echolot/server/internal/selftest/selftest.go

// SPDX-FileCopyrightText: 2026 Echolot contributors
// SPDX-License-Identifier: GPL-3.0-or-later

// Package selftest lets the daemon prove its own host is a clean measurement
// target: the kernel isn't silently altering what clients measure, and the
// server's own egress reaches full MTU. If the server side is already broken,
// client-side results (especially MTU/PMTUD) measure the server, not the
// client — so the daemon says so.
package selftest

import (
	"os"
	"strconv"
	"strings"
)

// Severity of a check result.
type Severity string

const (
	OK   Severity = "ok"
	Warn Severity = "warn"
)

// Check is one sysctl (or derived) assertion.
type Check struct {
	Name     string   `json:"name"`
	Got      string   `json:"got"`
	Want     string   `json:"want"`
	Severity Severity `json:"severity"`
	Why      string   `json:"why"`
}

// MTUResult is one egress path-MTU probe outcome.
type MTUResult struct {
	Target       string `json:"target"`
	DiscoveredMTU int   `json:"discovered_mtu"`
	FullMTU      bool   `json:"full_mtu"` // >= 1500
	Err          string `json:"err,omitempty"`
}

// Report is the whole self-test.
type Report struct {
	Sysctls   []Check     `json:"sysctls"`
	EgressMTU []MTUResult `json:"egress_mtu"`
	// SysctlOK / MTUOK are the compact "server proven good" signals; the
	// profile surfaces these so a client can skip MTU tests the server can't
	// support honestly.
	SysctlOK bool `json:"sysctl_ok"`
	MTUOK    bool `json:"mtu_ok"`
}

// readSysctl reads /proc/sys/<dotted.name>. Empty string if unavailable.
func readSysctl(name string) string {
	p := "/proc/sys/" + strings.ReplaceAll(name, ".", "/")
	b, err := os.ReadFile(p)
	if err != nil {
		return ""
	}
	return strings.TrimSpace(string(b))
}

// sysctlChecks are the measurement-fidelity assertions. Each closure returns
// OK/Warn given the read value; a missing value (non-Linux / restricted) is
// reported as Warn "unreadable" but never fatal.
var sysctlChecks = []struct {
	name string
	want string
	why  string
	ok   func(v string) bool
}{
	{"net.ipv6.conf.all.accept_ra", "0", "static v6 host must not let RAs mutate routing (the very thing Echolot detects)", eq("0")},
	{"net.ipv4.conf.all.accept_redirects", "0", "ICMP redirects could alter routing mid-measurement", eq("0")},
	{"net.ipv4.conf.all.send_redirects", "0", "an endpoint should not emit ICMP redirects", eq("0")},
	{"net.ipv4.icmp_echo_ignore_all", "0", "server must answer ping so clients can measure to it", eq("0")},
	{"net.ipv4.ip_no_pmtu_disc", "0", "server must honor path MTU on its own sends", eq("0")},
	{"net.ipv4.tcp_sack", "1", "so a missing SACK in mss_observed is the path's fault, not the server's", eq("1")},
	{"net.ipv4.tcp_timestamps", "1", "so TCP-timestamp absence reflects the path, not the server", eq("1")},
	{"net.ipv4.tcp_window_scaling", "1", "so wscale absence reflects the path, not the server", eq("1")},
	{"net.ipv4.icmp_ratelimit", "0", "nonzero throttles the server's ICMP errors → false loss/black-hole readings", eq("0")},
}

func eq(want string) func(string) bool { return func(v string) bool { return v == want } }

// Sysctls runs the sysctl audit.
func Sysctls() []Check {
	out := make([]Check, 0, len(sysctlChecks))
	for _, c := range sysctlChecks {
		got := readSysctl(c.name)
		sev := Warn
		switch {
		case got == "":
			got = "(unreadable)"
		case c.ok(got):
			sev = OK
		}
		out = append(out, Check{Name: c.name, Got: got, Want: c.want, Severity: sev, Why: c.why})
	}
	return out
}

// Run performs the full self-test: sysctl audit + egress MTU probes to the
// given targets (each "host" — port is irrelevant for PMTUD).
func Run(mtuTargets []string) Report {
	r := Report{Sysctls: Sysctls()}
	r.SysctlOK = true
	for _, c := range r.Sysctls {
		if c.Severity == Warn {
			r.SysctlOK = false
		}
	}
	r.MTUOK = true
	for _, t := range mtuTargets {
		res := probeEgressMTU(t)
		r.EgressMTU = append(r.EgressMTU, res)
		if !res.FullMTU {
			r.MTUOK = false
		}
	}
	if len(r.EgressMTU) == 0 {
		r.MTUOK = false // couldn't prove it
	}
	return r
}

var _ = strconv.Atoi