topic Re: ftp connection pauses before asking for a login prompt in Operating System - Linux

ftp connection pauses before asking for a login prompt

Debbie Fleith — Tue, 09 Dec 2008 22:41:12 GMT

What would cause an ftp session to take several minutes before it issues a login prompt?

I have this problem even from the Linux server itself. I can start ftp and then issue "open localhost 3021" (a custom version of ftp is running on 3021) and I get my ftp login right away as normal. However, if I try to "open ip_address 3021", it takes a very long time to get my login prompt. This used to work fine and I'm not sure what may have changed.

Re: ftp connection pauses before asking for a login prompt

Steven Schweda — Tue, 09 Dec 2008 23:13:13 GMT

"localhost" is, I assume, 127.0.0.1, while
"ip_address" is, I assume, some other IP
address. (If you would post actual commands
with actual output, I might not need to
_assume_ anything.) The usual explanation
for a pause like this is that the server
(here, the FTP server) is trying to look up
the name associated with the IP address of
the client which is connecting to it. It
would be unusual if such a reverse look-up of
127.0.0.1 were anything but trouble-free, but
it's more likely that there's a DNS problem
which causes a reverse look-up of
"ip_address" to fail (often slowly). As a
quick test, you could try (on the server):

nslookup ip_address

If that's slow, then you probably have a
reverse DNS problem.

Re: ftp connection pauses before asking for a login prompt

James R. Ferguson — Wed, 10 Dec 2008 02:57:55 GMT

Hi Debbie:

I agree with Steven that a probable explanation for the delay is due to a DNS reverse name lookup.

Another way to verify that this is causing your problem is to add the ip_address in question to your '/etc/hosts' file. If the lookup order in '/etc/hosts.conf' is "hosts,bind" then the '/etc/hosts' will be consulted before the DNS server. The elimination of the delay would point to a reverse-name lookup.

Regards!

...JRF...

Re: ftp connection pauses before asking for a login prompt

Mark McDonald_2 — Wed, 10 Dec 2008 03:39:53 GMT

I have seen this before, and I agree with the above, it was a DNS problem. My machine only had one nameserver in resolv.conf. That server had gone down.

Check nslookup both on the ip and the hostname, how long do these take?

Re: ftp connection pauses before asking for a login prompt

Fredrik.eriksson — Wed, 10 Dec 2008 09:15:44 GMT

This is quite a usual occurance (atleast for me :P) in opensshd.
Since I'm not sure which ftpd you're running I'm just going to suggest to look for some config option telling your daemon to reverse lookup addresses or to use DNS at all. Normal operation doesn't need to use dns lookups for client connections (unless you're Ã¼ber paranoid and want to use FQDN/FQHN).

Anyway, hope this gives you a lead :)

Best regards
Fredrik Eriksson

Re: ftp connection pauses before asking for a login prompt

Fredrik.eriksson — Wed, 10 Dec 2008 09:18:30 GMT

Oh almost forgot, it can be in some ftpd's that it's trying to do a ident check or auth check. Should be config options for that also.

Best regards
Fredrik Eriksson

Re: ftp connection pauses before asking for a login prompt

Brendan Peter Murphy — Wed, 10 Dec 2008 10:32:56 GMT

Aside from DNS being the most likely culprit, similar problems can also sometimes be a mismatch between the NIC speed & the switch port speed. Check the NIC speed with ethtool,

eg :
# ethtool eth0
Settings for eth0:
Supported ports: [ TP ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Half 1000baseT/Full
Supports auto-negotiation: Yes
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Half 1000baseT/Full
Advertised auto-negotiation: Yes
Speed: 100Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD: 1
Transceiver: internal
Auto-negotiation: on
Supports Wake-on: g
Wake-on: g
Current message level: 0x000000ff (255)
Link detected: yes

Then check your network switch port is matches what you NIC speed.... in this case 100Mb/Full Duplex, card set to autonegotiate.

Regards,

Brendan

Re: ftp connection pauses before asking for a login prompt

Steven Schweda — Wed, 10 Dec 2008 13:45:59 GMT

> I have this problem even from the Linux
> server itself. [...]

> [...] mismatch between the NIC speed & the
> switch port speed. [...]

I'd expect such a hardware configuration
problem not to have much effect when the bits
don't actually leave the one system. And if
it did, I'd expect more trouble than a
one-time delay when the connection is formed.

> [...] I'm not sure what may have changed.

That'll probably be the real mystery.

Re: ftp connection pauses before asking for a login prompt

Debbie Fleith — Wed, 10 Dec 2008 17:06:55 GMT

More info in response to above questions:

1. nslookup by ip_address never returns an address:

nslookup -silent 10.100.0.22
Server: 10.100.0.19
Address: 10.100.0.19#53

** server can't find 22.0.100.10.in-addr.arpa: SERVFAIL

2. more /etc/host.conf
order hosts,bind

3. /etc/hosts entries:
127.0.0.1 localhost.localdomain localhost
10.100.0.22 vmms.brown1.local vbrown

4. nslookup by hostname works:
nslookup -silent vmms.brown1.local
[root@vbrown gbg]# nslookup vmms.brown1.local
Server: 10.100.0.19
Address: 10.100.0.19#53

Name: vmms.brown1.local
Address: 10.100.0.22

5. The switch settings match the settings returned by ethtool.

Any other ideas would be appreciated.

Re: ftp connection pauses before asking for a login prompt

James R. Ferguson — Wed, 10 Dec 2008 17:23:46 GMT

Hi (again) Debbie:

If the 'nslookup' by name works but the lookup by IPaddress doesn't, then the DNS server is missing the PTR record to enable reverse lookups.

Speak to whoever manages your DNS server and have the record added.

Regards!

...JRF...

Re: ftp connection pauses before asking for a login prompt

Steven Schweda — Wed, 10 Dec 2008 19:33:00 GMT

> ** server can't find 22.0.100.10.in-addr.arpa: SERVFAIL

And was this slow enough to account for the
delay when opening an FTP session? Sure
smells like a reverse DNS look-up problem to
me.