Add dns_cache so server addresses are cached and invalidated when DNS changes.

Adds a module to deal with dns_cache feature. It's
main struct is CachedResolver, which is a simple thread safe
hostname <-> Ips cache with the ability to refresh resolutions
every `dns_max_ttl` seconds. This way, a client can check whether its
ip address has changed.

Author: magec

Cargo.lock

@@ -34,6 +34,7 @@ rustls-pemfile = "1"
 hyper = { version = "0.14", features = ["full"] }
 phf = { version = "0.11.1", features = ["macros"] }
 exitcode = "1.1.2"
+trust-dns-resolver = "0.22"
 
 [target.'cfg(not(target_env = "msvc"))'.dependencies]
 jemallocator = "0.5.0"

Cargo.toml

@@ -276,6 +276,8 @@ The config can be reloaded by sending a `kill -s SIGHUP` to the process or by qu
 | `default_role`          | no                   |
 | `primary_reads_enabled` | no                   |
 | `query_parser_enabled`  | no                   |
+| `dns_max_ttl`           | no                   |
+| `dns_cache_enabled`     | no                   |
 
 
 ## Benchmarks

README.md

@@ -41,6 +41,15 @@ log_client_disconnections = false
 # Reload config automatically if it changes.
 autoreload = false
 
+# If enabled, hostname resolution will be cached and
+# and server connections will be invalidated if a change on the ip is
+# detected. This check is done every `dns_max_ttl` seconds.
+# dns_cache_enabled = false
+
+# The number of seconds to wait until we check again the
+# cached hostnames resolution. 30 seconds by default.
+# dns_max_ttl = 30
+
 # TLS
 # tls_certificate = "server.cert"
 # tls_private_key = "server.key"