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Configuration options

Spectrum is a global TCP and UDP proxy running on Cloudflare's edge nodes. It does not terminate the connection. Instead it passes through the packets to the backend server.

IP Addresses

When a Spectrum application is created, it is assigned a unique IPv4 and IPv6 address. These addresses are not static, they may change over time. The best way to look up what the current addresses are is using DNS. The DNS name of the Spectrum application will always return the IP's currently dedicated to the application.

These addresses are Anycasted from all Cloudflare data centers, with the exception of data centers in China.


Spectrum can act as a TCP load balancer in front of an SMTP server. Spectrum will not act as an intermediary mail server. Instead, it will simply pass data through to your origin. The client IP shown on mail will be the Cloudflare edge IP. If the mail server requires knowing the true client IP, it should use Proxy Protocol to get the source IP from Cloudflare. Cloudflare recommends enabling Proxy Protocol on applications configured to proxy SMTP.

SMTP servers may perform a series of checks on servers attempting to send messages through it. These checks are intended to filter requests from illegitimate servers. Messages may be rejected if:

  • A reverse DNS lookup on the IP address of the connecting server returns a negative response.
  • The reverse DNS lookup produces a different hostname than what was sent in the SMTP HELO/EHLO message.
  • The reverse DNS lookup produces a different hostname than what is advertised in your SMTP server's banner.
  • The result of a reverse DNS lookup does not match a corresponding forward DNS lookup.

Spectrum applications do not have reverse DNS entries.

Additionally, SMTP servers may perform a DNS lookup to find the MX records for a domain. Messages from your server may be rejected if an MX record for your domain is associated with a Spectrum application, as the IP address of server will not match the Spectrum IP address.


Cloudflare supports all TCP ports.

Port Ranges

Spectrum applications can be configured to proxy traffic on ranges of ports.

For direct origins:

{  "protocol": "tcp/1000-2000",  "dns": {    "type": "CNAME",    "name": ""  },  "origin_direct": ["tcp://"]}

For DNS origins:

{  "protocol": "tcp/1000-2000",  "dns": {    "type": "CNAME",    "name": ""  },  "origin_dns": {    "name": "",    "ttl": 1200  },  "origin_port": "3000-4000"}

Note: the number of ports in an origin port range must match the number of ports specified in the protocol field. Connections to a port within a port range at the edge will be proxied to the equivalent port offset in the origin range. For example, in the configurations above, a connection to would be proxied to port 3005 on the origin.

IP Access Rules

If the IP Access Rules toggle is enabled for a Spectrum application, then Cloudflare will respect the IP Access Rules created under the Tools tab in for that domain. Cloudflare only respects rules created for specific IP addresses, IP blocks, countries or ASNs for Spectrum applications. Spectrum will also only respect rules created with the actions 'allow' or 'block'.

Edge TLS Termination

Spectrum offers four modes of TLS termination: 'Passthrough', 'Flexible', 'Full', and 'Full (Strict)'.

'Passthrough' disables TLS termination at the edge. This is the required setting if the underlying traffic is not encrypted with TLS. If the connection is encrypted, 'Passthrough' allows traffic to flow from client to origin using the TLS session negotiated directly with the origin.

'Flexible' enables termination of the client connection at the edge, but does not enable TLS from Cloudflare to your origin. Traffic will be sent over an encrypted connection from the client to Cloudflare, but not from Cloudflare to the origin.

'Full' specifies that traffic from Cloudflare to the origin will also be encrypted but without certificate validation. When set to 'Full (Strict)', traffic from Cloudflare to the origin will also be encrypted with strict validation of the origin certificate.

TLS traffic can still be sent through Spectrum with TLS set to 'Passthrough'. Cloudflare will not terminate it. Instead, it will act as a pure passthrough.

Spectrum supports versions TLS 1.1, TLS 1.2, and TLS 1.3.

Origin TLS Termination

Below are the cipher suites that we present to origins during an SSL/TLS handshake. If you're looking for cipher suites that we support at our edge, i.e., that are presented to browsers and other user agents, see here instead.

Note that the cipher suites below are ordered based on how they appear in the ClientHello, communicating our preference to the origin.

Supported Cipher Suites by Protocol

OpenSSL NameTLS 1.1TLS 1.2TLS 1.3

  1. Although TLS 1.3 uses the same cipher suite space as previous versions of TLS, TLS 1.3 cipher suites are defined differently, only specifying the symmetric ciphers, and cannot be used for TLS 1.2. Similarly, TLS 1.2 and lower cipher suites cannot be used with TLS 1.3 (IETF TLS 1.3 draft 21). BoringSSL also hard-codes cipher preferences in this order for TLS 1.3.