DNS Discovery

1
DNS Discovery

• Dave Thaler
• Microsoft

2
Overview of design team recommendation

• Transport Mechanism
• Q How do discovery messages get from devices to
  servers (if any)?
• A Site-scoped anycast
• Content Mechanism
• Q Whats the format of the discovery messages?
• A DNS SRV lookups

3
Anycast deployment scenarios

• How do you securely inject anycast host route?
• Run DNS servers on routers
• Run routing daemons on DNS servers to talk to
  routers
• Run multiple servers on same link, configure
  servers routers with static route
• Upgrade servers routers to advertise route based
  on ND responses for anycast address
• Develop a real host-router anycast group joining
  protocol (draft-haberman-ipngwg-host-anycast-00.tx
  t)

4
Response Content Requirements

• Response must be able to contain
• 1 or more DNS server addresses (ordered?)
• Could just be the anycast address
• Multiple addresses allow end nodes to more
  quickly fail over to other servers
• Clients domain name
• Clients search path (ordered)
• Response must fit in 512 bytes

5
Other Requirements

• Allow single config for entire site
• Allow per-subnet config as an option
• Request should include subnet information
• Want a single request message
• Implies a single query in practice

6
Anycast for name resolution

• DNS server list MAY just contain the anycast
  address itself
• Nodes just send UDP queries to the DNS server
  anycast address to resolve names
• Deployment notes
• Already tried (w/o domain name or search path)
• Trivial to obtain maintain server list, but
  slower convergence time on server failure
• Reply doesnt come from request destination

7
Using DNS SRV messages

• Resolver does a lookup using a well-known domain
  name and service name
• IPNG WG will define details of how to embed
  necessary information in SRV records
• Mechanism can be disabled using an existing flag
  in Router Advertisements that says to use DHCP
  for other configuration

8
SRV record basics

• Specified in RFC 2782
• Record has format
• _Service._Proto.Name TTL Class SRV Priority
  Weight Port Target
• Query for _Service._Proto.Name
• Example _ldap._tcp.example.com
• Priority field imposes an ordering
• Weight field used for probabilistic load
  balancing within a priority
• Target is a name

9
Example from RFC 2782

• ORIGIN example.com.
• ...
• foobar - use old-slow-box or new-fast-box if
• either is available, make three quarters of the
• logins go to new-fast-box.
• _foobar._tcp SRV 0 1 9 old-slow-box.example.com
  .
• SRV 0 3 9 new-fast-box.example.com
  .
• if neither old-slow-box or new-fast-box is up,
• switch to using the sysdmin's box and the
  server
• SRV 1 0 9 sysadmins-box.example.co
  m.
• SRV 1 0 9 server.example.com.
• NO other services are supported
• ._tcp SRV 0 0 0 .
• ._udp SRV 0 0 0 .

10
Basic questions

• What name do you ask for?
• How do you do longest match, so that you get
  subnet-specific information if available, and
  site information otherwise?
• How encode 3 types of information?

11
Proposal (Queries)

• Well known (domain) Name (local.arpa?)
• Well known Protocol e.g. _dnsinfo
• Dont need to use this field for anything else
• Allows use of Service field for longest match
  wildcarding
• Put SLA in Service field of query
• Example
• Client is on subnet fec001234/64
• Query for _1234._dnsinfo.local.arpa

12
Proposal (Server config)

• Use Port to encode type (e.g., 0domain name,
  1search path, 2server)
• Use Priority to specify ordering within a type
• Only server Targets need address records

13
Site-specific example

• ORIGIN local.arpa.
• ...
• Specify default domain name
• ._dnsinfo SRV 1 0 0 seattle.example.com.
• Specify default search path
• ._dnsinfo SRV 1 0 1 seattle.example.com.
• ._dnsinfo SRV 2 0 1 example.com.
• ._dnsinfo SRV 3 0 1 foobar.com.
• Specify server list. Split among dns1-3 if
• available, else fall back to dns.foobar.com.
• ._dnsinfo SRV 1 1 2 dns1.example.com.
• ._dnsinfo SRV 1 1 2 dns2.example.com.
• ._dnsinfo SRV 1 1 2 dns3.example.com.
• ._dnsinfo SRV 2 0 2 dns.foobar.com.

14
Subnet-specific example

• Specify default domain name
• _1234._dnsinfo SRV 1 0 0 marketing.example.com.
• Specify default search path
• _1234._dnsinfo SRV 1 0 1 seattle.example.com.
• _1234._dnsinfo SRV 2 0 1 example.com.
• Specify server list. Split among dns1-3.
• _1234._dnsinfo SRV 1 1 2 dns1.example.com.
• _1234._dnsinfo SRV 1 1 2 dns2.example.com.
• _1234._dnsinfo SRV 1 1 2 dns3.example.com.

15
Conclusions

• Can encode multiple types of information
• Can support 2-level longest match
• Site-specific and subnet-specific
• No changes to existing DNS servers