i have a problem. I've developed a web-app using WebRtc for one-to-one videocall via browser using WebRtc with signalling server on node js (listening e.g. on 8181 port).
Now i would implement MITM attack. I was thinking that, wheen Peer_1 should invoke two rtc peer connection, one for the second peer (Peer_2), one to the MITM. The same thing for the second peer.
Now, i was thinking that signalling server needs to listen on another port, for each rtc peer connection received from the two peers (e.g. 8282 for Peer_1 and 8383 for Peer_2).
Am i right? I think that because signalling server's implementation is to one-to-one communication.
In this way, signalling server on port 8181 allows end-to-end communication for Peer_1 and Peer_2, on 8282 there is the signalling path for Peer_1 and the MITM, and on 8383 for MITM and Peer_2.
Am i right or not? Thanks for the support.
Man in the middle refers to interception during transmission, which WebRTC itself is secured against using DTLS and key exchange, so the weak point is usually the signaling server chosen by an application instead.
But what you describe however sounds like Man on both ends. You have to trust the service (the server) to guarantee whom you're being connected to. If that server is compromised, or either client is compromised - say by injection - then there's no guarantee whom you're talking to, since a client can easily forward a transmission to another party.
As far as I know, the role of the signaling server is to exchange sdp between the web browsers you want to communicate with.
I understood the process as follows.
Web browser passes sdp to signaling server through websocket
The server passes each other's sdp back to the web browsers.
When the transfer is complete, close the web socket.
After that, web browsers can communicate with each other without a signaling server
Did I get it right?
It depends on whether you are using vanilla or trickle ICE.
In trickle ICE, ICE candidates are not discovered via STUN, for example, prior to sending the SDP. In this case your webRTC endpoints may not be able to communicate because they will not be able to communicate with each other yet. The signaling layer is still required to relay the ICE candidates to establish the peer connection. Once the peer connection is established, signaling is no longer required. Most implementations I have seen use trickle ICE because it usually reduces the latency to establish the peer connection.
In vanilla ICE, the ICE candidates are discovered prior to sending the SDP. In this case, the signaling is complete after sending the SDP.
I am trying to get a browser client to connect with my C++ linux application using WebRTC. So my environment is not the typical triangle WebRTC where 2 browsers setup a WebRTC call thru a server. Instead, the browser client side is typical, but my application is acting as the server and the remote client, so it does the signalling and also streams the SRTP media using gstreamer.
I am successful up to a point. I have successfully exchanged the ice candidates and the offer/answer SDP exchange is also successful. The browser ICE connection state successfully goes to "checking" and at that point I am stuck.
Question: Is the server or remote browser involved in the ice checking operations? That is, does the browser do the ICE checking with the STUN server or with the actual candidate address from the remote end. That would then imply that my C++ application has to be involved in that checking process.
Thanks,
-Andres
your server needs to respond to STUN binding requests at least which are sent as part of ICE.
If your server always has a public IP, using ice-lite (see RFC 5245) will make your life a lot easier.
Is the signaling server used only the first time to establish a connection between 2 peers or is it also used to send and receive data-streams between the peers?
According to the w3c proposal:
An RTCPeerConnection allows two users to communicate directly, browser to browser. Communications are coordinated via a signaling channel which is provided by unspecified means, but generally by a script in the page via the server, e.g. using XMLHttpRequest.
So the Server is only used for signalig not for data transmission. But signaling is not limited to establishing the first connection. The signaling channel is also used for transmitting error messages, metadata such as codecs, codec settings, networkdata and keys for secure transmission.
This depends on the network configuration.
If at least one of the peers is not behind a NAT firewall, the peer that is directly on the internet acts as server, and the signalling server is no longer used after the connection is established.
If both peers are behind a NAT appliance, under certain circumstances it might be possible to negociate a client server connection between the peers, and the data is again sent directly between the two peers.
If both peers are behind a NAT firewall that is locked down, all the traffic between the peers passes through the signalling server.
Notice also that in the first two cases, a STUN server is used to establish the connection. If the full data is relayed through the server, a TURN server is used.
Look at a good explanation in the article an video on html5rocks. They claim only about 14% of all connexions need TURN, which seems a really low number to me (This corresponds to only 37% of all clients are behind a locked down NAT router).
I'm interested in Peer-to-Peer connections in the browser. Since this seems to be possible with WebRTC, I'm wondering how it works exaclty.
I've read some explanations and saw diagrams about it and now it's clear to me, that the connection establishmet works over the server. The server seems to exchange some data between the client that are willing to connect to each other, so that they can start a direct connection, that is independent of the server.
But that's exaclty what I don't understand. Until now, I thought the only way to create connections is to listen on a port on computer A and connect to that port from computer B. But this does not seem to be the case in WebRTC. I think none of the clients starts to listen on a port. Somehow, they can create a connection without listening on ports and accepting connections. Neither client A, nor client B starts acting as a server.
But how? What data is exchanged over the WebRTC server, that the clients can use to connect to each other?
Thanks for your explanations for this :)
Edit
I found this article. It's not related to WebRTC, but I think it answers a part of my question. I'm not sure, tough. It still would be cool, if someone could explain it to me and give me some additional links.
WebRTC gives SDP Offer to the client JS app to send (however the JS app wants) to the other device, which uses that to generate an SDP Answer.
The trick is that the SDP includes ICE candidates (effectively "try to talk to me at this IP address and this port"). ICE works to punch open ports in the firewalls; though if both sides are symmetric NATs it won't be possible generally, and an alternative candidate (on a TURN server) can be used.
Once they're talking directly (or via TURN, which is effectively a packet-mirror), they can open a DTLS connection and use it to key the SRTP-DTLS media streams, and to send DataChannels over DTLS.
Edit:
Acronyms here: http://blog.1click.io/10-jargons-abbreviations-for-webrtc-fans/ for the rest, there is Google. Most of these are defined by the IETF (http://ietf.org/)
Edit 2:
Firefox and Chrome (and the spec) have moved to using "trickle" for ICE candidates, so the ICE candidates are generally added after-the-face to the PeerConnection and exchanged independently of the initial SDP (though you can wait until the initial candidates are ready before sending an offer, and bundle them together).
See https://webrtcglossary.com/trickle-ice/ and https://datatracker.ietf.org/doc/draft-ietf-ice-trickle/
How WebRTC Works
This document provides a quick and abstract introduction to WebRTC. In order to get more information about WebRTC please look at the Further Reading section at the end of this document.
WebRTC
WebRTC(Web Real-Time Communication) is a set of technologies that is developed for peer to peer duplex real-time communication between browsers. As its name mentions it is compatible with Web and it is a standard in W3C One of the important feature of WebRTC is that it works even behind NAT addresses.
WebRTC uses several technologies to provide real-time peer to peer communication between browsers. These technologies are
SDP (Session Description Protocol)
ICE (Interactivity Connection Establishment)
RTP (Real Time Protocol)
There is one more thing which is Signalling Server is needed for running WebRTC. However, there is no defined standart in implementing signalling server. Each implementation creates its own style. There will give some more information about Signalling Server later in this section.
Let's give some quick info about technologies above.
SDP (Session Description Protocol)
SDP is a simple protocol and it is used for which codecs are supported in browsers. For instance, assume that there are two peers(Client A and Client B) which will be connected through WebRTC. Client A and Client B create SDP strings that defines which codecs they support. For example, Client A may support H264, VP8 and VP9 codecs for video, Opus and PCM codecs for audio. Client B may support only H264 for video and only Opus codec for audio. For this case, the codecs that will be used between Client A and Client B are H264 and Opus. If there are no common codecs between peers, peer to peer communication cannot be established.
You may have a question about how these SDP strings are sent between each others. This is where Signalling Server takes place.
ICE (Interactivity Connection Establishment)
ICE is the magic that establishes connection between peers even if they are behind NAT. Let's assume again Client A and Client B will get connected and take a look at how ICE is used for that.
Client A finds out their local address and public Internet address by using STUN server and sends these address to Client B through Signalling Server. Each addresses received from STUN server is called ICE candidate
In the image above, there are two servers. One of them is STUN and other of them is TURN server.
STUN server is used to let Client A learn its all addresses. Let me give an example for this, our computers generally has one local address in the 192.168.0.0 network and there is a second address we see when we connect to www.whatismyip.com, this IP address is actually the Public IP address of our Internet Gateway(modem, router, etc.) so let's define STUN server; STUN servers lets peers know theirs Public and Local IP addresses. Btw, Google provides free STUN server(stun.l.google.com:19302).
There is a one more server, TURN Server, in the image. TURN Server is used when peer to peer connection cannot be established between peers. TURN server just relays the data between peers.
Client B does the same, gets local and public IP addresses from STUN server and sends these addresses to Client A through Signalling Server.
Client A receives Client B's addresses and tries each IP addresses by sending special pings in order to create connection with Client B. If Client A receives response from any IP addresses, it puts that address in a list with its response time and other performance credentials. At last Client A choose the best addresses according to its performance.
Client B does the same in order to connect to Client A
RTP (Real Time Protocol)
RTP is a mature protocol for transmitting real-time data. It is based on UDP. Audio and Video are transmitted with RTP in WebRTC. There is a sister protocol of RTP which name is RTCP(Real-time Control Protocol) which provides QoS in RTP communication. RTP is also used in RTSP(Real-time Streaming Protocol)
Signalling Server
The last part is the Signalling Server which is not defined in WebRTC. As mentioned above, Signalling Server is used to send SDP strings and ICE Candidates between Client A and Client B. Signalling Server also decides which peers get connected to each other. WebSocket technology is generally used in Signalling Servers for communication.
Compatibility
In the last one year, all browsers including Safari, Edge have released new versions supporting WebRTC. Chrome, Firefox and Opera have already supported WebRTC for a while. The video codec that is common to browsers are H264. For the audio, Opus is common in browsers. PCM can also be used for audio codec but AAC is not used even if AAC is supported in all browsers because of licensing issues. IP Cameras generally support H264 for video codec and PCM or AAC for audio codec.
Further Reading and References
WebRTC Samples
ICE Wikipedia
SDP
RTP RFC
Getting Started with WebRTC
WebRTC.org
STUN Server
RTSP Wikipedia
Btw, I am developer at Ant Media Server which supports scalable one-to-many WebRTC and peer to peer WebRTC connection
Establishing a p2p WebRTC connection has 3 steps (10.000 feet overview) :
Step 1: Signaling: both peers connect to a signaling server (using websockets over 80/443, comet, SIP,etc..) and exchange information (about their media capabilities, public IP:port pairs when they become available, etc.)
Step 2: Discovery: Devices connected to LAN or mobile networks are not aware of their public IP (and port) where they can be reached at so they use STUN/TURN servers located on the public Internet to discover their ip:port pair (ICE candidates). In the process they punch a hole through the NAT/router which is used in step3:
Step 3: P2P connection: once the ICE candidates are exchanged through the initial signaling channel each peer is aware of each other's ip:port (and holes have been punched in NATs/routers) so a peer to peer UDP connection can be established.
The scheme above explains the process with 2 devices connected to local networks. It's part of an article I wrote that deals with troubleshooting connection issues but it does a good job of explaining how WebRTC works.
A very good explanation can be found in this book "High Performance Browser Networking (O'Reilly)" http://chimera.labs.oreilly.com/books/1230000000545/ch03.html#STUN_TURN_ICE
which provides the fundamentals on how WebRTC uses ICE technology.
In particular assuming the IP address of the STUN server is known, the WebRTC application first sends a binding request to the STUN server. The STUN server replies with a response that contains the public IP address and port of the client as seen from the public network.
Now the application discovers its public IP and port tuple which can send to the other peer through SDP. (note that SDP are sent over an external signalling channel, f.i. websocket established through a web service)
With this mechanism in place, whenever two peers want to talk to each other over UDP, they can then use the established public IP and port tuples to exchange data.
Unfortunately, in some cases UDP may be blocked by a firewall. To address this issue, whenever STUN fails, we can use the Traversal Using Relays around NAT (TURN) protocol as a fallback, which can run over UDP and switch to TCP if all else fails.
WebRTC connection starts with WebRTC offer.
Caller creates the WebRTC offer and posts it to the Signaling server which will pass the offer to the callee. Users actually passing their SDP (Session Description Protocol) information each other.
Then we need to exchange the internet connection details. it allows clients to discover their public IP address and the type of NAT they are behind. this information is used to establish the media connection. This is handled by STUN server. this process is also known as getting the ICE Candidates. This data is also exchanged via the Signalling Server.
Final step is to exchange audio and video streams true TURN (Traversal Using Relay NAT) server. This ensures the connection even thought users are behind the firewall. this server process a lot of heavy calculations so its cost is high. when you test your app in dev with different browsers, you are directly connecting each other, you are not using TURN server