docs: add troubleshooting

docs/getting-started/README.md

@@ -176,3 +176,7 @@ routing {
 See more at [example.dae](https://github.com/daeuniverse/dae/blob/main/example.dae).
 
 If you use PVE, refer to [#37](https://github.com/daeuniverse/dae/discussions/37).
+
+### Troubleshooting
+
+See [Troubleshooting](troubleshooting.md).

docs/getting-started/kernel-upgrade.md

@@ -30,7 +30,7 @@ Various Linux distributions have different methods to upgrade the Linux kernel.
    uname -r
    ```
 
-3. Download corresponding kernel debs from [daeuniverse/armbian-btf-kernel](https://github.com/daeuniverse/armbian-btf-kernel/releases).
+3. Download corresponding kernel debs from [daeuniverse/armbian-btf-kernel](https://github.com/daeuniverse/armbian-btf-kernel).
 
 4. Extract debs and install
 

docs/getting-started/troubleshooting.md

@@ -0,0 +1,59 @@
+# Troubleshooting
+
+### Unknown bpf_trace_printk
+
+```
+invalid argument: unknown func bpf_trace_printk
+```
+
+Solution:
+
+Compile dae with CFLAG `-D__REMOVE_BPF_PRINTK`. See [build-by-yourself](build-by-yourself.md).
+
+### Bind to WAN but no network
+
+**1. Troubleshoot local DNS service**
+
+If you use `systemd-resolved`, `dnsmasq` or other local DNS service in `/etc/resolv.conf`, do not use their DNS service.
+
+Methods to disable their DNS service:
+
+1. Method 1: by modifying their configurations.
+2. Method 2: by modifying the content of `/etc/resolv.conf` to `nameserver 223.5.5.5` or other DNS.
+
+Alternatively, if you really want to use them, refer to [external-dns](external-dns.md).
+
+**2. Troubleshoot firewall**
+
+If you bind to wan, make sure firewall is closed or `12345` is allowed by firewall. Don't worry about the security of this port because this port has its own firewall rule.
+
+Usual firewalls on Linux:
+
+```
+ufw
+firewalld
+```
+
+**3. Troubleshoot PPPoE**
+
+dae does not support PPPoE yet. We are working on it. However, binding to LAN for other machines should work fine.
+
+### Bind to LAN but bad DNS in other machines
+
+To verify:
+
+```
+curl -i 1.1.1.1
+curl -i google.com
+```
+
+If the first line has a response and the second line doesn't, check whether port `53` is occupied by others on dae's machine.
+
+```
+netstat -ulpen|grep 53
+# or
+# lsof -i:53 -n
+```
+
+If does, stop the service process or change its listening port from 53 to others.
+