Squid access log is very informative if you know how to dig something out of it. Following is an a line from access.log file.

1201172176.719 1190 127.0.0.1 TCP_MISS/200 529 GET http://www.blogger.com/status.g? – DIRECT/72.14.221.191 application/xml

This line can be written as:

Timestamp, Total time, Source, Action/Code, Size, Method, URL, Ident, Hierarchy/From, Content type

1:	Timestamp	Time when the request was completed.
2:	Total time	Total time taken to complete the request
3:	Source	IP address of the client
4:	Action/Code	Action taken for the request
5:	Size	Total size of the request in bytes
6:	Method	Whether the request was GET or POST
7:	URL	The actual request
8:	Ident	Usually –
9:	Hierarchy/From	How the object is fetched and from where
10:	Content type	Type of object

Action

“TCP_” refers to requests on the HTTP port (3128)
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	An expired copy of the requested object was in the cache. Squid made an If-Modified-Since request and the response was “Not Modified.”
TCP_REFRESH_FAIL_HIT	An expired copy of the requested object was in the cache. Squid attempted to make an If-Modified-Since request, but it failed. The old (stale) object was delivered to the client.
TCP_REFRESH_MISS	An expired copy of the requested object was in the cache. Squid made an If-Modified-Since request and received a new, different object.
TCP_CLIENT_REFRESH	The client issued a request with the “no-cache” pragma. (“reload” – handled as MISS)
TCP_IMS_HIT	An If-Modified-Since GET request was received from the client. A valid copy of the object was in the cache (fresh).
TCP_IMS_MISS	An If-Modified-Since GET request was received from the client. The requested object was not in the cache (stale).
TCP_SWAPFAIL	The object was believed to be in the cache, but could not be accessed.
TCP_DENIED	Access was denied for this request.

“UDP_” refers to requests on the ICP port (3130)
UDP_HIT	A valid copy of the requested object was in the cache
UDP_HIT_OBJ	Same as UDP_HIT, but the object data was small enough to be sent in the UDP reply packet. Saves the following TCP request.
UDP_MISS	The requested object was not in the cache
UDP_DENIED	Access was denied for this request
UDP_INVALID	An invalid request was received.
UDP_RELOADING	The neighbor cache is reloading its disk store metadata and does not want any TCP requests for MISSES until it is finished.

Errors
ERR_READ_TIMEOUT	The remote site or network is unreachable – may be down.
ERR_LIFETIME_EXP	The remote site or network may be too slow or down.
ERR_NO_CLIENTS_BIG_OBJ	All Clients went away before tranmission completed and the object is too big to cache.
ERR_READ_ERROR	The remote site or network may be down.
ERR_CLIENT_ABORT	Client dropped connection before transmission completed. Squid fetches the Object according to its settings for `quick_abort’.
ERR_CONNECT_FAIL	The remote site or server may be down.
ERR_INVALID_REQ	Invalid HTTP request
ERR_UNSUP_REQ	Unsupported request
ERR_INVALID_URL	Invalid URL syntax
ERR_NO_FDS	Out of file descriptors
ERR_DNS_FAIL	DNS name lookup failure
ERR_NOT_IMPLEMENTED	Protocol Not Supported
ERR_CANNOT_FETCH	The requested URL can not currently be retrieved.
ERR_NO_RELAY	There is no WAIS relay host defined for this cache.
ERR_DISK_IO	The system disk is out of space or failing.
ERR_ZERO_SIZE_OBJECT	The remote server closed the connection before sending any data.
ERR_FTP_DISABLED	This cache is configured to NOT retrieve FTP objects.
ERR_PROXY_DENIED	Access Denied. The user must authenticate himself before accessing this cache.

CODE

Code											Reason phrase	RFC 2616 section
0											No Response Received (Squid-specific)	N/A
1xx											Informational	10.1
100											Continue	10.1.1
101											Switching Protocols	10.1.2
2xx											Successful	10.2
200											OK	10.2.1
201											Created	10.2.2
202											Accepted	10.2.3
203											Non-Authoritative Information	10.2.4
204											No Content	10.2.5
205											Reset Content	10.2.6
206											Partial Content	10.2.7
3xx											Redirection	10.3
300											Multiple Choices	10.3.1
301											Moved Permanently	10.3.2
302											Found	10.3.3
303											See Other	10.3.4
304											Not Modified	10.3.5
305											Use Proxy	10.3.6
306											(Unused)	10.3.7
307											Temporary Redirect	10.3.8
4xx											Client Error	10.4
400											Bad Request	10.4.1
401											Unauthorized	10.4.2
402											Payment Required	10.4.3
403											Forbidden	10.4.4
404											Not Found	10.4.5
405											Method Not Allowed	10.4.6
406											Not Acceptable	10.4.7
407											Proxy Authentication Required	10.4.8
408											Request Timeout	10.4.9
409											Conflict	10.4.10
410											Gone	10.4.11
411											Length Required	10.4.12
412											Precondition Failed	10.4.13
413											Request Entity Too Large	10.4.14
414											Request-URI Too Long	10.4.15
415											Unsupported Media Type	10.4.16
416											Requested Range Not Satisfiable	10.4.17
417											Expectation Failed	10.4.18
5xx											Server Error	10.5
500											Internal Server Error	10.5.1
501											Not Implemented	10.5.2
502											Bad Gateway	10.5.3
503											Service Unavailable	10.5.4
504											Gateway Timeout	10.5.5
505											HTTP Version Not Supported	10.5.6
6xx											Proxy Error	N/A
600											Unparseable Response Headers (Squid-specific)	N/A

Methods

GET	Request URL
HEAD	Request only HTTP headers of the supplied URL and no document body
POST	Transfer data to the supplied URL
PUT	Store data under the supplied URL
CONNECT	Forward data to SSL-Server:Port
ICP_QUERY	Request from a Parent/Neighbor for the supplied URL
NONE	Request of an unsupported method

Hierarchy

NONE	The object requested by a sibling, was not in my cache.
DIRECT	The object has been requested from the origin server.
SIBLING_HIT	The object was requested from a neighbor cache which replied with a UDP_HIT (formerly logged as NEIGHBOR_HIT).
PARENT_HIT	The object was requested from a parent cache which replied with a UDP_HIT.
DEFAULT_PARENT	The object was requested from a default parent cache appropriate for this URL.
SINGLE_PARENT	The object was requested from the only parent cache appropriate for this URL.
FIRST_UP_PARENT	The object has been requested from the first available parent in your list.
NO_PARENT_DIRECT	The object was requested from the origin server because no parent caches exist for the URL.
FIRST_PARENT_MISS	The object has been requested from the parent cache with the fastest weighted round trip time.
ROUNDROBIN_PARENT	No ICP queries were received from any parent caches. This parent was chosen because it was marked as ‘default’ in the config file and it had the lowest round-robin use count.
CLOSEST_PARENT_MISS	This parent was selected because it included the lowest RTT measurement to the origin server. This only appears with ‘query_icmp on’ set in the config file.
CLOSEST_DIRECT	The object was fetched directly from the origin server because this cache measured a lower RTT than any of the parent caches.
LOCAL_IP_DIRECT	The object has been requested from the origin server because the origin host IP address matched your ‘local_ip’ list.
FIREWALL_IP_DIRECT	The object has been requested from the origin server because the origin host IP address is inside your firewall.
NO_DIRECT_FAIL	The object could not be requested because of firewall restrictions and no parent caches were available.
SOURCE_FASTEST	The object was requested from the origin server because the ‘source_ping’ reply arrived first.
SIBLING_UDP_HIT_OBJ	The object was received in a UDP_HIT_OBJ reply from a neighbor cache (formerly logged as UDP_HIT_OBJ).
PARENT_UDP_HIT_OBJ	The object was received in a UDP_HIT_OBJ reply from a parent cache (formerly logged as UDP_HIT_OBJ).
PASSTHROUGH_PARENT	The neighbor or proxy defined in the config option ‘passthrough_proxy’ was used.
SSL_PARENT_MISS	The neighbor or proxy defined in the config option ‘ssl_proxy’ was used.