mogul@Shasta.ARPA (03/08/85)
Index: sys/netinet/ip_icmp.c 4.2BSD Fix
Index: sys/netinet/ip_input.c 4.2BSD Fix
Index: sys/netinet/tcp_input.c 4.2BSD Fix
Description:
IP, TCP, and ICMP packets with valid checksums can occasionally
be discarded due to a "bad checksum" (about 1 time in 2^16).
The problem comes when a checksum that Unix would calculate
as 0xFFFF is sent as 0x0 by the peer host. Since these are
one's complement numbers, 0xFFFF == 0x0, and Unix should
accept either.
Repeat-By:
One way to repeat this problem is to use the "ping" (ICMP
Echo) program written by Larry Allen of MIT. (The BRL
ping program does not repeatably provoke the bug.) When
pinging a TOPS-20 host, replies will be discarded by Unix
because of an apparently bad checksum.
The MIT ping program sends ICMP Echo requests that have
all fields zero, except for the Type and Checksum fields.
The Type value for ICMP echo reply is zero, and thus the
echoing host returns a packet that is entirely zero,
except for the checksum. The "proper" value for the
checksum (the one Unix expects) is 0xFFFF, but TOPS-20
hosts (and some others, such as WISCVM) convert this
to the "equivalent" value 0x0.
Fix:
Jon Postel says that while it might be a good idea to
change the TOPS-20 code to send 0xFFFF in this case,
the robustness principle implies that Unix should
accept either checksum.
This change must be made to ip_icmp.c, ip_input.c,
and tcp_input.c. It need not be made to udp_usrreq.c,
because of the special UDP convention for null
checksums.
Line numbers are for comparison only; your line numbers
will vary.
*** ip_icmp.c.bad Wed Mar 6 12:52:37 1985
--- ip_icmp.c Wed Mar 6 14:34:48 1985
***************
*** 121,126
int (*ctlfunc)(), code;
extern u_char ip_protox[];
struct sockaddr_in *sin;
/*
* Locate icmp structure in mbuf, and check
--- 121,127 -----
int (*ctlfunc)(), code;
extern u_char ip_protox[];
struct sockaddr_in *sin;
+ u_short cksm;
/*
* Locate icmp structure in mbuf, and check
***************
*** 145,151
m->m_len -= hlen;
m->m_off += hlen;
icp = mtod(m, struct icmp *);
! if (in_cksum(m, icmplen)) {
icmpstat.icps_checksum++;
goto free;
}
--- 146,157 -----
m->m_len -= hlen;
m->m_off += hlen;
icp = mtod(m, struct icmp *);
! if (cksm = in_cksum(m, icmplen)) {
! if (~cksm == 0) {
! /* sum is good, one's complement of zero is zero */
! if (icmpprintfs)
! printf("icmp_input: -0 sum, src %x\n", ip->ip_src);
! } else {
icmpstat.icps_checksum++;
goto free;
}
***************
*** 148,153
if (in_cksum(m, icmplen)) {
icmpstat.icps_checksum++;
goto free;
}
#ifdef ICMPPRINTFS
--- 154,160 -----
} else {
icmpstat.icps_checksum++;
goto free;
+ }
}
#ifdef ICMPPRINTFS
****************************************************************
*** ip_input.c.bad Wed Mar 6 14:32:26 1985
--- ip_input.c Wed Mar 6 14:36:34 1985
***************
*** 110,115
}
if (ipcksum)
if (ip->ip_sum = in_cksum(m, hlen)) {
ipstat.ips_badsum++;
goto bad;
}
--- 110,121 -----
}
if (ipcksum)
if (ip->ip_sum = in_cksum(m, hlen)) {
+ if (~(ip->ip_sum) == 0) {
+ /* sum is good, one's complement of zero is zero */
+ #ifdef DEBUG
+ printf("ipintr: -0 sum, src %x\n", ip->ip_src);
+ #endif DEBUG
+ /* packet may be forwarded, change -0 to +0 */
+ ip->ip_sum = 0;
+ } else {
ipstat.ips_badsum++;
goto bad;
}
***************
*** 112,117
if (ip->ip_sum = in_cksum(m, hlen)) {
ipstat.ips_badsum++;
goto bad;
}
/*
--- 118,124 -----
} else {
ipstat.ips_badsum++;
goto bad;
+ }
}
/*
****************************************************************
*** tcp_input.c.old Wed Mar 6 14:40:20 1985
--- tcp_input.c Wed Mar 6 14:40:24 1985
***************
*** 81,86
ti->ti_len = (u_short)tlen;
ti->ti_len = htons((u_short)ti->ti_len);
if (ti->ti_sum = in_cksum(m, len)) {
if (tcpprintfs)
printf("tcp sum: src %x\n", ti->ti_src);
tcpstat.tcps_badsum++;
--- 81,93 -----
ti->ti_len = (u_short)tlen;
ti->ti_len = htons((u_short)ti->ti_len);
if (ti->ti_sum = in_cksum(m, len)) {
+ if (~(ti->ti_sum) == 0) {
+ /* sum is good, one's complement of zero is zero */
+ if (tcpprintfs)
+ printf("tcp_input: -0 sum, src %x\n", ti->ti_src);
+ /* preserve original behaviour, change -0 to +0 */
+ ti->ti_sum = 0;
+ } else {
if (tcpprintfs)
printf("tcp sum: src %x\n", ti->ti_src);
tcpstat.tcps_badsum++;
***************
*** 85,90
printf("tcp sum: src %x\n", ti->ti_src);
tcpstat.tcps_badsum++;
goto drop;
}
}
--- 92,98 -----
printf("tcp sum: src %x\n", ti->ti_src);
tcpstat.tcps_badsum++;
goto drop;
+ }
}
}