introduce shortlived in-memory dns cache and prefill ip addresses into

it during configuration.

src/configure.rs

@@ -13,6 +13,8 @@ mod auto_mozilla;
 mod auto_outlook;
 pub(crate) mod server_params;
 
+use std::net::IpAddr;
+
 use anyhow::{Context as _, Result, bail, ensure, format_err};
 use auto_mozilla::moz_autoconfigure;
 use auto_outlook::outlk_autodiscover;
@@ -557,6 +559,25 @@ async fn configure(ctx: &Context, param: &EnteredLoginParam) -> Result<Option<&'
     let ctx2 = ctx.clone();
     let update_device_chats_handle = task::spawn(async move { ctx2.update_device_chats().await });
 
+    if !param.dns_prefill.is_empty() {
+        let mut ips: Vec<IpAddr> = Vec::new();
+        for ip in &param.dns_prefill {
+            match ip.parse::<IpAddr>() {
+                Ok(ip) => ips.push(ip),
+                Err(err) => {
+                    error!(
+                        ctx,
+                        "IP address prefill failed: parsing of address '{ip}' failed: {err}"
+                    );
+                }
+            }
+        }
+        ctx.dns_memory_cache
+            .write()
+            .await
+            .insert(param.imap.server.clone(), ips);
+    }
+
     let configured_param = get_configured_param(ctx, param).await?;
     let proxy_config = ProxyConfig::load(ctx).await?;
     let strict_tls = configured_param.strict_tls(proxy_config.is_some());

src/context.rs

@@ -318,6 +318,11 @@ pub struct InnerContext {
             ) -> mail_builder::mime::MimePart<'a>,
         >,
     >,
+
+    /// Short lived DNS cache which only lives in memory.
+    /// Used for configuration from `dcaccount` links with ip address.
+    /// Like `dcaccount:example.org?a=127.0.0.1,[::1]`
+    pub(crate) dns_memory_cache: Arc<RwLock<HashMap<String, Vec<std::net::IpAddr>>>>,
 }
 
 /// The state of ongoing process.

src/net/dns.rs

@@ -860,6 +860,14 @@ pub(crate) async fn lookup_host_with_cache(
                 }
             }
         }
+        if let Some(ips) = context.dns_memory_cache.read().await.get(hostname) {
+            for ip in ips {
+                let addr = SocketAddr::new(*ip, port);
+                if !cache.contains(&addr) {
+                    cache.push(addr);
+                }
+            }
+        }
 
         merge_with_cache(resolved_addrs, cache)
     } else {