Initial commit: VPS proxy server configs and security audit
This commit is contained in:
Gan, Jimmy
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# Issue Fixing Report & VPS Security Audit
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**Date:** 2026-02-14
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**VPS:** `oracle-tokyo` (158.101.140.85, Ubuntu 24.04, Oracle Cloud Japan)
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---
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## Part 1: Tailscale Exit Node Fix ✅
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### Symptom
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With Tailscale exit node on, Claude Code / Antigravity / Google failed. Bing worked.
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### Root Cause
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**GFW DNS poisoning.** Tailscale's DNS servers (Cloudflare `1.1.1.1`, Google `8.8.8.8`) were queried as plaintext UDP, allowing the GFW to intercept and return poisoned IPs.
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### Fixes Applied
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| # | Change | Location | Persistent? |
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|---|---|---|---|
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| 1 | UFW forward policy → `ACCEPT` | VPS `/etc/default/ufw` | ✅ |
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| 2 | NAT MASQUERADE for `ens3` | VPS `/etc/ufw/before.rules` | ✅ |
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| 3 | DNS nameserver → `100.70.115.1` (AdGuard Home) | Tailscale Admin DNS | ✅ |
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| 4 | "Use with exit node" → ON | Tailscale Admin DNS | ✅ |
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| 5 | Removed Cloudflare/Google public DNS | Tailscale Admin DNS | ✅ |
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---
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## Part 2: VPS Security Audit
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### ✅ Things That Are Good
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| Item | Status |
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|---|---|
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| SSH root login | Disabled ✅ |
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| SSH password auth | Disabled ✅ |
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| fail2ban | Active, 108 IPs banned historically ✅ |
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| Unattended upgrades | Enabled, last update Feb 13 ✅ |
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| `/etc/shadow` permissions | Properly restricted (`640`) ✅ |
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| rpcbind | Disabled ✅ |
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| Kernel | `6.8.0-1026-oracle` (recent) ✅ |
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| INPUT policy | `DROP` (deny by default) ✅ |
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---
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### ⚠️ Security Issues Found
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#### 🔴 HIGH: AdGuard Home DNS exposed publicly on `0.0.0.0:53`
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AdGuard Home binds DNS to **all interfaces** including the public IP `158.101.140.85:53`. This makes your VPS an **open DNS resolver**, which can be abused for DNS amplification DDoS attacks.
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**Fix:**
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```bash
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# Edit /opt/AdGuardHome/AdGuardHome.yaml
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# Change:
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# bind_hosts:
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# - 0.0.0.0
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# To:
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# bind_hosts:
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# - 100.70.115.1
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# - 127.0.0.1
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sudo systemctl restart AdGuardHome
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```
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> [!CAUTION]
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> An open DNS resolver is one of the most commonly exploited misconfigurations on the internet. This should be fixed immediately. OCI's security list may be blocking port 53 inbound, but defense in depth is critical.
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---
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#### 🟡 MEDIUM: X11Forwarding enabled in SSH
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`X11Forwarding yes` is set in `/etc/ssh/sshd_config`. This is unnecessary for a headless server and expands the attack surface.
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**Fix:**
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```bash
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sudo sed -i 's/^X11Forwarding yes/X11Forwarding no/' /etc/ssh/sshd_config
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sudo systemctl restart sshd
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```
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---
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#### 🟡 MEDIUM: Xray config world-readable (contains private key)
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`/usr/local/etc/xray/config.json` has permissions `-rw-r--r--` and contains the Reality private key.
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**Fix:**
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```bash
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sudo chmod 600 /usr/local/etc/xray/config.json
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```
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---
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#### 🟡 MEDIUM: Port 80 open in iptables (nothing listening)
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The INPUT chain accepts traffic on port 80 (386K packets), but nothing is listening. This is leftover from nginx and should be removed to reduce attack surface.
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**Fix:**
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```bash
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# Find and delete the rule
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sudo iptables -D INPUT -p tcp --dport 80 -j ACCEPT
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```
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---
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#### 🟡 MEDIUM: WireGuard PostUp/PostDown references wrong interface `eth0`
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In `/etc/wireguard/wg0.conf`, the NAT rules reference `eth0` but the actual interface is `ens3`. WireGuard NAT is currently broken.
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**Fix:**
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```bash
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# Edit /etc/wireguard/wg0.conf
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# Replace all 'eth0' with 'ens3' in PostUp and PostDown lines
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sudo wg-quick down wg0 && sudo wg-quick up wg0
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```
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---
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#### 🟢 LOW: `opc` user has a shell
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The default Oracle Cloud `opc` user still has `/bin/sh`. It has no SSH keys but exists as a potential entry point.
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**Fix:**
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```bash
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sudo usermod -s /usr/sbin/nologin opc
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```
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---
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#### 🟢 LOW: `ubuntu` user has passwordless sudo
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The cloud-init user `ubuntu` has `NOPASSWD:ALL` sudo. If you don't use it, consider disabling.
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**Fix:**
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```bash
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sudo usermod -s /usr/sbin/nologin ubuntu
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# Or remove the sudoers entry:
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# sudo rm /etc/sudoers.d/90-cloud-init-users
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```
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---
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### Summary of Findings
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| Severity | Issue | Status |
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|---|---|---|
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| 🔴 HIGH | AdGuard DNS on `0.0.0.0:53` (open resolver) | ✅ Fixed — bound to `100.70.115.1` + `127.0.0.1` |
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| 🟡 MED | X11Forwarding enabled | ✅ Fixed — disabled, SSH restarted |
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| 🟡 MED | Xray config world-readable (has private key) | ✅ Fixed — `chmod 600` |
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| 🟡 MED | Port 80 open in iptables (unused) | ✅ Fixed — rule removed |
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| 🟡 MED | WireGuard uses wrong interface `eth0` | ✅ Fixed — changed to `ens3` |
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| 🟢 LOW | `opc` user has shell | ✅ Fixed — set to nologin |
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| 🟢 LOW | `ubuntu` user has passwordless sudo | ✅ Fixed — set to nologin |
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> [!TIP]
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> If you want, I can apply all these fixes for you right now via SSH.
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