Port Blockade

透過網路傳染的病毒種類與數量愈來愈多
也讓各大企業公司開始封鎖許多容易遭到攻擊的 port
甚至為了防止私人郵件傳遞時造成的病毒感染
連 SMTP (port 25) 和 POP3 (port 110) 都加以封鎖
只准員工利用公司的 Webmail 系統來收發信
但 Webmail 在某些情況下其實並不方便
因此當上有政策的時候, 自然下有對策
解決的方法常見的有

1. 使用 VPN(Virtual Private Network, 私人虛擬網路)

這個方法幾乎可以解決所有 port 被擋的問題
比較麻煩的是 VPN 的 server / client 端都需要做一些煩雜的設定
特別是安全認證的部份

2. 更改各個 Service 的 Listen Port, 例如跳過 25 / 110 這些被封鎖的 Port

這是治標不治本的方法
但可以很簡單很快速的設定完成
在這邊就以 FreeBSD 中我所使用的 postfix 和 qpopper 兩個 SMTP / POP3 軟體來說明

+ Postfix 2.3.20050726

1.修改 /etc/services 增加下列兩項
smtp-2 5000/tcp
smtp-2 5000/udp

2.修改 /usr/local/etc/postfix/master.cf
在 smtp 後面增加 smtp-2 如下
smtp inet n – n – – smtpd
smtp-2 inet n – n – – smtpd

重新啟動 postfix 後, SMTP 便會同時監聽 25 和 5000 兩個 port
這時將 outlook 等寄信軟體的 smtp port 更改至 5000 即可閃過被封鎖的 port 25

+ qpopper-4.0.8

1.修改 /etc/services 增加下列兩項
pop3-2 5001/tcp
pop3-2 5001/udp

2. 修改 /etc/inetd.conf (我是使用 inetd 去啟動 qpopper)
在 pop3 後面增加 pop3-2 如下
pop3 stream tcp nowait root /usr/local/libexec/qpopper qpopper -s
pop3-2 stream tcp nowait root /usr/local/libexec/qpopper qpopper -s

重新啟動 inetd 後, POP3 便會同時監聽 110 和 5001 兩個 port
這時將 outlook 等收信軟體的 pop3 port 更改至 5001 即可閃過被封鎖的 port 110

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