curl ootw: –keepalive-time

(previously blogged about options are listed here.)

This option is named --keepalive-time even if the title above ruins the double-dash (thanks for that WordPress!). This command line option was introduced in curl 7.18.0 back in early 2008. There’s no short version of it.

The option takes a numerical argument; number of seconds.

What’s implied in the option name and not spelled out is that the particular thing you ask to keep alive is a TCP connection. When the keepalive feature is not used, TCP connections typically don’t send anything at all if no data is transmitted.

Idle TCP connections

Silent TCP connections typically cause the two primary issues:

  1. Middle-boxes that track connections, such as your typical NAT boxes (home WiFi routers most notoriously) will consider silent connections “dead” after a certain period of time and drop all knowledge about them, leading to the connection non functioning when the client (or server) later wants to resume operation of it.
  2. Neither side of the connection will notice when the network between them breaks, as it takes actual traffic to do so. This is of course also a feature, because there’s no need to be alarmed by a breakage if there’s no traffic as it might be fine again when it eventually gets used again.

TCP stacks then typically implement a low-level feature where they can send a “ping” frame over the connection if it has been idle for a certain amount of time. This is the keepalive packet.

--keepalive-time <seconds> therefor sets the interval. After this many seconds of “silence” on the connection, there will be a keepalive packet sent. The packet is totally invisible to the applications on both sides but will maintain the connection through NATs better and if the connection is broken, this packet will make curl detect it.

Keepalive is not always enough

To complicate issues even further, there are also devices out there that will still close down connections if they only send TCP keepalive packets and no data for certain period. Several protocols on top of TCP have their own keepalive alternatives (sometimes called ping) for this and other reasons.

This aggressive style of closing connections without actual traffic TCP traffic typically hurts long-going FTP transfers. This, because FTP sets up two connections for a transfer, but the first one is the “control connection” and while a transfer is being delivered on the “data connection”, nothing happens over the first one. This can then result in the control connection being “dead” by the time the data transfer completes!

Default

The default keepalive time is 60 seconds. You can also disable keepalive completely with the --no-keepalive option.

The default time has been selected to be fairly low because many NAT routers out there in the wild are fairly aggressively and close idle connections already after two minutes (120) seconds.

For what protocols

This works for all TCP-based protocols, which is what most protocols curl speaks use. The only exception right now is TFTP. (See also QUIC below.)

Example

Change the interval to 3 minutes:

curl --keepalive-time 180 https://example.com/

Related options

A related functionality is the --speed-limit amd --speed-time options that will cancel a transfer if the transfer speed drops below a given speed for a certain time. Or just the --max-time that sets a global timeout for an entire operation.

QUIC?

Soon we will see QUIC getting used instead of TCP for some protocols: HTTP/3 being the first in line for that. We will have to see what exactly we do with this option when QUIC starts to get used and what the proper mapping and behavior shall be.

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