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TCP connection test between Windows and Ubuntu

Popularity：488 ℃/2025-03-10 23:44:48

The reason is that I want to use Synergy on Ubuntu, but I found that I can't connect to you no matter how I love you. After fiddling for a long time, I found that it seemed that Ubuntu monitors, Windows tested TCP connections and was always unsuccessful, but it was successful. Perhaps the problem is here. I will record it before I have solved it.

Basic situation

Two machines, one running Ubuntu 22.04 and the other running Windows 10. The machine is connected directly with a network cable, and the Ubuntu end is manually equipped with the address 192.168.1.1/24, and the other side is equipped with 192.168.1.2/24.

  • The two can ping
  • It can ensure that the Ubuntu side firewall is not turned on, and iptables is also empty; it can almost be confirmed that the Windows side firewall is normal
  • When both machines were running Windows 10, Synergy could be used normally, indicating that the 24800 port on Windows could be established normally.

Windows Server ← Ubuntu Client

Windows side Powershell run script waiting for connection

$listener = []::new([]::Any, 24800)
$()
Write-Host "Listening on port 24800..."
$client = $()
Write-Host "Connection received!"
$stream = $()
$reader = New-Object ($stream)
while ($) {
    $data = $()
    if ($data) {
        Write-Host "Received: $data"
    }
}
$()
$()

Ubuntu uses ncat to send requests

nc 192.168.1.2 24800

Establish a normal connection, and output any windows side transmitted in the terminal

Ubuntu Server ← Windows Client

Ubuntu side uses ncat to listen to port

nc -l 24800

At this time, check that the operation of nc is not abnormal, and the inspection behavior is as follows:

  • Execute at this timess -tlnp | grep 24800Check the port status and get feedbackLISTEN 0 1 0.0.0.0:24800 0.0.0.0:* users:(("nc",pid=30444,fd=3)), prove that nc has a 24800 port that is listening carefully 0.0.0.0.
    LISTEN 0      1            0.0.0.0:24800      0.0.0.0:*    users:(("nc",pid=30444,fd=3))
  • Running nownc 127.0.0.1 24800Try to connect yourself, find that you can indeed connect and transmit data correctly, prove that NC can listen correctly and establish connection

However, if you try to establish a connection with a Windows side Test-NetConnection or use a script, it cannot succeed

Test-NetConnection 192.168.1.1 -Port 24800

During this process, I tried to enable tcpdump on the Ubuntu side to view the packets. I found that there were indeed SYN packets coming in. It should be that the Ubuntu side did not give a response; probably the problem lies in some Ubuntu configurations.

sudo tcpdump -i any port 24800 -nn

 ## There is a monitoring content
 tcpdump: data link type LINUX_SLL2
 tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
 listening on any, link-type LINUX_SLL2 (Linux cooked v2), snapshot length 262144 bytes
 23:13:52.328526 enp38s0 In IP 192.168.1.2.6786 > 192.168.1.1.24800: Flags [S], seq 4189060361, win 64240, options [mss 1460,nop,wscale 8,sackOK,TS val 1190883235 ecr 0], length 0
 23:13:53.338401 enp38s0 In IP 192.168.1.2.6786 > 192.168.1.1.24800: Flags [S], seq 4189060361, win 64240, options [mss 1460,nop,wscale 8,sackOK,TS val 1190884245 ecr 0], length 0
 23:13:55.349621 enp38s0 In IP 192.168.1.2.6786 > 192.168.1.1.24800: Flags [S], seq 4189060361, win 64240, options [mss 1460,nop,wscale 8,sackOK,TS val 1190886256 ecr 0], length 0
 23:13:59.358816 enp38s0 In IP 192.168.1.2.6786 > 192.168.1.1.24800: Flags [S], seq 4189060361, win 64240, options [mss 1460,nop,wscale 8,sackOK,TS val 1190890265 ecr 0], length 0
 23:14:07.373485 enp38s0 In IP 192.168.1.2.6786 > 192.168.1.1.24800: Flags [S], seq 4189060361, win 64240, options [mss 1460,nop,wscale 8,sackOK,TS val 1190898280 ecr 0], length 0
 ^C
 5 packets captured
 5 packets received by filter
 0 packets dropped by kernel
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