14. IP Cache and FQDN Cache

14.1 Introduction

The IP cache is a built-in component of squid providing Hostname to IP-Number translation functionality and managing the involved data-structures. Efficiency concerns require mechanisms that allow non-blocking access to these mappings. The IP cache usually doesn't block on a request except for special cases where this is desired (see below).

14.2 Data Structures

The data structure used for storing name-address mappings is a small hashtable (static hash_table *ip_table), where structures of type ipcache_entry whose most interesting members are:

        struct _ipcache_entry {
                char *name;
                time_t lastref;
                ipcache_addrs addrs;
                struct _ip_pending *pending_head;
                char *error_message;
                unsigned char locks;
                ipcache_status_t status:3;
        }

14.3 External overview

Main functionality is provided through calls to:

ipcache_nbgethostbyname(const char *name, IPH *handler, void *handlerdata)

where name is the name of the host to resolve, handler is a pointer to the function to be called when the reply from the IP cache (or the DNS if the IP cache misses) and handlerdata is information that is passed to the handler and does not affect the IP cache.

ipcache_gethostbyname(const char *name,int flags)

is different in that it only checks if an entry exists in it's data-structures and does not by default contact the DNS, unless this is requested, by setting the flags to IP_BLOCKING_LOOKUP or IP_LOOKUP_IF_MISS.

ipcache_init()

is called from mainInitialize() after disk initialization and prior to the reverse fqdn cache initialization

ipcache_restart()

is called to clear the IP cache's data structures, cancel all pending requests. Currently, it is only called from mainReconfigure.

14.4 Internal Operation

Internally, the execution flow is as follows: On a miss, ipcache_getnbhostbyname checks whether a request for this name is already pending, and if positive, it creates a new entry using ipcacheAddNew with the IP_PENDING flag set . Then it calls ipcacheAddPending to add a request to the queue together with data and handler. Else, ipcache_dnsDispatch() is called to directly create a DNS query or to ipcacheEnqueue() if all no DNS port is free. ipcache_call_pending() is called regularly to walk down the pending list and call handlers. LRU clean-up is performed through ipcache_purgelru() according to the ipcache_high threshold.