DynDNS: Automatic External IP Tracking & Periodic Host Resolution

[p4bpj]

Discussion Invitation

This PR proposes adding a DynDNS-based external IP tracking fallback to complement (or replace when unavailable) UPnP-based discovery. It's aimed at improving inbound reachability for home users who disable UPnP but rely on static/manual port forwarding plus a dynamic DNS hostname. Feedback on ergonomics, defaults, naming, and refresh strategy is welcome before finalizing. Also checkout PR #2

Summary

Introduces an optional DynDNS resolver service that periodically resolves a user-specified hostname and updates the node’s advertised external address (best_local_address) when the underlying WAN IP changes. If UPnP successfully yields a public external IP, DynDNS is skipped. If UPnP is disabled or produces no routable address, the DynDNS path performs:

1. A minimal synchronous one-shot resolve at startup (non-blocking except for a single DNS lookup) to seed the initial external IP before P2P connects.
2. A background DynDnsExtender task that refreshes on an adaptive interval (min→exponential backoff up to max) and notifies registered sinks upon change.

The result: nodes behind manually forwarded routers (with UPnP off) can automatically keep their advertised address current without restarts when their ISP-assigned IP updates.

Motivation

Problem: Many home operators disable UPnP but still forward a port manually and operate behind a dynamic WAN IP served via an ISP or consumer router. Previously:

• Without specifying --externalip, the node could fail to advertise a valid routable address if interface auto-discovery yielded none.
• Even if initially correct (e.g. via manual --externalip), future WAN IP rotations (DHCP/PPPoE lease renewal) required manual restarts to propagate the new address.
• Users who already maintain a DynDNS hostname (common with manual port forwarding) gain no automatic benefit unless they hard-code resolved IPs.

Goal: Seamless external IP lifecycle management when UPnP is unavailable, leveraging an operator’s existing DynDNS hostname.

Key Changes

• AddressManager
  • Extended constructor now returns both optional UPnP port-mapping extender and optional DynDnsExtender.
  • Adds minimal initial DynDNS resolve (try_initial_dyndns_resolve) if UPnP not active and a hostname is configured; prefers IPv4 in Auto mode.
  • Provides set_best_local_address already used by both UPnP and DynDNS flows.
  • Maintains a list of ExternalIpChangeSink subscribers (unchanged pattern) reused for DynDNS notifications.
• DynDnsExtender (new file components/addressmanager/src/dyndns_extender.rs)
  • Periodically resolves the configured hostname with adaptive interval (reset to min on success; exponential backoff up to max on failures / no public result).
  • Filters to publicly routable IPs; selects according to IpVersionMode (Ipv4|Ipv6|Auto with IPv4 preference).
  • Detects changes and updates AddressManager + asynchronously notifies sinks (on_external_ip_changed(new, old)).
• Config / CLI
  • New arguments:
    • --external-dyndns-host=<HOSTNAME>
    • --external-dyndns-min-refresh-sec=<N> (default 30)
    • --external-dyndns-max-refresh-sec=<N> (default 300)
    • --external-dyndns-ip-version=<auto|ipv4|ipv6> (default auto; auto prefers IPv4 if present)
  • Existing --disable-upnp and --disable-ipv6-interface-discovery continue to apply.
  • Config struct extended with DynDNS fields and IpVersionMode.
• Config Builder
  • Injects defaults (host = None; refresh bounds; mode = Auto).
• Daemon
  • Integrates new triple return from AddressManager::new, wiring (if present) the dyn DNS extender service similarly to the UPnP extender (service start handled elsewhere in lifecycle).
• Cleanup / Refactor
  • Removed unused outbound reconnect helper remnants (pre-existing logic now internally managed).
  • Minor logging simplifications and removal of stale debug/imports in subsequent refactor commit.

Behavior

Decision tree:

1. Startup:
   • Attempt local routable interface discovery (respecting --disable-ipv6-interface-discovery).
   • If none and UPnP enabled: attempt UPnP external IP + port mapping.
   • If UPnP fails (or disabled) AND a DynDNS host is configured: perform a one-shot resolve and set initial external IP if found.
2. Background:
   • If UPnP succeeded: UPnP extender handles periodic external IP refresh (pre-existing behavior); DynDNS skipped.
   • Else if DynDNS configured: DynDnsExtender resolves host on a loop (min interval → backoff).
3. On IP change (DynDNS or UPnP path):
   • best_local_address updated (retains port from existing best or default P2P port).
   • All registered ExternalIpChangeSinks notified asynchronously (fire-and-forget).

Configuration

Examples:

# Typical manual port forward + DynDNS hostname
kaspad --disable-upnp \
       --external-dyndns-host=mynode.exampleddns.org \
       --external-dyndns-min-refresh-sec=30 \
       --external-dyndns-max-refresh-sec=600 \
       --external-dyndns-ip-version=auto

IPv6-only preference:

--external-dyndns-host=myv6node.example \
--external-dyndns-ip-version=ipv6

Force IPv4 only:

--external-dyndns-ip-version=ipv4

IP Selection Logic

Given resolved set R (filtered to publicly routable):

• Mode ipv4: first IPv4 in R.
• Mode ipv6: first IPv6 in R.
• Mode auto: first IPv4 if any else first element (possibly IPv6).
  No persistence of multiple candidates—single best address advertised.

Testing / Verification Suggestions

Manual:

1. UPnP enabled & working + DynDNS configured:
   • Expect UPnP success, DynDNS skip log: [AddrMan] UPnP extender active; DynDNS skipped.
2. UPnP disabled, DynDNS host valid:
   • On startup see initial resolve log and Initial DynDNS external IP set....
   • Modify DNS A record to new IP; after min-refresh interval expect change log + sink callbacks.
3. Failure then success:
   • Point hostname to unroutable/private IP (or remove A record): observe warnings + interval growth (doubling until max).
   • Restore public A record: observe resolution success and interval reset to min.
4. --external-dyndns-ip-version=ipv6 with dual-stack host:
   • Ensure selected IP is IPv6 even if IPv4 present.
5. --disable-ipv6-interface-discovery with only IPv6 routable interface + DynDNS providing IPv4:
   • Should still use DynDNS IPv4 (flag only affects interface scanning, not DynDNS selection).

Limitations / Future Work

• Resolver currently uses ToSocketAddrs (blocking DNS semantics) for the initial single resolve; background loop also synchronous per cycle—could offload to dedicated async DNS (e.g. trust-dns) if needed.
• Does not (yet) attempt multi-address advertisement or simultaneous IPv4+IPv6 publication.

Backward Compatibility

• If both UPnP and DynDNS host configured, UPnP remains first-class and unchanged unless it fails.

Operational Guidance

• Operators already using --externalip can continue; DynDNS is optional and may reduce manual maintenance.
• Recommended for users with:
  • Disabled UPnP
  • Reliable manual port forwarding
  • Existing dynamic DNS hostname

Authorship Note

This description was drafted with AI assistance and reviewed by the author (p4bpj) to ensure accuracy of the represented changes.