packet.c

Go to the documentation of this file.

/*
 *   This 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.
 *
 *   This 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 this library; if not, write to the Free Software
 *   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA
 */
 
/**
 * $Id: a467adab8192a98126c59d74dc792c0a134b63d2 $
 *
 * @file protocols/dhcpv4/packet.c
 * @brief Functions to encode/decode DHCP packets.
 *
 * @copyright 2008,2017 The FreeRADIUS server project
 * @copyright 2008 Alan DeKok (aland@deployingradius.com)
 */
#include <freeradius-devel/util/pair.h>
#include <freeradius-devel/util/rand.h>
#include <freeradius-devel/protocol/dhcpv4/rfc2131.h>
 
#include "dhcpv4.h"
#include "attrs.h"
 
/** Retrieve a DHCP option from a raw packet buffer
 *
 *
 */
uint8_t const *fr_dhcpv4_packet_get_option(dhcp_packet_t const *packet, size_t packet_size, fr_dict_attr_t const *da)
{
        int overload = 0;
        int field = DHCP_OPTION_FIELD;
        size_t where, size;
        uint8_t const *data;
 
        if (packet_size < MIN_PACKET_SIZE) return NULL;
 
        /*
         *      This is needed for UBSAN on MacOS, that doesn't
         *      allow misaligned accesses.  Because the packet
         *      structure is flat, we don't need to deref the
         *      packet pointer at any point, we just need to
         *      calculate the offsets relative to the pointer
         *      value and use those... Whatever actually deals
         *      with the option is just expecting a uint8_t *.
         */
#define ALIGNED_ACCESS(packet, field) \
                (uint8_t const *)packet + offsetof(dhcp_packet_t, field)
 
        where = 0;
        size = packet_size - offsetof(dhcp_packet_t, options);
 
        /*
         *      Alignment fix.  We can't just deref a pointer
         */
        data = ALIGNED_ACCESS(packet, options);
        while (where < size) {
                if (data[0] == 0) { /* padding */
                        where++;
                        data++;
                        continue;
                }
 
                if (data[0] == 255) { /* end of options */
                        if ((field == DHCP_OPTION_FIELD) && (overload & DHCP_FILE_FIELD)) {
                                data = ALIGNED_ACCESS(packet, file);
                                where = 0;
                                size = sizeof(packet->file);
                                field = DHCP_FILE_FIELD;
                                continue;
 
                        } else if ((field == DHCP_FILE_FIELD || field == DHCP_OPTION_FIELD) && (overload & DHCP_SNAME_FIELD)) {
                                data = ALIGNED_ACCESS(packet, sname);
                                where = 0;
                                size = sizeof(packet->sname);
                                field = DHCP_SNAME_FIELD;
                                continue;
                        }
 
                        return NULL;
                }
 
                /*
                 *      We MUST have a real option here.
                 */
                if ((where + 2) > size) {
                        fr_strerror_printf("Options overflow field at %u",
                                           (unsigned int) (data - (uint8_t const *) packet));
                        return NULL;
                }
 
                if ((where + 2 + data[1]) > size) {
                        fr_strerror_printf("Option length overflows field at %u",
                                           (unsigned int) (data - (uint8_t const *) packet));
                        return NULL;
                }
 
                if (data[0] == da->attr) return data;
 
                if ((data[0] == 52) && (data[1] > 0)) { /* overload sname and/or file */
                        overload = data[2];
                }
 
                where += data[1] + 2;
                data += data[1] + 2;
        }
 
        return NULL;
}
 
int fr_dhcpv4_decode(TALLOC_CTX *ctx, fr_pair_list_t *out, uint8_t const *data, size_t data_len, unsigned int *code)
{
        size_t          i;
        uint8_t const   *p = data;
        uint32_t        giaddr;
        fr_pair_list_t  tmp;
        fr_pair_t       *vp;
        fr_pair_t       *maxms, *mtu, *netaddr;
        fr_value_box_t  box;
        fr_dhcpv4_ctx_t *packet_ctx;
 
        fr_pair_list_init(&tmp);
 
        if (data[1] > 1) {
                fr_strerror_printf("Packet is not Ethernet: %u",
                      data[1]);
                return -1;
        }
 
        packet_ctx = talloc_zero(ctx, fr_dhcpv4_ctx_t);
        if (!packet_ctx) return -1;
        packet_ctx->tmp_ctx = talloc(packet_ctx, uint8_t);
        packet_ctx->root = fr_dict_root(dict_dhcpv4);
 
        /*
         *      Decode the header.
         */
        for (i = 0; i < dhcp_header_attrs_len; i++) {
                fr_dict_attr_t const *da = *dhcp_header_attrs[i];
 
                vp = fr_pair_afrom_da(ctx, da);
                if (!vp) {
                        fr_strerror_const_push("Cannot decode packet due to internal error");
                error:
                        talloc_free(vp);
                        fr_pair_list_free(&tmp);
                        talloc_free(packet_ctx);
                        return -1;
                }
 
                switch (vp->vp_type) {
                case FR_TYPE_STRING:
                        /*
                         *      According to RFC 2131, these are null terminated strings.
                         *      We don't trust everyone to abide by the RFC, though.
                         */
                        if (*p != '\0') {
                                uint8_t *q;
 
                                q = memchr(p, '\0', dhcp_header_sizes[i]);
                                fr_pair_value_bstrndup(vp, (char const *)p, q ? q - p : dhcp_header_sizes[i], true);
                        } else {
                                TALLOC_FREE(vp);
                        }
                        break;
 
                        /*
                         *      The DHCP header size for CHADDR is not
                         *      6, so the value_box function doesn't
                         *      like it.  Just do the copy manually.
                         */
                case FR_TYPE_ETHERNET:
                        if ((data[1] != 1) || (data[2] != 6)) {
                                TALLOC_FREE(vp);
                                break;
                        }
 
                        memcpy(vp->vp_ether, p, sizeof(vp->vp_ether));
                        break;
 
                default:
                        if (fr_value_box_from_network(vp, &vp->data, vp->vp_type, vp->da,
                                                      &FR_DBUFF_TMP(p, (size_t)dhcp_header_sizes[i]),
                                                      dhcp_header_sizes[i], true) < 0) goto error;
                        break;
                }
                p += dhcp_header_sizes[i];
 
                if (!vp) continue;
 
                fr_pair_append(&tmp, vp);
        }
 
        /*
         *      Nothing uses tail after this call, if it does in the future
         *      it'll need to find the new tail...
         */
        {
                uint8_t const           *end;
                ssize_t                 len;
 
                p = data + 240;
                end = p + (data_len - 240);
 
                /*
                 *      Loop over all the options data
                 */
                while (p < end) {
                        len = fr_dhcpv4_decode_option(ctx, &tmp, p, (end - p), packet_ctx);
                        if (len <= 0) {
                        fail:
                                fr_pair_list_free(&tmp);
                                talloc_free(packet_ctx);
                                return -1;
                        }
                        p += len;
                }
 
                if (code) {
                        vp = fr_pair_find_by_da(&tmp, NULL, attr_dhcp_message_type);
                        if (vp) {
                                *code = vp->vp_uint8;
                        }
                }
 
                /*
                 *      If option Overload is present in the 'options' field, then fields 'file' and/or 'sname'
                 *      are used to hold more options. They are partitioned and must be interpreted in sequence.
                 */
                vp = fr_pair_find_by_da(&tmp, NULL, attr_dhcp_overload);
                if (vp) {
                        if ((vp->vp_uint8 & 1) == 1) {
                                /*
                                 *      The 'file' field is used to hold options.
                                 *      It must be interpreted before 'sname'.
                                 */
                                p = data + 44;
                                end = p + 64;
                                while (p < end) {
                                        len = fr_dhcpv4_decode_option(ctx, &tmp,
                                                                      p, end - p, packet_ctx);
                                        if (len <= 0) goto fail;
                                        p += len;
                                }
                                fr_pair_delete_by_da(&tmp, attr_dhcp_boot_filename);
                        }
                        if ((vp->vp_uint8 & 2) == 2) {
                                /*
                                 *      The 'sname' field is used to hold options.
                                 */
                                p = data + 108;
                                end = p + 128;
                                while (p < end) {
                                        len = fr_dhcpv4_decode_option(ctx, &tmp,
                                                                      p, end - p, packet_ctx);
                                        if (len <= 0) goto fail;
                                        p += len;
                                }
                                fr_pair_delete_by_da(&tmp, attr_dhcp_server_host_name);
                        }
                }
        }
 
        /*
         *      If DHCP request, set ciaddr to zero.
         */
 
        /*
         *      Set broadcast flag for broken vendors, but only if
         *      giaddr isn't set.
         */
        memcpy(&giaddr, data + 24, sizeof(giaddr));
        if (giaddr == htonl(INADDR_ANY)) {
                /*
                 *      DHCP Opcode is request
                 */
                vp = fr_pair_find_by_da(&tmp, NULL, attr_dhcp_opcode);
                if (vp && vp->vp_uint8 == 1) {
                        /*
                         *      Vendor is "MSFT 98"
                         */
                        vp = fr_pair_find_by_da(&tmp, NULL, attr_dhcp_vendor_class_identifier);
                        if (vp && (vp->vp_length == 7) && (memcmp(vp->vp_strvalue, "MSFT 98", 7) == 0)) {
                                vp = fr_pair_find_by_da(&tmp, NULL, attr_dhcp_flags);
 
                                /*
                                 *      Reply should be broadcast.
                                 */
                                if (vp) vp->vp_uint16 |= 0x8000;
                        }
                }
        }
 
        /*
         *      Determine the address to use in looking up which subnet the
         *      client belongs to based on packet data.  The sequence here
         *      is based on ISC DHCP behaviour and RFCs 3527 and 3011.  We
         *      store the found address in an internal attribute of
         *      Network-Subnet
         *
         *
         *      All of these options / fields are type "ipv4addr", so
         *      we need to decode them as that.  And then cast it to
         *      "ipv4prefix".
         */
        vp = fr_pair_afrom_da(ctx, attr_dhcp_network_subnet);
        if (!vp) return -1;
 
        /*
         *      First look for Relay-Link-Selection
         */
        netaddr = fr_pair_find_by_da_nested(&tmp, NULL, attr_dhcp_relay_link_selection);
        if (!netaddr) {
                /*
                 *      Next try Subnet-Selection-Option
                 */
                netaddr = fr_pair_find_by_da(&tmp, NULL, attr_dhcp_subnet_selection_option);
        }
 
        if (netaddr) {
                /*
                 *      Store whichever address we found from options and ensure
                 *      the data type matches the pair, i.e address to prefix
                 *      conversion.
                 */
                if (fr_value_box_cast(vp, &vp->data, vp->vp_type, vp->da, &netaddr->data) < 0) return -1;
 
        } else if (giaddr != htonl(INADDR_ANY)) {
                /*
                 *      Gateway address is set - use that one
                 */
                if (fr_value_box_from_network(vp, &box, FR_TYPE_IPV4_ADDR, NULL,
                                          &FR_DBUFF_TMP(data + 24, 4), 4, true) < 0) return -1;
                if (fr_value_box_cast(vp, &vp->data, vp->vp_type, vp->da, &box) < 0) return -1;
 
        } else {
                /*
                 *      else, store client address whatever it is
                 */
                if (fr_value_box_from_network(vp, &box, FR_TYPE_IPV4_ADDR, NULL,
                                          &FR_DBUFF_TMP(data + 12, 4), 4, true) < 0) return -1;
                if (fr_value_box_cast(vp, &vp->data, vp->vp_type, vp->da, &box) < 0) return -1;
        }
 
        fr_pair_append(&tmp, vp);
 
        /*
         *      Client can request a LARGER size, but not a smaller
         *      one.  They also cannot request a size larger than MTU.
         */
        maxms = fr_pair_find_by_da(&tmp, NULL, attr_dhcp_dhcp_maximum_msg_size);
        mtu = fr_pair_find_by_da(&tmp, NULL, attr_dhcp_interface_mtu_size);
 
        if (mtu && (mtu->vp_uint16 < DEFAULT_PACKET_SIZE)) {
                fr_strerror_const("Client says MTU is smaller than minimum permitted by the specification");
                return -1;
        }
 
        /*
         *      Client says maximum message size is smaller than minimum permitted
         *      by the specification: fixing it.
         */
        if (maxms && (maxms->vp_uint16 < DEFAULT_PACKET_SIZE)) maxms->vp_uint16 = DEFAULT_PACKET_SIZE;
 
        /*
         *      Client says MTU is smaller than maximum message size: fixing it
         */
        if (maxms && mtu && (maxms->vp_uint16 > mtu->vp_uint16)) maxms->vp_uint16 = mtu->vp_uint16;
 
        /*
         *      FIXME: Nuke attributes that aren't used in the normal
         *      header for discover/requests.
         */
        fr_pair_list_append(out, &tmp);
 
        return 0;
}
 
int fr_dhcpv4_packet_encode(fr_packet_t *packet, fr_pair_list_t *list)
{
        ssize_t         len;
        fr_pair_t       *vp;
 
        if (packet->data) return 0;
 
        packet->data_len = MAX_PACKET_SIZE;
        packet->data = talloc_zero_array(packet, uint8_t, packet->data_len);
 
        /* XXX Ugly ... should be set by the caller */
        if (packet->code == 0) packet->code = FR_DHCP_NAK;
 
        /* store xid */
        if ((vp = fr_pair_find_by_da(list, NULL, attr_dhcp_transaction_id))) {
                packet->id = vp->vp_uint32;
        } else {
                packet->id = fr_rand();
        }
 
        len = fr_dhcpv4_encode(packet->data, packet->data_len, NULL, packet->code, packet->id, list);
        if (len < 0) return -1;
 
        packet->data_len = len;
 
        return 0;
}
 
fr_packet_t *fr_dhcpv4_packet_alloc(uint8_t const *data, ssize_t data_len)
{
        fr_packet_t *packet;
        uint32_t        magic;
        uint8_t const   *code;
 
        code = fr_dhcpv4_packet_get_option((dhcp_packet_t const *) data, data_len, attr_dhcp_message_type);
        if (!code) return NULL;
 
        if (data_len < MIN_PACKET_SIZE) return NULL;
 
        /* Now that checks are done, allocate packet */
        packet = fr_packet_alloc(NULL, false);
        if (!packet) {
                fr_strerror_const("Failed allocating packet");
                return NULL;
        }
 
        /*
         *      Get XID.
         */
        memcpy(&magic, data + 4, 4);
 
        packet->data_len = data_len;
        packet->code = code[2];
        packet->id = ntohl(magic);
 
        /*
         *      FIXME: for DISCOVER / REQUEST: src_port == dst_port + 1
         *      FIXME: for OFFER / ACK       : src_port = dst_port - 1
         */
 
        /*
         *      Unique keys are xid, client mac, and client ID?
         */
        return packet;
}