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doc: firewall: add tcpdump example

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Solène Rapenne 2023-11-03 12:17:28 +01:00
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user/security-in-qubes/firewall.md
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@ -509,3 +509,13 @@ Firewall troubleshooting
Firewall logs are stored in the systemd journal of the qube the firewall is running in (probably `sys-firewall`).
You can view them by running `sudo journalctl -u qubes-firewall.service` in the relevant qube.
Sometimes these logs can contain useful information about errors that are preventing the firewall from behaving as you would expect.
An effective console utility to troubleshoot network is [tcpdump](https://www.tcpdump.org/), it can be used to display network packets entering or leaving network interfaces.
For instance, if you want to check if your network interface `eth0` is receiving packets on port TCP 22 from the network 192.168.x.y, you can run this command:
```
tcpdump -i eth0 -nn dst port 22 and src net 192.168.x.y/24
```
This can be used effectively in a destination qube and its Network VM to see if forwarding / NAT rules are working.