/*- * Copyright (c) 2008 Michael J. Silbersack. * All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * 1. Redistributions of source code must retain the above copyright * notice unmodified, this list of conditions, and the following * disclaimer. * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */ #include __FBSDID("$FreeBSD$"); /* * IP ID generation is a fascinating topic. * * In order to avoid ID collisions during packet reassembly, common sense * dictates that the period between reuse of IDs be as large as possible. * This leads to the classic implementation of a system-wide counter, thereby * ensuring that IDs repeat only once every 2^16 packets. * * Subsequent security researchers have pointed out that using a global * counter makes ID values predictable. This predictability allows traffic * analysis, idle scanning, and even packet injection in specific cases. * These results suggest that IP IDs should be as random as possible. * * The "searchable queues" algorithm used in this IP ID implementation was * proposed by Amit Klein. It is a compromise between the above two * viewpoints that has provable behavior that can be tuned to the user's * requirements. * * The basic concept is that we supplement a standard random number generator * with a queue of the last L IDs that we have handed out to ensure that all * IDs have a period of at least L. * * To efficiently implement this idea, we keep two data structures: a * circular array of IDs of size L and a bitstring of 65536 bits. * * To start, we ask the RNG for a new ID. A quick index into the bitstring * is used to determine if this is a recently used value. The process is * repeated until a value is returned that is not in the bitstring. * * Having found a usable ID, we remove the ID stored at the current position * in the queue from the bitstring and replace it with our new ID. Our new * ID is then added to the bitstring and the queue pointer is incremented. * * The lower limit of 512 was chosen because there doesn't seem to be much * point to having a smaller value. The upper limit of 32768 was chosen for * two reasons. First, every step above 32768 decreases the entropy. Taken * to an extreme, 65533 would offer 1 bit of entropy. Second, the number of * attempts it takes the algorithm to find an unused ID drastically * increases, killing performance. The default value of 8192 was chosen * because it provides a good tradeoff between randomness and non-repetition. * * With L=8192, the queue will use 16K of memory. The bitstring always * uses 8K of memory. No memory is allocated until the use of random ids is * enabled. */ #include #include #include #include #include #include #include #include #include #include #include #include #include #include static MALLOC_DEFINE(M_IPID, "ipid", "randomized ip id state"); static u_int16_t *id_array = NULL; static bitstr_t *id_bits = NULL; static int array_ptr = 0; static int array_size = 8192; static int random_id_collisions = 0; static int random_id_total = 0; static struct mtx ip_id_mtx; static void ip_initid(void); static int sysctl_ip_id_change(SYSCTL_HANDLER_ARGS); MTX_SYSINIT(ip_id_mtx, &ip_id_mtx, "ip_id_mtx", MTX_DEF); SYSCTL_DECL(_net_inet_ip); SYSCTL_PROC(_net_inet_ip, OID_AUTO, random_id_period, CTLTYPE_INT|CTLFLAG_RW, &array_size, 0, sysctl_ip_id_change, "IU", "IP ID Array size"); SYSCTL_INT(_net_inet_ip, OID_AUTO, random_id_collisions, CTLFLAG_RD, &random_id_collisions, 0, "Count of IP ID collisions"); SYSCTL_INT(_net_inet_ip, OID_AUTO, random_id_total, CTLFLAG_RD, &random_id_total, 0, "Count of IP IDs created"); static int sysctl_ip_id_change(SYSCTL_HANDLER_ARGS) { int error, new; new = array_size; error = sysctl_handle_int(oidp, &new, 0, req); if (error == 0 && req->newptr) { if (new >= 512 && new <= 32768) { mtx_lock(&ip_id_mtx); array_size = new; ip_initid(); mtx_unlock(&ip_id_mtx); } else error = EINVAL; } return (error); } /* * ip_initid() runs with a mutex held and may execute in a network context. * As a result, it uses M_NOWAIT. Ideally, we would always do this * allocation from the sysctl contact and have it be an invariant that if * this random ID allocation mode is selected, the buffers are present. This * would also avoid potential network context failures of IP ID generation. */ static void ip_initid(void) { mtx_assert(&ip_id_mtx, MA_OWNED); if (id_array != NULL) { free(id_array, M_IPID); free(id_bits, M_IPID); } random_id_collisions = 0; random_id_total = 0; array_ptr = 0; id_array = (u_int16_t *) malloc(array_size * sizeof(u_int16_t), M_IPID, M_NOWAIT | M_ZERO); id_bits = (bitstr_t *) malloc(bitstr_size(65536), M_IPID, M_NOWAIT | M_ZERO); if (id_array == NULL || id_bits == NULL) { /* Neither or both. */ if (id_array != NULL) { free(id_array, M_IPID); id_array = NULL; } if (id_bits != NULL) { free(id_bits, M_IPID); id_bits = NULL; } } } u_int16_t ip_randomid(void) { u_int16_t new_id; mtx_lock(&ip_id_mtx); if (id_array == NULL) ip_initid(); /* * Fail gracefully; return a fixed id if memory allocation failed; * ideally we wouldn't do allocation in this context in order to * avoid the possibility of this failure mode. */ if (id_array == NULL) { mtx_unlock(&ip_id_mtx); return (1); } /* * To avoid a conflict with the zeros that the array is initially * filled with, we never hand out an id of zero. */ new_id = 0; do { if (new_id != 0) random_id_collisions++; arc4rand(&new_id, sizeof(new_id), 0); } while (bit_test(id_bits, new_id) || new_id == 0); bit_clear(id_bits, id_array[array_ptr]); bit_set(id_bits, new_id); id_array[array_ptr] = new_id; array_ptr++; if (array_ptr == array_size) array_ptr = 0; random_id_total++; mtx_unlock(&ip_id_mtx); return (new_id); }