Hi!
I am setting up Kamailio as SBC with upper registration. I can forward REGISTER messages to my softswitch, but 401 UNAUTHORIZED from it is not coming to my UAC.
Config section for registration route:

route[REGISTER_MY] {
    rewritehost("3.3.3.3");
    t_relay();
}

What I see with ngrep (1.1.1.1 - my external UAC ip, 2.2.2.2 - Kamailio ip, 3.3.3.3 - mera softswitch ip):

U 1.1.1.1:5060 -> 2.2.2.2:5060
REGISTER sip:2.2.2.2 SIP/2.0.
Via: SIP/2.0/UDP 10.2.1.35:5060;rport;branch=z9hG4bKPj85d21f7f-5c1d-4b0b-8e82-940c5cc9e21d.
Max-Forwards: 70.
From: <sip:101@2.2.2.2>;tag=b00a1df7-94bd-4b2e-8f63-15fea1a9015b.
To: <sip:101@2.2.2.2>.
Call-ID: e789b66c-d96c-45ca-b3b8-06bf65a2c3d1.
CSeq: 59629 REGISTER.
User-Agent: SFLphone.
Contact: <sip:101@10.2.1.35:5060>.
Expires: 600.
Allow: PRACK, SUBSCRIBE, NOTIFY, REFER, INVITE, ACK, BYE, CANCEL, UPDATE, INFO, REGISTER, OPTIONS, MESSAGE.
Content-Length:  0.
.


U 2.2.2.2:5060 -> 1.1.1.1:5060
SIP/2.0 200 OK.
Via: SIP/2.0/UDP 10.2.1.35:5060;rport=5060;branch=z9hG4bKPj85d21f7f-5c1d-4b0b-8e82-940c5cc9e21d;received=1.1.1.1.
From: <sip:101@2.2.2.2>;tag=b00a1df7-94bd-4b2e-8f63-15fea1a9015b.
To: <sip:101@2.2.2.2>;tag=2fb3028f366408eaa444e96dfd845447.8865.
Call-ID: e789b66c-d96c-45ca-b3b8-06bf65a2c3d1.
CSeq: 59629 REGISTER.
Contact: <sip:101@10.2.1.35:5060>;expires=600.
Server: kamailio (3.3.3 (x86_64/linux)).
Content-Length: 0.
.


U 2.2.2.2:5060 -> 3.3.3.3:5060
REGISTER sip:3.3.3.3 SIP/2.0.
Via: SIP/2.0/UDP 2.2.2.2;branch=z9hG4bK929f.8339e446.0.
Via: SIP/2.0/UDP 10.2.1.35:5060;received=1.1.1.1;rport=5060;branch=z9hG4bKPj85d21f7f-5c1d-4b0b-8e82-940c5cc9e21d.
Max-Forwards: 69.
From: <sip:101@2.2.2.2>;tag=b00a1df7-94bd-4b2e-8f63-15fea1a9015b.
To: <sip:101@2.2.2.2>.
Call-ID: e789b66c-d96c-45ca-b3b8-06bf65a2c3d1.
CSeq: 59629 REGISTER.
User-Agent: SFLphone.
Contact: <sip:101@10.2.1.35:5060>.
Expires: 600.
Allow: PRACK, SUBSCRIBE, NOTIFY, REFER, INVITE, ACK, BYE, CANCEL, UPDATE, INFO, REGISTER, OPTIONS, MESSAGE.
Content-Length:  0.
.


U 3.3.3.3:5060 -> 2.2.2.2:5060
SIP/2.0 401 Unauthorized.
Via: SIP/2.0/UDP 2.2.2.2:5060;branch=z9hG4bK929f.8339e446.0.
Via: SIP/2.0/UDP 10.2.1.35:5060;received=1.1.1.1;rport=5060;branch=z9hG4bKPj85d21f7f-5c1d-4b0b-8e82-940c5cc9e21d.
From: <sip:101@2.2.2.2>;tag=b00a1df7-94bd-4b2e-8f63-15fea1a9015b.
To: <sip:101@2.2.2.2>;tag=d67d5778704611e2a3c600505691470a.
Call-ID: e789b66c-d96c-45ca-b3b8-06bf65a2c3d1.
CSeq: 59629 REGISTER.
WWW-Authenticate: Digest realm="SIP-REGISTRAR", nonce="475a20c7".
Server: MERA MVTS3G v.4.3.0-38t.
Content-Length: 0.
.


U 2.2.2.2:5060 -> 1.1.1.1:5060
SIP/2.0 401 Unauthorized.
Via: SIP/2.0/UDP 10.2.1.35:5060;received=1.1.1.1;rport=5060;branch=z9hG4bKPj85d21f7f-5c1d-4b0b-8e82-940c5cc9e21d.
From: <sip:101@2.2.2.2>;tag=b00a1df7-94bd-4b2e-8f63-15fea1a9015b.
To: <sip:101@2.2.2.2>;tag=d67d5778704611e2a3c600505691470a.
Call-ID: e789b66c-d96c-45ca-b3b8-06bf65a2c3d1.
CSeq: 59629 REGISTER.
WWW-Authenticate: Digest realm="SIP-REGISTRAR", nonce="475a20c7".
Server: MERA MVTS3G v.4.3.0-38t.
Content-Length: 0.
.

Thanks in advance!

---
Alex Elokhov