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Linux Advanced
Networking and surroundings
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e5cb65b5
Commit
e5cb65b5
authored
Apr 21, 2020
by
JackV
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Add debug section
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@@ -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
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