Add debug section

slides/content.md

@@ -40,6 +40,71 @@ all these topics can come together
 
 ---
 
+## You can't connect to the Internet, now what?
+
+When setting up a network it might happen that everything
+breaks, we will go through some tools that might help debug
+network configuration problems
+
+---
+
+### Network checklist
+
+* Does the network have working routing?
+* Does the network resolve domain names?
+* Are response packets coming back?
+* Are there problems in upper layers?
+
+---
+
+### Quick routing check: ping & traceroute
+
+* **ping** is useful to check that we have working routing
+  a quick `ping 1.1.1.1` can tell us if we can reach a known
+  ip (in this case cloudflare DNS)
+
+* **traceroute** can help with identifying routing
+  nodes that are dropping packets
+
+---
+
+### DNS check: ping (again) & dig
+
+* Again **ping** can be used to quickly check if DNS resolution
+  is working as intended, `ping poul.org` will return an ip
+  address is DNS is working
+
+* **dig** can help to check more complex problems,
+  using `dig poul.org` will tell us if the system DNS
+  (found in `/etc/resolv.conf`) is working, whereas
+  `dig @1.1.1.1 poul.org` will tell us if there is some
+  external problem related to DNS
+
+---
+
+### Checking for responses
+
+* Tools like **tcpdump** and **Wireshark** can help debug
+  problems related to responses not coming back, just point to
+  the right interface and filter for the traffic (usually __icmp__)
+
+* Only have ssh but want to have a gui? No problem!<br>
+  ```
+  ssh root@<ip> 'tcpdump -i eth0 -s0 -w -' | wireshark -k -i -
+  ```
+
+---
+
+### Upper Layer Problems
+
+So far we only talked about L1-3 problems, but there can be just as many
+in L4-7, however these usually require specific tools to debug
+
+* **curl** can be used to detect a [TCP Blackhole](https://en.wikipedia.org/wiki/Path_MTU_Discovery#Problems) and a whole other lot of problems
+  ```
+  curl -vv https://example.org
+  ```
+
 ----
 
 # Wireguard