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/* Convert a DNS packet to a human-readable representation.
Copyright (C) 2016-2025 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#include <support/format_nss.h>
#include <arpa/inet.h>
#include <resolv.h>
#include <stdbool.h>
#include <support/check.h>
#include <support/support.h>
#include <support/xmemstream.h>
struct in_buffer
{
const unsigned char *data;
size_t size;
};
static inline bool
extract_16 (struct in_buffer *in, unsigned short *value)
{
if (in->size < 2)
return false;
*value = (in->data[0] << 8) | in->data[1];
in->data += 2;
in->size -= 2;
return true;
}
static inline bool
extract_32 (struct in_buffer *in, unsigned *value)
{
if (in->size < 4)
return false;
unsigned a = in->data[0];
unsigned b = in->data[1];
unsigned c = in->data[2];
unsigned d = in->data[3];
*value = (a << 24) | (b << 16) | (c << 8) | d;
in->data += 4;
in->size -= 4;
return true;
}
static inline bool
extract_bytes (struct in_buffer *in, size_t length, struct in_buffer *value)
{
if (in->size < length)
return false;
*value = (struct in_buffer) {in->data, length};
in->data += length;
in->size -= length;
return true;
}
struct dname
{
char name[MAXDNAME + 1];
};
static bool
extract_name (struct in_buffer full, struct in_buffer *in, struct dname *value)
{
const unsigned char *full_end = full.data + full.size;
/* Sanity checks; these indicate buffer misuse. */
TEST_VERIFY_EXIT
(!(in->data < full.data || in->data > full_end
|| in->size > (size_t) (full_end - in->data)));
int ret = dn_expand (full.data, full_end, in->data,
value->name, sizeof (value->name));
if (ret < 0)
return false;
in->data += ret;
in->size -= ret;
return true;
}
static void
extract_name_data (struct in_buffer full, struct in_buffer *rdata,
const struct dname *owner, const char *typename, FILE *out)
{
struct dname name;
if (extract_name (full, rdata, &name))
fprintf (out, "data: %s %s %s\n", owner->name, typename, name.name);
else
fprintf (out, "error: malformed CNAME/PTR record\n");
}
char *
support_format_dns_packet (const unsigned char *buffer, size_t length)
{
struct in_buffer full = { buffer, length };
struct in_buffer in = full;
struct xmemstream mem;
xopen_memstream (&mem);
unsigned short txnid;
unsigned short flags;
unsigned short qdcount;
unsigned short ancount;
unsigned short nscount;
unsigned short adcount;
if (!(extract_16 (&in, &txnid)
&& extract_16 (&in, &flags)
&& extract_16 (&in, &qdcount)
&& extract_16 (&in, &ancount)
&& extract_16 (&in, &nscount)
&& extract_16 (&in, &adcount)))
{
fprintf (mem.out, "error: could not parse DNS header\n");
goto out;
}
if (qdcount != 1)
{
fprintf (mem.out, "error: question count is %d, not 1\n", qdcount);
goto out;
}
struct dname qname;
if (!extract_name (full, &in, &qname))
{
fprintf (mem.out, "error: malformed QNAME\n");
goto out;
}
unsigned short qtype;
unsigned short qclass;
if (!(extract_16 (&in, &qtype)
&& extract_16 (&in, &qclass)))
{
fprintf (mem.out, "error: malformed question\n");
goto out;
}
if (qtype != T_A && qtype != T_AAAA && qtype != T_PTR)
{
fprintf (mem.out, "error: unsupported QTYPE %d\n", qtype);
goto out;
}
fprintf (mem.out, "name: %s\n", qname.name);
for (int i = 0; i < ancount; ++i)
{
struct dname rname;
if (!extract_name (full, &in, &rname))
{
fprintf (mem.out, "error: malformed record name\n");
goto out;
}
unsigned short rtype;
unsigned short rclass;
unsigned ttl;
unsigned short rdlen;
struct in_buffer rdata;
if (!(extract_16 (&in, &rtype)
&& extract_16 (&in, &rclass)
&& extract_32 (&in, &ttl)
&& extract_16 (&in, &rdlen)
&& extract_bytes (&in, rdlen, &rdata)))
{
fprintf (mem.out, "error: malformed record header\n");
goto out;
}
/* Skip non-matching record types. */
if ((rtype != qtype && rtype != T_CNAME) || rclass != qclass)
continue;
switch (rtype)
{
case T_A:
if (rdlen == 4)
fprintf (mem.out, "address: %d.%d.%d.%d\n",
rdata.data[0],
rdata.data[1],
rdata.data[2],
rdata.data[3]);
else
fprintf (mem.out, "error: A record of size %d: %s\n",
rdlen, rname.name);
break;
case T_AAAA:
{
if (rdlen == 16)
{
char buf[100];
if (inet_ntop (AF_INET6, rdata.data, buf, sizeof (buf)) == NULL)
fprintf (mem.out, "error: AAAA record decoding failed: %m\n");
else
fprintf (mem.out, "address: %s\n", buf);
}
else
fprintf (mem.out, "error: AAAA record of size %d: %s\n",
rdlen, rname.name);
}
break;
case T_CNAME:
extract_name_data (full, &rdata, &rname, "CNAME", mem.out);
break;
case T_PTR:
extract_name_data (full, &rdata, &rname, "PTR", mem.out);
break;
}
}
out:
xfclose_memstream (&mem);
return mem.buffer;
}
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