ZDNS

ZDNS is a command-line utility that provides high-speed DNS lookups. ZDNS is written in Go and contains its own recursive resolution code and a cache optimized for performing lookups of a diverse set of names. We use https://github.com/zmap/dns to construct and parse raw DNS packets. For more information about ZDNS's architecture and performance, check out the following paper appearing at ACM's Internet Measurement Conference '22.

As an example, the following will perform MX lookups and a secondary A lookup for the IPs of MX servers for the domains in the Alexa Top Million:

cat top-1m.csv | ./zdns MX --ipv4-lookup --alexa

Install

ZDNS can be installed by checking out the repository and running go build.

git clone https://github.com/zmap/zdns.git
cd zdns
go build

Usage

ZDNS was originally built as a CLI tool only. Work has been done to convert this into a library with a CLI that calls this library. Currently, the library has been separated out and a new, separate CLI has been added. Work is ongoing to clean up the interface between the CLI (or any other client program of the ZDNS library) and the ZDNS library itself.

The ZDNS library lives in github.com/zmap/zdns/pkg/zdns. A function there, zdns.Run(), is used to start the ZDNS tool and do the requested lookups. Currently, this tool is intended to accept a zdns.GlobalConf object, plfag flags, and other information, but this interface is undergoing revisions to be more generally usable and continue to decouple the CLI from the library.

The CLI for this library lives in github.com/zmap/zdns under the main package. Its functionality is described below.

ZDNS provides several types of modules:

Raw DNS modules provide the raw DNS reponse from the server similar to dig, but in JSON. There is a module for (nearly) every type of DNS record
Lookup modules provide more helpful responses when multiple queries are required (e.g., completing additional A lookup if a CNAME is received)
Misc modules provide other additional means of querying servers (e.g., bind.version)

We detail the modules below:

Raw DNS Modules

The A, AAAA, AFSDB, ANY, ATMA, AVC, AXFR, BINDVERSION, CAA, CDNSKEY, CDS, CERT, CNAME, CSYNC, DHCID, DMARC, DNSKEY, DS, EID, EUI48, EUI64, GID, GPOS, HINFO, HIP, HTTPS, ISDN, KEY, KX, L32, L64, LOC, LP, MB, MD, MF, MG, MR, MX, NAPTR, NID, NINFO, NS, NSAPPTR, NSEC, NSEC3, NSEC3PARAM, NSLOOKUP, NULL, NXT, OPENPGPKEY, PTR, PX, RP, RRSIG, RT, SVCBS, MIMEA, SOA, SPF, SRV, SSHFP, TALINK, TKEY, TLSA, TXT, UID, UINFO, UNSPEC, and URI modules provide the raw DNS response in JSON form, similar to dig.

For example, the command:

echo "censys.io" | ./zdns A

returns:

{
  "name": "censys.io",
  "class": "IN",
  "status": "NOERROR",
  "data": {
    "answers": [
      {
        "ttl": 300,
        "type": "A",
        "class": "IN",
        "name": "censys.io",
        "data": "216.239.38.21"
      }
    ],
    "additionals": [
      {
        "ttl": 34563,
        "type": "A",
        "class": "IN",
        "name": "ns-cloud-e1.googledomains.com",
        "data": "216.239.32.110"
      },
    ],
    "authorities": [
      {
        "ttl": 53110,
        "type": "NS",
        "class": "IN",
        "name": "censys.io",
        "data": "ns-cloud-e1.googledomains.com."
      },
    ],
    "protocol": "udp",
    "resolver": "30.128.52.190:53"
  }
}

Lookup Modules

Raw DNS responses frequently do not provide the data you want. For example, an MX response may not include the associated A records in the additionals section requiring an additional lookup. To address this gap and provide a friendlier interface, we also provide several lookup modules: alookup and mxlookup.

mxlookup will additionally do an A lookup for the IP addresses that correspond with an exchange record. alookup acts similar to nslookup and will follow CNAME records.

For example,

echo "censys.io" | ./zdns mxlookup --ipv4-lookup

returns:

{
  "name": "censys.io",
  "status": "NOERROR",
  "data": {
    "exchanges": [
      {
        "name": "aspmx.l.google.com",
        "type": "MX",
        "class": "IN",
        "preference": 1,
        "ipv4_addresses": [
          "74.125.28.26"
        ],
        "ttl": 288
      },
      {
        "name": "alt1.aspmx.l.google.com",
        "type": "MX",
        "class": "IN",
        "preference": 5,
        "ipv4_addresses": [
          "64.233.182.26"
        ],
        "ttl": 288
      }
    ]
  }
}

Other DNS Modules

ZDNS also supports special "debug" DNS queries. Modules include: BINDVERSION.

Threads, Sockets, and Performance

ZDNS performance stems from massive parallelization using light-weight Go routines. This architecture has several cavaets:

Every Go routine uses its own dedicated network socket. Thus, you need to be able to open as many sockets (in terms of both max file descriptors and ephemeral ports) as you have threads specified (via --threads). By default, ZDNS uses 1,000 threads, which is less than Linux's default max number of 1024 open FDs. However, it is greater than Mac OS's default of 256. You can view the maximum number of open FDs (and thus sockets) permitted by running unlimit -n. If you want to run with a greater number of threads than this number, you need to increase the number of open files at the OS level. If you fail to do this, you'll encounter a fatal error similar to FATA[0000] unable to create socketlisten udp <client IP address>:0: socket: too many open files. If you want to run more threads than you have ephemeral ports available, you will need to use multiple client IP addresses: --local-addr=A,B,C.
By default, ZDNS "reuses" UDP sockets by creating an unbound UDP socket for each light-weight routine at launch and using it for all queries (regardless of destination IP). This dramatically improves performance because ZDNS and the host OS don't need to setup and tear down a socket to send each individual packet (since DNS queries/responses tend to be one packet each). However, this means that ZDNS will preallocate a socket for each thread at launch. This may not be optimal if you're only looking up a small number of names. For example, if you only need to lookup 100 names, but use the default 1,000 threads, you'll bind but never use 900 UDP sockets. Instead, of worrying about recycling sockets, we recommend that you specify a reasonable number of threads for your use case (since this also foregoes any work to start those threads in the first place). This is why, though, you can get an error about being unable to open a large number of sockets even though you're only looking up a single name. If it's important to create a fresh socket for each query, you can disable this reuse by specifying --recycle-sockets=false.
Go is happy to use all CPU cores that are available to it, and can use a tremendous amount of CPU if you specify a large number of threads. CPU is primarily used for parsing and JSON encoding. If you want to limit the number of CPU cores, you can do so by including the --go-processes=n flag or setting the GOMAXPROCS environment variable.
Typically we recommend using around 1000-5000 threads. Unless you're on an underesourced system, you'll likely be throwing away free performance with only tens or hundreds of threads (since you'll be waiting on network communication). We typically don't see significant improvement in performance with over 5,000 threads, and don't have any cases where more than 10,000 threads improved performance.

Local Recursion

ZDNS can either operate against a recursive resolver (e.g., an organizational DNS server) [default behavior] or can perform its own recursion internally. If you are performing a small number of lookups (i.e., millions) and using a less than 10,000 go routines, it is typically fastest to use one of the common recursive resolvers like Cloudflare or Google. Cloudflare is nearly always faster than Google. This is particularly true if you're looking up popular names because they're cached and can be answered in a single round trip. When using tens of thousands of concurrent threads, consider performing iteration internally in order to avoid DOS'ing and/or rate limiting your recursive resolver.

To perform local recursion, run zdns with the --iterative flag. When this flag is used, ZDNS will round-robin between the published root servers (e.g., 198.41.0.4). In iterative mode, you can control the size of the local cache by specifying --cache-size and the timeout for individual iterations by setting --iteration-timeout. The --timeout flag controls the timeout of the entire resolution for a given input (i.e., the sum of all iterative steps).

Output Verbosity

DNS includes a lot of extraneous data that is not always useful. There are four result verbosity levels: short, normal (default), long, and trace:

short: Short is the most terse result output. It contains only information about the responses
normal: Normal provides everything included in short as well as data about the responding server
long: Long outputs everything the server included in the DNS packet, including flags.
trace: Trace outputs everything from every step of the recursion process

Users can also include specific additional fields using the --include-fields flag and specifying a list of fields, e.g., --include-fields=flags,resolver. Additional fields are: class, protocol, ttl, resolver, flags.

Name Server Mode

By default ZDNS expects to receive a list of names to lookup on a small number of name servers. For example:

echo "google.com" | ./zdns A --name-servers=8.8.8.8,8.8.4.4

However, there are times where you instead want to lookup the same name across a large number of servers. This can be accomplished using name server mode. For example:

echo "8.8.8.8" | ./zdns A --name-server-mode --override-name="google.com"

Here, every line piped in ZDNS is sent an A query for google.com. ZDNS also supports mixing and matching both modes by piping in a comma-delimited list of name,nameServer. For example:

echo "google.com,8.8.8.8" | ./zdns A will send an A query for google.com to 8.8.8.8 regardless of what name servers are specified by --name-servers= flag. Lines that do not explicitly specify a name server will use the servers specified by the OS or --name-servers flag as would normally happen.

Querying all Nameservers

There is a feature available to perform a certain DNS query against all nameservers. For example, you might want to get the A records from all nameservers of a certain domain. To do so, you can do:

echo "google.com" | ./zdns A --all-nameservers

Running ZDNS

By default, ZDNS will operate with 1,000 light-weight go routines. If you're not careful, this will overwhelm many upstream DNS providers. We suggest that users coordinate with local network administrators before performing any scans. You can control the number of concurrent connections with the --threads and --go-processes command line arguments. Alternate name servers can be specified with --name-servers. ZDNS will rotate through these servers when making requests. We have successfully run ZDNS with tens of thousands of light-weight routines.

Unsupported Types

If zdns encounters a record type it does not support it will generate an output record with the type field set correctly and a representation of the underlying data structure in the unparsed_rr field. Do not rely on the presence or structure of this field. This field (and its existence) may change at any time as we expand support for additional record types. If you find yourself using this field, please consider submitting a pull-request adding parser support.

License

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See LICENSE for the specific language governing permissions and limitations under the License.

zmap/zdns

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