number of SYN retries does not match value in /proc/sys/net/ipv4/tcp_syn_retries

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Environment

SUSE Linux Enterprise Server 11 Service Pack 1

Situation

The default value of syn retries, /proc/sys/net/ipv4/tcp_syn_retries is set to 5 but the actual number of syn retries appear to be smaller.
This is different from previous SUSE Linux Enterprise versions and with kernel up to 2.6.31, where the actual number of syn retries is the number in tcp_syn_retries + 1.
This results in less time spent in trying to establish the connection before the client gives up.
The old default connection timeout was ~180 seconds, while the new one is about ~23 seconds.
The reason is a change in kernel 2.6.32, using calculation instead of a direct definition of the retries number.

Resolution

A patch is adding a syn_set flag to the retransmits_timed_out() routine, to make sure the sysctl_syn_retries use the actual value. The patch is included in the SUSE Linux Enterprise 11 Service Pack 2, and the mismatch should be corrected.

As workaround on SUSE Linux Enterprise 11 Service Pack 1 the number of /proc/sys/net/ipv4/tcp_syn_retries can be increased.

Additional Information

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