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![David Howells](/assets/img/avatar_default.png)
Allow the DNS resolver to retrieve a set of servers and their associated addresses, ports, preference and weight ratings. In terms of communication with userspace, "srv=1" is added to the callout string (the '1' indicating the maximum data version supported by the kernel) to ask the userspace side for this. If the userspace side doesn't recognise it, it will ignore the option and return the usual text address list. If the userspace side does recognise it, it will return some binary data that begins with a zero byte that would cause the string parsers to give an error. The second byte contains the version of the data in the blob (this may be between 1 and the version specified in the callout data). The remainder of the payload is version-specific. In version 1, the payload looks like (note that this is packed): u8 Non-string marker (ie. 0) u8 Content (0 => Server list) u8 Version (ie. 1) u8 Source (eg. DNS_RECORD_FROM_DNS_SRV) u8 Status (eg. DNS_LOOKUP_GOOD) u8 Number of servers foreach-server { u16 Name length (LE) u16 Priority (as per SRV record) (LE) u16 Weight (as per SRV record) (LE) u16 Port (LE) u8 Source (eg. DNS_RECORD_FROM_NSS) u8 Status (eg. DNS_LOOKUP_GOT_NOT_FOUND) u8 Protocol (eg. DNS_SERVER_PROTOCOL_UDP) u8 Number of addresses char[] Name (not NUL-terminated) foreach-address { u8 Family (AF_INET{,6}) union { u8[4] ipv4_addr u8[16] ipv6_addr } } } This can then be used to fetch a whole cell's VL-server configuration for AFS, for example. Signed-off-by: David Howells <dhowells@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
167 lines
4.6 KiB
C
167 lines
4.6 KiB
C
/* Upcall routine, designed to work as a key type and working through
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* /sbin/request-key to contact userspace when handling DNS queries.
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*
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* See Documentation/networking/dns_resolver.txt
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*
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* Copyright (c) 2007 Igor Mammedov
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* Author(s): Igor Mammedov (niallain@gmail.com)
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* Steve French (sfrench@us.ibm.com)
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* Wang Lei (wang840925@gmail.com)
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* David Howells (dhowells@redhat.com)
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*
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* The upcall wrapper used to make an arbitrary DNS query.
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*
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* This function requires the appropriate userspace tool dns.upcall to be
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* installed and something like the following lines should be added to the
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* /etc/request-key.conf file:
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*
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* create dns_resolver * * /sbin/dns.upcall %k
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*
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* For example to use this module to query AFSDB RR:
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*
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* create dns_resolver afsdb:* * /sbin/dns.afsdb %k
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*
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* This library is free software; you can redistribute it and/or modify
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* it under the terms of the GNU Lesser General Public License as published
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* by the Free Software Foundation; either version 2.1 of the License, or
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* (at your option) any later version.
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*
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* This library is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See
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* the GNU Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public License
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* along with this library; if not, see <http://www.gnu.org/licenses/>.
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*/
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#include <linux/module.h>
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#include <linux/slab.h>
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#include <linux/cred.h>
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#include <linux/dns_resolver.h>
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#include <linux/err.h>
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#include <keys/dns_resolver-type.h>
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#include <keys/user-type.h>
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#include "internal.h"
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/**
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* dns_query - Query the DNS
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* @type: Query type (or NULL for straight host->IP lookup)
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* @name: Name to look up
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* @namelen: Length of name
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* @options: Request options (or NULL if no options)
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* @_result: Where to place the returned data (or NULL)
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* @_expiry: Where to store the result expiry time (or NULL)
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*
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* The data will be returned in the pointer at *result, if provided, and the
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* caller is responsible for freeing it.
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*
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* The description should be of the form "[<query_type>:]<domain_name>", and
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* the options need to be appropriate for the query type requested. If no
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* query_type is given, then the query is a straight hostname to IP address
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* lookup.
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*
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* The DNS resolution lookup is performed by upcalling to userspace by way of
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* requesting a key of type dns_resolver.
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*
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* Returns the size of the result on success, -ve error code otherwise.
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*/
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int dns_query(const char *type, const char *name, size_t namelen,
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const char *options, char **_result, time64_t *_expiry)
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{
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struct key *rkey;
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struct user_key_payload *upayload;
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const struct cred *saved_cred;
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size_t typelen, desclen;
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char *desc, *cp;
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int ret, len;
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kenter("%s,%*.*s,%zu,%s",
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type, (int)namelen, (int)namelen, name, namelen, options);
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if (!name || namelen == 0)
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return -EINVAL;
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/* construct the query key description as "[<type>:]<name>" */
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typelen = 0;
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desclen = 0;
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if (type) {
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typelen = strlen(type);
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if (typelen < 1)
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return -EINVAL;
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desclen += typelen + 1;
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}
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if (!namelen)
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namelen = strnlen(name, 256);
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if (namelen < 3 || namelen > 255)
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return -EINVAL;
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desclen += namelen + 1;
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desc = kmalloc(desclen, GFP_KERNEL);
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if (!desc)
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return -ENOMEM;
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cp = desc;
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if (type) {
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memcpy(cp, type, typelen);
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cp += typelen;
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*cp++ = ':';
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}
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memcpy(cp, name, namelen);
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cp += namelen;
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*cp = '\0';
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if (!options)
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options = "";
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kdebug("call request_key(,%s,%s)", desc, options);
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/* make the upcall, using special credentials to prevent the use of
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* add_key() to preinstall malicious redirections
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*/
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saved_cred = override_creds(dns_resolver_cache);
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rkey = request_key(&key_type_dns_resolver, desc, options);
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revert_creds(saved_cred);
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kfree(desc);
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if (IS_ERR(rkey)) {
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ret = PTR_ERR(rkey);
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goto out;
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}
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down_read(&rkey->sem);
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set_bit(KEY_FLAG_ROOT_CAN_INVAL, &rkey->flags);
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rkey->perm |= KEY_USR_VIEW;
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ret = key_validate(rkey);
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if (ret < 0)
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goto put;
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/* If the DNS server gave an error, return that to the caller */
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ret = PTR_ERR(rkey->payload.data[dns_key_error]);
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if (ret)
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goto put;
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upayload = user_key_payload_locked(rkey);
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len = upayload->datalen;
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if (_result) {
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ret = -ENOMEM;
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*_result = kmemdup_nul(upayload->data, len, GFP_KERNEL);
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if (!*_result)
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goto put;
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}
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if (_expiry)
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*_expiry = rkey->expiry;
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ret = len;
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put:
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up_read(&rkey->sem);
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key_put(rkey);
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out:
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kleave(" = %d", ret);
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return ret;
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}
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EXPORT_SYMBOL(dns_query);
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