Problem of DNS resolution exception causes SSH login system slow

Troubleshooting

Problem

SSH login system is slow.

Symptom

Customer login system with ssh, but login interface appears very slowly, need to find out the reason of SSH login system is slow.

Diagnosing The Problem

For SSH related problems, we can collect SSH debug data for analysis. The data collection method is as follows:

1> Create a temp testcase dir, ie
# mkdir /tmp/tc

2> Enable DEBUG3 for sshd
Edit sshd_config file and set the following lines as shown "LogLevel DEBUG3":
# vi /etc/ssh/sshd_config
LogLevel DEBUG3

3> Edit /etc/syslog.conf and add this line
*.debug /tmp/tc/syslog.out

4> Touch syslog out file
# touch /tmp/tc/syslog.out

5> Refresh syslogd
# refresh -s syslogd

6> Restart sshd
# stopsrc -s sshd
# startsrc -s sshd

7> Start iptrace
# startsrc -s iptrace -a "-a -L 500000000 -p 22 /tmp/tc/iptrace.pcap"

8> Recreate ssh delay

9> Stop iptrace
# stopsrc -s iptrace

10> Get copy of /etc/ssh dir
# cd /tmp/tc
# cp -pr /etc/ssh ssh.dir

11> Package data
# cd /tmp/tc
# tar -cvf./sshlog.tar *
# gzip sshlog.tar

After collection, we can analyse /tmp/tc/ sygoogle.out and iptrace data

After analysing SSH debug data, we found that the system spent 80 seconds parsing the 192.168.0.9 address during login:

May 2 18:43:34 erp-proxy auth|security:debug sshd[7274616]: debug3: Trying to reverse map address 192.168.0.9.
May 2 18:44:54 erp-proxy auth|security:debug sshd[7274616]: debug2: parse_server_config: config reprocess config len 236

Resolving The Problem

Adjust DNS resolution order of system by adding following to the /etc/netsvc.conf file:
hosts=local,bind4
---> Problem remains.

Disable DNS in /etc/ssh/sshd_config file:
UseDNS no
---> Problem disappear.

Therefore, this problem is caused by DNS Server resolution exception, resulting in SSH login slowly, need to check the configuration of DNS Server.

Note that except the DNS Settings on AIX, there are also DNS Settings in SSH configuration file, we need to check both of two DNS settings in case of related problems.

Document Location

Worldwide

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Document Information

More support for:
IBM Power Systems

Software version:
All Versions

Operating system(s):
AIX

Document number:
1165798

Modified date:
06 January 2020

UID

ibm11165798

IBM Support

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