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// shunt - DNS message parser
//
// Parses a response far enough to answer: which name was asked for, and
// which A/AAAA addresses came back. Never trusts a length off the wire.
//
// SPDX-License-Identifier: GPL-3.0-or-later
// Copyright (c) 2026 Dirk Brenken <dev@brenken.org>
// Hard limits: message size, name length, answers processed per message.
export const LIM = {
msg: 4096,
labels: 63,
name: 255,
answers: 64
};
export const TYPE = {
A: 1,
NS: 2,
CNAME: 5,
SOA: 6,
TXT: 16,
AAAA: 28,
OPT: 41
};
// Parser verdicts. These identifiers are contract - fixtures and the LuCI
// labels compare them verbatim.
export const ERR = {
SHORT: 'E_SHORT',
MSGLEN: 'E_MSGLEN',
NOTRESP: 'E_NOTRESP',
TRUNC: 'E_TRUNC',
RCODE: 'E_RCODE',
QDCOUNT: 'E_QDCOUNT',
QPTR: 'E_QPTR',
LABEL: 'E_LABEL',
NAMELEN: 'E_NAMELEN',
CHARSET: 'E_CHARSET',
RDLEN: 'E_RDLEN',
ANSMAX: 'E_ANSMAX'
};
const HDR_LEN = 12;
const RR_FIXED = 10;
const F_QR = 0x8000;
const F_TC = 0x0200;
const M_RCODE = 0x000f;
const LBL_MASK = 0xc0;
const LBL_PTR = 0xc0;
function u16at(buf, off) {
return (ord(buf, off) << 8) | ord(buf, off + 1);
}
function fmt4(buf, off) {
return sprintf('%d.%d.%d.%d',
ord(buf, off), ord(buf, off + 1),
ord(buf, off + 2), ord(buf, off + 3));
}
function fmt6(buf, off) {
let g = [];
for (let i = 0; i < 8; i++)
push(g, u16at(buf, off + i * 2));
let bs = -1, bl = 0, cs = -1, cl = 0;
for (let i = 0; i < 8; i++) {
if (g[i] != 0) {
cs = -1;
cl = 0;
continue;
}
if (cs < 0)
cs = i;
cl++;
if (cl > bl) {
bs = cs;
bl = cl;
}
}
if (bl < 2) {
bs = -1;
bl = 0;
}
let parts = [], i = 0;
while (i < 8) {
if (i == bs) {
push(parts, '');
i += bl;
continue;
}
push(parts, sprintf('%x', g[i]));
i++;
}
let out = join(':', parts);
if (bs == 0)
out = ':' + out;
if (bs >= 0 && bs + bl == 8)
out = out + ':';
return out;
}
export function decode_name(buf, off) {
let blen = length(buf), labels = [], total = 1;
while (true) {
if (off >= blen)
return { err: ERR.SHORT };
let len = ord(buf, off);
if ((len & LBL_MASK) == LBL_PTR)
return { err: ERR.QPTR };
if (len & LBL_MASK)
return { err: ERR.LABEL };
off++;
if (!len)
break;
total += len + 1;
if (total > LIM.name)
return { err: ERR.NAMELEN };
if (off + len > blen)
return { err: ERR.SHORT };
let lbl = lc(substr(buf, off, len));
for (let i = 0; i < len; i++) {
let c = ord(lbl, i);
if ((c >= 0x61 && c <= 0x7a) || (c >= 0x30 && c <= 0x39) ||
c == 0x2d || c == 0x5f)
continue;
return { err: ERR.CHARSET };
}
push(labels, lbl);
off += len;
}
return { name: join('.', labels), next: off };
};
export function skip_name(buf, off) {
let blen = length(buf);
while (true) {
if (off >= blen)
return null;
let len = ord(buf, off);
if ((len & LBL_MASK) == LBL_PTR)
return (off + 2 <= blen) ? off + 2 : null;
if (len & LBL_MASK)
return null;
off++;
if (!len)
return off;
off += len;
if (off > blen)
return null;
}
};
export function parse(buf) {
let blen = length(buf ?? '');
if (blen > LIM.msg)
return { ok: false, err: ERR.MSGLEN };
if (blen < HDR_LEN)
return { ok: false, err: ERR.SHORT };
let flags = u16at(buf, 2);
if (!(flags & F_QR))
return { ok: false, err: ERR.NOTRESP };
if (flags & F_TC)
return { ok: false, err: ERR.TRUNC };
if (flags & M_RCODE)
return { ok: false, err: ERR.RCODE };
if (u16at(buf, 4) != 1)
return { ok: false, err: ERR.QDCOUNT };
let ancount = u16at(buf, 6);
if (ancount > LIM.answers)
return { ok: false, err: ERR.ANSMAX };
let q = decode_name(buf, HDR_LEN);
if (q.err)
return { ok: false, err: q.err };
let off = q.next;
if (off + 4 > blen)
return { ok: false, err: ERR.SHORT };
let qtype = u16at(buf, off);
off += 4;
let a = [], aaaa = [];
for (let i = 0; i < ancount; i++) {
off = skip_name(buf, off);
if (off === null)
return { ok: false, err: ERR.SHORT };
if (off + RR_FIXED > blen)
return { ok: false, err: ERR.SHORT };
let rtype = u16at(buf, off);
let rdlen = u16at(buf, off + 8);
off += RR_FIXED;
if (off + rdlen > blen)
return { ok: false, err: ERR.RDLEN };
if (rtype == TYPE.A) {
if (rdlen != 4)
return { ok: false, err: ERR.RDLEN };
push(a, fmt4(buf, off));
}
else if (rtype == TYPE.AAAA) {
if (rdlen != 16)
return { ok: false, err: ERR.RDLEN };
push(aaaa, fmt6(buf, off));
}
off += rdlen;
}
return {
ok: true,
id: u16at(buf, 0),
qname: q.name,
qtype,
a,
aaaa
};
};
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