- It does not offer any way to dynamically look for the default DNS setup on your machine.
- It does not offer any cache, which is not a very good idea, even though the DNS server is usually "close" to you.
[PT] 26Feb03: I have sucked this into tcllib as resolv package alongside the dns package. Some improvements to the resolution algorithm - like trying a search list of domain suffixes and ensuring that the dns request has completed using [dns::wait $token].
##################
## Module Name -- resolv.tcl
## Original Author -- Emmanuel Frecon - emmanuel@sics.se
## Description:
##
## A super module on top of the dns module for host name resolution.
## There are two services provided on top of the regular Tcl library:
## Firstly, this module attempts to automatically discover the default
## DNS server that is setup on the machine that it is run on. This
## server will be used in all further host resolutions. Secondly, this
## module offers a rudimentary cache. The cache is rudimentary since it
## has no expiration on host name resolutions, but this is probably
## enough for short lived applications.
##
##################
package provide resolv 1.0
package require dns
namespace eval ::resolv {
namespace export resolve init ignore
array set R {
initdone 0
dns ""
dnsdefault ""
}
}
# Command Name -- ignore
# Original Author -- Emmanuel Frecon - emmanuel@sics.se
#
# Remove a host name resolution from the cache, if present, so that the
# next resolution will query the DNS server again.
#
# Arguments:
# hostname - Name of host to remove from the cache.
proc ::resolv::ignore { hostname } {
global ::resolv::Cache
catch "unset ::resolv::Cache($hostname)"
}
# Command Name -- init
# Original Author -- Emmanuel Frecon - emmanuel@sics.se
#
# Initialise this module with a known host name. This host (not mandatory)
# will become the default if the library was not able to find a DNS server.
# This command can be called several times, its effect is double: actively
# looking for the default DNS server setup on the running machine; and
# emptying the host name resolution cache.
#
# Arguments:
# defaultdns - Default DNS server
proc ::resolv::init { { defaultdns "" } } {
global ::resolv::R ::resolv::Cache
catch "unset ::resolv::Cache"
set ::resolv::R(dnsdefault) $defaultdns
set res [catch "exec nslookup 127.0.0.1" lkup]
if { $res == 0 } {
set l [split $lkup]
set nl ""
foreach e $l {
if { [string length $e] > 0 } {
lappend nl $e
}
}
set hostname ""
set len [llength $nl]
for { set i 0 } { $i < $len } { incr i } {
set e [lindex $nl $i]
if { [string match -nocase "*server*" $e] } {
set hostname [lindex $nl [expr $i + 1]]
break
}
}
if { $hostname != "" } {
set ::resolv::R(dns) $hostname
} else {
set ::resolv::R(dns) $::resolv::R(dnsdefault)
}
}
set ::resolv::R(initdone) 1
return $::resolv::R(dns)
}
# Command Name -- resolve
# Original Author -- Emmanuel Frecon - emmanuel@sics.se
#
# Resolve a host name to an IP address. This is a wrapping procedure around
# the basic services of the dns library.
#
# Arguments:
# hostname - Name of host
proc ::resolv::resolve { hostname } {
global ::resolv::R
global ::resolv::Cache
# Initialise if not already done. Auto initialisation cannot take
# any known DNS server (known to the caller)
if { ! $::resolv::R(initdone) } {
::resolv::init
}
# Check whether this is not simply a raw IP address. What about
# IPv6 ??
if { [regexp "\\d+\\.\\d+\\.\\d+\\.\\d+" $hostname] } {
return $hostname
}
# Look for hostname in the cache, if found return.
if { [array names ::resolv::Cache $hostname] != "" } {
return $::resolv::Cache($hostname)
}
# Scream if we don't have any DNS server setup, since we cannot do
# anything in that case.
if { $::resolv::R(dns) == "" } {
error "No dns server provided"
}
# Now resolve in a clean way
set t [::dns::resolve $hostname -server $::resolv::R(dns)]
set ip [lindex [::dns::address $t] 0]
::dns::cleanup $t
# And store the result of resolution in our cache for further use.
set ::resolv::Cache($hostname) $ip
return $ip
}