“Connection refused” vs “No route to host”，refusedroute

“Connection refused” vs “No route to host”，refusedroute

曾經在http://blog.csdn.net/bisal/article/details/42496583這篇博文中提到一個連接埠串連的驗證：

“ora10g@localhost.localdomain$telnet 172.101.19.57 1521
Trying 172.101.19.57...
telnet: connect to address 172.101.19.57: No route to host
如果連接埠未開，實際報錯：
ora10g@localhost.localdomain$telnet 172.27.19.56 1521
Trying 172.27.19.56...
telnet: connect to address 172.27.19.56: Connection refused
是不是防火牆的問題？？？
從資料庫伺服器關閉防火牆：
[root@dcsopen2Node ~]# service iptables stop
iptables: Flushing firewall rules: [  OK  ]
iptables: Setting chains to policy ACCEPT: nat mangle filter [  OK  ]
iptables: Unloading modules: [  OK  ]
再從遠程機器執行：
ora10g@localhost.localdomain$telnet 172.101.19.571521
Trying 172.101.19.57...
Connected to 172.101.19.57.
Escape character is '^]'.
說明連接埠已開，更重要的是，明確了，就是防火牆問題。”

通過實驗再次說明下“Connection refused”和“No route to host”的區別和問題診斷思路。

首先，網上有篇文章說明“Connection refused” vs “No route to host”(http://superuser.com/questions/720851/connection-refused-vs-no-route-to-host)：

"Connection refused" means that the target machine actively rejected the connection. With port 80 as the context, one of the following things is likely the reason:
    Nothing is listening on 127.0.0.1:80 and 132.70.6.157:80
    Nothing is listening on *:80
    The firewall is blocking the connection with REJECT
So check your Apache and iptables config.
"No route to host" refers to a network problem. It is not a reply from the target machine.
說的是“Connection refused”是目標主機明確拒絕了這次串連，有可能是該連接埠沒有啟動監聽，或者因為防火牆。“No route to host”則可能是一個網路問題，不是目標主機的回複。

一個實驗類比：

用戶端機器ip：172.1.1.1

目標機ip：172.1.2.1

1. 從用戶端telnet目標機的一個已啟動連接埠1521，但防火牆中未添加例外。

從172.1.1.1 telnet 172.1.2.1 1521，提示no route to host。

Trying 172.1.2.1...

telnet: connect to address 172.1.2.1: No route to host

在防火牆配置中iptables添加1521連接埠，telnet正常。

2. 從用戶端telnet目標機的一個未啟動監聽的連接埠

172.1.2.1的10001連接埠沒有啟用，netstat -an | grep 10001不存在。

從172.1.1.1 telnet 172.1.2.1 10001，提示connection refused。

Trying 172.1.2.1...

telnet: connect to address 172.1.2.1: Connection refused

結論：說明No route to host是防火牆的返回，先經過防火牆，不管連接埠有沒有。然後如果通過了防火牆，但監聽未啟動，則提示Connection refused的錯誤。