When working with Netcat (nc) for network testing or debugging, socket binding failures can be particularly frustrating. The error messages we're seeing suggest multiple potential issues:

$ nc -vvv -l 5349
5349: inverse host lookup failed: Unknown host
nnetfd reuseport failed : Protocol not available
listening on [any] 38541 ... : Protocol not available

From the provided information, we can note several important details:

• Netcat works fine on Ubuntu 14.04 but fails on Debian 8.0 Jessie
• The port (5349) is confirmed available via ss -tnl | grep 5349
• The IP address (10.1.6.189) is correctly configured on eth0
• The server is behind NAT
• Both specific IP and wildcard binding attempts fail

Several factors could contribute to this behavior:

1. Netcat Version Differences

Different Linux distributions may ship with different Netcat implementations. Let's check which variant we're using:

$ nc -h
# or
$ dpkg -l | grep netcat

The error message "nnetfd reuseport failed" suggests this might be the Nmap's Ncat version rather than traditional Netcat.

2. Protocol Support Issues

The "Protocol not available" error often indicates:

• Missing kernel support for socket options
• Firewall interference
• Network namespace restrictions

3. NAT Configuration Problems

While NAT shouldn't prevent binding, it can affect:

• Port forwarding rules
• Connection tracking
• IP masquerading behavior

Option 1: Try Traditional Netcat

On Debian, install the classic implementation:

$ sudo apt-get install netcat-traditional
$ sudo update-alternatives --config nc

Then retry binding:

$ nc.traditional -l -p 5349 -vvv

Option 2: Disable Advanced Socket Options

For Ncat versions, try disabling reuseport:

$ ncat -l 5349 -vvv --no-shutdown

Option 3: Verify Kernel Modules

Check relevant kernel modules are loaded:

$ lsmod | grep tcp
$ lsmod | grep ipv6

Option 4: Test Without NAT

If possible, test on a non-NATed interface or IP:

$ nc -l 127.0.0.1 5349 -vvv

For deeper investigation, consider these approaches:

1. Network Tracing

Use strace to see system calls:

$ strace nc -l 5349

2. Alternative Tools

Test with other networking tools:

$ socat - TCP-LISTEN:5349,reuseaddr,fork
$ python -m http.server 5349

3. Firewall Check

Verify iptables/nftables rules:

$ sudo iptables -L -n -v
$ sudo nft list ruleset

Based on the symptoms, the most likely solutions are:

1. Switch to traditional Netcat implementation
2. Use simpler socket options (--no-shutdown or --keep-open)
3. Verify kernel network protocol support

Remember that NAT typically affects routing, not binding, so the core issue likely lies with the Netcat implementation or system configuration rather than the NAT itself.

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When working with netcat (nc) on Linux systems, socket binding issues can be particularly frustrating. While basic listening works fine on my Ubuntu 14.04 workstation:

$ nc -l 5349 -vvv
Listening on [0.0.0.0] (family 0, port 5349)

The same command fails spectacularly on a Debian 8.0 server behind NAT:

$ nc -vvv -l 5349
5349: inverse host lookup failed: Unknown host
nnetfd reuseport failed : Protocol not available
listening on [any] 38541 ... : Protocol not available

First, let's verify the port availability:

$ ss -tnl | grep 5349
(empty)

And confirm the network interface configuration:

$ ip addr list
2: eth0:  mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 1e:89:93:f1:96:a4 brd ff:ff:ff:ff:ff:ff
    inet 10.1.6.189/19 brd 10.1.31.255 scope global eth0
    inet6 fe80::1c89:93ff:fef1:96a4/64 scope link 
       valid_lft forever preferred_lft forever

The error Protocol not available typically indicates one of several possibilities:

• Kernel-level socket options mismatch
• Netcat version differences between systems
• NAT/firewall interference
• Missing kernel modules or capabilities

Debian Jessie ships with the BSD variant of netcat by default. Try installing the traditional version:

$ sudo apt-get install netcat-traditional
$ sudo update-alternatives --config nc

Then test with the traditional syntax:

$ nc -l -p 5349 -vvv

If the protocol error persists, try these kernel parameter adjustments:

$ sudo sysctl -w net.ipv4.tcp_fin_timeout=30
$ sudo sysctl -w net.ipv4.tcp_tw_reuse=1

When netcat fails, consider these alternatives:

Using socat:

$ socat TCP-LISTEN:5349,reuseaddr,fork -

Using Python as a quick listener:

$ python -c 'import socket; s=socket.socket(); s.bind(("",5349)); s.listen(1); conn,addr=s.accept()'

For servers behind NAT, additional steps may be required:

1. Configure port forwarding on the NAT device
2. Ensure proper iptables rules:

$ sudo iptables -A INPUT -p tcp --dport 5349 -j ACCEPT

After applying fixes, verify with:

$ netstat -tulnp | grep 5349
$ ss -tulnp | grep 5349

And test connectivity from another machine:

$ nc -zv your.server.ip 5349