[LEDE-DEV] [PATCH mdns] Fix sending replies to PTR questions

[Cristian Morales Vega]

On 13 February 2017 at 15:42, Rafał Miłecki <zajec5 at gmail.com> wrote:
> From: Rafał Miłecki <rafal at milecki.pl>
>
> When we receive PTR question it includes hostname (instance), e.g.:
> mdnsd: parse_question (391): Q -> PTR lede._http._tcp.local
>
> First of all we should check if it matches hostname we use before trying
> to reply. Secondly service_reply expects service with domain appended
> (without hostname/instance) so we need to strip received string out of
> hostname before passing it.
>
> Signed-off-by: Rafał Miłecki <rafal at milecki.pl>
> ---
>  dns.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/dns.c b/dns.c
> index 91434f2..dac6f2c 100644
> --- a/dns.c
> +++ b/dns.c
> @@ -367,9 +367,19 @@ parse_question(struct interface *iface, char *name, struct dns_question *q)
>                 break;
>
>         case TYPE_PTR:
> -               if (!strcmp(name, sdudp))
> +               if (!strcmp(name, sdudp)) {
>                         service_announce_services(iface, announce_ttl);
> -               service_reply(iface, name, announce_ttl);
> +               } else {
> +                       /* First dot separates instance name from the rest */
> +                       char *dot = strchr(name, '.');
> +                       /* Length of queried instance */
> +                       size_t len = dot ? dot - name : 0;
> +
> +                       /* Make sure it's query for the instance name we use */
> +                       if (len && len == strlen(mdns_hostname) &&
> +                           !strncmp(name, mdns_hostname, len))
> +                               service_reply(iface, dot + 1, announce_ttl);
> +               }
>                 break;
>
>         case TYPE_AAAA:


Isn't this failing to take into account the case when there is no hostname?

With 480d7bc74eba20c03875aa06c1c25dbdb98e0b12 running on SKWB8:

$ sudo systemctl restart avahi-daemon
$ avahi-browse -ck '_ssh._tcp'
$ avahi-browse -tk '_ssh._tcp'
+ enp0s25 IPv6 linux-vf7n                                    _ssh._tcp
           local
+ enp0s25 IPv4 linux-vf7n                                    _ssh._tcp
           local
$ avahi-browse -atk
+ enp0s25 IPv6 SKWB8
_skhttp._tcp         local
+ enp0s25 IPv6 SKWB8
_skjitter._tcp       local
+ enp0s25 IPv6 SKWB8                                         _ssh._tcp
           local
+ enp0s25 IPv6 linux-vf7n                                    _ssh._tcp
           local
+ enp0s25 IPv4 linux-vf7n                                    _ssh._tcp
           local
+ enp0s25 IPv4 SKWB8
_skhttp._tcp         local
+ enp0s25 IPv4 SKWB8
_skjitter._tcp       local
+ enp0s25 IPv4 SKWB8                                         _ssh._tcp
           local
+ enp0s25 IPv6 linux-vf7n
_sftp-ssh._tcp       local
+ enp0s25 IPv6 linux-vf7n [14:da:e9:f7:1b:95]
_workstation._tcp    local
+ enp0s25 IPv4 linux-vf7n
_sftp-ssh._tcp       local
+ enp0s25 IPv4 linux-vf7n [14:da:e9:f7:1b:95]
_workstation._tcp    local
$ avahi-browse -ck '_ssh._tcp'
+ enp0s25 IPv6 SKWB8                                         _ssh._tcp
           local
+ enp0s25 IPv6 linux-vf7n                                    _ssh._tcp
           local
+ enp0s25 IPv4 SKWB8                                         _ssh._tcp
           local
+ enp0s25 IPv4 linux-vf7n                                    _ssh._tcp
           local
$ avahi-browse -tk '_ssh._tcp'
+ enp0s25 IPv6 SKWB8                                         _ssh._tcp
           local
+ enp0s25 IPv6 linux-vf7n                                    _ssh._tcp
           local
+ enp0s25 IPv4 SKWB8                                         _ssh._tcp
           local
+ enp0s25 IPv4 linux-vf7n                                    _ssh._tcp
           local
$

When running with -d umdns says things like

mdnsd: parse_question (366): Q -> PTR _skhttp._tcp.local
mdnsd: parse_question (366): Q -> PTR _skhttp._tcp.local
mdnsd: parse_question (366): Q -> PTR _skhttp._tcp.local
mdnsd: parse_question (366): Q -> PTR _skhttp._tcp.local
mdnsd: parse_question (366): Q -> PTR _services._dns-sd._udp.local
mdnsd: dns_send_answer (178): A <- A SKWB8.local
mdnsd: dns_send_answer (178): A <- AAAA SKWB8.local
mdnsd: dns_send_answer (178): A <- PTR _services._dns-sd._udp.local
mdnsd: dns_send_answer (178): A <- PTR _skhttp._tcp.local
mdnsd: dns_send_answer (178): A <- SRV SKWB8._skhttp._tcp.local
mdnsd: dns_send_answer (178): A <- TXT SKWB8._skhttp._tcp.local
mdnsd: dns_send_answer (178): A <- PTR _services._dns-sd._udp.local
mdnsd: dns_send_answer (178): A <- PTR _skjitter._tcp.local
mdnsd: dns_send_answer (178): A <- SRV SKWB8._skjitter._tcp.local
mdnsd: dns_send_answer (178): A <- TXT SKWB8._skjitter._tcp.local
mdnsd: dns_send_answer (178): A <- PTR _services._dns-sd._udp.local
mdnsd: dns_send_answer (178): A <- PTR _ssh._tcp.local
mdnsd: dns_send_answer (178): A <- SRV SKWB8._ssh._tcp.local
mdnsd: dns_send_answer (178): A <- TXT SKWB8._ssh._tcp.local

So it's seems it's not answering to the _ssh._tcp queries.