Re: [PATCH v2] dhcp, dhcpv6: Add hostname and client fqdn ops

[Stefano Brivio <sbrivio@redhat.com>]

Sorry for the delay.

On Mon, 18 Nov 2024 13:20:03 +0100
Enrique Llorente Pastora <ellorent@redhat.com> wrote:

> On Fri, Nov 15, 2024 at 12:29 PM Stefano Brivio <sbrivio@redhat.com> wrote:
> >
> > On Fri, 15 Nov 2024 10:25:33 +0100
> > Enrique Llorente Pastora <ellorent@redhat.com> wrote:
> >  
> > > On Thu, Nov 14, 2024 at 6:13 PM Stefano Brivio <sbrivio@redhat.com> wrote:  
> > > >
> > > > On Thu, 14 Nov 2024 09:47:27 +0100
> > > > Enrique Llorente <ellorent@redhat.com> wrote:
> > > >  
> > > > > Both DHCPv4 and DHCPv6 has the capability to pass the hostname to
> > > > > clients, the DHCPv4 uses option 12 (hostname) while the DHCPv6 uses option 39
> > > > > (client fqdn), for some virt deployments like kubevirt is expected to
> > > > > have the VirtualMachine name as the guest hostname.
> > > > >
> > > > > This change add the -H --hostname to configure the DHCPv4 and DHCPv6
> > > > > options to will send hostname to clients.
> > > > >
> > > > > Signed-off-by: Enrique Llorente <ellorent@redhat.com>
> > > > > ---
> > > > >  conf.c           | 13 ++++++++++---
> > > > >  dhcp.c           |  8 +++++++-
> > > > >  dhcpv6.c         | 44 +++++++++++++++++++++++++++++++++++++++++++-
> > > > >  passt.h          |  2 ++
> > > > >  test/lib/setup   | 10 +++++-----
> > > > >  test/passt.mbuto |  1 +
> > > > >  test/passt/dhcp  | 11 ++++++++++-
> > > > >  7 files changed, 78 insertions(+), 11 deletions(-)
> > > > >
> > > > > diff --git a/conf.c b/conf.c
> > > > > index 14411b4..ddb585c 100644
> > > > > --- a/conf.c
> > > > > +++ b/conf.c
> > > > > @@ -847,7 +847,8 @@ static void usage(const char *name, FILE *f, int status)
> > > > >               "  --freebind           Bind to any address for forwarding\n"
> > > > >               "  --no-map-gw          Don't map gateway address to host\n"
> > > > >               "  -4, --ipv4-only      Enable IPv4 operation only\n"
> > > > > -             "  -6, --ipv6-only      Enable IPv6 operation only\n");
> > > > > +             "  -6, --ipv6-only      Enable IPv6 operation only\n"
> > > > > +             "  -H, --hostname NAME Hostname to configure client with\n");  
> > > >
> > > > Use one tab instead of one space between "NAME" and "Hostname" so that
> > > > the output (and the code itself) is aligned.
> > > >  
> > > > >
> > > > >       if (strstr(name, "pasta"))
> > > > >               goto pasta_opts;
> > > > > @@ -1303,6 +1304,7 @@ void conf(struct ctx *c, int argc, char **argv)
> > > > >               {"map-guest-addr", required_argument,   NULL,           22 },
> > > > >               {"host-lo-to-ns-lo", no_argument,       NULL,           23 },
> > > > >               {"dns-host",    required_argument,      NULL,           24 },
> > > > > +             {"hostname",    required_argument,      NULL,           'H' },  
> > > >
> > > > Maybe this could go a bit more up, just after the "dns" and "search"
> > > > options. Two reasons: we have all the options without a short version
> > > > at the end, and this logically belongs together with other name-related
> > > > stuff we send to the guest.
> > > >  
> > > > >               { 0 },
> > > > >       };
> > > > >       const char *logname = (c->mode == MODE_PASTA) ? "pasta" : "passt";
> > > > > @@ -1325,9 +1327,9 @@ void conf(struct ctx *c, int argc, char **argv)
> > > > >       if (c->mode == MODE_PASTA) {
> > > > >               c->no_dhcp_dns = c->no_dhcp_dns_search = 1;
> > > > >               fwd_default = FWD_AUTO;
> > > > > -             optstring = "+dqfel:hF:I:p:P:m:a:n:M:g:i:o:D:S:46t:u:T:U:";
> > > > > +             optstring = "+dqfel:hF:I:p:P:m:a:n:M:g:i:o:D:S:46t:u:T:U:H:";  
> > > >
> > > > Same here: maybe after S: it makes more sense.
> > > >  
> > > > >       } else {
> > > > > -             optstring = "+dqfel:hs:F:p:P:m:a:n:M:g:i:o:D:S:461t:u:";
> > > > > +             optstring = "+dqfel:hs:F:p:P:m:a:n:M:g:i:o:D:S:461t:u:H:";
> > > > >       }
> > > > >
> > > > >       c->tcp.fwd_in.mode = c->tcp.fwd_out.mode = FWD_UNSET;
> > > > > @@ -1680,6 +1682,11 @@ void conf(struct ctx *c, int argc, char **argv)
> > > > >
> > > > >                       c->one_off = true;
> > > > >                       break;
> > > > > +             case 'H':
> > > > > +                     ret = snprintf(c->hostname.n, sizeof(c->hostname.n), "%s", optarg);  
> > > >
> > > > We (generally) wrap lines at 80 columns where doable:
> > > >
> > > >                         ret = snprintf(c->hostname.n, sizeof(c->hostname.n),
> > > >                                        "%s", optarg);
> > > >  
> > > > > +                     if (ret <= 0 || ret >= (int)sizeof(c->hostname.n))
> > > > > +                             die("Invalid hostname: %s", optarg);
> > > > > +                     break;
> > > > >               case 't':
> > > > >               case 'u':
> > > > >               case 'T':
> > > > > diff --git a/dhcp.c b/dhcp.c
> > > > > index a06f143..ec7e78a 100644
> > > > > --- a/dhcp.c
> > > > > +++ b/dhcp.c
> > > > > @@ -275,7 +275,7 @@ static void opt_set_dns_search(const struct ctx *c, size_t max_len)
> > > > >   */
> > > > >  int dhcp(const struct ctx *c, const struct pool *p)
> > > > >  {
> > > > > -     size_t mlen, dlen, offset = 0, opt_len, opt_off = 0;
> > > > > +     size_t mlen, dlen, offset = 0, opt_len, opt_off = 0, hostname_len;
> > > > >       char macstr[ETH_ADDRSTRLEN];
> > > > >       const struct ethhdr *eh;
> > > > >       const struct iphdr *iph;
> > > > > @@ -375,6 +375,12 @@ int dhcp(const struct ctx *c, const struct pool *p)
> > > > >               opts[6].slen += sizeof(uint32_t);
> > > > >       }
> > > > >
> > > > > +     hostname_len = strlen(c->hostname.n);
> > > > > +     if ( hostname_len > 0 ) {  
> > > >
> > > > No space around parentheses in expressions:
> > > >
> > > >         if (hostname_len > 0) {
> > > >  
> > > > > +             opts[12].slen = hostname_len;
> > > > > +             memcpy(opts[12].s, &c->hostname.n, hostname_len);
> > > > > +     }
> > > > > +
> > > > >       if (!c->no_dhcp_dns_search)
> > > > >               opt_set_dns_search(c, sizeof(m->o));
> > > > >
> > > > > diff --git a/dhcpv6.c b/dhcpv6.c
> > > > > index 14a5c7e..190dc0e 100644
> > > > > --- a/dhcpv6.c
> > > > > +++ b/dhcpv6.c
> > > > > @@ -48,6 +48,7 @@ struct opt_hdr {
> > > > >  # define  STATUS_NOTONLINK   htons_constant(4)
> > > > >  # define OPT_DNS_SERVERS     htons_constant(23)
> > > > >  # define OPT_DNS_SEARCH              htons_constant(24)
> > > > > +# define OPT_CLIENT_FQDN     htons_constant(39)  
> > > >
> > > > One tab instead of spaces.
> > > >  
> > > > >  #define   STR_NOTONLINK              "Prefix not appropriate for link."
> > > > >
> > > > >       uint16_t l;
> > > > > @@ -163,6 +164,18 @@ struct opt_dns_search {
> > > > >       char list[MAXDNSRCH * NS_MAXDNAME];
> > > > >  } __attribute__((packed));
> > > > >
> > > > > +/**
> > > > > + * struct opt_client_fqdn - Client FQDN option (RFC 4704)
> > > > > + * @hdr:             Option header
> > > > > + * @flags:           Flags as stated at RFC 4704  
> > > >
> > > > s/stated at/described by/
> > > >
> > > > Maybe mention that we don't really use those ("always zero for us").
> > > >  
> > > > > + * @hostname:                Client fqdn  
> > > >
> > > > A hostname is not necessarily a fully-qualified domain name. The RFC
> > > > calls this "domain name", so we could at least call this 'domain' in
> > > > the struct to make it clear we're referring to that.
> > > >
> > > > As to what it should contain: RFC 4702 defines option 81 (FQDN) for
> > > > DHCP, which is equivalent to DHCPv6's option 39 (RFC 4704). Should we
> > > > just call this whole thing "FQDN" and switch to option 81 for DHCP
> > > > instead? Would this work for KubeVirt?
> > > >
> > > > Or introduce two options: -H / --hostname (option 12, DHCP only), and
> > > > --fqdn (setting both DHCP option 81 and DHCPv6 option 39)?  
> > >
> > > Let me experiment with DHCPv4 option 81 with kubevirt machines, if it's enough
> > > we use 81 instead of 12 and call it --fqdn.
> > >
> > > If not we go with --fqdn and --hostname, but in this case it's not
> > > clear to me what kubevirt/libvirt
> > > has to pass though, also is this going to be expose at the domain xml
> > > or libvirt will directly pass always
> > > the vm name as one of them ? (for dual stack or single stack ipv4
> > > hostname is enough, problem is single stack ipv6)  
> >
> > Note that they are (of course) different things, and neither RFC 4702
> > nor RFC 4703 make any mention whatsoever of hostname and option 12.
> >
> > That is, you could legitimately pass x as hostname and y.tld as FQDN
> > (which gives 'y' as hostname... but RFCs don't specify which one wins).
> >
> > I think for libvirt we should propose to add one or two separate
> > attributes ("hostname" / "fqdn"), and not imply them from the name of
> > the machine or "domain" (the guest "domain", nothing to do with network
> > domain), because that's not necessarily what users want.
> >
> > About option 81 itself: it's not just a matter of changing the number,
> > unfortunately. You would need to set the 'E' flag. We have an
> > implementation of name compression according to RFC 1035, 4.1.4 in
> > opt_set_dns_search(), but RFC 4702 says we shouldn't use it.
> >
> > No other flags set, as those are for clients.
> 
> From some experiments I have do with networkmanager at a fedora 41,
> the DHCPv4 client fqdn is only used
> for clients to state the fqdn but never for client to configure it's
> own hostname
> 
> But the DHCPv6 client does update the client's hostname with single
> stack ipv6

Ouch. Of course we have to consider this for compatibility reasons, but
it's obviously inconsistent: RFC 4704 is without a doubt equivalent
to RFC 4702 for IPv6.

Neither says that a client must update its hostname, but if DHCPv6
option 39 causes NetworkManager to, it doesn't make a lot of sense to
me that DHCP option 81 doesn't.

I think support for option 39 (in the sense of setting the hostname,
not in general) was simply added later, by commit 1f74ea52f581
("policy: get the DHCPv6 hostname from the FQDN option"), and that
might simply be the reason.

Well, regardless of whether it makes sense to propose to change this
for NetworkManager, we have to deal with existing versions anyway.

> so I think we should do the following:
> - Have only one argument -H/--hostname

...taking a hostname or a FQDN?

> - Delegate the config to libvirt
> - For IPv4 Options(12) with it

What would we send in this option if we're given a FQDN? We shouldn't
(no MUST NOT, though) send a FQDN in it.

> - For IPv6 Option(39) with flags S and O <- so we can totally ignore the client

...and what would we send in this option if we're given a hostname?
Here, we MUST send a FQDN, not an unqualified hostname.

By the way, if the client sets 'S' to 0, I think that we should (even
though it's not a MUST) set 'O' to 0, because we're not overriding it.

It's not needed but it's less likely to cause issues with a buggy
DHCPv6 client.

> - Document this and if someone is not happy, don't use that option.
> 
> Also for DHCPv4 we have also the option (15) for domain names, so
> maybe if we detect that -H has a domain name we also add option(15).

I'm not sure if we should try to be clever about it. In general I agree
it's a good idea, but: 1. we can't fix the inconsistencies I presented
above (I think) and 2. we're actually losing generality because there
are configurations that a user can't cover this way, such as:

- setting DHCP option 81 itself

- setting hostname only, without FQDN, or FQDN without hostname

- setting both FQDN and hostname to different (non-conflicting) values

- setting a different domain for the guests (it's not possible at the
  moment but it could become a bit complicated to add this option in
  the future), say, the host is called "x" and we would like to call
  the guest "x.guests".

The RFCs are doing us a big favour by not coupling options together and
letting us do pretty much what we want.

Do you think it would be problematic to do this instead:

- -H / --hostname sets DHCP option 12

- --fqdn sets DHCP option 81 and DHCPv6 option 39

- ask libvirt to take zero, one or two different attributes between
  "fqdn" (setting --fqdn) and "hostname" (setting --hostname)

?

In the use case that KubeVirt needs the most (if I understood
correctly), the domain XML would include something like:

  <fqdn>abcd.example.com</fqdn><hostname>abcd</hostname>

which is not ideal, but we have a configuration front-end for it, and,
if NetworkManager ever changes to handling --fqdn as it handles
--hostname, we could skip <fqdn/> at that point.

Side note: with this item from the feature list on the website, at
https://passt.top/#services:

  ⌚ fine-grained configurability of DHCP, NDP, DHCPv6 options

...I was actually thinking of a mechanism where users could set
arbitrary options, some of which with type validation, in a similar
way as what dhcpcd (as client) supports:

$ /sbin/dhcpcd -V
[...]
DHCPv4 options:
[...]
001 subnet_mask                         	 ipaddress request
121 classless_static_routes             	 string rfc3442
002 time_offset                         	 int32
003 routers                             	 array ipaddress request
004 time_servers                        	 array ipaddress
[...]
DHCPv6 options:
00001 client_id                         	 binhex
00002 server_id                         	 binhex
00003 ia_na                             	 embed index norequest
[...]

but without support for fancy attributes such as "embed" or "index".

That is, supporting arbitrary "ia_na" options would be way too
complicated, but simple strings (such as the ones we have in option 12)
or numbers would be okay. Say: '--dhcp-opt 12 abcd'.

On the other hand, this doesn't solve the problem for FQDN options
because they have flags which need some logic in the server itself,
so... yes, this is just a side note, and I'm not saying we should
implement this now (or ever).

-- 
Stefano