Date: Thu, 21 Jul 2022 10:21:32 -0700 (PDT) From: "Rodney W. Grimes" <freebsd@gndrsh.dnsmgr.net> To: Mike Karels <mike@karels.net> Cc: rgrimes@freebsd.org, Mike Karels <karels@freebsd.org>, src-committers@freebsd.org, dev-commits-src-all@freebsd.org, dev-commits-src-main@freebsd.org Subject: Re: git: fb8ef16bab0d - main - IPv4: correct limit on loopback_prefix Message-ID: <202207211721.26LHLWwa078429@gndrsh.dnsmgr.net> In-Reply-To: <80467AB6-DB3E-44CA-A67F-A246420014B5@karels.net>
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> On 21 Jul 2022, at 11:21, Rodney W. Grimes wrote: > > >> The branch main has been updated by karels: > >> > >> URL: https://cgit.FreeBSD.org/src/commit/?id=fb8ef16bab0d23e185deed5a6b2e44e72ad53d43 > >> > >> commit fb8ef16bab0d23e185deed5a6b2e44e72ad53d43 > >> Author: Mike Karels <karels@FreeBSD.org> > >> AuthorDate: 2022-07-21 13:10:15 +0000 > >> Commit: Mike Karels <karels@FreeBSD.org> > >> CommitDate: 2022-07-21 14:38:17 +0000 > >> > >> IPv4: correct limit on loopback_prefix > >> > >> Commit efe58855f3ea allowed the net.inet.ip.loopback_prefix value > >> to be 32. However, with a 32-bit mask, 127.0.0.1 is not included > >> in the reserved loopback range, which should not be allowed. > >> Change the max prefix length to 31. > > > > Hummm... 127.0.0.1/32 specifices exactly and ONLY 127.0.0.1, and > > this should be fine. Looking at the mask calculated below with > > loopback_prefix=32 this should yeild a mask of 0xffffffff, which > > appears to be exactly what is correct. What DOES become an issue > > when /32 is used is that the loopback ROUTE 127.0.0.0/32 is wrong > > now, but then with a /32 you dont need a network route, as you > > should have a host route to exactly 127.0.0.1. > > > > Can you be more descriptive on what problem arrose with /32? > > You are thinking about this the way I did originally; but the mask > doesn?t apply to 127.0.0.1 directly. The test is > > #define IN_LOOPBACK(i) \ > (((in_addr_t)(i) & V_in_loopback_mask) == 0x7f000000) > > So if considering whether to forward 127.0.0.1, we?ll incorrectly > say it?s OK if the prefixlen is 32 (mask of 255.255.255.255). > In that case, only 127.0.0.0 is considered loopback. > > John Gilmore pointed out the problem. I see this issue now, but something is bugging me about being forced to use a /31 when a /32 *would* work if the 0x7f000000 was actually 0x7f000001, but that fails for other cases. Note oddly the code would work with /32 if I decided to use 127.0.0.0/32 as the IP address on lo0 so should the expression become: #define IN_LOOPBACK(i) \ (((in_addr_t)(i) & V_in_loopback_mask) == (0x7f000001 & V_in_loopback_mask)) > > Mike > >> --- > >> sys/netinet/in.c | 2 +- > >> 1 file changed, 1 insertion(+), 1 deletion(-) > >> > >> diff --git a/sys/netinet/in.c b/sys/netinet/in.c > >> index c3880c4ba983..1c44623bdec1 100644 > >> --- a/sys/netinet/in.c > >> +++ b/sys/netinet/in.c > >> @@ -297,7 +297,7 @@ sysctl_loopback_prefixlen(SYSCTL_HANDLER_ARGS) > >> error = sysctl_handle_int(oidp, &preflen, 0, req); > >> if (error || !req->newptr) > >> return (error); > >> - if (preflen < 8 || preflen > 32) > >> + if (preflen < 8 || preflen > 31) > >> return (EINVAL); > >> V_in_loopback_mask = 0xffffffff << (32 - preflen); > >> return (0); > >> > > > > -- > > Rod Grimes rgrimes@freebsd.org > > -- Rod Grimes rgrimes@freebsd.org
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