Squid Proxy Server: debugging problems

Mostly, we encounter problems that are well-known and are a result of configuration glitches or operating system limitations. So, those problems can be fixed easily by tweaking configuration files. However, sometimes we may face problems that cannot be solved directly or we may not even be able to identify them by simply looking at the log files.

By default, Squid only logs the essential information to cache.log. To inspect or debug problems, we need to increase the verbosity of the logs so that Squid can tell us more about the actions it's taking, which may help us find the source of the problem. We can extract information from Squid about its actions at our convenience by using the debug_options directive in the Squid configuration file.

Let's have a look at the format of the debug_options directive:

debug_options rotate=N section,verbosity [section,verbosity]...

The parameter rotate (rotate=N) specifies the number of cache.log files that will be maintained when Squid logs are rotated. The default value of N is 1. The rotate option helps in preventing disk space from being wasted due to excessive log messages when the verbosity level is high.

The parameter section is an integer identifying a particular component of Squid. It can have a special value, ALL, which represents all components of Squid. The verbosity parameter is also an integer representing the verbosity level for each section. Let's have a look at the meaning of different verbosity levels:

Verbosity levelDescription0Only critical or fatal messages will be logged.1Warnings and important problems will be logged.2At verbosity level 2, the minor problems, recovery, and regular high-level actions will be logged.3-5Almost everything useful is covered by verbosity level 5.6-9Above verbosity level 5, it is extremely verbose. Individual events, signals, and so on are described in detail.

The following is the default configuration:

debug_options rotate=1 ALL,1

The preceding configuration line sets the verbosity level for all sections of Squid to 1, which means that Squid will try to log the minimum amount of information possible.

The section number can be determined by looking at the source of the file. In most source files, we can locate a commented line, as shown in the following example, which is from access_log.cc:

/*
...
   * DEBUG: section 46 Access Log
...
*/

The previous comment tells us that the section number for the Access Log is 46. A list of section numbers and corresponding Squid components can be found at doc/debug-sections.txt in Squid's source code. The following table represents some of the important section numbers for Squid version 3.1.10:

Section numberSquid components0Announcement Server, Client Database, Debug Routines, DNS Resolver Daemon, UFS Store Dump Tool1Main Loop, Startup2Unlink Daemon3Configuration File Parsing, Configuration Settings4Error Generation6Disk I/O Routines9File Transfer Protocol (FTP)11Hypertext Transfer Protocol (HTTP)12Internet Cache Protocol (ICP)14IP Cache, IP Storage, and Handling15Neighbor Routines16Cache Manager Objects17Request Forwarding18Cache Manager Statistics20Storage Manager, Storage Manager Heap-based replacement, Storage Manager Logging Functions, Storage Manager MD5 Cache Keys, Storage Manager Swapfile Metadata, Storage Manager Swapfile Unpacker, Storage Manager Swapin Functions, Storage Manager Swapout Functions, Store Rebuild Routines, Swap Dir base object23URL Parsing, URL Scheme parsing28Access Control29Authenticator, Negotiate Authenticator, NTLM Authenticator31Hypertext Caching Protocol32Asynchronous Disk I/O34Dnsserver interface35FQDN Cache44Peer Selection Algorithm46Access Log50Log file handling51Filedescriptor Functions55HTTP Header56HTTP Message Body57HTTP Status-line58HTTP Reply (Response)61Redirector64HTTP Range Header65HTTP Cache Control Header66HTTP Header Tools67String68HTTP Content-Range Header70Cache Digest71Store Digest Manager72Peer Digest Routines73HTTP Request74HTTP Message76Internal Squid Object handling78DNS lookups, DNS lookups; interacts with lib/rfc1035.c79Disk IO Routines, Squid-side DISKD I/O functions, Squid-side Disk I/O functions, Storage Manager COSS Interface, Storage Manager UFS Interface84Helper process maintenance89NAT / IP Interception90HTTP Cache Control Header, Storage Manager Client-Side Interface92Storage File System