Wang Lei | 1a4240f | 2010-08-04 15:16:33 +0100 | [diff] [blame] | 1 | =================== |
| 2 | DNS Resolver Module |
| 3 | =================== |
| 4 | |
| 5 | Contents: |
| 6 | |
| 7 | - Overview. |
| 8 | - Compilation. |
| 9 | - Setting up. |
| 10 | - Usage. |
| 11 | - Mechanism. |
| 12 | - Debugging. |
| 13 | |
| 14 | |
| 15 | ======== |
| 16 | OVERVIEW |
| 17 | ======== |
| 18 | |
| 19 | The DNS resolver module provides a way for kernel services to make DNS queries |
| 20 | by way of requesting a key of key type dns_resolver. These queries are |
| 21 | upcalled to userspace through /sbin/request-key. |
| 22 | |
| 23 | These routines must be supported by userspace tools dns.upcall, cifs.upcall and |
| 24 | request-key. It is under development and does not yet provide the full feature |
| 25 | set. The features it does support include: |
| 26 | |
| 27 | (*) Implements the dns_resolver key_type to contact userspace. |
| 28 | |
| 29 | It does not yet support the following AFS features: |
| 30 | |
| 31 | (*) Dns query support for AFSDB resource record. |
| 32 | |
| 33 | This code is extracted from the CIFS filesystem. |
| 34 | |
| 35 | |
| 36 | =========== |
| 37 | COMPILATION |
| 38 | =========== |
| 39 | |
| 40 | The module should be enabled by turning on the kernel configuration options: |
| 41 | |
| 42 | CONFIG_DNS_RESOLVER - tristate "DNS Resolver support" |
| 43 | |
| 44 | |
| 45 | ========== |
| 46 | SETTING UP |
| 47 | ========== |
| 48 | |
| 49 | To set up this facility, the /etc/request-key.conf file must be altered so that |
| 50 | /sbin/request-key can appropriately direct the upcalls. For example, to handle |
| 51 | basic dname to IPv4/IPv6 address resolution, the following line should be |
| 52 | added: |
| 53 | |
| 54 | #OP TYPE DESC CO-INFO PROGRAM ARG1 ARG2 ARG3 ... |
| 55 | #====== ============ ======= ======= ========================== |
| 56 | create dns_resolver * * /usr/sbin/cifs.upcall %k |
| 57 | |
| 58 | To direct a query for query type 'foo', a line of the following should be added |
| 59 | before the more general line given above as the first match is the one taken. |
| 60 | |
| 61 | create dns_resolver foo:* * /usr/sbin/dns.foo %k |
| 62 | |
| 63 | |
Wang Lei | 1a4240f | 2010-08-04 15:16:33 +0100 | [diff] [blame] | 64 | ===== |
| 65 | USAGE |
| 66 | ===== |
| 67 | |
| 68 | To make use of this facility, one of the following functions that are |
| 69 | implemented in the module can be called after doing: |
| 70 | |
| 71 | #include <linux/dns_resolver.h> |
| 72 | |
| 73 | (1) int dns_query(const char *type, const char *name, size_t namelen, |
| 74 | const char *options, char **_result, time_t *_expiry); |
| 75 | |
| 76 | This is the basic access function. It looks for a cached DNS query and if |
| 77 | it doesn't find it, it upcalls to userspace to make a new DNS query, which |
| 78 | may then be cached. The key description is constructed as a string of the |
| 79 | form: |
| 80 | |
| 81 | [<type>:]<name> |
| 82 | |
| 83 | where <type> optionally specifies the particular upcall program to invoke, |
| 84 | and thus the type of query to do, and <name> specifies the string to be |
| 85 | looked up. The default query type is a straight hostname to IP address |
| 86 | set lookup. |
| 87 | |
| 88 | The name parameter is not required to be a NUL-terminated string, and its |
| 89 | length should be given by the namelen argument. |
| 90 | |
| 91 | The options parameter may be NULL or it may be a set of options |
| 92 | appropriate to the query type. |
| 93 | |
| 94 | The return value is a string appropriate to the query type. For instance, |
| 95 | for the default query type it is just a list of comma-separated IPv4 and |
| 96 | IPv6 addresses. The caller must free the result. |
| 97 | |
David Howells | ff9517a | 2010-08-06 03:13:52 +0100 | [diff] [blame] | 98 | The length of the result string is returned on success, and a negative |
| 99 | error code is returned otherwise. -EKEYREJECTED will be returned if the |
| 100 | DNS lookup failed. |
Wang Lei | 1a4240f | 2010-08-04 15:16:33 +0100 | [diff] [blame] | 101 | |
| 102 | If _expiry is non-NULL, the expiry time (TTL) of the result will be |
| 103 | returned also. |
| 104 | |
| 105 | |
David Howells | 1362fa0 | 2011-03-03 11:28:58 +0000 | [diff] [blame] | 106 | =============================== |
| 107 | READING DNS KEYS FROM USERSPACE |
| 108 | =============================== |
| 109 | |
| 110 | Keys of dns_resolver type can be read from userspace using keyctl_read() or |
| 111 | "keyctl read/print/pipe". |
| 112 | |
| 113 | |
Wang Lei | 1a4240f | 2010-08-04 15:16:33 +0100 | [diff] [blame] | 114 | ========= |
| 115 | MECHANISM |
| 116 | ========= |
| 117 | |
| 118 | The dnsresolver module registers a key type called "dns_resolver". Keys of |
| 119 | this type are used to transport and cache DNS lookup results from userspace. |
| 120 | |
| 121 | When dns_query() is invoked, it calls request_key() to search the local |
| 122 | keyrings for a cached DNS result. If that fails to find one, it upcalls to |
| 123 | userspace to get a new result. |
| 124 | |
| 125 | Upcalls to userspace are made through the request_key() upcall vector, and are |
| 126 | directed by means of configuration lines in /etc/request-key.conf that tell |
| 127 | /sbin/request-key what program to run to instantiate the key. |
| 128 | |
| 129 | The upcall handler program is responsible for querying the DNS, processing the |
| 130 | result into a form suitable for passing to the keyctl_instantiate_key() |
| 131 | routine. This then passes the data to dns_resolver_instantiate() which strips |
| 132 | off and processes any options included in the data, and then attaches the |
| 133 | remainder of the string to the key as its payload. |
| 134 | |
| 135 | The upcall handler program should set the expiry time on the key to that of the |
| 136 | lowest TTL of all the records it has extracted a result from. This means that |
| 137 | the key will be discarded and recreated when the data it holds has expired. |
| 138 | |
| 139 | dns_query() returns a copy of the value attached to the key, or an error if |
| 140 | that is indicated instead. |
| 141 | |
| 142 | See <file:Documentation/keys-request-key.txt> for further information about |
| 143 | request-key function. |
| 144 | |
| 145 | |
| 146 | ========= |
| 147 | DEBUGGING |
| 148 | ========= |
| 149 | |
| 150 | Debugging messages can be turned on dynamically by writing a 1 into the |
| 151 | following file: |
| 152 | |
| 153 | /sys/module/dnsresolver/parameters/debug |