I'd like to figure out what IP address a DCOM client is connecting from. I have an OPC server, and from the context of the COM/DCOM code, there is no way to get the client IP that I can find. This article appears to confirm that (http://blogs.msdn.com/b/sanpil/archive/2004/03/01/82302.aspx).
I tried using a C++ library that netstat uses (iphlpapi.lib, GetExtendedTcpTable) to get open connections with no luck. It looks like COM/DCOM defaults to UDP.
The only other thing I can think of is using WireShark pcap to try and capture inbound traffic on port 135?
Does anyone have any clever ideas?
It's said "Using DCOM through firewalls becomes problematic because it dynamically allocates one port per process (configurable through the registry) and requires UPD and TCP ports 135-139 to be open. ".
Related
I am working on a project need to send periodic alive message to https server.
Because of security issue, we need to use minimal number of ports (blocking unused ports as many as we can).
I am using c++ libcurl easy interface to send https request in linux.
I have tried to use the same curl handler object (CURL object) and set CURLOPT_LOCALPORT to a port number. The first request is ok. But in the second, libcurl verbose mode said address already in use.
However, when I comment out the port set through CURLOPT_LOCALPORT, it works on second connection also, and by setting VERBOSE to 1, I can see "Re-using existing connection" print out, which is missing in version setting up local port.
And I check with linux netstat, find out that it is using the same port.
I cannot figure out why setting up local port will make it failed.
And also, I have tried to close the connection using curl_easy_cleanup, but due to tcp time_wait state, we cannot reuse the port in a while, that's not what I want.
Could anyone provide a solution or suggestion to us? Thanks a lot.
Edit
My reason using one port is not to keep opening and closing connection too much.
Because of the security issue ...
There is no security issue. You need to get over this phobia about using multiple local outbound ports. There is zero security benefit in using fewer, or in constraining them in any way.
I'm writing a program on Windows using winsocks that can send messages to another computer. The client connects with the server in the other computer and begin exchanging data.
It works fine on my local network using local addresses(192.168.1.*), but I can't communicate with public addresses (216.185.45.129); not even my own. I can successfully connect to a website on port 80, but not to my laptop at home using its public IP address, regardless of what ports I use (unreserved ports).
So I did research online and the only solution that seems to work is port forwarding.
-But is there absolutely no other way to achieve this?
-How do other programs like Teamviewer connect to other computers on the network then?
-Is there an already open but typically unused port that I can use?
-At the very least, can I forward the ports on my router but not have the client do anything? Or maybe have my program forward the ports automatically.
The main problem is, that every router is using NAT to distinguish different computer in your lokal network against the WAN. He need to do this, because you got only one IP in the internet, but several devices in your home. To archive this, he uses groups of ports. That means, if you use to send maybe from port 2048 to a webserver in internet with two devices, the router gives one device another port (like 2049). The response has the Port of the requester, so the router can map it back. Unfortunately most router always map ports so you never now which port you have from the internet side.
There are two common ways to work around and archive your goal.
Port Fowarding
You can force most router not to map special ports but bind them to unique MAC addresses. You can use UPNP to config most router to do that, but I do not recommend that for security reasons and also it does not work in many enviroments where Router do not allow UPNP manipulation.
Most router have port forwarding abilities for gaming reasons (mostly it is used in P2P networks)
It works with TCP and UDP.
NAT Traversal
The common way is NAT traversal, also known as NAT hole punching. I will describe it in short for UDP. You can find a wiki explanation here for TCP and for UDP here. Unfortunately you need a server in the internet both clients can reach. Here the steps:
Both clients contact the server. The server now know IP and PORT of both clients.
Server send back the information to the clients.
Both(!) clients send now packages to each other on the known address.
It is necessary that both client send a UDP package and have to accept that the first package get lost. The reason is the router. Most router only accept packages from a source on a mapped PORT if a client has send a package to that source before.
UPDATE
Regarding to a comment of Remy Lebau I changed the Firewall piercing part to NAT Traversal as it was partly wrong.
I've got a problem in socket programming. I'm currently writing a simple server/client application which asks connecting clients to answer a survey (I'm following the exercises in the book: TCP/IP Sockets in C: Practical guide for Programmers). It works fine in my local network (using localhost to connect), but I can't make it work over the internet.
Since I changed some of the code found in the book, I tried to compile the original source code from the book but it still doesn't work. I assume this is not a code problem but a network problem.
I did some research and so I turned off my firewall, I forwarded the port I'm using (12543) in my router but it still doesn't work... I've got a remote windows server running Windows 7 for testing: when I run my server on it and try to connect from my computer it fails, when I try to run my server from my computer and try to connect from the remote windows server: it fails again. Even when I run my server on my computer and try to connect with the client from the same computer using my private IPv4 address 192.168.x.x or my public one, it fails ! Oh, and there is no firewall running on the windows server.
I really don't know what to do now... I can ping my windows server from my computer, I can ping my computer from my windows server, but it's impossible to connect to my application.
The source code can be found here: http://cs.ecs.baylor.edu/~donahoo/practical/CSockets2/textcode.html (SurveyServer2.cpp, SurveyClient2.cpp, SurveyCommon.h), but I don't think it's a code problem.
Please tell me if it's not clear enough. And excuse me if I did some grammatical errors, I'm french!
Any help would be appreciated, thanks!
EDIT : Ok, I know what's going on: it's a compatibility issue between IPv4 and IPv6! It's not properly working yet, but I now know what to fix. Thanks everybody :)
SECOND EDIT : Well, I think I finally understood. I was binding my server to a IPv6 address, but the host my server is running on only have a public IPv4 address. When I tried to connect, the DNS resolution only returned a IPv4 address so I was unable to connect to my server. I told to getaddrinfo to return only IPv4 addresses, so now it binds on a IPv4 address and it works fine.
There is surely a way to add a IPv6 address to my host but I don't think I will need it, it works okay right now, I hope this doesn't cause any trouble.
The code makes usage of getaddrinfo and need a hostname not a IP address.
It does sound like you have a connectivity issue or a DNS issue.
Ensure the name resolution is working:
Enter 'ping server_NAME' on the command line of the client machine.
If it can't find an IP address for the server name that could be your issue.
Ensure connectivity:
On the server command line:
enter 'netstat -an -p TCP'. This will show a list of all programs listening and their port numbers. You should see your server listening on port 12543.
On the machine where you are running your client program:
Use telnet to see if you can get through to the server. You can set the port number telnet uses with a command line option. Usually something like 'telnet -p 12543 server-ip'. If it says 'connection refused' then there's a connectivity issue (a firewall/etc). If it opens a connection you will get no error message and you will be able to type text to be sent to the server. You really only care if telnet was able to establish a tcp connection here.
If Telnet does connect then your issue is communication between the programs and not a network issue.
TCP standard has "simultaneous open" feature.
The implication of the feature, client trying to connect to local port, when the port is from ephemeral range, can occasionally connect to itself (see here).
So client think it's connected to server, while it actually connected to itself. From other side, server can not open its server port, since it's occupied/stolen by client.
I'm using RHEL 5.3 and my clients constantly tries to connect to local server.
Eventually client connects to itself.
I want to prevent the situation. I see two possible solutions to the problem:
Don't use ephemeral ports for server ports.
Agree ephemeral port range and configure it on your machines (see ephemeral range)
Check connect() as somebody propose here.
What do you thinks?
How do you handle the issue?
P.S. 1
Except of the solution, which I obviously looking for,
I'd like you to share your real life experience with the problem.
When I found the cause of the problem, I was "astonished" on my work place people are not familiar with it. Polling server by connecting it periodically is IMHO common practice,
so how it's that the problem is not commonly known.
When I stumbled into this I was flabbergasted. I could figure out that the outgoing
port number accidentally matches the incoming port number, but not why the TCP
handshake (SYN SYN-ACK ACK) would succeed (ask yourself: who is sending the ACK if
there is nobody doing a listen() and accept()???)
Both Linux and FreeBSD show this behavior.
Anyway, one solution is to stay out of the high range of port numbers for servers.
I noticed that Darwin side-steps this issue by not allowing the outgoing port
to be the same as the destination port. They must have been bitten by this as well...
An easy way to show this effect is as follows:
while true
do
telnet 127.0.0.1 50000
done
And wait for a minute or so and you will be chatting with yourself...
Trying 127.0.0.1...
telnet: Unable to connect to remote host: Connection refused
Trying 127.0.0.1...
telnet: Unable to connect to remote host: Connection refused
Trying 127.0.0.1...
telnet: Unable to connect to remote host: Connection refused
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
hello?
hello?
Anyway, it makes good job interview material.
Bind the client socket to port 0 (system assigns), check the system assigned port, if it matches the local server port you already know the server is down and and can skip connect().
For server you need to bind() socket to port. Once addr:port pair had socket bound, it will no longer be used for implicit binding in connect().
No problem, no trouble.
Note that this solution is theoretical and I have not tested it on my own. I've not experienced it before (or did not realize) and hopefully I won't experience it anymore.
I'm assuming that you cannot edit neither the client source code nor the server source. Additionally I'm assuming the real problem is the server which cannot start.
Launch the server with a starter application. If the target port that the server will bind is being used by any process, create an RST (reset packet) by using raw sockets.
The post below briefly describes what an RST packet is (taken from http://forum.soft32.com/linux/killing-socket-connection-cmdline-ftopict473059.html)
You have to look at a "raw socket" packet generator.
And you have to be superuser.
You probably need a network sniffer as well.
http://en.wikipedia.org/wiki/Raw_socket
http://kerneltrap.org/node/3072 - TCP RST attacks
http://search.cpan.org/dist/Net-RawIP/lib/Net/RawIP.pm - a Perl module
http://mixter.void.ru/rawip.html - raw IP in C
In the C version, you want a TH_RST packet.
RST is designed to handle the following case.
A and B establish a connection.
B reboots, and forgets about this.
A sends a packet to B to port X from port Y.
B sends a RST packet back, saying "what are you talking about? I don't
have a connection with you. Please close this connection down."
So you have to know/fake the IP address of B, and know both ports X
and Y. One of the ports will be the well known port number. The other
you have to find out. I thnk you also need to know the sequence
number.
Typically people do this with a sniffer. You could use a switch with a
packet mirroring function, or run a sniffer on either host A or B.
As a note, Comcast did this to disable P2P traffic.
http://www.eff.org/wp/packet-forgery-isps-report-comcast-affair
In our case we don't need to use a sniffer since we know the information below:
So you have to know/fake the IP address of B, and know both ports X
and Y
X = Y and B's IP address is localhost
Tutorial on http://mixter.void.ru/rawip.html describes how to use Raw Sockets.
NOTE that any other process on the system might also steal our target port from ephemeral pool. (e.g. Mozilla Firefox) This solution will not work on this type of connections since X != Y B's IP address is not localhost but something like 192.168.1.43 on eth0. In this case you might use netstat to retrieve X, Y and B's IP address and then create a RST packet accordingly.
Hmm, that is an odd problem. If you have a client / server on the same machine and it will always be on the same machine perhaps shared memory or a Unix domain socket or some other form of IPC is a better choice.
Other options would be to run the server on a fixed port and the client on a fixed source port. Say, the server runs on 5000 and the client runs on 5001. You do have the issue of binding to either of these if something else is bound to them.
You could run the server on an even port number and force the client to an odd port number. Pick a random number in the ephemeral range, OR it with 1, and then call bind() with that. If bind() fails with EADDRINUSE then pick a different odd port number and try again.
This option isn't actually implemented in most TCPs. Do you have an actual problem?
That's an interesting issue! If you're mostly concerned that your server is running, you could always implement a heartbeat mechanism in the server itself to report status to another process. Or you could write a script to check and see if your server process is running.
If you're concerned more about the actual connection to the server being available, I'd suggest moving your client to a different machine. This way you can verify that your server at least has some network connectivity.
In my opinion, this is a bug in the TCP spec; listening sockets shouldn't be able to send unsolicited SYNs, and receiving a SYN (rather than a SYN+ACK) after you've sent one should be illegal and result in a reset, which would quickly let the client close the unluckily-chosen local port. But nobody asked for my opinion ;)
As you say, the obvious answer is not to listen in the ephemeral port range. Another solution, if you know you'll be connecting to a local machine, is to design your protocol so that the server sends the first message, and have a short timeout on the client side for receiving that message.
The actual problem you are having seems to be that while the server is down, something else can use the ephemeral port you expect for your server as the source port for an outgoing connection. The detail of how that happens is separate to the actual problem, and it can happen in ways other than the way you describe.
The solution to that problem is to set SO_REUSEADDR on the socket. That will let you create a server on a port that has a current outgoing connection.
If you really care about that port number, you can use operating specific methods to stop it being allocated as an ephemeral port.
Is there any method in c++/vc++/mfc so that I can verify whether remote machine is on network or not. I don't want to use ping command. I tried to use WSAConnect method but it takes lot of time in searching if remote machine is not on network. In that case application windoe freezes for long time. I need alternate way to verify remote machine netwotk connectivity. Plz help
You can check to see if a host name exists and if DNS can resolve an IP address for it, but this won't tell you if it's online. In order to determine if it's online, you must exchange data with the host. For this to happen the host must be listening on a port, and must send some sort of response.
Generally, the lowest common denominator for hosts is the ICMP packet - aka ping. You can write your own service in C++ to send these UDP packets, and detect a reply.
Some hosts and some routers disable ICMP to appear 'invisible'. If you know that your host accepts connections on a TCP port, you can attempt to create a tcp socket. For example, from a shell you can test using nc
nc -z google.com 80
Connection to google.com 80 port [tcp/http] succeeded!
If you do raw socket programming, you can attempt to connect to a host via, and then call select() with a short timeout.
If you don't want your app to block, use a thread.
If the only issue is that calling WSAConnect locks up your UI, you can try calling it in nonblocking mode.
Call WSAEventSelect to set the socket to nonblocking mode prior to calling WSAConnect. You then call WSAAsyncSelect or WSAEventSelect to check the status of the connection.
MSDN's page on WSAConnect has some remarks about nonblocking mode here.
You can use tracert command. Example:
tracert -w 1000 192.168.0.77
You can check if the machine is connected to a network, but without trying to transmit data to a site residing on the internet I don't think there is a way to check if that network is actually online. For example your end user might as well be connected to a DSL router which is not online at the moment.
You can use arping for it.
Also some smart switch can talk about attached host.