How can I get all the IP addresses and associated host names in a LAN?
To get the list of interfaces and IP addresses, use getifaddrs().
Search for interfaces with ifa_addr->sa_family == AF_INET
The IP address is in sin_addr.s_addr.
You can then use gethostbyaddr() to look up the DNS name for that IP address.
Update:
It was pointed out to me that the OP was probably asking about discovering other hosts, rather than the addresses of interfaces on the local machine.
There is no reliable way to discover other machines on the local area network, but there are a couple of tricks.
Ping method: Use the ping utility (or a programatic equivalent) to ping the local broadcast address, then see who responds. The broadcast address can be found by listing the interfaces as shown above. I believe ICMP does not require root access under OSX. Note that many systems may have ICMP ping disabled or firewalled, so you will only get responses from the non-stealth ones.
ARP method: Check the system ARP cache to see what IP addresses have been recently active. This will only show systems which have broadcast packets on the same network segment in recent minutes.
Both methods can be blocked by firewalls, routers, and even switches, so the exact borders of the "LAN" can be pretty narrow. Both methods can be implemented programmatically, but it might be simpler and more portable to just call out to the command line ping or arp commands.
Related
I'm writing a macOS C++ application using Qt that acts as both a UDP client and UDP server. The server functionality allows the user to configure which port the UDP packets will be received on, and the client functionality allows specifying both a target host and a port number. The host can be either a hostname or an IP address, including addresses or hostnames that resolve to a multicast or broadcast address.
In order to help prevent circularity, when the server functionality is enabled I need to be able to warn the user when the host and port they've entered for the client would send the packets directly to the app's server. Of course it's easy to check if the port numbers match, but this means I need to know whether the host they've entered refers to the current system.
Examples of hostnames or IP addresses that would typically be problematic:
127.0.0.1
localhost
192.168.1.255 (assuming the system is on a 192.168.1.0/24 subnet)
any of the IP addresses assigned to the current system's network interfaces
the system's local DNS name
any other loopback addresses that may be configured other than 127.0.0.1
How could I go about detecting this?
Since this app is being written using Qt, a solution that exclusively uses Qt's framework would be ideal. But if that's not possible (since Qt doesn't handle every possible use case) a macOS-specific solution will work too.
QNetworkInterface class should provide the most information you may need.
You can obtain a list of IP addresses using QNetworkInterface::allAddresses() static function.
You can also get a list of all interfaces using QNetworkInterface::allInterfaces().
Calling QNetworkInterface::addressEntries() for each QNetworkInterface returned by QNetworkInterface::allInterfaces() will give you more information about address entries for each interface.
auto ifs = QNetworkInterface::allInterfaces();
foreach (auto interface , ifs){
auto addresses = interface.addressEntries();
foreach ( auto addy , addresses){
///play with the addy here.
}
}
You can also test hostnames against those ip addresses which you are going to ban, using QDnsLookup class.
I've just began reading information about multicast transfers using boost::asio and I'm somewhat puzzled by the following:
Why do we need a "listening address" in the following boost::asio example? What's the point of that? Why would one choose anything different than localhost?
http://www.boost.org/doc/libs/1_37_0/doc/html/boost_asio/example/multicast/receiver.cpp
Specifying the listening IP address is necessary when you have more than one network interface card (each NIC is bound to a different IP address).
In your apartment...
When you're working at home on your laptop, you probably don't care. The primary goal is usually to connect anything to everything it wants to within your machine, in which case localhost is just fine.
... but servers do care!
High-end servers, on the other hand, usually have more than one network card. Even better, high-performance network cards often have more than one physical plug, and both of them may be active with different DHCP leases.
Servers will also often be part of public and private networks, which may or may not include a VPN which has its own subnet and accessibility parameters. Sysadmins think about these addresses a lot, and they care deeply about which particular address each service is available. Is it a private service? Is there an untrusted subnet that shouldn't be making these requests?
These questions span both security and system organization concerns. It's not specific to multicast: the UNIX bind system call also takes a specific address for all of the above reasons.
I would like to send data between two ethernet interfaces that are connected with a crossover cable in Linux. The two ethernet interfaces are on the same box. I defined the ethernet interfaces to have different static ip addresses (1.2.3.4 and 5.6.7.8) and have been using sockets to send bytes from one IP address to the other. I want to emphasize that I want the data to leave the box one interface of the box and get received on the other interface of the same box. One consequence of this is that unplugging the cable would prevent communication between the client and server on the same box.
The kernel is smarter than me I guess and decides it doesn't need to send information out on the wire and routes the data directly between the sockets internally, thus negating the test. I have tried to use SO_BINDTODEVICE to force the client to send data out of a particular interface, but the server never sees it. I am really stuck and this doesn't seem like it should be this difficult.
There are two entries in the route -n table
Dest Gateway Genmask flags metric use interface
1.2.3.0 0.0.0.0 255.255.255.0 U 0 0 eth0
5.6.7.0 0.0.0.0 255.255.255.0 U 0 0 eth1
You can not communicate using IP between 1.2.3.4/24 to 5.6.7.8/24 without going though a router. The problem is that IP can only talk to other computers in the same network segment. To calulate the network address you need to do a logic AND between both the interface address and the subnet mask. This will give you the network addresses. If the two network addresses are different then a router will be required. In your example you will have network address 1.2.3.0 and 5.6.7.0. Because these are different it will will to send data.
More importantly most network stacks are smart enough to see that if both interfaces are on the same computer it will not send the data all the way to the phyical interface. It will probably only send the message though the IP stack. But again it would need to be vaild address for it to work.
You should even be able to test a similar setup using just loopback network devices. (virtual network cards.)
Try using the following two IP addresses:
192.168.64.1
192.168.64.2
I'm not sure this is the problem, but 1.2.3.4 and 5.6.7.8 aren't on the same subnet. (Because when you apply the mask to each one, you don't get the same network address, as the Dest column is showing you.)
I have a network of computers that they will connect to the a server with DHCP, so I don't know what Ip address a computer will get when I connects to the server. If 192.168.0.39 for example is connected to the server can I identify the real computer behinde this ip address? ( I can install an external application on each client in order to send some data to server for example mac address or so... )
If you are responsible for the DHCP server, you can configure it to hand out a specific IP to a specific MAC. Having done that, you can be reasonably confident of that mapping -- it is possible to spoof MACs, so if you are worried about security, you'll need a much more heavy duty approach. If this is a casual application where the risk of that is low, you configure your DHCP server to hand out IPs based on MACs and then make use of those mappings in your application.
You might not even need the IP address. On an Ethernet network, all communication from a computer, whether it's IPv4, IPv6, or even IPX will be labelled with a MAC address that's stable over time and unique per network card.
i have known that winpcap library enabled a person to obtain advanced information about installed devices...
given all the networking devices found in the computer, how will i know which one of this has an internet connection?!
thanks:)
You typically don't care about this. Normally you just ask the network stack to make a connection, and don't worry about details.
However, each "device" will have its own IP address. You can request the network stack to use this IP address when making a connection.
Now, to figure which devices have an intenet connection, iterate over all of them, obtain their IP address, and try to create a connection from that originating IP to a destination IP on the Internet. Typically you already know such a destination IP, e.g. your companies webserver.
GetAdaptersInfo() will give you a list of all network adapters installed in a form that you can then use in GetIfEntry(). This latter function will tell you the operational status of an adapter, so you can at least tell if the adapter is plugged into a live hub/switch/router.
If you are particularly interested in IP connectivity, you could look to see if a default gateway is configured for that interface. Don't forget you might have to support IPv6 as well as IP4 if you try this.
Correlating the network adapters found by GetAdaptersInfo() with those found by pcap_findalldevs() is left as an exercise for the reader. I don't remember the details but it was fairly obvious.