BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Finding a server on LAN Goto page 1, 2  Next          BorlandTalk.com Forum Index -> Delphi Internet Winsock View previous topic :: View next topic   Author Message Mike Warren Guest Posted: Thu Dec 30, 2004 10:23 pm    Post subject: Finding a server on LAN Hi, I intend to use TidTCPClient and TidTCPServer for communication between a server and several clients. Is there a way for my clients to find my server on a LAN when the clients don't know the IP Address or computer name where the server is? This could be used on a 10.x.x.x network so there are too many addresses to check each one. -Mike Back to top Eugene Mayevski Guest Posted: Thu Dec 30, 2004 10:35 pm    Post subject: Re: Finding a server on LAN Hello! You wrote on Fri, 31 Dec 2004 08:23:53 +1000: MW> Is there a way for my clients to find my server on a LAN when the MW> clients don't know the IP Address or computer name where the MW> server is? This could be used on a 10.x.x.x network so there are MW> too many addresses to check each one. Use UDP broadcast -- the client should send a packet which asks the server to report it's existence to the client. Also there can be some non-TCP methods like creating a predefined name (if the server is planned to be the only one in the LAN) or use active directory or something similat. With best regards, Eugene Mayevski Back to top Remy Lebeau (TeamB) Guest Posted: Thu Dec 30, 2004 11:09 pm    Post subject: Re: Finding a server on LAN "Eugene Mayevski" <mayevski (AT) eldos (DOT) org> wrote Quote: Use UDP broadcast -- the client should send a packet which asks the server to report it's existence to the client. The thing to watch out for is that UDP does not usually pass through routers, so if the network contains multiple subnets, the client on one subnet may not be able to find a server on another subnet even though TCP will be able to connect them fine. Gambit Back to top Mike Warren Guest Posted: Thu Dec 30, 2004 11:10 pm    Post subject: Re: Finding a server on LAN Hi Eugene, Quote: Is there a way for my clients to find my server on a LAN when the clients don't know the IP Address or computer name where the server is? Use UDP broadcast -- the client should send a packet which asks the server to report it's existence to the client. Thanks for the reply. I won't know anything about the network it is going to be on. Do you know of an example or more information to start me on learning how to use UDP broadcast. Regards -Mike Back to top Mike Warren Guest Posted: Thu Dec 30, 2004 11:54 pm    Post subject: Re: Finding a server on LAN Hi, Quote: Use UDP broadcast -- the client should send a packet which asks the server to report it's existence to the client. The thing to watch out for is that UDP does not usually pass through routers, so if the network contains multiple subnets, the client on one subnet may not be able to find a server on another subnet even though TCP will be able to connect them fine. The clients are setup by DHCP and have no user access so I can't do anything from the client end. The server will be running on a standard desktop PC. Maybe I should use a different method. Would it be better if my "client" machines were all servers and the "server" machine was actually accessing multiple servers from the one client program? Is that even possible? -Mike Back to top Eugene Mayevski Guest Posted: Fri Dec 31, 2004 12:26 am    Post subject: Re: Finding a server on LAN Hello! You wrote on Fri, 31 Dec 2004 09:10:21 +1000: MW> Do you know of an example or more information to start me on MW> learning how to use UDP broadcast. UDP broadcast is a UDP packet sent to 255.255.255.255. One thing to remember is that, as Remy said, the packets can be kiiled by some routers. So the best thing to do is for the server to register it's presence in some database (be it AD, DNS or other type of namespace system) which has predefined address. BTW why not let the users specify the name of the computer, on which the server is started? With best regards, Eugene Mayevski Back to top Remy Lebeau (TeamB) Guest Posted: Fri Dec 31, 2004 12:35 am    Post subject: Re: Finding a server on LAN "Mike Warren" <miwaNOSPAM (AT) iprimus (DOT) com.au> wrote Quote: The clients are setup by DHCP That does not matter. That only effects the client's locally-assigned IP. That does not effect the client's ability to connect to other machines on the same network, or on the Internet if the DHCP server also has routing capabilities. Quote: and have no user access so I can't do anything from the client end. Yes, you can. The UDP approach mentioned earlier is initiated from the client end. A client can send out a single UDP packet over the entire subnet. If your server is on the same subnet (or at least is reachable if the network routers allow UDP broadcasts to pass through), the server can receive the message and send its current IP address back to the client that sent the broadcast. The client can then receive that message and connect to the specified IP accordingly. Quote: The server will be running on a standard desktop PC. Does it have static IP, or at least a DNS-reachable hostname? This is exactly the kind of scenerio that DNS was designed for. If your network has a DNS server available, you can register a static hostname on it for the server PC, and the clients can then do DNS queries to find out what the current IP is. Quote: Would it be better if my "client" machines were all servers and the "server" machine was actually accessing multiple servers from the one client program? That approach requires the server to be explicitally configured to know the IP of every client that wants to use the server. A much different architecture then having all of your clients connect to the server instead. Gambit Back to top Remy Lebeau (TeamB) Guest Posted: Fri Dec 31, 2004 12:36 am    Post subject: Re: Finding a server on LAN "Mike Warren" <miwaNOSPAM (AT) iprimus (DOT) com.au> wrote Quote: I won't know anything about the network it is going to be on. Doesn't matter. As long as the client and server are on the same network, and there are no routers in between blocking the packets, then it will usually work fine. Gambit Back to top Remy Lebeau (TeamB) Guest Posted: Fri Dec 31, 2004 12:55 am    Post subject: Re: Finding a server on LAN "Eugene Mayevski" <mayevski (AT) eldos (DOT) org> wrote Quote: BTW why not let the users specify the name of the computer, on which the server is started? I agree. If a user wants to connect the client to a server, the user should specify where the server is located. This is much more flexible and accurate than using UDP, which is not a reliable transport in the first place. Gambit Back to top Mike Warren Guest Posted: Fri Dec 31, 2004 1:03 am    Post subject: Re: Finding a server on LAN Quote: BTW why not let the users specify the name of the computer, on which the server is started? I agree. If a user wants to connect the client to a server, the user should specify where the server is located. This is much more flexible and accurate than using UDP, which is not a reliable transport in the first place. That's my problem. The "client" machines are just boxes with no user interface running WinXP. They have a random computer name and dynamic IP address. What I need is to be able to access several of these machines from one desktop computer in an existing network to transfer data to and from them. Perhaps there is a better way of doing this? -Mike Back to top Remy Lebeau (TeamB) Guest Posted: Fri Dec 31, 2004 1:12 am    Post subject: Re: Finding a server on LAN "Eugene Mayevski" <mayevski (AT) eldos (DOT) org> wrote Quote: UDP broadcast is a UDP packet sent to 255.255.255.255. That depends. "255.255.255.255" is not always available or allowed (multiple NICs present, admin configurations, etc). In those cases, you have to calculate the broadcast address dynamically in order for the packet to be routed correctly. You would use the sender's own IP and subnet mask to calculate the target broadcast address, using the following algorithm: BroadcastIP := (SenderIP and SubnetMask) or (not SubnetMask); So, for example, on my home network I have an IP of 192.168.1.101 and a subnet of 255.255.255.0, so my broadcast address would be 192.168.1.255. Since I have multiple NICs installed, the packet can be routed to the appropriate network. If you use 255.255.255.255 with multiple NICs present, there are three possible outcomes: 1) the packet may be sent to all connected networks, creating more network traffic then needed 2) the packet may be sent to none of the connected networks, failing the broadcast 3) the packet may go to the wrong network since the OS does not know which NIC is supposed to be used to send the packet, so it makes a (somewhat educated) guess as to which NIC to use. Gambit Back to top Remy Lebeau (TeamB) Guest Posted: Fri Dec 31, 2004 1:20 am    Post subject: Re: Finding a server on LAN "Mike Warren" <miwaNOSPAM (AT) iprimus (DOT) com.au> wrote Quote: That's my problem. The "client" machines are just boxes with no user interface running WinXP. They have a random computer name and dynamic IP address. Why do they have random names? DHCP clients can have static computer names assigned to them. You can then use DNS to find their assigned IP addresses. Quote: What I need is to be able to access several of these machines from one desktop computer in an existing network to transfer data to and from them. Your design is backwards then. Each of the XP machines should be running server sockets, not client sockets, and the desktop PC should have a separate client socket for each XP machine that it wants to access. That way, you just specify to the desktop application which computer names you want to access, and it connects to each one accordingly. If static computer names are not available (a very bad idea, but so be it), you can still use the UDP approach that has already been described to you. Have each XP machine running a UDP server on a specific port number. The desktop application can send a UDP broadcasted packet to the network on that port number. When each XP machine receives the packet, it can report back its currently assigned IP address. The desktop application can then build up a list of the available XP machines that it can connect to based on the responses that it receives. Gambit Back to top Ben Hochstrasser Guest Posted: Fri Dec 31, 2004 1:21 am    Post subject: Re: Finding a server on LAN Mike Warren wrote: Quote: That's my problem. The "client" machines are just boxes with no user interface running WinXP. They have a random computer name and dynamic IP address. What I need is to be able to access several of these machines from one desktop computer in an existing network to transfer data to and from them. Perhaps there is a better way of doing this? As these computers have DHCP they probably know about a name server. All you have to do is to create an alias (CNAME) for your own server and have (via a service) the clients contacting the server behind that name and register themselves (open a connection to a specific port where they wait for instructions, for example). If you haven't got any DNS, use some rarely-used DHCP option (eg "impress server") and let the clients contact that one. These things require some stuff to be installed onto these clients in the first place. If that is not an option, scan the subnet, remotely open registries and look for distinctive keys/values. -- Ben Back to top Mike Warren Guest Posted: Fri Dec 31, 2004 1:40 am    Post subject: Re: Finding a server on LAN Quote: That's my problem. The "client" machines are just boxes with no user interface running WinXP. They have a random computer name and dynamic IP address. Why do they have random names? DHCP clients can have static computer names assigned to them. You can then use DNS to find their assigned IP addresses. The names are random when the machines are built but don't change each time they boot. I can probably supply a list of computer names that match each machine. Quote: What I need is to be able to access several of these machines from one desktop computer in an existing network to transfer data to and from them. Your design is backwards then. Each of the XP machines should be running server sockets, not client sockets, and the desktop PC should have a separate client socket for each XP machine that it wants to access. I thought that might be the case. I'll work in that direction. Since I don't know in advance how many machines may be available, should I create TidTCPClient components dynamically? -Mike Back to top Remy Lebeau (TeamB) Guest Posted: Fri Dec 31, 2004 2:15 am    Post subject: Re: Finding a server on LAN "Mike Warren" <miwaNOSPAM (AT) iprimus (DOT) com.au> wrote Quote: The names are random when the machines are built but don't change each time they boot. Then they are, in fact, static names. You can specify those names when your desktop application wants to connect to them. Then you do not need to know the IP addresses at all. The OS will handle the IP lookup internally for you. Quote: Since I don't know in advance how many machines may be available, should I create TidTCPClient components dynamically? Assuming that you want to connect to multiple machines at one time, then yes. If you are connecting to them one at a time, however, then you can re-use a single TIdTCPClient instance. 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