checker-ssh/checker/rules_reachability.go

// This file is part of the happyDomain (R) project.
// Copyright (c) 2020-2026 happyDomain
// Authors: Pierre-Olivier Mercier, et al.
//
// This program is offered under a commercial and under the AGPL license.
// For commercial licensing, contact us at <contact@happydomain.org>.
//
// For AGPL licensing:
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program.  If not, see <https://www.gnu.org/licenses/>.

package checker

import (
	"context"
	"fmt"
	"strconv"

	sdk "git.happydns.org/checker-sdk-go/checker"
)

// reachabilityRule reports per-endpoint TCP reachability. One state
// per probed (ip, port) pair so the UI distinguishes individual
// firewall / routing issues.
type reachabilityRule struct{}

func (r *reachabilityRule) Name() string { return "ssh.tcp_reachable" }
func (r *reachabilityRule) Description() string {
	return "Verifies that every probed (address, port) pair accepts a TCP connection."
}

func (r *reachabilityRule) Evaluate(ctx context.Context, obs sdk.ObservationGetter, _ sdk.CheckerOptions) []sdk.CheckState {
	data, errSt := loadSSHData(ctx, obs)
	if errSt != nil {
		return []sdk.CheckState{*errSt}
	}
	if len(data.Endpoints) == 0 {
		return []sdk.CheckState{noEndpointsState("ssh.tcp_reachable.no_endpoints")}
	}
	var states []sdk.CheckState
	for _, ep := range data.Endpoints {
		if ep.TCPConnected {
			continue
		}
		msg := "Cannot open TCP connection to " + ep.Address
		if ep.Error != "" {
			msg += ": " + ep.Error
		}
		states = append(states, sdk.CheckState{
			Status:  sdk.StatusCrit,
			Message: msg,
			Code:    "tcp_unreachable",
			Subject: ep.Address,
			Meta: map[string]any{
				"fix": "Check DNS, firewall (allow tcp/" + strconv.Itoa(int(ep.Port)) + " from the internet), and that sshd is running.",
			},
		})
	}
	if len(states) == 0 {
		return []sdk.CheckState{passState("ssh.tcp_reachable.ok", "All probed endpoints accept TCP connections.")}
	}
	return states
}

// handshakeRule reports per-endpoint SSH handshake progress: whether
// banner exchange and KEXINIT parsing completed. Endpoints that are
// TCP-unreachable are skipped (covered by reachabilityRule).
type handshakeRule struct{}

func (r *handshakeRule) Name() string { return "ssh.handshake" }
func (r *handshakeRule) Description() string {
	return "Verifies that the SSH banner exchange and KEXINIT parse succeed on every reachable endpoint."
}

func (r *handshakeRule) Evaluate(ctx context.Context, obs sdk.ObservationGetter, _ sdk.CheckerOptions) []sdk.CheckState {
	data, errSt := loadSSHData(ctx, obs)
	if errSt != nil {
		return []sdk.CheckState{*errSt}
	}
	if len(data.Endpoints) == 0 {
		return []sdk.CheckState{noEndpointsState("ssh.handshake.no_endpoints")}
	}
	var states []sdk.CheckState
	for _, ep := range data.Endpoints {
		if !ep.TCPConnected {
			continue
		}
		switch ep.Stage {
		case "banner":
			states = append(states, sdk.CheckState{
				Status:  sdk.StatusCrit,
				Code:    "no_ssh_banner",
				Subject: ep.Address,
				Message: fmt.Sprintf("Server on %s did not send an SSH-2.0 banner: %s", ep.Address, ep.Error),
				Meta:    map[string]any{"fix": "Check that an SSH daemon (not HTTP, mail, ...) listens on this port."},
			})
		case "banner_write":
			states = append(states, sdk.CheckState{
				Status:  sdk.StatusCrit,
				Code:    "banner_write_failed",
				Subject: ep.Address,
				Message: "Failed to send our client banner: " + ep.Error,
			})
		case "kexinit_read":
			states = append(states, sdk.CheckState{
				Status:  sdk.StatusCrit,
				Code:    "kexinit_read_failed",
				Subject: ep.Address,
				Message: "Server did not send KEXINIT after banner: " + ep.Error,
			})
		case "kexinit_parse":
			states = append(states, sdk.CheckState{
				Status:  sdk.StatusCrit,
				Code:    "kexinit_parse_failed",
				Subject: ep.Address,
				Message: "Malformed KEXINIT packet: " + ep.Error,
			})
		}
	}
	if len(states) == 0 {
		return []sdk.CheckState{passState("ssh.handshake.ok", "All reachable endpoints completed the SSH handshake.")}
	}
	return states
}