# reaction

a program that scans program outputs, such as logs,
for repeated patterns, such as failed login attempts,
and takes action, such as banning ips.

(adapted from [ fail2ban ](http://fail2ban.org)'s presentation 😄)

🚧 this program hasn't received external audit. however, it already works well on my servers 🚧

## rationale

i was using fail2ban since quite a long time, but i was a bit frustrated by its cpu consumption
and all its heavy default configuration.

in my view, a security-oriented program should be simple to configure (`sudo` is a very bad example!)
and an always-running daemon should be implemented in a fast language.

<a href="https://u.ppom.me/reaction.webm">📽️ french example</a>

## configuration

this configuration file is all that should be needed to prevent brute force attacks on an ssh server.

see [ reaction.service ](./config/reaction.service) and [reaction.yml](./app/reaction.yml) for the fully explained examples.

`/etc/reaction.yml`
```yaml
definitions:
  - &iptablesban [ "ip46tables" "-w" "-I" "reaction" "1" "-s" "<ip>" "-j" "block" ]
  - &iptablesunban [ "ip46tables" "-w" "-D" "reaction" "1" "-s" "<ip>" "-j" "block" ]

patterns:
  ip: '(([ 0-9 ]{1,3}\.){3}[0-9]{1,3})|([0-9a-fA-F:]{2,90})'

start:
  - [ "ip46tables", "-w", "-N", "reaction" ]
  - [ "ip46tables", "-w", "-A", "reaction", "-j", "ACCEPT" ]
  - [ "ip46tables", "-w", "-I", "reaction", "1", "-s", "127.0.0.1", "-j", "ACCEPT" ]
  - [ "ip46tables", "-w", "-I", "INPUT", "-p", "all", "-j", "reaction" ]

stop:
  - [ "ip46tables", "-w," "-D," "INPUT", "-p", "all", "-j", "reaction" ]
  - [ "ip46tables", "-w," "-F," "reaction" ]
  - [ "ip46tables", "-w," "-X," "reaction" ]

streams:
  ssh:
    cmd: [ "journalctl" "-fu" "sshd.service" ]
    filters:
      failedlogin:
        regex:
          - 'authentication failure;.*rhost=<ip>'
        retry: 3
        retryperiod: '6h'
        actions:
          ban:
            cmd: *iptablesban
          unban:
            cmd: *iptablesunban
            after: '48h'
```

jsonnet is also supported:

`/etc/reaction.jsonnet`
```jsonnet
local iptables(args) = [ 'ip46tables', '-w' ] + args;
local iptablesban(ip) = iptables([ '-A', 'reaction', '1', '-s', ip, '-j', 'DROP' ]);
local iptablesunban(ip) = iptables([ '-D', 'reaction', '1', '-s', ip, '-j', 'DROP' ]);
{
  patterns: {
    ip: {
      regex: @'(?:(?:[ 0-9 ]{1,3}\.){3}[0-9]{1,3})|(?:[0-9a-fA-F:]{2,90})',
    },
  },
  start: {
    iptables([ "-N", "reaction" ]),
    iptables([ "-A", "reaction", "-j", "ACCEPT" ]),
    iptables([ "-I", "reaction", "1", "-s", "127.0.0.1", "-j", "ACCEPT" ]),
    iptables([ "-I", "INPUT", "-p", "all", "-j", "reaction" ]),
  },
  stop: {
    iptables([ "-D,", "INPUT", "-p", "all", "-j", "reaction" ]),
    iptables([ "-F,", "reaction" ]),
    iptables([ "-X,", "reaction" ]),
  },
  streams: {
    ssh: {
      cmd: [ 'journalctl', '-fu', 'sshd.service' ],
      filters: {
        failedlogin: {
          regex: [ @'authentication failure;.*rhost=<ip>' ],
          retry: 3,
          retryperiod: '6h',
          actions: {
            ban: {
              cmd: iptablesban('<ip>'),
            },
            unban: {
              cmd: iptablesunban('<ip>'),
              after: '48h',
              onexit: true,
            },
          },
        },
      },
    },
  },
}
```

note that both yaml and jsonnet are extensions of json, so json is also inherently supported.

`/etc/systemd/system/reaction.service`
```systemd
[ Unit ]
WantedBy=multi-user.target

[ Service ]
ExecStart=/path/to/reaction -c /etc/reaction.yml

StateDirectory=reaction
RuntimeDirectory=reaction
WorkingDirectory=/var/lib/reaction
```

### database

the working directory of `reaction` will be used to create and read from the embedded database.
if you don't know where to start it, `/var/lib/reaction` should be a sane choice.

### socket

the socket allowing communication between the cli and server will be created at `/run/reaction/reaction.socket`.

### `ip46tables`

`ip46tables` is a minimal c program present in its own subdirectory with only standard posix dependencies.

it permits to configure `iptables` and `ip6tables` at the same time.
it will execute `iptables` when detecting ipv4, `ip6tables` when detecting ipv6 and both if no ip address is present on the command line.

### compilation

you'll need the go toolchain for reaction and a c compiler for ip46tables.
```shell
$ make
```

alternatively,
```shell
$ go build .
$ gcc ip46tables.d/ip46tables.c -o ip46tables
```

### nixos

in addition to the [ package ](https://framagit.org/ppom/nixos/-/blob/cf5448b21ae3386265485308a6cd077e8068ad77/pkgs/reaction/default.nix)
and [ module ](https://framagit.org/ppom/nixos/-/blob/cf5448b21ae3386265485308a6cd077e8068ad77/modules/common/reaction.nix)
that i didn't try to upstream to nixpkgs yet (although they are ready), i use extensively reaction on my servers. if you're using nixos,
consider reading and building upon [ my own building blocks ](https://framagit.org/ppom/nixos/-/blob/cf5448b21ae3386265485308a6cd077e8068ad77/modules/common/reaction-variables.nix),
[ my own non-root reaction conf, including conf for SSH, port scanning & Nginx common attack URLS ](https://framagit.org/ppom/nixos/-/blob/cf5448b21ae3386265485308a6cd077e8068ad77/modules/common/reaction-custom.nix),
and the configuration for [ nextcloud ](https://framagit.org/ppom/nixos/-/blob/cf5448b21ae3386265485308a6cd077e8068ad77/modules/musi/file.ppom.me.nix#L53),
[ vaultwarden ](https://framagit.org/ppom/nixos/-/blob/cf5448b21ae3386265485308a6cd077e8068ad77/modules/musi/vaultwarden.nix#L45),
and [ maddy ](https://framagit.org/ppom/nixos/-/blob/cf5448b21ae3386265485308a6cd077e8068ad77/modules/musi/mail.nix#L74). see also an [example](https://framagit.org/ppom/nixos/-/blob/cf5448b21ae3386265485308a6cd077e8068ad77/modules/musi/mail.nix#L85) where it does something else than banning IPs.