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I have the dumbest problem that has me perplexed.

enter image description here

SRV1 is the FTP server I'm trying to connect too... from inside my work network (CPU1) SRV2 is (random ftp server that I found, just to verify my computer can FTP out). CPU1 is a couple computers inside my work network. CPU2 is a computer outside of my work network.

I can connect to SRV1&2 from outside the network (CPU2). I know that the servers, users and passwords work on both servers from CPU2. I can connect to SRV2 (ftp.test) from inside my work network (multiple computers, multiple public IPs), but I can't connect to SRV1 (ISP hosted ftp server).

Firewall shouldn't be an issue, I can FTP out to SRV2 (as well as other servers). DNS isn't an issue (everything resolves correctly: I can view website on the same server).

Where can I go to troubleshoot this further? It's not a huge issue (workarounds already in place), but I just can't stand an unanswered riddle.

share|improve this question
And you're sure that your ISP isn't blocking FTP requests that don't originate from their own customers' IP's? – Ian Atkin Nov 29 '12 at 18:46
@IanAtkin They have our public IP's added their input rules. SRV1 wouldn't accept CPU2 otherwise. Forgot to mention that. – WernerCD Nov 29 '12 at 21:16
Please report what happens when doing telnet 21. You should get a classic FTP header like 220---------- Welcome to Pure-FTPd [Srv1] [TLS] ----------. – Sopalajo de Arrierez Apr 21 '14 at 21:14
Please report what happens when scanning port 21TCP to SRV1 from online scanner sites, like this one: – Sopalajo de Arrierez Apr 21 '14 at 21:15
This is longer advice, but could be useful too: if allowed, install nMap (installation of nMap is not agressive for operating systems): . Then do nmap -p 21 for FTP port scanning to your FTP SRV1, and later nmap to get a complete TCP port scanning. Paste the results in your initial post. – Sopalajo de Arrierez Apr 21 '14 at 21:18

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