别名最终解析到哪

按记录表，shop.example.com 跟着别名链走，最终解析到哪个 IP？

贯穿本节的记录表 RECORDS，每条是（名字, 类型, 值, 优先级）：A=域名对 IPv4、AAAA=对 IPv6、CNAME=别名指向另一个名字、MX=收信服务器（带优先级，数字小的先用）。type_of 交回一个名字最主要的记录类型，resolve 跟着 CNAME 链一路走到 A 记录的地址，mx_top 挑优先级最小的收信服务器。

RECORDS = [
    ("www.example.com", "A", "93.184.216.34", 0),
    ("www.example.com", "AAAA", "2606:2800:220:1:248:1893:25c8:1946", 0),
    ("example.com", "MX", "mail.example.com", 10),
    ("example.com", "MX", "backup.example.com", 20),
    ("shop.example.com", "CNAME", "www.example.com", 0),
    ("mail.example.com", "A", "93.184.216.35", 0),
    ("backup.example.com", "A", "93.184.216.36", 0),
]


def records_of(name):
    return [r for r in RECORDS if r[0] == name]


def type_of(name):
    # 这个名字最主要的记录类型（一个名字可能有多条，取第一条登记的）
    for n, t, v, p in RECORDS:
        if n == name:
            return t
    return "无"


def resolve(name):
    # 跟着 CNAME 链一路走到 A 记录的地址（别名指向别名也跟）
    seen = 0
    while seen < 16:
        seen += 1
        hit = None
        for n, t, v, p in RECORDS:
            if n == name and t == "CNAME":
                name = v
                hit = "cname"
                break
        if hit:
            continue
        for n, t, v, p in RECORDS:
            if n == name and t == "A":
                return v
        return "无"
    return "无"


def mx_top(domain):
    # 收信优先挑 MX 优先级数字最小的那台
    mxs = sorted((p, v) for n, t, v, p in RECORDS if n == domain and t == "MX")
    return mxs[0][1] if mxs else "无"

print(resolve("shop.example.com"))