The university where I study right now ABS – (Arnhem Business School) does some outbound port filtering.
Therefore I couldn’t access my Gmail email through POP3 neither send my emails via the SMTP protocol.
In other words the port numbers 25, 110, 995 and 143 are filtered by the University System Administrator, or whoeverdid built the University network.
This I found pretty annoying because I’ve noticed the Teachers who work on their Desktops are able to access their email addressesvia a normal POP3 and SMTP protocols .
Of course there is likely a good reason that the university filters the traffic for students accessing internet via their notebooks through the Internal built University wireless network, however as I feel very convenient with checking my mail via Thunderbird (Icedove), it’s really, really irritating to go for a browser each and every time I’d like to check my Gmail.
In that reason an idea come to my mind to get through the SMTP, POP and IMAP protocol access restrictions.
The idea is not something brilliant or something too smart, however it prooved to work so I was quite happy with it.
My idea was to simply use my personal Linux router as a jumping off place to access gmail.
To do so first think I did was to scan my Linux router and check which ports are filtered from Arnhem Business School University firewall and which one are allowed to pass traffic.
After a while I have found out that the ports range from 2010 up to 2050 are freely allowing traffic to pass without any firewall restrictions.
Thus my next logical step I took was to configure my Linux router to pass by all incoming traffic on ports 2050 and 2060 to Gmail servers for POP3 email access pop.gmail.com and the other one responsible for sending emails via the SMTP protocol – smtp.gmail.com
In order to fulfill my desired task I first experimented a bit with some iptables nat PREROUTING redirect to destination rules.
However after many tries without success I finally decided to abandom this approach and try with another one.
I’ve remembered that some time ago I’ve used a tiny Linux software called rinetd that makes the port forwardking, redirections a piece of cake
RINETD is really straight forward to install and use on Debian Linux. To make the actual port redirects first you will have to install rinetd
1. Install rinetd
debian-desktop:~# apt-get install rinetd
2. Configure rinetd by editting /etc/rinetd.conf to make the redirects to gmail or any other pop3 mail server host
Open up the /etc/rinetd.conf with your favourite text editor and for a gmail redirect place the lines:
220.127.116.11 2010 18.104.22.168 995
22.214.171.124 2050 126.96.36.199 25
In the above configuration directives the first IP address 188.8.131.52 should be changed and adjusted with your actual Linux router external IP address.
The second option 2010 is the port number to accept connections from your host to be redirected to the IP 184.108.40.206 , the last argument is the port number to where you desire to redirect 995.
In the above example I’ve used the IP 220.127.116.11 which actually a gmail server IP address I have obtained through resolving smtp.gmail.com and pop.gmail.com
In order to conduct the resolve I issued the commands:
3. Resolve pop.gmail.com and smtp.gmail.com to find out their actual IP addresses, which you will have to use in your port redirect
hipo@debian-desktop:~$ host pop.gmail.com
pop.gmail.com is an alias for gmail-pop.l.google.com.
gmail-pop.l.google.com has address 18.104.22.168
hipo@debian-desktop:~$ host smtp.gmail.com
smtp.gmail.com is an alias for gmail-smtp-msa.l.google.com.
gmail-smtp-msa.l.google.com has address 22.214.171.124
All left to be done to have the port redirections active is to run up the rinetd service, to do so issue:
debian-desktop:~# /etc/init.d/rinetd start
That’s all, now go to your Thunderbird, Outlook or any POP3 email client of choice and just change the mail server hosts and ports with your Linux router IP address and ports where you just binded the redirect.Of course the Linux router IP could also be used, if you don’t have a hostname associated with it.