lib: improve fletcher checksum validation
OVERVIEW
The checksum used in OSPF (rfc2328) is specified in rc905 annex B. There is an
sample implementation in rfc1008 which forms the basis of the quagga
implementation. This algorithm works perfectly when generating a checksum;
however, validation is a bit problematic.
The following LSA (generated by a non-quagga implementation) is received by
quagga and marked with an invalid checksum; however, it passes both the rfc905
and rfc1008 validation checks.
static uint8_t lsa_10_121_233_29[] = {
0x0e, 0x10, 0x02, 0x03,
0x09, 0x00, 0x35, 0x40,
0x0a, 0x79, 0xe9, 0x1d,
0x80, 0x00, 0x00, 0x03,
0x00, 0x8a, 0x00, 0x1c,
0xff, 0xff, 0xff, 0xe0,
0x00, 0x00, 0x36, 0xb0
};
LS Type: Summary-LSA (IP network)
LS Age: 3600 seconds
Do Not Age: False
Options: 0x02 (E)
Link-State Advertisement Type: Summary-LSA (IP network) (3)
Link State ID: 9.0.53.64
Advertising Router: 10.121.233.29 (10.121.233.29)
LS Sequence Number: 0x80000003
LS Checksum: 0x008a
Length: 28
Netmask: 255.255.255.224
Metric: 14000
You'll note that one byte of the checksum is 0x00; quagga would calculate the
checksum as 0xff8a.
It can be argued that the sourcing implementation generates an incorrect
checksum; however, rfc905 indicates that, for 1's complement arithmetic, the
value 255 shall be regarded as 0, thus either values are valid.
EXPLANATION
The quagga ospfd and ospf6d implementations operate by copying the PDU's
existing checksum in a holding variable, calculating the checksum, and comparing
the resulting checksum to the original. As a note, this implementation has the
side effect of modifying the contents of the PDU.
Evaluation of both rfc905 and rfc1008 shows that checksum validation should
involve calculating the sum over the PDU and checking that both resulting C0 and
C1 values are zero. This behavior is enacted in the rfc1008 implementation by
calling encodecc with k = 0 (checksum offset); however, this functionality had
been omitted from the quagga implementation.
PATCH
This patch adds the ability to call the quagga's fletcher_checksum() with a
checksum offset value of 0xffff (aka FLETCHER_CHECKSUM_VALIDATE) which returns
the sum over the buffer (a value of 0 indicates a valid checksum). This is
similar to the mechanism in rfc1008 when called with k = 0. The patch also
introduces ospf_lsa_checksum_valid().
ospf6d had it's own implementation of the fletcher checksum in
ospf6_lsa_checksum(); it's the same algorithm as in fletcher_checksum(). This
patch removes the local implementation in favor of the library's as well as creates
and uses ospf6_lsa_checksum_valid().
quagga's ISIS implementation suffers from the same problem; however, I do not
have the facilities to validate a fix to ISIS, thus this change has been left to
the ISIS maintainers. The function iso_csum_verify() should be reduced to
running the fletcher checksum over the buffer using an offset of 0.
Signed-off-by: JR Rivers <jrrivers@cumulusnetworks.com>
Reviewed-by: Scott Feldman <sfeldma@cumulusnetworks.com>
Reviewed-by: Nolan Leake <nolan@cumulusnetworks.com>
Reviewed-by: Ayan Banerjee <ayan@cumulusnetworks.com>
Reviewed-by: Shrijeet Mukherjee <shm@cumulusnetworks.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/lib/checksum.c b/lib/checksum.c
index 3ddde81..43940b7 100644
--- a/lib/checksum.c
+++ b/lib/checksum.c
@@ -51,6 +51,8 @@
/* To be consistent, offset is 0-based index, rather than the 1-based
index required in the specification ISO 8473, Annex C.1 */
+/* calling with offset == FLETCHER_CHECKSUM_VALIDATE will validate the checksum
+ without modifying the buffer; a valid checksum returns 0 */
u_int16_t
fletcher_checksum(u_char * buffer, const size_t len, const uint16_t offset)
{
@@ -62,13 +64,14 @@
checksum = 0;
- assert (offset < len);
- /*
- * Zero the csum in the packet.
- */
- csum = (u_int16_t *) (buffer + offset);
- *(csum) = 0;
+ if (offset != FLETCHER_CHECKSUM_VALIDATE)
+ /* Zero the csum in the packet. */
+ {
+ assert (offset < (len - 1)); /* account for two bytes of checksum */
+ csum = (u_int16_t *) (buffer + offset);
+ *(csum) = 0;
+ }
p = buffer;
c0 = 0;
@@ -89,7 +92,7 @@
left -= partial_len;
}
-
+
/* The cast is important, to ensure the mod is taken as a signed value. */
x = (int)((len - offset - 1) * c0 - c1) % 255;
@@ -98,17 +101,24 @@
y = 510 - c0 - x;
if (y > 255)
y -= 255;
-
- /*
- * Now we write this to the packet.
- * We could skip this step too, since the checksum returned would
- * be stored into the checksum field by the caller.
- */
- buffer[offset] = x;
- buffer[offset + 1] = y;
- /* Take care of the endian issue */
- checksum = htons((x << 8) | (y & 0xFF));
+ if (offset == FLETCHER_CHECKSUM_VALIDATE)
+ {
+ checksum = (c1 << 8) + c0;
+ }
+ else
+ {
+ /*
+ * Now we write this to the packet.
+ * We could skip this step too, since the checksum returned would
+ * be stored into the checksum field by the caller.
+ */
+ buffer[offset] = x;
+ buffer[offset + 1] = y;
+
+ /* Take care of the endian issue */
+ checksum = htons((x << 8) | (y & 0xFF));
+ }
return checksum;
}
diff --git a/lib/checksum.h b/lib/checksum.h
index da1d3cb..b310f74 100644
--- a/lib/checksum.h
+++ b/lib/checksum.h
@@ -1,2 +1,3 @@
extern int in_cksum(void *, int);
+#define FLETCHER_CHECKSUM_VALIDATE 0xffff
extern u_int16_t fletcher_checksum(u_char *, const size_t len, const uint16_t offset);
diff --git a/ospf6d/ospf6_flood.c b/ospf6d/ospf6_flood.c
index 670c5d1..b815972 100644
--- a/ospf6d/ospf6_flood.c
+++ b/ospf6d/ospf6_flood.c
@@ -733,7 +733,6 @@
{
struct ospf6_lsa *new = NULL, *old = NULL, *rem = NULL;
int ismore_recent;
- unsigned short cksum;
int is_debug = 0;
ismore_recent = 1;
@@ -751,8 +750,7 @@
}
/* (1) LSA Checksum */
- cksum = ntohs (new->header->checksum);
- if (ntohs (ospf6_lsa_checksum (new->header)) != cksum)
+ if (! ospf6_lsa_checksum_valid (new->header))
{
if (is_debug)
zlog_debug ("Wrong LSA Checksum, discard");
diff --git a/ospf6d/ospf6_lsa.c b/ospf6d/ospf6_lsa.c
index e65752d..ff061df 100644
--- a/ospf6d/ospf6_lsa.c
+++ b/ospf6d/ospf6_lsa.c
@@ -29,6 +29,7 @@
#include "command.h"
#include "memory.h"
#include "thread.h"
+#include "checksum.h"
#include "ospf6_proto.h"
#include "ospf6_lsa.h"
@@ -672,47 +673,36 @@
-/* enhanced Fletcher checksum algorithm, RFC1008 7.2 */
-#define MODX 4102
-#define LSA_CHECKSUM_OFFSET 15
+/* Fletcher Checksum -- Refer to RFC1008. */
+/* All the offsets are zero-based. The offsets in the RFC1008 are
+ one-based. */
unsigned short
ospf6_lsa_checksum (struct ospf6_lsa_header *lsa_header)
{
- u_char *sp, *ep, *p, *q;
- int c0 = 0, c1 = 0;
- int x, y;
- u_int16_t length;
+ u_char *buffer = (u_char *) &lsa_header->type;
+ int type_offset = buffer - (u_char *) &lsa_header->age; /* should be 2 */
- lsa_header->checksum = 0;
- length = ntohs (lsa_header->length) - 2;
- sp = (u_char *) &lsa_header->type;
+ /* Skip the AGE field */
+ u_int16_t len = ntohs(lsa_header->length) - type_offset;
- for (ep = sp + length; sp < ep; sp = q)
- {
- q = sp + MODX;
- if (q > ep)
- q = ep;
- for (p = sp; p < q; p++)
- {
- c0 += *p;
- c1 += c0;
- }
- c0 %= 255;
- c1 %= 255;
- }
+ /* Checksum offset starts from "type" field, not the beginning of the
+ lsa_header struct. The offset is 14, rather than 16. */
+ int checksum_offset = (u_char *) &lsa_header->checksum - buffer;
- /* r = (c1 << 8) + c0; */
- x = ((length - LSA_CHECKSUM_OFFSET) * c0 - c1) % 255;
- if (x <= 0)
- x += 255;
- y = 510 - c0 - x;
- if (y > 255)
- y -= 255;
+ return (unsigned short)fletcher_checksum(buffer, len, checksum_offset);
+}
- lsa_header->checksum = htons ((x << 8) + y);
+int
+ospf6_lsa_checksum_valid (struct ospf6_lsa_header *lsa_header)
+{
+ u_char *buffer = (u_char *) &lsa_header->type;
+ int type_offset = buffer - (u_char *) &lsa_header->age; /* should be 2 */
- return (lsa_header->checksum);
+ /* Skip the AGE field */
+ u_int16_t len = ntohs(lsa_header->length) - type_offset;
+
+ return (fletcher_checksum(buffer, len, FLETCHER_CHECKSUM_VALIDATE) == 0);
}
void
diff --git a/ospf6d/ospf6_lsa.h b/ospf6d/ospf6_lsa.h
index 7d93f5c..263411f 100644
--- a/ospf6d/ospf6_lsa.h
+++ b/ospf6d/ospf6_lsa.h
@@ -237,6 +237,7 @@
extern int ospf6_lsa_refresh (struct thread *);
extern unsigned short ospf6_lsa_checksum (struct ospf6_lsa_header *);
+extern int ospf6_lsa_checksum_valid (struct ospf6_lsa_header *);
extern int ospf6_lsa_prohibited_duration (u_int16_t type, u_int32_t id,
u_int32_t adv_router, void *scope);
diff --git a/ospfd/ospf_lsa.c b/ospfd/ospf_lsa.c
index 493209a..5579d8e 100644
--- a/ospfd/ospf_lsa.c
+++ b/ospfd/ospf_lsa.c
@@ -192,6 +192,18 @@
return fletcher_checksum(buffer, len, checksum_offset);
}
+int
+ospf_lsa_checksum_valid (struct lsa_header *lsa)
+{
+ u_char *buffer = (u_char *) &lsa->options;
+ int options_offset = buffer - (u_char *) &lsa->ls_age; /* should be 2 */
+
+ /* Skip the AGE field */
+ u_int16_t len = ntohs(lsa->length) - options_offset;
+
+ return(fletcher_checksum(buffer, len, FLETCHER_CHECKSUM_VALIDATE) == 0);
+}
+
/* Create OSPF LSA. */
diff --git a/ospfd/ospf_packet.c b/ospfd/ospf_packet.c
index 351fb21..ede5908 100644
--- a/ospfd/ospf_packet.c
+++ b/ospfd/ospf_packet.c
@@ -1590,7 +1590,7 @@
/* Validate the LSA's LS checksum. */
sum = lsah->checksum;
- if (sum != ospf_lsa_checksum (lsah))
+ if (! ospf_lsa_checksum_valid (lsah))
{
/* (bug #685) more details in a one-line message make it possible
* to identify problem source on the one hand and to have a better