Set SSH connection timeout

Question

I m trying to cut down the time ssh is trying to open a connection to a host  If I put for example ssh www google com it takes very long until the prompt comes back   I read about using ssh -o ConnectTimeout 10 www google com instead  but even this takes very long  Is there maybe a number of attemps I can modify to decrease the blocking time

User · Accepted Answer

The problem may be that ssh is trying to connect to all the different IPs that www.google.com resolves to. For example on my machine:

# ssh -v -o ConnectTimeout=1 -o ConnectionAttempts=1 www.google.com
OpenSSH_5.9p1, OpenSSL 0.9.8t 18 Jan 2012
debug1: Connecting to www.google.com [173.194.43.20] port 22.
debug1: connect to address 173.194.43.20 port 22: Connection timed out
debug1: Connecting to www.google.com [173.194.43.19] port 22.
debug1: connect to address 173.194.43.19 port 22: Connection timed out
debug1: Connecting to www.google.com [173.194.43.18] port 22.
debug1: connect to address 173.194.43.18 port 22: Connection timed out
debug1: Connecting to www.google.com [173.194.43.17] port 22.
debug1: connect to address 173.194.43.17 port 22: Connection timed out
debug1: Connecting to www.google.com [173.194.43.16] port 22.
debug1: connect to address 173.194.43.16 port 22: Connection timed out
ssh: connect to host www.google.com port 22: Connection timed out

If I run it with a specific IP, it returns much faster.

EDIT: I've timed it (with time) and the results are:

www.google.com - 5.086 seconds
173.94.43.16 - 1.054 seconds

User · Answer

The ConnectTimeout option allows you to tell your ssh client how long you re willing to wait for a connection before returning an error  By setting ConnectTimeout to 1  you re effectively saying  try for at most 1 second and then fail if you haven t connected yet    The problem is that when you connect by name  the DNS lookup can take several seconds  Connecting by IP address is much faster  and may actually work in one second or less  What sinelaw is experiencing is that every attempt to connect by DNS name is failing to occur within one second  The default setting of ConnectTimeout defers to the linux kernel connect timeout  which is usually pretty long

[ssh] Set SSH connection timeout

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