Iptables Block outbound url in Linux

iptables is a command-line firewall utility that uses policy chains to allow or block traffic. When a connection tries to establish itself on your system, iptables looks for a rule in its list to match it to. If it doesn’t find one, it resorts to the default action.

For this in particular, you’ll need a kernel compiled with Netfilter “String match support” enabled.

iptables -A OUTPUT -p tcp -m string --string "/webnus.net/plugin-api/verify" --algo kmp -j REJECT --reject-with tcp-reset
iptables -A OUTPUT -p tcp -m string --string "/webnus.net/addons-api/verify" --algo kmp -j REJECT --reject-with tcp-reset

• -A OUTPUT Appends a rule targeting the outgoing calls
• -p tcp Use the tcp protocol for the rule to do its checks
• -m string --string "PATTERN" --algo kmp
• -m string Use the match module with the string selector
• --string "PATTERN" Match the given pattern.
• --algo kmp Use the KMP algorithm to do the string matching. Can read more about it here
• https://stackoverflow.com/questions/16085201/when-would-you-use-kmp-over-boyer-moore
• https://stackoverflow.com/questions/12656160/what-are-the-main-differences-between-the-knuth-morris-pratt-and-boyer-moore-sea
• -j REJECT --reject-with tcp-reset Connection reset: instead of dropping the packet with -j DROP, we can reject it and immediately close the connection with -p tcp -j REJECT –reject-with tcp-reset.

Alternatively and for extra cookie points, you can always block all outgoing connections and whitelist only the ones are ok with you.