隨著對(duì)網(wǎng)絡(luò)安全需求的深入開(kāi)發(fā)，基于網(wǎng)絡(luò)的入侵檢測(cè)技術(shù)已經(jīng)成為一個(gè)重要且有意思的研究方向。想學(xué)習(xí)NIDS技術(shù)除了去讀一些現(xiàn)成的資料和一些開(kāi)源系統(tǒng)的源碼，最好的辦法莫過(guò)于自己去寫一個(gè)NIDS程序，只有那樣才能真正體會(huì)到一些NIDS的實(shí)現(xiàn)需求和設(shè)計(jì)妙處。
本質(zhì)上說(shuō)NIDS只是一種網(wǎng)絡(luò)流量的分析工具，通過(guò)對(duì)網(wǎng)絡(luò)流量的分析識(shí)別出一些已知或未知的攻擊行為，一個(gè)最簡(jiǎn)單的NIDS完成的主要工作也就是抓包->協(xié)議解碼->匹配，眾所周知PERL是極其強(qiáng)大的腳本語(yǔ)言，尤其是它的字符串處理能力可以方便地實(shí)現(xiàn)對(duì)于網(wǎng)絡(luò)流量中惡意特征進(jìn)行匹配。當(dāng)然PERL畢竟只是腳本語(yǔ)言，它的執(zhí)行效率不允許用于真正大流量生產(chǎn)性環(huán)境，但PERL的簡(jiǎn)單易學(xué)及強(qiáng)大功能對(duì)于實(shí)現(xiàn)一個(gè)簡(jiǎn)單的NIDS達(dá)到學(xué)習(xí)的目的無(wú)疑是非常好的，下面我介紹一個(gè)用PERL實(shí)現(xiàn)的簡(jiǎn)單NIDS框架，我們將在Linux下實(shí)現(xiàn)它，在其他操作系統(tǒng)上類似。
PERL的一個(gè)強(qiáng)大特性就在于它海量的CPAN模塊庫(kù)，很多你想實(shí)現(xiàn)的功能都可以找到現(xiàn)成的模塊，你所要做的只是安裝上那些模塊即可，關(guān)于PERL的模塊及面向?qū)ο筇匦缘墓芾砗褪褂迷谶@就不介紹了，請(qǐng)參看相關(guān)資料，比如O'REILLY出版的《高級(jí)Perl編程》。在用PERL編寫網(wǎng)絡(luò)流量分析腳本之前，需要安裝一些底層的抓包及基本的數(shù)據(jù)包解碼模塊，包括如下這些：
http://www.tcpdump.org/release/libpcap-0.8.1.tar.gz
底層基本的抓包庫(kù)。
http://www.cpan.org/authors/id/T/TI/TIMPOTTER/Net-Pcap-0.04.tar.gz
libpcap的PERL接口。
http://www.cpan.org/authors/id/T/TI/TIMPOTTER/Net-PcapUtils-0.01.tar.gz
Net-Pcap模塊的wrapper，包裝Net-Pcap的函數(shù)，可以更方便地在PERL里調(diào)用抓包。
http://www.cpan.org/authors/id/T/TI/TIMPOTTER/NetPacket-0.03.tar.gz
用于基本的IP/TCP/UDP等包解碼的模塊，剝除各種協(xié)議頭，抽取各個(gè)字段。
下面的代碼演示了一個(gè)帶有基本SMB和FTP協(xié)議解碼模塊的最簡(jiǎn)單NIDS框架，此程序?qū)崿F(xiàn)最簡(jiǎn)單的NIDS功能，面向單包，不關(guān)心包的狀態(tài)，不具備高級(jí)的商業(yè)NIDS產(chǎn)品諸如流重組，包狀態(tài)及應(yīng)用層協(xié)議的跟蹤等功能。為了提高檢測(cè)的準(zhǔn)確性，與Snort直接匹配數(shù)據(jù)區(qū)不同的是，這個(gè)腳本實(shí)現(xiàn)了兩個(gè)應(yīng)用層協(xié)議：SMB、FTP的簡(jiǎn)單解碼，解碼完全是面向NIDS的需要，代碼也沒(méi)有經(jīng)過(guò)仔細(xì)的測(cè)試可能存在問(wèn)題。
（一）perl-ids.pl 實(shí)現(xiàn)抓包及檢測(cè)分析的主程序。
復(fù)制代碼 代碼如下:
#!/usr/bin/perl
#
# Comments/suggestions to stardust at xfocus dot org
#
#
# $Id: perl-ids.pl,v 1.16 2004/03/04 21:51:12 stardust Exp $
#
# 引用所有相關(guān)的模塊
use Net::PcapUtils;
use NetPacket::Ethernet qw(:strip);
use NetPacket::TCP;
use NetPacket::IP qw(:protos);
use NetPacket::SMB;
use NetPacket::FTP;
# 定義日志文件名
$workingdir = "./";
$attacklog = "attack.log";
$monitorlog = "monitor.log";
# 以后臺(tái)進(jìn)程方式運(yùn)行
daemon ();
sub daemon {
unless (fork) {
SniffLoop ();
exit 0;
}
exit 1;
}
# 抓包循環(huán)
sub SniffLoop {
# 進(jìn)入工作目錄
chdir ("$workingdir");
# 打開(kāi)日志文件
open (ATTACKLOG,">> $attacklog");
open (MONITORLOG,">> $monitorlog");
# 設(shè)置文件讀寫為非緩沖模式
select(ATTACKLOG); $ ++; select(MONITORLOG); $ ++; select(STDOUT); $ ++;
# 設(shè)置信號(hào)處理函數(shù)，因?yàn)槌绦蜻\(yùn)行于后臺(tái)，退出時(shí)需要利用信號(hào)處理函數(shù)做些清理工作
$SIG{"INT"} = 'HandleINT';
$SIG{"TERM"} = 'HandleTERM';
# 進(jìn)入抓包回調(diào)函數(shù)
Net::PcapUtils::loop(&sniffit, SNAPLEN => 1800, Promisc => 1, FILTER => 'tcp or udp', DEV => 'eth0');
}
sub sniffit {
my ($args,$header,$packet) = @_;
# 解碼IP包
$ip = NetPacket::IP->decode(eth_strip($packet));
# TCP協(xié)議
if ($ip->{proto} == IP_PROTO_TCP) {
# 解碼TCP包
$tcp = NetPacket::TCP->decode($ip->{data});
# 檢查來(lái)自SMB客戶端的包
if (($tcp->{dest_port} == 139) ($tcp->{dest_port} == 445)) {
# 如果目的端口是139或445，認(rèn)為是SMB協(xié)議包，做相應(yīng)的檢查
SmbClientCheck ($ip->{src_ip},$tcp->{src_port},$ip->{dest_ip},$tcp->{dest_port},$tcp->{data});
} elsif ($tcp->{dest_port} == 21) {
# 如果目的端口是21，認(rèn)為是FTP協(xié)議，做相應(yīng)的檢查
FtpClientCheck ($ip->{src_ip},$tcp->{src_port},$ip->{dest_ip},$tcp->{dest_port},$tcp->{data});
} else {}
# UDP協(xié)議
} elsif ($ip->{proto} == IP_PROTO_UDP) {
} else {}
}
sub SmbClientCheck {
my ($src_ip,$src_port,$dest_ip,$dst_port,$data) = @_;
# 調(diào)用SMB解碼模塊解碼
$smb = NetPacket::SMB->decode($data);
# 如果解碼成功
if ($smb->{valid}) {
# 示例檢測(cè)新近公布eeye的那個(gè)ASN.1解碼錯(cuò)誤導(dǎo)致的堆破壞漏洞
# BID：9633,9635 CVEID：CAN-2003-0818 NSFOCUSID：6000
# 如果SMB命令是Session Setup AndX
if ($smb->{cmd} == 0x73) {
# 如果設(shè)置了Extended Security Negotiation位，表示有包里有Security Blob
if ($smb->{flags2} & F2_EXTSECURINEG) {
# 用正則表達(dá)式匹配通常會(huì)在攻擊包里出現(xiàn)的OID及引發(fā)錯(cuò)誤的畸形數(shù)據(jù)串
# 由于不是從原理上檢測(cè)加之ASN.1編碼的靈活性，這樣的檢測(cè)會(huì)導(dǎo)致漏報(bào)
if (($smb->{bytecount} > 0) && ($smb->{bytes} =~ m/x06x06x2bx06x01x05x05x02.*[xa1x05x23x03x03x01x07 x84xffxffxff]/)) {
# 記入日志文件
LogAlert ($src_ip,$src_port,$dest_ip,$dst_port,"ASN.1 malform encode attack!");
}
}
}
}
}
sub FtpClientCheck {
my ($src_ip,$src_port,$dest_ip,$dst_port,$data) = @_;
# 調(diào)用FTP解碼模塊解碼
$ftp = NetPacket::FTP->decode($data);
# 如果解碼成功
if ($ftp->{valid}) {
# 示例檢測(cè)新近公布的Serv-U < 5.0.0.4版FTP服務(wù)器MDTM命令溢出攻擊
# BID：9751 NSFOCUSID：6078
# 遍歷從數(shù)據(jù)包里解碼出來(lái)的FTP命令及其參數(shù)
for (my $i = 1;$i <= $ftp->{cmdcount};$i++) {
my $cmd = "cmd"."$i";
my $para = "para"."$i";
# 如果FTP命令是MDTM
if (uc($ftp->{$cmd}) eq "MDTM") {
# 用正則表達(dá)式匹配引發(fā)溢出的參數(shù)串，這里體現(xiàn)了正則
# 表達(dá)式的強(qiáng)大，用此匹配可以從原理上檢測(cè)到畸形參數(shù)串
if ($ftp->{$para} =~ m/d{14}[+ -]S{5,}s+S{1,}/) {
LogAlert ($src_ip,$src_port,$dest_ip,$dst_port,"Serv-U < v5.0.0.4 MDTM command long timezone string overflow attack!");
}
}
}
}
}
# 記錄攻擊告警
sub LogAlert {
my ($src_ip,$src_port,$dest_ip,$dst_port,$message) = @_;
my $nowtime = localtime;
printf ATTACKLOG ("%s %s:%s -> %s:%s %s ",$nowtime,$src_ip,$src_port,$dest_ip,$dst_port,$message);
printf ("%s %s:%s -> %s:%s %s ",$nowtime,$src_ip,$src_port,$dest_ip,$dst_port,$message);
}
# 記錄監(jiān)控信息
sub LogMonitor {
my ($src_ip,$src_port,$dest_ip,$dst_port,$message) = @_;
my $nowtime = localtime;
printf MONITORLOG ("%s %s:%s -> %s:%s %s ",$nowtime,$src_ip,$src_port,$dest_ip,$dst_port,$message);
printf ("%s %s:%s -> %s:%s %s ",$nowtime,$src_ip,$src_port,$dest_ip,$dst_port,$message);
}
# INT信號(hào)處理例程
sub HandleINT {
CleanUp ();
exit (0);
}
# TERM信號(hào)處理例程
sub HandleTERM {
CleanUp ();
exit (0);
}
# 清理，主要工作是關(guān)閉文件句柄
sub CleanUp {
close (ATTACKLOG); close (MONITORLOG);
}
（二）FTP.pm FTP協(xié)議解碼模塊，抽取數(shù)據(jù)包里的FTP命令及相應(yīng)的參數(shù)，此文件需要拷貝到NetPacket系列模塊所在的目錄，通常是在/usr/lib/perl5/site_perl/5.x.x/NetPacket/
復(fù)制代碼 代碼如下:
#
# NetPacket::FTP - Decode FTP packets
#
# Comments/suggestions to stardust at xfocus dot org
#
#
# $Id: FTP.pm,v 1.16 2004/03/03 l1:16:20 stardust Exp $
#
package NetPacket::FTP;
use strict;
use vars qw($VERSION @ISA @EXPORT @EXPORT_OK %EXPORT_TAGS);
use NetPacket;
my $myclass;
BEGIN {
$myclass = __PACKAGE__;
$VERSION = "0.01";
}
sub Version () { "$myclass v$VERSION" }
BEGIN {
@ISA = qw(Exporter NetPacket);
# Items to export into callers namespace by default
# (move infrequently used names to @EXPORT_OK below)
@EXPORT = qw(
);
# Other items we are prepared to export if requested
@EXPORT_OK = qw(
);
# Tags:
%EXPORT_TAGS = (
ALL => [@EXPORT, @EXPORT_OK],
);
}
#
# Decode the packet
#
# FTP協(xié)議文本參看RFC959，http://www.ietf.org/rfc/rfc0959.txt
# 常見(jiàn)的FTP命令
my @ftp_cmds = qw(ABOR ACCT ALLO APPE CDUP CWD DELE HELP LIST MKD MODE NLST
NOOP PASS PASV PORT PWD QUIT REIN REST RETR RMD RNFR RNTO
SITE SMNT STAT STOR STOU STRU SYST TYPE USER XCUP XCWD XMKD
XPWD XRMD LPRT LPSV ADAT AUTH CCC CONF ENC MIC PBSZ PROT
FEAT OPTS EPRT EPSV LANG MDTM MLSD MLST SIZE DIGT CLNT MACB
);
sub decode {
my $class = shift;
my($data) = @_;
my $self = {};
my $cmdhead = 0;
my $cmdtail = 0;
my @parts = ();
my $cmdcount = 0;
my $returnindex = 0;
my $data_len = length($data);
# 如果數(shù)據(jù)長(zhǎng)度過(guò)短則不處理
if ($data_len >= 4) {
# 一個(gè)包里的FTP命令個(gè)數(shù)
$self->{cmdcount} = 0;
# 搜索回車，之前認(rèn)為是一個(gè)命令行，需要注意的是一個(gè)包里可能包含多個(gè)FTP命令
while ( (($returnindex = index ($data,"x0a",$cmdhead)) >=0) (($returnindex < 0) && (($data_len - $cmdhead) >= 4))) {
# 調(diào)整一個(gè)命令行串尾指針
if ($returnindex < 0) {
$cmdtail = $data_len -1;
} else {
$cmdtail = $returnindex;
}
if ((my $cmdlen = ($cmdtail - $cmdhead + 1)) >= 4) {
# 取出命令行串
my $cmdline = substr($data,$cmdhead,$cmdlen);
# 從命令行里拆分出命令名和它的參數(shù)串
if (splitcmd($cmdline,@parts)) {
$self->{cmdcount}++;
my $cmdindex = "cmd"."$self->{cmdcount}";
my $paraindex = "para"."$self->{cmdcount}";
# 記錄到要返回到主程序的對(duì)象
$self->{$cmdindex} = $parts[0];
$self->{$paraindex} = $parts[1];
}
}
# 調(diào)整命令行串頭指針
$cmdhead = $cmdtail + 1;
}
# 如果命令個(gè)數(shù)大于0，則說(shuō)明解碼是有效的
if ($self->{cmdcount} == 0) {
$self->{valid} = 0;
} else {
$self->{valid} = 1;
}
} else {
$self->{valid} = 0;
}
# 返回對(duì)象
bless($self, $class);
return $self;
}
sub splitcmd {
my ($cmdline,$parts) = @_;
# 去除行尾的回車
chomp($cmdline);
# 用正則表達(dá)式抽取出命令名字和參數(shù)，既然效率不是考慮的主要問(wèn)題就“毫無(wú)顧忌”地使用正則表達(dá)式，因?yàn)榉奖?BR>if ($cmdline =~ m/^s*([a-zA-Z]{3,4})s+(.*)/) {
my $valid_cmd = 0;
# 檢查抽出來(lái)的命令名字是否是一個(gè)已知的合法FTP命令
for (my $i=0;$i<@ftp_cmds;$i++) {
if ($ftp_cmds[$i] eq uc($1)) {
$valid_cmd = 1;
last;
}
}
# 如果是合法的命令則返回給調(diào)用函數(shù)
if ($valid_cmd) {
${$parts}[0] = $1;
${$parts}[1] = $2;
return 1;
} else {
return 0;
}
} else {
return 0;
}
}
#
# Module initialisation
#
1;
# autoloaded methods go after the END token (&& pod) below
__END__
（三）SMB.pm 對(duì)SMB包頭結(jié)構(gòu)的簡(jiǎn)單解碼模塊，此文件需要拷貝到NetPacket系列模塊所在的目錄，通常是在/usr/lib/perl5/site_perl/5.x.x/NetPacket/
復(fù)制代碼 代碼如下:
#
# NetPacket::SMB - Decode SMB packets
#
# Comments/suggestions to stardust at xfocus dot org
#
#
# $Id: SMB.pm,v 1.16 2004/02/23 12:25:17 stardust Exp $
#
package NetPacket::SMB;
use strict;
use vars qw($VERSION @ISA @EXPORT @EXPORT_OK %EXPORT_TAGS);
use NetPacket;
my $myclass;
# SMB flags
use constant F2_LONGNAMEALLW => 0x0001;
use constant F2_EXTATTRIBUTE => 0x0002;
use constant F2_SECURITYSIGN => 0x0004;
use constant F2_LONGNAMEUSED => 0x0040;
use constant F2_EXTSECURINEG => 0x0800;
use constant F2_DONTRESOLDFS => 0x1000;
use constant F2_EXECONLYREAD => 0x2000;
use constant F2_ERRORCODTYPE => 0x4000;
use constant F2_UNICODSTRING => 0x8000;
use constant F_LOCKANDREAD => 0x01;
use constant F_RCVBUFFPOST => 0x02;
use constant F_CASESENSITV => 0x08;
use constant F_CANONICPATH => 0x10;
use constant F_OPLOCKSREQU => 0x20;
use constant F_NOTIFYONOPN => 0x40;
use constant F_REQUERESPON => 0x80;
BEGIN {
$myclass = __PACKAGE__;
$VERSION = "0.01";
}
sub Version () { "$myclass v$VERSION" }
BEGIN {
@ISA = qw(Exporter NetPacket);
# Items to export into callers namespace by default
# (move infrequently used names to @EXPORT_OK below)
@EXPORT = qw(F2_LONGNAMEALLW F2_EXTATTRIBUTE F2_SECURITYSIGN
F2_LONGNAMEUSED F2_EXTSECURINEG F2_DONTRESOLDFS
F2_EXECONLYREAD F2_ERRORCODTYPE F2_UNICODSTRING
F_LOCKANDREAD F_RCVBUFFPOST F_CASESENSITV
F_CANONICPATH F_OPLOCKSREQU F_NOTIFYONOPN
F_REQUERESPON
);
# Other items we are prepared to export if requested
@EXPORT_OK = qw(smb_strip
);
# Tags:
%EXPORT_TAGS = (
ALL => [@EXPORT, @EXPORT_OK],
strip => [qw(smb_strip)],
);
}
#
# Strip header from packet and return the data contained in it
#
undef &smb_strip;
*smb_strip = &strip;
# 剝除SMB頭的函數(shù)
sub strip {
my ($data) = @_;
my $smb_obj = NetPacket::SMB->decode($data);
return $smb_obj->{data};
}
#
# Decode the packet
#
sub decode {
my $class = shift;
my($data) = @_;
my $self = {};
my $data_len = 0;
my $temp = "";
$data_len = length ($data);
# 如果數(shù)據(jù)區(qū)長(zhǎng)度小于39字節(jié)（4+32+3），則認(rèn)為不是一個(gè)可解碼的SMB包
if ($data_len < 39) {
$self->{valid} = 0;
} else {
# 取SMB的標(biāo)志串
my $smb_mark = substr ($data,4,4);
# 是否符合標(biāo)志串
if ($smb_mark ne "xffx53x4dx42") {
$self->{valid} = 0;
} else {
$self->{valid} = 1;
# Decode SMB packet
if (defined($data)) {
# 用PERL的unpack函數(shù)解碼32字節(jié)長(zhǎng)的SMB頭結(jié)構(gòu)，頭結(jié)構(gòu)可
# 參考 http://www.cs.uml.edu/~bill/cs592/cifs.chm
# 感謝小四（scz at nsfocus dot com）對(duì)于SMB頭結(jié)構(gòu)中字段字節(jié)序的提醒
($self->{nbt_type}, $self->{nbt_flag}, $self->{nbt_len},
$self->{mark}, $self->{cmd}, $self->{status},
$self->{flags}, $self->{flags2}, $self->{ext},
$self->{ext2}, $self->{ext3}, $self->{tid},
$self->{pid}, $self->{uid}, $self->{mid},
$self->{data}) = unpack("CCna4CVCvVVVvvvva*", $data);
($self->{wordcount},$temp) = unpack("Ca*",$self->{data});
if ((36 + 1 + $self->{wordcount} * 2) <= ($data_len - 2)) {
# 解碼SMB結(jié)構(gòu)下的wordcount字節(jié)及bytecount字節(jié)數(shù)據(jù)
my $wordbytes = $self->{wordcount} * 2;
($self->{wordcount},$self->{words},$self->{bytecount},$self->{bytes}) = unpack("C"."a"."$wordbytes"."va*",$self->{data});
} else {
($self->{wordcount},$self->{words}) = unpack("Ca*",$self->{data});
$self->{bytecount} = -1; $self->{bytes} = "";
}
}
}
}
# 返回對(duì)象
bless($self, $class);
return $self;
}
#
# Module initialisation
#
1;
# autoloaded methods go after the END token (&& pod) below
__END__
