I'm writing a TCP client to connect, send messages to, and read responses from nodes on a busy network. I do not have access to code running on those nodes, so I have to sculpt the TCP messages I send out over the wire very carefully.
I've decided that I'm going to gather a bunch of live TCP data off of the network and use that as the basis for testing my client - given these input parameters, make sure that the transmitted binary renders into the hex I expect to see.
At this point I'm about to either fire up Wireshark and figure out how to filter for the packets that I'm sending or implement a really simple TCP server that waits for connections, spits the transmitted binary to disk (maybe processed to hex) and then test that I'm sending what I expect to send; neither of which feel like robust or professional solutions.
So: how would you recommend setting up a test harness for a TCP client to verify the right hex messages are coming out?
You should check out aleph for tcp and gloss for protocols. here is a tutorial and an example.
a simple tcp echo-server could be:
(defn echo-handler [ch client-info]
(siphon ch ch)) ;; a simple echo handler
(defn start-server [] (start-tcp-server
echo-handler
{:port 9997}))
(start-server)
gloss will be very useful at spitting the received messages either as hex dump or as values if you encode your protocol in it and use "decode".
Related
I have a trouble to tune TCP client-server communication.
My current project has a client, running on PC (C#) and a server,
running on embedded Linux 4.1.22-ltsi.
Them use UDP communication to exchanging data.
The client and server work in blocking mode and
send short messages one to 2nd
(16, 60, 200 bytes etc.) that include either command or set of parameters.
The messages do note include any header with message length because
UDP is message oriented protocol. Its recvfrom() API returns number of received bytes.
For my server's program structure is important to get and process entire alone message.
The problem is raised when I try to implement TCP communication type instead of UDP.
The server's receive buffer (recv() TCP API) is 2048 bytes:
#define UDP_RX_BUF_SIZE 2048
numbytes = recv(fd_connect, rx_buffer, UDP_RX_BUF_SIZE, MSG_WAITALL/*BLOCKING_MODE*/);
So, the recv() API returns from waiting when rx_buffer is full, i.e after it receives
2048 bytes. It breaks all program approach. In other words, when client send 16 bytes command
to server and waits an answer from it, server's recv() keeps the message
"in stomach", until it will receive 2048 bytes.
I tried to fix it as below, without success:
On client side (C#) I set the socket parameter theSocket.NoDelay.
When I checked this on the sniffer I saw that client sends messages "as I want",
with requested length.
On server side I set TCP_NODELAY socket option to 1
int optval= 1;
setsockopt(fd,IPPROTO_TCP, TCP_NODELAY, &optval, sizeof(optval);
On server side (Linux) I checked socket options SO_SNDLOWAT/SO_RCVLOWAT and they are 1 byte each one.
Please see the attached sniffer's log picture. 10.0.0.10 is a client. 10.0.0.106 is a server. It is seen, that client activates PSH flag (push), informing the server side to move the incoming data to application immediately and do not fill a buffer.
Additional question: what is SSH encrypted packets that runs between the sides. I suppose that it is my Eclipse debugger on PC (running server application through the same Ethernet connection) sends them. Am I right?
So, my problem is how to cause `recv() API to return each short message (16, 60, 200 bytes etc.) instead of accumulating them until receiving buffer fills.
TCP is connection oriented and it also maintains the order in which packets are sent and received.
Having said that, in TCP client, you will receive the stream of bytes and not the individual udp message as in UDP. So you will need to send the packet length and marker as the initial bytes.
So client can first find the packet length and then read data till packet length is reached and then expect new packet length.
You can also check for library like netty, zmq to do this extra work
This question already has answers here:
TCP Connection Seems to Receive Incomplete Data
(5 answers)
Closed 3 years ago.
I'm attempting to implement the Remote Frame Buffer protocol using Ada's Sockets library and I'm having trouble controlling the length of the packets that I'm sending.
I'm following the RFC 6143 specification (https://tools.ietf.org/pdf/rfc6143.pdf), see comments in the code for section numbers...
-- Section 7.1.1
String'Write (Comms, Protocol_Version);
Put_Line ("Server version: '"
& Protocol_Version (1 .. 11) & "'");
String'Read (Comms, Client_Version);
Put_Line ("Client version: '"
& Client_Version (1 .. 11) & "'");
-- Section 7.1.2
-- Server sends security types
U8'Write (Comms, Number_Of_Security_Types);
U8'Write (Comms, Security_Type_None);
-- client replies by selecting a security type
U8'Read (Comms, Client_Requested_Security_Type);
Put_Line ("Client requested security type: "
& Client_Requested_Security_Type'Image);
-- Section 7.1.3
U32'Write (Comms, Byte_Reverse (Security_Result));
-- Section 7.3.1
U8'Read (Comms, Client_Requested_Shared_Flag);
Put_Line ("Client requested shared flag: "
& Client_Requested_Shared_Flag'Image);
Server_Init'Write (Comms, Server_Init_Rec);
The problem seems to be (according to wireshark) that my calls to the various 'Write procedures are causing bytes to queue up on the socket without getting sent.
Consequently two or more packet's worth of data are being sent as one and causing malformed packets. Sections 7.1.2 and 7.1.3 are being sent consecutively in one packet instead of being broken into two.
I had wrongly assumed that 'Reading from the socket would cause the outgoing data to be flushed out, but that does not appear to be the case.
How do I tell Ada's Sockets library "this packet is finished, send it right now"?
To enphasize https://stackoverflow.com/users/207421/user207421 comment:
I'm not a protocols guru, but from my own experience, the usage of TCP (see RFC793) is often misunderstood.
The problem seems to be (according to wireshark) that my calls to the various 'Write procedures are causing bytes to queue up on the socket without getting sent.
Consequently two or more packet's worth of data are being sent as one and causing malformed packets. Sections 7.1.2 and 7.1.3 are being sent consecutively in one packet instead of being broken into two.
In short, TCP is not message-oriented.
Using TCP, sending/writing to socket results only append data to the TCP stream. The socket is free to send it in one exchange or several, and if you have lengthy data to send and message oriented protocol to implement on top of TCP, you may need to handle message reconstruction. Usually, an end of message special sequence of characters is added at the end of the message.
Processes transmit data by calling on the TCP and passing buffers of data as arguments. The TCP packages the data from these buffers into segments and calls on the internet module to transmit each segment to the destination TCP. The receiving TCP places the data from a segment into the receiving user's buffer and notifies the receiving user. The TCPs include control information in the segments which they use to ensure reliable ordered data transmission.
See also https://stackoverflow.com/a/11237634/7237062, quoting:
TCP is a stream-oriented connection, not message-oriented. It has no
concept of a message. When you write out your serialized string, it
only sees a meaningless sequence of bytes. TCP is free to break up
that stream up into multiple fragments and they will be received at
the client in those fragment-sized chunks. It is up to you to
reconstruct the entire message on the other end.
In your scenario, one would typically send a message length prefix.
This way, the client first reads the length prefix so it can then know
how large the incoming message is supposed to be.
or TCP Connection Seems to Receive Incomplete Data, quoting:
The recv function can receive as little as 1 byte, you may have to call it multiple times to get your entire payload. Because of this, you need to know how much data you're expecting. Although you can signal completion by closing the connection, that's not really a good idea.
Update:
I should also mention that the send function has the same conventions as recv: you have to call it in a loop because you cannot assume that it will send all your data. While it might always work in your development environment, that's the kind of assumption that will bite you later.
I'm using Wireshark packet analyzer & when I filter for all "Websocket" packets I see what I am sending /receiving to the host. When I check individual packets mine always show as [MASKED], but you can 'Umask Payload' which shows the data in clear text that looks like this:
<IC sid="52ccc752-6080-4668-8f55-662020d83979" msqid="120l93l9l114l30l104"/>
However, if I 'Follow TCP stream & look at that same packet, the data shows up as encoded in some way like this:
....K#....../...y#..|...}...f...s...~...}...{G..r...kN.."G..z...r...'...'...z...d.
The problem is all Websocket packets I receive from the host come as encoded, it is NOT SSL & I can't figure out how to decode them, I have no idea what they are even encoded as (but yet my browser can decode it).
I assume that whatever method they are coming back to me as encoded data is the same method that my data is encoded when I use 'Follow TCP stream'.
Can someone please help me figure out how to decode the data the host is sending me? See host data below
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Client to server data is XORed with a mask (included in the dataframe). Some people suggest this is in order to throw off bad caching mechanisms responding to new websocket requests with server messages from older sessions. The masking makes sure that even messages containing identical data will appear differently to applications that do not understand websockets.
Also note that there are many different size options for the headers themselves.
Refer to RFC 6455 Section 5 which defines the masking/unmasking process for payloads sent from the client to the server.
https://www.rfc-editor.org/rfc/rfc6455
If you find any freeware VBA code to do the job of forming packets let me know! :-)
I am wondering what is happening between the creating a TCP packet and a [Ethernet[IP[TCP-packet]]] leaving the network adapter.
When i use for example a TCP program and want to send a single packet ( or could be more in fact TCP using byte streaming).
So i set up in any language a function called socket(...);
So my OS, refering to any documenation, creating me an interface with a specified port on which I can receive and send data over.
And if I create a TCP package (for example sendto(...), it will be send to the socket.
But what kind processes are done now [1], until my packet will leave the network adapter with an Ethernet + IP Header?
[1]: Where are the following steps happening (OS/Network adapter) and how does it exactly work?
Hope you understand me.. and please correct me if I missunderstood something wrong.
I wrote a TCP socket program,and define a text protocol format like: "length|content",
to make it simple, the "length" is always 1-byte-long and it define the number of bytes of "content"
My problem is:
when attackers send packets like "1|a51",it will stay in tcp's receive buffer
the program will parse it wrong and the next packet would start like "5|1XXXX"，
then the rest of the packets remain in the buffer would all parsed wrong，
how to solve this problem?
If you get garbage, just close the connection. It's not your problem to figure out what they meant, if anything.
instead of length|content only, you also need to provide a checksum, if the checksum is not correct, you should drop the connection to avoid partial receive.
this is a typical problem in tcp protocol, since the tcp is stream based. but just as http, which is an application of tcp protocol, it has a structure of request / response to make sure each end of the connection knows when the data has been fully transferred.
but your scenario is a little bit tricky, since the hacker can only affect the connection of his own. while it cannot change the data from other connections, only if he can control the route / switcher between your application and the users.