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The following code, with my patch, is from the netbsd 1.5 branch's
netinet/ip_input.c:ip_forward().  It is how ip_forward() handles an error 
from ip_output() that the packet has an sp policy, the DF bit set and is too 
large for the tunnel's MTU.  This code may have a bug in looking up
ro->ro_rt->rt_ifp->if_mtu' and not looking at the per-route MTU in reentry's 
rt_rmx.

(In case of bad line wrapping, it is included inline and as an attachment.)

thanks,
-david waitzman

(the rcs ids are bogus because we imported netbsd into our own tree)
Index: ip_input.c
===================================================================
RCS file: netbsd/src/sys/netinet/ip_input.c,v
retrieving revision 1.1.1.1
diff -c -r1.1.1.1 ip_input.c
*** ip_input.c  2002/05/10 01:51:02     1.1.1.1
--- ip_input.c  2002/06/06 18:42:03
***************
*** 1585,1592 ****
                                   && sp->req->sav->sah != NULL) {
                                          ro = &sp->req->sav->sah->sa_route;
                                          if (ro->ro_rt && ro->ro_rt->rt_ifp) {
!                                               dummyifp.if_mtu =
!                                                  ro->ro_rt->rt_ifp->if_mtu;
                                                  dummyifp.if_mtu -= ipsechdr;
                                                  destifp = &dummyifp;
                                          }
--- 1585,1597 ----
                                   && sp->req->sav->sah != NULL) {
                                          ro = &sp->req->sav->sah->sa_route;
                                          if (ro->ro_rt && ro->ro_rt->rt_ifp) {
!                                               dummyifp.if_mtu =
!                                                   o->ro_rt->rt_rmx.rmx_mtu;
!                                               if (dummyifp.if_mtu == 0)
!                                                       dummyifp.if_mtu =
!                                                               ro->ro_rt->
!                                                            rt_ifp->if_mtu;
!
                                                  dummyifp.if_mtu -= ipsechdr;
                                                  destifp = &dummyifp;
                                          }



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Index: ip_input.c
===================================================================
RCS file: /KEPLER-CVS/netbsd/src/sys/netinet/ip_input.c,v
retrieving revision 1.1.1.1
diff -c -r1.1.1.1 ip_input.c
*** ip_input.c  2002/05/10 01:51:02     1.1.1.1
--- ip_input.c  2002/06/06 18:42:03
***************
*** 1585,1592 ****
                                 && sp->req->sav->sah != NULL) {
                                        ro = &sp->req->sav->sah->sa_route;
                                        if (ro->ro_rt && ro->ro_rt->rt_ifp) {
!                                               dummyifp.if_mtu =
!                                                   ro->ro_rt->rt_ifp->if_mtu;
                                                dummyifp.if_mtu -= ipsechdr;
                                                destifp = &dummyifp;
                                        }
--- 1585,1597 ----
                                 && sp->req->sav->sah != NULL) {
                                        ro = &sp->req->sav->sah->sa_route;
                                        if (ro->ro_rt && ro->ro_rt->rt_ifp) {
!                                               dummyifp.if_mtu =
!                                                   ro->ro_rt->rt_rmx.rmx_mtu;
!                                               if (dummyifp.if_mtu == 0)
!                                                       dummyifp.if_mtu =
!                                                               ro->ro_rt->
!                                                               rt_ifp->if_mtu;
! 
                                                dummyifp.if_mtu -= ipsechdr;
                                                destifp = &dummyifp;
                                        }


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