Date: Thu, 21 Jul 2022 11:51:27 -0500 From: Mike Karels <mike@karels.net> To: rgrimes@freebsd.org Cc: 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: <80467AB6-DB3E-44CA-A67F-A246420014B5@karels.net> In-Reply-To: <202207211621.26LGLkNn078062@gndrsh.dnsmgr.net> References: <202207211621.26LGLkNn078062@gndrsh.dnsmgr.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=3Dfb8ef16bab0d23e185deed5= a6b2e44e72ad53d43 >> >> 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=3D32 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=E2=80=99t apply to 127.0.0.1 directly. The test is #define IN_LOOPBACK(i) \ (((in_addr_t)(i) & V_in_loopback_mask) =3D=3D 0x7f000000) So if considering whether to forward 127.0.0.1, we=E2=80=99ll incorrectly= say it=E2=80=99s 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. 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 =3D 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 =3D 0xffffffff << (32 - preflen); >> return (0); >> > > -- = > Rod Grimes rgrimes@free= bsd.org
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