How to Send Hex Bytes Over TCP: netcat and xxd vs a GUI

Short answer: on Linux and macOS, echo <hex> | xxd -r -p | nc host port sends exact bytes. On Windows, use PowerShell or Python. When you need TLS, readable binary replies, or the same test many times, a GUI such as InterceptSuite Replay is quicker.

Last updated 6 October 2026.

netcat and xxd (Linux, macOS)

xxd -r -p turns a plain hex string into raw bytes, and nc sends them.

echo '2a310d0a24340d0a50494e470d0a' | xxd -r -p | nc 127.0.0.1 6379

That sends the Redis PING command. To see the reply as hex too:

echo '2a310d0a24340d0a50494e470d0a' | xxd -r -p | nc -w 2 127.0.0.1 6379 | xxd

Common problems:

• nc exits before the reply. Add a timeout such as -w 2, or keep stdin open with (echo ... | xxd -r -p; sleep 1) | nc .... Flags differ between netcat versions, so check nc -h.
• A newline gets added. echo appends \n. Use printf '%s' '2a31...' or echo -n.
• Spaces in the hex. xxd -r -p ignores whitespace, so 2a 31 0d 0a works too.

PowerShell (Windows)

$bytes = [byte[]](0x2a,0x31,0x0d,0x0a,0x24,0x34,0x0d,0x0a,0x50,0x49,0x4e,0x47,0x0d,0x0a)
$client = New-Object System.Net.Sockets.TcpClient('127.0.0.1', 6379)
$stream = $client.GetStream()
$stream.Write($bytes, 0, $bytes.Length)
Start-Sleep -Milliseconds 300
$buf = New-Object byte[] 4096
$n = $stream.Read($buf, 0, $buf.Length)
($buf[0..($n-1)] | ForEach-Object { '{0:x2}' -f $_ }) -join ' '
$client.Close()

Python (any system)

import socket

payload = bytes.fromhex("2a310d0a24340d0a50494e470d0a")
with socket.create_connection(("127.0.0.1", 6379), timeout=3) as s:
    s.sendall(payload)
    print(s.recv(4096).hex(" "))

Where command-line tools get awkward

• TLS. nc sends plain bytes. For TLS you switch to openssl s_client and its quirks.
• Binary replies. You have to pipe through xxd to read them.
• Repeating a test. Editing one byte means editing and re-running a long command.
• UDP and DTLS. Possible with nc -u, but DTLS needs other tools.
• Comparing runs. Shell history is not a record you can reopen.

The same test in Replay

1. Open Replay and click + New Session.
2. Enter host 127.0.0.1, port 6379, protocol TCP.
3. Open the Hex tab and enter 2a 31 0d 0a 24 34 0d 0a 50 49 4e 47 0d 0a.
4. Click Send. The reply appears in the conversation panel, with Raw and Hex views.
5. Change a byte and send again. Use another tab to keep the first result.

For TLS, choose TLS and set the ALPN value. For UDP, choose UDP. Replay sends the payload exactly as shown. See the Replay docs and the hex editor.

Which to use

Situation	Use
Quick plaintext check on a Linux box	netcat and xxd
Windows, no tools installed	PowerShell
Automated test in a repo	Python script
TLS, binary replies, repeated edits, saved sessions	Replay

Related

• Postman for raw TCP, UDP and TLS
• Debug a custom binary protocol
• Packet Sender alternative

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