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 (Alpha) 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
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
Customer provides an FQDN that resolves to their SBC/public SIP gateway.
Exotel adds this FQDN in the trunk configuration.
During each SIP call, Exotel queries DNS to resolve the IP.
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 200 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 = <your_public_ip>
localnet = 192.168.0.0/16
[exotelvsip-fqdn]
type = friend
context = incoming
fromdomain = <accountsid>.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
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 CopyEdit
INVITE sip:+91XXXXXXXXXX@sip.customer.com:5070;transport=tcp SIP/2.0
Via: SIP/2.0/TCP pstn.in2.exotel.com;branch=...
From: "+91AAAAAAAAAA" <sip:+91AAAAAAAAAA@exotelt.pstn.exotel.com>
To: sip:+91XXXXXXXXXX@sip.customer.com
Call-ID: abcd1234efgh@pstn.mum1.exotel.com
CSeq: 102 INVITE
Contact: sip:+91AAAAAAAAAA@<exotel-media-ip>:port;transport=tcp
X-Exotel-CallSid: <auto-generated-by-exotel>
X-Exotel-LegSid: <leg-id>
X-Exotel-TrunkSid: <trunk-id>
P-Asserted-Identity: sip:+91AAAAAAAAAA@pstn.in2.exotel.com
P-Early-Media: supported
Content-Type: application/sdp
Content-Length: ...
Inbound SIP Header Reference (Exotel → Customer via FQDN)
Key take-aways for FQDN mode
DNS resolution – Exotel resolves your FQDN on every call; keep TTL ≤ 60s for rapid fail-over.
Certificates (TLS) – For secure trunks, your SBC’s cert CN/SAN should match the FQDN Exotel is dialling.
Logging – Always capture X-Exotel-CallSid + Call-ID to join SIP pcaps with Exotel dashboards.
9. Troubleshooting
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
Version: Alpha-FQDN | Last updated: June 2025