Posts Tagged ‘acl’

Use haproxy to dynamically modify haproxy load balancer variables, view stastics, errors and much more via stats UNIX socket with socat via command line

Friday, December 15th, 2023

haproxy-modify-dynamic-through-haproxy-unix-sock-via-socat-netcat

Haproxy could be configured to use the listen stats interface to provide a tiny web interface with statistics on all configured haproxy frontends / backends state status (UP / DOWN), current connections to proxy, errors and other interesting bandwidth information.

That is mostly useful but not every haproxy has it configured and if you did not configure the HAproxy load balancer machines on your own it might be, the previous person who build the LB infrastructure did not create the haproxy listener. 

If that is the case and you still need to get various statistics on how haproxy performs and the status of active connections towards Frotnend i/ Backend interfaces this is still possible via configured stats socket (usually this is in Global or some of the other haproxy.cfg config sections..

It is possible to do many things with haproxy such as disable / enable frotnends / backends / servers

Lets say your Haproxy has a global section that looks like this:
 

global
        stats socket /var/run/haproxy/haproxy.sock mode 0600 level admin #Creates Unix-Like socket to fetch stats
        log /dev/log    local0
        log /dev/log    local1 notice
        chroot /var/lib/haproxy
        user haproxy
        group haproxy
        daemon
        maxconn 99999
        nbproc          1
        nbthread 2
        cpu-map         1 0
        cpu-map         2 1

1. Listing all available options that can be send via the haproxy.sock UNIX socket interface

root@pcfreak:/home/hipo/info# echo "show help" | socat stdio /var/run/haproxy/haproxy.sock
Unknown command. Please enter one of the following commands only :
  help           : this message
  prompt         : toggle interactive mode with prompt
  quit           : disconnect
  show tls-keys [id|*]: show tls keys references or dump tls ticket keys when id specified
  set ssl tls-key [id|keyfile] <tlskey>: set the next TLS key for the <id> or <keyfile> listener to <tlskey>
  add ssl crt-list <filename> <certfile> [options] : add a line <certfile> to a crt-list <filename>
  del ssl crt-list <filename> <certfile[:line]> : delete a line <certfile> in a crt-list <filename>
  show ssl crt-list [-n] [
] : show the list of crt-lists or the content of a crt-list <filename>
  new ssl cert <certfile> : create a new certificate file to be used in a crt-list or a directory
  set ssl cert <certfile> <payload> : replace a certificate file
  commit ssl cert <certfile> : commit a certificate file
  abort ssl cert <certfile> : abort a transaction for a certificate file
  del ssl cert <certfile> : delete an unused certificate file
  show ssl cert [
] : display the SSL certificates used in memory, or the details of a <certfile>
  set maxconn global : change the per-process maxconn setting
  set rate-limit : change a rate limiting value
  set severity-output [none|number|string] : set presence of severity level in feedback information
  set timeout    : change a timeout setting
  show env [var] : dump environment variables known to the process
  show cli sockets : dump list of cli sockets
  show cli level   : display the level of the current CLI session
  show fd [num] : dump list of file descriptors in use
  show activity : show per-thread activity stats (for support/developers)
  operator       : lower the level of the current CLI session to operator
  user           : lower the level of the current CLI session to user
  clear counters : clear max statistics counters (add 'all' for all counters)
  show info      : report information about the running process [desc|json|typed]*
  show stat      : report counters for each proxy and server [desc|json|typed]*
  show schema json : report schema used for stats
  show sess [id] : report the list of current sessions or dump this session
  shutdown session : kill a specific session
  shutdown sessions server : kill sessions on a server
  disable agent  : disable agent checks (use 'set server' instead)
  disable health : disable health checks (use 'set server' instead)
  disable server : disable a server for maintenance (use 'set server' instead)
  enable agent   : enable agent checks (use 'set server' instead)
  enable health  : enable health checks (use 'set server' instead)
  enable server  : enable a disabled server (use 'set server' instead)
  set maxconn server : change a server's maxconn setting
  set server     : change a server's state, weight or address
  get weight     : report a server's current weight
  set weight     : change a server's weight (deprecated)
  show startup-logs : report logs emitted during HAProxy startup
  clear table    : remove an entry from a table
  set table [id] : update or create a table entry's data
  show table [id]: report table usage stats or dump this table's contents
  add acl        : add acl entry
  clear acl <id> : clear the content of this acl
  del acl        : delete acl entry
  get acl        : report the patterns matching a sample for an ACL
  show acl [id]  : report available acls or dump an acl's contents
  add map        : add map entry
  clear map <id> : clear the content of this map
  del map        : delete map entry
  get map        : report the keys and values matching a sample for a map
  set map        : modify map entry
  show map [id]  : report available maps or dump a map's contents
  show events [
] : show event sink state
  show threads   : show some threads debugging information
  show peers [peers section]: dump some information about all the peers or this peers section
  disable frontend : temporarily disable specific frontend
  enable frontend : re-enable specific frontend
  set maxconn frontend : change a frontend's maxconn setting
  show servers conn [id]: dump server connections status (for backend <id>)
  show servers state [id]: dump volatile server information (for backend <id>)
  show backend   : list backends in the current running config
  shutdown frontend : stop a specific frontend
  set dynamic-cookie-key backend : change a backend secret key for dynamic cookies
  enable dynamic-cookie backend : enable dynamic cookies on a specific backend
  disable dynamic-cookie backend : disable dynamic cookies on a specific backend
  show errors    : report last request and response errors for each proxy
  show resolvers [id]: dumps counters from all resolvers section and
                     associated name servers
  show pools     : report information about the memory pools usage
  show profiling : show CPU profiling options
  set  profiling : enable/disable CPU profiling
  show cache     : show cache status
  trace <module> [cmd [args…]] : manage live tracing
  show trace [
] : show live tracing state
 

2. View haproxy running threads

root@pcfreak:/home/hipo/info# echo "show threads" | socat stdio /var/run/haproxy/haproxy.sock
  Thread 1 : id=0x7f87b6e2c1c0 act=0 glob=0 wq=1 rq=0 tl=0 tlsz=0 rqsz=0
             stuck=0 prof=0 harmless=1 wantrdv=0
             cpu_ns: poll=3061065069437 now=3061065077880 diff=8443
             curr_task=0
* Thread 2 : id=0x7f87b6e20700 act=1 glob=0 wq=1 rq=0 tl=0 tlsz=0 rqsz=0
             stuck=0 prof=0 harmless=0 wantrdv=0
             cpu_ns: poll=2969050092523 now=2969050197848 diff=105325
             curr_task=0x7f87b006f740 (task) calls=1 last=0
               fct=0x560978846340(task_run_applet) ctx=0x7f87b0190720(<CLI>)
             strm=0x56097a763560 src=unix fe=GLOBAL be=GLOBAL dst=<CLI>
             rqf=c48200 rqa=0 rpf=80008000 rpa=0 sif=EST,200008 sib=EST,204018
             af=(nil),0 csf=0x56097a776ef0,8200
             ab=0x7f87b0190720,9 csb=(nil),0
             cof=0x56097a77fb00,1300:PASS(0x7f87b019a680)/RAW((nil))/unix_stream(22)
             cob=(nil),0:NONE((nil))/NONE((nil))/NONE(0)

3. Show haproxy server connections

root@pcfreak:/home/hipo/info# echo "show servers conn" | socat stdio /var/run/haproxy/haproxy.sock
# bkname/svname bkid/svid addr port – purge_delay used_cur used_max need_est unsafe_nb safe_nb idle_lim idle_cur idle_per_thr[2]
http-websrv/ha1server-1 3/1 192.168.0.209 80 – 5000 0 12 12 0 0 -1 0 0 0
http-websrv/ha1server-2 3/2 192.168.0.200 80 – 5000 1 142 142 0 0 -1 0 0 0
http-websrv/ha1server-3 3/3 192.168.1.30 80 – 5000 0 0 0 0 0 -1 0 0 0
http-websrv/ha1server-4 3/4 192.168.1.14 80 – 5000 0 0 0 0 0 -1 0 0 0
http-websrv/ha1server-5 3/5 192.168.0.1 80 – 5000 0 13 13 0 0 -1 0 0 0
https-websrv/ha1server-1 5/1 192.168.0.209 443 – 5000 0 59 59 0 0 -1 0 0 0
https-websrv/ha1server-2 5/2 192.168.0.200 443 – 5000 11 461 461 0 0 -1 0 0 0
https-websrv/ha1server-3 5/3 192.168.1.30 443 – 5000 0 0 0 0 0 -1 0 0 0
https-websrv/ha1server-4 5/4 192.168.1.14 443 – 5000 0 0 0 0 0 -1 0 0 0
https-websrv/ha1server-5 5/5 192.168.0.1 443 – 5000 1 152 152 0 0 -1 0 0 0
MASTER/cur-1 6/1 – 0 – 0 0 0 0 0 0 0 0

4. Show Load balancer servers state

root@pcfreak:/home/hipo/info# echo "show servers state" | socat stdio /var/run/haproxy/haproxy.sock
1
# be_id be_name srv_id srv_name srv_addr srv_op_state srv_admin_state srv_uweight srv_iweight srv_time_since_last_change srv_check_status srv_check_result srv_check_health srv_check_state srv_agent_state bk_f_forced_id srv_f_forced_id srv_fqdn srv_port srvrecord
3 http-websrv 1 ha1server-1 192.168.0.209 2 0 254 254 3929 6 3 4 6 0 0 0 – 80 –
3 http-websrv 2 ha1server-2 192.168.0.200 2 0 255 255 3928 6 3 4 6 0 0 0 – 80 –
3 http-websrv 3 ha1server-3 192.168.1.30 2 0 252 252 3927 6 3 4 6 0 0 0 – 80 –
3 http-websrv 4 ha1server-4 192.168.1.14 2 0 253 253 3929 6 3 4 6 0 0 0 – 80 –
3 http-websrv 5 ha1server-5 192.168.0.1 2 0 251 251 1708087 6 3 4 6 0 0 0 – 80 –
5 https-websrv 1 ha1server-1 192.168.0.209 2 0 254 254 3929 6 3 4 6 0 0 0 – 443 –
5 https-websrv 2 ha1server-2 192.168.0.200 2 0 255 255 3928 6 3 4 6 0 0 0 – 443 –
5 https-websrv 3 ha1server-3 192.168.1.30 2 0 252 252 3927 6 3 4 6 0 0 0 – 443 –
5 https-websrv 4 ha1server-4 192.168.1.14 2 0 253 253 3929 6 3 4 6 0 0 0 – 443 –
5 https-websrv 5 ha1server-5 192.168.0.1 2 0 251 251 1708087 6 3 4 6 0 0 0 – 443 –
6 MASTER 1 cur-1 – 2 0 0 0 1708087 1 0 0 0 0 0 0 – 0 –

5. Get general haproxy info on variables that can be used for Load Balancer fine tuning

root@pcfreak:/home/hipo/info# echo "show info" | socat stdio /var/run/haproxy/haproxy.sock
Name: HAProxy
Version: 2.2.9-2+deb11u5
Release_date: 2023/04/10
Nbthread: 2
Nbproc: 1
Process_num: 1
Pid: 3103635
Uptime: 19d 18h11m49s
Uptime_sec: 1707109
Memmax_MB: 0
PoolAlloc_MB: 1
PoolUsed_MB: 0
PoolFailed: 0
Ulimit-n: 200059
Maxsock: 200059
Maxconn: 99999
Hard_maxconn: 99999
CurrConns: 8
CumConns: 19677218
CumReq: 2740072
MaxSslConns: 0
CurrSslConns: 0
CumSslConns: 0
Maxpipes: 0
PipesUsed: 0
PipesFree: 0
ConnRate: 1
ConnRateLimit: 0
MaxConnRate: 2161
SessRate: 1
SessRateLimit: 0
MaxSessRate: 2161
SslRate: 0
SslRateLimit: 0
MaxSslRate: 0
SslFrontendKeyRate: 0
SslFrontendMaxKeyRate: 0
SslFrontendSessionReuse_pct: 0
SslBackendKeyRate: 0
SslBackendMaxKeyRate: 0
SslCacheLookups: 0
SslCacheMisses: 0
CompressBpsIn: 0
CompressBpsOut: 0
CompressBpsRateLim: 0
ZlibMemUsage: 0
MaxZlibMemUsage: 0
Tasks: 32
Run_queue: 1
Idle_pct: 100
node: pcfreak
Stopping: 0
Jobs: 13
Unstoppable Jobs: 0
Listeners: 4
ActivePeers: 0
ConnectedPeers: 0
DroppedLogs: 0
BusyPolling: 0
FailedResolutions: 0
TotalBytesOut: 744390344175
BytesOutRate: 30080
DebugCommandsIssued: 0
Build info: 2.2.9-2+deb11u5
 

root@pcfreak:/home/hipo/info# echo "show errors" | socat stdio /var/run/haproxy/haproxy.sock
Total events captured on [14/Dec/2023:17:29:17.930] : 0

6. View all opened sessions and, the session age (time since it has been opened) and session exp (expiry)

root@pcfreak:/home/hipo/info# echo "show sess" | socat stdio /var/run/haproxy/haproxy.sock
0x56097a763560: proto=tcpv4 src=113.120.74.123:54651 fe=https-in be=https-websrv srv=ha1server-2 ts=00 age=37s calls=3 rate=0 cpu=0 lat=0 rq[f=848000h,i=0,an=00h,rx=1m58s,wx=,ax=] rp[f=80048202h,i=0,an=00h,rx=,wx=1m58s,ax=] s0=[8,200000h,fd=24,ex=] s1=[8,40018h,fd=25,ex=] exp=1m51s
0x56097a812830: proto=tcpv4 src=190.216.236.134:35526 fe=https-in be=https-websrv srv=ha1server-2 ts=00 age=17s calls=3 rate=0 cpu=0 lat=0 rq[f=848202h,i=0,an=00h,rx=1m42s,wx=,ax=] rp[f=80048202h,i=0,an=00h,rx=1m42s,wx=,ax=] s0=[8,200008h,fd=40,ex=] s1=[8,200018h,fd=41,ex=] exp=12s
0x56097a784ad0: proto=tcpv4 src=103.225.203.131:33835 fe=https-in be=https-websrv srv=ha1server-2 ts=00 age=17s calls=2 rate=0 cpu=0 lat=0 rq[f=848202h,i=0,an=00h,rx=1m44s,wx=,ax=] rp[f=80048202h,i=0,an=00h,rx=1m44s,wx=,ax=] s0=[8,200008h,fd=20,ex=] s1=[8,200018h,fd=21,ex=] exp=13s
0x7f87b0082cc0: proto=tcpv4 src=190.216.236.134:35528 fe=https-in be=https-websrv srv=ha1server-2 ts=00 age=14s calls=3 rate=0 cpu=0 lat=0 rq[f=848202h,i=0,an=00h,rx=1m46s,wx=,ax=] rp[f=80048202h,i=0,an=00h,rx=1m46s,wx=,ax=] s0=[8,200008h,fd=34,ex=] s1=[8,200018h,fd=35,ex=] exp=15s
0x7f87b0089e10: proto=tcpv4 src=40.130.105.242:50669 fe=https-in be=https-websrv srv=ha1server-2 ts=00 age=11s calls=2 rate=0 cpu=0 lat=0 rq[f=848202h,i=0,an=00h,rx=1m49s,wx=,ax=] rp[f=80048202h,i=0,an=00h,rx=1m49s,wx=,ax=] s0=[8,200008h,fd=15,ex=] s1=[8,200018h,fd=16,ex=] exp=18s
0x7f87b010b450: proto=tcpv4 src=64.62.202.82:37562 fe=https-in be=https-websrv srv=ha1server-2 ts=00 age=7s calls=2 rate=0 cpu=0 lat=0 rq[f=848202h,i=0,an=00h,rx=1m52s,wx=,ax=] rp[f=80048202h,i=0,an=00h,rx=1m52s,wx=,ax=] s0=[8,200008h,fd=26,ex=] s1=[8,200018h,fd=27,ex=] exp=22s
0x56097a7b8bc0: proto=tcpv4 src=85.208.96.211:54226 fe=https-in be=https-websrv srv=ha1server-2 ts=00 age=0s calls=2 rate=2 cpu=0 lat=0 rq[f=848202h,i=0,an=00h,rx=1m59s,wx=,ax=] rp[f=80048202h,i=0,an=00h,rx=1m59s,wx=,ax=] s0=[8,200008h,fd=22,ex=] s1=[8,200018h,fd=23,ex=] exp=29s
0x7f87b008ec00: proto=tcpv4 src=3.135.192.206:60258 fe=http-in be=http-websrv srv=ha1server-2 ts=00 age=0s calls=2 rate=2 cpu=0 lat=0 rq[f=848000h,i=0,an=00h,rx=1m59s,wx=1m59s,ax=] rp[f=80008000h,i=0,an=00h,rx=1m59s,wx=1m59s,ax=] s0=[8,200008h,fd=28,ex=] s1=[8,200018h,fd=29,ex=] exp=29s
0x56097a7b2490: proto=tcpv4 src=45.147.249.119:62283 fe=https-in be=https-websrv srv=ha1server-2 ts=00 age=0s calls=3 rate=3 cpu=0 lat=0 rq[f=848202h,i=0,an=00h,rx=1m59s,wx=,ax=] rp[f=80048202h,i=0,an=00h,rx=1m59s,wx=,ax=] s0=[8,200008h,fd=17,ex=] s1=[8,200018h,fd=18,ex=] exp=29s
0x7f87b0114f90: proto=unix_stream src=unix:1 fe=GLOBAL be=<NONE> srv=<none> ts=00 age=0s calls=1 rate=1 cpu=0 lat=0 rq[f=c48200h,i=0,an=00h,rx=,wx=,ax=] rp[f=80008002h,i=0,an=00h,rx=,wx=,ax=] s0=[8,200008h,fd=30,ex=] s1=[8,204018h,fd=-1,ex=] exp=

root@pcfreak:/home/hipo/info#

7. Disabling an haproxy frontend via UNIX socket

If you get some frontend that gets broken and this is monitored in Zabbix or other monitoring tool used to monitor you can use the haproxy stats interface to disable frontend

root@pcfreak:/home/hipo/info# echo "disable frontend https-websrv" | socat stdio /var/run/haproxy/haproxy.sock

8. Show general haproxy statistics (could tell you much about customer connections health state) and state of connection to backend

Lets check uptime details for frontends / backends, that is done with show stat command.

root@pcfreak:/home/hipo/info# echo "show stat" | socat stdio /var/run/haproxy/haproxy.sock
#

 

pxname,svname,qcur,qmax,scur,smax,slim,stot,bin,bout,dreq,dresp,ereq,econ,eresp

,wretr,wredis,status,weight,act,bck,chkfail,chkdown,lastchg,downtime,qlimit,
pid,iid,sid,throttle,lbtot,tracked,type,rate,rate_lim,rate_max,check_status

,check_code,check_duration,hrsp_1xx,hrsp_2xx,hrsp_3xx,hrsp_4xx

,hrsp_5xx,hrsp_other,hanafail,req_rate,req_rate_max,req_tot,cli_abrt

,srv_abrt,comp_in,comp_out,comp_byp,comp_rsp,lastsess,last_chk

,last_agt,qtime,ctime,rtime,ttime,agent_status,agent_code,

agent_duration,check_desc,agent_desc,check_rise,

check_fall,check_health,agent_rise,

agent_fall,agent_health,addr,cookie,mode,

algo,conn_rate,conn_rate_max,conn_tot,intercepted

,dcon,dses,wrew,connect,reuse,cache_lookups,

cache_hits,srv_icur,src_ilim,qtime_max,ctime_max,

rtime_max,ttime_max,eint,idle_conn_cur,

safe_conn_cur,used_conn_cur,need_conn_est,

    http-in,FRONTEND,,,0,142,99999,371655,166897324,

1462777381,0,0,62,,,,,OPEN,,,,,,,,,1,2,0,,,,0,0,0,

1080,,,,,,,,,,,0,0,0,,,0,0,0,0,,,,,,,,,,,,,,,,,,,,,tcp,,0,1080,

371655,,0,0,0,,,,,,,,,,,0,,,,,

    http-websrv,ha1server-1,0,0,0,12,,9635,3893561

,64880833,,0,,0,3,15,0,UP

,254,0,1,41,9,4686,34728,,1,3,1,,4924,,2,0,,56,L4OK

,,0,,,,,,,,,,,900,168,,,,,1292679,,,0,0,0,2843,,,,

Layer4 check passed,,2,3,4,,,,192.168.0.209:80,,tcp,,,,,,,,

0,9635,0,,,0,,0,15024,0,672888,0,0,0,0,12,

    http-websrv,ha1server-2,0,0,0,142,,321867,

149300590,1350577153,,0,,

1,4,30,0,UP,255,1,0,37,10,4685,89418,,1,3,2,,111864,,2

,0,,1080,L4OK,,0,,,,,,,,,,,37161,4822,,,,,6,,,0,12,0,

2120,,,,Layer4 check passed,,2,3,4,,,,192.168.0.200:80,,tcp,,,,,,,,0,321867,

0,,,0,,0,30223,0,1783442,0,0,0,0,142,

List continues here
….

..
.

 

 

 

 

9. Using netcat to view UNIX socket instead of socat

If you don't have the socat command on the server but you have netcat installed, you can also send the commands to the running haproxy daemon via nc's capability to send via UNIX socket via nc -U option.

   -U      Use UNIX-domain sockets.  Cannot be used together with -F or -x.

 

root@pcfreak:/home/hipo/info# echo "set server"|nc -U /var/run/haproxy/haproxy.sock
Require 'backend/server'.

10. Get only statistics about running LB Backends and Frontends

To get only haproxy statistics about running Load Balancer BACKENDs and FRONTENDs

root@pcfreak:/home/hipo/info# echo "show stat" | sudo socat unix-connect:/var/run/haproxy/haproxy.sock stdio | awk -F '.' '/BACKEND/ {print $1, $6}'
http-websrv,BACKEND,0,0,2,142,10000,371880,167022255,1462985601,0,0,,1,7,46,0,UP

,255,1,4,,0,1709835,0,,1,3,0,,118878,,1,0,,1080,,,,,,,,,,,,,,38782,5001,0,0,0,0,5,,,0,8,0,2034

,,,,,,,,,,,,,,tcp,source,,,,,,,0,371864,0,,,,,0,30223,0,1783442,0,,,,,
https-websrv,BACKEND,0,0,5,461,10000,2374328,3083873321,740021649129,0,0,,28,42,626,0,UP
,255,1,4,,0,1709835,0,,1,5,0,,474550,,1,1,,1081,,,,,,,,,,,,,,451783,72307,0,0,0,0,0,,,0,0,0,6651

,,,,,,,,,,,,,,tcp,source,,,,,,,0,2374837,0,,,,,0,32794,0,46414141,0,,,,,

As you can see there are two configured BACKENDs that are in UP state, the other possibility is that they're DOWN if haproxy can't reach the backend.

root@pcfreak:/home/hipo/info# echo "show stat" | sudo socat unix-connect:/var/run/haproxy/haproxy.sock stdio | awk -F '.' '/FRONTEND/ {print $1, $6}'
http-in,FRONTEND,,,2,142,99999,371887,167024040,1462990718,0,0,62,,,,,OPEN

,,,,,,,,,1,2,0,,,,0,1,0,1080,,,,,,,,,,,0,0,0,,,0,0,0,0,,,,,,,,,,,,,,,,,,,,,tcp,,1,1080,371887,,0,0,0,,,,,,,,,,,0,,,,,
https-in,FRONTEND,,,4,461,99999,2374337,3083881912,740021909870,0,0,112,,,,,OPEN

,,,,,,,,,1,4,0,,,,0,1,0,1081,,,,,,,,,,,0,0,0,,,0,0,0,0,,,,,,,,,,,,,,,,,,,,,tcp,,1,1081,2374337,,0,0,0,,,,,,,,,,,0,,,,,
root@pcfreak:/home/hipo/info#

As you can see from the list of show help you can change maxconns supported, change the proxy rate-limit and even in real time change a haproxy.cfg configured section timeouts or even modify ACLs dynamicly for Backends and Frontends.

If you use those to make a modifications to the haproxy, that modifications should been written also to Haproxy's configured instance haproxy.cfg file.
If you want to check it reload the haproxy instance with the new written haproxy.cfg, through the Unix socket.

11. Shutting down specific opened sessions

Shutting down specific session that has been opened for too long is particularly useful to do, especially if you have some kind of VPN encryption device before the Haproxy server and an Application Backend server that is buggy and fails to properly close sessions at time, to cut off a specific sessions that has been hanging for days after reviewing it with "show sess".

root@pcfreak:/home/hipo/info# echo "shutdown session 0x56097a7707d0" | socat stdio /var/run/haproxy/haproxy.sock

12. Sending shutdown to backend on a certain configured LB service


To bring down a configured backend on a certain server after listing it:
 

root@pcfreak:/home/hipo/info# echo "disable server bk_mybackend/srv_myserver" | socat /var/run/haproxy.sock stdio


12. Sending multiple commands to haproxy socket

# echo "show info;show stat" | socat /var/run/haproxy/haproxy.sock stdio

 

13. Report table usage information or dump table data content


It is possible to view exact queued connections inside the sticky table. To get a list of available, available configured tables on the haproxy

root@pcfreak:/home/hipo/info# echo "show table" | socat /var/run/haproxy/haproxy.sock stdio
# table: https-websrv, type: ip, size:204800, used:498
# table: http-websrv, type: ip, size:204800, used:74


To get the exact record of queued IPs inside https-websrv.

root@pcfreak:/home/hipo/info# echo "show table https-websrv" | socat /var/run/haproxy/haproxy.sock stdio|head -10
# table: https-websrv, type: ip, size:204800, used:502
0x56097a7444e0: key=2.147.73.42 use=0 exp=1090876 server_id=2 server_name=ha1server-2
0x56097a792ac0: key=3.14.130.119 use=0 exp=1038004 server_id=2 server_name=ha1server-2
0x7f87b006a4e0: key=3.15.203.28 use=0 exp=1536721 server_id=2 server_name=ha1server-2
0x56097a7467f0: key=3.16.54.132 use=0 exp=387191 server_id=2 server_name=ha1server-2
0x7f87b0075f90: key=3.17.180.28 use=0 exp=353211 server_id=2 server_name=ha1server-2
0x56097a821b10: key=3.23.114.130 use=0 exp=1521100 server_id=2 server_name=ha1server-2
0x56097a7475b0: key=3.129.250.144 use=0 exp=121043 server_id=2 server_name=ha1server-2
0x7f87b004d240: key=3.134.112.27 use=0 exp=1182169 server_id=2 server_name=ha1server-2
0x56097a754c90: key=3.135.192.206 use=0 exp=1383882 server_id=2 server_name=ha1server-2

14. Show information about Haproxy startup

Sometimes, where logrotation is integrated on the server and haproxy's logs are log rotated to a central logging server, it might be hard to get information about Haproxy startup messages (warnings, errors etc.).
As digging through old haproxy logs might be tedious, you can simply get it via the stats interface.

root@pcfreak:/home/hipo/info# echo "show startup-logs" | socat unix-connect:/var/run/haproxy/haproxy.sock stdio              

[WARNING] 327/231534 (3103633) : parsing [/etc/haproxy/haproxy.cfg:62] : 'fullconn' ignored because frontend 'http-in' has no backend capability. Maybe you want 'maxconn' instead ?
[WARNING] 327/231534 (3103633) : parsing [/etc/haproxy/haproxy.cfg:69] : 'maxconn' ignored because backend 'http-websrv' has no frontend capability. Maybe you want 'fullconn' instead ?
[WARNING] 327/231534 (3103633) : parsing [/etc/haproxy/haproxy.cfg:114] : 'maxconn' ignored because backend 'https-websrv' has no frontend capability. Maybe you want 'fullconn' instead ?
[WARNING] 327/231534 (3103633) : config : missing timeouts for frontend 'http-in'.
   | While not properly invalid, you will certainly encounter various problems
   | with such a configuration. To fix this, please ensure that all following
   | timeouts are set to a non-zero value: 'client', 'connect', 'server'.
[WARNING] 327/231534 (3103633) : config : 'option forwardfor' ignored for frontend 'http-in' as it requires HTTP mode.
[WARNING] 327/231534 (3103633) : config : 'option forwardfor' ignored for backend 'http-websrv' as it requires HTTP mode.
[WARNING] 327/231534 (3103633) : config : missing timeouts for frontend 'https-in'.
   | While not properly invalid, you will certainly encounter various problems
   | with such a configuration. To fix this, please ensure that all following
   | timeouts are set to a non-zero value: 'client', 'connect', 'server'.
[WARNING] 327/231534 (3103633) : config : 'option forwardfor' ignored for frontend 'https-in' as it requires HTTP mode.
[WARNING] 327/231534 (3103633) : config : 'option forwardfor' ignored for backend 'https-websrv' as it requires HTTP mode.

15. Disable / Enable health check for haproxy configured backend

 Disable health checks is useful, especially on non production server environments, during integration phase of application with Haproxy load balancer.

The general syntax is like this:

> disable health backend/server1

 

root@pcfreak:/home/hipo/info# echo "show servers state" | socat unix-connect:/var/run/haproxy/haproxy.sock stdio             1
# be_id be_name srv_id srv_name srv_addr srv_op_state srv_admin_state srv_uweight srv_iweight srv_time_since_last_change srv_check_status srv_check_result srv_check_health srv_check_state srv_agent_state bk_f_forced_id srv_f_forced_id srv_fqdn srv_port srvrecord
3 http-websrv 1 ha1server-1 192.168.0.209 2 0 254 254 13709 6 3 4 6 0 0 0 – 80 –
3 http-websrv 2 ha1server-2 192.168.0.200 2 0 255 255 13708 6 3 4 6 0 0 0 – 80 –
3 http-websrv 3 ha1server-3 192.168.1.30 2 0 252 252 13707 6 3 4 6 0 0 0 – 80 –
3 http-websrv 4 ha1server-4 192.168.1.14 2 0 253 253 13709 6 3 4 6 0 0 0 – 80 –
3 http-websrv 5 ha1server-5 192.168.0.1 2 0 251 251 1717867 6 3 4 6 0 0 0 – 80 –
5 https-websrv 1 ha1server-1 192.168.0.209 2 0 254 254 13709 6 3 4 6 0 0 0 – 443 –
5 https-websrv 2 ha1server-2 192.168.0.200 2 0 255 255 13708 6 3 4 6 0 0 0 – 443 –
5 https-websrv 3 ha1server-3 192.168.1.30 2 0 252 252 13707 6 3 4 6 0 0 0 – 443 –
5 https-websrv 4 ha1server-4 192.168.1.14 2 0 253 253 13709 6 3 4 6 0 0 0 – 443 –
5 https-websrv 5 ha1server-5 192.168.0.1 2 0 251 251 1717867 6 3 4 6 0 0 0 – 443 –
6 MASTER 1 cur-1 – 2 0 0 0 1717867 1 0 0 0 0 0 0 – 0 –

 

Lets disable health checks for ha1server-1 server and http-websrv backend.

root@pcfreak:/home/hipo/info# echo "disable health http-websrv/ha1server-1" | socat unix-connect:/var/run/haproxy/haproxy.sock stdio

 

To enable back health checks 

root@pcfreak:/home/hipo/info# echo "enable health http-websrv/ha1server-1" | socat unix-connect:/var/run/haproxy/haproxy.sock stdio

16. Change weight for server

if you have a round-robin Load balancing configured and already have a predefined configuration on how many percentage of the server to be sent to which application server (e.g. have a configured weight to dynamically change it via UNIX sock iface).

# Change weight by percentage of its original value

# socat unix-connect:/var/run/haproxy/haproxy.sock stdio


> set server be_app/webserv1 weight 50%
 
# Change weight in proportion to other servers
> set server be_app/webserv1 weight 100

 

root@pcfreak:/home/hipo/info#  socat unix-connect:/var/run/haproxy/haproxy.sock stdio                                        
set server http-websrv/ha1server-1 weight 50%
Backend is using a static LB algorithm and only accepts weights '0%' and '100%'.

17. Draining traffic from server / backend App in case of Maintenance

You can gradually drain traffic away from a particular server if those backend Application server should be put in maintenance mode for update or whatever. The drain option is very interesting and combined with scripting does open a lot of possibilities for the Load balancer system administrator to put an extra automation.

To drain, set server command with the state argument set to drain:
 

# Drain traffic
> set server backend_app/server1 state drain

# Allow server to accept traffic again
> set server backend_app/server1 state ready

 


root@pcfreak:/home/hipo/info#  socat unix-connect:/var/run/haproxy/haproxy.sock stdio
set server http-websrv/ha1server-1 state drain

 

root@pcfreak:/home/hipo/info#  socat unix-connect:/var/run/haproxy/haproxy.sock stdio
set server http-websrv/ha1server-1 state ready

18. Run Interactive Mode connection to haproxy UNIX stats socket

For a haproxies that has multiple configured proxied rules backends / frontends, it is nice to use the interactive mode.
Instead of processing a single line of semicolon separate commands, HAProxy takes one command at a time and waits for the user.
In interactive mode, HAProxy sends a “>” character and waits for input command. After command is submitted, HAProxy sends back the result and waits for a new command.
The interactive mode is especially useful during phase of integrating a new haproxy towards an application, where multiple things has to be tuned on the fly without, reloading the haproxy again and again.

On RPM based distros socat is compiled to have the readline interactive capability. Thus to use the haproxy haproxy stats connect interactive mode on RHEL / CentOS / Fedora and other RPM based distros simply use:

# socat /var/run/haproxy.sock readline
> show info
Name: HAProxy
Version: 2.2.9-2+deb11u5
Release_date: 2023/04/10
Nbthread: 2
Nbproc: 1
Process_num: 1
Pid: 3103635
Uptime: 19d 20h48m50s
Uptime_sec: 1716530
Memmax_MB: 0
PoolAlloc_MB: 1
PoolUsed_MB: 0
PoolFailed: 0
Ulimit-n: 200059
Maxsock: 200059
Maxconn: 99999
Hard_maxconn: 99999
CurrConns: 9
CumConns: 19789176
CumReq: 2757976
MaxSslConns: 0
CurrSslConns: 0
CumSslConns: 0
Maxpipes: 0
PipesUsed: 0
PipesFree: 0
ConnRate: 0
ConnRateLimit: 0
MaxConnRate: 2161
SessRate: 0
SessRateLimit: 0
MaxSessRate: 2161
SslRate: 0
SslRateLimit: 0
MaxSslRate: 0
SslFrontendKeyRate: 0
SslFrontendMaxKeyRate: 0
SslFrontendSessionReuse_pct: 0
SslBackendKeyRate: 0
SslBackendMaxKeyRate: 0
SslCacheLookups: 0
SslCacheMisses: 0
CompressBpsIn: 0
CompressBpsOut: 0
CompressBpsRateLim: 0
ZlibMemUsage: 0
MaxZlibMemUsage: 0
Tasks: 35
Run_queue: 1
Idle_pct: 100
node: pcfreak
Stopping: 0
Jobs: 14
Unstoppable Jobs: 0
Listeners: 4
ActivePeers: 0
ConnectedPeers: 0
DroppedLogs: 0
BusyPolling: 0
FailedResolutions: 0
TotalBytesOut: 744964070459
BytesOutRate: 0
DebugCommandsIssued: 0
Build info: 2.2.9-2+deb11u5

On Deb (Debian) based distributions such as Debian, Ubuntu Mint Linux, unfortunately the readline inractive mode is disabled due to licensing issues that makes readline not GPL license compliant.

root@pcfreak:/home/hipo/info# socat -V|awk 'NR < 5 || tolower($0) ~ /readline/'
socat by Gerhard Rieger and contributors – see www.dest-unreach.org
socat version 1.7.4.1 on Feb  3 2021 12:58:17
   running on Linux version #1 SMP Debian 5.10.179-3 (2023-07-27), release 5.10.0-23-amd64, machine x86_64
features:
  #undef WITH_READLINE

There is a workaround to emulate the Intearactive mode on Debians however like this:

root@pcfreak:/home/hipo/info# while [ 1 ]; do socat – /var/run/haproxy/haproxy.sock ; done

show table
# table: https-websrv, type: ip, size:204800, used:511
# table: http-websrv, type: ip, size:204800, used:67

show sess
0x56097a784ad0: proto=tcpv4 src=45.61.161.66:51416 fe=https-in be=https-websrv srv=ha1server-2 ts=00 age=1m13s calls=3 rate=0 cpu=0 lat=0 rq[f=848000h,i=0,an=00h,rx=47s,wx=,ax=] rp[f=80048000h,i=0,an=00h,rx=47s,wx=,ax=] s0=[8,200008h,fd=17,ex=] s1=[8,200018h,fd=23,ex=] exp=47s
0x56097a7707d0: proto=tcpv4 src=47.128.41.242:39372 fe=https-in be=https-websrv srv=ha1server-2 ts=00 age=16s calls=2 rate=0 cpu=0 lat=0 rq[f=848202h,i=0,an=00h,rx=1m45s,wx=,ax=] rp[f=80048202h,i=0,an=00h,rx=1m45s,wx=,ax=] s0=[8,200008h,fd=35,ex=] s1=[8,200018h,fd=36,ex=] exp=14s
0x56097a781300: proto=tcpv4 src=54.36.148.40:17439 fe=https-in be=https-websrv srv=ha1server-2 ts=00 age=13s calls=2 rate=0 cpu=0 lat=0 rq[f=848202h,i=0,an=00h,rx=1m47s,wx=,ax=] rp[f=80048202h,i=0,an=00h,rx=1m47s,wx=,ax=] s0=[8,200008h,fd=26,ex=] s1=[8,200018h,fd=28,ex=] exp=17s
0x56097a7fca80: proto=tcpv4 src=18.217.94.243:4940 fe=https-in be=https-websrv srv=ha1server-2 ts=00 age=7s calls=2 rate=0 cpu=0 lat=0 rq[f=848202h,i=0,an=00h,rx=1m53s,wx=,ax=] rp[f=80048202h,i=0,an=00h,rx=1m53s,wx=,ax=] s0=[8,200008h,fd=21,ex=] s1=[8,200018h,fd=22,ex=] exp=23s
0x7f87b00778c0: proto=tcpv4 src=85.208.96.206:51708 fe=https-in be=https-websrv srv=ha1server-2 ts=00 age=4s calls=3 rate=0 cpu=0 lat=0 rq[f=848202h,i=0,an=00h,rx=1m56s,wx=,ax=] rp[f=80048202h,i=0,an=00h,rx=1m56s,wx=,ax=] s0=[8,200008h,fd=20,ex=] s1=[8,200018h,fd=24,ex=] exp=26s
0x56097a80c1e0: proto=unix_stream src=unix:1 fe=GLOBAL be=<NONE> srv=<none> ts=00 age=3s calls=1 rate=0 cpu=0 lat=0 rq[f=c48202h,i=0,an=00h,rx=10s,wx=,ax=] rp[f=80008002h,i=0,an=00h,rx=,wx=,ax=] s0=[8,200008h,fd=15,ex=] s1=[8,204018h,fd=-1,ex=] exp=7s


To end the eternal loop press CTRL + z and kill first detached job %1 run:

# kiill %1


Sum it up what learned

What we learned in this article is how to use socat and netcat to connect and manage dynamically haproxy via its haproxy stats interface, without reloading the proxqy itself. We learned how to view various statistics and information on the proxy, its existing tables, caches, session information (such as age, and expiry). Also you've seen how to disable / enable configured backends as well as get available backends and frontends and their state.
You've seen how the drained option could be used to slowly drain connections towards configured backend, in case if you need to a maintenance on a backend node.
Also was pointed how to shutdown a specific long lived sessions that has been hanging and creating troubles towards app backends.

Finally, you've seen how to open an interactive connection towards the haproxy socket and send commands in a raw with socat (on distros where compiled with readline support) as well shown how to emulate the interactive mode of rest of distros whose socat is missing the readline support. 

Configure aide file integrity check server monitoring in Zabbix to track for file changes on servers

Tuesday, March 28th, 2023

zabbix-aide-log-monitoring-logo

Earlier I've written a small article on how to setup AIDE monitoring for Server File integrity check on Linux, which put the basics on how this handy software to improve your server overall Security software can be installed and setup without much hassle.

Once AIDE is setup and a preset custom configuration is prepared for AIDE it is pretty useful to configure AIDE to monitor its critical file changes for better server security by monitoring the AIDE log output for new record occurs with Zabbix. Usually if no files monitored by AIDE are modified on the machine, the log size will not grow, but if some file is modified once Advanced Linux Intrusion Detecting (aide) binary runs via the scheduled Cron job, the /var/log/app_aide.log file will grow zabbix-agentd will continuously check the file for size increases and will react.

Before setting up the Zabbix required Template, you will have to set few small scripts that will be reading a preconfigured list of binaries or application files etc. that aide will monitor lets say via /etc/aide-custom.conf
 

1. Configure aide to monitor files for changes


Before running aide, it is a good idea to prepare a file with custom defined directories and files that you plan to monitor for integrity checking e.g. future changes with aide, for example to capture bad intruders who breaks into server which runs aide and modifies critical files such as /etc/passwd /etc/shadow /etc/group or / /usr/local/etc/* or /var/* / /usr/* critical files that shouldn't be allowed to change without the admin to soon find out.

# cat /etc/aide-custom.conf

# Example configuration file for AIDE.
@@define DBDIR /var/lib/aide
@@define LOGDIR /var/log/aide
# The location of the database to be read.
database=file:@@{DBDIR}/app_custom.db.gz
database_out=file:@@{DBDIR}/app_aide.db.new.gz
gzip_dbout=yes
verbose=5

report_url=file:@@{LOGDIR}/app_custom.log
#report_url=syslog:LOG_LOCAL2
#report_url=stderr
#NOT IMPLEMENTED report_url=mailto:root@foo.com
#NOT IMPLEMENTED report_url=syslog:LOG_AUTH

# These are the default rules.
#
#p:      permissions
#i:      inode:
#n:      number of links
#u:      user
#g:      group
#s:      size
#b:      block count
#m:      mtime
#a:      atime
#c:      ctime
#S:      check for growing size
#acl:           Access Control Lists
#selinux        SELinux security context
#xattrs:        Extended file attributes
#md5:    md5 checksum
#sha1:   sha1 checksum
#sha256:        sha256 checksum
#sha512:        sha512 checksum
#rmd160: rmd160 checksum
#tiger:  tiger checksum

#haval:  haval checksum (MHASH only)
#gost:   gost checksum (MHASH only)
#crc32:  crc32 checksum (MHASH only)
#whirlpool:     whirlpool checksum (MHASH only)

FIPSR = p+i+n+u+g+s+m+c+acl+selinux+xattrs+sha256

#R:             p+i+n+u+g+s+m+c+acl+selinux+xattrs+md5
#L:             p+i+n+u+g+acl+selinux+xattrs
#E:             Empty group
#>:             Growing logfile p+u+g+i+n+S+acl+selinux+xattrs

# You can create custom rules like this.
# With MHASH…
# ALLXTRAHASHES = sha1+rmd160+sha256+sha512+whirlpool+tiger+haval+gost+crc32
ALLXTRAHASHES = sha1+rmd160+sha256+sha512+tiger
# Everything but access time (Ie. all changes)
EVERYTHING = R+ALLXTRAHASHES

# Sane, with multiple hashes
# NORMAL = R+rmd160+sha256+whirlpool
NORMAL = FIPSR+sha512

# For directories, don't bother doing hashes
DIR = p+i+n+u+g+acl+selinux+xattrs

# Access control only
PERMS = p+i+u+g+acl+selinux

# Logfile are special, in that they often change
LOG = >

# Just do sha256 and sha512 hashes
LSPP = FIPSR+sha512

# Some files get updated automatically, so the inode/ctime/mtime change
# but we want to know when the data inside them changes
DATAONLY =  p+n+u+g+s+acl+selinux+xattrs+sha256

##############TOUPDATE
#To delegate to app team create a file like /app/aide.conf
#and uncomment the following line
#@@include /app/aide.conf
#Then remove all the following lines
/etc/zabbix/scripts/check.sh FIPSR
/etc/zabbix/zabbix_agentd.conf FIPSR
/etc/sudoers FIPSR
/etc/hosts FIPSR
/etc/keepalived/keepalived.conf FIPSR
# monitor haproxy.cfg
/etc/haproxy/haproxy.cfg FIPSR
# monitor keepalived
/home/keepalived/.ssh/id_rsa FIPSR
/home/keepalived/.ssh/id_rsa.pub FIPSR
/home/keepalived/.ssh/authorized_keys FIPSR

/usr/local/bin/script_to_run.sh FIPSR
/usr/local/bin/another_script_to_monitor_for_changes FIPSR

#  cat /usr/local/bin/aide-config-check.sh
#!/bin/bash
/sbin/aide -c /etc/aide-custom.conf -D

# cat /usr/local/bin/aide-init.sh
#!/bin/bash
/sbin/aide -c /etc/custom-aide.conf -B database_out=file:/var/lib/aide/custom-aide.db.gz -i

 

# cat /usr/local/bin/aide-check.sh

#!/bin/bash
/sbin/aide -c /etc/custom-aide.conf -Breport_url=stdout -B database=file:/var/lib/aide/custom-aide.db.gz -C|/bin/tee -a /var/log/aide/custom-aide-check.log|/bin/logger -t custom-aide-check-report
/usr/local/bin/aide-init.sh

 

# cat /usr/local/bin/aide_app_cron_daily.txt

#!/bin/bash
#If first time, we need to init the DB
if [ ! -f /var/lib/aide/app_aide.db.gz ]
   then
    logger -p local2.info -t app-aide-check-report  "Generating NEW AIDE DATABASE for APPLICATION"
    nice -n 18 /sbin/aide –init -c /etc/aide_custom.conf
    mv /var/lib/aide/app_aide.db.new.gz /var/lib/aide/app_aide.db.gz
fi

nice -n 18 /sbin/aide –update -c /etc/aide_app.conf
#since the option for syslog seems not fully implemented we need to push logs via logger
/bin/logger -f /var/log/aide/app_aide.log -p local2.info -t app-aide-check-report
#Acknoledge the new database as the primary (every results are sended to syslog anyway)
mv /var/lib/aide/app_aide.db.new.gz /var/lib/aide/app_aide.db.gz

What above cron job does is pretty simple, as you can read it yourself. If the configuration predefined aide database store file /var/lib/aide/app_aide.db.gz, does not
exists aide will create its fresh empty database and generate a report for all predefined files with respective checksums to be stored as a comparison baseline for file changes. 

Next there is a line to write aide file changes via rsyslog through the logger and local2.info handler


2. Setup Zabbix Template with Items, Triggers and set Action

2.1 Create new Template and name it YourAppName APP-LB File integrity Check

aide-itengrity-check-zabbix_ Configuration of templates

Then setup the required Items, that will be using zabbix's Skip embedded function to scan file in a predefined period of file, this is done by the zabbix-agent that is
supposed to run on the server.

2.2 Configure Item like

aide-zabbix-triggers-screenshot
 

*Name: check aide log file

Type: zabbix (active)

log[/var/log/aide/app_aide.log,^File.*,,,skip]

Type of information: Log

Update Interval: 30s

Applications: File Integrity Check

Configure Trigger like

Enabled: Tick On

images/aide-zabbix-screenshots/check-aide-log-item


2.3 Create Triggers with the respective regular expressions, that would check the aide generated log file for file modifications


aide-zabbix-screenshot-minor-config

Configure Trigger like
 

Enabled: Tick On


*Name: Someone modified {{ITEM.VALUE}.regsub("(.*)", \1)}

*Expression: {PROD APP-LB File Integrity Check:log[/var/log/aide/app_aide.log,^File.*,,,skip].strlen()}>=1

Allow manual close: yes tick

*Description: Someone modified {{ITEM.VALUE}.regsub("(.*)", \1)} on {HOST.NAME}

 

2.4 Configure Action

 

aide-zabbix-file-monitoring-action-screensho

Now assuming the Zabbix server has  a properly set media for communication and you set Alerting rules zabbix-server can be easily set tosend mails to a Support email to get Notifications Alerts, everytime a monitored file by aide gets changed.

That's all folks ! Enjoy being notified on every file change on your servers  !
 

How to test if imap and pop mail server service is working with Telnet cmd

Monday, August 8th, 2011

test-if-imap-pop3-is-working-with-telnet-command-logo-imap
I’ve recently built new mail qmail server with vpopmail to serve pop3 connectins and courierimap and courierimaps to take care for IMAP IMAPS.

I further used telnet to test if the Linux server pop3 service on (110) and imap on (143) worked fine, straight after the completed qmail install.
Here is how to test mail server with vpopmail listening for connections on pop3 port :

debian:~# telnet mail.mymailserver.com 110
Trying 111.222.333.444...
Connected to mail.mymailserver.com.
Escape character is '^]'.
+OK <2813.1312745988@mymailserver.com>
USER hipo@mymailserver.com
+OK
PASS here_goes_my_secret_pass
+OK
LIST
1 309783
2 64053
3 2119
4 64357
5 317893
RETR 1
My first mail content retrieved with RETR commandgoes here ....
quit
+OK
Connection closed by foreign host.

You see I have 5 messages in my mailbox, as you can see I used RETR command to check the content of my mail, this is handy as I can read my mails straight with telnet (if the mail is in plain text), of course it’s a bit more complicated if I have to read encrypted or html mail, though still its easy to write a tiny parser and pipe the content produced by telnet command to lynx or some other text based browser.

Now another sys admin handy tip is the use of telnet to check my mail servers IMAP servers is correctly operating.
Here is how:

debian:~# telnet mail.mymailserver.com 143
Trying 111.222.333.444...
Connected to localhost.
Escape character is '^]'.
* OK [CAPABILITY IMAP4rev1 UIDPLUS CHILDREN NAMESPACE THREAD=ORDEREDSUBJECT THREAD=REFERENCES SORT QUOTA IDLE ACL ACL2=UNION STARTTLS] Courier-IMAP ready. Copyright 1998-2010 Double Precision, Inc. See COPYING for distribution information.
01 LOGIN hipo@mymailserver.com here_goes_my_secret_pass
A OK LOGIN Ok.
02 LIST "" *
* LIST (Unmarked HasNoChildren) "." "INBOX"
02 OK LIST completed
03 SELECT INBOX
* FLAGS (Draft Answered Flagged Deleted Seen Recent)
* OK [PERMANENTFLAGS (* Draft Answered Flagged Deleted Seen)] Limited
* 5 EXISTS
* 5 RECENT
* OK [UIDVALIDITY 1312746907] Ok
* OK [MYRIGHTS "acdilrsw"] ACL
03 OK [READ-WRITE] Ok
04 STATUS INBOX (MESSAGES)
* STATUS "INBOX" (MESSAGES 5)
04 OK STATUS Completed.
05 FETCH 1 ALL
...
06 FETCH 1 BODY
...
07 FETCH 1 ENVELOPE
...
As you can see according to standard to send commands to IMAP server from console after a telnet connection you will have to always include a command line number like 01, 02, 03 .. etc.

Using such a line numbering is not obligitory and also letters like A, B, C could be use still line numbering with numbers is generally a good idea since it’s easier for reading on the screen.

Now line 02 shows you available mailboxes, line 03 SELECT INBOX selects the imap Inbox to be further operated with, 04 STATUS INBOX cmd displays status about current mailboxes in folder.
FETCH 1 ALL instructs the imap server to get list of all IMAP message headers. Next command in line 05 FETCH 1 BODY will display the message body of the first message in list.
The 07 FETCH 1 ENVELOPE will display the mail headers for the 1 message.

Few other IMAP commands which might be helpfun on connection are:

08 FETCH 1 FULL
09 FETCH * FULL

First one would fetch complete content of a message numbered one from the imap server and the second one 09 FETCH * FULL will get all the mail content for all messages located on the remote IMAP server.

The STATUS command aforementioned earlier could take the following list of arguments:

MESSAGES, UNSEEN, RECENT UIDNEXT UIDVALIDITY

These commands are a gold mine for me as a sysadmin as it helps quickly solve problems, hope they would help to somebody out there as well 😉
This way is a way shorter than bothering each time to check, if some customer e-mail account is improperly configured by creating setting up a new account in Thunderbird.