tcpdump能帮助我们捕捉并保存网络包,保存下来的网络包可用于分析网络负载情况,包可通过tcpdump命令解析,也可以保存成后缀为pcap的文件,使用wireshark等软件进行查看。
以下将给出9个使用tcpdump的例子,以说明tcpdump的具体使用方法。
1.针对特定网口抓包(-i选项)
当我们不加任何选项执行tcpdump时,tcpdump将抓取通过所有网口的包;使用-i选项,我们可以在某个指定的网口抓包:
linux:/tmp/lx # tcpdump -i eth0tcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes10:50:28.607429 IP 10.70.121.92.autodesk-lm > 10.71.171.140.ssh: . ack 116 win 6495110:50:28.607436 IP 10.71.171.140.ssh > 10.70.121.92.autodesk-lm: P 116:232(116) ack 1 win 1286410:50:30.384195 arp who-has 128.128.128.35 tell 128.128.128.35
以上例子中,tcpdump抓取所有通过eth0的包。
2.抓取指定数目的包(-c选项)
默认情况下tcpdump将一直抓包,直到按下”ctrl+c”中止,使用-c选项我们可以指定抓包的数量:
linux:/tmp/lx # tcpdump -c 2 -i eth0tcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes10:58:05.656104 IP 10.71.171.140.ssh > 10.70.121.92.autodesk-lm: P 1210443473:1210443589(116) ack 2583117929 win 1286410:58:05.657074 IP 10.70.121.92.autodesk-lm > 10.71.171.140.ssh: . ack 116 win 652112 packets captured6 packets received by filter0 packets dropped by kernel
以上例子中,只针对eth0网口抓2个包。
3.将抓到包写入文件中(-w选项)
使用-w选项,我们可将抓包记录到一个指定文件中,以供后续分析
linux:/tmp/lx # tcpdump -w 20120606.pcap -i eth0tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes75 packets captured150 packets received by filter0 packets dropped by kernel
应当保存为.pcap后缀的文件,方便我们使用wireshark等工具读取分析。
4.读取tcpdump保存文件(-r选项)
对于保存的抓包文件,我们可以使用-r选项进行读取:
linux:/tmp/lx # tcpdump -r 20120606.pcapreading from file 20120606.pcap, link-type EN10MB (Ethernet)11:01:57.392907 IP 10.71.171.140.ssh > 10.70.121.92.autodesk-lm: P 1210446405:1210446457(52) ack 2583119957 win 1286411:01:57.392917 IP 10.71.171.140.ssh > 10.70.121.92.autodesk-lm: P 52:168(116) ack 1 win 1286411:01:57.393649 IP 10.70.121.92.autodesk-lm > 10.71.171.140.ssh: . ack 52 win 65327
5.抓包时不进行域名解析(-n选项)
默认情况下,tcpdump抓包结果中将进行域名解析,显示的是域名地址而非ip地址,使用-n选项,可指定显示ip地址。
6.增加抓包时间戳(-tttt选项)
使用-tttt选项,抓包结果中将包含抓包日期:
linux:/tmp/lx # tcpdump -n -tttt -i eth02012-06-06 11:14:59.539736 IP 10.71.171.140.22 > 10.70.121.95.1787: P 1:53(52) ack 100 win 75042012-06-06 11:14:59.539754 IP 10.71.171.140.22 > 10.70.121.95.1787: P 53:105(52) ack 100 win 75042012-06-06 11:14:59.539770 IP 10.71.171.140.22 > 10.70.121.95.1787: P 105:157(52) ack 100 win 7504
7.指定抓包的协议类型
我们可以只抓某种协议的包,tcpdump支持指定以下协议:ip,ip6,arp,tcp,udp,wlan等。以下例子只抓取arp协议的包:
linux:/tmp/lx # tcpdump -i eth0 arptcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes11:22:26.948656 arp who-has 10.10.1.30 tell 10.10.1.2611:22:27.017406 arp who-has 10.10.1.30 tell 10.10.1.2611:22:27.078803 arp who-has 10.10.1.30 tell 10.10.1.26
8.指定抓包端口
如果想要对某个特定的端口抓包,可以通过以下命令:
linux:/tmp/lx # tcpdump -i eth0 port 22tcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes11:41:04.387547 IP 10.70.121.92.autodesk-lm > 10.71.171.140.ssh: . ack 1216136825 win 6475111:41:04.387891 IP 10.71.171.140.ssh > 10.70.121.92.autodesk-lm: P 1:233(232) ack 0 win 1608011:41:04.398973 IP 10.70.121.92.autodesk-lm > 10.71.171.140.ssh: P 0:52(52) ack 233 win 64519
9.抓取特定目标ip和端口的包
网络包的内容中,包含了源ip地址、端口和目标ip、端口,我们可以根据目标ip和端口过滤tcpdump抓包结果,以下命令说明了此用法:
linux:/tmp/lx # tcpdump -i eth0 dst 10.70.121.92 and port 22
Reference: Packet Analyzer: 15 TCPDUMP Command Examples