My GO version is 1.1.1
the sever recieved messages after connection close, but NoDelay was setted.
Is there something wrong
addr, _ := net.ResolveTCPAddr("tcp", "localhost:5432")
conn, err := net.DialTCP("tcp", nil, addr)
defer conn.Close()
if err != nil {
fmt.Println("connect fail")
return
}
err = conn.SetNoDelay(true)
if err != nil {
fmt.Println(err.Error())
}
for {
var message string
_, err := fmt.Scanln(&message)
if err != nil && err.Error() != "unexpected newline" {
fmt.Println("input finished", err)
break
}
if message == "" {
fmt.Println("no input, end")
break
}
// message = fmt.Sprintf("%s\n",message)
//fmt.Fprintf(conn, message) // send immediately but following message won't send any more
conn.Write([]byte(message)) // won't send until connection close
}
There doesn't seem to be anything vitally wrong with your code so I'm guessing the error is on the server end.
If you create a local TCP server on port 5432 you can test this.
Try running the below server code and then test your client code against it. It just echos all received data to stdout.
package main
import (
"io"
"log"
"net"
"os"
)
func main() {
l, err := net.Listen("tcp", "localhost:5432")
if err != nil {
log.Fatal(err)
}
defer l.Close()
for {
conn, err := l.Accept()
if err != nil {
log.Fatal(err)
}
go func(c net.Conn) {
defer c.Close()
io.Copy(os.Stdout, c)
}(conn)
}
}
You should see each line sent to the client printed (without the newline) as soon as you hit enter.
the problem is on the server end.
func handleConnection(conn net.Conn) {
// I didn't put it in for loop
message, err := bufio.NewReader(conn).ReadString('\n')
}
Related
Talk is cheap, so here we go the simple code:
package main
import (
"fmt"
"time"
"net"
)
func main() {
addr := "127.0.0.1:8999"
// Server
go func() {
tcpaddr, err := net.ResolveTCPAddr("tcp4", addr)
if err != nil {
panic(err)
}
listen, err := net.ListenTCP("tcp", tcpaddr)
if err != nil {
panic(err)
}
for {
if conn, err := listen.Accept(); err != nil {
panic(err)
} else if conn != nil {
go func(conn net.Conn) {
buffer := make([]byte, 1024)
n, err := conn.Read(buffer)
if err != nil {
fmt.Println(err)
} else {
fmt.Println(">", string(buffer[0 : n]))
}
conn.Close()
}(conn)
}
}
}()
time.Sleep(time.Second)
// Client
if conn, err := net.Dial("tcp", addr); err == nil {
for i := 0; i < 2; i++ {
_, err := conn.Write([]byte("hello"))
if err != nil {
fmt.Println(err)
conn.Close()
break
} else {
fmt.Println("ok")
}
// sleep 10 seconds and re-send
time.Sleep(10*time.Second)
}
} else {
panic(err)
}
}
Ouput:
> hello
ok
ok
The Client writes to the Server twice. After the first read, the Server closes the connection immediately, but the Client sleeps 10 seconds and then re-writes to the Server with the same already closed connection object(conn).
Why can the second write succeed (returned error is nil)?
Can anyone help?
PS:
In order to check if the buffering feature of the system affects the result of the second write, I edited the Client like this, but it still succeeds:
// Client
if conn, err := net.Dial("tcp", addr); err == nil {
_, err := conn.Write([]byte("hello"))
if err != nil {
fmt.Println(err)
conn.Close()
return
} else {
fmt.Println("ok")
}
// sleep 10 seconds and re-send
time.Sleep(10*time.Second)
b := make([]byte, 400000)
for i := range b {
b[i] = 'x'
}
n, err := conn.Write(b)
if err != nil {
fmt.Println(err)
conn.Close()
return
} else {
fmt.Println("ok", n)
}
// sleep 10 seconds and re-send
time.Sleep(10*time.Second)
} else {
panic(err)
}
And here is the screenshot:
attachment
There are several problems with your approach.
Sort-of a preface
The first one is that you do not wait for the server goroutine
to complete.
In Go, once main() exits for whatever reason,
all the other goroutines still running, if any, are simply
teared down forcibly.
You're trying to "synchronize" things using timers,
but this only works in toy situations, and even then it
does so only from time to time.
Hence let's fix your code first:
package main
import (
"fmt"
"log"
"net"
"time"
)
func main() {
addr := "127.0.0.1:8999"
tcpaddr, err := net.ResolveTCPAddr("tcp4", addr)
if err != nil {
log.Fatal(err)
}
listener, err := net.ListenTCP("tcp", tcpaddr)
if err != nil {
log.Fatal(err)
}
// Server
done := make(chan error)
go func(listener net.Listener, done chan<- error) {
for {
conn, err := listener.Accept()
if err != nil {
done <- err
return
}
go func(conn net.Conn) {
var buffer [1024]byte
n, err := conn.Read(buffer[:])
if err != nil {
log.Println(err)
} else {
log.Println(">", string(buffer[0:n]))
}
if err := conn.Close(); err != nil {
log.Println("error closing server conn:", err)
}
}(conn)
}
}(listener, done)
// Client
conn, err := net.Dial("tcp", addr)
if err != nil {
log.Fatal(err)
}
for i := 0; i < 2; i++ {
_, err := conn.Write([]byte("hello"))
if err != nil {
log.Println(err)
err = conn.Close()
if err != nil {
log.Println("error closing client conn:", err)
}
break
}
fmt.Println("ok")
time.Sleep(2 * time.Second)
}
// Shut the server down and wait for it to report back
err = listener.Close()
if err != nil {
log.Fatal("error closing listener:", err)
}
err = <-done
if err != nil {
log.Println("server returned:", err)
}
}
I've spilled a couple of minor fixes
like using log.Fatal (which is
log.Print + os.Exit(1)) instead of panicking,
removed useless else clauses to adhere to the coding standard of keeping the main
flow where it belongs, and lowered the client's timeout.
I have also added checking for possible errors Close on sockets may return.
The interesting part is that we now properly shut the server down by closing the listener and then waiting for the server goroutine to report back (unfortunately Go does not return an error of a custom type from net.Listener.Accept in this case so we can't really check that Accept exited because we've closed the listener).
Anyway, our goroutines are now properly synchronized, and there is
no undefined behaviour, so we can reason about how the code works.
Remaining problems
Some problems still remain.
The more glaring is you making wrong assumption that TCP preserves
message boundaries—that is, if you write "hello" to the client
end of the socket, the server reads back "hello".
This is not true: TCP considers both ends of the connection
as producing and consuming opaque streams of bytes.
This means, when the client writes "hello", the client's
TCP stack is free to deliver "he" and postpone sending "llo",
and the server's stack is free to yield "hell" to the read
call on the socket and only return "o" (and possibly some other
data) in a later read.
So, to make the code "real" you'd need to somehow introduce these
message boundaries into the protocol above TCP.
In this particular case the simplest approach would be either
using "messages" consisting of a fixed-length and agreed-upon
endianness prefix indicating the length of the following
data and then the string data itself.
The server would then use a sequence like
var msg [4100]byte
_, err := io.ReadFull(sock, msg[:4])
if err != nil { ... }
mlen := int(binary.BigEndian.Uint32(msg[:4]))
if mlen < 0 {
// handle error
}
if mlen == 0 {
// empty message; goto 1
}
_, err = io.ReadFull(sock, msg[5:5+mlen])
if err != nil { ... }
s := string(msg[5:5+mlen])
Another approach is to agree on that the messages do not contain
newlines and terminate each message with a newline
(ASCII LF, \n, 0x0a).
The server side would then use something like
a usual bufio.Scanner loop to get
full lines from the socket.
The remaining problem with your approach is to not dealing with
what Read on a socket returns: note that io.Reader.Read
(that's what sockets implement, among other things) is allowed
to return an error while having had read some data from the
underlying stream. In your toy example this might rightfully
be unimportant, but suppose that you're writing a wget-like
tool which is able to resume downloading of a file: even if
reading from the server returned some data and an error, you
have to deal with that returned chunk first and only then
handle the error.
Back to the problem at hand
The problem presented in the question, I beleive, happens simply because in your setup you hit some TCP buffering problem due to the tiny length of your messages.
On my box which runs Linux 4.9/amd64 two things reliably "fix"
the problem:
Sending messages of 4000 bytes in length: the second call
to Write "sees" the problem immediately.
Doing more Write calls.
For the former, try something like
msg := make([]byte, 4000)
for i := range msg {
msg[i] = 'x'
}
for {
_, err := conn.Write(msg)
...
and for the latter—something like
for {
_, err := conn.Write([]byte("hello"))
...
fmt.Println("ok")
time.Sleep(time.Second / 2)
}
(it's sensible to lower the pause between sending stuff in
both cases).
It's interesting to note that the former example hits the
write: connection reset by peer (ECONNRESET in POSIX)
error while the second one hits write: broken pipe
(EPIPE in POSIX).
This is because when we're sending in chunks worth 4k bytes,
some of the packets generated for the stream manage to become
"in flight" before the server's side of the connection manages
to propagate the information on its closure to the client,
and those packets hit an already closed socket and get rejected
with the RST TCP flag set.
In the second example an attempt to send another chunk of data
sees that the client side already knows that the connection
has been teared down and fails the sending without "touching
the wire".
TL;DR, the bottom line
Welcome to the wonderful world of networking. ;-)
I'd recommend buying a copy of "TCP/IP Illustrated",
read it and experiment.
TCP (and IP and other protocols above IP)
sometimes works not like people expect them to by applying
their "common sense".
I'm developing a fast dns client in go just to mess around with But I'm facing troubles at the time of reading from server responses cause it never arrives and I know it actually did because I have WireShark open and it read the packet.
Here is the code sample(8.8.8.8 is Google DNS and the hex msg is a valid DNS query):
package main
import (
"fmt"
"net"
"encoding/hex"
"bufio"
)
func CheckError(err error) {
if err != nil {
fmt.Println("Error: " , err)
}
}
func main() {
Conn, err := net.Dial("udp", "8.8.8.8:53")
CheckError(err)
defer Conn.Close()
msg, _ := hex.DecodeString("5ab9010000010000000000001072312d2d2d736e2d68357137646e65650b676f6f676c65766964656f03636f6d0000010001")
scanner := bufio.NewScanner(Conn)
buf := []byte(msg)
_, err1 := Conn.Write(buf)
if err1 != nil {
fmt.Println(msg, err1)
}
for scanner.Scan() {
fmt.Println(scanner.Bytes())
}
}
Here you have the proof that it actually arrives:
WireShark Screen Capture
I've testes reading directly from conn with:
func main() {
Conn, err := net.Dial("udp", "8.8.8.8:53")
CheckError(err)
defer Conn.Close()
msg, _ := hex.DecodeString("5ab9010000010000000000001072312d2d2d736e2d68357137646e65650b676f6f676c65766964656f03636f6d0000010001")
buf := []byte(msg)
_, err1 := Conn.Write(buf)
if err1 != nil {
fmt.Println(msg, err1)
}
Reader(Conn)
}
func Reader(conn net.Conn) {
var buf []byte
for {
conn.Read(buf)
fmt.Println(buf)
}
}
You can't use bufio around a UDP connection. UDP is not a stream oriented protocol, so you need to differentiate the individual datagrams yourself, and avoid partial reads to prevent data loss.
In order to read from an io.Reader, you must have space allocated to read into, and you need to use the bytes read value returned from the Read operation. Your example could be reduced to:
conn, err := net.Dial("udp", "8.8.8.8:53")
if err != nil {
log.Fatal(err)
}
defer conn.Close()
msg, _ := base64.RawStdEncoding.DecodeString("WrkBAAABAAAAAAAAEHIxLS0tc24taDVxN2RuZWULZ29vZ2xldmlkZW8DY29tAAABAAE")
resp := make([]byte, 512)
conn.Write(msg)
n, err := conn.Read(resp)
if err != nil {
log.Fatal(err)
}
fmt.Printf("%q\n", resp[:n])
I am trying to connect to two remote mongodb servers using ssh port-forwarding in golang which are used by my graphql server for querying. The intermediary host for the two tunnels is same. So let's say the intermediary host is 123.45.678.678 and the two remote mongodb servers are 1.23.45.67 and 1.23.45.78, I create the tunnels like this,
conn, err := ssh.Dial("tcp", 123.45.678.678, config)
if err != nil {
panic(err)
}
remote1, err := conn.Dial("tcp", "1.23.45.67:27017")
if err != nil {
panic(err)
}
remote2, err := conn.Dial("tcp", "1.23.45.78:27017")
if err != nil {
panic(err)
}
local1, err := net.Listen("tcp", "localhost:27018")
if err != nil {
panic(err)
}
local2, err := net.Listen("tcp", "localhost:27019")
if err != nil {
panic(err)
}
Now i forward traffic from local1 to remote1 and local2 to remote2 like this
go func() {
for {
l, err := local1.Accept()
if err != nil {
panic(err)
}
go func() {
_, err := io.Copy(l, remote1)
if err != nil {
panic(err)
}
}()
go func() {
_, err := io.Copy(remote1, l)
if err != nil {
panic(err)
}
}()
}
}()
go func() {
for {
l, err := local2.Accept()
if err != nil {
panic(err)
}
go func() {
_, err := io.Copy(l, remote2)
if err != nil {
panic(err)
}
}()
go func() {
_, err := io.Copy(remote2, l)
if err != nil {
panic(err)
}
}()
}
}()
And I create two mongo sessions using mgo.Dial and export these two sessions to the graphql whenever this function is called. For some queries ( not all queries, only some complex queries ) which need both the sessions, i see the write: broken pipe error
panic: readfrom tcp 127.0.0.1:27019->127.0.0.1:53128: write tcp 127.0.0.1:27019->127.0.0.1:53128: write: broken pipe
When I debugged this, i figured out that this error occurs when the io.copy happens between l and remote2 in the code snippet above which i guess is due to the disconnection of remote2 tunnel.
The tcpdump showed that the intermediary host is sending the finish flag to the remote2 server after sometime which inturn is leading to the termination of the connection. I am wondering how I can resolve this.
I'm trying to write a golang program to control mpv via issuing commands to a unix socket running at /tmp/mpvsocket.
This is what I've tried so far:
func main() {
c, err := net.Dial("unix", "/tmp/mpvsocket")
if err != nil {
panic(err)
}
defer c.Close()
_, err = c.Write([]byte(`{"command":["quit"]}`))
if err != nil {
log.Fatal("write error:", err)
}
}
This should cause mpv to quit but nothing happens.
This command can be issued via the command line to get the expected results:
echo '{ "command": ["quit"] }' | socat - /tmp/mpvsocket
It uses socat to send the JSON to the socket. How can I send this to the socket using Golang?
Thanks to #AndySchweig in the comments above, I needed a new line after my JSON.
The fixed line:
_, err = c.Write([]byte(`{"command":["quit"]}` + "\n"))
The full block of fixed code:
func main() {
c, err := net.Dial("unix", "/tmp/mpvsocket")
if err != nil {
panic(err)
}
defer c.Close()
_, err = c.Write([]byte(`{"command":["quit"]}` + "\n"))
if err != nil {
log.Fatal("write error:", err)
}
}
I am new to go,
I was trying to prepare client server in go language and tried to write code, but it's not giving any output. It's not giving any error but just listening.
Please someone help me, I want to create authentication system using go where server authenticate client using Username password..
server :
package main
import (
"fmt"
"net"
)
func main() {
service := "0.0.0.0:8080"
tcpAddr, err := net.ResolveTCPAddr("tcp", service)
checkError(err)
listener, err := net.ListenTCP("tcp", tcpAddr)
checkError(err)
for {
conn, err := listener.Accept()
//fmt.Println("Server listerning")
_, err = conn.Read([]byte("HEAD"))
if err != nil {
conn.Close()
}
if err != nil {
continue
}
}
}
func checkError(err error) {
if err != nil {
fmt.Println("Fatal error ", err.Error())
}
}
client :
package main
import (
"bufio"
"fmt"
"net"
"os"
"strings"
)
func main() {
if len(os.Args) != 2 {
fmt.Println("Usage: ", os.Args[0], "host")
os.Exit(1)
}
host := os.Args[1]
conn, err := net.Dial("tcp", host+":8080")
checkError(err)
_, err = conn.Write([]byte("HEAD"))
reader := bufio.NewReader(os.Stdin)
for {
line, err := reader.ReadString('\n')
ftm.Println(err)
line = strings.TrimRight(line, " \t\r\n")
if err != nil {
conn.Close()
break
}
}
}
func checkError(err error) {
if err != nil {
fmt.Println("Fatal error ", err.Error())
}
}
I'm not sure you need to resolve your address in order to listen.
You should be able to do just this :
listener, err := net.Listen("tcp", ":8080")
And you don't seem to do anything with the received bytes server side (you discard the result of Read), which explains why you think you receive nothing.
Note that your code can only handle one connection at a time. You should handle each opened connection in a new goroutine.
Here's an example of client-server communication over TCP in a related question.
I'm not sure exactly what output your expecting, as it seems to me that you are not printing anything out, just reading input in the client. As #dystroy pointed out, the server then discards what it received, only checking for errors in the communication.
Also, it seems that your server should just sit there listening, as you have it in a loop doing just that. If you look in the docs for the net package, it gives an example of how to run a server and client. Here's a sample that I made from that, that worked for me.
Server:
ln, err := net.Listen("tcp", ":8080")
// handle error
for {
conn, err := ln.Accept()
// handle error
var cmd []byte
fmt.Fscan(conn, &cmd)
fmt.Println("Message:", string(cmd))
}
Client:
conn, err := net.Dial("tcp", "127.0.0.1:8080")
// handle error
fmt.Fprintf(conn, "message\n")
Edited to add: The expected result of this program is that you run the server and it sits waiting. You then run the client in a different terminal, which immediately exits, but now the server has printed "Message: message". If you run the client again, the server will print the same message again. I tested this code on my machine (just adding code to respond to errors) and it worked as advertised.