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SSH Login Stuck At : “debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP” CentOS/RHEL 7

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The Problem

Trying to login via SSH, the session hangs. Adding “-vvv” option, shows that it hangs with below messages:

debug1: expecting SSH2_MSG_KEX_ECDH_REPLY

The Solution

MTU was set to 9000 :

# ip add| grep -i eth0
2: eth0:  mtu 9000 qdisc pfifo_fast state UP group default qlen 1000
inet x.x.x.x/24 brd x.x.x.x scope global noprefixroute eth0

Tcpdump captured for both nodes shows that a packet whose length is greater than 1500 was being retransmitted.

5 11:13:10.261757 x.x.x.x y.y.y.y TCP 66 44545 → 22 [ACK] Seq=3850969779 Ack=2311909736 Win=26880 Len=0 TSval=232224265 TSecr=231193894
6 11:13:10.264136 x.x.x.x y.y.y.y SSHv2 87 Client: Protocol (SSH-2.0-OpenSSH_7.4)
7 11:13:10.266977 x.x.x.x y.y.y.y TCP 66 22 → 44545 [ACK] Seq=2311909736 Ack=3850969800 Win=26880 Len=0 TSval=231193904 TSecr=232224268
8 11:13:10.813477 x.x.x.x y.y.y.y SSHv2 87 Server: Protocol (SSH-2.0-OpenSSH_7.4)
9 11:13:10.813678 x.x.x.x y.y.y.y TCP 66 44545 → 22 [ACK] Seq=3850969800 Ack=2311909757 Win=26880 Len=0 TSval=232224817 TSecr=231194452
10 11:13:10.814416 x.x.x.x y.y.y.y SSHv2 1562 Client: Key Exchange Init
11 11:13:10.924128 x.x.x.x y.y.y.y SSHv2 1346 Server: Key Exchange Init
12 11:13:10.963964 x.x.x.x y.y.y.y TCP 66 44545 → 22 [ACK] Seq=3850971296 Ack=2311911037 Win=29440 Len=0 TSval=232224968 TSecr=231194563
13 11:13:11.020996 x.x.x.x y.y.y.y SSHv2 114 Client: Diffie-Hellman Key Exchange Init
14 11:13:11.021435 x.x.x.x y.y.y.y TCP 78 [TCP Window Update] 22 → 44545 [ACK] Seq=2311911037 Ack=3850969800 Win=28032 Len=0 TSval=231194660 TSecr=232224817 SLE=3850971296 SRE=3850971344
15 11:13:11.022944 x.x.x.x y.y.y.y TCP 1562 [TCP Out-Of-Order] 44545 → 22 [PSH, ACK] Seq=3850969800 Ack=2311911037 Win=29440 Len=1496 TSval=232225027 TSecr=231194660
16 11:13:11.228988 x.x.x.x y.y.y.y TCP 1562 [TCP Retransmission] 44545 → 22 [PSH, ACK] Seq=3850969800 Ack=2311911037 Win=29440 Len=1496 TSval=232225233 TSecr=231194660
17 11:13:11.641942 x.x.x.x y.y.y.y TCP 1562 [TCP Retransmission] 44545 → 22 [PSH, ACK] Seq=3850969800 Ack=2311911037 Win=29440 Len=1496 TSval=232225646 TSecr=231194660
18 11:13:12.467981 x.x.x.x y.y.y.y TCP 1562 [TCP Retransmission] 44545 → 22 [PSH, ACK] Seq=3850969800 Ack=2311911037 Win=29440 Len=1496 TSval=232226472 TSecr=231194660
19 11:13:14.119969 x.x.x.x y.y.y.y TCP 1562 [TCP Retransmission] 44545 → 22 [PSH, ACK] Seq=3850969800 Ack=2311911037 Win=29440 Len=1496 TSval=232228124 TSecr=231194660
20 11:13:15.566176 x.x.x.x y.y.y.y TCP 78 22 → 44544 [FIN, ACK] Seq=4255038057 Ack=3492427307 Win=219 Len=0 TSval=231199205 TSecr=232109567 SLE=3492428803 SRE=3492428852
21 11:13:15.566249 x.x.x.x y.y.y.y TCP 54 44544 → 22 [RST] Seq=3492427307 Win=0 Len=0
22 11:13:17.419955 x.x.x.x y.y.y.y TCP 1562 [TCP Retransmission] 44545 → 22 [PSH, ACK] Seq=3850969800 Ack=2311911037 Win=29440 Len=1496 TSval=232231424 TSecr=231194660
23 11:13:24.027991 x.x.x.x y.y.y.y TCP 1562 [TCP Retransmission] 44545 → 22 [PSH, ACK] Seq=3850969800 Ack=2311911037 Win=29440 Len=1496 TSval=232238032 TSecr=231194660
24 11:13:37.259952 x.x.x.x y.y.y.y TCP 1562 [TCP Retransmission] 44545 → 22 [PSH, ACK] Seq=3850969800 Ack=2311911037 Win=29440 Len=1496 TSval=232251264 TSecr=231194660
25 11:14:03.691987 x.x.x.x y.y.y.y TCP 1562 [TCP Retransmission] 44545 → 22 [PSH, ACK] Seq=3850969800 Ack=2311911037 Win=29440 Len=1496 TSval=232277696 TSecr=231194660
26 11:14:10.067551 x.x.x.x y.y.y.y TCP 66 44545 → 22 [FIN, ACK] Seq=3850971344 Ack=2311911037 Win=29440 Len=0 TSval=232284071 TSecr=231194660
27 11:14:10.067984 x.x.x.x y.y.y.y TCP 78 [TCP Dup ACK 7#1] 22 → 44545 [ACK] Seq=2311911037 Ack=3850969800 Win=28032 Len=0 TSval=231253707 TSecr=232224817 SLE=3850971296 SRE=3850971345

In order to resolve the issue, set the MTU to 1500 in the network configuration files in /etc/sysconfig/network-scripts/ for the respective interfaces.

Note : If 9k is required for each NIC then all network devices need to be set to 9k and support it.

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