ServiceRegistration/wireutils.c

/* wireutils.c
 *
 * Copyright (c) 2019-2021 Apple Inc. All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 * DNS wire-format utility functions.
 *
 * Functions that are neither necessary for very simple DNS packet generation, nor required for parsing
 * a message, e.g. compression, name printing, etc.
 */

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <ctype.h>
#include "srp.h"
#include "dns-msg.h"

#include "mDNSEmbeddedAPI.h"
#include "DNSCommon.h"

#undef LogMsg
#define LogMsg(...)

// We need the compression routines from DNSCommon.c, but we can't link to it because that
// pulls in a _lot_ of stuff we don't want.   The solution?   Define STANDALONE (this is done
// in the Makefile, and include DNSCommon.c.
//
// The only functions that aren't excluded by STANDALONE are FindCompressionPointer and
// putDomainNameAsLabels.

#ifndef STANDALONE
#define STANDALONE
#endif
#include "../mDNSCore/DNSCommon.c"

void dns_name_free(dns_label_t *name)
{
    dns_label_t *next;
    if (name == NULL) {
        return;
    }
    next = name->next;
    free(name);
    if (next != NULL) {
        return dns_name_free(next);
    }
}

dns_name_t *
dns_name_copy(dns_name_t *original)
{
    dns_name_t *ret = NULL, **cur = &ret;
    dns_name_t *next;

    for (next = original; next; next = next->next) {
        *cur = calloc(1, 1 + next->len + (sizeof (dns_name_t)) - DNS_MAX_LABEL_SIZE);
        if (*cur == NULL) {
            if (ret != NULL) {
                dns_name_free(ret);
            }
            return NULL;
        }
        if (next->len) {
            memcpy((*cur)->data, next->data, next->len + 1);
        }
        (*cur)->len = next->len;
        cur = &((*cur)->next);
    }
    return ret;
}

// Needed for TSIG (RFC2845).
void
dns_u48_to_wire_(dns_towire_state_t *NONNULL txn,
                 uint64_t val, int line)
{
    if (!txn->error) {
        if (txn->p + 6 >= txn->lim) {
            txn->error = ENOBUFS;
            txn->truncated = true;
            txn->line = line;
            return;
        }
        *txn->p++ = (val >> 40) & 0xff;
        *txn->p++ = (val >> 32) & 0xff;
        *txn->p++ = (val >> 24) & 0xff;
        *txn->p++ = (val >> 16) & 0xff;
        *txn->p++ = (val >> 8) & 0xff;
        *txn->p++ = val & 0xff;
    }
}

void
dns_concatenate_name_to_wire_(dns_towire_state_t *towire, dns_name_t *labels_prefix, const char *prefix,
                              const char *suffix, int line)
{
    dns_wire_t namebuf;
    dns_towire_state_t namewire;
    mDNSu8 *ret;
    namebuf.data[0] = 0;

    // Don't do all this work if we're already past an error.
    if (towire->error) {
        return;
    }
    memset(&namewire, 0, sizeof namewire);
    namewire.message = &namebuf;
    namewire.lim = &namebuf.data[DNS_DATA_SIZE];
    namewire.p = namebuf.data;
    if (prefix != NULL) {
        dns_name_to_wire(NULL, &namewire, prefix);
    } else if (labels_prefix != NULL) {
        size_t bytes_written;

        if (!namewire.error) {
            bytes_written = (size_t)(namewire.lim - namewire.p);
            if (bytes_written > INT16_MAX) {
                towire->error = true;
                towire->line = __LINE__;
                return;
            }
            bytes_written = dns_name_to_wire_canonical(namewire.p, bytes_written, labels_prefix);
            // This can never occur with a valid name.
            if (bytes_written == 0) {
                namewire.truncated = true;
            } else {
                namewire.p += bytes_written;
            }
        }
    }
    if (suffix != NULL) {
        dns_full_name_to_wire(NULL, &namewire, suffix);
    }
    if (namewire.error) {
        towire->truncated = namewire.truncated;
        towire->error = namewire.error;
        towire->line = line;
    }

    ret = putDomainNameAsLabels((DNSMessage *)towire->message, towire->p, towire->lim, (domainname *)namebuf.data);
    if (ret == NULL) {
        towire->error = ENOBUFS;
        towire->truncated = true;
        towire->line = line;
        return;
    }

    // Shouldn't happen
    if (ret > towire->lim) {
        towire->error = ENOBUFS;
        towire->truncated = true;
        towire->line = line;
    } else {
        towire->p = ret;
    }
}

// Convert a dns_name_t to presentation format.   Stop conversion at the specified limit.
// A trailing dot is only written if a null label is present.
const char *NONNULL
dns_name_print_to_limit(dns_name_t *NONNULL name, dns_name_t *NULLABLE limit, char *buf, size_t bufmax)
{
    dns_label_t *lp;
    size_t ix = 0;
    size_t i;

    // Copy the labels in one at a time, putting a dot between each one; if there isn't room
    // in the buffer (shouldn't be the case), copy as much as will fit, leaving room for a NUL
    // termination.
    for (lp = name; lp != limit && lp != NULL; lp = lp->next) {
        if (ix != 0) {
            if (ix + 2 >= bufmax) {
                break;
            }
            buf[ix++] = '.';
        }
        for (i = 0; i < lp->len; i++) {
            if (isascii(lp->data[i]) && (lp->data[i] == ' ' || isprint(lp->data[i]))) {
                if (ix + 2 >= bufmax) {
                    break;
                }
                buf[ix++] = lp->data[i];
            } else {
                if (ix + 5 >= bufmax) {
                    break;
                }
                buf[ix++] = '\\';
                buf[ix++] = '0' + (lp->data[i] / 100);
                buf[ix++] = '0' + (lp->data[i] /  10) % 10;
                buf[ix++] = '0' + lp->data[i]         % 10;
            }
        }
        if (i != lp->len) {
            break;
        }
    }
    buf[ix++] = 0;
    return buf;
}

const char *NONNULL
dns_name_print(dns_name_t *NONNULL name, char *buf, size_t bufmax)
{
    return dns_name_print_to_limit(name, NULL, buf, bufmax);
}

bool
dns_labels_equal(const char *label1, const char *label2, size_t len)
{
    unsigned i;
    for (i = 0; i < len; i++) {
        if (isascii(label1[i]) && isascii(label2[i])) {
            if (tolower(label1[i]) != tolower(label2[i])) {
                return false;
            }
        }
        else {
            if (label1[i] != label2[i]) {
                return false;
            }
        }
    }
    return true;
}

bool
dns_names_equal(dns_label_t *NONNULL name1, dns_label_t *NONNULL name2)
{
    if (name1->len != name2->len) {
        return false;
    }
    if (name1->len != 0 && !dns_labels_equal(name1->data, name2->data, name1->len) != 0) {
        return false;
    }
    if (name1->next != NULL && name2->next != NULL) {
        return dns_names_equal(name1->next, name2->next);
    }
    if (name1->next == NULL && name2->next == NULL) {
        return true;
    }
    return false;
}

// Note that "foo.arpa" is not the same as "foo.arpa."
bool
dns_names_equal_text(dns_label_t *NONNULL name1, const char *NONNULL name2)
{
    const char *ndot;
    const char *s, *t;
    int tlen = 0;
    ndot = strchr(name2, '.');
    if (ndot == NULL) {
        ndot = name2 + strlen(name2);
    }
    for (s = name2; s < ndot; s++) {
        if (*s == '\\') {
            if (s + 4 <= ndot) {
                tlen++;
                s += 3;
            } else {
                return false;  // An invalid name can't be equal to anything.
            }
        } else {
            tlen++;
        }
    }
    if (name1->len != tlen) {
        return false;
    }
    if (name1->len != 0) {
        t = name1->data;
        for (s = name2; s < ndot; s++, t++) {
            if (*s == '\\') { // already bounds checked
                int v0 = s[1] - '0';
                int v1 = s[2] - '0';
                int v2 = s[3] - '0';
                int val = v0 * 100 + v1 * 10 + v2;
                if (val > 255) {
                    return false;
                } else if (isascii(*s) && isascii(*t)) {
                    if (tolower(*s) != tolower(*t)) {
                        return false;
                    }
                } else if (val != *t) {
                    return false;
                }
                s += 3;
            } else {
                if (*s != *t) {
                    return false;
                }
            }
        }
    }
    if (name1->next != NULL && *ndot == '.') {
        return dns_names_equal_text(name1->next, ndot + 1);
    }
    if (name1->next == NULL && *ndot == 0) {
        return true;
    }
    return false;
}

// Find the length of a name in uncompressed wire format.
static size_t
dns_name_wire_length_in(dns_label_t *NONNULL name, size_t ret)
{
    // Root label.
    if (name == NULL)
        return ret;
    return dns_name_wire_length_in(name->next, ret + name->len + 1);
}

size_t
dns_name_wire_length(dns_label_t *NONNULL name)
{
    return dns_name_wire_length_in(name, 0);
}

// Copy a name we've parsed from a message out in canonical wire format so that we can
// use it to verify a signature.   As above, not actually needed for copying to a message
// we're going to send, since in that case we want to try to compress.
static size_t
dns_name_to_wire_canonical_in(uint8_t *NONNULL buf, size_t max, size_t ret, dns_label_t *NONNULL name)
{
    if (name == NULL) {
        return ret;
    }
    if (max < name->len + 1) {
        return 0;
    }
    *buf = name->len;
    memcpy(buf + 1, name->data, name->len);
    return dns_name_to_wire_canonical_in(buf + name->len + 1,
                                         max - name->len - 1, ret + name->len + 1, name->next);
}

size_t
dns_name_to_wire_canonical(uint8_t *NONNULL buf, size_t max, dns_label_t *NONNULL name)
{
    return dns_name_to_wire_canonical_in(buf, max, 0, name);
}

// Parse a NUL-terminated text string into a sequence of labels.
dns_name_t *
dns_pres_name_parse(const char *pname)
{
    const char *dot, *s, *label;
    dns_label_t *next, *ret, **prev = &ret;
    size_t len;
    char *t;
    char buf[DNS_MAX_LABEL_SIZE];
    ret = NULL;

    label = pname;
    dot = strchr(label, '.');
    while (true) {
        if (dot == NULL) {
            dot = label + strlen(label);
        }
        len = (size_t)(dot - label);
        if (len > 0) {
            t = buf;
            for (s = label; s < dot; s++) {
                if (*s == '\\') { // already bounds checked
                    int v0 = s[1] - '0';
                    int v1 = s[2] - '0';
                    int v2 = s[3] - '0';
                    int val = v0 * 100 + v1 * 10 + v2;
                    if (val > 255) {
                        goto fail;
                    }
                    s += 3;
                    *t++ = (char)val;
                } else {
                    *t++ = *s;
                }
                if ((size_t)(t - buf) >= sizeof(buf)) {
                    goto fail;
                }
            }
            len = (size_t)(t - buf);
        }
        next = calloc(1, len + 1 + (sizeof *next) - DNS_MAX_LABEL_SIZE);
        if (next == NULL) {
            goto fail;
        }
        *prev = next;
        prev = &next->next;
        next->len = (uint8_t)len;
        if (next->len > 0) {
            memcpy(next->data, buf, next->len);
        }
        next->data[next->len] = 0;
        if (dot[0] == '.' && len > 0) {
            dot = dot + 1;
        }
        if (*dot == '\0') {
            if (len > 0) {
                label = dot;
            } else {
                break;
            }
        } else {
            label = dot;
            dot = strchr(label, '.');
        }
    }
    return ret;

fail:
    if (ret) {
        dns_name_free(ret);
    }
    return NULL;
}

// See if name is a subdomain of domain.   If so, return a pointer to the label in name
// where the match to domain begins.
dns_name_t *
dns_name_subdomain_of(dns_name_t *name, dns_name_t *domain)
{
    int dnum = 0, nnum = 0;
    dns_name_t *np, *dp;

    for (dp = domain; dp; dp = dp->next) {
        dnum++;
    }
    for (np = name; np; np = np->next) {
        nnum++;
    }
    if (nnum < dnum) {
        return NULL;
    }
    for (np = name; np; np = np->next) {
        if (nnum-- == dnum) {
            break;
        }
    }
    if (np != NULL && dns_names_equal(np, domain)) {
        return np;
    }
    return NULL;
}

const char *
dns_rcode_name(int rcode)
{
    switch(rcode) {
    case dns_rcode_noerror:
        return "No Error";
    case dns_rcode_formerr:
        return "Format Error";
    case dns_rcode_servfail:
        return "Server Failure";
    case dns_rcode_nxdomain:
        return "Non-Existent Domain";
    case dns_rcode_notimp:
        return "Not Implemented";
    case dns_rcode_refused:
        return "Query Refused";
    case dns_rcode_yxdomain:
        return "Name Exists when it should not";
    case dns_rcode_yxrrset:
        return "RR Set Exists when it should not";
    case dns_rcode_nxrrset:
        return "RR Set that should exist does not";
    case dns_rcode_notauth:
        return "Not Authorized";
    case dns_rcode_notzone:
        return "Name not contained in zone";
    case dns_rcode_dsotypeni:
        return "DSO-Type Not Implemented";
    case dns_rcode_badvers:
        return "TSIG Signature Failure";
    case dns_rcode_badkey:
        return "Key not recognized";
    case dns_rcode_badtime:
        return "Signature out of time window";
    case dns_rcode_badmode:
        return "Bad TKEY Mode";
    case dns_rcode_badname:
        return "Duplicate key name";
    case dns_rcode_badalg:
        return "Algorithm not supported";
    case dns_rcode_badtrunc:
        return "Bad Truncation";
    case dns_rcode_badcookie:
        return "Bad/missing Server Cookie";
    default:
        return "Unknown rcode.";
    }
}

bool
dns_keys_rdata_equal(dns_rr_t *key1, dns_rr_t *key2)
{
    if ((key1->type == dns_rrtype_key && key2->type == dns_rrtype_key) &&
        key1->data.key.flags == key2->data.key.flags &&
        key1->data.key.protocol == key2->data.key.protocol &&
        key1->data.key.algorithm == key2->data.key.algorithm &&
        key1->data.key.len == key2->data.key.len &&
        !memcmp(key1->data.key.key, key2->data.key.key, key1->data.key.len))
    {
        return true;
    }
    return false;
}

void
dns_txt_data_print(char *txt_buf, size_t buf_size, uint16_t txt_length, uint8_t *txt_data)
{
    uint16_t index = 0;
    char *sp = txt_buf;
    char *lim = sp + buf_size;
    const char *continuation_string = "";
    const char *commasp = ", ";
    size_t continuation_length = 0;
    txt_buf[0] = 0;
    while (sp < lim && index < txt_length) {
        uint16_t hunk_len = txt_data[index];
        uint16_t next_index = index + hunk_len + 1;

        // Overflow or past the end of data?
        if (next_index > txt_length || next_index < index) {
            break;
        }

        // Out of space, shouldn't be possible.
        if (sp + continuation_length + 1 >= lim) {
            break;
        }
        if (hunk_len != 0) {
            if (continuation_length != 0 && sp + continuation_length + 1 < lim) {
                memcpy(sp, continuation_string, continuation_length);
                sp += continuation_length;
                *sp = 0;
            }
            continuation_string = commasp;
            continuation_length = 2;

            for (int i = index + 1; i < index + 1 + hunk_len; i++) {
                if (isascii(txt_data[i]) && isprint(txt_data[i])) {
                    if (sp + 1 < lim) {
                        *sp++ = txt_data[i];
                        *sp = 0;
                    }
                } else {
                    if (sp + 5 < lim) {
                        size_t ret = snprintf(sp, 5, "\%o", txt_data[i]);
                        sp += ret; // Note that this might push sp past lim, which is fine because we'll then exit the loops.
                    }
                }
            }
        }
        index = next_index;
    }
}

bool
dns_rrs_equal(dns_rr_t *a, dns_rr_t *b, bool rdata_present)
{
    // Obvious stuff first...  We do not compare TTL.
    if (a->type != b->type || a->qclass != b->qclass) {
        return false;
    }
    if (!dns_names_equal(a->name, b->name)) {
        return false;
    }
    if (!rdata_present) {
        return true;
    }

    switch(a->type) {
        // There's no reason to compare invalid RRs, but if we do, they are all equally invalid.
    case dns_invalid_rr:
        return true;

        // Anything we don't have a specific format for we store as binary data.
    default:
        if (a->data.unparsed.len == b->data.unparsed.len) {
            return memcmp(a->data.unparsed.data, b->data.unparsed.data, a->data.unparsed.len) == 0;
        }
        break;

        // All have a single name as the data
    case dns_rrtype_ptr:
    case dns_rrtype_ns:
    case dns_rrtype_cname:
        return dns_names_equal(a->data.ptr.name, b->data.ptr.name);

    case dns_rrtype_srv:
        if (a->data.srv.priority == b->data.srv.priority &&
            a->data.srv.weight == b->data.srv.weight && a->data.srv.port == b->data.srv.port)
        {
            return dns_names_equal(a->data.srv.name, b->data.srv.name);
        }
        break;

    case dns_rrtype_a:
        return a->data.a.s_addr == b->data.a.s_addr;

    case dns_rrtype_aaaa:
        return in6addr_compare(&a->data.aaaa, &b->data.aaaa) == 0;

        // We could compare signatures, but it doesn't really make sense.
    case dns_rrtype_sig:
        break;

    case dns_rrtype_key:
        return dns_keys_rdata_equal(a, b);

    case dns_rrtype_txt:
        if (a->data.txt.len == b->data.txt.len) {
            return memcmp(a->data.txt.data, b->data.txt.data, a->data.txt.len) == 0;
        }
    }
    return false;
}

bool
dns_rr_to_wire(dns_towire_state_t *towire, dns_rr_t *rr, bool question)
{
    uint8_t *revert = towire->p;

    if (towire->truncated) {
        return false;
    }

    // Copy out the invariants.
    dns_concatenate_name_to_wire(towire, rr->name, NULL, NULL);
    dns_u16_to_wire(towire, rr->type);
    dns_u16_to_wire(towire, rr->qclass);

    // Questions don't have RDATA.
    if (!question) {
        dns_ttl_to_wire(towire, rr->ttl);
        dns_rdlength_begin(towire);
        switch(rr->type) {
            // There's no reason to compare invalid RRs, but if we do, they are all equally invalid.
        case dns_invalid_rr:
            ERROR("invalid rr!");
            towire->error = EINVAL;
            break;

            // Anything we don't have a specific format for we store as binary data.
        default:
            dns_rdata_raw_data_to_wire(towire, rr->data.unparsed.data, rr->data.unparsed.len);
            break;

            // All have a single name as the data
        case dns_rrtype_ptr:
        case dns_rrtype_ns:
        case dns_rrtype_cname:
            dns_concatenate_name_to_wire(towire, rr->data.ptr.name, NULL, NULL);
            break;

        case dns_rrtype_srv:
            dns_u16_to_wire(towire, rr->data.srv.priority);
            dns_u16_to_wire(towire, rr->data.srv.weight);
            dns_u16_to_wire(towire, rr->data.srv.port);
            dns_concatenate_name_to_wire(towire, rr->data.ptr.name, NULL, NULL);
            break;

        case dns_rrtype_a:
            dns_rdata_raw_data_to_wire(towire, &rr->data.a, sizeof(rr->data.a));
            break;

        case dns_rrtype_aaaa:
            dns_rdata_raw_data_to_wire(towire, &rr->data.aaaa, sizeof(rr->data.aaaa));
            break;

            // We could compare signatures, but it doesn't really make sense.
        case dns_rrtype_sig:
            ERROR("signature not valid here!");
            towire->error = EINVAL;
            break;

        case dns_rrtype_key:
            ERROR("key not valid here!");
            towire->error = EINVAL;
            break;

        case dns_rrtype_txt:
            dns_rdata_raw_data_to_wire(towire, rr->data.txt.data, rr->data.txt.len);
            break;
        }
        dns_rdlength_end(towire);
    }

    if (towire->truncated || towire->error) {
        towire->p = revert;
        return false;
    }
    return true;
}

void
dns_message_rrs_to_wire(dns_towire_state_t *towire, dns_message_t *message)
{
    bool question = true;
    for (int i = 0; i < 4; i++) {
        int count = 0;
        dns_rr_t *rrs = NULL;
        switch(i) {
        case 0: count = message->qdcount; rrs = message->questions; break;
        case 1: count = message->ancount; rrs = message->answers; break;
        case 2: count = message->nscount; rrs = message->authority; break;
        case 3: count = message->arcount; rrs = message->additional; break;
        }

        for (int j = 0; j < count; j++) {
            dns_rr_t *rr = &rrs[j];
            if (!dns_rr_to_wire(towire, rr, question)) {
                // XXX if it's TCP we really need to embiggen here.
                ERROR("no space in message for rr %d/%d %d", i, j, rr->type);
            }
        }
        question = false;
    }
}

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