wiki: add SELinux fail2ban execmem fix + pending articles

New article: selinux-fail2ban-execmem-fix.md — custom policy module
for fail2ban grep execmem denial on Fedora 43.

Also includes previously uncommitted:
- n8n-proxy-trust-x-forwarded-for.md
- fail2ban-apache-404-scanner-jail.md updates

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

02-selfhosting/security/selinux-fail2ban-execmem-fix.md

@@ -0,0 +1,86 @@
+# SELinux: Fixing Fail2ban grep execmem Denial on Fedora
+
+## The Problem
+
+After a reboot on Fedora 43, Netdata fires a `selinux_avc_denials` WARNING alert. The audit log shows:
+
+```
+avc: denied { execmem } for comm="grep"
+scontext=system_u:system_r:fail2ban_t:s0
+tcontext=system_u:system_r:fail2ban_t:s0
+tclass=process permissive=0
+```
+
+Fail2ban spawns `grep` to scan log files when its jails start. SELinux denies `execmem` (executable memory) for processes running in the `fail2ban_t` domain. The `fail2ban-selinux` package does not include this permission.
+
+## Impact
+
+- Fail2ban still functions — the denial affects grep's memory allocation strategy, not its ability to run
+- Netdata will keep alerting on every reboot (fail2ban restarts and triggers the denial)
+- No security risk — this is fail2ban's own grep subprocess, not an external exploit
+
+## The Fix
+
+Create a targeted SELinux policy module that allows `execmem` for `fail2ban_t`:
+
+```bash
+cd /tmp
+
+cat > my-fail2ban-grep.te << "EOF"
+module my-fail2ban-grep 1.0;
+
+require {
+    type fail2ban_t;
+    class process execmem;
+}
+
+allow fail2ban_t self:process execmem;
+EOF
+
+# Compile the module
+checkmodule -M -m -o my-fail2ban-grep.mod my-fail2ban-grep.te
+
+# Package it
+semodule_package -o my-fail2ban-grep.pp -m my-fail2ban-grep.mod
+
+# Install at priority 300 (above default policy)
+semodule -X 300 -i my-fail2ban-grep.pp
+```
+
+## Verifying
+
+Confirm the module is loaded:
+
+```bash
+semodule -l | grep fail2ban-grep
+# Expected: my-fail2ban-grep
+```
+
+Check that no new AVC denials appear after restarting fail2ban:
+
+```bash
+systemctl restart fail2ban
+ausearch -m avc --start recent | grep fail2ban
+# Expected: no output (no new denials)
+```
+
+## Why Not `audit2allow` Directly?
+
+The common shortcut `ausearch -c grep --raw | audit2allow -M my-policy` can fail if:
+
+- The AVC events have already rotated out of the audit log
+- `ausearch` returns no matching records (outputs "Nothing to do")
+
+Writing the `.te` file manually is more reliable and self-documenting.
+
+## Environment
+
+- **OS:** Fedora 43
+- **SELinux:** Enforcing, targeted policy
+- **Fail2ban:** 1.1.0 (`fail2ban-selinux-1.1.0-15.fc43.noarch`)
+- **Kernel:** 6.19.x
+
+## See Also
+
+- [Docker & Caddy Recovery After Reboot (Fedora + SELinux)](../../../05-troubleshooting/docker-caddy-selinux-post-reboot-recovery.md) — another SELinux fix for post-reboot service issues
+- [SELinux: Fixing Dovecot Mail Spool Context](../../../05-troubleshooting/selinux-dovecot-vmail-context.md) — custom SELinux context for mail spool