拒绝服务攻击原理及解决方法.txt

BBS的精华之处

#define DNS_SERVER_IP "10.0.0.8" 
Ngrep通过监视DNS服务器的53端口的UDP包来跟踪向内的DNS请求（只有UDP）。因此，n 
grep需要知道您的DNS服务器的IP地址。 
我们的设备可能会有多个DNS服务器，但我们认为对一台DNS服务器的支持足以证明这项 
技术的能力。 
#define TTL_THRESHOLD 150 
tfn2k SYN flood 攻击使用的 TTL值通常在200-255的范围内。估计到攻击者与目标主机 
之间不止50跳，因此我们可以只查找TTL时间高于150的包。假如您相信攻击者在50跳左 
右，那么您可以对TTL的限制进行一下更改。 
编译更改过的 ngrep 
编译和安装都非常简单。您仅需要使用以下之一来取代ngrep.c 文件。处于方便起见， 
我们可以详细说明。 
这段代码只是在RedHat 6.1 和Mandrake 6.5 Linux上测试过。 
首先您需要在 http://www.packetfactory.net/ngrep/ 下载ngrep，我们测试的是1.35 
版。 
然后在 ftp://ftp.ee.lbl.gov/libpcap.tar.Z下载libpcap 我们使用的是 0.40版。 
把文件放在临时文件夹里并解包， 
tar xvzf libpcap.tar.Z 
然后进行编译 
cd libpcap-0.4; ./configure; make; make install; make install-incl 
假如您遇到了困难，可以参见在libpcap-0.4目录里的README或INSTALL文件。根据我们 
实验的经验，如果/usr/local/include 和/usr/local/include/net目录在linux系统中 
不存在的话，安装会失败。加入您在安装时遇到了pcap.h 或 bpf.h的错误时你可以运行 
  
mkdir /usr/local/include; mkdir /usr/local/include/net然后重新运行'make inst 
all-incl'。然后我们需要编译ngrep (使用我们修改过的版本)。首先解包 
tar xvzf ngrep-1.35.tar.gz 
然后进行配置 
cd ngrep; ./configure 
然后把ngrep.c复制到ngrep目录里。你可以覆盖也可以备份原始的ngrep.c文件。在这里 
，您应当回顾在修改过的ngrep.c里的配置，至少您应当把DNS_SERVER_IP更改为您所使 
用的DNS的地址。更改完毕后你就可以运行'make'，这样就建立了ngrep应用程序。 
Modified ngrep.c source code 
/* this code is available for download from http://www.wiretrip.net/na/ngrep 
.c */ 
/* 
* $Id: ngrep.c,v 1.35 1999/10/13 16:44:16 jpr5 Exp $ 
* 
*/ 
/* TFN detection code added by Rain Forest Puppy / [e-mail] 
and Night Axis / [e-mail] */ 
/********* TFN detection defines *******************************/ 
/* how many DNS and ICMP requests to track */ 
#define DNS_REQUEST_MAX 5000 
#define ICMP_REQUEST_MAX 1000 
/* flood threshold is matches per 10 seconds */ 
#define FLOOD_THRESHOLD 20 
/* IP of your DNS server */ 
#define DNS_SERVER_IP "10.9.100.8" 
/* TFN syn uses ttl between 200-255. Assuming less than 50 hops, 
flag stuff with ttl > TTL_THRESHOLD (other critera are used 
as well) */ 
#define TTL_THRESHOLD 150 
/**************************************************************/ 
#include <stdlib.h> 
#include <string.h> 
#include <signal.h> 
#ifdef LINUX 
#include <getopt.h> 
#endif 
#if defined(BSD) || defined(SOLARIS) 
#include <unistd.h> 
#include <sys/types.h> 
#include <ctype.h> 
#include <sys/socket.h> 
#include <netinet/in.h> 
#include <netinet/in_systm.h> 
#include <net/if.h> 
#endif 
#if defined(LINUX) && !defined(HAVE_IF_ETHER_H) 
h> 
#include <pcap.h> 
#include <net/bpf.h> 
#include "regex.h" 
#include "ngrep.h" 
static char rcsver[] = "$Revision: 1.35 $"; 
int snaplen = 65535, promisc = 1, to = 1000; 
int show_empty = 0, show_hex = 0, quiet = 0; 
int match_after = 0, keep_matching = 0, invert_match = 0; 
int matches = 0, max_matches = 0; 
char pc_err[PCAP_ERRBUF_SIZE], *re_err; 
int (*match_func)(); 
int re_match_word = 0, re_ignore_case = 0; 
struct re_pattern_buffer pattern; 
char *regex, *filter; 
struct bpf_program pcapfilter; 
struct in_addr net, mask; 
char *dev = NULL; 
int link_offset; 
pcap_t *pd; 
/**************** TFN2K detection **********************************/ 
unsigned int udp_flood_count=0, syn_flood_count=0; 
unsigned int targa_flood_count=0, icmp_flood_count=0; 
unsigned long my_dns, targ1, targ2, rfp1, icmp_flood=1; 
time_t t; 
unsigned long dns_circbuff[DNS_REQUEST_MAX]; 
unsigned int dns_cb_ptr=0; 
unsigned long icmp_circbuff[ICMP_REQUEST_MAX]; 
unsigned int icmp_cb_ptr=0; 
void add_dns (unsigned long ipadd){ 
dns_circbuff[dns_cb_ptr++]=ipadd; 
if (dns_cb_ptr==DNS_REQUEST_MAX) dns_cb_ptr=0;} 
void add_icmp (unsigned long ipadd){ 
icmp_circbuff[icmp_cb_ptr++]=ipadd; 
if (icmp_cb_ptr==ICMP_REQUEST_MAX) dns_cb_ptr=0;} 
void anti_tfn_init (void) { 
unsigned int x; 
for(x=0;x<DNS_REQUEST_MAX;x++) dns_circbuff[x]=0; 
for(x=0;x<ICMP_REQUEST_MAX;x++) icmp_circbuff[x]=0; 
my_dns=inet_addr(DNS_SERVER_IP); 
printf("Ngrep with TFN detection modifications by wiretrip / www.wiretrip.ne 
t\n"); 
printf("Watching DNS server: %s\n",inet_ntoa(my_dns)); 
targ1=htons(16383); targ2=htons(8192); 
rfp1=htons(~(ICMP_ECHO << 8)); /* hopefull this is universal ;) */ 
alarm(20);} 
void print_circbuffs (void) { 
unsigned int x; 
printf("Last (%u) DNS requests:\n",DNS_REQUEST_MAX); 
for(x=0;x<DNS_REQUEST_MAX;x++) 
if(dns_circbuff[x]>0) printf("%s\n",inet_ntoa(dns_circbuff[x])); 
printf("\nLast (%u) ICMP echo requests (pings):\n",ICMP_REQUEST_MAX); 
for(x=0;x<ICMP_REQUEST_MAX;x++) 
if (icmp_circbuff[x]>0) printf("%s\n",inet_ntoa(icmp_circbuff[x]));} 
void reset_counters (int sig) { 
udp_flood_count=syn_flood_count=targa_flood_count=icmp_flood_count=0; 
alarm(10);} 
void tfn_attack_detected (char* attack_type){ 
if(icmp_flood==0) return; 
(void)time(&t); 
printf("\n%s",ctime(&t)); 
printf("A TFN2K %s attack has been detected!\n\n",attack_type); 
print_circbuffs(); 
printf("\nIncoming realtime ICMP echo requests (pings):\n"); 
icmp_flood=0;} 
/*********************************************************************/ 
int main(int argc, char **argv) { 
char c; 
signal(SIGINT,dealloc); 
signal(SIGQUIT,dealloc); 
signal(SIGABRT,dealloc); 
signal(SIGPIPE,dealloc); 
signal(SIGALRM,reset_counters); 
anti_tfn_init(); 
while ((c = getopt(argc, argv, "d:")) != EOF) { 
switch (c) { 
case 'd': 
dev = optarg; 
break;}} 
if (!dev) 
if (!(dev = pcap_lookupdev(pc_err))) { 
perror(pc_err); 
exit(-1);} 
if ((pd = pcap_open_live(dev, snaplen, promisc, to, pc_err)) == NULL) { 
perror(pc_err); 
exit(-1);} 
if (pcap_lookupnet(dev,&net.s_addr,&mask.s_addr, pc_err) == -1) { 
perror(pc_err); 
exit(-1);} 
printf("interface: %s (%s/", dev, inet_ntoa(net)); 
printf("%s)\n",inet_ntoa(mask)); 
switch(pcap_datalink(pd)) { 
case DLT_EN10MB: 
case DLT_IEEE802: 
link_offset = ETHHDR_SIZE; 
break; 
case DLT_SLIP: 
link_offset = SLIPHDR_SIZE; 
break; 
case DLT_PPP: 
link_offset = PPPHDR_SIZE; 
break; 
case DLT_RAW: 
link_offset = RAWHDR_SIZE; 
break; 
case DLT_NULL: 
link_offset = LOOPHDR_SIZE; 
break; 
default: 
fprintf(stderr,"fatal: unsupported interface type\n"); 
exit(-1); 
} while (pcap_loop(pd,0,(pcap_handler)process,0));} 
void process(u_char *data1, struct pcap_pkthdr* h, u_char *p) { 
struct ip* ip_packet = (struct ip *)(p + link_offset); 
switch (ip_packet->ip_p) { 
case IPPROTO_TCP: { 
struct tcphdr* tcp = (struct tcphdr *)(((char *)ip_packet) + ip_packet->ip_h 
l*4); 
if(tcp->th_flags==0x22 && ip_packet->ip_ttl > TTL_THRESHOLD){ 
if(++syn_flood_count > FLOOD_THRESHOLD) tfn_attack_detected("SYN");} 
if(ip_packet->ip_ttl==0 && 
(ip_packet->ip_off==targ1 || ip_packet->ip_off==targ2)){ 
if(++targa_flood_count > FLOOD_THRESHOLD) tfn_attack_detected("TARGA"); 
}} break; 
case IPPROTO_UDP: { 
struct udphdr* udp = (struct udphdr *)(((char *)ip_packet) + ip_packet->ip_h 
l*4); 
#ifdef HAVE_DUMB_UDPHDR 
if ((ntohs(udp->source) + ntohs(udp->dest)) == 65536) { 
#else 
if ((ntohs(udp->uh_sport) + ntohs(udp->uh_dport)) == 65536) { 
#endif 
if(++udp_flood_count > FLOOD_THRESHOLD) tfn_attack_detected("UDP");} 
if(ip_packet->ip_dst.s_addr==my_dns && 
#ifdef HAVE_DUMB_UDPHDR 
ntohs(udp->dest) == 53) { 
#else 
ntohs(udp->uh_dport) == 53) { 
#endif 
add_dns(ip_packet->ip_src.s_addr); 

}} break; 
icmp_cksum==rfp1 && ip_packet->ip_ttl==0){ 
if(++icmp_flood_count > FLOOD_THRESHOLD) tfn_attack_detected("ICMP"); 
} else { if(icmp_flood>0) add_icmp(ip_packet->ip_src.s_addr); 
else printf("%s\n",inet_ntoa(ip_packet->ip_src));}}}}} 
void dealloc(int sig) { 
if (filter) free(filter); 
exit(0);} 
-- 
世界上有两件东西能够深深地震撼人们的心灵 
一件是我们心中崇高的道德准则 
另一件是我们头顶上灿烂的星空 
  
                                ——伊曼努尔·康德 
  
  
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