总之，尽管我设置了允许tcp：80的防火墙规则，但是位于“默认”网络上的GCE实例不接受与端口80的连接。看来我的实例上仅打开了端口22。我可以对其进行ping操作，但在64跳以下无法跟踪到它。

接下来的是我的调查，这些调查使我得出了这些结论。

gcloud beta compute firewall-rules list

NAME                    NETWORK  DIRECTION  PRIORITY  ALLOW                         DENY
default-allow-http      default  INGRESS    1000      tcp:80
default-allow-https     default  INGRESS    1000      tcp:443
default-allow-icmp      default  INGRESS    65534     icmp
default-allow-internal  default  INGRESS    65534     tcp:0-65535,udp:0-65535,icmp
default-allow-rdp       default  INGRESS    65534     tcp:3389
default-allow-ssh       default  INGRESS    65534     tcp:22
temp                    default  INGRESS    1000      tcp:8888


gcloud compute instances list
NAME   ZONE        MACHINE_TYPE  PREEMPTIBLE  INTERNAL_IP  EXTERNAL_IP    STATUS
ssrf3  us-west1-c  f1-micro      true         10.138.0.4   35.197.33.182  RUNNING


gcloud compute instances describe ssrf3
...
name: ssrf3
networkInterfaces:
- accessConfigs:
  - kind: compute#accessConfig
    name: external-nat
    natIP: 35.197.33.182
    type: ONE_TO_ONE_NAT
  kind: compute#networkInterface
  name: nic0
  network: https://www.googleapis.com/compute/v1/projects/hack-170416/global/networks/default
  networkIP: 10.138.0.4
  subnetwork: https://www.googleapis.com/compute/v1/projects/hack-170416/regions/us-west1/subnetworks/default
...
tags:
  fingerprint: 6smc4R4d39I=
  items:
  - http-server
  - https-server

我将ssh放入35.197.33.182（这是ssrf3实例）并运行：

sudo nc -l -vv -p 80

在本地计算机上，我运行：

nc 35.197.33.182 80 -vv
hey

但什么也没发生。因此，我尝试对主机执行ping操作。看起来很健康：

ping 35.197.33.182 
PING 35.197.33.182 (35.197.33.182): 56 data bytes
64 bytes from 35.197.33.182: icmp_seq=0 ttl=57 time=69.172 ms
64 bytes from 35.197.33.182: icmp_seq=1 ttl=57 time=21.509 ms

Traceroute经过64跳后退出，但未到达35.197.33.182目标。

因此，我检查了使用nmap打开的端口：

nmap 35.197.33.182

Starting Nmap 7.12 ( https://nmap.org ) at 2017-06-18 16:39 PDT
Note: Host seems down. If it is really up, but blocking our ping probes, try -Pn
Nmap done: 1 IP address (0 hosts up) scanned in 3.06 seconds



nmap 35.197.33.182 -Pn

Starting Nmap 7.12 ( https://nmap.org ) at 2017-06-18 16:39 PDT
Nmap scan report for 182.33.197.35.bc.googleusercontent.com (35.197.33.182)
Host is up (0.022s latency).
Not shown: 999 filtered ports
PORT   STATE SERVICE
22/tcp open  ssh

Nmap done: 1 IP address (1 host up) scanned in 6.84 seconds

…即使我nc -l -p 80以35.197.33.182运行。