--- title: Exotel Virtual SIP Trunking – FQDN-based slug: dynamic-sip-trunking/exotel-virtual-sip-trunking-fqdn-based docTags: createdAt: 2025-09-29T14:42:06.230Z --- This support article explains how to configure Exotel’s Virtual SIP Trunking (vSIP) using Fully Qualified Domain Name (FQDN) based trunking instead of fixed IPs. It details how the FQDN-based approach works, when it should be used, and why it offers benefits in enterprise-grade SIP deployments. ## 1. Product Overview Exotel’s FQDN-based Virtual SIP Trunking allows customers to use DNS-resolvable hostnames (e.g., trunk.customer.com) instead of static IPs to establish SIP trunking with Exotel's PoPs. This method enables greater flexibility and reliability, especially for deployments on cloud infrastructure or load-balanced SIP setups. ## 2. What is an FQDN-based Trunk? An FQDN (Fully Qualified Domain Name) is a domain name that maps dynamically to one or more IPs. In this model: - Your SIP gateway/SBC is reachable via a DNS-resolvable FQDN - Exotel performs DNS resolution during SIP call setup - No static IP whitelisting is required from customer side Exotel supports both **static FQDN** and **dynamic DNS-based FQDN** approaches. ## 3. When to Use FQDN Instead of Static IP | **Scenario** | **Recommendation** | | ---------------------------------------------------- | ------------------ | | SIP infra hosted on cloud (AWS, Azure, GCP) | Use FQDN | | Customer SBC has load balancer or HA proxy | Use FQDN | | IP addresses change due to autoscaling/load shifting | Use FQDN | | On-prem setup with fixed static IPs | Use static IP | FQDN is best suited when your IP address is not guaranteed to remain fixed or you have redundant/public-facing SIP endpoints. ## 4. How It Works 1. Customer provides an FQDN that resolves to their SBC/public SIP gateway. 2. Exotel adds this FQDN in the trunk configuration. 3. During each SIP call, Exotel queries DNS to resolve the IP. 4. Call is routed based on the returned A/AAAA record(s). **Note:** Exotel performs DNS resolution on each call attempt to support failover and dynamic environments. **Note – vSIP Throttling** Exotel enforces a default vSIP rate-limit of 20 calls per minute (CPM) per trunk to safeguard carrier capacity and call quality. If your traffic profile requires a higher burst rate, raise a request via your CSM or Support ticket. The capacity-planning team will review historical traffic, carrier limits, and QoS requirements and can increase the throttling threshold accordingly. ## 5. Configuration Requirements ### On Customer Side - Expose SIP server using a public FQDN (e.g., sip.companydomain.com) - Ensure DNS A or AAAA records point to valid, routable IP(s) - SIP server must be reachable on TCP or TLS as applicable - Support G.711 codecs and standard RTP port ranges ### On Exotel Side (done by Exotel Ops) - Default\:Trunk is configured with FQDN instead of IP - Default\:DNS lookups are enabled in trunk routing logic - Default\:Media IP range is kept open as per standard PoP setup ## 6. SIP Trunk Parameters (via FQDN) ### Sample Configuration (Asterisk) \[general] `externip = ` `localnet = 192.168.0.0/16` `[exotelvsip-fqdn]` `type = friend` `context = incoming` `fromdomain = .pstn.exotel.com` `host = sip.customerdomain.com` `port = 5070 ; or 443 for TLS` `transport = tcp ; or tls if using encrypted trunk` `disallow = all` `allow = alaw` `allow = ulaw` `nat = force_rport` `insecure = port` `canreinvite = no` `sendrpid = yes` `trustrpid = yes` `relaxdtmf = yes` `encryption = yes ; only if using TLS with SRTP` ### SIP Parameters Summary | Parameter | Example | | --------------- | ---------------------- | | SIP Peer FQDN | sip.customerdomain.com | | Port | 5070 (TCP) / 443 (TLS) | | Transport | TCP or TLS | | Media IP Range | 10000–40000 (UDP) | | DNS Record Type | A (IPv4) / AAAA (IPv6) | | Parameter | Example | | --------------- | ---------------------- | | SIP Peer FQDN | sip.customerdomain.com | | Port | 5070 (TCP) / 443 (TLS) | | Transport | TCP or TLS | | Media IP Range | 10000–40000 (UDP) | | DNS Record Type | A (IPv4) / AAAA (IPv6) | ## 7. Advantages of FQDN-based Approach - **No need for IP whitelisting**: Reduces manual ACL changes - **Supports redundancy**: Load balancer IPs can be rotated behind the same FQDN - **Cloud-friendly**: Ideal for cloud-native SIP deployments - **Failover-friendly**: Multiple A-records supported for active/passive routing ## 8. Best Practices - TTL for DNS entries should be low (e.g., 30s–60s) for fast failover - Use reliable DNS hosting with high availability - Ensure SIP firewall rules allow calls from Exotel media IPs - Avoid using CNAME records for SIP endpoints (use A/AAAA directly) ## 8.1 SIP Message Format and Header Behavior ## SIP Message Format (FQDN Mode – TCP Transport) ### A. INVITE from Exotel to Customer via FQDN (Inbound) This is how Exotel delivers a call to your SBC or PBX using a **public DNS hostname** instead of a static IP. **Sample INVITE (Inbound via FQDN)** Sip `INVITE sip:+91XXXXXXXXXX@sip.customer.com:5070;transport=tcp SIP/2.0` `Via: SIP/2.0/TCP ;branch=...` `From: "+91AAAAAAAAAA" ` `To: sip:+91XXXXXXXXXX@sip.customer.com` `Call-ID: abcd1234efgh@` `CSeq: 102 INVITE` `Contact: sip:+91AAAAAAAAAA@:port;transport=tcp` `X-Exotel-CallSid: ` `X-Exotel-LegSid: ` `X-Exotel-TrunkSid: ` `P-Asserted-Identity: sip:+91AAAAAAAAAA@` `P-Early-Media: supported` `Content-Type: application/sdp` `Content-Length: ...` **Inbound SIP Header Reference (Exotel → Customer via FQDN)** | **Header** | **Mandatory** | **Description** | | ------------------- | ------------- | --------------------------------------------------------------- | | Request URI | Yes | Your registered Exophone and SIP FQDN | | Via | Yes | Exotel’s signaling IP + transport (TCP) | | From | Yes | Caller CLI (could be real, masked, or Exophone) | | To | Yes | Your PBX’s FQDN URI target | | Call-ID | Yes | Dialog identifier for SIP trace correlation | | CSeq | Yes | Command sequence, starts with INVITE | | Contact | Yes | Exotel’s signaling contact (for BYE, ACK, etc.) | | Content-Type | Yes | application/sdp | | Content-Length | Yes | SDP payload length | | Max-Forwards | Yes | Loop control | | X-Exotel-CallSid | Yes | Exotel’s global call ID — key for webhook/CDR correlation | | X-Exotel-LegSid | Yes | Call leg identifier | | X-Exotel-TrunkSid | Yes | Identifies the virtual SIP trunk provisioned to you | | P-Asserted-Identity | Yes | Validated CLI presented by Exotel | | P-Early-Media | Optional | Indicates early RTP media (if Exotel connects media pre-200 OK) | **Key take-aways for FQDN mode** 1. **DNS resolution** – Exotel resolves your FQDN on every call; keep TTL ≤ 60s for rapid fail-over. 2. **Certificates (TLS)** – For secure trunks, your SBC’s cert CN/SAN should match the FQDN Exotel is dialling. 3. **Logging** – Always capture X-Exotel-CallSid + Call-ID to join SIP pcaps with Exotel dashboards. ### 9. Troubleshooting | Issue | Cause | Fix | | ----------------------- | ----------------------------------- | ----------------------------------------- | | Call fails to reach SBC | FQDN not resolving or DNS TTL delay | Use dig/nslookup to verify resolution | | One-way audio | Media ports blocked | Open UDP 10000–40000 | | Repeated 408 errors | DNS record TTL mismatch | Reduce DNS TTL and ensure IP is reachable | ## 10. Support For FQDN-based trunk configuration: - Share your FQDN and port with your Exotel account manager - Exotel will provision the trunk using DNS-based routing - Test connectivity using SIPp, sngrep, or call traces For help: - Contact support.exotel.com with FQDN, DNS setup, and logs