A fix is available
APAR status
Closed as program error.
Error description
SMTP has it's own resolver function and uses the UDP protocol to communicate with the DNS. If a DNS response has the truncate bit set (meaning important information is missing from the reply because the information would not fit in a UDP packet), SMTP discards the reply. This can cause mail to not be delivered if SMTP is not able to resolve the hostname to an IP address. The only way to tell this is happening is to turn on RESOLVER TRACE in SMTP and look for message EZA5526I 01/26/11 14:35:41# 1903 Truncation Bit set in Nameserver Response . SMTP should issue a message to the console when it discards the first truncated DNS reply. This provides a way to know a problem may be going on that needs to be investigated. . KEYWORDS: SMTP SMTPD truncation truncated not sending mail delivering hung . VERIFICATION STEPS: F smtp_jobname,SMSG,QU will show mail is on the resolver queues: EZA5594I SMSG QUEUE Output ----- Mail Queues ----- Spool Queue: 0 Undeliverable Queue: 0 --- Resolver Queues --- Process Queue: 0 Send Queue: 0 Wait Queue: 100 Retry Queue: 50 Completed Queue: 0 Error Queue: 0 *END OF OUTPUT* . Turn on resolver trace with: F smtp_jobname,SMSG,TRACE Resolver trace will show message EZA5526I . To turn off resolver trace, isssue: F smtp_jobname,SMSG,NOTRACE
Local fix
There are a couple of things that can be done: 1) Change SMTP to use IPMAILERADDRESS x.x.x.x where x.x.x.x is the IP address of the mail server and code RESOLVERUSAGE NO. 2) Change the DNS so that it doesn't return so much information so that it does not turn on the truncate bit in the replies
Problem summary
**************************************************************** * USERS AFFECTED: All users of the IBM Communications Server * * for z/OS Version 1 Release(s) 11, and 12 * * IP: SMTP * **************************************************************** * PROBLEM DESCRIPTION: SMTP does not write a message to the * * system console when it discards a * * truncated DNS response * **************************************************************** * RECOMMENDATION: * **************************************************************** SMTP does not generate a console message when it discards a truncated DNS response. The customer must do a TSO command "SMSG <SMTP_STARTED_PROCEDURE> QUEUE" to see all the mail messages that are still on the resolver queues waiting for retry. They would then activate resolver traces by doing a TSO command "SMSG <SMTP_STARTED_PROCEDURE> TRACE". In reviewing the resolver traces the following message is found: EZA5526I 02/03/11 13:25:08# 1 Truncation Bit set in Nameserver Response +-------------------------------------------------------------+ + Please check our Communications Server for OS/390 homepages + + for common networking tips and fixes. The URL for these + + homepages can be found in Informational APAR II11334. + +-------------------------------------------------------------+
Problem conclusion
SMTP will generate a new message EZA5208E once when this condition is first detected. Customers can automate on this message to then activate resolver trace to determine which domain name server(s) have the problem and scope of the problem. For example, if all mail messages are being blocked or just a few due to an unusual mailbox name. **************************************************************** * FUNCTION AFFECTED: Communications Server TCP/IP (PM31474) * * SMTP * **************************************************************** * DESCRIPTION : Documentation updates * **************************************************************** * TIMING : None * **************************************************************** Added new message EZA5208E and updated z/OS IP Diagnostic Guide. ---------------------------------------------------------------- z/OS Communications Server IP Messages: Volume 1 (EZA) Document Number SC31-8783 ---------------------------------------------------------------- EZA5208E <mm/dd/yy> <hh:mm:ss> DNS response was truncated so a name cannot be resolved Explanation: SMTP has received a response to a domain name server (DNS) query. The response was truncated because it was too large to fit in a UDP packet. SMTP supports only UDP protocol for name resolution. Because the DNS response was incomplete, it was discarded. As a result, the mail messages remain on the resolver queues and will be retried. This message will be generated only the first time the situation is detected. <mm/dd/yy> is the date (month/day/year) when the DNS response was received. <hh:mm:ss> is the time (hours:minutes:seconds) when the DNS response was received. Examples: SMTP output file with resolver traces active: EZA5526I 02/03/11 13:25:08# 1 Truncation Bit set in Nameserver Response EZA5208E 02/03/11 13:25:08 DNS response was truncated so a name cannot be resolved System console: $HASP100 SMTP ON STCINRDR IEF695I START SMTP WITH JOBNAME SMTP IS ASSIGNED TO USER IBMUSER, GROUP SYS1 $HASP373 SMTP STARTED IEF403I SMTP - STARTED - TIME=13.40.18 EZY1876I SMTP STACK FUNCTIONS STARTED WITH PARAMETER SMTP,PARM=, ERRFILE(SYSERR),NOSPIE/. EZA5208E 02/03/11 13:40:20 DNS response was truncated so a name cannot be resolved System Action: SMTP continues. Operator Response: None. User Response: Not applicable. System Programmer Response: Contact the administrator for the name server and inform them of the problem. If the problem can be resolved at the DNS, SMTP will automatically recover because it will retry the DNS query. The retry time of the DNS query depends on your SMTP configuration setting, see the RESOLVERRETRYINT statement in the z/OS IP Configuration Reference. If you are using the IPMAILERNAME statement and the problem cannot be resolved at the DNS, you can switch to using the IPMAILERADDRESS statement until the DNS is modified. If you do switch to the address indicated by the IPMAILERADDRESS statement and you want all the mail sent to that address, then set the RESOLVERUSAGE statement to NO. See the SMTP configuration statements IPMAILERNAME and IPMAILERADDRESS in the z/OS IP Configuration Reference. You can determine if the problem is resolved by entering the TSO command SMSG <SMTP_STARTED_TASK_NAME> QUEUE. If the problem is resolved, the number of mail messages on the resolver queues will decrease. Problem Determination: If you are using multiple DNS servers, you might need to determine which server returned this response. You can dynamically activate resolver traces by entering the TSO command SMSG <SMTP_STARTED_TASK_NAME> TRACE. Review the resolver traces in the SMTP output file. Look for message EZA5526I, which contains the DNS query number, to determine which DNS is issuing this response and what name is being resolved. Source: z/OS Communications Server TCP/IP: SMTP Module: SMTPRES Routing code: 2,10 Descriptor code: 4 Automation: This message is a candidate for automation. You can use this message to alert you to a DNS server problem. ---------------------------------------------------------------- z/OS Communications Server IP Diagnosis: Document Number GC31-8782 ---------------------------------------------------------------- Chapter: Diagnosing Simple Mail Transfer Protocol (SMTP) problem Under the following section add new first bullet: If the problem is that mail was not forwarded to a remote TCP/IP host: Has the host name been resolved to an IP address? 1. Check the system console or the SMTP output log for an EZA5208E message. If this is present, then SMTP has received a domain name server (DNS) response with the truncation bit on in the packet. SMTP discards these packets because the DNS response is not complete. SMTP will continue to retry these DNS queries for a configured amount of time. But if the DNS response is the same the mail message will eventually become undeliverable. Use RESOLVER traces to determine the DNS sending the response and what is the unresolved name. Look for message EZA5526I in the resolver traces. Changes are needed to the DNS to resolve this problem. Note that the CSSMTP application handles the DNS truncation bit differently since it uses the system resolver whereas SMTP has its own resolution code. So migration to the new CSSMTP application would avoid this problem. 2. Run RESOLVER trace to see if the host name is resolved correctly. The RESOLVER trace is explained in SMTP RESOLVER trace. * Cross Reference between External and Internal Names EZAAA00H (MSSMTP ) EZABB01X (SMTP ) EZABB025 (SMTPRES ) EZAAA00H (MSSMTP ) EZABB01X (SMTP ) EZABB025 (SMTPRES )
Temporary fix
Comments
APAR Information
APAR number
PM31474
Reported component name
TCP/IP V3 MVS
Reported component ID
5655HAL00
Reported release
1B0
Status
CLOSED PER
PE
NoPE
HIPER
NoHIPER
Special Attention
NoSpecatt / Xsystem
Submitted date
2011-01-28
Closed date
2011-02-21
Last modified date
2011-04-04
APAR is sysrouted FROM one or more of the following:
APAR is sysrouted TO one or more of the following:
UK65090 UK65091
Modules/Macros
EZAAA00H EZABB01X EZABB025
GC31878211 | SC31878311 | GC31878210 | SC31878310 |
Fix information
Fixed component name
TCP/IP V3 MVS
Fixed component ID
5655HAL00
Applicable component levels
Fix is available
Select the PTF appropriate for your component level. You will be required to sign in. Distribution on physical media is not available in all countries.
[{"Business Unit":{"code":"BU054","label":"Systems w\/TPS"},"Product":{"code":"SG19M","label":"APARs - z\/OS environment"},"Component":"","ARM Category":[],"Platform":[{"code":"PF025","label":"Platform Independent"}],"Version":"1B0","Edition":"","Line of Business":{"code":"","label":""}},{"Business Unit":{"code":"BU054","label":"Systems w\/TPS"},"Product":{"code":"SSCY4DZ","label":"DO NOT USE"},"Component":"","ARM Category":[],"Platform":[{"code":"PF025","label":"Platform Independent"}],"Version":"1B0","Edition":"","Line of Business":{"code":"","label":""}}]
Document Information
Modified date:
04 April 2011