Rules (original) (raw)

• Format
• Performance / Important notes
  • localhost connections
• Best practices
• For servers

Format

Rules are stored as JSON files inside the -rule-path directory (by default /etc/opensnitchd/rules), in its simplest form, a rule looks like this:

{ "created": "2018-04-07T14:13:27.903996051+02:00", "updated": "2018-04-07T14:13:27.904060088+02:00", "name": "deny-simple-www-google-analytics-l-google-com", "enabled": true, "precedence": false, "action": "deny", "duration": "always", "operator": { "type": "simple", "sensitive": false, "operand": "dest.host", "data": "www-google-analytics.l.google.com" } }

Some considerations

All the fields you select when defining a rule will be used to match connections, for example:

• Rule: allow -> port 443 -> Dst IP 1.1.1.1 -> Protocol TCP -> Host www.site.test
  • This rule will match connections to port 443 AND IP 1.1.1.1 AND protocol TCP AND host www.site.test
  • connections to IP 2.2.2.2 won't match, connections to port 80 won't match, etc...
• Rule: allow -> port 53 -> [x] domains list -> [x] network ranges list
  • This rule will match connections to port 53 AND domains in the list AND IPs in the network ranges list
• Rule: allow -> port ^(53|80|443)$ -> UID 1000 -> Path /app/bin/test -> [x] domains list
  • This rule will match connections to ports (53 OR 80 OR 443) AND UID 1000 AND Path /app/bin/test AND domains in the specified.
• If you select multiple lists on the same rule, bear in mind that all the lists must match in order to apply an action:Read this disccussion to learn more

Rule precedence: When a connection is attempted, OpenSnitch evaluates each of the enabled rules. The rules are sorted in the alphabetical order of rule names (since v.1.2.0). OpenSnitch goes through the list and as soon as it encounters a Deny/Reject rule or an Important ([x] Priority) rule (since v1.2.0) that matches the connection, that rule will be immediately selected as the effective rule. If no such rule is found, then the last non-Important Allow rule that matched will be selected. If no rule matched, it shows a pop-up dialogue, or applys the default action if that's not possible.

• In the following example, the Deny rule takes precedence over the Allow rules:

000-allow-chrome-to-specific-domains
001-allow-not-so-important-rule
001-deny-chrome

• In the following example, the first Allow rule takes precedence because it is set to Priority and it comes first in the alphabetical order:

000-allow-chrome-to-specific-domains [x] Priority
001-allow-not-so-important-rule
001-deny-chrome

This way you can not only prioritize critical connections (like VPNs), but also gain performance.

More on rules performance

As already mentioned, the order of rules is critical. If you use Firefox and prioritize Allow rules to allow Firefox's connections, web navegation will be faster.

But the type of rule also impacts the rule's performance. regexp and list types are slower than simple because regexp and list types check multiple parameters while simple rules check just one. And regexp is the slowest, because is the more complex type.

An example with a regular expression:

{ "created": "2018-04-07T14:13:27.903996051+02:00", "updated": "2018-04-07T14:13:27.904060088+02:00", "name": "deny-any-google-analytics", "enabled": true, "precedence": false, "action": "deny", "duration": "always", "operator": { "type": "regexp", "sensitive": false, "operand": "dest.host", "data": "(?i)" } }

An example whitelisting a process path:

{ "created": "2018-04-07T15:00:48.156737519+02:00", "updated": "2018-04-07T15:00:48.156772601+02:00", "name": "allow-simple-opt-google-chrome-chrome", "enabled": true, "precedence": false, "action": "allow", "duration": "always", "operator": { "type": "simple", "sensitive": false, "operand": "process.path", "data": "/opt/google/chrome/chrome" } }

Example of a complex rule using the operator list, saved from the GUI (Note: version v1.2.0):

{
  "created": "2020-02-07T14:16:20.550255152+01:00",
  "updated": "2020-02-07T14:16:20.729849966+01:00",
  "name": "deny-list-type-simple-operand-destip-data-1101-type-simple-operand-destport-data-23-type-simple-operand-userid-data-1000-type-simple-operand-processpath-data-usrbintelnetnetkit",
  "enabled": true,
  "precedence": false,
  "action": "deny",
  "duration": "always",
  "operator": {
    "type": "list",
    "operand": "list",
    "list": [
      {
        "type": "simple",
        "operand": "dest.ip",
        "sensitive": false,
        "data": "1.1.0.1",
        "list": null
      },
      {
        "type": "simple",
        "operand": "dest.port",
        "sensitive": false,
        "data": "23",
        "list": null
      },
      {
        "type": "simple",
        "operand": "user.id",
        "sensitive": false,
        "data": "1000",
        "list": null
      },
      {
        "type": "simple",
        "operand": "process.path",
        "sensitive": false,
        "data": "/usr/bin/telnet.netkit",
        "list": null
      }
    ]
  }
}

localhost connections

Some applications have components that communicate in localhost. For example KDE uses kdeinit5 and kwin, Xfce and others use xbrlapi , and GnuPG dirmngr. If you change daemon's default action to deny these applications will stop working. For example you may notice a delay login to the Desktop Environment (See issues #982 and #965 for more information).

The solution is to allow either localhost connections, or these binaries in particular.

Here's a rule to allow localhost connections:

{ "created": "2023-07-05T10:46:47.904024069+01:00", "updated": "2023-07-05T10:46:47.921828104+01:00", "name": "000-allow-localhost", "enabled": true, "precedence": true, "action": "allow", "duration": "always", "operator": { "type": "network", "operand": "dest.network", "sensitive": false, "data": "127.0.0.0/8", "list": [] } }

If you want to restrict it further, under the Addresses tab you can review what binaries established localhost connections, and then add the absolute path to the rule + destination port.

Best practices

• Allow DNS queries only to your configured DNS nameservers:
  ⚠️ DNS protocol can be used to exfiltrate information from local networks.
  • Allow systemd-resolved, dnsmasq, dnscrypt-proxy, etc, to connect only to your DNS nameservers + port 53 + UID.
  • Besides allowing connections to remote DNS servers (9.9.9.9 for example), you may need to allow connections to localhost IPs (127.0.0.1, etc)
  • If you already allowed these stub resolvers, the easiest way would we to delete the existing rule, let it ask you again to allow/deny it, click on the [+] button and then select from the pop-up from this command line AND to IP x.x.x.x AND_ to port xxx
• Limit what an application can do as much as possible:
  • Filter by executable + command line: You don't want to allow curl or wget system wide. Instead, allow only a particular command line, for example:
    command launched: $ wget https://mirror.karneval.cz/pub/linux/fedora/linux/releases/34/Workstation/x86_64/iso/Fedora-Workstation-Live-x86_64-34-1.2.iso
    Instead of allowing from this executable: wget, use allow from this executable + from this command line
    You can narrow it further, by allowing from this command line + from this User ID + to this IP + to this port
    Or just let it prompt you to allow/deny it if you don't these commands usually, and allow it for 30s.
• Don't allow python3, perl or ruby binaries system-wide:
  • As explained above, filter by executable + command line + (... more parameters ...) If you allow python3for example, you'll allow ANY python3 script, so be careful.
    https://github.com/evilsocket/opensnitch/wiki/Rules-examples#filtering-python-scripts-applicable-to-java-and-others-interpreters
• Disable unprivileged namespaces to prevent rules bypass
  If /proc/sys/kernel/unprivileged_userns_clone is set to 1, change it to 0.
  Or enable checksums verifying from the Preferences -> Nodes -> Rules.

For servers

These recommendations also apply to the Linux Desktop, but are specially important on servers.

Why? If someone gets access to the system, usually there're a few directories where everyone can write files: /tmp, /var/tmp or /dev/shm. Thus these directories are usually used to drop malicious files or download remote binaries to escalate privileges, mine cryptocoins, etc.

Usually the attackers use wget, curl or bash to establish outbound connections (malware examples). So, if you don't need these binaries, just uninstall them.

There're two approaches to secure a server with OpenSnitch:

1. restrict everything by default (DefaultAction set to deny/reject in the default-config.json file) and allow only system binaries and needed apps. Incoming connections will keep working, but NEW outbound connections will be denied.
2. allow everything by default, and deny connections from specific locations, or by binary / destination.

• If you need curl or wget and the DefaultAction is not allow, restrict their outbound connections as much as possible (this practice applies to any other binary of the server):
  {
  "created": "2020-02-07T14:16:20.550255152+01:00",
  "updated": "2020-02-07T14:16:20.729849966+01:00",
  "name": "allow-curl-net-proxy",
  "description": "allow curl only to 10.168.10.164 on port 8081",
  "enabled": true,
  "precedence": false,
  "action": "allow",
  "duration": "always",
  "operator": {
  "type": "list",
  "operand": "list",
  "list": [
  {
  "type": "simple",
  "operand": "process.path",
  "sensitive": false,
  "data": "/usr/bin/curl",
  "list": null
  },
  {
  "type": "simple",
  "operand": "dest.ip",
  "sensitive": false,
  "data": "10.168.10.164",
  "list": null
  },
  {
  "type": "simple",
  "operand": "dest.port",
  "sensitive": false,
  "data": "8081",
  "list": null
  }
  ]
  }
  }
  Or for example you can allow everything only to the local lan, and let the rest of outbound connections be denied by the DefaultAction:
  {
  "created": "2023-05-20T20:39:33.765468194+02:00",
  "updated": "2023-05-20T20:39:33.7655761+02:00",
  "name": "000-allow-lan",
  "description": "",
  "enabled": true,
  "precedence": true,
  "nolog": false,
  "action": "allow",
  "duration": "always",
  "operator": {
  "type": "network",
  "operand": "dest.network",
  "sensitive": false,
  "data": "LAN",
  "list": []
  }
  }
• When the DefaultAction is allow, don't allow connections opened by binaries located under certain directories: /dev/shm, /tmp, /var/tmp or /memfd:
  There're ton of malware examples that drop malicious files to temporary directories (more common on servers than on the desktop):
  Collection of Linux malware payloads
  https://github.com/timb-machine/linux-malware

(*) Deny  
[x] From this executable: ^(/memfd|/tmp/|/var/tmp/|/dev/shm/|/var/run|/var/lock).*  

/etc/opensnitchd/rules/000-deny-tmp.json:
{
"created": "2025-04-26T09:58:03.704090244+02:00",
"updated": "2025-04-26T09:58:03.704216578+02:00",
"name": "000-deny-tmp",
"enabled": true,
"precedence": true,
"action": "reject",
"duration": "always",
"operator": {
"type": "regexp",
"operand": "process.path",
"sensitive": false,
"data": "^(/var/tmp|/dev|/memfd|/tmp).*",
"list": []
}
}

• You can also block outbound connections to crypto mining pools and malware domains/ips with blocklists rules.
  One of the common reason to compromise servers is to mine cryptos. Denying connections to the mining pools, disrupts the operation.
• Think also if your web server, database server, etc, needs to establish connections to remote IPs.
  In some cases, the download of malicious files is executed from common applications: pg_mem campaign
  Note that the default policy should be deny everything unless explicitely allowed. But by creating a rule to deny specifically these directories, you can have a place where to monitor these executions.