Domain Lookup Tree

by Jason A. Donenfeld (Jason@zx2c4.com)

Some applications, like dnsmasq, need to find the closest-match domain name. One way is by iterating through a list of domains and finding the one that has the longest length match. But a more efficient way is to build a tree of domain name components.

This project provides a very simple C API for a domain name matching tree. The source code is simple, extremely well commented, and easy to read.

API

`struct domain_lookup_tree* init_dlt()`

Constructs a new tree head and returns a pointer to it.

`void* find_dlt(struct domain_lookup_tree head, const char domain)`

Returns the prior inserted data in head for a given domain.

`int insert_dlt(struct domain_lookup_tree head, const char domain, void *data)`

Inserts data into a tree given by head for a given domain. If domain has length of zero or is equal to #, it will act as the default match for non-matching lookups.

Example

struct domain_lookup_tree *domains = init_dlt();

insert_dlt(domains, "zx2c4.com", "zx2c4 root node");
insert_dlt(domains, "data.zx2c4.com", "zx2c4 data node");
insert_dlt(domains, "yahoo.co.uk", "yahoo.co.uk node");
insert_dlt(domains, "#", "generic node");

char *data = find_dlt(domains, "cheese.somewhere.zx2c4.com");
printf("zx2c4 root node == %s\n", data);

Benchmarks

With gcc 4.7.2 on an Intel Core i7-3820QM, we show a ~587x speed improvement over dnsmasq's old algorithm:

zx2c4@thinkpad ~/Projects/domain-lookup-tree $ make
cc -march=native -pipe -fomit-frame-pointer -flto -O3    benchmark.c domain-lookup.c domain-lookup.h   -o benchmark

zx2c4@thinkpad ~/Projects/domain-lookup-tree $ ./benchmark 
[+] Populating in-memory word list from /usr/share/dict/words.
[+] Truncating in-memory word list to 300 random words.
[+] Creating random lists of 100000 domains to insert and 100000 domains to query.
[+] Populating domain lookup tree.
[+] Performing lookup benchmarks:
    [*] New method took 0.34 seconds.
    [*] Old method took 199.45 seconds.
    [*] The new method is 586.617647 times faster than the old method.
[+] Verifying that new and old methods produced identical results:
    [*] New and old methods produced the same results.

Limitations & Improvements

Currently no helper functions for removing and freeing nodes
Instead of strcmp'ing, a hash function could be used