I need to find the port number of a server, I have the host name and the IP address.
Is this possible?
I need this as when I try to connect to this server through putty its throwing a Network error:Connection refused error, which may be because of the wrong port number
So you are looking for the port number the ssh server on that system listens on. Usually that is port 22 (well known ssh port), but you are right, this can be changed in the ssh server configuration. If so there are two possibilities yo have:
ask the administrator of the ssh server for the port number
make a network scan of the server which shows up all open ports. Note however that this can be regarded as offensive behavior and may be blocked in mid way.
But most likely you are facing another problem: some firewall blocking your requests or the ssh server not listening to request from outside at all.
And a side note: a server is a service, often listening on a port, you can interact with it typically by "speaking" a specific protocol. A system might refer to a computer running software, typically reachable via network these days. Many servers can be operated on a system. A system can be identified by its ip address. Many people confuse this and speak of a "server" when referring to such a "system" which is simply wrong and creates confusion from a technical point of view.
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I have been trying to create a server in one network but the people in another network cant connect to my server? Even though my IP is dynamic (dhcp :yes)
I want to create a server in one network but want to connect people to that server present in another network. I started a server "eg: 103.251.9.85:27015"
even though my IP is dynamic, they can't connect to my server? Help me ...
who ever knows my IP address and port number will connect to my server, but when I am starting a server they cant connect.
Your server needs a DNS record.
DHCP is great for allocating an IP address - but you then have to manually tell everyone what the IP Address is.
You should define your Server in your DHCP configuration, assuming that there is a DNS Server also.
By default the DHCPD informs the DNS Server - assuming they are both under your control.
First ensure that you have network connectivity between the nodes - there can be NATs, Firewalls and a lot of different reasons why you can not connect. My advice for you would be to try and use netcat.
Once you have netcat on your computer - start it in a server mode. Then ask your friend to download netcat on his/her computer and connect to your IP address. If you can establish connection - great. Then make a question regarding your server program. If netcat fails - then there is network connectivity issue and you will find better help for those issues in the power user or network engineering Stack Exchange sites.
I've been investigating networking for use in a two-player game I'm writing, and I'm still not clear on when a device must have a port forwarded in order to communicate with the outside world.
From what I've seen in other games, port forwarding is always required in order to host a server, but is not required on the client. In addition, there are other situations, such as skype (which, to my understanding is ultimately client to client), where neither end must forward a port.
So my question is, in over-the-Internet communication, when is and isn't port forwarding necessary, and what steps can i take as a developer to make it so my users don't have to worry about it? Thanks in advance!
Port forwarding is needed when a machine on the Internet needs to initiate a connection to a machine that's behind a firewall or NAT router. If the connection is initiated by the machine behind the firewall, the firewall/router automatically recognizes the reply traffic and sends it to the machine that opened the connection.
But if a packet arrives on the external interface, and it's not a part of such a connection, the router needs to know what to do with it. By default, it will reject it. But if forwarding is configured for the port, that tells it what internal machine to send it to.
Put another way: you need port forwarding if you want to run a server behind the NAT firewall/router, you don't need it if you're just running a client.
There is reason why Skype don't (not always) need manual setting of port forwarding:
When you install Skype, a port above 1024 is chosen at random as the
port for incoming connections. You can configure Skype to use a
different port for incoming connections if you wish, but if you do,
you must open the alternative port manually.
If the port chosen for incoming connections becomes unavailable, by
default ports 80 and 443 will be used as alternatives. If another
application (such as Apache HTTP server or IIS) uses these ports, you
can either configure the application to use other ports, or you can
configure Skype to not use these ports.
Port forwarding is must if you host a server.
You can use same technique as Skype...
I am not sure if there is any other option...
Port forwarding (occurs) when a NAT, firewall or some other device blocks communication on all or some ports.
To answer your question as an example, most commercial routers use NAT to allow multiple people to use the same IP(As view from the outside world) provided by ISPs. Most ISP's use NAT to allow multiple customers to use the same IP(As viewed from the outside world). To get this to work, the NAT changes the internal IP and the port number of a communication to THE(there is only one for the entire sub network) external IP and a new port number. By doing this, the router/isp/ect can tell which internal IP and port each external communication goes to.
Anytime one of the computers communicating over the internet are behind a NAT, port forwarding is required. I'm sure there are way more situations than this, and the solution to each can be quite complicated. But this covers the vast majority.
everyone! I have a list of http proxy servers, some of then may stop running http proxy process. Of course, there are two cases: the port is not listened any more; and the port may be listened for another service. I have to write a program to judge if a given server is still running http proxy. How can I do that? If I can connect to a port, how can I judge whether it provides http proxy service or not? Is there protocol relative thing I can employ?
Most proxy servers use port numbers like 8080, which are not assigned to any other services. In those cases, if you can connect, it is usually safe to assume it is still a proxy.
In the case that the proxy server uses a standard port like 80, you may want to make a single request to a known host on the internet. That way you can know that it is still forwarding requests to the outside.
If you're trying to see what services are available on any given server or port, something like nmap may be useful. nmap can usually identify the type of service running on any given port.
IMPORTANT: Running a full port scan on a remote host is (almost always) illegal unless you have written permission from the owner of that host. Sometimes it is illegal even if you have written permission.
Scanning one targeted port using nmap is probably okay.
I don't really understand the differences between the different ports. I couldn't find any useful resources online that would go into detail about it. From what I can understand, it acts as some kind of filter that blocks out any other request not using the same port number.
When will I, if ever, need to change the port number when typing in a website in my browser? What about if I am running my own website? Are there any risks I take when using port 80? As in, are there ways to use different port numbers to hack into a website?
And since a Simple Mail Transfer Protocol uses port 25, could I use this knowledge to send emails using self written programs?
By conventions and standards, defined protocols generally listen on defined ports by default. You can set any service to listen on any port you want, just be aware that changing from a standard port means that standard tools would need to be explicitly told to use your non-standard port.
For example, if you run a web server then by default it's probably listening on port 80. So all someone needs to do to visit your server is use the address:
http://www.yourserver.com
However, if you change it to listen on a non-standard port, such as 81, then any user who wants to visit your site will need to specify the port number:
http://www.yourserver.com:81
The standard convention of just using the address wouldn't be sufficient, because you've broken convention. Now, this is fine if you don't want people randomly using your site and only want it to be available to people to whom you've told the port number. This is called "security through obscurity." It doesn't actually secure your site in any way or filter out anything, it just adds an additional step to the use of the site.
As for SMTP, yes, you can write programs to connect to port 25 on SMTP services and send data to those services. You may indeed be able to spoof emails in this manner. However, most services have other built-in checks and balances to prevent such spoofing.
TCP/IP supports 2^16 ports on a machine. A server program is said to listen on a specific port; other machines contact a server program at a machine using the port number and the name of the machine. It's like the address on a letter.
Some port numbers are "well known", which just means they're officially assigned for a particular service: port 80 is for web servers, port 25 for SMTP, port 25 for telnet, etc.
But any server can run on any port; it's just a matter of changing the code or the configuration. Sometimes you may way to run two separate web servers on one machine; one might listen on port 80, and the second one on some other port (8080 is a common choice for servers used during development.)
So the client can't meaningfully change the port to some random number: there has to be something listening on that port, on that machine, or it doesn't do anything.
There is no 'difference' between running a web server on any port number, from a purely technical perspective, as long as the client and the server both know which port to use. When the client connects to the server, it has to know the IP address as well as the port.
By convention, several ports are used by standard services. For example, port 25 is SMTP, port 80 is the http port, 22 is the ssh port, etc. Because of these conventions, ports less than 1024 are reserved by the operating systems and can only be opened by a root process.
Unless there is a specfiic reason, it is generally a good idea to stick with the standard port numbers. Advanced port scanners/ analysis tools will try all ports on a system, so there isn't much benefit to running a service on a non-standard port.
The port numbers are a way of multiplexing communications over IP links. This is commonly used to provide specific services on accepted port numbers. HTTP servers have been allocated port 80 for listening for incoming client connections, though this is not a lock - there is nothing stopping you constructing/configuring an HTTP server that listens on port 9001, or whatever. If you did use a non-standard port number for your server, then you would have to specifically instruct a browser to connect to that port, rather than defaulting to 80 as it normally does.
SMTP in port 25 using your own program - sure, you can do this. Any mail app is some developers' 'own program' .
Rgds,
Martin
In short: How to reliably discover a server running somewhere on a (presumably multi-segmented) local area network with zero client configuration
My client application has to locate the server application without knowing the server IP address. It has to work on a local LAN that may be split into segments with hubs or other switching devices.
I already have a working solution, but it is a bit cumbersome to get it working on multi-segment networks. It works as follows:
When the client starts up, it sends UDP broadcasts on its own network segment. If the server is running on the same segment, it works without any issues - the server responds with the appropriate messages.
If the server and client are running on networks separated by a hub / switch that won't forward UDP (the most likely case), then I have a server instance running on each segment, and they forward client requests to each other via TCP - but I need to configure this for the server instances (simple, but still a pain for tech support.) This is the main problem that I need to address. There are sites where we have hundreds of clients running on 5 or 6 separate segments.
The problems I'm facing:
1. Although my application installer enables the appropriate ports on the firewall, sometimes I come across situations where this doesn't seem to happen correctly.
2. Having to run multiple server instances (and therefore configure and maintain them) on hub/switched networks that won't forward UDP
Finally I need a solution that will work without maintenance on a minimal Windows network (XP / 2000 / Vista) that probably doesn't have Active Directory or other lookup services configured.
I don't want to tag on any runtime stuff for this - should be able to do it with plain VC++ or Delphi.
What approaches do commercial apps usually take? I know that SQL Server uses a combination of broadcast and NetBEUI calls (I may be wrong about this).
Thanks in advance.
You have a few terminology issues:
Where you say "network segment" you appear to mean "IP subnet". Devices on the same network segment can see the same IP broadcasts.
Where you say "hub/switch" you appear mean "IP router".
Where you say "won't forward UDP", the problem is actually "won't forward IP broadcasts".
Once we get past that, you have a few options:
Your servers could register themselves under a well-known name in DNS, if you have a DNS server that allows dynamic DNS updates. You should probably use a SRV record as specified in RFC2782. The clients then do a DNS lookup to find the server(s).
You could statically assign your server(s) well-known names in the organisation's DNS, perhaps with a SRV record as with the previous option.
Your servers could join an IP multicast group, if your routers support IP multicast. The clients then send their initial discovery request as a UDP packet to the (pre-ordained) multicast address.
If you have domain server, I would go with small service on it. You can connect with other services to it and use it as distribution point.
Why domain server? It is relatively easy to find it's name (DsGetDcName).
Other choices would include DHCP server, DNS server or something of that kind that needs to be filled by maintenance staff anyhow.