# Suricata & Iptables cheatsheet

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## Iptables ### Chains Iptables chains are just lists of rules, processed in order. You will always find the following 3, but others such as NAT might also be supported. * **Input** – This chain is used to control the behavior of incoming connections. * **Forward** – This chain is used for incoming connections that aren’t being delivered locally. Think of a router – data is always being sent to it but rarely actually destined for the router itself; the data is just forwarded to its target. Unless you’re doing some kind of routing, NATing, or something else on your system that requires forwarding, you won’t even use this chain. * **Output** – This chain is used for outgoing connections. ```bash # Delete all rules iptables -F # List all rules iptables -L iptables -S # Block IP addresses & ports iptables -I INPUT -s ip1,ip2,ip3 -j DROP iptables -I INPUT -p tcp --dport 443 -j DROP iptables -I INPUT -s ip1,ip2 -p tcp --dport 443 -j DROP # String based drop ## Strings are case sensitive (pretty easy to bypass if you want to check an SQLi for example) iptables -I INPUT -p tcp --dport -m string --algo bm --string '' -j DROP iptables -I OUTPUT -p tcp --sport -m string --algo bm --string 'CTF{' -j DROP ## You can also check for the hex, base64 and double base64 of the expected CTF flag chars # Drop every input port except some iptables -P INPUT DROP # Default to drop iptables -I INPUT -p tcp --dport 8000 -j ACCEPT iptables -I INPUT -p tcp --dport 443 -j ACCEPT # Persist Iptables ## Debian/Ubuntu: apt-get install iptables-persistent iptables-save > /etc/iptables/rules.v4 ip6tables-save > /etc/iptables/rules.v6 iptables-restore < /etc/iptables/rules.v4 ##RHEL/CentOS: iptables-save > /etc/sysconfig/iptables ip6tables-save > /etc/sysconfig/ip6tables iptables-restore < /etc/sysconfig/iptables ``` ## Suricata ### Install & Config ```bash # Install details from: https://suricata.readthedocs.io/en/suricata-6.0.0/install.html#install-binary-packages # Ubuntu add-apt-repository ppa:oisf/suricata-stable apt-get update apt-get install suricata # Debian echo "deb http://http.debian.net/debian buster-backports main" > \ /etc/apt/sources.list.d/backports.list apt-get update apt-get install suricata -t buster-backports # CentOS yum install epel-release yum install suricata # Get rules suricata-update suricata-update list-sources #List sources of the rules suricata-update enable-source et/open #Add et/open rulesets suricata-update ## To use the dowloaded rules update the following line in /etc/suricata/suricata.yaml default-rule-path: /var/lib/suricata/rules rule-files: - suricata.rules # Run ## Add rules in /etc/suricata/rules/suricata.rules systemctl suricata start suricata -c /etc/suricata/suricata.yaml -i eth0 # Reload rules suricatasc -c ruleset-reload-nonblocking ## or set the follogin in /etc/suricata/suricata.yaml detect-engine: - rule-reload: true # Validate suricata config suricata -T -c /etc/suricata/suricata.yaml -v # Configure suricata as IPs ## Config drop to generate alerts ## Search for the following lines in /etc/suricata/suricata.yaml and remove comments: - drop: alerts: yes flows: all ## Forward all packages to the queue where suricata can act as IPS iptables -I INPUT -j NFQUEUE iptables -I OUTPUT -j NFQUEUE ## Start suricata in IPS mode suricata -c /etc/suricata/suricata.yaml -q 0 ### or modify the service config file as: systemctl edit suricata.service [Service] ExecStart= ExecStart=/usr/bin/suricata -c /etc/suricata/suricata.yaml --pidfile /run/suricata.pid -q 0 -vvv Type=simple systemctl daemon-reload ``` ### Rules Definitions A rule/signature consists of the following: * The **action**, determines what happens when the signature matches. * The **header**, defines the protocol, IP addresses, ports and direction of the rule. * The **rule options**, define the specifics of the rule. ![](<../../../.gitbook/assets/image (642) (3).png>) #### **Valid actions are** * alert - generate an alert * pass - stop further inspection of the packet * **drop** - drop packet and generate alert * **reject** - send RST/ICMP unreachable error to the sender of the matching packet. * rejectsrc - same as just _reject_ * rejectdst - send RST/ICMP error packet to the receiver of the matching packet. * rejectboth - send RST/ICMP error packets to both sides of the conversation. #### **Protocols** * tcp (for tcp-traffic) * udp * icmp * ip (ip stands for ‘all’ or ‘any’) * _layer7 protocols_: http, ftp, tls, smb, dns, ssh... (more in the [**docs**](https://suricata.readthedocs.io/en/suricata-6.0.0/rules/intro.html)) #### Source and Destination Addresses It supports IP ranges, negations and a list of addresses: | Example | Meaning | | ------------------------------ | ---------------------------------------- | | ! 1.1.1.1 | Every IP address but 1.1.1.1 | | !\[1.1.1.1, 1.1.1.2] | Every IP address but 1.1.1.1 and 1.1.1.2 | | $HOME\_NET | Your setting of HOME\_NET in yaml | | \[$EXTERNAL\_NET, !$HOME\_NET] | EXTERNAL\_NET and not HOME\_NET | | \[10.0.0.0/24, !10.0.0.5] | 10.0.0.0/24 except for 10.0.0.5 | #### Source and Destination Ports It supports port ranges, negations and lists of ports | Example | Meaning | | --------------- | -------------------------------------- | | any | any address | | \[80, 81, 82] | port 80, 81 and 82 | | \[80: 82] | Range from 80 till 82 | | \[1024: ] | From 1024 till the highest port-number | | !80 | Every port but 80 | | \[80:100,!99] | Range from 80 till 100 but 99 excluded | | \[1:80,!\[2,4]] | Range from 1-80, except ports 2 and 4 | #### Direction It's possible to indicate the direction of the communication rule being applied: ``` source -> destination source <> destination (both directions) ``` #### Keywords There are **hundreds of options** available in Suricata to search for the **specific packet** you are looking for, here it will be mentioned if something interesting is found. Check the [**documentation** ](https://suricata.readthedocs.io/en/suricata-6.0.0/rules/index.html)for more! ```bash # Meta Keywords msg: "description"; #Set a description to the rule sid:123 #Set a unique ID to the rule rev:1 #Rule revision number config classification: not-suspicious,Not Suspicious Traffic,3 #Classify reference: url, www.info.com #Reference priority:1; #Set a priority metadata: key value, key value; #Extra metadata # Filter by geolocation geoip: src,RU; # ICMP type & Code itype:<10; icode:0 # Filter by string content: "something" content: |61 61 61| #Hex: AAA content: "http|3A|//" #Mix string and hex content: "abc"; nocase; #Case insensitive reject tcp any any -> any any (msg: "php-rce"; content: "eval"; nocase; metadata: tag php-rce; sid:101; rev: 1;) # Replaces string ## Content and replace string must have the same length content:"abc"; replace: "def" alert tcp any any -> any any (msg: "flag replace"; content: "CTF{a6st"; replace: "CTF{u798"; nocase; sid:100; rev: 1;) ## The replace works in both input and output packets ## But it only modifies the first match # Filter by regex pcre:"//opts" pcre:"/NICK .*USA.*[0-9]{3,}/i" drop tcp any any -> any any (msg:"regex"; pcre:"/CTF\{[\w]{3}/i"; sid:10001;) # Other examples ## Drop by port drop tcp any any -> any 8000 (msg:"8000 port"; sid:1000;) ```

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