DNS – ColinPaice

This came out of a question. It is another of the little questions that get much bigger.

Background to Domain Name System(DNS)

DNS allows you to get an IP address from a string such as “WWW.MY.COM”.

You can have some files on your local system which provide this mapping, or you can exploit DNS Servers in the big internet.

Some people configure their system so it tries the internet first, and if that fails, uses local files.

You can do reverse DNS lookup, mapping an IP address to a string. For example you want to allow access from sites in WWW.MYFRIEND.COM. When a connection is started, you get the IP address, and can then do a reverse DNS lookup to get a name, which you can check in your “allow” list.

DNS commands for the end user

You can use the “old” tso command NSLOOKUP http://www.ibm.com, or the “new” command dig http://www.ibm.com. Neither of which seemed to give me any output!

The NSLOOKUP and DIG commands send their output to SYSOUT. In my TSO system, SYSOUT has been configured to JES. If I use SDSF, and display the output of my TSO userid, there is a SYSOUT, with the output in it!

NSLOOKUP

The NSLOOKUP command

NSLOOKUP http://www.my.com

NSLOOKUP http://www.my.com this.dns.site

NSLOOKUP 10.1.1.2

Tracing a DNS request

This does not provide much useful information! It does not tell you what happened, or what failed. It is described here.

Starting and stopping the DNS

This is not obvious. At IPL the ADCD.Z24C.PARMLIB(BPXPRM00) member has

RESOLVER_PROC(RESOLVER)

the resolver procedure must be in a data set that is specified by the IEFPDSI DD card specification of the MSTJCLxx PARMLIB member.

If you use D A,L it does not show up.

D A,RESOLVER gives you the normal output.

When I issued

P RESOLVER
S RESOLVER

It used the RESOLVER procedure from USER.Z24C.PROCLIB, the normal concatenation.

Displaying and changing the configuration.

You can display some of the current resolver configuration using

f resolver,display

The output is like

EZZ9298I RESOLVERSETUP - USER.Z24C.TCPPARMS(GBLRESOL)                   
EZZ9298I DEFAULTTCPIPDATA - USER.Z24C.TCPPARMS(GBLTDATA)                
EZZ9298I GLOBALTCPIPDATA - /etc/resolv.conf                             
EZZ9298I DEFAULTIPNODES - ADCD.Z24C.TCPPARMS(ZPDTIPN1)                  
EZZ9298I GLOBALIPNODES - /etc/hosts                                     
EZZ9304I COMMONSEARCH                                                   
EZZ9304I CACHE                                                          
EZZ9298I CACHESIZE - 200M                                               
EZZ9298I MAXTTL - 2147483647                                            
EZZ9298I MAXNEGTTL - 2147483647                                         
EZZ9304I NOCACHEREORDER                                                 
EZZ9298I UNRESPONSIVETHRESHOLD - 25

The only way I could display all of the resolver configuration was to get a resolver trace!

//IBMRESO JOB 1,MSGCLASS=H 
//S1  EXEC PGM=IKJEFT01,REGION=0M 
//SYSPRINT DD SYSOUT=* 
//SYSTSPRT DD SYSOUT=* 
//SYSTCPT DD SYSOUT=* 
//SYSPRINT DD SYSOUT=* 
//SYSTSIN DD * 
NSLOOKUP 99.99.99.99 
/*

This gave me in //SYSTCPT

Resolver Trace Initialization Complete -> 2023/02/26 18:01:56.725504                      
res_init Parse error on line     1: /etc/resolv.conf                                      
                                                                                          
res_init Resolver values:                                                                 
 Setup file warning messages = No                                                         
 CTRACE TRACERES option = No                                                              
 Global Tcp/Ip Dataset  = /etc/resolv.conf                                                
 Default Tcp/Ip Dataset = USER.Z24C.TCPPARMS(GBLTDATA)                                    
 Local Tcp/Ip Dataset   = None                                                            
 Translation Table      = TCPIP.STANDARD.TCPXLBIN                                         
 UserId/JobName         = IBMUSER                                                         
 Caller API             = TCP/IP C Sockets                                                
 Caller Mode            = EBCDIC                                                          
 System Name            = S0W1 (from VMCF)                                                
 UnresponsiveThreshold  = 25                                                              
 (D) DataSetPrefix = TCPIP                                                                
 (D) HostName      = S0W1                                                                 
 (D) TcpIpJobName  = TCPIP                                                                
 (*) DomainOrigin  = None                                                                   
 (*) NameServer(s) = None                                                                   
 (*) NsPortAddr    = 53            (*) ResolverTimeout    = 5                               
 (*) ResolveVia    = UDP           (*) ResolverUdpRetries = 1                               
 (*) Options NDots = 1                                                                      
 (D) Trace Resolver                (*) SockNoTestStor                                       
 (D) AlwaysWto     = NO            (D) MessageCase        = MIXED                           
 (*) LookUp        = DNS LOCAL                                                              
 (*) Cache                                                                                  
 (*) NoCacheReorder                                                                         
res_init Succeeded                                                                          
res_init Started: 2023/02/26 18:01:56.794280                                                
res_init Ended: 2023/02/26 18:01:56.794305

This is documented here.

The source of the value is

(*) Default value
(A) Modified by application
(D) Default file (not used if the local file is found)
(E) Environment variable
(G) Global file
(L) Local file

This means the “LookUp = DNS LOCAL ” value came from the default value.

The resolver JCL in USER.Z24C.PROCLIB had

//SETUP DISP=SHR,DSN=USER.Z24C.TCPPARMS(GBLRESOL)

When I changed this member to have LOOKUP LOCAL DSN, and used the F RESOLVER,REFRESH command, this changed the value.

Sample hosts file

The sample host file in TCPIP.SEZAINST(HOSTS) has

; The format of this file is documented in RFC 952, "DoD Internet 
; Host Table Specification". 
; 
; The format for entries is: 
; 
; NET : ADDR : NETNAME : 
; GATEWAY : ADDR, ALT-ADDR : HOSTNM : CPUTYPE : OPSYS : PROTOCOLS : 
; HOST : ADDR, ALT-ADDR : HOSTNM, NICKNM : CPUTYPE : OPSYS : PROTOCOLS : 
; 
; Where: 
;   ADDR, ALT-ADDR = IP address in decimal, e.g., 26.0.0.73 
;   HOSTNM, NICKNM = the fully qualified host name and any nicknames 
;   CPUTYPE = machine type (PDP-11/70, VAX-11/780, IBM-3090, C/30, etc.) 
;   OPSYS = operating system (UNIX, TOPS20, TENEX, VM/SP, etc.) 
;   PROTOCOLS = transport/service (TCP/TELNET,TCP/FTP, etc.) 
;   : (colon) = field delimiter 
;   :: (2 colons) = null field 
; *** CPUTYPE, OPSYS, and PROTOCOLS are optional fields. 
; 
;   MAKESITE does not allow continuation lines, as described in 
;   note 2 of the section "GRAMMATICAL HOST TABLE SPECIFICATION" 
;   in RFC 952.  Entries should be specified on a single line of 
;   up to a maximum of 512 characters per line. 
HOST : 129.34.128.245, 129.34.128.246 : YORKTOWN, WATSON :::: 
; 
NET  : 9.67.43.0 : RALEIGH.IBM.COM : 
; 
GATEWAY : 129.34.0.0 : YORKTOWN-GATEWAY ::::

Unix application trace

Enable the trace by issuing the Unix command

export RESOLVER_TRACE=~/trace

Run the command

pip install mfpandas

gave

-[33mWARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x500B209580>: 
Failed to establish a new connection: 
[Errno 1] EDC9501I The name does not resolve for the supplied parameters.')': /simple/mfpandas/-[0m-[33m

Look at the trace file

oedit trace