auth.log 中的“sshd: error: connect_to ... failed”是什么意思？

Question

我注意到/var/log/auth.log服务器上多次重复出现以下错误消息：

Aug 10 09:10:16 hostname sshd[661]: error: connect_to 1.1.1.1 port
25: failed.

我已经更改了实际的 IP 地址，它们是通常属于邮件服务器的外部地址。

我不明白的部分是谁试图连接到这些地址以及 sshd 与它有什么关系。sshd 在端口 22 上运行，该服务器上的端口 25 上没有运行任何内容。

这行到底是什么意思，谁在发起连接，为什么涉及 sshd？

Answer 1

您可以通过设置 SSH 动态端口转发来重现这一点：

man ssh：

 -D [bind_address:]port
         Specifies a local “dynamic” application-level port forwarding.  This works by allocating a socket
         to listen to port on the local side, optionally bound to the specified bind_address.  Whenever a
         connection is made to this port, the connection is forwarded over the secure channel, and the
         application protocol is then used to determine where to connect to from the remote machine.  Cur?
         rently the SOCKS4 and SOCKS5 protocols are supported, and ssh will act as a SOCKS server.  Only
         root can forward privileged ports.  Dynamic port forwardings can also be specified in the configu?
         ration file.

         IPv6 addresses can be specified by enclosing the address in square brackets.  Only the superuser
         can forward privileged ports.  By default, the local port is bound in accordance with the
         GatewayPorts setting.  However, an explicit bind_address may be used to bind the connection to a
         specific address.  The bind_address of “localhost” indicates that the listening port be bound for
         local use only, while an empty address or ‘*’ indicates that the port should be available from all
         interfaces.

在本地主机，端口 2302 上启动 SOCKS 代理：

$ ssh -v -ND 2302 user@host

要通过此隧道路由 HTTP 流量，请在 Firefox 中：

编辑 -> 首选项 -> 高级 -> 网络选项卡 -> 设置 -> 手动代理配置 -> SOCKS 主机：本地主机和端口：2302

为了将 SOCKS 代理与其他流量一起使用，您可以使用像这样的socksifier 程序tsocks：

[I] net-proxy/tsocks
     Available versions:  1.8_beta5-r3 ~1.8_beta5-r4 1.8_beta5-r5 ~1.8_beta5-r6 {tordns}
     Installed versions:  1.8_beta5-r5(10:08:28 AM 06/15/2010)(-tordns)
     Homepage:            http://tsocks.sourceforge.net/
     Description:         Transparent SOCKS v4 proxying library

在我的 Gentoo 上，编辑/etc/socks/tsocks.conf如下：

# Otherwise we use the server
server = 127.0.0.1
server_port = 2302

测试：

$ tsocks telnet 255.255.255.255 25

您将/var/log/secure在 SSH 服务器上看到类似的内容：

sshd[28491]: error: connect_to 255.255.255.255 port 25: failed.

我不明白的部分是谁试图连接到这些地址

要缩小范围，请查看/var/log/secure(auth.log在您的发行版上)，并检查在此之前登录的人：

sshd[26898]: pam_unix(sshd:session): session opened for user quanta

Answer 2

也许登录到您服务器的人正在尝试建立到 1.1.1.1:25 的 ssh 隧道？