475 lines
18 KiB
TeX
475 lines
18 KiB
TeX
\chapter{DNS Client Library}\label{cha:dns}
|
|
%
|
|
\begin{description}
|
|
\item[Used files:] dns.scm
|
|
\item[Name of the package:] dns
|
|
\end{description}
|
|
%
|
|
\section{Overview}
|
|
The \ex{dns} structure contains a library for querying DNS servers.
|
|
The library contains sophisticated replacements for scsh's interface
|
|
to the \ex{gethostbyname} and \ex{gethostbyaddr} and many extensions
|
|
to these functions.
|
|
|
|
The main features of the libraray include:
|
|
\begin{itemize}
|
|
\item Complete implementation of the DNS protocol
|
|
\item Concurrent contacting of multiple DNS servers without blocking
|
|
the scsh process
|
|
\item Internal caching of DNS responses
|
|
\item Parsing of \texttt{resolv.conf}, including \texttt{search}
|
|
entries to generate FQDNs from unqualified host names
|
|
\item Rich condition hierarchie
|
|
\end{itemize}
|
|
|
|
\section{Conditions}
|
|
|
|
The library defines a set of conditions raised by the procedures of
|
|
the library. The supertype of these conditions is \exi{dns-error}.
|
|
\defun{dns-error?}{thing}{\boolean}
|
|
\begin{desc}
|
|
The predicate for \ex{dns-error} conditions.
|
|
\end{desc}
|
|
\defun{dns-error->string} {dns-error-condition} {\str}
|
|
\begin{desc}
|
|
Returns a string with the description of the condition.
|
|
\end{desc}
|
|
|
|
\defvar{parse-error}{condition}
|
|
\defvarx{unexpected-eof-from-server}{condition}
|
|
\defvarx{bad-address}{condition}
|
|
\defvarx{no-nameservers}{condition}
|
|
\defvarx{bad-nameserver}{condition}
|
|
\defvarx{not-a-hostname}{condition}
|
|
\defvarx{not-a-ip} {condition}
|
|
|
|
\begin{desc}
|
|
|
|
\end{desc}
|
|
\defvar {dns-format-error} {condition}
|
|
\defvarx {dns-server-failure} {condition}
|
|
\defvarx {dns-name-error} {condition}
|
|
\defvarx {dns-not-implemented} {condition}
|
|
\defvarx {dns-refused} {condition}
|
|
\begin{desc}
|
|
These conditons correspond to errors returned by the DNS server.
|
|
They are all subtypes of the \exi{dns-server-error} condition which
|
|
in turn is a subtype of \ex{dns-error}.
|
|
\end{desc}
|
|
\defun{dns-server-error?}{thing}{\boolean}
|
|
\begin{desc}
|
|
The predicate for \ex{dns-server-error} conditions.
|
|
\end{desc}
|
|
|
|
\defun{parse-error?}{thing} {\boolean}
|
|
\defunx{unexpected-eof-from-server?}{thing} {\boolean}
|
|
\defunx{bad-address?}{thing} {\boolean}
|
|
\defunx{no-nameservers?}{thing} {\boolean}
|
|
\defunx{bad-nameserver?}{thing} {\boolean}
|
|
\defunx{not-a-hostname?}{thing} {\boolean}
|
|
\defunx{not-a-ip?}{thing} {\boolean}
|
|
\defunx{dns-format-error?} {thing} {\boolean}
|
|
\defunx{dns-server-failure?} {thing} {\boolean}
|
|
\defunx{dns-name-error?} {thing} {\boolean}
|
|
\defunx{dns-not-implemented?} {thing} {\boolean}
|
|
\defunx{dns-refused?} {thing} {\boolean}
|
|
\begin{desc}
|
|
The type predicates for the conditions above.
|
|
\end{desc}
|
|
|
|
\section{High-level Interface}
|
|
\def\ipaddr{IP-address\xspace}
|
|
\def\ipstring{IP-string\xspace}
|
|
\def\fqdn{FQDN\xspace}
|
|
|
|
The library uses an internal store to cache data obtained from DNS
|
|
servers. All procedures take a boolean flag \var{use-cache?} that
|
|
indicates whether the cache should be used or not. \var{use-cache?}
|
|
defaults to true.
|
|
|
|
\defun{dns-clear-cache!}{}{\undefined}
|
|
\begin{desc}
|
|
This procedure erases all information stored in the internal cache.
|
|
\end{desc}
|
|
|
|
The library is further capable of parsing the contents of
|
|
\texttt{/etc/resolv.conf} (see Section~\ref{sec:dns-rc}). The
|
|
nameservers listed there are the default value for the optional
|
|
argument \var{nameserver list} which many procedures of the library
|
|
accept. \var{Nameserver} is either a \ipaddr or a dotted IP string.
|
|
|
|
\defun{dns-lookup-name}{\fqdn [nameserver list][use-cache?]}{\ipaddr}
|
|
\begin{desc}
|
|
Given the FQDN of a host, \ex{dns-lookup-ip} returns the IP address.
|
|
The optional argument specifes the name servers to query, it defaults
|
|
to the ones found in \texttt{/etc/resolv.conf}.
|
|
\end{desc}
|
|
|
|
\defun{dns-lookup-ip}{\ipstring/\ipaddr [nameserver list][use-cache?]}{\fqdn}
|
|
\begin{desc}
|
|
Looks up the FQDN for the given IP address. The optional argument
|
|
specifes the name servers to query, it defaults to the ones found in
|
|
\texttt{/etc/resolv.conf}.
|
|
\end{desc}
|
|
|
|
\defun{dns-lookup-nameserver}{\ipstring/\ipaddr [nameserver list][use-cache?]}{\ipaddr list}
|
|
\begin{desc}
|
|
Looks up an authoritative name server for a hostname, returns a list
|
|
of name servers.
|
|
\end{desc}
|
|
|
|
\defun{dns-lookup-mail-exchanger}{\ipstring/\ipaddr [nameserver list][use-cache?]}{\fqdn list}
|
|
\begin{desc}
|
|
Looks up mail-exchangers for a hostname und returns them in a list
|
|
sorted by preference.
|
|
\end{desc}
|
|
\defun{socket-address->fqdn}{socket-address [nameserver list][use-cache?]}{\fqdn}
|
|
\begin{desc}
|
|
Returns the FQDN for of the address bound to argument. The argument
|
|
\var{cache?} indicates whether the internal cache may be queried to
|
|
obtain the information.
|
|
\end{desc}
|
|
|
|
\defun{maybe-dns-lookup-name}{\fqdn [nameserver list][use-cache?]}{\ipaddr or \sharpf}
|
|
\defunx{maybe-dns-lookup-ip}{\ipstring/\ipaddr [nameserver list][use-cache?]}{\fqdn{} or \sharpf}
|
|
\begin{desc}
|
|
These procedures provide the same functionality as
|
|
\ex{dns-lookup-name} and \ex{dns-lookup-ip} but return \sharpf{} in
|
|
case of an \ex{dns-error}.
|
|
\end{desc}
|
|
|
|
\defun{host-fqdn} {name/socket-address [nameserver list][use-cache?]}{\fqdn}
|
|
\defunx{system-fqdn}{[nameserver list][use-cache?]}{\fqdn}
|
|
\begin{desc}
|
|
\ex{host-fqdn} returns the fully qualified domain name (FQDN) for
|
|
its argument which can be either a unqualified host name or a socket
|
|
address. The procedure \ex{system-fqdn} returns the FQDN of the
|
|
local host. These procedures use a list of domain names obtained
|
|
from \texttt{/etc/resolv.conf} to the generate FQDNs and try to
|
|
resolve these FQDNs.
|
|
\end{desc}
|
|
|
|
\section{Low-level Interface}
|
|
|
|
This section describes a set of data structures and procedures which
|
|
directly correspond to the data flow of the DNS protocol. The central
|
|
entity is a \var{message}, the abstraction of the packet sent to the
|
|
server or received from the server (The DNS protocol uses the same
|
|
data format for both directions). A \var{dns-message} encapsulates the
|
|
query message sent to the server, the response message received from
|
|
the server, and some additional information the library gathered while
|
|
generating the \var{dns-message}.
|
|
|
|
\defunx{dns-get-information}{message protocol answer-okay? [nameserver
|
|
list][use-cache?]}{dns-message}
|
|
\begin{desc}
|
|
Most general way to submit a DNS query. The message is sent to the
|
|
name servers via \var{protocol} which can be either
|
|
\ex{(network-procotcol tcp)} or {(network-protocol udp)}, both
|
|
members of of the enumerated type \ex{network-protocol}. After
|
|
receiving the reply, \ex{dns-get-information} applies the predicate
|
|
\var{answer-okay?} to the message. If it returns \sharpf{} and the
|
|
answer is not authoritative additional name servers sent with the
|
|
reply are checked until an authoritative answer is found. If the
|
|
predicate returns \sharpf{} but the answer is authoritative a
|
|
\var{bad-address} condition is signalled.
|
|
\end{desc}
|
|
|
|
\dfn{network-protocol}{protocol-name}{network-protocol}{syntax}
|
|
\defunx{network-protocol?}{thing}{\boolean}
|
|
\begin{desc}
|
|
Constructor and predicate for the enumerated type
|
|
\ex{network-protocol} with the possible protocol names \ex{tcp} and
|
|
\ex{udp}.
|
|
\end{desc}
|
|
\defun{dns-lookup}{\ipstring/\ipaddr type [nameserver list][use-cache?]}{dns-message}
|
|
\begin{desc}
|
|
Convenient shortcut to submit a DNS query. The return value
|
|
is a \ex{dns-message} structure:
|
|
\end{desc}
|
|
|
|
\defun{dns-message?}{thing}{\boolean}
|
|
\defunx{dns-message-query}{dns-message}{message}
|
|
\defunx{dns-message-reply}{dns-message}{message}
|
|
\defunx{dns-message-cache?}{dns-message}{\boolean}
|
|
\defunx{dns-message-protocol}{dns-message}{protocol}
|
|
\defunx{dns-message-tried-nameservers}{dns-message}{}
|
|
\begin{desc}
|
|
A \var{dns-message} records the query sent to the server and the
|
|
reply from the server. It also contains information whether the
|
|
library took the reply from the cache, which protocol was used and
|
|
to which nameservers the query was sent.
|
|
\end{desc}
|
|
|
|
\defun{pretty-print-dns-message}{dns-message [output-port]}{\undefined}
|
|
\begin{desc}
|
|
Pretty prints a DNS message to \var{out-port} which defaults to the
|
|
current output port.
|
|
\end{desc}
|
|
|
|
\defun{message?}{thing}{\boolean}{}
|
|
\defunx{message-header}{message}{header}
|
|
\defunx{message-questions}{message}{question list}
|
|
\defunx{message-answers}{message}{resource-record list}
|
|
\defunx{message-nameservers}{message}{resource-record list}
|
|
\defunx{message-additionals}{message}{resource-record list}
|
|
\defunx{message-source}{message}{char list}
|
|
\begin{desc}
|
|
A \ex{message} represents the data sent to the DNS server or
|
|
received from the DNS server. The DNS protocol uses the same message
|
|
format for queries and replies. In queries only the header and the
|
|
questions is present, a reply may contain answers, name servers and
|
|
and additional informations as resource records. \ex{Message-source}
|
|
returns the actual data sent over the network.
|
|
\end{desc}
|
|
|
|
\defun{make-query-message header}{header question [questions]}{message}
|
|
\begin{desc}
|
|
The procedure generates a message the supplied questions,
|
|
\var{header}, and the standard message values for queries.
|
|
\end{desc}
|
|
\defun{make-simple-query-message}{name type class}{message}
|
|
\begin{desc}
|
|
This simplified constructor generates a message with one question
|
|
which is built from the parameters, and the standard header flags
|
|
for queries and the standard message values for queries.
|
|
\end{desc}
|
|
|
|
\defun{header?}{thing}{\boolean}
|
|
\defunx{header-id}{header}{number}
|
|
\defunx{header-flags}{header}{flags}
|
|
\defunx{header-question-count}{header}{number}
|
|
\defunx{header-answer-count}{header}{number}
|
|
\defunx{header-nameserver-count}{header}{number}
|
|
\defunx{header-additional-count}{header}{number}
|
|
\begin{desc}
|
|
Every DNS message contains a header which stores information about
|
|
the data present in the message and contains flags for the query.
|
|
\end{desc}
|
|
|
|
\defun{flags?}{thing}{\boolean}
|
|
\defunx{flags-query-type}{flags}{'query or 'response}
|
|
\defunx{flags-opcode}{flags}{number}
|
|
\defunx{flags-authoritative?}{flags}{\boolean}
|
|
\defunx{flags-truncated?}{flags}{\boolean}
|
|
\defunx{flags-recursion-desired?}{flags}{\boolean}
|
|
\defunx{flags-recursion-available?}{flags}{\boolean}
|
|
\defunx{flags-z}{flags}{0}
|
|
\defunx{flags-response-code}{flags}{number}
|
|
\begin{desc}
|
|
Flags occur within the header of a DNS message. The boolean value
|
|
returned from \ex{flags-authoritative} indicates whether the message
|
|
was sent from a authoritative server, \ex{flags-truncated?} should
|
|
always be \sharpf as the library automatically uses the TCP protocol
|
|
is the UDP message size is not sufficied.
|
|
\end{desc}
|
|
|
|
\defun{question?}{thing}{\boolean}
|
|
\defunx{question-name}{question}{\str}
|
|
\defunx{question-type}{question}{message-type}
|
|
\defunx{question-class}{question}{message-class}
|
|
\begin{desc}
|
|
A question sent to the DNS server.
|
|
\end{desc}
|
|
The type and class of the question and answer are elements of
|
|
enumerated types:
|
|
\dfn{message-class}{class-name}{message-class}{syntax}
|
|
\defunx{message-class?}{thing}{\boolean}
|
|
\defunx{message-class-name}{message-class}{symbol}
|
|
\defunx{message-class-number}{message-class}{number}
|
|
\begin{desc}
|
|
\ex{message-class} constructs a member of the enumerated type,
|
|
\ex{message-class?} is the type predicate, \ex{message-class-name}
|
|
returns the symbol and \ex{message-class-number} the number used for
|
|
the class in the DNS protocol.
|
|
\end{desc}
|
|
The possible names for the classes are:
|
|
\begin{description}
|
|
\item[\ex{in}] The Internet
|
|
\item[\ex{cs}] obsolete
|
|
\item[\ex{ch}] the CHAOS class
|
|
\item[\ex{hs}] Hesoid
|
|
\end{description}
|
|
|
|
\dfn{message-type}{type-name}{message-type}{syntax}
|
|
\defunx{message-type?}{thing}{\boolean}
|
|
\defunx{message-type-name}{message-type}{symbol}
|
|
\defunx{message-type-index}{message-type}{number}
|
|
\begin{desc}
|
|
\ex{message-type} constructs a member of the enumeration from name
|
|
\synvar{type-name} listed in Table~\ref{tab:message-types}.
|
|
\ex{message-type?} is the type predicate, \ex{message-type-name}
|
|
returns the name, and \ex{message-type-number} the number used for
|
|
the class the DNS protocol.
|
|
|
|
\end{desc}
|
|
\begin{table}[htb]
|
|
\centering
|
|
\begin{tabular}{|l|l|}
|
|
\hline
|
|
\ex{a}& a host address\\\hline
|
|
\ex{ns}&an authoritative name server\\\hline
|
|
\ex{md}&(obsolete)\\\hline
|
|
\ex{mf}&(obsolete)\\\hline
|
|
\ex{cname}&the canonical name for an alias\\\hline
|
|
\ex{soa}& marks the start of a zone of authority\\\hline
|
|
\ex{mb}&(experimental)\\\hline
|
|
\ex{mg}&(experimental)\\\hline
|
|
\ex{mr}&(experimental)\\\hline
|
|
\ex{null}& (experimental)\\\hline
|
|
\ex{wks}& a well known service description\\\hline
|
|
\ex{ptr}& a domain name pointer\\\hline
|
|
\ex{hinfo}& host information\\\hline
|
|
\ex{minfo}& (experimental)\\\hline
|
|
\ex{mx}& mail exchange\\\hline
|
|
\ex{txt}& text strings\\\hline
|
|
\end{tabular}
|
|
\caption{Message types}
|
|
\label{tab:message-types}
|
|
\end{table}
|
|
|
|
\defun{resource-record?}{thing}{\boolean}
|
|
\defunx{resource-record-name}{resource-record}{\str}
|
|
\defunx{resource-record-type}{resource-record}{message-type}
|
|
\defunx{resource-record-class}{resource-record}{message-class}
|
|
\defunx{resource-record-ttl}{resource-record}{number}
|
|
\defunx{resource-record-data}{resource-record}{resource-record-data-\dots}
|
|
\begin{desc}
|
|
A resource record as returned from the DNS server. The actual data
|
|
of the record is stored in the \texttt{resource-record-data} field.
|
|
It is one of the record types for resource record data described
|
|
below.
|
|
\end{desc}
|
|
|
|
\defun{resource-record-data-a?}{thing}{\boolean}
|
|
\defunx{resource-record-data-a-ip}{resource-record-data-a}{\ipaddr}
|
|
\begin{desc}
|
|
An address resource record which holds an internet address.
|
|
\end{desc}
|
|
|
|
\defun{resource-record-data-ns?}{thing}{\boolean}
|
|
\defunx{resource-record-data-ns-name}{resource-record-data-ns}{\fqdn}
|
|
\begin{desc}
|
|
A name server resource record containing the FQDN of the name server.
|
|
\end{desc}
|
|
|
|
\defun{resource-record-data-cname?}{thing}{\boolean}
|
|
\defunx{resource-record-data-cname}{resource-record-data-cname}{\fqdn}
|
|
\begin{desc}
|
|
A canonical name resource record which contains the canonical or
|
|
primary name of the owner.
|
|
\end{desc}
|
|
|
|
\defun{resource-record-data-mx?}{thing}{\boolean}
|
|
\defunx{resource-record-data-mx-preference}{resource-record-data-mx}{number}
|
|
\defunx{resource-record-data-mx-exchanger}{resource-record-data-mx}{\fqdn}
|
|
\begin{desc}
|
|
A mail exchange resource record with the preference and the FQDN of
|
|
a host willing to act as a mail exchange.
|
|
\end{desc}
|
|
|
|
\defun{resource-record-data-ptr?}{thing}{\boolean}
|
|
\defunx{resource-record-data-ptr-name}{resource-record-data-ptr}{\str}
|
|
\begin{desc}
|
|
A pointer resource record which points to some other domain name.
|
|
\end{desc}
|
|
|
|
\defun{resource-record-data-soa?}{thing}{\boolean}
|
|
\defunx{resource-record-data-soa-mname}{resource-record-data-soa}{\fqdn}
|
|
\defunx{resource-record-data-soa-rname}{resource-record-data-soa}{\fqdn}
|
|
\defunx{resource-record-data-soa-serial}{resource-record-data-soa}{number}
|
|
\defunx{resource-record-data-soa-refresh}{resource-record-data-soa}{number}
|
|
\defunx{resource-record-data-soa-retry}{resource-record-data-soa}{number}
|
|
\defunx{resource-record-data-soa-expire}{resource-record-data-soa}{number}
|
|
\defunx{resource-record-data-soa-minimum}{resource-record-data-soa}{number}
|
|
\begin{desc}
|
|
A start of a zone of authority resource record.
|
|
\end{desc}
|
|
The protocol specifies other possible values for the \texttt{resource-record-data}
|
|
field but we where no able to find test cases for them.
|
|
|
|
|
|
\defun{cache?}{thing}{\boolean}
|
|
\defunx{cache-answer}{cache}{dns-message}
|
|
\defunx{cache-ttl}{cache}{number}
|
|
\defunx{cache-time}{cache}{number}
|
|
\begin{desc}
|
|
A cache data structure corresponds to a saved answer to a previous
|
|
query. \ex{cache-answer} returns the saved message, \ex{cache-ttl}
|
|
returns the time when the cache entry expires and \ex{cache-time}
|
|
returns the time the entry was created.
|
|
\end{desc}
|
|
|
|
\section{Parsing \texttt{/etc/resolv.conf}}
|
|
\label{sec:dns-rc}
|
|
|
|
\defvar{resolv.conf-parse-error} {condition}
|
|
\defun{resolv.conf-parse-error?}{thing}{\boolean}
|
|
\begin{desc}
|
|
The code signals the condition \var{resolv.conf-parse-error} if a
|
|
parse error occurs while scanning \texttt{/etc/resolv.conf}. It is a
|
|
subtype of the \var{dns-error} condition.
|
|
\ex{resolv.conf-parse-error?} is the type predicate for this
|
|
condition.
|
|
\end{desc}
|
|
|
|
\defun{resolv.conf}{}{{symbol$\rightarrow$string} alist}
|
|
\begin{desc}
|
|
Returns the contents of \texttt{/etc/resolv.conv} as an alist with
|
|
the possible keys \texttt{nameserver}, \texttt{domain},
|
|
\texttt{search}, \texttt{sortlist} and \texttt{options}.
|
|
|
|
Note that the library caches the contents of
|
|
\texttt{/etc/resolv.conv} and \ex{resolv.conf} only really opens the
|
|
file if its modification time is more recent than the modification
|
|
time of the cache.
|
|
\end{desc}
|
|
\defun{parse-resolv.conf!}{}{\undefined}
|
|
\begin{desc}
|
|
Parses the contents of \texttt{/etc/resolv.conv} and updates the
|
|
internal cache of the library.
|
|
\end{desc}
|
|
\defun{dns-find-nameserver-list}{}{\fqdn list}
|
|
\begin{desc}
|
|
Returns a list of name servers from \texttt{/etc/resolv.conf}
|
|
\end{desc}
|
|
\defun{dns-find-nameserver}{}{\fqdn}
|
|
\begin{desc}
|
|
Returns the first name servers found in \texttt{/etc/resolv.conf}.
|
|
\ex{dns-find-nameserver} raises \ex{no-nameservers} if
|
|
\texttt{/etc/resolv.conf} does not contain a \texttt{nameserver}
|
|
entry.
|
|
\end{desc}
|
|
\defun{domains-for-search}{}{\str{} list}
|
|
\begin{desc}
|
|
Parses \texttt{/etc/resolv.conf} and extracts the domains specified
|
|
by the \texttt{search} keyword.
|
|
\end{desc}
|
|
|
|
|
|
\section{IP Addresses as Dotted Strings}
|
|
%
|
|
\begin{description}
|
|
\item[Used files:] ip.scm
|
|
\item[Name of the package:] ips
|
|
\end{description}
|
|
%
|
|
|
|
The structure \ex{ips} provides a small set of procedures for turning
|
|
the human-readable form of IP addresses (``dotted strings'') into 32
|
|
bits numbers.
|
|
|
|
\defun{address32->ip-string}{\ipaddr}{ip-string}
|
|
|
|
\defun{ip-string->address32}{ip-string}{\ipaddr}
|
|
|
|
\defun{ip-string?}{string}{\boolean}
|
|
\begin{desc}
|
|
Tests whether \var{string} is a valid dotted string for an IP
|
|
address.
|
|
\end{desc}
|
|
%%% Local Variables:
|
|
%%% mode: latex
|
|
%%% TeX-master: "man"
|
|
%%% End:
|