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>Using TCP keepalive under Linux</TITLE
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><DIV
CLASS="sect1"
><H1
CLASS="sect1"
><A
NAME="usingkeepalive"
></A
>3. Using TCP keepalive under Linux</H1
><P
>&#13; Linux has built-in support for keepalive. You need to enable TCP/IP
networking in order to use it. You also need <TT
CLASS="literal"
>procfs</TT
>
support and <TT
CLASS="literal"
>sysctl</TT
> support to be able to configure the
kernel parameters at runtime.
</P
><P
>&#13; The procedures involving keepalive use three user-driven variables:
<P
></P
><DIV
CLASS="variablelist"
><DL
><DT
><TT
CLASS="varname"
>tcp_keepalive_time</TT
></DT
><DD
><P
>&#13; the interval between the last data packet sent (simple ACKs are not
considered data) and the first keepalive probe; after the connection
is marked to need keepalive, this counter is not used any further
</P
></DD
><DT
><TT
CLASS="varname"
>tcp_keepalive_intvl</TT
></DT
><DD
><P
>&#13; the interval between subsequential keepalive probes, regardless of
what the connection has exchanged in the meantime
</P
></DD
><DT
><TT
CLASS="varname"
>tcp_keepalive_probes</TT
></DT
><DD
><P
>&#13; the number of unacknowledged probes to send before considering the
connection dead and notifying the application layer
</P
></DD
></DL
></DIV
>
</P
><P
>&#13; Remember that keepalive support, even if configured in the kernel, is not
the default behavior in Linux. Programs must request keepalive control for
their sockets using the <TT
CLASS="literal"
>setsockopt</TT
> interface. There are
relatively few programs implementing keepalive, but you can easily add
keepalive support for most of them following the instructions explained
later in this document.
</P
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="configuringkernel"
></A
>3.1. Configuring the kernel</H2
><P
>&#13; There are two ways to configure keepalive parameters inside the kernel via
userspace commands:
<P
></P
><UL
><LI
><P
><TT
CLASS="literal"
>procfs</TT
> interface</P
></LI
><LI
><P
><TT
CLASS="literal"
>sysctl</TT
> interface</P
></LI
></UL
>
</P
><P
>&#13; We mainly discuss how this is accomplished on the procfs interface because
it's the most used, recommended and the easiest to understand. The sysctl
interface, particularly regarding the <SPAN
CLASS="citerefentry"
><SPAN
CLASS="refentrytitle"
>&#13; <TT
CLASS="function"
>sysctl</TT
></SPAN
>(2)</SPAN
> syscall and not the <SPAN
CLASS="citerefentry"
><SPAN
CLASS="refentrytitle"
><B
CLASS="command"
>&#13; sysctl</B
></SPAN
>(8)</SPAN
>
tool, is only here for the purpose of background knowledge.
</P
><DIV
CLASS="sect3"
><H3
CLASS="sect3"
><A
NAME="procfsinterface"
></A
>3.1.1. The <TT
CLASS="literal"
>procfs</TT
> interface</H3
><P
>&#13; This interface requires both <TT
CLASS="literal"
>sysctl</TT
> and <TT
CLASS="literal"
>&#13; procfs</TT
> to be built into the kernel, and <TT
CLASS="literal"
>procfs
</TT
> mounted somewhere in the filesystem (usually on <TT
CLASS="filename"
>&#13; /proc</TT
>, as in the examples below). You can read the values for
the actual parameters by <SPAN
CLASS="QUOTE"
>"catting"</SPAN
> files in <TT
CLASS="filename"
>&#13; /proc/sys/net/ipv4/</TT
> directory:
<DIV
CLASS="informalexample"
><A
NAME="AEN133"
></A
><P
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>&#13; <TT
CLASS="prompt"
># </TT
><TT
CLASS="userinput"
><B
>cat /proc/sys/net/ipv4/tcp_keepalive_time</B
></TT
>
<TT
CLASS="computeroutput"
>7200</TT
>
<TT
CLASS="prompt"
># </TT
><TT
CLASS="userinput"
><B
>cat /proc/sys/net/ipv4/tcp_keepalive_intvl</B
></TT
>
<TT
CLASS="computeroutput"
>75</TT
>
<TT
CLASS="prompt"
># </TT
><TT
CLASS="userinput"
><B
>cat /proc/sys/net/ipv4/tcp_keepalive_probes</B
></TT
>
<TT
CLASS="computeroutput"
>9</TT
>
</PRE
></FONT
></TD
></TR
></TABLE
><P
></P
></DIV
>
</P
><P
>&#13; The first two parameters are expressed in seconds, and the last is the
pure number. This means that the keepalive routines wait for two hours
(7200 secs) before sending the first keepalive probe, and then resend it
every 75 seconds. If no ACK response is received for nine consecutive
times, the connection is marked as broken.
</P
><P
>&#13; Modifying this value is straightforward: you need to write new values
into the files. Suppose you decide to configure the host so that
keepalive starts after ten minutes of channel inactivity, and then send
probes in intervals of one minute. Because of the high instability of
our network trunk and the low value of the interval, suppose you also
want to increase the number of probes to 20.
</P
><P
>&#13; Here's how we would change the settings:
<DIV
CLASS="informalexample"
><A
NAME="AEN147"
></A
><P
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>&#13; <TT
CLASS="prompt"
># </TT
><TT
CLASS="userinput"
><B
>echo 600 &#62; /proc/sys/net/ipv4/tcp_keepalive_time</B
></TT
>
<TT
CLASS="prompt"
># </TT
><TT
CLASS="userinput"
><B
>echo 60 &#62; /proc/sys/net/ipv4/tcp_keepalive_intvl</B
></TT
>
<TT
CLASS="prompt"
># </TT
><TT
CLASS="userinput"
><B
>echo 20 &#62; /proc/sys/net/ipv4/tcp_keepalive_probes</B
></TT
>
</PRE
></FONT
></TD
></TR
></TABLE
><P
></P
></DIV
>
</P
><P
>&#13; To be sure that all succeeds, recheck the files and confirm these new
values are showing in place of the old ones.
</P
><P
>&#13; Remember that <TT
CLASS="literal"
>procfs</TT
> handles special files, and you
cannot perform any sort of operation on them because they're just an interface within the kernel space, not real
files, so try your
scripts before using them, and try to use simple access methods as in
the examples shown earlier.
</P
><P
>&#13; You can access the interface through the <SPAN
CLASS="citerefentry"
><SPAN
CLASS="refentrytitle"
>&#13; <B
CLASS="command"
>sysctl</B
></SPAN
>(8)</SPAN
> tool, specifying what you want to read or write.
<DIV
CLASS="informalexample"
><A
NAME="AEN163"
></A
><P
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>&#13; <TT
CLASS="prompt"
># </TT
><TT
CLASS="userinput"
><B
>sysctl \</B
></TT
>
<TT
CLASS="prompt"
>&#62; </TT
><TT
CLASS="userinput"
><B
>net.ipv4.tcp_keepalive_time \</B
></TT
>
<TT
CLASS="prompt"
>&#62; </TT
><TT
CLASS="userinput"
><B
>net.ipv4.tcp_keepalive_intvl \</B
></TT
>
<TT
CLASS="prompt"
>&#62; </TT
><TT
CLASS="userinput"
><B
>net.ipv4.tcp_keepalive_probes</B
></TT
>
<TT
CLASS="computeroutput"
>net.ipv4.tcp_keepalive_time = 7200
net.ipv4.tcp_keepalive_intvl = 75
net.ipv4.tcp_keepalive_probes = 9</TT
>
</PRE
></FONT
></TD
></TR
></TABLE
><P
></P
></DIV
>
</P
><P
>&#13; Note that <TT
CLASS="literal"
>sysctl</TT
> names are very close to <TT
CLASS="literal"
>&#13; procfs</TT
> paths. Write is performed using the <TT
CLASS="option"
>-w</TT
>
switch of <SPAN
CLASS="citerefentry"
><SPAN
CLASS="refentrytitle"
><B
CLASS="command"
>sysctl</B
>
</SPAN
>(8)</SPAN
>:
<DIV
CLASS="informalexample"
><A
NAME="AEN182"
></A
><P
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>&#13; <TT
CLASS="prompt"
># </TT
><TT
CLASS="userinput"
><B
>sysctl -w \</B
></TT
>
<TT
CLASS="prompt"
>&#62; </TT
><TT
CLASS="userinput"
><B
>net.ipv4.tcp_keepalive_time=600 \</B
></TT
>
<TT
CLASS="prompt"
>&#62; </TT
><TT
CLASS="userinput"
><B
>net.ipv4.tcp_keepalive_intvl=60 \</B
></TT
>
<TT
CLASS="prompt"
>&#62; </TT
><TT
CLASS="userinput"
><B
>net.ipv4.tcp_keepalive_probes=20</B
></TT
>
<TT
CLASS="computeroutput"
>net.ipv4.tcp_keepalive_time = 600
net.ipv4.tcp_keepalive_intvl = 60
net.ipv4.tcp_keepalive_probes = 20</TT
>
</PRE
></FONT
></TD
></TR
></TABLE
><P
></P
></DIV
>
</P
><P
>&#13; Note that <SPAN
CLASS="citerefentry"
><SPAN
CLASS="refentrytitle"
><B
CLASS="command"
>sysctl</B
>
</SPAN
>(8)</SPAN
> doesn't use
<SPAN
CLASS="citerefentry"
><SPAN
CLASS="refentrytitle"
><TT
CLASS="function"
>sysctl</TT
></SPAN
>(2)</SPAN
> syscall, but reads and writes
directly in the <TT
CLASS="literal"
>procfs</TT
> subtree, so you will need
<TT
CLASS="literal"
>procfs</TT
> enabled in the kernel and mounted in the
filesystem, just as you would if you directly accessed the files within
the <TT
CLASS="literal"
>procfs</TT
> interface. <SPAN
CLASS="citerefentry"
><SPAN
CLASS="refentrytitle"
>&#13; <B
CLASS="command"
>Sysctl</B
></SPAN
>(8)</SPAN
> is just a different way to do the same thing.
</P
></DIV
><DIV
CLASS="sect3"
><H3
CLASS="sect3"
><A
NAME="sysctlinterface"
></A
>3.1.2. The <TT
CLASS="literal"
>sysctl</TT
> interface</H3
><P
>&#13; There is another way to access kernel variables: <SPAN
CLASS="citerefentry"
><SPAN
CLASS="refentrytitle"
><TT
CLASS="function"
>sysctl</TT
></SPAN
>(2
)</SPAN
> syscall. It can be useful when you don't
have <TT
CLASS="literal"
>procfs</TT
> available because the communication with
the kernel is performed directly via syscall and not through the
<TT
CLASS="literal"
>procfs</TT
> subtree. There is currently no program that
wraps this syscall (remember that <SPAN
CLASS="citerefentry"
><SPAN
CLASS="refentrytitle"
><B
CLASS="command"
>&#13; sysctl</B
></SPAN
>(8)</SPAN
>
doesn't use it).
</P
><P
>&#13; For more details about using <SPAN
CLASS="citerefentry"
><SPAN
CLASS="refentrytitle"
><TT
CLASS="function"
>&#13; sysctl</TT
></SPAN
>(2)</SPAN
>
refer to the manpage.
</P
></DIV
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="makepersistchanges"
></A
>3.2. Making changes persistent to reboot</H2
><P
>&#13; There are several ways to reconfigure your system every time it boots up.
First, remember that every Linux distribution has its own set of init
scripts called by <SPAN
CLASS="citerefentry"
><SPAN
CLASS="refentrytitle"
><B
CLASS="command"
>init</B
>
</SPAN
>(8)</SPAN
>. The most common
configurations include the <TT
CLASS="filename"
>/etc/rc.d/</TT
> directory, or
the alternative, <TT
CLASS="filename"
>/etc/init.d/</TT
>. In any case, you can
set the parameters in any of the startup scripts, because keepalive
rereads the values every time its procedures need them. So if you change
the value of <TT
CLASS="varname"
>tcp_keepalive_intvl</TT
> when the connection is
still up, the kernel will use the new value going forward.
</P
><P
>&#13; There are three spots where the initialization commands should logically
be placed: the first is where your network is configured, the second is
the <TT
CLASS="filename"
>rc.local</TT
> script, usually included in all
distributions, which is known as the place where user configuration setups
are done. The third place may already exist in your system. Referring back
to the <SPAN
CLASS="citerefentry"
><SPAN
CLASS="refentrytitle"
><B
CLASS="command"
>sysctl</B
>
</SPAN
>(8)</SPAN
> tool, you can see
that the <TT
CLASS="option"
>-p</TT
> switch loads settings from the <TT
CLASS="filename"
>&#13; /etc/sysctl.conf</TT
> configuration file. In many cases your init
script already performs the <B
CLASS="command"
>sysctl</B
> <TT
CLASS="option"
>-p</TT
>
(you can <SPAN
CLASS="QUOTE"
>"grep"</SPAN
> it in the configuration directory for
confirmation), and so you just have to add the lines in <TT
CLASS="filename"
>&#13; /etc/sysctl.conf</TT
> to make them load at every boot. For more
information about the syntax of <SPAN
CLASS="citerefentry"
><SPAN
CLASS="refentrytitle"
><TT
CLASS="filename"
>&#13; sysctl.conf</TT
></SPAN
>(5)</SPAN
>, refer to the manpage.
</P
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