first commit

backend/app/routers/dns_setup.py

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+import ipaddress
+
+from fastapi import APIRouter, Depends, HTTPException, status
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.config import get_settings
+from app.database import get_postgres_session
+from app.deps.dns_setup_auth import require_dns_setup_auth
+from app.schemas import DnsSetupRequest, DnsSetupResponse
+from app.services.domain_provisioning import (
+    build_deployment_dns_records,
+    get_domain_by_name,
+    validate_domain,
+)
+
+router = APIRouter(prefix="/dns-setup", tags=["dns-setup"])
+
+
+def validate_mail_server_ip(value: str) -> str:
+    candidate = value.strip()
+    try:
+        return str(ipaddress.ip_address(candidate))
+    except ValueError as error:
+        raise ValueError("Enter a valid IPv4 or IPv6 address.") from error
+
+
+@router.post("/preview", response_model=DnsSetupResponse)
+async def preview_dns_setup(
+    payload: DnsSetupRequest,
+    _: None = Depends(require_dns_setup_auth),
+    session: AsyncSession = Depends(get_postgres_session),
+) -> DnsSetupResponse:
+    settings = get_settings()
+    try:
+        domain_name = validate_domain(payload.domain)
+        mail_server_ip = validate_mail_server_ip(payload.mail_server_ip)
+    except ValueError as error:
+        raise HTTPException(
+            status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
+            detail=str(error),
+        ) from error
+
+    domain = await get_domain_by_name(session, domain_name)
+    sections = build_deployment_dns_records(
+        domain_name=domain_name,
+        mail_server_ip=mail_server_ip,
+        dkim_selector=settings.dkim_selector,
+        dkim_public_key=domain.dkim_public_key if domain else None,
+        app_public_host=settings.app_public_host,
+    )
+    mail_host = f"mail.{domain_name}"
+    return DnsSetupResponse(
+        domain=domain_name,
+        mail_host=mail_host,
+        app_public_host=settings.app_public_host,
+        mail_server_ip=mail_server_ip,
+        is_platform_domain=domain_name == settings.platform_domain,
+        sections=sections,
+        notes=_build_notes(
+            domain_name,
+            settings.app_public_host,
+            mail_server_ip,
+        ),
+    )
+
+
+def _build_notes(
+    domain_name: str,
+    app_public_host: str,
+    mail_server_ip: str,
+) -> list[str]:
+    return [
+        "Add these records in Namecheap Advanced DNS for the zone.",
+        f"Point mail.{domain_name} and {app_public_host} A records to {mail_server_ip}.",
+        f"Keep the root {domain_name} A record on your main website server if it hosts the marketing site.",
+        f"Set {domain_name} MX to mail.{domain_name} (priority 10) so inbound mail reaches Postfix on this host.",
+        "Postfix accepts inbound mail on :25 and delivers outbound mail for the app over the Docker network.",
+        f"Users open the web UI at https://{app_public_host} (Caddy terminates TLS).",
+        f"Ask your VPS provider to set reverse DNS (PTR) for {mail_server_ip} → mail.{domain_name}.",
+        "SPF, DKIM, and DMARC authenticate outbound mail signed by Agmail (selector agmail).",
+    ]