Fixed broken ICMP error generation, unified conversion of IP header

fields between host and network byte order.  The details:

o icmp_error() now does not add IP header length.  This fixes the problem
  when icmp_error() is called from ip_forward().  In this case the ip_len
  of the original IP datagram returned with ICMP error was wrong.

o icmp_error() expects all three fields, ip_len, ip_id and ip_off in host
  byte order, so DTRT and convert these fields back to network byte order
  before sending a message.  This fixes the problem described in PR 16240
  and PR 20877 (ip_id field was returned in host byte order).

o ip_ttl decrement operation in ip_forward() was moved down to make sure
  that it does not corrupt the copy of original IP datagram passed later
  to icmp_error().

o A copy of original IP datagram in ip_forward() was made a read-write,
  independent copy.  This fixes the problem I first reported to Garrett
  Wollman and Bill Fenner and later put in audit trail of PR 16240:
  ip_output() (not always) converts fields of original datagram to network
  byte order, but because copy (mcopy) and its original (m) most likely
  share the same mbuf cluster, ip_output()'s manipulations on original
  also corrupted the copy.

o ip_output() now expects all three fields, ip_len, ip_off and (what is
  significant) ip_id in host byte order.  It was a headache for years that
  ip_id was handled differently.  The only compatibility issue here is the
  raw IP socket interface with IP_HDRINCL socket option set and a non-zero
  ip_id field, but ip.4 manual page was unclear on whether in this case
  ip_id field should be in host or network byte order.

1 files changed, 7 insertions, 1 deletions

diff --git a/sys/netinet/ip_icmp.c b/sys/netinet/ip_icmp.c
index 58a4915..91c0bc0 100644
--- a/sys/netinet/ip_icmp.c
+++ b/sys/netinet/ip_icmp.c
@@ -191,7 +191,13 @@ icmp_error(n, type, code, dest, destifp)
 	icp->icmp_code = code;
 	bcopy((caddr_t)oip, (caddr_t)&icp->icmp_ip, icmplen);
 	nip = &icp->icmp_ip;
-	nip->ip_len = htons((u_short)(nip->ip_len + oiplen));
+
+	/*
+	 * Convert fields to network representation.
+	 */
+	HTONS(nip->ip_len);
+	HTONS(nip->ip_id);
+	HTONS(nip->ip_off);
 
 	/*
 	 * Now, copy old ip header (without options)