Project: Network Diagnostic Report တစ်ခု တည်ဆောက်ခြင်း

import ipaddress
import textwrap

PASS, FAIL, INFO = "PASS", "FAIL", "INFO"

# Hardcoded observations. In a real tool these come from `ip addr`, `ip route`,
# a resolver call and a connect() attempt -- here they are frozen so the
# diagnosis is reproducible.
SCENARIOS = [
    {
        "name": "laptop-a  (freshly re-imaged)",
        "host_ip": "192.168.1.50", "mask": "255.255.255.192",
        "gateway": "192.168.1.100",
        "gateway_reachable": False,
        "target_name": "api.example.com", "target_port": 443,
        "dns_result": "93.184.216.34",
        "tcp_result": "timeout",
    },
    {
        "name": "web-tier-03  (deploy just failed)",
        "host_ip": "10.20.5.31", "mask": "255.255.255.0",
        "gateway": "10.20.5.1",
        "gateway_reachable": True,
        "target_name": "db.internal", "target_port": 5432,
        "dns_result": "10.20.9.14",
        "tcp_result": "refused",
    },
    {
        "name": "workstation-7  (baseline, known good)",
        "host_ip": "172.16.4.9", "mask": "255.255.255.0",
        "gateway": "172.16.4.1",
        "gateway_reachable": True,
        "target_name": "cache.internal", "target_port": 6379,
        "dns_result": "172.16.4.60",
        "tcp_result": "open",
    },
]


def check_host_address(obs, ctx):
    iface = ipaddress.ip_interface("%s/%s" % (obs["host_ip"], obs["mask"]))
    ctx["network"] = iface.network
    if iface.ip in ipaddress.ip_network("169.254.0.0/16"):
        return FAIL, "link-local address -- DHCP never answered", \
            "Fix DHCP or assign a static address."
    return PASS, "%s in %s" % (iface.ip, iface.network), None


def check_gateway_local(obs, ctx):
    gw = ipaddress.ip_address(obs["gateway"])
    net = ctx["network"]
    if gw not in net:
        return FAIL, "gateway %s is OUTSIDE %s" % (gw, net), \
            ("The mask is wrong. /%d only spans %s-%s, so the host can never "
             "ARP for its own gateway." % (net.prefixlen, net[0], net[-1]))
    return PASS, "gateway %s is inside %s" % (gw, net), None


def check_gateway_reachable(obs, ctx):
    if not obs["gateway_reachable"]:
        return FAIL, "no reply from gateway %s" % obs["gateway"], \
            "Layer 1/2 problem: cable, VLAN, or a down switch port."
    return PASS, "gateway %s replies" % obs["gateway"], None


def check_dns(obs, ctx):
    if not obs["dns_result"]:
        return FAIL, "%s did not resolve" % obs["target_name"], \
            "Name problem, not a reachability problem. Check the resolver."
    ctx["target_ip"] = ipaddress.ip_address(obs["dns_result"])
    return PASS, "%s -> %s" % (obs["target_name"], obs["dns_result"]), None


def check_route(obs, ctx):
    target, net = ctx["target_ip"], ctx["network"]
    if target in net:
        return INFO, "%s is on-link, delivered directly" % target, None
    return INFO, "%s is off-link, sent via %s" % (target, obs["gateway"]), None


def check_tcp(obs, ctx):
    result = obs["tcp_result"]
    if result == "open":
        return PASS, "handshake completed on port %d" % obs["target_port"], None
    if result == "refused":
        return FAIL, "RST on port %d" % obs["target_port"], \
            ("The host is reachable and answered -- nothing is listening on "
             "that port. A dead service, not a network fault.")
    return FAIL, "no SYN-ACK on port %d" % obs["target_port"], \
        "Silent drop: a firewall or ACL is discarding the SYN."


CHECKS = [
    ("L3", "Host address and mask", check_host_address),
    ("L3", "Gateway within subnet", check_gateway_local),
    ("L2", "Gateway reachable", check_gateway_reachable),
    ("L7", "DNS resolution", check_dns),
    ("L3", "Routing decision", check_route),
    ("L4", "TCP connect", check_tcp),
]


def diagnose(obs):
    print("SCENARIO: %s" % obs["name"])
    print("-" * 70)
    print("  target %s:%d" % (obs["target_name"], obs["target_port"]))
    print()
    ctx = {}
    root_cause = None
    for layer, label, fn in CHECKS:
        if root_cause is not None:
            print("  %-3s %-24s %-6s %s" % (layer, label, "SKIP", "not reached"))
            continue
        status, detail, advice = fn(obs, ctx)
        print("  %-3s %-24s %-6s %s" % (layer, label, status, detail))
        if status == FAIL:
            root_cause = (layer, label, advice)
    print()
    if root_cause is None:
        print("  VERDICT: healthy -- every layer checks out.")
    else:
        layer, label, advice = root_cause
        print("  VERDICT: first failure at %s -- %s" % (layer, label))
        for i, line in enumerate(textwrap.wrap(advice, 60)):
            print("  %s %s" % ("CAUSE  :" if i == 0 else "        ", line))
    print()


print("LAYERED NETWORK DIAGNOSTIC REPORT")
print("=" * 70)
print()
for scenario in SCENARIOS:
    diagnose(scenario)