[getdns-users] Question on transport fallback option of the getdns API

[Marcel van Garderen]

Hi getdns users,


My name is Marcel and this is my 1st post to the user's list...


I have a question on the usage of the getdns API function 
getdns_context_set_dns_transport_list().

With this function the user can provide an ordered list of transport 
protocols that will be used for DNS lookups.

Fallback options are specified by including multiple values in the list.


I did some experiments with transport fallback using the following dns 
context(lib version 1.0.0b2).

Here, the transport list is 'first try UDP and if it fails try TCP (the 
single IP address is masked)

{#012 "all_context":#012  {#012    "append_name": 
GETDNS_APPEND_NAME_TO_SINGLE_LABEL_FIRST,#012 
"dns_transport_list":#012    [#012 GETDNS_TRANSPORT_UDP,#012      
GETDNS_TRANSPORT_TCP#012 ],#012    "dnssec_allowed_skew": 0,#012    
"edns_do_bit": 0,#012    "edns_extended_rcode": 0,#012 
"edns_maximum_udp_payload_size": 512,#012    "edns_version": 0,#012    
"follow_redirects": GETDNS_REDIRECTS_FOLLOW,#012 "idle_timeout": 
5000000,#012    "limit_outstanding_queries": 0,#012    
"namespaces":#012    [#012 GETDNS_NAMESPACE_LOCALNAMES,#012 
GETDNS_NAMESPACE_DNS#012    ],#012    "suffix": [],#012 "timeout": 
1000,#012    "tls_authentication": GETDNS_AUTHENTICATION_NONE,#012 
"upstream_recursive_servers":#012    [#012      {#012 "address_data": 
<bindata for XXX.XXX.XXX.XXX>,#012 "address_type": <bindata of 
"IPv4">#012      }#012 ]#012  },#012  "implementation_string": <bindata 
of "https://getdnsapi.net">,#012  "resolution_type": 
GETDNS_RESOLUTION_STUB,#012  "version_string": <bindata of "1.0.0b2">#012}


The issue is that I was not seeing any transport fallback attempts 
(using the same context).

When the DNS lookup failed (using UDP, timeout), there was no TCP 
attempt during any of the next 10 lookups. The library just kept on 
trying UDP.

Changing the order (TCP with fallback to UDP) did not make any 
difference: it just kept on trying TCP.


Any feedback would be greatly appreciated!


Regards,

Marcel



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