Cheatsheet

Networking

The tools, numbers and records you need to find out why two machines cannot talk.

Look at the host

```
ping -c 4 8.8.8.8
```
Is the network reachable at all? (an IP, no DNS involved)
```
ping -c 4 example.com
```
Does name lookup work as well?

traceroute example.com  |  tracepath example.com

The path the packets take, hop by hop

curl -w "%{http_code} %{time_total}s\n" -o /dev/null -s URL

Status code and total time

```
dig example.com
```
Ask for the A record
```
dig +short example.com MX
```
Just the answer, for one record type
```
dig @1.1.1.1 example.com
```
Ask a specific resolver
```
dig +trace example.com
```
Follow the lookup from the root servers down
```
dig -x 203.0.113.10
```
Reverse lookup of an address
```
A / AAAA / CNAME / MX / TXT / NS
```
Address, IPv6 address, alias, mail server, text (SPF, DKIM), name server
```
TTL
```
How long a record may be cached: lower it before a change

```
200 / 301 / 302 / 304
```
OK / moved permanently / found (temporary) / not modified
```
400 / 401 / 403 / 404 / 429
```
Bad request / not logged in / not allowed / not found / too many requests
```
500 / 502 / 503 / 504
```
Server error / bad gateway / unavailable / gateway timeout

curl -X POST -H "Content-Type: application/json" -d '{"a":1}' URL

Send JSON

openssl s_client -connect example.com:443 -servername example.com

Look at the certificate a server presents

openssl x509 -in cert.pem -noout -dates -subject

Validity dates and subject of a certificate

10.0.0.0/8  172.16.0.0/12  192.168.0.0/16

The private IPv4 ranges

```
/24 = 256 addresses  /16 = 65,536  /32 = one host
```
The number after the slash is the length of the network part
```
ipcalc 10.0.4.0/22
```
Work out the range, mask and broadcast of a network (if installed)
```
127.0.0.1  ::1
```
This machine, over IPv4 and IPv6
```
169.254.0.0/16
```
Link-local: what you get when DHCP failed

3306  MySQL  |  5432  PostgreSQL  |  6379  Redis

Common databases and caches

sudo tcpdump -i eth0 host 203.0.113.10 -w out.pcap

Record traffic to one host for later

```
sudo iptables -L -n -v
```
List firewall rules with packet counts
```
sudo ufw allow 443/tcp && sudo ufw enable
```
Open a port with the simple firewall front end
```
ssh -L 8080:localhost:80 user@host
```
Forward a local port to the server's port 80 through SSH

Cheatsheet for the Networking path · Deboistech · All cheatsheets