作为一名网管，经常会接收用户反映某个网址有恶意程序，希望我们过滤一下，我们单位上网是通过 PIX520防火墙作NAT的，因此也就涉及到如何在PIX520防火墙上限制对于某些IP地址访问的问题，为此，就结合自己的实际工作经验写了这篇文章。（网络拓扑如图1所示）

图1

一、得到某网址与IP地址的对应关系

比如要封www.ttsou.cn，有两种方法可以得到该网址对应的IP地址，第一是ping该网址，如下所示：

C:>ping www.ttsou.cn
Pinging www.ttsou.cn [58.61.155.44] with 32 bytes of data:
Reply from 58.61.155.44: bytes=32 time=80ms TTL=116
Reply from 58.61.155.44: bytes=32 time=78ms TTL=116
Reply from 58.61.155.44: bytes=32 time=92ms TTL=116
Reply from 58.61.155.44: bytes=32 time=85ms TTL=116
Ping statistics for 58.61.155.44:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 78ms, Maximum = 92ms, Average = 83ms

从中我们可以得到www.ttsou.cn对应的IP地址为58.61.155.44.但是这种方法存在一个缺陷，即如果该网址对应有多个IP地址的话，用ping的方法不可能得到所有对应的IP地址，我们可以用nslookup来解决，如下所示：

C:>nslookup
Default Server:  ns.jncatv.net
Address:  222.175.169.91
> www.ttsou.cn
Server:  ns.jncatv.net
Address:  222.175.169.91
Non-authoritative answer:
Name:    www.ttsou.cn
Address:  58.61.155.44
> www.sina.com.cn
Server:  ns.jncatv.net
Address:  222.175.169.91
Non-authoritative answer:
Name:    hydra.sina.com.cn
Addresses:  218.30.108.58, 218.30.108.59, 218.30.108.61, 218.30.108.62
          218.30.108.64, 218.30.108.65, 218.30.108.66, 218.30.108.67, 218.30.108.68
          218.30.108.69, 218.30.108.72, 218.30.108.73, 218.30.108.74, 218.30.108.55
          218.30.108.56, 218.30.108.57
Aliases:  www.sina.com.cn, jupiter.sina.com.cn

从以上的结果我们可以看出，www.ttsou.cn确实是只对应了一个IP地址，但是象www.sina.com.cn这样的网址就对应了大量的IP地址。

二、在PIX520防火墙上了解当前访问列表的使用情况。

由于我们在PIX520防火墙上作了限制TELNET访问的限制，只有192.168的网段可以通过TELNET的方式登录上去，所以我们要先登录3层交换机（192.168.3.1），再从3层交换机上登录过去，先看一下当前配置：

telnet 192.168.201.1
Trying 192.168.201.1 ... Open
User Access Verification
Password:
Type help;or;'?' for a list of available commands.
pixfirewall> en
Password: ******
pixfirewall# show run
: Saved
:
PIX Version 6.2(2)
nameif ethernet0 outside security0
nameif ethernet1 inside security100

（以下省略）

出于安全方面的考虑，PIX防火墙的具体配置我就不列出了，把与本文有关的内容列出，重点应该看以下两条：

access-group acl_inside in interface outside
access-group acl_inside in interface inside

即当前应用的访问列表为acl_inside，然后再看acl_inside是如何写的：

access-list acl_inside deny udp any any eq tftp
access-list acl_inside deny tcp any any eq 135
access-list acl_inside deny udp any any eq 135
access-list acl_inside deny tcp any any eq 137
access-list acl_inside deny udp any any eq netbios-ns
access-list acl_inside deny tcp any any eq 138
access-list acl_inside deny udp any any eq netbios-dgm
access-list acl_inside deny tcp any any eq netbios-ssn
access-list acl_inside deny udp any any eq 139
access-list acl_inside deny tcp any any eq 445
access-list acl_inside deny tcp any any eq 593
access-list acl_inside deny tcp any any eq 4444
access-list acl_inside permit ip any any
access-list acl_inside permit tcp any any eq 1723
access-list acl_inside permit gre any any

从中我们可以看到原访问列表只是对某些端口的使用做了限制，而不涉及对某个IP地址进行访问的限制，为了稳妥起见，我们要先清楚的了解访问列表的格式，如下：

pixfirewall(config)# access-list ?
Usage:  [no] access-list compiled
[no] access-list  compiled
[no] access-list  deny|permit |object-group

  | object-group 
[  [] | object-group ]
  | object-group 
[  [] | object-group ]
[no] access-list  deny|permit icmp
  | object-group 
  | object-group 
[ | object-group ]

从帮助信息中大致了解到应该先写源IP，后写目标IP，因此对于我们想限制对于某个IP地址的访问就应该写成access-list acl_inside deny ip any host 58.61.155.44

三、具体的操作步骤

为了保障在添加一条对于某个IP地址限制的过程中PIX520的正常工作不受影响，我们应该按照以下步骤来进行操作

1、在内外端口上停掉访问控制列表

pixfirewall# conf t
pixfirewall(config)#access-group acl_inside in interface outside
pixfirewall(config)#access-group acl_inside in interface inside

2、去掉访问列表acl_inside

pixfirewall# conf t
pixfirewall(config)# no access-list acl-inside

3、重写access-list

pixfirewall(config)# access-list acl_inside deny udp any any eq tftp
pixfirewall(config)# access-list acl_inside deny tcp any any eq 135
pixfirewall(config)# access-list acl_inside deny udp any any eq 135
pixfirewall(config)# access-list acl_inside deny tcp any any eq 137
pixfirewall(config)# access-list acl_inside deny udp any any eq netbios -ns
pixfirewall(config)# access-list acl_inside deny tcp any any eq 138
pixfirewall(config)# access-list acl_inside deny udp any any eq netbios -dgm
pixfirewall(config)# access-list acl_inside deny tcp any any eq netbios -ssn
pixfirewall(config)# access-list acl_inside deny udp any any eq 139
pixfirewall(config)# access-list acl_inside deny tcp any any eq 445
pixfirewall(config)# access-list acl_inside deny tcp any any eq 593
pixfirewall(config)# access-list acl_inside deny tcp any any eq 4444
pixfirewall(config)# access-list acl_inside permit tcp any any eq 1723
pixfirewall(config)# access-list acl_inside permit gre any any
pixfirewall(config)# access-list acl_inside deny ip any host 58.61.155.44
pixfirewall(config)# access-list acl_inside permit ip any any

即保证permit ip any any这条命令是在最后面一行

4、在内外端口上应用访问列表

pixfirewall(config)#access-gropu acl_inside in inter outside
pixfirewall(config)#access-gropu acl_inside in inter outside

四、验证是否真正的对某个IP地址进行了限制

1、 进行完配置后肯定要先看一下当前配置：show run

2、可以通过tracert命令来验证，如下所示：

C:>tracert www.ttsou.cn
Tracing route to www.ttsou.cn [58.61.155.44]
over a maximum of 30 hops:
1    <1 ms    <1 ms    <1 ms  10.75.0.1
2     *        *        *     Request timed out.
3     *        *        *     Request timed out.
4     *        *        *     Request timed out.
5     *        *        *     Request timed out.

从中可以看出，对于www.ttsou.cn这个网址从三层交换机往上就不通了，证明在PIX520防火墙上已经成功的阻止了对于该网址的访问。