Implements better checks on incoming packets and LSAs in OSPF.

This commit is contained in:
Ondrej Zajicek 2009-10-29 23:57:42 +01:00
parent 9d4d38d1a5
commit a6bc04d591
10 changed files with 340 additions and 101 deletions

View file

@ -53,7 +53,7 @@ static void ospf_dump_dbdes(struct proto *p, struct ospf_dbdes_packet *pkt)
log(L_TRACE "%s: ddseq %u", p->name, ntohl(pkt->ddseq));
struct ospf_lsa_header *plsa = (void *) (pkt + 1);
int i, j;
unsigned int i, j;
j = (ntohs(op->length) - sizeof(struct ospf_dbdes_packet)) /
sizeof(struct ospf_lsa_header);
@ -247,11 +247,17 @@ void
ospf_dbdes_receive(struct ospf_packet *ps_i, struct ospf_iface *ifa,
struct ospf_neighbor *n)
{
struct ospf_dbdes_packet *ps = (void *) ps_i;
struct proto *p = &ifa->oa->po->proto;
u32 myrid = p->cf->global->router_id;
unsigned int size = ntohs(ps->ospf_packet.length);
unsigned int size = ntohs(ps_i->length);
if (size < sizeof(struct ospf_dbdes_packet))
{
log(L_ERR "Bad OSPF DBDES packet from %I - too short (%u B)", n->ip, size);
return;
}
struct ospf_dbdes_packet *ps = (void *) ps_i;
u32 ps_ddseq = ntohl(ps->ddseq);
u32 ps_options = ntoh_opt(ps->options);

View file

@ -45,12 +45,20 @@ void
ospf_hello_receive(struct ospf_packet *ps_i, struct ospf_iface *ifa,
struct ospf_neighbor *n, ip_addr faddr)
{
struct ospf_hello_packet *ps = (void *) ps_i;
u32 *pnrid;
struct proto *p = &ifa->oa->po->proto;
char *beg = "Bad OSPF HELLO packet from ", *rec = " received: ";
unsigned int size, i, twoway, oldpriority, eligible, peers;
u32 olddr, oldbdr, oldiface_id, tmp;
char *beg = "Bad OSPF hello packet from ", *rec = " received: ";
struct proto *p = (struct proto *) ifa->oa->po;
unsigned int size = ntohs(ps->ospf_packet.length), i, twoway, oldpriority, eligible = 0, peers;
u32 *pnrid;
size = ntohs(ps_i->length);
if (size < sizeof(struct ospf_hello_packet))
{
log(L_ERR "%s%I - too short (%u B)", beg, faddr, size);
return;
}
struct ospf_hello_packet *ps = (void *) ps_i;
OSPF_TRACE(D_PACKETS, "HELLO packet received from %I via %s%s", faddr,
(ifa->type == OSPF_IT_VLINK ? "vlink-" : ""), ifa->iface->name);

View file

@ -26,7 +26,7 @@ static void ospf_dump_lsack(struct proto *p, struct ospf_lsack_packet *pkt)
ASSERT(op->type == LSACK_P);
ospf_dump_common(p, op);
int i, j;
unsigned int i, j;
j = (ntohs(op->length) - sizeof(struct ospf_lsack_packet)) /
sizeof(struct ospf_lsa_header);
@ -138,13 +138,19 @@ void
ospf_lsack_receive(struct ospf_packet *ps_i, struct ospf_iface *ifa,
struct ospf_neighbor *n)
{
struct ospf_lsack_packet *ps = (void *) ps_i;
struct ospf_lsa_header lsa;
u16 nolsa;
struct top_hash_entry *en;
struct proto *p = &ifa->oa->po->proto;
unsigned int size = ntohs(ps->ospf_packet.length), i;
struct ospf_lsa_header lsa;
struct top_hash_entry *en;
unsigned int i, lsano;
unsigned int size = ntohs(ps_i->length);
if (size < sizeof(struct ospf_lsack_packet))
{
log(L_ERR "Bad OSPF LSACK packet from %I - too short (%u B)", n->ip, size);
return;
}
struct ospf_lsack_packet *ps = (void *) ps_i;
OSPF_PACKET(ospf_dump_lsack, ps, "LSACK packet received from %I via %s", n->ip, ifa->iface->name);
ospf_neigh_sm(n, INM_HELLOREC);
@ -152,17 +158,9 @@ ospf_lsack_receive(struct ospf_packet *ps_i, struct ospf_iface *ifa,
if (n->state < NEIGHBOR_EXCHANGE)
return;
nolsa = (size - sizeof(struct ospf_lsack_packet)) /
lsano = (size - sizeof(struct ospf_lsack_packet)) /
sizeof(struct ospf_lsa_header);
if ((nolsa < 1) || ((size - sizeof(struct ospf_lsack_packet)) !=
(nolsa * sizeof(struct ospf_lsa_header))))
{
log(L_ERR "Received corrupted LS ack from %I", n->ip);
return;
}
for (i = 0; i < nolsa; i++)
for (i = 0; i < lsano; i++)
{
ntohlsah(ps->lsh + i, &lsa);
u32 dom = ospf_lsa_domain(lsa.type, n->ifa);

View file

@ -291,6 +291,219 @@ lsa_comp(struct ospf_lsa_header *l1, struct ospf_lsa_header *l2)
return CMP_SAME;
}
#define HDRLEN sizeof(struct ospf_lsa_header)
static int
lsa_validate_rt(struct ospf_lsa_header *lsa, struct ospf_lsa_rt *body)
{
unsigned int i, max;
if (lsa->length < (HDRLEN + sizeof(struct ospf_lsa_rt)))
return 0;
struct ospf_lsa_rt_link *rtl = (struct ospf_lsa_rt_link *) (body + 1);
max = lsa_rt_count(lsa);
#ifdef OSPFv2
if (body->links != max)
return 0;
#endif
for (i = 0; i < max; i++)
{
u8 type = rtl[i].type;
if (!((type == LSART_PTP) ||
(type == LSART_NET) ||
#ifdef OSPFv2
(type == LSART_STUB) ||
#endif
(type == LSART_VLNK)))
return 0;
}
return 1;
}
static int
lsa_validate_net(struct ospf_lsa_header *lsa, struct ospf_lsa_net *body)
{
if (lsa->length < (HDRLEN + sizeof(struct ospf_lsa_net)))
return 0;
return 1;
}
#ifdef OSPFv2
static int
lsa_validate_sum(struct ospf_lsa_header *lsa, struct ospf_lsa_sum *body)
{
if (lsa->length < (HDRLEN + sizeof(struct ospf_lsa_sum)))
return 0;
/* First field should have TOS = 0, we ignore other TOS fields */
if ((body->metric & LSA_SUM_TOS) != 0)
return 0;
return 1;
}
#define lsa_validate_sum_net(A,B) lsa_validate_sum(A,B)
#define lsa_validate_sum_rt(A,B) lsa_validate_sum(A,B)
static int
lsa_validate_ext(struct ospf_lsa_header *lsa, struct ospf_lsa_ext *body)
{
if (lsa->length < (HDRLEN + sizeof(struct ospf_lsa_ext)))
return 0;
/* First field should have TOS = 0, we ignore other TOS fields */
if ((body->metric & LSA_EXT_TOS) != 0)
return 0;
return 1;
}
#else /* OSPFv3 */
static inline int
pxlen(u32 *buf)
{
return *buf >> 24;
}
static int
lsa_validate_sum_net(struct ospf_lsa_header *lsa, struct ospf_lsa_sum_net *body)
{
if (lsa->length < (HDRLEN + sizeof(struct ospf_lsa_sum_net) + 4))
return 0;
u8 pxl = pxlen(body->prefix);
if (pxl > MAX_PREFIX_LENGTH)
return 0;
if (lsa->length != (HDRLEN + sizeof(struct ospf_lsa_sum_net) +
IPV6_PREFIX_SPACE(pxl)))
return 0;
return 1;
}
static int
lsa_validate_sum_rt(struct ospf_lsa_header *lsa, struct ospf_lsa_sum_rt *body)
{
if (lsa->length != (HDRLEN + sizeof(struct ospf_lsa_sum_rt)))
return 0;
return 1;
}
static int
lsa_validate_ext(struct ospf_lsa_header *lsa, struct ospf_lsa_ext *body)
{
if (lsa->length < (HDRLEN + sizeof(struct ospf_lsa_ext) + 4))
return 0;
u8 pxl = pxlen(body->rest);
if (pxl > MAX_PREFIX_LENGTH)
return 0;
int len = IPV6_PREFIX_SPACE(pxl);
if (body->metric & LSA_EXT_FBIT) // forwardinf address
len += 16;
if (body->metric & LSA_EXT_TBIT) // route tag
len += 4;
if (*body->rest & 0xFFFF) // referenced LS type field
len += 4;
if (lsa->length != (HDRLEN + sizeof(struct ospf_lsa_ext) + len))
return 0;
return 1;
}
static int
lsa_validate_pxlist(struct ospf_lsa_header *lsa, u32 pxcount, unsigned int offset, u8 *pbuf)
{
unsigned int bound = lsa->length - HDRLEN - 4;
u32 i;
for (i = 0; i < pxcount; i++)
{
if (offset > bound)
return 0;
u8 pxl = pxlen((u32 *) (pbuf + offset));
if (pxl > MAX_PREFIX_LENGTH)
return 0;
offset += IPV6_PREFIX_SPACE(pxl);
}
if (lsa->length != (HDRLEN + offset))
return 0;
return 1;
}
static int
lsa_validate_link(struct ospf_lsa_header *lsa, struct ospf_lsa_link *body)
{
if (lsa->length < (HDRLEN + sizeof(struct ospf_lsa_link)))
return 0;
return lsa_validate_pxlist(lsa, body->pxcount, sizeof(struct ospf_lsa_link), (u8 *) body);
}
static int
lsa_validate_prefix(struct ospf_lsa_header *lsa, struct ospf_lsa_prefix *body)
{
if (lsa->length < (HDRLEN + sizeof(struct ospf_lsa_prefix)))
return 0;
return lsa_validate_pxlist(lsa, body->pxcount, sizeof(struct ospf_lsa_prefix), (u8 *) body);
}
#endif
/**
* lsa_validate - check whether given LSA is valid
* @lsa: LSA header
* @body: pointer to LSA body
*
* Checks internal structure of given LSA body (minimal length,
* consistency). Returns true if valid.
*/
int
lsa_validate(struct ospf_lsa_header *lsa, void *body)
{
switch (lsa->type)
{
case LSA_T_RT:
return lsa_validate_rt(lsa, body);
case LSA_T_NET:
return lsa_validate_net(lsa, body);
case LSA_T_SUM_NET:
return lsa_validate_sum_net(lsa, body);
case LSA_T_SUM_RT:
return lsa_validate_sum_rt(lsa, body);
case LSA_T_EXT:
return lsa_validate_ext(lsa, body);
#ifdef OSPFv3
case LSA_T_LINK:
return lsa_validate_link(lsa, body);
case LSA_T_PREFIX:
return lsa_validate_prefix(lsa, body);
#endif
default:
/* In OSPFv3, unknown LSAs are OK,
In OSPFv2, unknown LSAs are already rejected
*/
return 1;
}
}
/**
* lsa_install_new - install new LSA into database
* @po: OSPF protocol

View file

@ -20,6 +20,7 @@ u16 lsasum_check(struct ospf_lsa_header *h, void *body);
#define CMP_SAME 0
#define CMP_OLDER -1
int lsa_comp(struct ospf_lsa_header *l1, struct ospf_lsa_header *l2);
int lsa_validate(struct ospf_lsa_header *lsa, void *body);
struct top_hash_entry * lsa_install_new(struct proto_ospf *po, struct ospf_lsa_header *lsa, u32 domain, void *body);
void ospf_age(struct proto_ospf *po);
void flush_lsa(struct top_hash_entry *en, struct proto_ospf *po);

View file

@ -23,7 +23,7 @@ static void ospf_dump_lsreq(struct proto *p, struct ospf_lsreq_packet *pkt)
ASSERT(op->type == LSREQ_P);
ospf_dump_common(p, op);
int i, j;
unsigned int i, j;
j = (ntohs(op->length) - sizeof(struct ospf_lsreq_packet)) /
sizeof(struct ospf_lsreq_header);
@ -91,17 +91,23 @@ void
ospf_lsreq_receive(struct ospf_packet *ps_i, struct ospf_iface *ifa,
struct ospf_neighbor *n)
{
struct ospf_lsreq_packet *ps = (void *) ps_i;
struct ospf_area *oa = ifa->oa;
struct proto_ospf *po = oa->po;
struct proto *p = &po->proto;
struct ospf_lsreq_header *lsh;
struct l_lsr_head *llsh;
list uplist;
slab *upslab;
unsigned int size = ntohs(ps->ospf_packet.length);
int i, lsano;
struct ospf_area *oa = ifa->oa;
struct proto_ospf *po = oa->po;
struct proto *p = &po->proto;
unsigned int size = ntohs(ps_i->length);
if (size < sizeof(struct ospf_lsreq_packet))
{
log(L_ERR "Bad OSPF LSREQ packet from %I - too short (%u B)", n->ip, size);
return;
}
struct ospf_lsreq_packet *ps = (void *) ps_i;
OSPF_PACKET(ospf_dump_lsreq, ps, "LSREQ packet received from %I via %s", n->ip, ifa->iface->name);
if (n->state < NEIGHBOR_EXCHANGE)
@ -129,8 +135,7 @@ ospf_lsreq_receive(struct ospf_packet *ps_i, struct ospf_iface *ifa,
add_tail(&uplist, NODE llsh);
if (ospf_hash_find(po->gr, dom, hid, hrt, htype) == NULL)
{
log(L_WARN
"Received bad LS req from: %I looking: Type: %u, ID: %R, RT: %R",
log(L_WARN "Received bad LS req from: %I looking: Type: %u, ID: %R, RT: %R",
n->ip, htype, hid, hrt);
ospf_neigh_sm(n, INM_BADLSREQ);
rfree(upslab);

View file

@ -16,6 +16,7 @@ struct ospf_lsupd_packet
};
/* Beware of unaligned access */
void ospf_dump_lsahdr(struct proto *p, struct ospf_lsa_header *lsa_n)
{
struct ospf_lsa_header lsa;
@ -38,15 +39,16 @@ static void ospf_dump_lsupd(struct proto *p, struct ospf_lsupd_packet *pkt)
ASSERT(op->type == LSUPD_P);
ospf_dump_common(p, op);
/* We know that ntohs(op->length) >= sizeof(struct ospf_lsa_header) */
u8 *pbuf= (u8 *) pkt;
int offset = sizeof(struct ospf_lsupd_packet);
int bound = ntohs(op->length) - sizeof(struct ospf_lsa_header);
int i, j;
unsigned int offset = sizeof(struct ospf_lsupd_packet);
unsigned int bound = ntohs(op->length) - sizeof(struct ospf_lsa_header);
unsigned int i, j;
j = ntohl(pkt->lsano);
for (i = 0; i < j; i++)
{
if (offset > bound)
if ((offset > bound) || ((offset % 4) != 0))
{
log(L_TRACE "%s: LSA invalid", p->name);
return;
@ -405,13 +407,20 @@ void
ospf_lsupd_receive(struct ospf_packet *ps_i, struct ospf_iface *ifa,
struct ospf_neighbor *n)
{
struct ospf_lsupd_packet *ps = (void *) ps_i;
struct ospf_neighbor *ntmp;
struct ospf_lsa_header *lsa;
struct proto_ospf *po = ifa->oa->po;
struct proto *p = &po->proto;
unsigned int i, sendreq = 1, size = ntohs(ps->ospf_packet.length);
unsigned int i, max, sendreq = 1;
unsigned int size = ntohs(ps_i->length);
if (size < (sizeof(struct ospf_lsupd_packet) + sizeof(struct ospf_lsa_header)))
{
log(L_ERR "Bad OSPF LSUPD packet from %I - too short (%u B)", n->ip, size);
return;
}
struct ospf_lsupd_packet *ps = (void *) ps_i;
OSPF_PACKET(ospf_dump_lsupd, ps, "LSUPD packet received from %I via %s", n->ip, ifa->iface->name);
if (n->state < NEIGHBOR_EXCHANGE)
@ -420,35 +429,29 @@ ospf_lsupd_receive(struct ospf_packet *ps_i, struct ospf_iface *ifa,
return;
}
if (size <=
(sizeof(struct ospf_lsupd_packet) + sizeof(struct ospf_lsa_header)))
{
log(L_WARN "Received lsupd from %I is too short!", n->ip);
return;
}
ospf_neigh_sm(n, INM_HELLOREC); /* Questionable */
lsa = (struct ospf_lsa_header *) (ps + 1);
unsigned int offset = sizeof(struct ospf_lsupd_packet);
unsigned int bound = size - sizeof(struct ospf_lsa_header);
for (i = 0; i < ntohl(ps->lsano); i++,
lsa = (struct ospf_lsa_header *) (((u8 *) lsa) + ntohs(lsa->length)))
max = ntohl(ps->lsano);
for (i = 0; i < max; i++)
{
struct ospf_lsa_header lsatmp;
struct top_hash_entry *lsadb;
unsigned diff = ((u8 *) lsa) - ((u8 *) ps), lenn = ntohs(lsa->length);
u16 chsum;
if (((diff + sizeof(struct ospf_lsa_header)) >= size)
|| ((lenn + diff) > size))
if (offset > bound)
{
log(L_WARN "Received lsupd from %I is too short!", n->ip);
ospf_neigh_sm(n, INM_BADLSREQ);
break;
return;
}
if ((lenn <= sizeof(struct ospf_lsa_header))
|| (lenn != (4 * (lenn / 4))))
struct ospf_lsa_header *lsa = (void *) (((u8 *) ps) + offset);
unsigned int lsalen = ntohs(lsa->length);
if (((offset + lsalen) > size) || ((lsalen % 4) != 0) ||
(lsalen <= sizeof(struct ospf_lsa_header)))
{
log(L_WARN "Received LSA from %I with bad length", n->ip);
ospf_neigh_sm(n, INM_BADLSREQ);
@ -456,14 +459,13 @@ ospf_lsupd_receive(struct ospf_packet *ps_i, struct ospf_iface *ifa,
}
/* pg 143 (1) */
chsum = lsa->checksum;
u16 chsum = lsa->checksum;
if (chsum != lsasum_check(lsa, NULL))
{
log(L_WARN "Received bad lsa checksum from %I: %x %x", n->ip, chsum, lsa->checksum);
continue;
}
#ifdef OSPFv2
/* pg 143 (2) */
if ((lsa->type < LSA_T_RT) || (lsa->type > LSA_T_EXT))
@ -520,7 +522,6 @@ ospf_lsupd_receive(struct ospf_packet *ps_i, struct ospf_iface *ifa,
if ((lsadb == NULL) || (lsa_comp(&lsatmp, &lsadb->lsa) == CMP_NEWER))
{
struct ospf_iface *ift = NULL;
void *body;
int self = (lsatmp.rt == p->cf->global->router_id);
DBG("PG143(5): Received LSA is newer\n");
@ -618,10 +619,20 @@ ospf_lsupd_receive(struct ospf_packet *ps_i, struct ospf_iface *ifa,
} /* FIXME lsack? */
/* pg 144 (5d) */
body =
mb_alloc(p->pool, lsatmp.length - sizeof(struct ospf_lsa_header));
void *body = mb_alloc(p->pool, lsatmp.length - sizeof(struct ospf_lsa_header));
ntohlsab(lsa + 1, body, lsatmp.type,
lsatmp.length - sizeof(struct ospf_lsa_header));
/* We will do validation check after flooding and
acknowledging given LSA to minimize problems
when communicating with non-validating peer */
if (lsa_validate(&lsatmp, body) == 0)
{
log(L_WARN "Received invalid LSA from %I", n->ip);
mb_free(body);
continue;
}
lsadb = lsa_install_new(po, &lsatmp, domain, body);
DBG("New LSA installed in DB\n");

View file

@ -438,6 +438,8 @@ struct ospf_lsa_ext
u32 tag;
};
#define LSA_SUM_TOS 0xFF000000
#define LSA_EXT_TOS 0x7F000000
#define LSA_EXT_EBIT 0x80000000
/* Endianity swap for lsa->type */
@ -545,6 +547,9 @@ lsa_net_count(struct ospf_lsa_header *lsa)
#ifdef OSPFv3
#define IPV6_PREFIX_SPACE(x) ((((x) + 63) / 32) * 4)
#define IPV6_PREFIX_WORDS(x) (((x) + 63) / 32)
static inline u32 *
lsa_get_ipv6_prefix(u32 *buf, ip_addr *addr, int *pxlen, u8 *pxopts, u16 *rest)
{
@ -575,6 +580,29 @@ lsa_get_ipv6_addr(u32 *buf, ip_addr *addr)
return buf + 4;
}
static inline u32 *
put_ipv6_prefix(u32 *buf, ip_addr addr, u8 pxlen, u8 pxopts, u16 lh)
{
*buf++ = ((pxlen << 24) | (pxopts << 16) | lh);
if (pxlen > 0)
*buf++ = _I0(addr);
if (pxlen > 32)
*buf++ = _I1(addr);
if (pxlen > 64)
*buf++ = _I2(addr);
if (pxlen > 96)
*buf++ = _I3(addr);
return buf;
}
static inline u32 *
put_ipv6_addr(u32 *buf, ip_addr addr)
{
*(ip_addr *) buf = addr;
return buf + 4;
}
#endif

View file

@ -286,6 +286,7 @@ ospf_rx_hook(sock * sk, int size)
return 1;
}
/* This is strange! */
if ((ifa->oa->areaid != 0) && (ntohl(ps->areaid) == 0))
{
WALK_LIST(iff, po->iface_list)
@ -300,17 +301,16 @@ ospf_rx_hook(sock * sk, int size)
DBG("%s: RX_Hook called on interface %s.\n", p->name, sk->iface->name);
osize = ntohs(ps->length);
if ((unsigned) size < sizeof(struct ospf_packet))
{
log(L_ERR "%s%I - too short (%u bytes)", mesg, sk->faddr, size);
return 1;
}
if ((osize > size) || (osize != (4 * (osize / 4))))
osize = ntohs(ps->length);
if ((osize > size) || ((osize % 4) != 0))
{
log(L_ERR "%s%I - size field does not match (%d/%d)", mesg, sk->faddr, ntohs(ps->length), size );
log(L_ERR "%s%I - size field does not match (%d/%d)", mesg, sk->faddr, osize, size);
return 1;
}
@ -324,7 +324,7 @@ ospf_rx_hook(sock * sk, int size)
#ifdef OSPFv2
if ((ps->autype != htons(OSPF_AUTH_CRYPT)) &&
(!ipsum_verify(ps, 16, (void *) ps + sizeof(struct ospf_packet),
ntohs(ps->length) - sizeof(struct ospf_packet), NULL)))
osize - sizeof(struct ospf_packet), NULL)))
{
log(L_ERR "%s%I - bad checksum", mesg, sk->faddr);
return 1;
@ -358,17 +358,17 @@ ospf_rx_hook(sock * sk, int size)
return 1;
}
if (((unsigned) size > sk->rbsize) || (ntohs(ps->length) > sk->rbsize))
if ((unsigned) size > sk->rbsize)
{
log(L_ERR "%s%I - packet is too large (%d-%d vs %d)",
mesg, sk->faddr, size, ntohs(ps->length), sk->rbsize);
log(L_ERR "%s%I - packet is too large (%d vs %d)",
mesg, sk->faddr, size, sk->rbsize);
return 1;
}
/* This is deviation from RFC 2328 - neighbours should be identified by
* IP address on broadcast and NBMA networks.
*/
n = find_neigh(ifa, ntohl(((struct ospf_packet *) ps)->routerid));
n = find_neigh(ifa, ntohl(ps->routerid));
if(!n && (ps->type != HELLO_P))
{

View file

@ -133,37 +133,6 @@ lsab_end(struct proto_ospf *po)
return ((byte *) po->lsab) + po->lsab_used;
}
#ifdef OSPFv3
#define IPV6_PREFIX_SPACE(x) ((((x) + 63) / 32) * 4)
#define IPV6_PREFIX_WORDS(x) (((x) + 63) / 32)
static inline u32 *
put_ipv6_prefix(u32 *buf, ip_addr addr, u8 pxlen, u8 pxopts, u16 lh)
{
*buf++ = ((pxlen << 24) | (pxopts << 16) | lh);
if (pxlen > 0)
*buf++ = _I0(addr);
if (pxlen > 32)
*buf++ = _I1(addr);
if (pxlen > 64)
*buf++ = _I2(addr);
if (pxlen > 96)
*buf++ = _I3(addr);
return buf;
}
static inline u32 *
put_ipv6_addr(u32 *buf, ip_addr addr)
{
*(ip_addr *) buf = addr;
return buf + 4;
}
#endif
static int
configured_stubnet(struct ospf_area *oa, struct ifa *a)