MobileRead Forums - View Single Post - Calibre-Server and Port Forwarding Issues

davidfor · 01-28-2016, 08:58 PM

"LISTEN" means that an application on the PC is waiting for something to come in on the port. It doesn't mean anything can reach it. "ESTABLISHED" means that something has reached that port.

I'm not sure on the rest. But, some of them work by the local application initiating the connection. They can hold it open or check periodically. They might also negotiate another port to use. The ISP is usually blocking incoming ports, not outgoing, so applications that work like this will be OK.

It's also common for ISPs to block some ports, but not others. Blocking port 25 is common to help prevent spam. My ISP blocks 25 outgoing, 135, 137, 138, 139, 445 both ways and 80, 443, 22, 23, 8080, 3128 incoming. But, I can also turn that off. Another policy I have seen is to block all low numbered ports, but allow high numbers.

Something you can try is to telnet to the ports. Or run a traceroute to the port from outside. Using telnet is handy as is the simplest way of contacting the port. The traceroute can help see where the block is. Unless you know the network well, it will still be a guess, but the domain names of the devices can give a good idea of where the block happens.

And of course, are you sure the where ever you are testing from isn't blocking the port? I have access to several networks at work. They are all restricted, but one of them is almost completely closed to the outside world.

01-28-2016, 08:58 PM	#21
davidfor Grand Sorcerer Posts: 24,905 Karma: 47303824 Join Date: Jul 2011 Location: Sydney, Australia Device: Kobo:Touch,Glo, AuraH2O, GloHD,AuraONE, ClaraHD, Libra H2O; tolinoepos	"LISTEN" means that an application on the PC is waiting for something to come in on the port. It doesn't mean anything can reach it. "ESTABLISHED" means that something has reached that port. I'm not sure on the rest. But, some of them work by the local application initiating the connection. They can hold it open or check periodically. They might also negotiate another port to use. The ISP is usually blocking incoming ports, not outgoing, so applications that work like this will be OK. It's also common for ISPs to block some ports, but not others. Blocking port 25 is common to help prevent spam. My ISP blocks 25 outgoing, 135, 137, 138, 139, 445 both ways and 80, 443, 22, 23, 8080, 3128 incoming. But, I can also turn that off. Another policy I have seen is to block all low numbered ports, but allow high numbers. Something you can try is to telnet to the ports. Or run a traceroute to the port from outside. Using telnet is handy as is the simplest way of contacting the port. The traceroute can help see where the block is. Unless you know the network well, it will still be a guess, but the domain names of the devices can give a good idea of where the block happens. And of course, are you sure the where ever you are testing from isn't blocking the port? I have access to several networks at work. They are all restricted, but one of them is almost completely closed to the outside world.