Implementing DNS Push in IKEv2 VPN Profiles: Secure, Seamless Client Name Resolution

Implementing DNS push in IKEv2 VPN profiles is a crucial step for delivering a seamless, secure name resolution experience to clients. For site owners, enterprise administrators, and developers building VPN services, properly configuring DNS via IKEv2 eliminates common issues such as DNS leakage, split-brain resolution, and inconsistent client behavior across platforms. This article dives into the technical mechanics of DNS push with IKEv2, practical server/client features, sample configuration patterns, security considerations, and troubleshooting tips to help you implement a robust solution.

Why pushing DNS via IKEv2 matters

When a VPN client establishes a tunnel, merely routing traffic through the tunnel does not guarantee that DNS queries will follow. If the client continues to use its local or ISP-provided resolvers, sensitive metadata and hostnames can leak outside the protected network. By pushing DNS settings during the IKEv2 negotiation, the VPN server instructs the client to use specific resolver IP addresses and search domains for the duration of the tunnel. The advantages include:

• Consistent name resolution: Clients always resolve internal hostnames correctly without manual configuration.
• Reduced configuration burden: Administrators can manage DNS centrally and avoid per-client provisioning scripts.
• Privacy and security: Prevents DNS leaks to untrusted third parties; easier enforcement of enterprise DNS filtering/monitoring.
• Support for split-DNS: Push selective domain search suffixes so internal names resolve to internal IPs while public domains use normal resolvers.

How DNS is delivered in IKEv2

IKEv2 supports configuration attributes that can be carried in the IKE Configuration Payload (CFG payload) or using vendor-specific extensions. During the IKE_AUTH or CREATE_CHILD_SA exchanges, the server can include configuration attributes that tell the client which internal IP to use, and which DNS servers and search domains to adopt. Typical attributes you can push include:

• Internal DNS server IP addresses (IPv4 and/or IPv6)
• Domain search suffixes
• Split-DNS directives (client OS-dependent)

Most modern VPN stacks—strongSwan, Libreswan, Windows built-in IKEv2, Apple IKEv2 clients, and Android’s IKEv2 implementations—support receiving DNS server addresses and domain search lists via these attributes. The exact attribute encoding may vary by implementation; strongSwan for instance maps its cfg attribute names to the appropriate IKE payloads automatically.

Configuration vs. DHCP over IKE

Two common approaches exist to give a client network configuration over a secure tunnel: the IKEv2 configuration payload and in-tunnel DHCP (DHCP-proxy or DHCP server reachable via the tunnel). The IKEv2 configuration payload is preferred for immediate, atomic delivery during tunnel setup because it eliminates the need for additional DHCP traffic and works even before in-tunnel reachability is established. DHCP may be used where you need more advanced lease or option management, but it complicates startup and split-DNS behavior.

Server-side implementation patterns

Below are practical patterns for popular servers. Replace addresses and identifiers with your environment’s values.

strongSwan (swantlctl / ipsec.conf)

strongSwan supports pushing DNS and domain search using the “rightsourceip”, “rightdns”, and “rightdomain” options (ipsec.conf) or the equivalent keys in swanctl.conf. Example (conceptual):

conn vpn-net
left=%any
leftcert=server-cert.pem
right=%any
rightauth=eap-mschapv2
rightsourceip=10.10.50.0/24
rightdns=10.10.50.5,10.10.50.6
rightdomain=corp.internal
auto=add

When a client connects, strongSwan includes the DNS server addresses in the IKEv2 configuration payload. For split-DNS you can also use the “dpd” and virtual-IP based routing rules along with policy-based routing to ensure only certain FQDNs go to internal resolvers.

Windows Server RRAS / Windows clients

Windows RRAS and the built-in IKEv2 client accept DNS servers and search suffixes pushed by the server, but historically behavior differed by Windows version. Windows tends to set the pushed DNS servers as the primary resolvers for the VPN adapter. To support split-DNS you may need to combine pushed DNS with conditional forwarders on the server-side DNS infrastructure or configure NRPT (Name Resolution Policy Table) via Group Policy for fine-grained per-domain resolution.

Apple/iOS/macOS

Apple IKEv2 clients accept DNS server addresses and domain search lists pushed from the server. macOS and iOS also support “per-domain” DNS settings for VPN configurations using their managed configuration APIs — useful when deploying via MDM to ensure consistent split-DNS behaviors.

Linux clients and Libreswan/strongSwan

Linux clients running strongSwan typically use ip-up scripts or NetworkManager plugins to install pushed DNS servers into resolv.conf or systemd-resolved. For systemd-resolved-based systems, strongSwan provides integration to set DNS servers in the proper resolvectl scope to avoid race conditions.

Implementing split-DNS

Split-DNS means that queries for internal zones (e.g., corp.internal) are forwarded to internal resolvers while public queries continue to use local or public DNS. There are a few approaches:

• Client-side policy: OS-level features such as Windows NRPT, macOS per-domain VPN DNS settings, or systemd-resolved split rules on Linux.
• Server-side forwarding: Push internal DNS servers and also push domain search suffixes; clients will query internal servers for any domain that matches the suffix.
• Conditional forwarders at perimeter DNS: If the client cannot enforce split-DNS, configure your perimeter DNS to forward queries for internal zones to internal servers when appropriate.

The most reliable results come from combining IKEv2 DNS push with client-side policy support. For example, push DNS server IPs and the search domain, and configure a Group Policy or MDM to ensure the client treats those DNS servers as authoritative for the domain(s) you control.

Security considerations

Pushing DNS via IKEv2 helps mitigate leaks, but you must also consider the security of DNS resolution itself:

• Authentication and confidentiality: IKEv2 configuration payloads are integrity-protected and encrypted once the IKE SA keys are established, so DNS server addresses delivered this way are secure in transit.
• Protect DNS lookups: Use encrypted DNS inside the tunnel when possible — run DNS-over-TLS (DoT) or DNS-over-HTTPS (DoH) endpoints reachable via the tunnel, or ensure internal resolvers support DNSSEC validation.
• Restrict forwarded traffic: Use firewall rules and split-tunneling policies carefully to prevent bypass of the VPN for DNS.
• Validate client behavior: Different operating systems handle pushed settings uniquely — verify clients actually honor pushed DNS servers and do not fall back to insecure resolvers.

Testing and verification

After configuring DNS push, validate it across client OS families:

• On Windows: run ipconfig /all to confirm DNS servers on the VPN adapter; check the NRPT for domain policies with PowerShell: Get-DnsClientNrptRule.
• On macOS: run scutil –dns and networksetup -getinfo to confirm DNS servers and search domains for the VPN interface.
• On Linux: use resolvectl status or inspect /etc/resolv.conf (depending on resolver) to confirm the pushed DNS entries.
• Packet capture: use tcpdump or Wireshark on the client to ensure queries go to the intended IPs after the tunnel is up. Capture on the client’s tunnel interface or at the server egress to verify no DNS leakage occurs.
• Query internal-only names to confirm they resolve, and public names to confirm they either resolve via internal servers (if intended) or via fallback resolvers if split-DNS is configured.

Troubleshooting common issues

Here are frequent pitfalls and remedies:

• Clients ignore pushed DNS: Some OS versions or third-party VPN clients may ignore IKEv2 configuration attributes. Use client-side profiles (MDM, Group Policy) or client scripts to enforce settings.
• Order of resolver entries: Ensure your server pushes resolver IPs in the desired priority. OSes may reorder based on reachability tests.
• Race conditions on Linux: NetworkManager and strongSwan may fight over resolv.conf updates. Use the strongSwan plugin for systemd-resolved or configure NetworkManager to accept DNS from VPN plugins.
• Split-DNS not working: Confirm the client supports per-domain routing or configure per-domain DNS policies via NRPT/MDM/systemd-resolved.
• DNSSEC failures: If you push internal resolvers that act as validating resolvers, ensure they are properly configured and reachable over the VPN; otherwise DNSSEC validation can fail and block resolution.

Operational recommendations

For production deployments, follow these best practices:

• Centralize DNS management: Keep internal zones and forwarders on authoritative/internal recursive servers accessible only through the VPN.
• Monitor for leaks: Deploy periodic automated checks from client endpoints that report the operational resolver and whether queries leak.
• Document platform differences: Maintain platform-specific guidance for administrators to troubleshoot client behavior.
• Use encryption for DNS: Wherever possible, prefer DoT/DoH inside the tunnel to protect queries from observation within shared networks inside the VPN environment.

Implementing DNS push via IKEv2 is a powerful tool to deliver secure, predictable name resolution for VPN users. By understanding how configuration payloads are transmitted, how different clients respond, and how to combine server-side policies with client-side controls, you can design a resilient and secure DNS strategy for your VPN service. For enterprise deployments and managed client fleets, pair IKEv2 DNS push with MDM or Group Policy to ensure consistent behavior across operating systems.

For more guidance and practical configurations tailored to different VPN stacks, visit Dedicated-IP-VPN at https://dedicated-ip-vpn.com/.