MajorLinux
6592eb4fea
- Fixed 4 broken markdown links (bad relative paths in See Also sections) - Corrected n8n port binding to 127.0.0.1:5678 (matches actual deployment) - Updated SnapRAID article with actual majorhome paths (/majorRAID, disk1-3) - Converted 67 Obsidian wikilinks to relative markdown links or plain text - Added YAML frontmatter to 35 articles missing it entirely - Completed frontmatter on 8 articles with missing fields Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
129 lines
3.9 KiB
Markdown
129 lines
3.9 KiB
Markdown
---
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title: "Mail Client Stops Receiving: Fail2ban IMAP Self-Ban"
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domain: troubleshooting
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category: networking
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tags: [fail2ban, imap, dovecot, email, self-ban]
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status: published
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created: 2026-04-02
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updated: 2026-04-02
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---
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# Mail Client Stops Receiving: Fail2ban IMAP Self-Ban
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## 🛑 Problem
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A mail client stops receiving new email on one device while other devices (e.g., phone on cellular) continue to work normally. The mail server is healthy — Postfix is delivering to maildir and Dovecot is running — but the affected device receives no new messages and is never prompted for credentials.
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---
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## 🔍 Diagnosis
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### Step 1 — Confirm the mail server is delivering
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```bash
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ssh root@<mailserver> "tail -20 /var/log/maillog | grep 'status=sent'"
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```
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If you see `status=sent (delivered to maildir)`, the server is working. The issue is on the client side.
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---
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### Step 2 — Check if the client is connecting at all
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```bash
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ssh root@<mailserver> "grep '<client_ip>' /var/log/maillog | tail -10"
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```
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If there are **zero results**, the client is not reaching the server at all — not a credentials or sync issue.
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---
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### Step 3 — Get the client's current public IP
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Run this on the affected machine:
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```bash
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curl -s https://api.ipify.org
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```
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---
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### Step 4 — Check if that IP is banned by Fail2ban
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```bash
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ssh root@<mailserver> "fail2ban-client status postfix-sasl"
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ssh root@<mailserver> "fail2ban-client status dovecot-invalid"
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```
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Look for the IP in the `Banned IP list`. If it's there, that's your problem.
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---
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### Step 5 — Find when the ban was applied
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```bash
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ssh root@<mailserver> "grep '<client_ip>' /var/log/fail2ban.log | tail -20"
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```
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This shows the exact timestamps of auth failures and the ban event.
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---
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## ✅ Fix
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Unban the IP from all relevant jails:
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```bash
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fail2ban-client set postfix-sasl unbanip <IP>
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fail2ban-client set dovecot-invalid unbanip <IP>
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```
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Mail should resume immediately without restarting any services.
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---
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## 🔁 Why This Happens
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| Symptom | Cause |
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|---|---|
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| One device stops getting mail, others don't | Each device has a different public IP. Only the banned IP is blocked. |
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| No auth prompt in the mail client | Fail2ban issues a TCP `REJECT` — the client sees a connection error, not an auth failure, so it doesn't prompt for new credentials. It silently retries on a timer. |
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| Multiple failed attempts before ban | The mail client retried with a previously valid session token or stale auth state, triggering the `maxretry` threshold. |
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| Ban on `postfix-sasl` blocks IMAP (port 993) | The `postfix-sasl` jail monitors ports 25, 465, 587, 143, 993, 110, and 995 — not just SMTP. A failed SMTP auth attempt can ban access to IMAP as well. |
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---
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## ⚠️ Key Notes
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- **Fail2ban `postfix-sasl` covers all mail ports** — a single jail can lock out SMTP *and* IMAP simultaneously.
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- **The affected device's IP is never logged in Dovecot** once banned — the rejection happens at the iptables/Fail2ban layer before Dovecot sees the connection.
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- **Tailscale doesn't help** if the mail client connects via the public interface — the ban applies to the public IP, not the Tailscale IP.
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- Check when the ban happened: `grep '<IP>' /var/log/fail2ban.log` shows exact timestamps.
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---
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## 🔎 Quick Diagnostic Commands
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```bash
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# Get your current public IP (run on affected machine)
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curl -s https://api.ipify.org
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# Check all Fail2ban jail statuses
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fail2ban-client status
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# Check a specific jail for a banned IP
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fail2ban-client status postfix-sasl
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fail2ban-client status dovecot-invalid
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# Unban from a specific jail
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fail2ban-client set postfix-sasl unbanip <IP>
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fail2ban-client set dovecot-invalid unbanip <IP>
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# Unban from all jails at once
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for jail in $(fail2ban-client status | grep "Jail list" | sed 's/.*://;s/,/ /g'); do
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fail2ban-client set $jail unbanip <IP> 2>/dev/null && echo "Unbanned from $jail"
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done
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# Find when a specific IP was banned
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grep '<IP>' /var/log/fail2ban.log | tail -20
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```
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