Debug traces are not logged at level 0 or level 1. These levels are reserved for important and critical administrative messages.

• FATAL messages indicate a problem which has killed the Squid process. Affecting all current client traffic being supplied by that Squid instance. If these occur when starting or configuring a Squid component it must be resolved before you can run Squid.
• ERROR messages indicate a serious problem which has broken an individual client transaction and may have some effect on other clients indirectly. But has not completely aborted all traffic service.
• These can also occur when starting or configuring Squid components. In which case any service actions which that component would have supplied will not happen until it is resolved and Squid reconfigured.

  Some log level 0 error messages inherited from older Squid versions exist without any prioritization tag.

• WARNING messages indicate problems which might be causing problems to the client, but Squid is capable of working around automatically. These usually only display at log level 1 and higher.

  Some log level 1 warning messages inherited from older Squid versions exist without any prioritization tag.

• SECURITY ERROR messages indicate problems processing a client request with the security controls which Squid has been configured with. Some impossible condition is required to pass the security test. This is commonly seen when testing whether to accept a client request based on some reply detail which will only be available in the future.
• SECURITY ALERT messages indicate security attack problems being detected. This is only for problems which are unambiguous. ‘Attacks’ signatures which can appear in normal traffic are logged as regular WARNING.
  • A complete solution to these usually requires fixing the client, which may not be possible.
  • Administrative workarounds (extra firewall rules etc) can assist Squid in reducing the damage to network performance.
  • Attack notices may seem rather critical, but occur at level 1 since in all cases Squid also has some workaround it can perform.
• SECURITY NOTICE messages can appear during startup and reconfigure to indicate security related problems with the configuration file setting. These are accompanied by hints for better configuration where possible, and an indication of what Squid is going to do instead of the configured action.

Some of the more frequently questioned messages and what they mean are outlined in the KnowledgeBase:

1. Excess Data
2. Failed to select source
3. Host Header Forgery
4. Queue congestion
5. Too Many Queued Requests
6. Unparseable Header

🔗 access.log

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

Squid allows the administrators to configure their logfile format and log output method with great flexibility.

🔗 Squid result codes

The Squid result code is composed of several tags (separated by underscore characters) which describe the response sent to the client.

• One of these tags always exists to describe how it was delivered:

  TCP	Requests on the HTTP port (usually 3128).
  UDP	Requests on the ICP port (usually 3130) or HTCP port (usually 4128). If ICP logging was disabled using the log_icp_queries option, no ICP replies will be logged.
  NONE	Squid delivered an unusual response or no response at all. Seen with cachemgr requests and errors, usually when the transaction fails before being classified into one of the above outcomes. Also seen with responses to CONNECT requests.

• These tags are optional and describe why the particular handling was performed or where the request came from:

  CF	At least one request in this transaction was collapsed. See collapsed_forwarding for more details about request collapsing. Support for this tag has been added to Squid v5 on 2018-06-18 (commit d2a6dc). It may not be available in earlier Squid versions.
  CLIENT	The client request placed limits affecting the response. Usually seen with client issued a “no-cache”, or analogous cache control command along with the request. Thus, the cache has to validate the object.
  IMS	The client sent a revalidation (conditional) request.
  ASYNC	The request was generated internally by Squid. Usually this is background fetches for cache information exchanges, background revalidation from stale-while-revalidate cache controls, or ESI sub-objects being loaded.
  SWAPFAIL	The object was believed to be in the cache, but could not be accessed. A new copy was requested from the server.
  REFRESH	A revalidation (conditional) request was sent to the server.
  SHARED	This tag is not supported yet. This request was combined with an existing transaction by collapsed forwarding. NOTE: the existing request is not marked as SHARED.
  REPLY	The HTTP reply from server or peer. Usually seen on DENIED due to http_reply_access ACLs preventing delivery of servers response object to the client.

• These tags are optional and describe what type of object was produced:

  NEGATIVE	Only seen on HIT responses. Indicating the response was a cached error response. e.g. “404 not found”
  STALE	The object was cached and served stale. This is usually caused by stale-while-revalidate or stale-if-error cache controls.
  OFFLINE	The requested object was retrieved from the cache during offline_mode. The offline mode never validates any object.
  INVALID	An invalid request was received. An error response was delivered indicating what the problem was.
  FAIL	Only seen on REFRESH to indicate the revalidation request failed. The response object may be the server provided network error or the stale object which was being revalidated depending on stale-if-error cache control.
  MODIFIED	Only seen on REFRESH responses to indicate revalidation produced a new modified object.
  UNMODIFIED	Only seen on REFRESH responses to indicate revalidation produced a 304 (Not Modified) status. The client gets either a full 200 (OK), a 304 (Not Modified), or (in theory) another response, depending on the client request and other details.
  REDIRECT	Squid generated an HTTP redirect response to this request.

• These tags are optional and describe whether the response was loaded from cache, network, or otherwise:

  HIT	The response object delivered was the local cache object.
  MEM	Additional tag indicating the response object came from memory cache, avoiding disk accesses. Only seen on HIT responses.
  MISS	The response object delivered was the network response object.
  DENIED	The request was denied by access controls.
  NOFETCH	A ICP specific type. Indicating service is alive, but not to be used for this request. Sent during “-Y” startup, or during frequent failures, a cache in hit only mode will return either UDP_HIT or UDP_MISS_NOFETCH. Neighbours will thus only fetch hits.
  TUNNEL	A binary tunnel was established for this transaction.

• These tags are optional and describe some error conditions which occured during response delivery (if any):

  ABORTED	A client-to-Squid or Squid-to-server connection was closed unexpectedly, usually due to an I/O error or clean transport connection closure in the middle of some higher-level protocol message/negotiation. Before Squid v6, this tag was primarily seen when the client closed its connection to Squid before Squid could deliver the entire response. Since Squid v6, the tag also appears when Squid communication with an origin server or cache_peer is impossible (e.g., the server is refusing TCP connections) or aborted (e.g., an EOF in the middle of a chunked HTTP response body transfer).
  TIMEOUT	The response was not completed due to a connection timeout.
  IGNORED	While refreshing a previously cached response A, Squid got a response B that was older than A (as determined by the Date header field). Squid ignored response B (and attempted to use A instead). This “ignore older responses” logic complies with RFC 7234 Section 4 requirement: a cache MUST use the most recent response (as determined by the Date header field).

🔗 HTTP status codes

These are taken from RFC 1945 (HTTP/1.0), 2616 (HTTP/1.1) and verified for Squid. Squid uses almost all codes except 416 (Request Range Not Satisfiable). Extra codes used in the Squid logs (but not live traffic) 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 and 4918 (WebDAV). Yes, there are really two entries for status code 424:

Status	Description	RFC(s)
000	Used mostly with UDP traffic.	N/A
	Informational
100	Continue	2616
101	Switching Protocols	2616
102	Processing	2518
	Successful Transaction
200	OK	1945, 2616
201	Created	1945, 2616
202	Accepted	1945, 2616
203	Non-Authoritative Information	2616
204	No Content	1945, 2616, 4918
205	Reset Content	2616
206	Partial Content	2616
207	Multi Status	2518, 4918
	Redirection
300	Multiple Choices	1945, 2616, 4918
301	Moved Permanently	1945, 2616, 4918
302	Moved Temporarily	1945, 2616, 4918
303	See Other	2616, 4918
304	Not Modified	1945, 2616
305	Use Proxy	2616, 4918
307	Temporary Redirect	2616, 4918
	Client Error
400	Bad Request	1945, 2616, 4918
401	Unauthorized	1945, 2616
402	Payment Required	2616
403	Forbidden	1945, 2616, 4918
404	Not Found	1945, 2616
405	Method Not Allowed	2616
406	Not Acceptable	2616
407	Proxy Authentication Required	2616
408	Request Timeout	2616
409	Conflict	2616, 4918
410	Gone	2616
411	Length Required	2616
412	Precondition Failed	2616, 4918
413	Request Entity Too Large	2616
414	Request URI Too Large	2616, 4918
415	Unsupported Media Type	2616
416	Request Range Not Satisfiable	2616
417	Expectation Failed	2616
422	Unprocessable Entity	2518, 4918
424	Locked	(broken WebDAV implementations??)
424	Failed Dependency	2518, 4918
433	Unprocessable Entity
	Server Errors
500	Internal Server Error	1945, 2616
501	Not Implemented	1945, 2616
502	Bad Gateway	1945, 2616
503	Service Unavailable	1945, 2616
504	Gateway Timeout	2616
505	HTTP Version Not Supported	2616
507	Insufficient Storage	2518, 4918
	Broken Server Software
600	Squid: header parsing error
601	Squid: header size overflow detected while parsing
601	roundcube: software configuration error
603	roundcube: invalid authorization

🔗 Request methods

Squid recognizes several request methods as defined in RFC 2616 and 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.

Note that since Squid 3.1, methods not listed here (such as PATCH) are supported “out of the box.”

🔗 Hierarchy Codes

• 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.
• PINNED The server connection was pinned by NTLM or Negotiate authentication requirements.
• ORIGINAL_DST The server connection was limited to the client provided destination IP. This occurs on interception proxies when Host security is enabled, or client_dst_passthru transparency is enabled.
• ANY_OLD_PARENT (former ANY_PARENT?) Squid used the first considered-alive parent it could reach. This happens when none of the specific parent cache selection algorithms (e.g., userhash or carp) were enabled, all enabled algorithms failed to find a suitable parent, or all suitable parents found by those algorithms failed when Squid tried to forward the request to them.
• 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.

🔗 store.log

This file covers the objects currently kept on disk or removed ones. As a kind of transaction log (or journal) 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 released 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.

🔗 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.

🔗 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.

🔗 useragent.log

Starting from Squid-3.2 this log has become one of the default access.log formats and is always available for use. It is no longer a special separate log 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

It is a bad idea to disable the cache.log because this file contains many important status and debugging messages. However, if you really want to, you can

If /dev/null is specified to any of the above log files, logfile_rotate MUST also be set to 0 or else risk Squid rotating away /dev/null making t a plain log file

Instead of disabling the log files, it is advisable to use a smaller value for logfile_rotate and properly rotating Squid’s log files in your cron. That way, your log files are more controllable and self-maintained by your system

🔗 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.

Logging is very important to Squid. In fact, it is so important that it will shut itself down if it can’t write to its logfiles. This includes cases such as a full log disk, or logfiles getting too big.

🔗 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 rotate=N} 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.

🔗 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.

Some basic advice 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.

Logging is very important to Squid. In fact, it is so important that it will shut itself down if it can’t write to its logfiles.

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

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

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