7 Inet configuration
7.1 Introduction
This chapter tells you how the Erlang runtime system is configured
for IP communication. It also explains how you may configure it
for your own particular needs by means of a configuration file.
The information here is mainly intended for users with special
configuration needs or problems. There should normally be no need
for specific settings for Erlang to function properly on a correctly
IP configured platform.
When Erlang starts up it will read the kernel variable
inetrc which, if defined, should specify the location and
name of a user configuration file. Example:
% erl -kernel inetrc '"./cfg_files/erl_inetrc"'
Note that the usage of a .inetrc file, which was
supported in earlier Erlang versions, is now obsolete.
A second way to specify the configuration file is to set the
environment variable ERL_INETRC to the full name of the file. Example (bash):
% export ERL_INETRC=./cfg_files/erl_inetrc
Note that the kernel variable inetrc overrides this environment variable.
If no user configuration file is specified and Erlang is started
in non-distributed or short name distributed mode, Erlang will use
default configuration settings and a native lookup method that should
work correctly under most circumstances. Erlang
will not read any information from system inet configuration files
(like /etc/host.conf, /etc/nsswitch.conf, etc) in these modes,
except for /etc/resolv.conf and /etc/hosts that is read and monitored
for changes on Unix platforms for the internal DNS client
inet_res.
If Erlang is started in long name distributed mode, it needs to
get the domain name from somewhere and will read system inet
configuration files for this information. Any hosts and resolver
information found then is also recorded, but not
used as long as Erlang is configured for native lookups. (The
information becomes useful if the lookup method is changed to
'file' or 'dns', see below).
Native lookup (system calls) is always the default resolver method. This
is true for all platforms except VxWorks and OSE Delta where 'file'
or 'dns' is used (in that order of priority).
On Windows platforms, Erlang will search the system registry rather than
look for configuration files when started in long name distributed mode.
7.2 Configuration Data
Erlang records the following data in a local database if found in system
inet configuration files (or system registry):
-
Host names and addresses
-
Domain name
-
Nameservers
-
Search domains
-
Lookup method
This data may also be specified explicitly in the user
configuration file. The configuration file should contain lines
of configuration parameters (each terminated with a full
stop). Some parameters add data to the configuration (e.g. host
and nameserver), others overwrite any previous settings
(e.g. domain and lookup). The user configuration file is always
examined last in the configuration process, making it possible
for the user to override any default values or previously made
settings. Call inet:get_rc() to view the state of the
inet configuration database.
These are the valid configuration parameters:
-
{file, Format, File}.
-
Format = atom()
File = string()
Specify a system file that Erlang should read configuration
data from. Format tells the parser how the file should be
interpreted: resolv (Unix resolv.conf), host_conf_freebsd
(FreeBSD host.conf), host_conf_bsdos (BSDOS host.conf),
host_conf_linux (Linux host.conf), nsswitch_conf
(Unix nsswitch.conf) or hosts (Unix hosts). File should
specify the name of the file with full path.
-
{resolv_conf, File}.
-
File = string()
Specify a system file that Erlang should read resolver
configuration from for the internal DNS client
inet_res,
and monitor for changes, even if it does not exist.
The path must be absolute.
This may override the configuration parameters
nameserver and
search depending on the contents
of the specified file. They may also change any time in the future
reflecting the file contents.
If the file is specified as an empty string "",
no file is read nor monitored in the future. This emulates
the old behaviour of not configuring the DNS client when
the node is started in short name distributed mode.
If this parameter is not specified it defaults to
/etc/resolv.conf unless the environment variable
ERL_INET_ETC_DIR is set which defines
the directory for this file to some maybe other than
/etc.
-
{hosts_file, File}.
-
File = string()
Specify a system file that Erlang should read resolver
configuration from for the internal hosts file resolver
and monitor for changes, even if it does not exist.
The path must be absolute.
These host entries are searched after all added with
{file, hosts, File} above or
{host, IP, Aliases} below when the lookup option
file is used.
If the file is specified as an empty string "",
no file is read nor monitored in the future. This emulates
the old behaviour of not configuring the DNS client when
the node is started in short name distributed mode.
If this parameter is not specified it defaults to
/etc/hosts unless the environment variable
ERL_INET_ETC_DIR is set which defines
the directory for this file to some maybe other than
/etc.
-
{registry, Type}.
-
Type = atom()
Specify a system registry that Erlang should read configuration
data from. Currently, win32 is the only valid option.
-
{host, IP, Aliases}.
-
IP = tuple()
Aliases = [string()]
Add host entry to the hosts table.
-
{domain, Domain}.
-
Domain = string()
Set domain name.
-
{nameserver, IP [,Port]}.
-
IP = tuple()
Port = integer()
Add address (and port, if other than default) of primary
nameserver to use for
inet_res.
-
{alt_nameserver, IP [,Port]}.
-
IP = tuple()
Port = integer()
Add address (and port, if other than default) of secondary
nameserver for
inet_res.
-
{search, Domains}.
-
Domains = [string()]
Add search domains for
inet_res.
-
{lookup, Methods}.
-
Methods = [atom()]
Specify lookup methods and in which order to try them.
The valid methods are: native (use system calls),
file (use host data retrieved from
system configuration files and/or
the user configuration file) or dns
(use the Erlang DNS client
inet_res
for nameserver queries).
-
{cache_size, Size}.
-
Size = integer()
Set size of resolver cache. Default is 100 DNS records.
-
{cache_refresh, Time}.
-
Time = integer()
Set how often (in millisec)
the resolver cache for
inet_res.
is refreshed (i.e. expired DNS records are deleted).
Default is 1 h.
-
{timeout, Time}.
-
Time = integer()
Set the time to wait until retry (in millesec) for DNS queries
made by
inet_res.
Default is 2 sec.
-
{retry, N}.
-
N = integer()
Set the number of DNS queries
inet_res
will try before giving up.
Default is 3.
-
{inet6, Bool}.
-
Bool = true | false
Tells the DNS client
inet_res
to look up IPv6 addresses. Default is false.
-
{usevc, Bool}.
-
Bool = true | false
Tells the DNS client
inet_res
to use TCP (Virtual Circuit) instead of UDP. Default is false.
-
{edns, Version}.
-
Version = false | 0
Sets the EDNS version that
inet_res
will use. The only allowed is zero. Default is false
which means to not use EDNS.
-
{udp_payload_size, Size}.
-
N = integer()
Sets the allowed UDP payload size
inet_res
will advertise in EDNS queries. Also sets the limit
when the DNS query will be deemed too large for UDP
forcing a TCP query instead, which is not entirely
correct since the advertised UDP payload size of the
individual nameserver is what should be used,
but this simple strategy will do until a more intelligent
(probing, caching) algorithm need be implemented.
The default is 1280 which stems from the
standard Ethernet MTU size.
-
{udp, Module}.
-
Module = atom()
Tell Erlang to use other primitive UDP module than inet_udp.
-
{tcp, Module}.
-
Module = atom()
Tell Erlang to use other primitive TCP module than inet_tcp.
-
clear_hosts.
-
Clear the hosts table.
-
clear_ns.
-
Clear the list of recorded nameservers (primary and secondary).
-
clear_search.
-
Clear the list of search domains.
7.3 User Configuration Example
Here follows a user configuration example.
Assume a user does not want Erlang to use the native lookup method,
but wants Erlang to read all information necessary from start and use
that for resolving names and addresses. In case lookup fails, Erlang
should request the data from a nameserver (using the Erlang
DNS client, set to use EDNS allowing larger responses).
The resolver configuration will be updated when
its configuration file changes, furthermore, DNS records
should never be cached. The user configuration file
(in this example named erl_inetrc, stored
in directory ./cfg_files) could then look like this
(Unix):
%% -- ERLANG INET CONFIGURATION FILE --
%% read the hosts file
{file, hosts, "/etc/hosts"}.
%% add a particular host
{host, {134,138,177,105}, ["finwe"]}.
%% do not monitor the hosts file
{hosts_file, ""}.
%% read and monitor nameserver config from here
{resolv_conf, "/usr/local/etc/resolv.conf"}.
%% enable EDNS
{edns,0}.
%% disable caching
{cache_size, 0}.
%% specify lookup method
{lookup, [file, dns]}.
And Erlang could, for example, be started like this:
% erl -sname my_node -kernel inetrc '"./cfg_files/erl_inetrc"'
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