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grigoryvp
Asked: 2010-12-30 01:15:56 +0800 CST

How to check TCP timeout in linux / macos?

  • 772

I have a network problem on my MacOS that i need to troubleshoot. I know that TCP socket has internal timeout that will close connection if remote party is not responding (but no graceful disconnect either). Is it any command/tool i can use to check exact value of this timeout(s)?

mac-osx tcp

4 Answers

  1. Best Answer

    You can see all system-set tcp values with

    $ sysctl net.inet.tcp

    Interpreted from tcp_var.h, tcp_subr.c, and tcp_timer.c:

    • net.inet.tcp.keepidle = keepalive idle timer
    • net.inet.tcp.keepintvl = interval to send keepalives
    • net.inet.tcp.keepinit = timeout for establishing syn
    • net.inet.tcp.mssdflt = Default TCP Maximum Segment Size
    • net.inet.tcp.v6mssdflt = Default TCP Maximum Segment Size for IPv6
    • net.inet.tcp.minmss = Minmum TCP Maximum Segment Size
    • net.inet.tcp.minmssoverload = Number of TCP Segments per Second allowed to be under the MINMSS Size
    • net.inet.tcp.rfc1323 = Enable rfc1323 (high performance TCP) extensions
    • net.inet.tcp.rfc1644 = Enable rfc1644 (TTCP) extensions
    • net.inet.tcp.do_tcpdrain = Enable tcp_drain routine for extra help when low on mbufs
    • net.inet.tcp.pcbcount = Number of active PCBs
    • net.inet.tcp.icmp_may_rst = Certain ICMP unreachable messages may abort connections in SYN_SENT
    • net.inet.tcp.strict_rfc1948 = Determines if RFC1948 is followed exactly
    • net.inet.tcp.isn_reseed_interval = Seconds between reseeding of ISN secret
    • net.inet.tcp.background_io_enabled = Background IO Enabled
    • net.inet.tcp.rtt_min = min rtt value allowed
    • net.inet.tcp.randomize_ports = Randomize TCP port numbers
    • net.inet.tcp.tcbhashsize = Size of TCP control-block hashtable
    • net.inet.tcp.msl = Maximum segment lifetime
    • net.inet.tcp.always_keepalive = Assume SO_KEEPALIVE on all TCP connections
    • net.inet.tcp.broken_peer_syn_rxmit_thres = Number of retransmitted SYNs before TCP disables rfc1323 and rfc1644 during the rest of attempts
    • net.inet.tcp.pmtud_blackhole_detection = Path MTU Discovery Black Hole Detection
    • net.inet.tcp.pmtud_blackhole_mss = Path MTU Discovery Black Hole Detection lowered MSS

    I believe by default 8 keepalives will be sent before the connection is closed if SO_KEEPALIVE is set. Times are in milliseconds.

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  2. For linux you can use:

    $ sysctl -a | grep net.ipv4
    • 6

  3. I'm not sure if this is what you're looking for, but you can check the keep-alive value with:

    $ netstat -o
    • 3

  4. Maybe unrelated, but works for me:

    I'm trying to determine the ssh/tcp interactive timeout to one of our servers, so i just:

    date; ssh host.domain 'sleep 10000'; date

    example output:

    Thu May 24 12:22:39 CEST 2018
packet_write_wait: Connection to 172.29.1.27 port 22: Broken pipe
Thu May 24 14:22:40 CEST 2018
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