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Networking

Log Connections

tcpdump / tshark

Register ICMP replies from 10.10.13.38:

$ sudo tcpdump -n -i eth0 -XSs 0 'src 10.10.13.38 and icmp[icmptype]==8'

Redirect to text file adding timestamps:

$ sudo tcpdump -i eth0 -tttt -l icmp | tee icmp.txt

iptables

Add rule to register new (does not watch for related, established) connections to your machine:

$ sudo iptables -A INPUT -p tcp -m state --state NEW -j LOG --log-prefix "IPTables New-Connection: " -i tun0

Check the logs:

$ sudo grep IPTables /var/log/messages

Delete rule:

$ sudo iptables -D INPUT -p tcp -m state --state NEW -j LOG --log-prefix "IPTables New-Connection: " -i tun0

Tools

dhclient

Release the current lease on eth0 and obtain a fresh IP via DHCP in Linux:

$ sudo dhclient -v -r eth0
$ sudo dhclient -v eth0

iptables

List rules in all chains (default table is filter, there are mangle, nat and raw tables beside it):

$ sudo iptables -L -n --line-numbers [-t filter]

Print rules for all chains (for a specific chains):

$ sudo iptables -S [INPUT [1]]

fail2ban

  • /etc/fail2ban/filter.d - filters which turn into user-defined fail2ban iptables rules (automatically).

Status:

$ sudo service fail2ban status
$ sudo fail2ban-client status
$ sudo fail2ban-client status sshd

Unban:

$ sudo fail2ban-client unban --all
$ sudo fail2ban-client set sshd unbanip <IP>

OpenVPN

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