I'm working on a network client program that connects to public servers, specified by the user. If the user gives me a hostname to connect to that has both IPv4 and IPv6 addresses (commonly, a DNS name with both A and AAAA records), I'm not sure how I should decide which address I should connect to.
The problem is that it's quite common for machines to support both IPv4 and IPv6, but only to have global connectivity over IPv4. The most common case of this is when only IPv6 link-local addresses are configured. At the moment the best alternatives I can come up with are:
Try the IPv6 address(es) first - if the connection fails, try the IPv4 address(es); or
Just let the user specify it as a config setting ("prefer_ipv6" versus "prefer_ipv4").
The problem I can see with option 1 is that the connection might not fail straight away - it might take quite a while to time out.
Please do try IPv6. In the significant majority of installations, trying to create an IPv6 connection will fail right away if it can't succeed for some reason:
if the system doesn't support IPv6 sockets, creating the socket will fail
if the system does support IPv6, and has link-local addresses configured, there won't be any routing table entry for the global IPv6 addresses. Again, the local kernel will report failure without sending any packets.
if the system does have a global IP address, but some link necessary for routing is missing, the source should be getting an ICMPv6 error message, indicating that the destination cannot be reached; likewise if the destination has an IPv6 address, but the service isn't listening on it.
There are of course cases where things can break, e.g. if a global (or tunnel) address is configured, and something falsely filters out ICMPv6 error messages. You shouldn't worry about this case - it may be just as well that IPv4 connectivity is somehow broken.
Of course, it's debatable whether you really need to try the IPv6 addresses first - you might just as well try them second. In general, you should try addresses in the order in which they are returned from getaddrinfo. Today, systems support configuration options that let administators decide in what order addresses should be returned from getaddrinfo.
Subsequent to the question being asked the IETF has proposed an answer to this question with RFC6555, a.k.a. Happy Eyeballs.
The pertinent point being the client and server may both have IPv4 and IPv6 but a hop in between may not so it is impossible to reliably predict which path will work.
You should let the system-wide configuration decide thanks to getaddrinfo(). Just like Java does. Asking every single application to try to cater for every single possible IPv6 (mis)configuration is really not scalable! In case of a misconfiguration it is much more intuitive to the user if all or none applications break.
On the other hand you want to try to log annoying delays and time-outs profusely, so users can quickly identify what to blame. Just like every other delays ideally, including (very common) DNS time-outs.
This talk has the solution. To summarize;
Sometimes there are problems with either DNS lookups or the subsequent connection to the resolved address
You don't want to wait for connecting to an IPv6 address to timeout before connecting to the IPv4 address, or vice versa
You don't want to wait for a lookup for an AAAA record to timeout before looking for an A record or vice versa
You don't want to stall while waiting for both AAAA and A records before attempting to connect with whichever record you get back first.
The solution is to lookup AAAA and A records simultaneously and independently, and to connect independently to the resolved addresses. Use whatever connection succeeds first.
The easiest way to do this is to allow the networking API do it for you using connect-by-name networking APIs. For example, in Java:
InetSocketAddress socketAddress = new InetSocketAddress("www.example.com", 80);
SocketChannel channel = SocketChannel.open(socketAddress);
channel.write(buffer);
The slide notes say at this point:
Here we make an opaque object called an InetSocketAddress from a host
and port, and then when we open that SocketChannel, that can complete
under the covers, doing whatever is necessary, without the
application ever seeing an IP address.
Windows also has connect-by-name APIs. I don’t have code fragments for
those here.
Now, I’m not saying that all implementations of these APIs necessarily
do the right thing today, but if applications are using these APIs,
then the implementations can be improved over time.
The di!erence with getaddrinfo() and similar APIs is that they
fundamentally can’t be improved over time. The API definition is that
they return you a full list of addresses, so they have to wait until
they have that full list to give you. There’s no way getaddrinfo can
return you a partial list and then later give you some more.
Some ideas:
Allow the user to specify the preference on a per-site basis.
Try IPv4 first.
Attempt IPv6 in parallel upon the first connection.
On subsequent connections, use IPv6 if the connection was successful previously.
I say to try IPv4 first because that is the protocol which is better established and tested.
Related
when i ask from DNS Server about somewhere.com it resolve 4 IP for me in this order:
192.168.1.1
192.168.1.2
192.168.1.3
192.168.1.4
and my question is what happen if first IP(192.168.1.1) doesn't work?
which service or protocol switch to next IP?
and what about other services like telnet,... (telnet somewhere.com 443)?
First, when the lookup is made, the returned list is not in any kind of order, it is somewhat random. This helps ensure one system is not overloaded.
What happens if the first address in list fails, will depend on how well the application is written.
What is supposed to happen, is that if the first IP address fails, then the application should then try the next and iterate though the list until the list is exhausted or a connection is made. You can see this with many pieces of software. However, many developers are lazy, and only try the first returned. I can think of quite a few applications where this happens.
It's not fun when you see applications request ANY expecting an IPv4 address and the returned list contains an IPv6 address, only for the software to barf.
I'm using ForcebindIP to point an app at a specific network adapter, like this:
forcebindip -i 192.168.0.5 MyCSharpApp.exe
This works fine and the app isn't aware (or doesn't access) any of the other network adapters on the PC.
Is it possible to restrict ForceBindIP to outbound traffic only leaving the app to receive data from any local network adapter? Or even to specify a network adapter for outbound and another for inbound traffic?
I can't find an extra startup parameter for ForceBindIP that does this.
I'd appreciate any help with this.
If I get your problem correctly, you want to bind your application to listen for packets on all available interfaces but return packets to only through one given interface. I also assume it's a server application and you don't have neiter source code nor control over its behaviour.
Disclosure: I do not know how ForceBindIP works internally, I'm basing my understanding of it on this passage from the website:
it will then inject a DLL (BindIP.dll) which loads WS2_32.DLL into memory and intercepts the bind(), connect(), sendto(), WSAConnect() and WSASendTo() functions, redirecting them to code in the DLL which verifies which interface they will be bound to and if not the one specified, (re)binds the socket
Problems to overcome
I don't believe your desired configuration is possible with just one application level DLL injector. I'll list a few issues that ForceBindIP will have to overcome to make it work:
to listen to a socket, application has to bind() it to a unique protocol-address-port combination first. An application can bind itself to either a specific address or a wildcard (i.e. listen on all interfaces). Apparently, one can bind to wildcard and specific address simultaneously as outlined in this SO question. This however will be two different sockets from the application standpoint. Therefore your application will have to know how to handle this sort of traffic.
When accepting client connection, accept() will create a new socket and parameters on that are managed by Windows, I don't believe there's an API to intercept binding here - by this time the connection is considered established.
Now imagine, we somehow got a magic socket. We can receive packets on one interface and send to another. The client (and all routing equipment on the way) will have to be aware that two packets originating from two different source IP addresses are actually part of the same connection and be able to assemble the TCP session (or correctly merge UDP streams).
You can have multiple gefault gateways with different priorities and rules (which is a whole different topic to explore) but as far as I'm aware that's not going to solve your particular issue: majority of routing protocols assume links are symmetric and expect packets to keep within same interface. There are special cases like asymmetric routing and network interface teaming but they have to be implemented on per-interface level.
One potential solution
One way to achieve what you're after (I don't know enough about your environment to claim it will work), will be to create a virtual interface, set it into yet another IP network, bind your application to it, then use firewall (to, say, allow multicast backets into the "virtual" network) and routing from that network to required default gateway with metric set to 1. I also suspect just any Windows will not be that flexible, so you might need like a Server Edition.
I am sorry this didn't turn out to be the ready-to-fly solution, I however am hoping this gives you more context to the problem you are facing and points you into other directions to explore.
You can use Set-NetAdapterAdvancedProperty command in Powershell to set the flow control of your specified adapter
To get the names and properties of all the network adapter :-
Get-NetAdapterAdvancedProperty -Name "*"
Suppose you want the network adapter named "Ethernet 2" to be only used to receive data from internet then type :-
Set-NetAdapterAdvancedProperty -Name "Ethernet 2" -DisplayName "Flow Control" -DisplayValue "Rx Enabled"
You can find more in :
https://learn.microsoft.com/en-us/powershell/module/netadapter/set-netadapteradvancedproperty?view=win10-ps
Microsoft winsock example has a usage in their example for limiting a socket to only send or receive mode. It might help.
https://learn.microsoft.com/en-us/windows/win32/winsock/complete-client-code
Outbount and Inbount limits are not imposed while binding. But latter or when connection is established.
Line of code pertaining to this in client code is toward the end.
// shutdown the connection since no more data will be sent
iResult = shutdown(ConnectSocket, SD_SEND);
I have been writing my own version of the 802.11 protocol with network stack. This is mostly a learning experience to see more in depth on how networks work.
My question is, is there a standard for replying to client devices that a certain protocol is unsupported?
I have an android device connecting to my custom wifi device and immediately sending a TON of requests at the DNS port of my UDP protocol. Since I would like to test out other protocols I would very much like a way for my wifi device to tell the android device that DNS is not available and get it to quite down a little.
Thanks in advance!
I don't see a possibility to send a reply that a service is not available.
I can't find anything about this case in the UDP specification.
One part of the DNS specification assumes that there are multiple DNS servers and defines how to handle communication with them. This explains part of the behavior in your network, but does not provide much information how to handle it.
4.2.1 Messages - format - UDP usage
The optimal UDP retransmission policy will vary with performance of the
Internet and the needs of the client, but the following are recommended:
The client should try other servers and server addresses
before repeating a query to a specific address of a server.
The retransmission interval should be based on prior
statistics if possible. Too aggressive retransmission can
easily slow responses for the community at large. Depending
on how well connected the client is to its expected servers,
the minimum retransmission interval should be 2-5 seconds.
7.2 Resolver Implementation - sending the queries
If a resolver gets a server error or other bizarre response
from a name server, it should remove it from SLIST, and may
wish to schedule an immediate transmission to the next
candidate server address.
According to this you could try to send garbage back to the client, but this is rather a hack, or an error, but how does an error look like? Such a solution assumes that you have knowledge about the service that you don't support.
I believe that the DNS - requests can be avoided by using DHCP. DHCP allows to specify DNS-servers as listed in the linked page. This is the usual way that I know for a DNS-resolver in a LAN to get initial DNS servers although I don't find anything about this in the DNS specification. You can give the Android - device a DNS-server with DHCP so that it does to need to try to query your device. Querying your device could be a fallback.
Additionally to DNS there is mDNS which uses multicasts in the network to send queries. This seems not to be the protocol you have to do with because it uses the special port 5353.
Not possible to stop DNS in the way you intend. However, only for your tests you can check the UDP messages and find out the names the device is looking for. Then you update the hosts file (google how to do it: http://www.howtogeek.com/140576/how-to-edit-the-hosts-file-on-android-and-block-web-sites/) and add those names with some localoop IP address. That might work for your test.
Other possibility is to change DNS server to some localloop IP address: http://xslab.com/2013/08/how-to-change-dns-settings-on-android/
Again, this is only to avoid having all the DNS messages through the wifi connection.
What I'm trying to do is get all the ip addresses in my network, and I thought, assuming I know the address of all subnets could use arp requests to achieve that if there was a way to forward these requests over different subnets.
For example , assume I had two hosts
192.168.0.2/24 and 192.168.1.2/24
connected via router using IP addresses 192.168.0.1/16 192.168.1.1/16.
I would like to send an arp request from 192.168.0.2/16 to 192.168.1.2/16.
I thought maybe if the arp request was encapsulated in layer 3 header containing 192.168.1.2/24, or 192.168.1.255/24 as the dsetination this will work.
If it is possible and you know a tool that does that I will be happy to know about this tool.
If it isn't, I would like to know what happens to a packet like the one I described above
I would like to know what happens to a packet like the one I described above
If you encapsulate some info into standard IP-packet, then, naturally, it will be routed to the IP-destination host. Yet if the remote host knew nothing about this non-standard packet, then nothing would happen.
If you really want to get something out of this, you need to have up and running some software server on that remote host, which is able to process your requests. That is, you need some Proxy ARP: either existing implementation, or made of your own.
If you don't have such "an agent" in the target subnetwork, then you're out of luck. Go with sequential IP-scanning until be banned by admin.
I have 3 applications: a MasterServer, a Server and a Client.
The MasterServer is running on: 70.105.155.5:15555 (port forwarded with UPnP)
I create a server and let the MasterServer know I exist. The MasterServer keeps my public ip and port. The port that the MS gets is randomly assigned by my router (lets say: 70.105.155.5:16666). The server keeps messaging the MasterServer each 10sec to keep that same port open.
I open up the client, on which it asks the MasterServer for the public ip and port of a server. The MasterServer returns: 70.105.155.5:16666. I know 100% sure that the server's public port 16666 is still open because I can check that in my logs.
But all messages sent from Client => Server are never received. At the same time the Server is still getting messages from MasterServer through 16666.
So this is really puzzling. Am I forgetting something? Is my understanding of NAT punch flawed?
Thanks for any help!
There are multiple issues here, and it depends on the router's security configuration too, often in ways the user cannot control. The general excuse is that it's a security precaution, but really firewalling and NAT are two separate concerns. Anyway, most home users are stuck with whatever they've got. They do usually have the option to explicitly map a port, and UPnP can help you too if the router supports it.
But going back to NAT, to begin with you're likely to have a problem if your server and client are sitting behind the same NAT, which seems to be the case given the addresses you quoted above. Most NATs only rewrite incoming packets from the public interface - so packets that physically arrive on the private interface, even if they're addressed to the public IP, won't be forwarded. To support this configuration in the wild you need the devices to advertise their private addresses to the MasterServer, and detect when they want to talk to other devices behind the same NAT, and if so, use the private address rather than going through the NAT. This is flawed in many ways, especially with nested NATs, but I think it's the best you can do.
Beyond that, in the more common case where all the devices are behind different NATs, some routers will only allow incoming traffic on a forwarding port if it's from the place they originally sent the outgoing traffic to (which resulted in the port opening up in the first place). Some also require it to come from the same source port on the remote device.
The workaround is for the MasterServer to do a bit more work. The gist is that it should tell both peers to send a packet to each other; these may or may not get through, but simply sending the packet out through the Server's NAT to the Client's public IP address may be enough to get the Server's NAT to correctly forward later packets from the Client. And vice versa.
In practice it is even more complicated, because the Server's NAT may use a different port when talking to Client to what it used when talking to MasterServer. So while this may open up a port for the Client to talk back to the Server, but it might not be the one the Client is expecting to use. If the NAT behaves like this, then you need to look at how predictable its choice of port numbering is. Some just increase one by one (but bear in mind there may be other devices behind the same NAT causing the number to jump more than one step at a time). For these ones, the Client needs to spam a range of Server ports to try to figure out which one got opened up. Again the MasterServer is in the best position to coordinate this.
Others seem totally random in port allocations, and there's not much you can do with those. But it's only terminal if both ends are behind these random NATs. So long as one end is more amenable to being opened up, the random end won't matter.
Also note that some NATs use a different outgoing port for each port on the target - this also makes prediction a lot harder, even in the case that the ports are not randomly assigned. Again, so long as one end of the connection is flexible, you can tolerate these NATs but in a peer-to-peer context they will be a nightmare in the end because they just can't talk to each other.