route -blackhole option behaves differently on localhost from a normal host

[Nick Hibma]

After doing a

	route add <host> 127.0.0.1 -blackhole

on a FreeBSD 8.3 host I noticed that nc <host> 80 would instantly return with ENETUNRACH, instead of timing out, while blackhole routing to a real host would time out. Checking in CURRENT (october 2012) reveals the same behaviour:

  # netstat -nr
  Routing tables

  Internet:
  Destination        Gateway            Flags    Refs      Use  Netif Expire
  default            10.0.2.2           UGS         0        1    vtnet0
  10.0.2.0/24        link#1             U           0     1295    vtnet0
  10.0.2.15          link#1             UHS         0        0    lo0
  10.0.3.0/24        link#2             U           0        0    vtnet1
  10.0.3.4           link#2             UHS         0        0    lo0
  127.0.0.1          link#3             UH          0     1479    lo0

  Internet6:
  Destination                       Gateway                       Flags      Netif Expire
  ::/96                             ::1                           UGRS        lo0
  ::1                               link#3                        UH          lo0
  ::ffff:0.0.0.0/96                 ::1                           UGRS        lo0
  fe80::/10                         ::1                           UGRS        lo0
  fe80::%vtnet1/64                  link#2                        U           vtnet1
  fe80::a00:27ff:fea4:1a9a%vtnet1   link#2                        UHS         lo0
  fe80::%lo0/64                     link#3                        U           lo0
  fe80::1%lo0                       link#3                        UHS         lo0
  ff01::%vtnet1/32                  fe80::a00:27ff:fea4:1a9a%vtnet1 U         vtnet1
  ff01::%lo0/32                     ::1                           U           lo0
  ff02::/16                         ::1                           UGRS        lo0
  ff02::%vtnet1/32                  fe80::a00:27ff:fea4:1a9a%vtnet1 U         vtnet1
  ff02::%lo0/32                     ::1                           U           lo0
  # route add 4.2.2.1 127.0.0.1 -blackhole
  add host 4.2.2.1: gateway 127.0.0.1
  # route add 4.2.2.2 10.0.2.15 -blackhole
  add host 4.2.2.2: gateway 10.0.2.15
  # time nc -v -w 3 4.2.2.2 80
  nc: connect to 4.2.2.2 port 80 (tcp) failed: Operation timed out

  real  0m3.089s
  user  0m0.000s
  sys   0m0.014s
  # time nc -v -w 3 4.2.2.1 80
  nc: connect to 4.2.2.1 port 80 (tcp) failed: Network is unreachable

  real  0m0.014s
  user  0m0.000s
  sys   0m0.014s
  # uname -a
  FreeBSD citske.localdomain 10.0-CURRENT FreeBSD 10.0-CURRENT #3 r241988M: Wed Oct 24 15:21:27 CEST 2012     root at citske.localdomain:/usr/obj/usr/src/sys/CITSKE  i386

A quick grep -r through the kernel did not provide me with any obvious mistake, but a colleague suggested that for localhost an if 'statement' like the following might be missing somewhere:

	if (rt && rt->rt_flags & (RTF_REJECT|RTF_BLACKHOLE)) {
		m_freem(m);
		return (rt->rt_flags & RTF_BLACKHOLE ? 0 :
			rt->rt_flags & RTF_HOST ? EHOSTUNREACH : ENETUNREACH);
	}

Any pointers would be appreciated

Nick Hibma
nick at van-laarhoven.org

GTD: Time management for chaotic people.