#!/usr/bin/env python3
"""
wg_mtu_auto.py — Auto-detect egress IF, optionally probe Path MTU to one or more targets,
compute the correct WireGuard MTU, and apply it.

Examples:
  sudo ./wg_mtu_auto.py
  sudo ./wg_mtu_auto.py --force-egress-mtu 1452
  sudo ./wg_mtu_auto.py --pmtu-target 46.4.224.77 --pmtu-target 2a01:4f8:2201:4695::2
  sudo ./wg_mtu_auto.py --pmtu-target 46.4.224.77,2a01:4f8:2201:4695::2 --pmtu-policy min
  ./wg_mtu_auto.py --dry-run
"""
import argparse, os, re, subprocess, sys, pathlib, ipaddress, statistics

def run(cmd):  # -> str
    return subprocess.run(cmd, check=False, stdout=subprocess.PIPE, stderr=subprocess.DEVNULL, text=True).stdout.strip()

def rc(cmd):  # -> int
    return subprocess.run(cmd, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL).returncode

def exists_iface(iface):  # -> bool
    return pathlib.Path(f"/sys/class/net/{iface}").exists()

def get_default_ifaces():  # -> list[str]
    devs = []
    for cmd in (["ip","-4","route","show","default"], ["ip","-6","route","show","default"]):
        out = run(cmd)
        for line in out.splitlines():
            m = re.search(r"\bdev\s+(\S+)", line)
            if m: devs.append(m.group(1))
    if not devs:
        for cmd in (["ip","route","get","1.1.1.1"], ["ip","-6","route","get","2606:4700:4700::1111"]):
            out = run(cmd)
            m = re.search(r"\bdev\s+(\S+)", out)
            if m: devs.append(m.group(1))
    uniq = []
    for d in devs:
        if not d or d == "lo" or re.match(r"^(wg|tun)\d*$", d) or not exists_iface(d): continue
        if d not in uniq: uniq.append(d)
    return uniq

def read_mtu(iface):  # -> int
    with open(f"/sys/class/net/{iface}/mtu","r") as f:
        return int(f.read().strip())

def set_mtu(iface, mtu, dry):
    if dry:
        print(f"[wg-mtu] DRY-RUN: ip link set mtu {mtu} dev {iface}")
    else:
        subprocess.run(["ip","link","set","mtu",str(mtu),"dev",iface], check=True)

def require_root(dry):
    if not dry and os.geteuid() != 0:
        print("[wg-mtu][ERROR] Please run as root (sudo) or use --dry-run.", file=sys.stderr)
        sys.exit(1)

def is_ipv6(addr):  # -> bool
    try:
        return isinstance(ipaddress.ip_address(addr), ipaddress.IPv6Address)
    except ValueError:
        return ":" in addr

def ping_ok(payload, target, timeout_s):  # -> bool
    base = ["ping","-M","do","-c","1","-s",str(payload),"-W",str(max(1, int(round(timeout_s))))]
    if is_ipv6(target):
        base.insert(1, "-6")
    return rc(base + [target]) == 0

def probe_pmtu(target, lo_payload=1200, hi_payload=1472, timeout=1.0):  # -> int|None
    """Binary-search the largest payload that passes with DF. Return Path-MTU (payload + hdr) or None."""
    hdr = 48 if is_ipv6(target) else 28
    # ensure the lower bound works; if not, try slightly smaller floors
    if not ping_ok(lo_payload, target, timeout):
        for p in (1180, 1160, 1140):
            if ping_ok(p, target, timeout):
                lo_payload = p
                break
        else:
            return None
    lo, hi, best = lo_payload, hi_payload, None
    while lo <= hi:
        mid = (lo + hi) // 2
        if ping_ok(mid, target, timeout):
            best = mid
            lo = mid + 1
        else:
            hi = mid - 1
    return (best + hdr) if best is not None else None

def choose_effective(pmtus, policy="min"):  # -> int
    """Pick an effective PMTU from a list of successful PMTUs."""
    if not pmtus:
        raise ValueError("no PMTUs to choose from")
    if policy == "min":
        return min(pmtus)
    if policy == "max":
        return max(pmtus)
    if policy == "median":
        return int(statistics.median(sorted(pmtus)))
    raise ValueError(f"unknown policy {policy}")

def main():
    ap = argparse.ArgumentParser(description="Compute/apply WireGuard MTU based on egress MTU and optional multi-target PMTU probing.")
    ap.add_argument("--egress-if", help="Explicit egress interface (auto-detected if omitted).")
    ap.add_argument("--force-egress-mtu", type=int, help="Force this MTU on the egress interface before computing wg MTU.")
    ap.add_argument("--wg-if", default=os.environ.get("WG_IF","wg0"), help="WireGuard interface name (default: wg0).")
    ap.add_argument("--wg-overhead", type=int, default=int(os.environ.get("WG_OVERHEAD","80")), help="Bytes of WG overhead to subtract (default: 80).")
    ap.add_argument("--wg-min", type=int, default=int(os.environ.get("WG_MIN","1280")), help="Minimum allowed WG MTU (default: 1280).")
    # PMTU (multi-target)
    ap.add_argument("--pmtu-target", action="append", help="Target hostname/IP to probe PMTU. Can be given multiple times OR comma-separated.")
    ap.add_argument("--pmtu-timeout", type=float, default=1.0, help="Timeout (seconds) per ping probe (default: 1.0).")
    ap.add_argument("--pmtu-min-payload", type=int, default=1200, help="Lower bound payload for PMTU search (default: 1200).")
    ap.add_argument("--pmtu-max-payload", type=int, default=1472, help="Upper bound payload for PMTU search (default: 1472 ~ 1500-28).")
    ap.add_argument("--pmtu-policy", choices=["min","median","max"], default="min",
                    help="How to choose effective PMTU across multiple targets (default: min).")
    ap.add_argument("--dry-run", action="store_true", help="Show actions without applying changes.")
    args = ap.parse_args()

    require_root(args.dry_run)

    # Detect egress
    egress = args.egress_if or (get_default_ifaces()[0] if get_default_ifaces() else None)
    if not egress:
        print("[wg-mtu][ERROR] Could not detect egress interface (use --egress-if).", file=sys.stderr)
        sys.exit(2)
    if not exists_iface(egress):
        print(f"[wg-mtu][ERROR] Interface {egress} does not exist.", file=sys.stderr); sys.exit(3)
    print(f"[wg-mtu] Detected egress interface: {egress}")

    # Egress MTU
    if args.force_egress_mtu:
        print(f"[wg-mtu] Forcing egress MTU {args.force_egress_mtu} on {egress}")
        set_mtu(egress, args.force_egress_mtu, args.dry_run)
        base_mtu = args.force_egress_mtu
    else:
        base_mtu = read_mtu(egress)
    print(f"[wg-mtu] Egress base MTU: {base_mtu}")

    # PMTU over multiple targets
    effective_mtu = base_mtu
    pmtu_targets = []
    if args.pmtu_target:
        # flatten comma-separated + repeated flags
        for item in args.pmtu_target:
            pmtu_targets.extend([x.strip() for x in item.split(",") if x.strip()])

    if pmtu_targets:
        results = {}
        good = []
        print(f"[wg-mtu] Probing Path MTU for: {', '.join(pmtu_targets)} (policy={args.pmtu_policy})")
        for t in pmtu_targets:
            p = probe_pmtu(t, args.pmtu_min_payload, args.pmtu_max_payload, args.pmtu_timeout)
            results[t] = p
            if p:
                good.append(p)
            print(f"[wg-mtu]  - {t}: {'%s' % p if p else 'probe failed'}")
        if good:
            chosen = choose_effective(good, args.pmtu_policy)
            print(f"[wg-mtu] Selected Path MTU (policy={args.pmtu_policy}): {chosen}")
            effective_mtu = min(base_mtu, chosen)
        else:
            print("[wg-mtu] WARNING: All PMTU probes failed. Falling back to egress MTU.")

    # Compute WG MTU
    wg_mtu = max(args.wg_min, effective_mtu - args.wg_overhead)
    print(f"[wg-mtu] Computed {args.wg_if} MTU: {wg_mtu}  (overhead={args.wg_overhead}, min={args.wg_min})")

    # Apply
    if exists_iface(args.wg_if):
        set_mtu(args.wg_if, wg_mtu, args.dry_run)
        print(f"[wg-mtu] Applied: {args.wg_if} MTU {wg_mtu}")
    else:
        print(f"[wg-mtu] NOTE: {args.wg_if} not present yet. Start WireGuard first, then re-run this script.")

    print(f"[wg-mtu] Done. Summary: egress={egress} mtu={base_mtu}, effective_mtu={effective_mtu}, {args.wg_if}_mtu={wg_mtu}")

if __name__ == "__main__":
    main()