Сaching Dns proxy in the form of GNU/Linux kernel module.
Module was written and tested on Ubuntu 12.04.5 Server 32-bit (3.13 kernel), GCC 4.6.3.
Restrictions:
- support only protocol IPv4
- handles only requests of type A (i.e. conversion of the domain to the protocol IPv4)
- limited cache size (specified during configuration)
- delete cache on shutdown or restart (cache is located in RAM)
Scheme of work module:
+----------------------------------------------+
/| /|
+----------------------------------------------+ |
| | | |
| | Gateway machine | |
| | | |
| | ________________________________ | |
+---------+ | | | | | | +----------+
/ /| | | | Kernel | | | / /|
+---------+ | | | | | | | +----------+ |
| | | | | | +-------+ +----------+ | | | | Dns | |
| Client | |-----|-------|-->| Dns |---->| iptables |---|-----|------>| Server | |
|(browser)| |<----|-------|---| proxy |<----| (NAT) |<--|-----|-------| on | |
| |/ | | | +-------+ +----------+ | | | | Internet |/
+---------+ | | | | ^ | | | +----------+
| | | V | | | |
| | | +-------+ | | |
| | | | Cache | | | |
| | | +-------+ | | |
| | |________________________________| | |
| | | |
| |____________________________________________|_|
|/ |/
+----------------------------------------------+The path of the package inspection in the system Netfilter:
+------------------+
/ /|
+------------------+ |
| | |
| Network driver | |
| |/
+------------------+
|
V
+-----------------------+
/ /|
+-----------------------+ |
| | |
| NF_INET_PRE_ROUTING | |
| |/
+-----------------------+
|
V
+-----------+ +-----------------------+
/ /| / /|
+-----------+ | +-----------------------+ |
| | | | | |
| Routing | | --------> | NF_INET_LOCAL_IN | |
| |/ | |/
+-----------+ +-----------------------+
| |
| V
+-----------------------+ +--------------------+
/ /| / /|
+-----------------------+ | +--------------------+ |
| | | | | |
| NF_INET_FORWARD | | | Local IP service | |
| |/ | |/
+-----------------------+ +--------------------+
| |
V V
+-----------+ +-----------------------+
/ /| / /|
+-----------+ | +-----------------------+ |
| | | | | |
| Routing | | <-------- | NF_INET_LOCAL_OUT | |
| |/ | |/
+-----------+ +-----------------------+
|
V
+-----------------------+
/ /|
+-----------------------+ |
| | |
| NF_INET_POST_ROUTING | |
| |/
+-----------------------+
|
V
+------------------+
/ /|
+------------------+ |
| | |
| Network driver | |
| |/
+------------------+The format of the network packet:
+----------+----------+----------+--------+
/ / / / /|
+----------+----------+----------+--------+ |
| | | | | |
| MAC | IP | UPD | DNS | |
| header | header | header | data | |
| | | | |/
+----------+----------+ ---------+--------+The structure of the DNS packet:
+----------+------------+----------+-------------+--------------+
/ / / / / /|
+----------+------------+----------+-------------+--------------+ |
| | | | | | |
| Header | Question | Answer | Authority | Additional | |
| | | | | |/
+----------+------------+ ---------+-------------+--------------+The entire cache is stored in RAM PC. Cache is implemented on a data structure: the hash table.
The hash table is static and has a fixed size. The size is defined in the header file dnsproxy.h constant HASHTAB_SIZE.
Used the RS hash function (for strings).
Function listing:
static unsigned int rs_hash(unsigned char *str, unsigned int len)
{
unsigned int b = 378551;
unsigned int a = 63689;
unsigned int hash = 0;
unsigned int i = 0;
for (i = 0; i < len; str++, i++){
hash = hash * a + (unsigned char)(*str);
a *= b;
}
return (hash % HASHTAB_SIZE);
}Advantage hash function:
- fast calculation of the hash code
- deterministic
- uniform distribution of hash values
Building:
$ makeInstalling:
$ make installUninstalling:
$ make uninstallThis software is licensed under the terms of the GNU General Public License version 2.
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