Ephemeral port

An ephemeral port is a short-lived transport protocol port for Internet Protocol (IP) communications allocated automatically from a predefined range by the IP stack software. An ephemeral port is typically used by the Transmission Control Protocol (TCP), User Datagram Protocol (UDP), or the Stream Control Transmission Protocol (SCTP) as the port assignment for the client end of a client–server communication to a well-known port on a server.

On servers, ephemeral ports may also be used as the port assignment on the server end of a communication. This is done to continue communications with a client that initially connected to one of the server's well-known service listening ports. File Transfer Protocol (FTP) and Remote Procedure Call (RPC) applications are two protocols that can behave in this manner. Note that the term "server" here includes workstations running network services that receive connections initiated from other clients (e.g. Remote Desktop Protocol).

The allocations are temporary and only valid for the duration of the communication session. After completion of the communication session. After an OS specific completion timeout is reached, the ports become available for reuse.^{[note 1]} Since the ports are used on a per request basis they are also called dynamic ports.

Range

The Internet Assigned Numbers Authority (IANA) suggests the range 49152 to 65535 (2¹⁵+2¹⁴ to 2¹⁶−1) for dynamic or private ports.^[1]

Many Linux kernels use the port range 32768 to 61000.^{[note 2]} FreeBSD has used the IANA port range since release 4.6. Previous versions, including the Berkeley Software Distribution (BSD), use ports 1024 to 5000 as ephemeral ports.^[2]

Microsoft Windows operating systems through XP use the range 1025–5000 as ephemeral ports by default.^[3] Windows Vista, Windows 7, and Server 2008 use the IANA range by default.^[4] Windows Server 2003 uses the range 1025–5000 by default, until Microsoft security update MS08-037 from 2008 is installed, after which it uses the IANA range by default.^[5] Windows Server 2008 with Exchange Server 2007 installed has a default port range of 1025–60000.^[6] In addition to the default range, all versions of Windows since Windows 2000 have the option of specifying a custom range anywhere within 1025–65535.^[7][8]

Configuration characteristics

If certain server software is used, that uses non ephemeral custom port ranges for initiating some further connections, it needs to be ensured by configuration that this custom port range and the ephemeral port range do not overlap.

Notes

1. Most implementations may simply increment the last used port number until the ephemeral port range is exhausted.
2. The effective range is accessible via the /proc file system at node /proc/sys/net/ipv4/ip_local_port_range.

See also

• Internet Protocol Suite
• List of TCP and UDP port numbers

References

1. IANA port number assignments
2. UNIX Network Programming, Volume 1, Second Edition: Networking APIs: Sockets and XTI. Prentice Hall. 1998. pp. 42–43. ISBN 0-13-490012-X.
3. Microsoft Windows Technet Library
4. Microsoft KB Article 929851
5. Microsoft KB Article 956188
6. Microsoft KB Article 929851
7. Microsoft KB Article 929851
8. Microsoft KB Article 196271

External links

• The Ephemeral Port Range at NcFTP.com
• Ephemeral Source Port Selection Strategies at Team Cymru

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.