一个在容器外用 tcpdump 命令对容器内的网络请求抓包的方法

前言

如题所述，记录一种在不进入容器的情况下对容器进行抓包的方法。

方法介绍

简单来说就是找到容器所用的网卡，然后在 host 机器上对该网卡进行抓包就可以了 :joy

操作示例

创建一个测试容器:

$ docker run -d --rm --name test2 busybox sh -c 'while true; do wget baidu.com -O /tmp/test ; sleep 1; done'
1adfbbc5e3519eed16334590934c48cce778728660508a255f6fafe600c6bcd1

找到这个容器使用的网卡:

$ PID=$(docker inspect --format {{.State.Pid}} test2)

$ nsenter -n -t $PID ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
18: eth0@if19: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default
    link/ether 02:42:ac:11:00:02 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 172.17.0.2/16 brd 172.17.255.255 scope global eth0
       valid_lft forever preferred_lft forever

上面的那个 @if19 是关键信息，然后在 host 机器上找到这个 @if19 对应的网卡:

# ip addr
...
19: veth504d7e6@if18: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master docker0 state UP group default
    link/ether 9a:e8:ea:58:9d:f1 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet6 fe80::98e8:eaff:fe58:9df1/64 scope link
       valid_lft forever preferred_lft forever

这个 19: 就是我们要找的 @if19 网卡信息，抓包的时候指定对应的网卡 veth504d7e6 ， 现在可以抓包了:

$ tcpdump -i veth504d7e6 port 80 -A
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on veth504d7e6, link-type EN10MB (Ethernet), capture size 262144 bytes
05:39:43.181244 IP 172.17.0.2.33430 > 39.156.69.79.http: Flags [S], seq 2619505737, win 29200, options [mss 1460,sackOK,TS val 812639981 ecr 0,nop,wscale 7], length 0
E..<Vj@.@..S....'.EO...P."|I......r..-.........
0o..........
05:39:43.270550 IP 39.156.69.79.http > 172.17.0.2.33430: Flags [S.], seq 820750215, ack 2619505738, win 8192, options [mss 1452,sackOK,nop,nop,nop,nop,nop,nop,nop,nop,nop,nop,nop,wscale 5], length 0
E..<Vj@.,..O'.EO.....P..0...."|J.. ..:......................
05:39:43.270608 IP 172.17.0.2.33430 > 39.156.69.79.http: Flags [.], ack 1, win 229, length 0
E..(Vk@.@..f....'.EO...P."|J0...P.......
05:39:43.270686 IP 172.17.0.2.33430 > 39.156.69.79.http: Flags [P.], seq 1:73, ack 1, win 229, length 72: HTTP: GET / HTTP/1.1
E..pVl@.@.......'.EO...P."|J0...P....a..GET / HTTP/1.1
Host: baidu.com
User-Agent: Wget
Connection: close

总结

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