Here's a summary you can send to the Windows agent:

---

**DIAGNOSTIC SUMMARY: Cannot connect to Debian server (10.1.2.1) from Mac (10.1.2.7)**

### WHAT WORKS
- ✅ Traffic between Mac (10.1.2.7) and Windows (10.1.3.12) flows fine (RDP works)
- ✅ TCP traffic flows through Debian server as gateway between 10.1.2.x and 10.1.3.x subnets
- ✅ Existing SSH connections to 10.1.2.1 show in lsof as ESTABLISHED (but hung)

### WHAT FAILS
- ❌ **NEW** TCP connections to 10.1.2.1 fail with "No route to host"
- ❌ Ping to 10.1.2.1 fails: "sendto: No route to host"
- ❌ SSH to 10.1.2.1: "No route to host"
- ❌ HTTP to 10.1.2.1: "Failed to connect to port 80: No route to host"
- ❌ All protocols fail for NEW connections

### WEIRD: nmap from Mac shows server is UP with all ports open:
```
nmap -sT -Pn 10.1.2.1
22/tcp   open  ssh
80/tcp   open  http
443/tcp  open  https
3306/tcp open  mysql
3389/tcp open  ms-wbt-server
5432/tcp open  postgresql
... (many more)
```

### MAC ROUTING TABLE (current)
```
default    192.168.88.1    UGScg    en1       (Wi-Fi)
default    10.1.2.1       UGScIg   en0       (Ethernet)
10.1.2/24  link#7         UCS     en0
```

### QUESTIONS TO INVESTIGATE ON DEBIAN SERVER

1. **Check firewall rules** - Is Mac (10.1.2.7) specifically blocked?
   ```bash
   sudo iptables -L -n
   sudo iptables -L INPUT -n
   sudo ip6tables -L -n
   ```

2. **Check for IP blocking/fail2ban** - Is Mac IP blocked?
   ```bash
   sudo fail2ban-client status
   sudo iptables -L -n | grep 10.1.2.7
   ```

3. **Check SSH config** - Any restrictions?
   ```bash
   sudo cat /etc/ssh/sshd_config | grep -iE "listen|permit|deny|allow"
   ```

4. **Check logs** - Any blocks logged?
   ```bash
   sudo tail -50 /var/log/auth.log
   sudo tail -50 /var/log/syslog
   ```

5. **Check network interfaces** - Is SSH listening on all interfaces?
   ```bash
   sudo ss -tlnp | grep :22
   sudo netstat -tlnp | grep :22
   ```

6. **Test from Debian server itself** - Can you SSH to localhost?
   ```bash
   ssh admin@127.0.0.1
   ```

---

**The fact that established connections work but new ones fail suggests either:**
- A firewall on the Debian server blocking new connections from Mac specifically
- Some rate-limiting or fail2ban blocking the Mac
- The server's SSH daemon is misconfigured

Please run these commands on the Debian server and share the output.