usbloadergx/source/network/dns.c

#include "dns.h"

/**
 * Resolves a domainname to an ip address
 * It makes use of net_gethostbyname from libogc, which in turn makes use of a Wii BIOS function
 * Just like the net_gethostbyname function this function is NOT threadsafe!
 *
 * @param char* The domain name to resolve
 * @return u32 The ipaddress represented by four bytes inside an u32 (in network order)
 */
u32 getipbyname(char *domain)
{
    //Care should be taken when using net_gethostbyname,
    //it returns a static buffer which makes it not threadsafe
    //TODO: implement some locking mechanism to make below code atomic
    struct hostent *host = net_gethostbyname(domain);

    if (host == NULL)
    {
        return 0;
    }

    u32 *ip = (u32*) host->h_addr_list[0];
    return *ip;
}

//Defines how many DNS entries should be cached by getipbynamecached()
#define MAX_DNS_CACHE_ENTRIES 20

//The cache is defined as a linked list,
//The last resolved domain name will always be at the front
//This will allow heavily used domainnames to always stay cached
struct dnsentry
{
    char *domain;
    u32 ip;
    struct dnsentry *nextnode;
};

static struct dnsentry *firstdnsentry = NULL;
static int dnsentrycount = 0;

/**
 * Performs the same function as getipbyname(),
 * except that it will prevent extremely expensive net_gethostbyname() calls by caching the result
 */
u32 getipbynamecached(char *domain)
{
    //Search if this domainname is already cached
    struct dnsentry *node = firstdnsentry;
    struct dnsentry *previousnode = NULL;

    while (node != NULL)
    {
        if (strcmp(node->domain, domain) == 0)
        {
            //DNS node found in the cache, move it to the front of the list
            if (previousnode != NULL) previousnode->nextnode = node->nextnode;

            if (node != firstdnsentry) node->nextnode = firstdnsentry;
            firstdnsentry = node;

            return node->ip;
        }
        //Go to the next element in the list
        previousnode = node;
        node = node->nextnode;
    }
    u32 ip = getipbyname(domain);

    //No cache of this domain could be found, create a cache node and add it to the front of the cache
    struct dnsentry *newnode = malloc(sizeof(struct dnsentry));
    if (newnode == NULL)
    {
        return ip;
    }

    newnode->ip = ip;
    newnode->domain = malloc(strlen(domain) + 1);
    if (newnode->domain == NULL)
    {
        free(newnode);
        return ip;
    }
    strcpy(newnode->domain, domain);

    newnode->nextnode = firstdnsentry;
    firstdnsentry = newnode;
    dnsentrycount++;

    //If the cache grows too big delete the last (and probably least important) node of the list
    if (dnsentrycount > MAX_DNS_CACHE_ENTRIES)
    {
        struct dnsentry *node = firstdnsentry;
        struct dnsentry *previousnode = NULL;

        //Fetch the last two elements of the list
        while (node->nextnode != NULL)
        {
            previousnode = node;
            node = node->nextnode;
        }

        if (node == NULL)
        {
            printf("Configuration error, MAX_DNS_ENTRIES reached while the list is empty\n");
            exit(1);
        }
        else if (previousnode == NULL)
        {
            firstdnsentry = NULL;
        }
        else
        {
            previousnode->nextnode = NULL;
        }

        free(node->domain);
        free(node);
        dnsentrycount--;
    }

    return newnode->ip;
}